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[THESIS

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Univ crsity

      
   

THE ARGENTWI‘X’RIC DiuT EEHINATION OF THE HALIDES
USIHG THE DEAD-STOP ENDFL‘DH'

By

Mary Laud.” Huston

A THESIS

Snbnittad to the School of Graduate 8M1” of Michigan
State College of Agriculture and Applied Science
in partial fulfillment of the requirements
for the degree at

EASIER 0P EDIENCE

Department of Chemistry

1952

AC KNGJLLWT

The author viehee to emcee her einom
appreciation to Dr. Kenneth 6. Stone for hie
helpful counaol and guidance in the preparation
of this work. She also would like to than):
many of tho other nowhere of the Chenietry
Departaont for their advice and uaiotame dur-
ing her etay at Michigan State College.

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5. Determination of One Halide in the Promo of Another.
6. The Titration of Throo Halide: in a 151nm............

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PAGE

The Allah—31. 0: U.3.P. Pou’dm Emmdeeeeeeoeeeeeeeeneoee
The Titration of Chloride in Nitric Acid Media............
The Titration of Chloride in Acetic Acid Hedi\.m............
The Titration of Bromide in Nitric Acid Hedim........ .....
The Titration of Bromide in Ammonium Carbonate Median” ..
The Titration of Bromide in Acetic Acid Medim.............
The Titration of Iodide in Neutral Hedium..................
The Titration of Iodide in Armenian Hydroxide Media-u...”
The Titration of Iodide in Amonium Carbonate Hedim...” ..
The Titration of Iodide in Acetic Acid Hedim..............
The Titration of Iodide in the Presence of Chloride in
Amnim Cwbonat. mam-m....0.0.0.0.0....I'OIOOOOIOOOOOOI.
The Titration of Iodide in the Preeonce of Bromide in
main Cu'bcmu "wimeeeeeeeeoe-eeeeeeeeeeeeeeeeeeeeeeo
The Titration of Bromide in the Prosonco of Chloride in
manila cvboBQte “wimeeOeeeeeeeeeeleeeeeeeeeeeeeeeeeeee
Titration of Bromide in the Preeence oi' Chloride in Acetic

“m ”filmOIIIDOIOCOOCCOC...0.0COCOOIO'COCIOOOOOOOOOOOO.I.

The Titration of Halide hixtm‘ee in Acetic Acid Median..."

Typical Titration of a Halide Mixture with Silver nitrate
muOtio Acid nedilmCOOOOOOO00.0006...COOOOQDOOOOOOQOO.DO.

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33

I . INTRODUCTION

Recent studies using the dead-atop endpoint technique have been
primarily concerned with ite application to oxidation-reduction systems.
The polarization effect theory propoeed by Foulk and Bevdcn (10) and
Willard and Pentick (32) has been further tested by Stone and Scholten
(27) who shoved that the applications of the dead-stop endpoint are
auch more moron; than thoee originally preposed. tron their studiee
of‘different redo: syetens, Stone and Scholten suggested that the dead-
etop phenomenon was not a polarisation effect based on gas adsorption as
suggested by hull: and Bauden but concluded that the endpoint depended
on electrolytic oxidation at the anode coupled with electrolytic reduction
at the cathode. Stone and Scholten proposed that the dead-stop technique
could he used when an electrolytic procese was poesihle even if a redo:
eyeten were not present.

The deed-stop technique is an electranetric method which 11 noted
for its simple nethod of ascertaining the equivalence point by observing
the current flow on a sensitive galvanometer. The endpoint ie character-
ised, in general, by three possible raye in which the galvanoneter may
function: (a) the appearance of a shall flow of current at the etoichio-
netric point; (b) the disappearance of a mall flow of current at the
stoichionetric point} or (c) the gradual disappearance of the current
followed by a current increaee after the endpoint. The functioning of
the current in these endpoints nay be explained ae follovet

(a) an electrolytic «u begine to function ae the etoichiceetric point
in relched land therefore, a current Oppeareg (b) an electrolytic cell
fuetione until the endpoint ie reached causing a current to flee up to
the etoichialetric paint and then dieappeer) or (c) one electrolytic cell
oeaeee to function ae the endpoint ie reached and e eecond electrolytic
cell begin to “notion at the etoichionetric point. The-e endpointe
are not e epecial cm of the nonel memetric titration. In the dead-
etep lethod, two half-cell reactione ere involved and the current which
flow between then ie honored, while in the uperoutric lethod, only
one half-cell reaction ie important with the amount of oxidation or
reduction et the dropping lei-cm or rotating platimn electrode being
the hportent factor.

the upercletric eethcd differe free the dead-etop in that with the
form- the cal-rem ie eheerved throughout the titration and the endpoint
ie detenined graphicelly whereae only a eedden incroeee or ecdden
meetion of current 10 noted in the dead-etc}: lethcd. The cocluetrie
technique differe free the enpcrceetric and dead-etcp technique in that
it ie baled on the exact mnt of the quantity of electricity need
in the detenination. The quantity of electricity peeeed through the
eolction ie med and the anonnt of eubetence ie calculated directly
fr- !eredq'e LII. A con-tent current method in but eince tile can
be accretely loomed. An indicator in the eolction new he need to
deter-inc the approxinate endpoint, or potentioeetric eethode are poeeible.
tn-pereeetrieeethedfordeter-iningthe endpointbaebeenapplied to
ecllmetrio titration which nee electrolytically genereted halogen.

Actually, thie no a dead-atop technique eince two indicator electrodce
were coed and the firet large current increaee due to an emeee of the
generated halogen lurked the endpoint of the titration.

The endpointe noted in theee caeee are good emplee of the firet
endpoint type in which a current appeare or greatly increaeu at the
etoichionetric point. Kycre and Swift (20) need no.- uumd in the
coulonetric titration of areenic by cache of electrolytically generated
bromine. An inteheediate electrode reaction concieting of the anedic
oxidation of bromide to bromine wee need and the first exceee bromine
«need a current increaee between a second pair of platinum electrodea.
Thie cmwent increaae was caeily noted on a eeneitive galvanceeter.

Il‘he eecond pair of electrodea which were referred to aa 'indicator
electrodee" had a mall potential difference iepreeeed acroee thee ae do
the electrodea in the dead-atop eethod. Brown and Swift (3) tpplied the
acne method to the coulonetric titration of antiecny while‘ seaee , Iienann,
and Swift (26) need electrolytically generated bromine in the detoninaa

_ tin of thioglycol. Smilax-1y, mar, Hyere, and Swift (19) applied it

to the titration of chronate and vanadate using electrolytically generated
cuprcue copper. A method uaing electrolytically generated iodine for the
titration of areenic wee developed by My, Parrington, and Swift (21)
and an atteept to apply electrolytically generated chlorine to the
detenination of areenic wee made by Fara-instep and Mft (8) . The
indicator reeponee wae elcwer for chlorine than for bromine or iodine,
eating chlorine the hut eatiefactory of the three. In hie couloeetric
noticed for the titration of B-quinolonol with electrolytically generated

bro-inc, 3810!! (’4) need an indicator eycton which he described ee e
hybrid of the enpemnotric indicator or Hyore end swift and the deed-etop
indicator of Pool}: end Ban-don.

To determine the approximate endpoint in the above nentioned titra-
tione, the indicator current one vetched on a excitable galvanmcter end
the titration nee etOpped ae econ no the current began to riee. Then
all]. incremente of the halogen were again generated into the eyetu and
galmceeter readings were taken. A plot or the indicator con-ent term
the tine of generation was conetructmi from the data obtained in thie
manner and the line use extrewlatad to the time axis to give what is
referred to by the euthore as an 'uperonetrio“ endpoint.

The tint endpoint type ha aleo been applied by Weir and Shoo: (7)
to the determination or bromine addition nubere. The flee or current
et the equivalence point nee detected by the opening or an electric eye
of a cathode ray tube. Brace (2) need 4!. einiler method in the mercuric
ion-catalyted brownetion or double bonds And found that it gem a good
endpoint. Another lpplioetion can that or Seinehart (29) who applied the
deed-atop technique to the titration of etenmun chloride with iodine end
of zinc ione with terrooyenide ions. The enceee etennoue chloride gave
e enell eteady manometer deflection when 50 nillivolte were applied
men the pletimn electrodce. Iodine wee added end u the endpoint
wee approached the eeneitive galvanoneter ehomd enell deflectione which
returned to the original eteedy readings. The endpoint we considered
to be the tint penchant deflection. The nettwd for the determination
of nine m chiller. A Inclined deed-etc; netl'uod woe need by Gale and

Hoeher (11) in the damminatdnn of ninigran quantitiae of media. in
tho praaonoe or waning. Werninont and Hopkinaon (30) applied the dead-
atOp nothod to the microtitration of selenium. They added an exneae of
thioanltate and titrated thie excoaa with the iodine liberated from an
iodata-iodine ablation. At the endpoint, as in the above canoe, the
galvanouoter deflected away from zero. The attemt to ram" thia titre;
tion and to titrate‘ the liberated iodine was not neonatal. One of the
mat recent applications of tho dead-step technique was that by Scholton and
Stone (2L) and Ferraro and Brazuin (9) to the datomination of winery
aninea by diuotization.

Le nontioned before, the above cited titrationa are exanploe of the
first dead-atop endpoint type in which a current begins to now at the
endpoint. Thia typo in probably the nest common of the three. It» hoe
been pointed out by Kenny, Cooke, and Yunnan (22) that in a rovereible
ayaton, the current initially and at the endpoint abould approach eero
but that the dead-atop method in applied neatly to irreversible mtma.
The scope of thia firet endpoint type vaa widened by Clippingor and
Foulk‘a (6) application of "aloctronetric indicate re' no that thn dead-
atop technique night be need in neutralization and precipitation methode.
Clippingor and Fault mod the iodida-iodata ayatel la the anodio de-
polariear in the neutralixation reactione and eodiun nitrite in the
precipitation reactiona.

Tho aooond endpoint typo in which the current ocean to now at the
endpoint 1a amplified by the original work of Foul): and Baden (10) in
{which the dead-atop lethod I” accidentally diacovered. In their experiment,

 

1lmlnnfluflg

Paul]: and Burden titrated iodine with thiosulfate and the light apot
remained of! the gelnmnetor ocalo and no deflected back to zero at

the endpoint. Thia ia in agreement with the theory ‘of the amnd end-
point typo aa the thioauli'ote-tetrathionato system io irreveraible and
no current would be expected to flow at the endpoint. Another example
of thio type canoe tron the work of Bohr-coder, Kay, and hille, (25) who
applied the dead-atop technique to the determination of uino acido by
the iodonotrio titration of their copper oelta. Probably one or the loot
useful adaptation of the dead-atop technique io that devioed by Vornincnt
and Bopkinoon (31) for the Karl Fiooher method at determining water. In
their work, Worninont and £10an round that 10-15 millivolta applied
potential m enough to deflect the pointer off the acalo where it re-
uined. during the addition of tho otandard water in methyl alcohol
aolntion. The pointer returned to ooro at the endpoint. The rovoroo
titration nae attaspted but the endpoint nae not ae good.

The thind endpoint type can only ocenr in redo: work when too ro-
voroiblo oyatena are present, one preceding and one following the endpoint
of the titration. An ample oi’ thia ia round in Vooeter, l'arrington,
and sum- (33) work with the coulonotrio titration of iodide nth
electrolytically generated broninc. The galvanonoter regiatorod an
initial indicator current believed to be due to the iodine rooulting from
air oxidation. Thio current incremd to a broad nan-u- corruponding
to the oxidation of iodato to iodine, followed by a nininul near the
equivalence point corroopcnding to the oxidation or the iodine to the

nonovalent poeitive ion, and finally followed by a current increaee due
to the exceee bromine. Often.in those titretione it'uae sound necessary

to plot the current verm the tine of generation end extrapolate to get
the accurate endpoint.

II. THE THRATICIN O? HALIDES

There have been eeveral etudiee of the application or electruotric
mthode to the titration of the halides with a etandard eilver eolution.
The "galvanoeetric endpoint" developed by Salonon (22) in very einilar
to the work carried out in thie experiment. Salomon'e nethod m
developed tron an attempt to dotencine the actuality of a eilver nitrate
eolution using a 0.01 I potaaeiun chloride eolution. The eolution to be
titrated wee prepared by accurately-rennin; about twenty millilitere
of the potaaeinl chloride eolution into a beaker and diluting with water.
Silver electrodes clamped to a eeneitive gclvanoneter were Succeed in
thie eolution and an e.e.i’. or 100 nillivolte wee ilpreaeed aemee then.
The lilver nitrate eolntion, when normality wee to he detenined, nae
added elowly tron a hunt and the galvanoneter deflection eae obeemd.
The firet drop or eilvor eolution eetabliahed a current which no noted
on the galvanoeeter. Every drop or edlver nitrate added owed a alight
rise in the flow or current. The firet exceee drop oi’ the ailver nitrate
after the etoichionetrio point caused a rapid increcee in the current
nee. Thie amp nae deecribed by Selmn ae being nan-om; eharp on a
‘eeneitive golvemeter. The Yolhard nethod m md ae the control and
the reenlta of the two methode agreed within one part per thousand. The
advantage of the eharp endpoint and the poeeibility of overtitrating and
etill determining the endpoint nae pointed out by salmon. Thin m the
only attamt in the ‘eilver halide eyeteu, however, which Sale-on aade.

The method need in thie experincnt ie einilar to Salomon‘e in that
the etandard ailver ion eolution is added to the halide eolution and the
current flow: ie observed. A web caller e.e.f. of about 10 IV. ia
applied acroee the eilver electrodes and a different type of endpoint ia
obeerved. This will be described in detail in the eminentel notion.

Potontionetrie titratione using a eilver electrode ae the indicator
electrode were applied by Bohrend (l) to the titration of halidea with
silver nitrate. Behrond found that it was poeoible to titrete halide
paire by adding menial: hydroxide to increaee the solubility of the
more eolnble eilvor halide. The first drop in potential when working in
moniacal eolution was equivalent to the mount of the lean eoluhle
halide. Then, nitric acid was added and the eecond potential drop
corresponded to the total halide present. Bohr-end could not titrete
bromide in the presence of chloride in thie leaner. Clark (5) , who wee
primarily interested in photomphio muleione, did further work in which
he neaenred the e.a.f. of a eilver electrode connected to a normal electrode
after adding eilver nitrate in mall incroaente. Thie method was quite
eucoeeeful for eingle halidee, hit the firet endpointe were late in the
titration of brouide-chloride and iodideobroaidc combinations in water
eolution while the iodide-chloride aixturee were only in alight error.

A 5% barium nitrate aediun gave better reunite for both halide paire and
the combination of the three halides in photographic anulaione. Clark'e
paper aleo containe a good bibliography of previous work in thie field.

It baa been found that the potentiometric method can be applied to
the deteuination of alkali halides in concentration ae lee ae 0.001 N

10

- but that the eilver electrodee were aloe in attaining equilibrim.
However, in Sutton'e book (28) , meeetone etatee that although Iixturee
of halidee can be analysed in thin manner, the reeulte are not alwaye
accurate and the inflectione on the potential curvee are not alum die-
tinct.

The coulonetric technique ha been applied to the deter-nation of
the halidee by Lingane and small (18) . The method m based on the
aeaeurenent of the quantity or electricity required to achieve a quantita—
tive reaction of the halide ione according to the reaction

is e I. —e- Ag! 0 e.
The halide eolution wee electrolyeed with a eilver anode eheee potential
no controlled very carefully and a platinun cathode nae need. Lingane
and Slall'e reeulte for the eeparate determination of 0.5-100 ng. of
the halide ione compared favorably with the claeaical nethode. Iodide-
bromide and iodide-chloride nixturea were analyzed with better accuracy
than the argontonetrio titration but the bromide-chloride combination
nae poor einoe eilver chloride ”precipitated with the eilver broaide .
During the titration the current decreaeed exponentially with tine and
finally drapped to virtually aero when the electrolyaie wee complete.
The quantity of electricity paeaed through the eolution was neaeured by
a couloneter and the amount of halide preeent u. calculated using
Paradey'e Law.

The nee of the dead-atop endpoint in titrationa using electro-

. lytically generated halogene ha been mentioned previoualy. in superc-
letric titration using rotating platimn electrodee hae been applied to

tho titration of tho holidoo with o otmdord oilvor solution. Loitinon
and Kolthoi‘t (l?) tirot ottuptod thio in tho titrotion of oilvor, which
hoo o ditfnoion omont, with potaooiun chloride, which hu no diffusion
our-rout. mun-n m1 mum: notod um tho ohrront doorooood pro-
portionally with tho dooroooing oilvor ion oonoontrotion ond rooohod o
vory lull rooiduol curt-out with tho prounoo or o lorgo omou of
potoooim chlorido. Thio ondpoint night ho oonporod to tho third dood-
otop typo in which tho ourront doorooooo and "ooh” ooro ot tho otoiohio-
ootrio point on! than inmoooo ogoin upon tho oddition of onooo titront.
tho otoichioootrio point omoopondod to tho intoroootion of thooo two
otroizht linoo. Very lot-go ourronto oompuoblo to tho originol difmoion
out-rout woro found ot o potentiol of 50 ow. worm tho 8.0.5. "ovon ortor
tho oquimomo point hod boon pooood. Thooo ohoomtiono woro ottributod
to tho reduction or oilvor ion- fru portioloo of tho oilvor chlorido
precipitoto in onoponoion and colloidol oolution.

Kolthoi’t and ans-m (13) and oilvor nitroto ond m Imporonotrio
noth in tho titrotion of oorooptono . Whoa working in on monioool
oodiul to provont tho intortoronoo of chlorido on! hronido, tho ourront
woo found to ho oooll or zero when tho oilvor woo not in omooo. Tho
donation ortor tho ondpoint oorroopondod to tho dittwoion our-rout of
tho onooo oilvor. _

Soon ottor thio, Loitinon, Joaningo, and Pork (15) ottupt'od to
rovoroo tho mlior work with tho holidoo by uoing tho otondord oilvor
oolution oo tho titront. Tho uporonotrio ondpoint with rototing plotinun
olootrodoo woo ogain nod. Tho detominotion of chloride was found to ho

12

loot mooootul in o oolntion 0.8 h with nitric ooid and contoining 0.1%
gelotin. In oolutiono loco than 0.002 I! in chlorido, I 50-75% ocotono
oolution we need to docroooo tho oolubility of tho precipitate. Similar
ottowpto with tho bromide ohowed that golotin woe unnecoooory in aqueous
oolution and ocotono woo ogoin need for low concentrotiono. To prevent
intori'oronco, o 0.01-0.02 l moniocol oolution woo hood. The titrotion
of iodide in o nitric ocid, noutrol, or dilnto monioool oolution was
found to he nocoooml. A oolution 0.1-0.3 l in mania hydroxide was
need to prevent the interference of chloride ond bronide. Loitinon,
Jominco, end ram (16) followed thoir oorlieo' work with the application
of tho one nethod to tho dotorninotion of holido wirturoo. Inclmtlod in
thio pope:- io on ortonoivo bibliography or proviouo electromotrio mothodo
need in holide doterninotiono. With this and their own proviono work u
o bockgronnd, Loitinon, Jenninge, and Parke found the boot method to be
ono in which the oolution woo node 0.1 I in monio for tho iodide, 0.8 I
in nitric acid for the mm, and 0.11 in galotin for the ohlorido
dotmimtiono. In cuporing thoir method to tho potontionotric technique,
thoy ototod that tho Imporomotrio method no more rapid but not too
occuroto, oopeeiolly in dilute oolutiono.

The dood-otop method using plotinuo eloctrodoo hoo boon applied to
tho determination of tho holideo by Clippingor ond Foul): (6) who propoood
the an of 'olootronetric indiootoro' in precipitation end neutrolizotion
deter-incubus. They and oodiul nitrite to tho onodic depolarizor during
the titration of on tho halidoo but tho iodide which in on modio de~
polorioor itoolf. A oharp reproducible endpoint was claimed end the

13

onthore theorized thot the first exceoo silver ion depolorieed the
cethode.

The deed-etop procedure used in this work roploceo the pletinun
olectrodoo of the previous uperonetrio end deedaotOp experinents with
the silver electrodes of the potentiometric titretions. However, the
enrront rather then the potentiel is cheer-Nd in order to escortein the
equivelence point of the titrotion. totally, the deed-stop endpoint
neing silver electrodes es epplied to voluotrio precipitetiono end the
'golvenonetrio titretion’ of bloom ere idonticol except thet Solo-on
need the helide nether then the silver solution es the stenderd solution
end observed e current inoreeee rether then e decreeoe in current et the
endpoint. Hie work, however, could be considered to be e prelinisery
experinent in the epplioetion of the then “blown deed-stop technique to
voluetrie precipitetion .

In order to toot the newly proposed nethod, o otndy wee node of its
opplicetion in different media to the wtitotive deter-initial: o! single
helices, one holide in the preeenco of enothor, end the eonhimtion of the
three holideo.

III . EXPWAL

1. Appsrstus

A convenient systo- for epplying the potentiel to the electrodes end
assuring the current thet flows is described by Nomi-om end Hoan
(31). The Fisher Scientific Coupon: Elecdropodo as found to ho very '
convenient for this work. The dropping nercury electrode wee removed
' from the Llecdropode end the loads fro: the two silver eloctrodeo were
inserted in its pleco. It woo then noodto epply the notentiol to the
electrodes end the enclosed sensitive ‘nlvenoneter wee need to neoonro
the flow of current. The ectnsl current wee not known but could he
mortunsd by the calibration of the gelvononeter. However, only the
relative current was necessery in this cm.

The silver electrodes were prepored by Joining silver and capper with
silver solder. This was then sealed to gloss tubing with De Khotinoky
cenent snowing epproxinetely one on. of silver wire to protrude fro. the
end of the cool. The cenent eeel woo then coeted with glyptel to increeoe
its weter resistance. The upper wire extending free the other end of
the tubing wee clnped into the leads of the necdropodo.

In leter work, silver foil electrodes were need which were properod
by Joining e piece of silver foil shout 2.5 by h on. to silver wire which
wee, in turn, soldered to cOpper wire 1nd sealed in close tubing in the

m “01'.

15

2. insects
Il'he stendsrd 0.1 I silver nitrste solution used in this experiment

wee preprod free mor's Anely-eod silver nitrete in one liter quentitios .
The epproxineto enount necesssry for one liter of tenth noreel solution
woo weiaknd cut end dried in en oven for one hour at 110° C. It was then
weighed on on endytioel belence end the solution see prepared by diluting
to one liter in e retested volumetric flask iron which it we transferred
to en moor bottle for storege end use.

' Baker's Analysed eodion chloride was dried in the oven st 110" for
one hour before use. rho potassium bromide need use of U.S.P. purity
end therefore, its purity wee checked gre-vinotrioelly so silver bromide.
The results of the enelyeis ere tabulated in Table I. The potassium
bromide was found to be 99 .93 pure or better. It wee dried in the oven
for one hour at 110° C before being weighed out for us. Pctsseiun iodide

TABLE I
THE AMISIS OI UJJ’. POTASSIUH BROMIDE

 

Seeplo 30. Weight Er. I:
. Token ound

l O.h638 0 .1163}:
2 o .5129 O .5123
3 O .1666 . 0 .hSSB

Vi

(Fisher's A.c.8.) woe dried in the oven in the sons eenner before being
weighed out.

15

The 2% dextrin solution was prepared by weighing out epproxinetely
20 gene of dextrin end dissolving it in one liter of distilled ester.
The stanchion cerbonete solution was prepared by weighing one mole of
reagent-grade moniun carbonate end diluting it to one liter with die-

tilled water .

3. Genet-e1 Procedure

The senple which was to be enslyted wee weighed into e 250 nl. beeker
sith the «epic size being limited so es to require between 15 end 60 ml.
of 0.1000 I silver nitrete solution. One hundred milliliters of distilled
ureter were edded slang with 5 ml. of the 2S dextrin solution to prevent
the edherenoe of the helide precipitate to the silver electrodes. The
electrodes were then inset-led with only s short length of the protruding
silver wire ectuelly being in the solution. The stirrer wee started end
the zelvenoneter wee set et zero. Then e potentiel of 10 Iv. wee epplied
scross the electrodes.

The mount of epplisd potential is very inportent es the fund-lentil
requirement for the production or the deed-stop endpoint is the use of
the highest potentiel possible between the electrodes which still esuses
little current to flow. This potentiel on best be eecerteined experiment-
elly by propel-in; e solution of the weter end dextrin end edding one drop
of the silver ion solution. The electrodes ere incl-sod, the stirrer
stertsd, end the potential is spplied in epproxiaetely 5 nv. increments
while noting the gelvenoeeter deflection. The potentiel et which e rsther
1033. current incrseee is noted approxiletes thet which should be epplied

17

in tho detorminstion. Sometimes this method does not give too slurp a
break and the only m to ascertain the correct. potential is through the
"trill and error" method. The largest potential possible should. be im-
pressed across the electrodes in order to obtain the sharpest endpoint.
In working with the silver-silver ion agate: it use found that s current.
jump occurred when 5 IV. were spplied but s sluggish endpoint was obtained.
The oyster gave s much sharper emipo int. when 10 It. was applied although
the increase in potential had little effect on the original current.
registered on the zelvsmmetsr. Since the highest. sensitivity possible
was required in those titrations, the gelmeter of the mocfiropode was
Operated st the full sensitivity of 0.011 ucmmporos per m. st all
times.

After the preparation of the sample as outlined shove end the small--
cation or the potential, the 0.1 I silver solution as sdded from s 50
ll. burst in such s ”mar that rapid drops “rather than s stress: or titrsnt
entered the solution. The spproaoh of the endpoint was usually indicated
by fluctuations in current followed by s gradual decrease in current flow.
When this occurred the titrsnt m added dropuise snowing time for
equilibration before the addition of the next drop. M. this point in some
titrstions, the current deoresss us very slow end one to two silliliters
of silver solution but to be sdded in this owner while in other cases
loss than 0.5 or s milliliter was sdded to reach the final endpoint.

Also of utmost importance as the sensitivity of the electrodes.

In order to move :11 adhering precipitste end impurities from than
following s titrstion, the electrodes were inersed in s oomentrsted

18

sodios thiosultste solution end washed with distilled wster before being
used sgsin. Electrodes which hed been exposed to sir for some time
sppesred to be sluggish when used in s titrstion. They seued to be
casted with s culling film which was thought to be an oxide. In ouch
uses the electrodes sore scraped with s sharp object end then int-creed
in s thicenltste solution before using.

' h. Dcterninstion oi' the Single halide

s. Chloride

The first sttanpts using this method were in the dotorninntion of
the chloride. A sample of sodium chloride was prepared as described above
and the titretion was csrried out in s solution containing five drape of
concentrsted nitric scid per 100 ml. The endpoint in this case was not
too sharp, probably due to the foot that the silver chloride is the most
Soluble of the three silver hslids salts. When the potential was first
spplied to the solution the current surged upward and then dropped slowly
to s constsnt rennin near zero. Upon the sodition o! the silver solution
the current began to rise quits rapidly and then I hit more slowly,
finally reaching s unison st which s current reversal occurred. The
endpoint us not merited by s sharp current docroseo but rather the current
dropped to s low st the endpoint upon the drcpwicc sdditlon of the eilvsr
solution sud “holly tho addition of one drop caused s grsdnsl ctu'rent
incresse. The duct-sue in current es the endpoint was spprosohed was very
zrsdnsl end st this point s short time wss sllcwed tor squilibrstion after
esch drop was eddod. A few eness drape of the silver solution, on the
other bend, ceased s rsther lsrge incresee in current. In Table II ere

19

recorded the results or some titretions of chloride in the nitric acid
neditsa. Those titretions were carried out very slowly in the region of

TABLE I:
m: THEATER or CHLORIDE Ill mam ACID mm

Sample Rog Weight MCI. 3,
Islam ound

 

1 O .1780 0 .1187
2 0.2553 0.2%
3 0.2 6148 0.2623
b 0.2295 0.2293
5 0 .2321 0 .2317
6 0.2330 0.2 329
7 o .26h2 o .26h9
8 O .1881 0 .1878
9 0.2530 0.2531;

 

the endpoint so the electrodes could equilibrsts end sore scour-etc results
could be obteined.

The chloride as elso titreted in scetio ecid eediun. lten drops of
alsciel scetic acid were edded to 100 ll. of the solution. the endpoint
use quite siniler to that found in the nitric ecid nediun but use s bit
eesier to escortein. Tshle III lists the results of these titrstions.

TABLE III
THE TITRATIDN OF CHLORIDE II ACHIC ACID HEDIUH

Sample Ho. Heigt lfll. I:
A Tsken ound

 

1 0.2361 0.2367
2 0.25% 0.2516
3 0 .2337 0 .2310
1: 0.2071: 0.2071
5 0.21.86 0.2h82

20

in ettenpt to increase the sensitivity of the chloride detersinstion
wee node by using the larger silver foil electrodes. these did give s
slightly sore repid end lsrcor gelrenosetor deflection st the endpoint

in ecetio eeid sodium liking it s bit euier to follow the titretion.
the toil electrodes did serve to isprove the chloride endpoint.

b. Bronide

The nitric acid "dim use the first one tried for the titration
of hc'onide. The current bchsved such so it had during the chloride
deter-instions. It epprosched s nexillul shout helt-wsy through the
titretion end neer the endpoint the eta-rent reversel wee indiceted by the
needle shitting shout the center of the solvencseter, tinelly going right
repidly end then left. Again, the endpoint occurred st the lowest
selvenoneter reading which was found by sdding the silver solution drop-
Iiss when the epproech of the endpoint see indiceted end slloving equili-
hretion with such drop. fl. find Midpoint required spproxilstely
0.1043 .15 Ill. of titrsnt sfter the first lsrge current revered. However,
it use essier to sscertein then the chloride endpoint. stle IV gives
the results of these titratione. It should be stressed thst the operetor
lust be familier with the endpoint before reproducible results csn be

obtsined .
TABLE IV

THE TITRATION 0F WIDE II NITRIC ACID mun

 

Sample lo. Height KBrI g.
Token ound

1 0.3108 , o .ahoa
2 0.3595 0.3558
3 o .1656 ' c .161.

h 0.3335 0.3326

 

21

In order to study the behavior of bromide in monieosl solution,
titrations were ettonpted after adding 5 ml. of monim csrbonate solu-
tion rather than nitric scid to the prepared simple solution. The titre-
tion proceeded such es the previous one but the gelvsnoneter deflection
use slightly less sherp in the moniecsl solution es would be ewud
due to the slight solubility of silver wayside in nonin- hydroxide.

The endpoint wee egein taken st the lowest current flow indicsted by the
lowest gclvenoseter resding end nore rspid titrstions resulted in larger-
errors. Tshle V lists the results of the titretione of bronide in
whim carbonete solution. It will be noted that the lest deter-ins-
tione in this sequence gave the best reeults. This further substantistee
. up stetuent thet these titretions mom be done by so experienced '

Operltol‘ e

TABLE V
THE THEATRE OF BRSflIDE II mom CARBOHATE HEDIHM

~_ k L __
*w T _ _‘ A

sample to. Weight mr, g,

 

Taken Found M‘
1 0.2896 0.2883
2 0.3351 0.3330
3 0 .3218 0 .3208
h 0.2889 0 .2856
5 0 .2931: 0 .2939
6 0 MM: 0 .h'lhh

 

The third sediun in which the bromide wee titrated use one in which
ten drops of glacial scetic acid were edded to the originel bromide solu-
tion. The course token by this titration woe sinilsr to the others.

22

As the endpoint wee epprceched the current docrcescd quits rapiily end

the gelvencneter light shot of! the loft aide. This sharp deflection wee
caused by the sddition or only one drop of the silver solution end the
light returned after s short equilibretion time. The stoiohimctric

point in this use still coincided with the lovcat reeding obteined sfter
ellcwin; s short equilibration end en excess of silver ezein csused the
current to rise. The results of these titretions ere tsbuleted in Tshle VI.

TABLE VI
THE THEATER OF BRCHIDS IN mum ACID HEDIUH

W

 

ample lo. ' . Height Br. I:
. Tnkcn Found
1 0.5666 0.5663
2 0.5308 0 .5307
3 o .h636 0 M31;
1: O .1099 O .1696
5 o .1316? O .hléa
6 0 .5701 0 .5703
7 0 $3788 0 $779
8 0 .11625 0 .1361?
9 0.3832 O .3830
10 0.3661: 0 .3669

 

In guard, the bromide endpoints in ecid nedis were seeier to
escertsin then the chloride endpoints . The lower solubility of the silver
hronide would indicste this.

c. Iodide
The titration of iodide clone use not sttonptod in nitric ecid
nedin es mu s very small mount of the acid hed s tendency to releeee

23

iodine end gave incorrect results. Because of this , the first iodide
titretions were carried out in e neutral, aqueous solution. The midpoint
was s bit more obvious in these titrations. The current increased upon
the eddition of silver solution and the first clue of the approaching
endpoint was given by s sudden current reversal indicated by e shift of
the gelvencneter off the left side of the scale. The silver solution was
stopped st this point and the gnlvenoneter light returned slowly. It wee
noted that cosguletion of the precipitsts usually occurred st this time.
After this, the silver solution was edded dropwise allowing time for
equilibration end the galvsnoneter usually repented these fluctuetions
until one drop sensed s repid end large current increase noted by e shift
of the light completely off the right side of the galvancneter scale
where it rennined. This was taken es the finnl endpoint end corresponded
closely to the true stoichiometric point. The results are shown in

 

 

 

 

 

Table VII.
TABLE VII
THE TITRATIOH O? IODIDE IN NEUTRAL mm
M v Tr
Simple lo. Wei ht II
A 7 Taken Found
1 0.6595 0.6571
2 0.7711: 0.7711:
3 03351 0.5576
h 0.13573 o.h870
5 0.6113 0.6121
6 0.6337 0.6315
7 0.6182 0.6181;
8 0.8361 0.8.951;
9 0.61m 0.61.09

 

2h

The next sttenpt use the titretion of iodide in monisosl solution
prepsred by the eddition or 5 ml. or eonsentrsted menin- hvdroxide to
the mp1s solution. It Its very difficult to rind. sny (nanometer
behavior which could setuelly be celled en endpoint in these titretione.
A rather rspid current inoreeee indiosted by s repid shift to the right
ususllq' occurred in the genera region of the endpoint. The eosguletien
of the eolloidel silver iodide preeipitete occurred et the me the es
the shift. Further draperies sddition of the silver solution stter this
nun bed 1.11.1.1. effect on the current end the manna monoxide solution
seemed to esuse both s lets endpoint end s lste eeeguletion es will be
noted from the dete in treble VIII.

TABLE VIII
THE TITBATIOH OF IODIDE II WOW HYDROIIDE HEDIUH

 

Snple Ho. Take ‘1th II. 5., und
1: o
1 0 .3717 0 .3398
2 . 0.50h1 0.516l
3 0 .1172? 0 $797
’4 0 .5230 0 .5373
S 0.7hh3 0.7h5h
6 0 .5101: 0 52111
7 0 .8133 O .8171
3 0.5258 0.5356
9 0 .5153 0 .52 35
10 0 .59148 0 .6017

 

Sines eomentrsted moniun hydroxide geve such poor results and the
success of the titretion of iodide in s-onisesl solution use thought to
be of sons isportsnoe, the next sttmts were with the titrstion or iodide

25

in monit- esrbonste nediun. In these titretions, 5 ml. or the «nonin-
osrbonete solution were edded to the prepared sample end the titretions
were carried out in the uses]. nennor. The endpoint was such like thet
found in the neutral solution. deer the endpoint, one drop of the silver
solution «used the occurrence of e rapid current reversal with the
needle shifting to the left, eonetines going pest sero sad completely off
the selvenoseter soele. After e short equilibretion, the selvenoneter
light returned to the previous reading. ‘The next drop or titrent'nsuslly
caused e lerze inoroese in current. In the first ettonpts this endpoint
wee difficult to find and both my: end low results were obtained. After
observing e umber of titretions, however, the endpoint boom more
obvious. is will be noted by the date in.rehle IX, ell or these endpoints

occurred slightly early.

TABLE II
THE THEATION 0]" 10131138 H mm CARBWATE mm

m— w I
”Z M I

 

 

fl

Semle lo. Weight XI. 3.

Teken

to one!

1 0.5132 0.5116
2 0 .5158 0 .5124?
3 0.7ou3 0.702h
h 0.5052 0 .5031:
S mute o.L123
6 O.h906 0 .haeh
1 0 51.81 0 .Shao
8 O.h263 o.h2h9
9 0.5355 0.53M
10 o .3672 o .3661

 

26

Acetio eoid we the third nedim ettmpted for the iodide titration.
Ton drape of 3mm eoetio laid sore added to the ~10 before “W
ing. The endpoint in this one wee leee definite than before but gave
the new general type of current fluctuation. The eotnel endpoint
oocurrod at the point where the current greatly inoroeocd end the 53173.ne-
notcr light ehot rapidly off the right side and did not return following
e ehort equilibration. Thie usually coincided elnoet exactly with the
ooegnletion of the precipitate . Some data trot: those titration are
given in Table I.

TABLE X
T33 THEATER OP IODIDE Ill ACLLTIC ACID W

 

 

 

 

 

m wo v W~ mu» wW
Stools no. . m1 m. n
_ w: m _ Token . ' quad
1 0.5020 0.5017
2 0.2.225 mm:
3 0.5556 0.53552
g 9.5191 Debi-99 '

0.1.517 o.h813

S. Detereinetion of One Belide in the Presence of heather

The first ettenpte to detenine one halide in the presence of another
were made with the iodide-chloride eyeten. Thie titration would he
expected to be moeeei‘ul beceuee or the lover solubility of the eilm
iodide. Einoe Kolthefr (12) hed been sucoeeei‘ul neing monim carbonate
in hie work with edsorption indieetors end the halide determinetione, the
first ettenpte were node in this medias. The solution conteining known
iodide ees propered in the ueuel name:- except thet ”prominently one green

27

or the Beker'e Anelyeed sodium chloride wee edded. It was observed thet
5 rd. of the main: carbonete solution were not enough to give e sherp
endpoint so 10 ll. were need end the endpoint wee ilpreved. Again the
endpoint coincided with the lowest current resding registered on the
gelvenoneter during the find dropwiee eddition or the silver solution.

A smell current flow use registered on the culvenoneter even st the end-
point. !‘urthsr silver ion eddition eeueed e repid deflection to the
right due to e lerge current incl-em. is will he noted tron the results
in 'i'eble II, the empoint in ell the titretione wee slightly lets. In his
work with edsorption indicators, Iolthoft found 1:0 Iillip'ne of poteeeim
iodide could be deternined in the presence of one gren of poteeeiun
chloride nu: 1x eccurecy. The siniler‘enelyeie by the emu-mp eethod
gsve en even-eye error of less then three perte per thcueend tor the staple
renge of 0.]; to 0.8 grams of potaeeiun iodide in the presence of epprcxi-
netely one gra or sodiun chloride.

The next etteepte were with the determination or iodide in the
preeence of bromide. The sample was prepared in the some nannor with
epproxinetely one gran of 0.8.1’. pure poteeeiun bromide being edded to
the unple or poteeeiul iodide. The endpoint us not definite in this
titration. The current beg-n to drop within shout one eilliliter o: the
oelouleted stoiohioeetrie point end from thst tine the silver solution use
sddsd drapuise elloving tine for equilibretion. However, not: drop caused
only e slight lowering of the elm-rent em no definite endpoint res ob-
served long exter the eelculeted one bed been peesed.

25

TABLE II

TIE TITfmTION 0? WEEK IN 1‘51}? PI‘T'JRfCi OF CRUDE-"LIBS
IN mm CMxBOKfiTl‘l HEDIW

A

 

Sample no. Helga; t KII g,

Taken Found

1 0.1019 0.1023
2 0.1562 0.14313;
3 0.1.81? 0.L827
h 0.5952 0.5955
5 0.781;? 0.7th
6 0.6115 0.6136
7 0.5775 0.5807
8 0.63336 0.6330
9 0.5618 0.5630
10 oxma 0.6025

Teble III gives the results or some of. those titretione in‘v‘nioh the
endpoint could only be spproxinetsd.

no: rmmm or mom: III THE 912-61.. or Bit-1:19;;
In mom amounts: mama

Seaple Ho. weight XII 3,
Taken mm

1 O .6010 O .6863
2 0 .5672 0 .5996
3 o .6627 0 .7387
h 0.5714? 0 .6573
5 O .2 671 0 .2961;

A seven]. nillilitor exooee or silvar caused the oment to rise.

It was obvious that the endpoint had been ovoretopped es s colloidal

precipitate which did mt resemble the silver iodide wee beginning to torn

29

in the supernetent solution. This precipitate derkened rapidly upon
steading in the sunlight. in ettenpt wee nede to increase the sensitivity
by using the lerger foil electrodes. These electrodes eppeered to be
more sensitive in the titretion but still did not give en endpoint which
could be definitely observed through the gelvenoneter behevior. In his
work with the determination of iodide in the preeeme of bromide,
Iolthoi‘f (12) else round that the endpoint ems much too lets when adsorp-
tion indioetore were need. Since silver iodide end silver bromide ere
skill: in their solubilities, s definite Midpoint would not be entioim
petod.

The final ettonpte were epplied to deter-lining bromide in the presence
of chloride. is would be expected from their solubilities, the bromide
endpoint wee not sherp end one each too lete in monito- eerbonete Iedim
us will be noticed in table 1111, end further work in this mediu- wee

TABLE XIII

n12: TITMTIOE or BROMIDE n: we PRdSiflBE or swarm;
m mom amorous mm

 

#

Sample lo. , Wei ht IBr
. Teken ’ Found

1 0 4:678 0 .1383?
2 0 .3587 0 .3652
3 0 .5735 0 .5386
k 0 .1655 0 .1160?

ebendoned. Kolthotf else round the endpoint wee too lete end too in-
definite.

so

Attempts were elso nede to determine bromide in the presence of
chloride in eoetio eoid nediue. The endpoints in those deter-inetione
were such sherper end the results ere recorded in Table XIV.

TABLE XIV

TITHATIOH 01" 811011138 Ill THE PRESDEE G CHLORIDE
n ACMIC ACID HEDIIM

 

Senple no. Weig t KBrI g,
Token ound

1 0.5566 0.5671
2 0. 308 ‘ 0. 309
3 o. 636 o 6&2
1: 0.14399 0.1097
S 0.1%? 0.11171
6 0.5710

0 .5707

 

No more work was done in this group elthongh the epplicetion or e
more strongly monieoel solution might serve to improve the endpoint.

6. The Titretion of Three Relidee in e Hitters

The titretion of halide sixtnres wee originally only en ettupt to
determine totel helides. Staples containing chloride , bro-ide, end
iodide, which would require twenty-five to fifty nilliliters of tenth
nonlll silver nitrete solution, were weighed out. Five drape of dilete
nitric said were edded to the first prepered eenple end e visible snout
of iodine wee libel-steel. The next seeplee were titrehd in the presence
of ten end twenty drops of glecisl ecetio ecid, respectively. The lerger
esonnt of sold did not improve the endpoint. At the beginning of the
titretion, the current beheved in the need sinner. After sose silver

31

solution had been edded, the current inoreeeod rapidly end the pointer

was deflected completely of! the right hand side of the scale. However,
the galvenonoter light did return to the sale and leave again st leeet
once during the titration end e fair current revel-eel followed by the
reaching of the lowest current flow occurred very close to the true
equivalence point corresponding, to the total halides. Upon further study
of the signirioenoe of the first return of the gelvenoneter light to the
scale during the course of the titration, it it” found thet the milliliters
sddod et thet point corresponded spproxinetely to the mother of milli-
equivelente o: iodide plus bromide in the sample.

Another sample we titreted with drop-tine eddition of the silver
solution when the galvenoaeter pointer first returned to the scale. The
current dropped slowly to e low point where it meinod while en exoees
or ebout one milliliter of silver solution use sdded before the current
egein inoremd. it the endpoint equivelent to the tote]. halides, the
selvenoneter light egein returned to the scale end e low point in amount
fies ruched by dropuiee oddition o! the titrent. Coeguletion one observed
shortly before the endpoint res reached. Further work with these whine-
tions shoved on olose observation that mother drop in current occurred
betueen the beginning of the titretion end the endpoint due to the chloride
plus brouide present. It vest thought thet this could he equivalent to the
number of lillieqninlents of iodide in the Maple end in another smle,
the silver solution we pegein sdded dropvise st this point. This tiret
endpoint hed s tendency to arrive e bit lete end to. in; even more then the
iodide-bromide endpoint es one end oneg-helf to two milliliters of excess

32

silver solution were needed to dense s new inoreese in oment otter the
current low use reached. The next drop in current corresponding to the
iodide-bromide endpoint in the seaple me quite scourete. The Approeoh
of the final, endpoint was cheater-iced by e repid current revereel due
to the eddition of one dump of the silver solution. Further slow drape
vise eddition or the solution oeueed e decrease in current to the lowest
reeding followed by e slight current increase. The first excess drop of
silver caused s new decrease in current which did not go quite es low es
the preceding one followed by s repid increese in current. i lerger
eneesot eilverionesueedthe gdvmeterneedleteeeheederinite
shift to the right. It wee only otter soverel such titretione were
«tried out thet the operetcr could be sure of the three endpoints.

Tobie XV. gives the-results or some consecutive titretione suede in the
shove doeoribed manner. Since only the number of silliequivelents or t
the three halides had been recorded, the table stetos only the milliliters
required for the three seperete endpoints elong with the other of milli-
litere eotuelly used. | _

It an be noted that the intermediate endpoints usuallyoene
slightly late due to the leg which occurred when the less soluble helide
ves‘neerly ell precipitated end the next less soluble halide‘vos beginning
to preoipitete. Becmee or this everlepping, one nuet work with these
titretions for some tine before the intemediete endpoints ere recognised.

A generel titration curve plotting the current versus the milliliters
of silver solution sdded is shown in figure I. This shows the leg found
to be present in ttnse titretions. The first minimum corresponds to the

33

mam: 11:
me rrrmrmx or was rumors m 13:21:: 1210 1mm
_ ~ ~ __._ A v *2:

Sample Io. Iodide g Iociide+Bromide Total Halide:
_ m 111 Used lie fin can a: Re

 

 

El

        

 

   

1 12.00 12 .09 28.28 28.51 h2.69 113.30
2 7 .hl 7 .123 21 .75 22 .03 39.6% 39.90
E S .29 5 .32 11 .09 11 .15 143.96 1111.19
‘ 7.98 8.00 18.82 19.10 h6.83 h6.76
5 8.hh 8.h2 2e.ho 25.75 51.13 e1.18
6 6.92 6.92 20 .18 20.13 55.07 511.92
7 9 .20 9 .15 22 .28 22 .22 53.91». 53 .90
8 8 .97 9c: 21 .06 21 .10 35' .10 33‘ .c
- 9 6.31. 6.38 18 .10 113.10 115 .87 Mn:
10 12 .13 12 .11 311.97 $.86 53.51: 53.111

amount of iodide present, the second to the bromide plus iodide, and the
third to the total amount of halides as would be expected due to the in-
creeeing solubility at eilver can. in m. order.

Since the intemediete endpoints were not too sharp, e titration was
node using the luger silver toil electrodes. These electrodes did not
gently improve the first two endpoints but did make the total hslide
endpoint sppeer to be e little sore definite. A titretion was slso
sttenpted in dilute nitric acid solution but the endpoints were not merly
on WP.

Since mtimn, Jennings, and Parks (16) in their uperometric work
suggested I henge 12$ men‘ie totincreeee the sensitivity of the three
different endpoints, similar media were tried in this experiment. In the
first ettenpt, ten milliliters of the moniun’ carbonate solution rather
than pure mooniue hydroxide were edfled but the encipoint was no more

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detini’a. Fin drapa of concentrated nitric acid were adder} for the
humid: endpoint which I100 lagged. The ndditinn of five milliliters of
the dentin Iolution gave the usual chloride endpoint. and the use of five
drops of cancanu‘tted manila! hydroflde for the iodide determination did
improve the endpoint slightly. However, the arflpoint nu not churp in
my of the titrutiona in which ammonium hydroxide was used. Amounts vary-
ing from ten to twenty drops at both concantratod nitric and glacial
lactic mid were tried for the bromide endpoint ad the twenty drape of
concentra’ud nitric acid seemed to give the but endpoint. The tin-stint:
using twenty drops of the 5mm acetic acid on the other fund was not.
too successful. the Imitivity at tho total halide determination an
increased by the use of ten “that than five Iillilitorl of. tho 2% dextrin
Iolution. h : wholc, the India suggested by 'Lnitinon, Jennings, and
Park! in their war}: war. not found to be too maessrul in this experi-
ment. Huck nor-o work would be done in smegma media which might improve
the sensitivity of the triple endpoint titration.

I? . COIEUISIORS

It is evident iron: the above emerimwts that the dead-atop teami-
que can be applied to come vol‘imotrie precipitation reactions vim-out
the use of emdic depolarizere when electrodes of the cane metal as the
titrant era used. This notions! oaamt b3 applied to all metallic syctme
due to the tendemy of new metal electrodes to be sluggish in reaching
equilibrimn in e eolution. The epplication of the deed-atop method in
thie determination euggeete the need or e theory to ezplein the phenomenon
which would also be in agreement with previous ttxoorice concerning the
deed-atop endpoint. Part of the dead-atop theory in bued. on oxicntion
taking piece et the nude accompanied by 0. reduction prooeee et the
cathode. Here, the two eilver electrodoe me the identity of an mode
and eethode when e potezhel 1e epplied acroee then in eolution. Before
the titretion in actually begun, the electrodee will be eomundod by
at emcee of negative helide ione and the current regietered on the gel-
venoeeter when the mall potential in applied will be equal to or moor
eero when equilim-im ie etteined between the electrodes end the eolution.
When the eilm eolution in added, I eilm halide precipitete ie formed
at once end eoee of thie precipitate will come in content with both
electrodoe. it the mode, which he ettrected e large number of the
negative halide ione, oxidation in taking piece end eome at the eilver
metal of the mode in being oxidieed to eilver ione which react with the
halide ion in Inlution to precipitate out to e eilver halide. At

37

tho cathode, only o wall mount of the halide ion: on prooent and tho
momtion of the ailver halide to tron .1116: plus a negative halido ion
io tilting ploco. Ao the large ones: of halide ions in decroaaed by
their precipitation with the added oilvor ions, the wront increases
quito rapidly to indicated by tho tact rim of tho galvanomter pointer.
But u the mount of oilvar iono coded to tho oolution and the halide
ions which hovo precipitated out with the oilvor iono bocono more nearly
011211, on oquilibrixm between the reactions taking ploco at tho two
oloctrodeo is reached. This

i I A31 o O '3‘!- Ag + 1'
omulihritno oocounto for tho Madam in tho titrotion corn and no more
oilvcr icon no coded, precipitation continua to occur coming 1 ohirt
in tho oqnilibrim. Booms: o: thio, tho cur-root demoooo quito rapidly
at first ond than noro slowly u the loot bit of the mom. ion is pre-
cipitated. it tho otoichiomotric point tho amt ohonld ogoin In at
tho original can point but duo to tho olngymhnooo of tho ”tonic
olootx-odco it on]: roach“ olov point. Tho rirot'omcoo oilm ions
cmoo mother rapid imraaso in current by tho elootrolgrtic oouplo,
A: - Ag” + o. ‘X'hio thoory, along with tho knoolodgo at tho lolnbility of
tho oilnr holido alto, would account for tho noximo ond plotcouo soon
in Figaro I. It oloo would allow for the application of ouch o deter-
nimtion to othor how octol oyctno. An ItMPt a: manic to coo tho
lou-lood ion oyoton in tho tits-own o: chromto. 80mm, the method
on found to ho motiotoctory duo to the olnggiohnooo of tho load
electrode. oo tool]. to to the high oxidation potentiol of tho chromiu-

chrcnoto oyotom in the media used.

38

The accuracy of the single halide determinations carried out by one
familiar with the endpoint in cmnparabla to 122131: or the angercme tria and
potentiometric methods. Since the coulometric method was carried out.
with much smaller comantrations, the results 1.1:. mt comparable. The
dead-atop matmd 1! acre rapid than the potentiomatric titration and
requires & very simple apparatus as well 13 not requiring the plotting
of & graph ta detemina the exact. endpoint. This method has the uifi'antaga
that. dark-misread substances would not interfara with the endpcint as time
dc in the new, F5331“, and Volhard meténda. Alan, it is t direct dao'
termination and I in: rigarc-ua control of pH is required.

The accuracy of em halide determination in the preaanco a! large
“flaunts cf nmflmr has already been campared to Kolthoff’a work with
adsorption indium". A true cmparia—an canmt bs made as Kolflmfi'
state-:1 his accuracy {of I more dilute salution than was used in these
experiment... 30 results or accurlcios were stated by many of the former
workers in this field so that. no actual comp-rim m be made. The
method outlined in this paper for the determinatian a: the three halides
is a rapid am with a fair degree of accuracy. The potentiomatric
techniqae in not too good time only small potenfiial breaks of about 8.1
volt m round for the diffs-rent halides. ’

Rush more exparmmfition should be done before the study of the
numb-silver ion Iyatam will be complete. Other media should be triad
with tho tingle halides. In the determination of one halide in the
presence of another, different concentrations of amenity.) hydroxide might.

be tried u well 33 Clark“ (5) barium nitrate media. These mathods

39

should also b0 tried. with lower halide cement-rational. In the mixture of
the three, more study of media should be made in order to induce & manger
endpoint.

Also, than m w determinations in which tho silver electrode-
Iilver ion mite! could be attempted. An mpommetric method has :1-
ready boon doveloPod for the titration of nomaptans with silver nitrate
(l3) and maorption indiutors have been used in the titration of mar:-
uptobmothiazolo (lb) . Other applications might be mud.- in the
detemination of thiocyanato, telewoyamte, eyauate, cyanide, carbonate ,
iodate, oxalate, plmsphate, neonate, chromate, and moimto ions, higher
fatty acids and bar‘bitwio acid derivatives. Attempts haw boon made to
tpply the Valhard method in these determinations and m of the suggested
prooeciumn are outlinad by Kolthoi‘f uni Stetiger. (1h) .' Of course, the
moon of the detamdmtion wold depend on tho conditions necessary for
the Itoiohiometry at tho reaction, the solubility at the sum- cults,
tad the reactions taking place at the anode and cathoda in tho titration.

The dead-atop onzzlpoint teclmiqun is a simple, rapid, and occur-nu:
method of determining the halides. This application to volumetric Iro—
oipitation only further proves that the applications of the dead~stop
endpoint are definitely more mercu- than those preposed by Foulk and
Burden and do deptnd on the immity of the Lollytioal ohwiat u Stan:
and Soholton (27) but outed.

(1)
(2)
(3)

( k)
( S)
( 6)

( 7)
( a)

( 9)
(10)

(11)
(12)
(13)

(1L)

(15)

(16)

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Carson, w. 11., Jr., Anal.Chen., g3, 1555-1368, (195:2).
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Kolthoff, I. 14., and Wager, V. A” Wolmaatric A .10, Vol. II“,
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mm H. A. Jennings W. P. and Parks ‘1‘. D. Ind. Eng. Chm.
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. rmgton P. 3. «1:15th 3.1!. Am1.Cham.
3;. msv-lhéo.’(19m. ' ' ' ' '

 

 

 

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3 133