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ELECTROHETRIC TITRATION
OF CHROHIUM
WI T H
POTASS UM PERMANGANATE

 

Thesis
Submitted to the Faculty
of
Michigan State College
In Partial Fulfillment of the
Requirements for the Degree
of

Master of Science

 

BY

Alfred M. Malloy

June

1927

I‘. Y D Til}? T.

f.” 5....fl,“

1023637

TABLE OF CONTENTS

 

I. Introduction
II. General Theory
III. Materials and Apparatus
I IV} Procedure
V. Data and Discussion
VI. Conclusions

VII. References

ELECTROHETRIC TITRATION OF CHROMIUH

WITH POTPSFI'A PTFHANGAHATE

introduction

 

While working on the electrodeposition of

1 found no method

metallic chromium, Ewing and Malloy
available in the literature for readily determining
the chromium content and state of oxidation of the
constituents of the solutions which were used in the
investigation. The present work is an attempt to
surmount this difficulty, presenting a short, rapid
method, capable of a high degree of accuracy in the

hands of a trained experimenter.

The solution containing chromium is poured through
a modified Jones reductor, the reduced solution caught
in an air-free container and titrated with standard
potassium permanganate solution, the end point being

determined by means of a potentiometer.

The technique of the potentiometric titration of
oxidizing and reducing substances has been modified and
improved and the chemistry of divalent chromium has

been investigated, leading to an appreciation of the

possibilities of reduced chromium as one of the most

intense reducing agents known.

H. D. Newton2 reduced titanium by means of
metallic zinc and titrated the resulting solution with
potassium permanganate. The method required elaborate
precautions to insure ccmplete solution of the zinc
and an atmosphere of hydrogen was constantly kept above

the liquid during the determination.

Shimer and Shimer3 modified the method of Newton by
replacing the reduction in a flask with the use of 3
Jones reductor. However, the slow passage of the
liquid through the long, thin tube presented little
advantage over the tedious method preposed by Newton.
Here also, there still remained the difficulty of pre-

venting reoxidaticn of the reduced titanium by the air.

C. Van Brunt” has described a method for the
reduction of potassium dichromate by means of an elec-
trically heated Jones reductor, the reduced solution
being caught in a beaker containing excess of ferric
ammonium sulphate and the resulting ferrous salt
oxidized with standard potassium permanganate solution,

using the color of excess permanganate to determine

the end point. He indicates an accuracy of three parts

per thousand. This method obviates the difficulty

of keeping the reduced solution from being oxidized
by the air and presents an avenue of escape from the
slowness and inconvenience of the long column used by

Shimer and Shimer.

On the other hand, it has the disadvantage of being
indirect, ferrous iron being the intermediate ocmpound
formed in the reaction, A direct method would have

inherent advantages which are obvious.

The method proposed by the writer combines the
advantages of the Newton and the Shimer and Shimer
methods by reduction of the chromium in a beaker with
zinc-mercury amalgam and pouring the mixture through
a long, thin Jones reductor, the column retaining
particles of amalgam unconsumed in the preliminary
reduction. Electrical heating is supplanted by steam
heating, a process more adaptable to ordinary labora-
tory practice, The ferric sulphate of Van Brunt gives
way to potassium permanganate for anticipating the
action of the air on the reduced chromium and an
atmosphere of carbon dioxide replaces the lighter

hsd rcgen used by Newton.

The electrometric method used by the writer for

++

determining the end point of the reaction: Cr -—v Cr+++'
is very accurate, one drOp of reagent permanganate in
excess of the required quantity changing the potential
reading of the cell by as much as six-tenths of a volt.
_Since the resulting chromic sulphate solution isgreen,
the end point could not have been detected by color

change, even on the addition of a large excess of

permanganate.

- GENERAL THEORY -

Jean Piccards studied the autooxidation of
chromous salts and concluded that in neutral or acid
solution atmospheric oxygen in excess forms not only
chromic salts but chromic acid and compounds of an
intermediate degree which are stable for a measurable
time.

6 found that salts of divalent

Trahbe and Passarge
chromium1reduce water directly when the latter is
warmed or acidified. The divalent salts also reduce
the carbon compounds having double or triple bonds,

in acid solution.

These investigations show the instability of
chromous chromium and explain the precautions necessary

in the work of Newton, Shimer and Shimer, and VanBrunt.

When a platinum wire is immersed in a solution
containing chromous and chromic ions and connected by
means of an external circuit with a mercurous sulphate
cell, which in turn makes contact with the chromous-
chromic solution by means of a salt bridge, a potential
difference is established between the platinum wire
and the metallic electrode of the mercurous sulphate
cell. This potential difference may be measured by
means of a potentiometer.

The equation -

 

E _ E _ El. 10; activity of product of oxidation
' 0 NF ° activity of product of reduction

expresses the electromctive force of this cell, where -

E = the observed potential difference

E0 = the potential difference of a like cell when

the ratio activity of product of oxidation
activity of product of reduction

 

is exactly 1.

R = the gas constant in the van't Hoff equation.
Measured in volts X coulombs.

T : the absolute temperature

N the difference in valence of the metal in final

and initial states.

F : the value of the Faraday, or 96,530 coulombs.
In the case of chromium, the equation takes the form
+-t
”:0 o‘- 01‘
E 3 E0 + N” 10°10 ’6???

Forbes and Richter7 have determined the value of
Eo and found a deviation from the theoretical value
of the coefficient of the logarithm. They give the

value, referred to the normal hydrogen electrode as

zero, of
++
77. : o.‘400+0.065 10s .33....
. Cr+++

This determination was made at mercury electrodes,
the potentials on platinum reaching a.maximum about

0.16 volts lower, with evolution of hydrogen.

The value of E is a measure of the ratio of the
concentrations of Orw*and Cr+++ions. On adding the
oxidizing agent the falue of this ratio changes until
the concentration of the Cr++ ion becomes vanishingly
small, when there is a corresponding sudden rise in
potential. This point represents the end point of the

reaction.

If the measured electromotive force isplotted as
ordinate against quantity of oxidizing agent added, as
abscissa, a curve, which expresses the given function,
results. The point where the second derivative equals

zero is taken as the end point of the reaction.

The absolute value of the potential reading will vary

from the value obtained against the hydrogen electrode
by the value of this half-cell:-

+
Pt/Hg/ngsou (s), H2804 (.5), waasoLL (.3). H (l)/H2(g)/Pt

The initial readings actually recorded in this
work were around -O.8 volt and the end point around -O.3

volt, the final readings around-+0.15 volt.
- MATERIALS AND APPARATUS -

The apparatus used is illustrated in the accompan-
ying diagram,(Fig.l). It was especially designed for
this work and will be found to be very easy to duplicate.
It consists essentially of a beaker(A) of 500 cc.
capacity, fitted with a rubber stopper. An inlet tube
for carbon dioxide gas; an outlet tube for the gas; a
thermometer (T); a platinum wire (pt), six inches in
length; a stirrer (S), run by a small l/50th horse
power motor; the end of the delivery tube of the reduc-
tor and the longer arm of the salt bridge extend through
the rubber atOpper into the beaker(A). The delivery tube
reaches three-fourths of the way to the bottcm of the

beaker.

The half-cell consists of a hard glass test tube (G),
into the bottom of which a small platinum wire has been
fused, the tube containing sufficient mercury to cover the'

protruding end of the platinium, then a paste of

mercurous sulphate and mercury, on top of which is a
paste of mercurous sulphate and normal sulphuric acid.
The rest of the space in the tube is filled with
normal sulphuric acid which had been saturated with

mercurous sulphate.

A glass arm filled with the cell acid, sulphuric
acid saturated with mercurous sulphate, makes contact
with an intermediate vessel (I), containing a solution
of sodium sulphate, 49 grams per liter. An Ft? shaped
bridge (B) filled with the solium sulphate solution
makes contact with the vessel (A) by means of an arm
curved upward at the extremity. This curve prevents
the admittance of air bubbles when the vessel (A) is

removed.

The function of the intermediate vessel (I) is
.to allow for any desired change in the acidity of the
solution in the reaction vessel (A). In fact, a.basic
solution in (A) would not invalidate the applicability
of the cell(C) as the half-cell. Thus the set-up is

very elastic in its application.

The vessel (D) consists of a 5-liter bottle

containing the sodium sulphate solution used in the

salt arm (B). On opening the pinch clamp, the arm (B)

may be flushed into the reaction vessel, insuring no loss
of its contents, due to diffusion. The density of the
sodium sulphate was so chosen as to out down the

possible diffusion to a minimum.

The reductor column (E) consists of a Pyrex tube, to
one end of which a stop-cock length is fused. The other
and carries a Pyrex test tube fused to a piece of glass
of the same bore as the reductor tube. The upper end
of the column is fitted with a stOpper with an inlet tube
for carbon dioxide, for forcing the charge down through
the reductor column. The column contains a large plug
of glass wool (F) just above the stop cock and is filled
to a height of 500m. with Zn-Hg amalgam. The tube is
encased with a rubber sack into which steam is admitted.
The steam and water of condensation leave through the

opening in the bottom of the sack.

The platinum electrode from the reaction vessel (A)
and the platinum electrode from the mercurous sulphate
half—cell are connected to two of the terminals of a
Leeds and Northrup Student Type Potentiometer. Being
standard equipment, no attempt will be made here to

explain the potentiometer set-up.

_ 10 -

- PREPARATION AFB ANALYSIS OF EtT?PIALS -

The burettes used were calibrated at the Bureau
of Standards, Washington D.C., and checked. The pipettes
were calibrated at the Bureau of Standards and also
checked. The weights were calibrated in strict accord-
ance with the method of T.W. Richards8 on a very good
Voland and Sons Analytical Balance, using the method

of weighing by vibrations.

The water used was laboratory re-distilled water

and Baker's best chemicals were used.

The permanganate used was made up a month in
advance and allowed to "age". It was then kept in a
black bottle to prevent change in strength. The normal-
ity of the solution was determined against sodium
oxalate, certified by the Bureau of Standards and

containing 9 .9% sodium oxalate.

The potassium dichromate was standardized against
Mohr's salt, the Mohr's salt being previously checked
by analysis with the standard permanganate and found to

be 99 06¢) p‘dreo

The chromic acid solution was standardized against

the same Mghr's salt.

- 11 -

In order to standardize the solution of chromic
sulphate it was found necessary to use iodimetric

methods.

A standard sodium thiosulphate solution was made
up and checked against the potassium dichromate, which
had been previously standarized in terms of the primary

standard used in this work, the sodium oxalate.

The chromic sulphate was then oxidized with
sodium peroxide and the chromium estimated by means of

.L

the thiosulphate solution.

As a final check, the thiosulphate was standardized

against the standard permanganate.

- 12 -

- STANDARDIZATION DATA -

Standardizing KMnOu against Sodium Oxalate

 

    
   

Sample No.

Weight of

 

sample gme. .25415 .25633 .2557u .25uso .25uau
Corr. weight

sample .25390 .25699 .25548 .25455 .25459
cc Krnou 43.50 M3.87 £3.77 n3.60 A3.61
Normality .os7120 .oa713o .087121 .0871n .087136

 

 

 

 

 

Sample calculation of normality -

NaZC O

2 g; a 66.937 = equivalent weight
2
N = .25390 x 1000

 

" v087120

66.997 x 43.50

Average of 5 determinations of normality = .OS?13:t.COC09

- 13 _

 

 

 

 

 

 

 

Standardizing F380.4 . (NH4)2804 . 6H20 against
the standard permansanate.
Sample No. #6 #7 #5 #9
Weight of
sample gms. 1.53661 1.53634 1.53638 1.53744
cc KMnOu 44.79 , 44.78 44.77 44.79
Percent Fe 14.1a2 14.1s1 14.178 14.174
Sample calculation of percent icon:
Percentage = ”.64 x 44. x .08 l x 100

 

1.53661 x 1000

Average of 4 determinations of percent iron = 14.179:t.005

_ 14 -

Standardizing K20r207 against mohr's salt.

 

 

Sample No. #10 #11 #12 #13
Weight of , w

sample gms. 11.38567 11.3a526 11.35459 11.38572
ch20r207 44.81 44.80 44.79 44.81
Normality .64519 .64531 .64541 .64519

 

 

 

 

 

Sample calculation of normality -

N = 11.38567 x .14179 x 1000

-- .64519
55.84 x 44.81

 

Average of 4 determinations of normality - .6453:t.0001

Standardizing H20r04 against Mohr's salt.

 

 

Sample no. #14 #15 #16
Weight of

sample gms. 5.1021 5.1019 5.1017
cc Hacrou 40.60 40.57 40.54
Normality .3191 3193 3195

 

 

 

 

Sample calculation of normality -

N : 5.1021 x .14179 x 1000
55 .84 x 400%

 

Average of 3 determinations of normality = .31931:.0002

_ 16 -

Standardizing Sodium Thiosulphafe Solution Against

Standard Potassiun LichrOmate Solution.

##Ih'k *iktt

 

Sample NO. #17 #15 £19 #20 #21
ch2 CrZO7 10.00 10.00 10.00 10.00 10.00

 

cc Thioeul-
phate 33.81 33.82 33.80 33.82 33.80

Normality .19086 .19081 .19092 .19081 .19092

 

 

 

 

 

 

Sample calculation of normality —

 

N - 10.00 x .64
' 53 f .19086
33.81
Avera;e 0f 5 determinations of normality = .l9086:t.00006

_ 17 -

Standarizing Cr2(SO,+)3 against Thiosulphate

 

 

Sample No. #22 #23 #24

co Cr2(804)3 10.00 10.00 10.00
00 Thiosulphate 30.u9 30.50 30.50
Normality .29 37 .29106 .29106

 

 

 

 

Sample calculation of normality -

‘

 

F . 86 0.4
- 190 x 3 9 _ .2909?
2 x 10.00 '

Average of 3 determinations of normality . .291031t.00006

Standardizing Thtosulphate against standard KMnOu

 

 

Sample Ho. #25 #26 #27 £28
ocKHnOu 100.00 100.00 100100 100.00
00 Thiosulphate A5.ao - u5.82 u5.82 A5.79
Kormality .19024 .19016 .19024 .19025

 

 

 

 

Sample calculation of normality -

N - 100.00 x 008]13

$5.80 ' .19024

Average of 4 determinations of normality . .19023zt.00007

SCHEHE OF STAHDARDIZATION

Sodium Oxalate

KMnO
4\‘
Sodium Thiosulphate

L

!
Mchr's Salt

 

 

H2010“ K2 0:207
- Figure 2 -

The above scheme shows quite clearly the method

of standardization employed. Since in the actual data

taking during the body of this thesis the potassium

permanganate is taken as the standard, there Can be very

little chance for error in this entire work.

It will be noted that whereas the normality of

the thiosulphate by the K20r207 oxidation is .19086, it

- go -

is found to be .19023 by the KMnOu oxidation, a discrepancy
of .33%. However, this need occasion no alarm, for the
dichromate oxidation of KI to iodine differs from the
permanganate oxidation of KI t0 iodine in the rate of
reaction and degree of efficiency. Since the thiosul-
phate was standardized against dichromate and the chromic
sulphate was first oxidized to dichromate and then caused
to react with the thiosulphate, thus behaving the same as
dichrOmate, the value to be accepted for the thiosulphate
is that obtained on stardardization against dichromate,

rather than that obtained with potassium permanganate.

The Zn used in the reductor column consists of

particles which pass a lO-mesh sieve but are retained by a
20-mesh sieve. It was amalgamated by stirring in a mixture
of 25 cc of normal mercuric chloride solution, 25 cc of
hydrochloric acid (specific gravity 1.12) and 252 cc of
water for three minutes. The solution was then poured

off and the zinc washed repeatedly with water. The Zn-Hg

1 .1

amalgam was Kept under water when not in use.
The carbon dioxide gas was purified by passing through

a U—tube filled with calcium chloride and then through a

column filled with glass wool, the latter removing solid

particles.

The sodium sulphate solution in the storage bottle
(E) used for flushing out salt bridge (E) was treated with
just sufficient permanganate to oxidize the slight amount
of reducing substances present, in order to prevent the
subsequent reaction of these with the contents of the

reaction vessel (A).

- PROCEDURE -

The reductor column was first washed with 430 cc. of
5% sulphuric acid ( 5 cc. concentrated acid in 100 cc. of
solution), thus removing all traces of foreign material and
remains of previous runs. Then 25 cc. of 16% sulphuric
as d was run through the column. The side arm of the salt
br dge (B) was then flushed thorou hly, after which the
platinum electrode in beaker (p) was washed with acid

and flamed to remove adherent impurities.

Meanwhile, the steam was started through the rubber
sack enclosini the column and allowed to issue freely from
the lower opening. The beaker (A) was rinsed andi?t was
placed 20 grams of C.P. anhydrous, fused sodium acetate and
20 cc. of water. The Carbon dioxide gas was then allowed
to sweep through the vessel for five minutes, displacing
the air.

When the vessel was free from air, the mixture to be

- 22 -

analyzed was diluted to 50 cc., 4 cc. of concentrated
sulphuric acid and about 5 grams of Zn-Hg amalgam added
and the whole boiled for five minutes, the solution
assuming first a green color (formation of chromic
sulphate), and then a light blue color (formation of
chromous sulphate). The solution was then poured into
the hot reductor column and allowed to enter the reac-
tion vessel at a slow rate, in no case faster than rapid

drippi ng .

The beaker in which the preliminary reduction took
plate was rinsed with a 9% sulphuric acid solution, the

washings being added to the contents of the columh.

The column was washed with 100 cc. of 5% sulphuric
acid, the air at no time having access to the tap of the
Zneng column, although now this precaution is believed
to be unnecessary. A final rinsing was accomplished with
distilled water. The passage of the wash acid through
the column was often hurried by applying pressure to the

top by means of a cylinder of compressed carbon dilxide.

The reaction vessel was kept air-free by continually
blowing carbon dioxide through it during a run. The
reduced chromium solution was then titrated with the

permanganate from bumette (G), the reading of the potentio-

meter being recorded at each addition.

-23-

When near the end point of the titration, the salt
bridge (B) was flushed into the vessel IA) to include

any solution which may have diffused into the side-arm of

(B).

The vessel was kept at 70° C. during the entire run

by means of a small microaburner.

The voltage readings were then plotted on coordinate
paper againstzco Kano, added, the end point being taken
as the abscissa corresponding to_the point one-half of

the way up the rise in the titration curve. Pram the

normality of the permanganate used, the number of
grams of chromium per cc of the solution analysed is

readily Obtained.

If the oxidizing power of the solution were previously
determined against any standard reducing agent, a simple
calculation would give the proportion of hexavalent chromium
to trivalent chromium in the original solution, providing
it were free from divalent chromium. This can be assured
by aerating the sample for a few minutes, when the
divalent chromium is immediately oxidized completely to
trivalent chromium. However, in the Opinion of the writer,
divalent chromium cannot exist in the presence of the

strongly oxidizing hexavalent chromium, so the pnecaution

- 24 -

noted is not at all necessary.

In the later work it was found advantageous to
introduce 1 so less than the required amount of KMnou in
the beaker (A) and to allow the reduced chromium solution
to enter, thus anticipating any oxidizing action of re-
sidual air. The end point was then determined by adding
the rest of the KMnOu and taking potential readings, as

before.

To prevent the slight diffusion of the contents of
beaker (A) into the salt bridge, the side-arm of bridge
(B) was lowered into the beaker only when potential
readings were to be taken. This procedure was used in
connection with the method in the above paragraph. In the
earlier work, where the KMnOu was added dr0p by drOp to
the contents of the beaker, :‘the sidsf-srm remained in

position throughout the entire experiment.

It was found necessary to wait for the potential
readings near the end point to remain constant, requiring
in some cases as long as twenty minutes. It is not
permissible to add more KMnOu until equilibrium is
established, when the potential reading remains fairly

constant.

- 25 -

In a portion of the work it was found of decided
advantage to add the KMnOu continuously, keeping a
little excess in the beaker up until the last stages of

the reduction.

 

 

FO‘TINTIOWTER

 

 

 

 

 

 

 

 

 

 

 

 

 

fly. 1

 

- 26 -

- DATA -
Table I

Using 20.00 cc. 0:2(son)3 ; Normality = .2910}

 

 

 

 

 

 

 

 

Theoretiqal Actual

Sample cc cc Grams Cr Grams 0r Percent

No. KunOu Required Added found error
1. 44.54 44.60 0.2018 0.2021 +"15
2. '44.54 44.39 0.2018 0.2011 ~55
3 . 44.54 44.55 0 .2018 0 .2018 $.00
4. 44.54 44.52 0.2018 0.2017 -.05
5. 44.54 44.52 0.2018 0.2017 -.05
6. 44.54 44.52 0.2018 0.2017 -.05
7. 44.54V 44.52 0.2018 0.2017 ~.05
8. 44.54 44.90 0.2018 0.2016 -.10
9. 22.27 22.26 0.1009 0.1009 $.00

* Using 10.00 cc. 01:2(8034)3

Normality in divalent chromium . 2/3 of .2910} a .19402

Equivalent weight of Cr : 52.0

Weight of Cr in one liter of solution : 10.08904

Weight of Cr in 20.00 00 of solution : 0.20178

Mean [omitting sample #2 as in_err0r) of eight determinations -
Reduction 3 99 e98%

Table II

Using 20.00 cc. K Crao7 ; Normality - .6453

 

 

 

 

 

 

2

Theoretical Actual Grams Grams Pere.

Sample No. 00 cc Cr Cr Error
- KMnOn Required Added found

10. 49.37 49.30 0.2237 0.2234 —.13
11. 49.37 49.37 0.2237 0.2237 1.00
12. 49.37 49.35 0.2237 0.2236 -.04
13. 49137 49.33 0.2237 0.2235 -.09
14.: 24.69 24.67 0.1119 0.1118 -.09

 

* Using 10.00 cc. K20r207

Normality in divalent chromium - 1/3 of .6453 = .2151

EQuivalent weight of Cr = 52.0
Weight of Cr in one liter of solution : 11.1852
Weight of Cr in 20.00 00. of solution : 0.2237

Mean of five determinations ; Reduction = 99.93%

 

Using 40.00 00. 320394 —

ngle III

 

Normality a .3193

 

Theoretical Actual rams rams Percent
Sample 0 cc 0r Cr
' No. Kfinou Required Added found error
17s 46.86 48.814» 0e2213 0e2212 -e05
18.* 24.43 24.41 0.1106 0.1105 -.09

 

 

 

 

 

 

* Using 20.00 00. 320194

Normality in divalent chromium - 1/3 of .3193 . .1064

EQuivalent weight of Cr

Weight of Cr in one liter of solution

Weight of Cr in 40.00 cc. of solution

Mean of four determinations -

Reduction

52.0

5.5333
0.2213

99.91%

 

- 29 -

- DISCUSSION OF DATA -

Samples of the standard Cr2(S04)3, I'ZCrao7 and
necrou solutions were analyzed in accordance with the
procedure outlined. The method of anticipation.hy

means of Kano“ was used and the results were excellent.

It is seen from Table I that the mean of eight
consecutive determinations of chromium in chromic sul-

phate yielded 99.98% of the quantity actually taken for

analysis.

Table II shows the results in the case of potassium
dichromate, the quantity of chromium found being 99.93%
of that added.

Table III shows like figures for chromic acid, the
quantity of chromium found being 99.91% of that added.
0n decreasing the quantity of chromium taken for analysis
as in samples #9,#14 and #18 equally excellent results

are obtained.

A blank of .01 cc. KMnOu has been subtracted from
all the tabulated figures. This represents the quantity of
Elnou consumed by the reagents.

- 3o -

It was found that the quantity of sulphuric acid
used during the reduction is of little importance, pro-
vided enough is present to effect the reduction. Of
course, tOOthOh acid.tends to sulphate the column and

consequently is deleterious to the reductor.

During the titration with KMnQu it was found that
a solution only slightly acid prevented to a large
degree the oxidation of chr0mous chromium, which was
often found to occur even in the presence of the inert
carbon dioxide. This oxidation is explained by supposing
the presence of oxygen in the partially purified gas and
the small oxidizing effect of the sulphuric acid in.more

concentrated solutions. The acidity was decreased.by

using sodium acetate, which yields NaOH on hydrolysis.
The difficultly soluble and slightly ionized chr0mous
acetate formed is less affected by atmospheric oxygen
than the sulphate. Thus the sodium acetate accomplishes

a twofold purpose.

In the course of this work,chromous acetate was is-
olated and found to be red in color. Chromous sulphate
is light blue in color. The change from acetate to
sulphate could be readily followed by means of this color

change.

wfi

ma

ma_

in

ma

NH

Ha

.m masmaa

OH 155$ .00 m m ,4. n

 

 

 

 

 

aomaomo codenaaam

onp.ca flemnq ovmcemnmenom
mo :00 new wmaumem.nmpmaofipcmwom

somepmm coapmamm on» mafiamnm-e>azo

Narut\i

:Ion

 

,g... ..

- 31 -

The curve in Fig. 3 represents the analysis of a
sample of KZCIZO7 and is typical of all the titration
curves obtained with all the compounds of chromium inves-

tigated.

On st0pping the carbon dioxide stream during a
titration the potential was found to rise as much as 0.2
volt, showing the necessity for excluding even traces of

atmospheric oxygen from the reaction vessel.

- CONCLUSIONS--

It was found that chromium compounds were quantita-
tively reduced to chr0mous chromium in a modified Jones
reductor and were determined volumetrically with standard
potassium permanganate. The course of the titration was
followed by means of potentiometer readings and the end
point accurately determined from the rise in the titration

curve at this point.

The method of anticipation of atmospheric oxidation,
by means of permanganate was: found preferable to that of

keeping a little excess of permanganate in the titration

vessel up until the later stages of the reduction.

The activity of chr0mous sulphate was found to be so

great that the method of total titration after complete

5334990

“”26

2

- 32 -

reduction was discarded in favor of the method of 2

anticipation'by means of permanganate.

The titration curves for chr0mous sulphate were
found to be ideal, one drop of permanganate causing a
sudden rise in potential of 0.5-0.8 volt.

In a given sample of hexavalent and trivalent
chromium, the former may be determined by means of
ordinary laboratory methods and the total chromium content
by means of the method proposed by the writer. A simple
calculation will then yield the quantity in each of the

two states of oxidation.

-33..

- REFERENCES -

I. Ewing and Kelley - "Electrodeposition of Metallic Chromium"
Bulletin #7, Michigan Engineering
Expt. Station.

2. H. D. Newton - Am. J. Sci., 25, 130 (1908)

3. Shimer and Shimer - Orig. Comm. 8th Intern. Congr. Appl-
Chem., _1_, 445 (1912)

4. C. Van Brunt - J. Am. Chem. Soc. 16, 1426 (1914)

5. Jean Piccard - Berichte der deutschen Chem. Gesellschaft
.46. 2477-66 (1913)

6. Traube and Passargs-Berichte der deutschen Chem. Gesellschaft
49, 1692-1700 (1916)

7. Forbes and Richter -J. Am. Chem. Soc. 39, 1140—8 (1917)
8. 'r. w. Richards - J. Am. Chem. Soc. _2_g_, 144 (1900)“

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