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TH _

STUDY OF SOLUTIONS OF THE
SODIUM-CADMiUM CYANIDE
COMPLEX

Thosis {or the Degree cf M‘ S.
MICE-“CAN STATE COLLEGE

John McCaHum
1943

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STUDY OF SOLUTIONS OF THE SODIUM.
GADMIUM.CYANIDE COMPLEX

by
IOHIiMCGEEUmH

A.THESIS
submitted to the Graduate School or lichigan
State College of Agriculture and Applied
Science in partial fulfilment of the
requirements for the degree of

HASTER OF SCIENCE

Department of Chemistry
1943

.r w". _
; 4' a“ 4" I!
\ _; f 2"
‘U

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swampemm

the author wishes to express his indebtedness to
Dr. D. T. Ewing, Professor of Physical Chemistry, for
hie helpful suggestions and guidance during the course
or this investigation. fhe triter is also indebted to
IProIessor A. J. Clark, Head of the Department 0: Chem»
ietry of Michigan State College, for the grant or an
eesietantship.

r «’1 '" I“; 5)

-1-

is stated by M. s. Tompson1--'During the last forty
years over one hundred papers have been published deal-
ing in one way or another with the constitution and prop-
ertiee of the cyanide plating baths“. Of the various
metals plated in this manner, formulae for the cadmium
complex are less definite than those for zinc, copper.
gold, silver, as well as those in the brass plating sol-
ution.

L. P. Westbrooka states that the cadmium complex
is usually written as Hazcd(6ll‘ but continues to use
Heedml)a throughout his article on cadmium plating.

S. Glasetone3 carried out an electrometrie titra-
tion of cadmium sulphate with sodium cyanide. He ob-
tained no direct evidence of the formation of ca¢cn)3
but states: “The shape of the titration curve when the
ratio Cl:6d exceeds 2:1. and the fact that the precip-
itate of cadmium cyanide dissolves completely when the
ratio is 3:521, suggests that at least two complex ions,
01(CBJE, or perhaps (cc(c3)zlszol’, and Gdtcnlz’ are

present in solution'.

1 4

u; R. fompson basing his calculation upon corbit'e
phase rule study of the cadmium cyanide complex, shows

that the content of ca(cn); in.a saturated solution con-
taining ICE and ca(cn)2 is appreciable although in this

case, the complex ion is chiefly Cdlcnlz’.

5 have studied the formation of the

Britton and Dodd
complexes in several different ways:

(1) The glass electrode

{2) metal electrode potentials

tel Conductivity measurements.

By all methods they found definite complex formations
for silver, zinc, etc., but cadmium was an exception.

Their pH measurements with the glass electrode
suggested the formation of Cd(CK)g and ce(cn);’ but
the evidence was not pronounced.

Their study of metal-ion concentration by means
of potential measurements and the electrode potential
equation gave a very irregular curve in the case of
cadmium. A slight inflextion.was obtained between the
points which would correspond to formation of 6d(CN)5
and cc(cs);’.

The conclusion they draw from conductivity meas-
urements states: ”In the cadmium curve, the end-point
is not difinits'. This curve was a eonductrmmetric
titration of ccso‘ with KER.

I. Hall‘s6 work most closely resembles the present
investigation. He determined by volumetric analyses,
the formula of the cadmium cyanide-sodium cyanide com-
plex in a sodium cyanide solution saturated with cadmium

oxide. He found the mole ratio-Nacn:cdo to be 3.8 and

concludes there from.that this ratio may be used in the

analyses of cadmium plating baths. For industrial pur-
poses, this empirical constant, which was also prOposed

by I. Wernick7

in 1929, may prove satisfactory. however,
there are several circumstances which may change the
above factor as this investigation will show.

In order to fully interpret the meaning of such an
empirical constant, it is necessary to consider the in-
fluence of such factors as: the mole ratio in an unsat-
urated solution of neon plus can. the combined ratio
change with change in.the total concentration of sacs
and one, change in unsaturated solutions if the lac! con-
centration is held constant but the sec concentration
varies, change with ageing of solutions, pH, the effect
of electrolysis upon.the complex.formula, the cadmium
salt in combination with sacs, temperature, the effect
of 332005 which is always present in cyanide plating
baths, and.alteration of the complex during the process
of volumetric titration. Ho attempt is made to discuss
all these details because of the limited time allotted
to this research.

A study of these matters by volumetric analysis
presents several difficulties in that there is to date

no accurate dependable method for the analyses needed.

ANALYSES
All titrations were carried out at room temperature.

Free Real -- a widely used method for determination of

 

"free't NacN in the cadmium plating bath is the direct
titration of the solution with.LgN03 using I1.as indica-
tor. 1. Hall6 adequately discusses this procedure but
as he points out, the results are not sufficiently re-
producible, and they are generally high. He explains
these high results as follows: ”Because of the very
high cyanide content, there is some decomposition result-
ing in the formation of sodium carbonate and ammonia,
the reaction being represented as followsaz

211ml t 2320 1* ZlaOH + 02 8 219.2603 + 2315
In the presence of ammonia, the endpoint does not occur
when all the free cyanide has been titrated, this is
probably due to the formation of the soluble sawmium.
ammonia complex which may be representated by the rever-
sible reaction: .

reaction), 4 m3 : caflnaldcs), 4 areas
some of the combined cyanide being liberated and titrated
as free cyanide, thus giving high results.“

The end point is not sharp in this titration. It
was the experience of the present author that different
lighting conditions have widely different results. Be-
cause this titration is so difficult and at times appar-

ently inaccurate, another method was attempted.

-5-

Titration of I'free" uses by case, was next tried.
the titration reactions could be
OdSO"+»filaCN : HaCdflm)3 4-sa2so‘
or caso"e-enacr' = Iazcd(0l)4 4-ls230‘
the end point is determined by the formation of a slight
turbidity caused by one of the following reactions:
«so, timzcoz _-, cacao,3 +sa2so,
or case, +£18.03 : catch), «excess,
Either of these precipitates is soluble in alkali cyan~
ids and hence will dissolve so long as any excess lac!
remains.
this method has its disadvantages but was used for
the maJor part of this investigation. It is difficult
to fix the exact endpoint. is the stoichiometric point

is approached, local excess of GdBO causes a precipitate

4
which is not readily soluble. This tends to make one
take his endpoint too soon. Obviously too, the end point
is dependant upon she solubility of 6d{03)2 or cccoz.
these solubilities in.turn are largely dependant upon
concentration of the 037 or 605' ions as predicted by

the law of mass action. mhis latter effect will be

brought out later in the discussion of results.

-5-

‘ggjglL§§§!.-- This analysis was quite thoroughly studied
and the following facts were obtained:

(1) There is a range of ammonium hydroxide over
which constant and reproducible results may be obtained.
Below this range, the complex cyanide is incompletely
broken down - giving low results. Above this range,
silver iodide is appreciably soluble - giving high re-
sults.

(2) The magnitude of this range is dependant upon
the solution being titrated.

(3) 10 ml. of 2% K1.in.the 100-150 ml. being titra-
ted is the maximum allowable concentration of indicator
ion. more than this maximum.gives low results.

(4) Within experimental error, the igsoz~sacs ti-
tration is independent of dilution prior to titration.

In view of these facts the following procedure was
adopted:

The cyanide solution is diluted to approximately
100 m1., 5 ml. of 2% :1 added, and is then titrated with
standard Lglo3 (0.1K) until a faint opalescence is noted.

5 ml. dilute maps is added.

A turbidity is again produced by addition of Agnoz.

The last two steps are repeated until addition of
5 ml. of dilute unionicauses no change in opalescence.
One can then assume that he is in the constant range

mentioned under (1) above.

-7-

Csdmium Analysis.--The standard method for cadmium anal-
ysis in the plating bath is the precipitation as sulphide
and titration with standard potassium ferrocyanide using
uranium acetate as outside indicator. This method is
rather lengthy and not entirely dependable.

The following procedure for electro-analysis was
adapted after trying a number of conditions:

(1) Quantitatively transfer the equivalent of less
than 0.25 g. of Cd to the electrolytic beaker.

(2) Add 1 drOp of phenolphthalein and then add
dilute sulphuric acid (noon) until the last trace of
pink disappears. I i

(5) Add two grams of NaOHipellets, dilute to approx-
imatelleO ml. and heat almost to boiling (HOOD).

(4) Electrolyze for 14 to 18 minutes with rotating
copper gauze. 5.0 amps/dmz for half this time and 6.0
amps/dmz for the remaining half.

(5) Remove cathode with current on, wash and dry
in the usual manner.

Equally accurate results may be obtained by deposi-
tion on a Cu of Cd surface. Anode area was found to be
important, probably because of the voltage relationship.
In this investigation a platinum electrode with an area

2

of approximately 1.5 cm . was used.

Voltage less than 11; greater than 7.

FORMULA OF THE CADMIUM CYANIDE-SODIUM CYANIDE COMPLEX AS
FORMED BY ADDITION OF CADMIUM SULPHATE TO SODIUM CYANIDE

Experimental.--Ihe cadmium sulphate solution was stan-
dardized according to the electrolytic method given by
Classen &.Hall9 except that the current densities given
under Cadmium Analysis were used. Concentration was
found to be 49.58 g/l. This standardized solution was
used throughout the entire research.

The sodium cyanide solution was standardized with
the procedure given by Willard and Furmanlo. Oincen-
tration was found to be 384.8 g/l.

To a sample of Baal was added 5 ml. of 20$Nazco3
and enough water to make the volume 100 ml. prior to
titration. The lone exception to this was the 10 Id..
sample of seen. Only 40 ml. of water was added in this
case because of the large amount of CdSO‘ necessary to
produce an end point. Sample #1 was titrated with a
cadmium sulphate solution which was exactly one-half

the concentration of the above standardized CdSO solution.

4
By carrying out these titrations in this manner,
the effect of concentration of the complex upon its form-
ula may be studied.
Assume that the titration reaction is

CdSO4 + macs +Complex 4» 39.2304

-9-

Since the number of grams of Nacfi titrated and the num-
ber of ml. of standard CdSO4 are known, the I.of the
above equation may be solved for in the following manner:

I 3 grams NaCl 1 [01. Wt. cesc
ml. Gd804 1 grams Cd804lml 1 Mol. Wt. nacn

 

 

(1) : _§5.80 x g. Nae!

 

Results are arranged in Table I.

iaBLE 1
ml .ml 3) .ml
men 1120 no: cuso4 x
l 94 0.3848 18.40(; 2) 3.589
2 93 0.7636 18.47 . 3.576
7 88 2.694 66.33 3.484
10 40 3.848 95.34 3.463

It may draw from table 1 at least two conclusions:

(1) Ehe mole ratio neon-cc is different in differ-
ent solutions. fhe above valuss do not even closely
check with the mole ratio of 3.8 obtained by 1. Han“
and others. ’

{2) this ratio (coordination number) decreases
upon increasing the complex concentration of the solu-

tion.

-10-

THE FORMULA OF THE CADMIUM CYANIDE~SODIUM OYAHIDE COMPLEX
AS FORMED BY ADDITIOR OF CADMIUM.0XIDE T0 SODIUM CYANIDE
Part I.

EXperimental.--fotal NaOn was found by titration with

 

igno3 in the presence of dilute ammonium hydroxide plus
5 ml. of 2%IKI. The amount of ammonia was determined as
recommended under ggtgl_01anide analysis.

cadmium oxide was weighed accurately in the solid
form and then quantitatively transferred to the macs
solution. Three different solutions were then electro-
analyzed by the procedure previously given. In this
manner representative results gave us the per cent pur-
ity of the CdO. This was found to be 96.39%.

12 g/l of sezcoz was added to each solution to
serve as indicator for titration with (M804 and the sol-
utions were analyzed 81 days after preparation.

In each of these solutions the ratio of NaCl to
cec is maintained constant but the total concentration
is varied.

Since the coordination number obtained for titration
of EaCN with CdSO‘ is on the average about 3.5, this num»
bar is used in the first series of calculations (by use
of equation (1)) to find the coordination number of Cd0
plus sacs.

-11-

the chemical equations used are as follows:

titration: Cd304 + 3.5Hacl- Complex «0- Hazso4
Original reaction: Gd0 {- near 1- 330 —~Complex + 1.280,

Results are organized in Table 2.

Table 2

Free lotal Combined

men are] seen 1

8.62 17.74 ‘ 9.12 4.86
30.82 69.42 38.60 5.15

67.58 142.4 74.82 4.98
111.5 210.9 99.40 4.42
136.7 279.9 143.2 4.78
169.6 346.1 176.5 4.71
211.7 435.2 223.5 4.77

it. c 4.81
It is evident from these data the confirmation of an ,
earlier conclusion; namely, IThe mole ratio NaCled is
different in different solutions". This follows from
the fact that a coordination number greater than four
is unlikely for this compound, which means that a differ-
ent titration reaction takes place in this solution than
was assumed.

It should be noted further that although widely
varying solutions are studied, a fairly constant coord~
ination number I.is obtained. This is to be expected
for two reasons.

The ratio of NaCR to one is constant in each solution.

The amount of sample taken from each solution was
varied and diluted to approximately the same concentra-
tion so that at the time of analysis, each solution was

almost identical. The fact that each solution was not

-12...

exactly identical at the time of titration probably ac-
counts for the large part of the discrepencies obtained
and indicates that concentration may have some effect.
Ehe effect may be ta) the stoichiometry of the titration
or (b) the complex may actually be different.

Ihese solutions therefore present us with three
unknowns:

(ll g/lof free cyanide

(2) ‘mole ratio in the original reaction

(3) mole ratio in the titration reaction
and since one cannot obtain three unknowns from two
equations, a further assumption must be made.

We have already shown that the same salt plus
alkali cyanide forms a different complex in a different
solution. Therefore let us assume that that complexis
inddpendant of the salt; that is, assume that the mole
ratios NaCE-GdO and NaCN-CdSO‘ are the same if present

in the same solution. Our reactions will then be:

Cd804 + INaCN -- Complex + HazSO‘

GdO 1' Heel! + HOB -—> Complex + arson
Lit n be the grams per liter of free cyanide then,

(2) n : ml.CdSOl x ngSOiIml. x x x M01. Wt. secs x 1000
M51. Ht. case, x ml. sample titrated

: 11.12 x.I.x 310 Cd504
m1. sample

 

Further more, total NaCN u n : combined, therefore

(a) rota; - n g 5/1 cao x 1.x:noi. Wt. sacs
~ 1751. st. 050

a 0.3818 x 3/1 (:50 x x.
(4) n a Total macs - 0.8818 x 5/1 CdO x.x
mebining (2) and (4} we obtain

11.1. "3» 095° : Total - (0.3818 x g/l cao x x)
. amp e

lotalgll sacs x H1. Sample
(11.12 x ml. 0d804} *'(0.3818 x gll 050 x ml. sample)

Using this equation we obtain the new set of values
shown in Table 3.

(5) I.

Table 3
3/1 total g/l n1. Sample n1. ceso‘ 71
seen 040 titrated

17.74 4.92 25 5.70 8.91
59.42 19.55 15 11.72 4.18
142.4 89.82 10 17.07 .4.08
210.9 58.99 5 12.82 4.02
279.9 78.65 5 17.27 3.98
845.1 98.88 5 21.42 8.98
485.2 122.9 2 10.68 3.98

AV 0 3 e

1!. (without 2) - 3.99

It is obvious from these data that we have a set of
solutions in which the complex ion is nearly all in

the form of sazce(cn)4.
is evidence that these statistics are nearly cor-

rect we can present results for free Baal as obtained
by the standard method with silver nitrate.

Table 4
Free Ice! Free sacs

11.58 g/l 10.55 g/l
43.83 36.51
80.16 79.68
124.0 119.8

60.6 161.4

22.3 200.2
247.0 249.2

iv. 3 127.1 g/l 19.. 122.5 5/1

1]]1 .

I'luloll- 1', Iv

eomeo an nos: .55“ a o 295254 an zoez eeeu u n - .N .msa

noes Hausa ”\m
one cos one cam and on

 

 

T1,: ,1--..-«i.--- \\ .13
t\\\\.

\E i. ONH

 

 

 

 

 

 

 

:ll 0,.

4)
H

seen 9947 1/3

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

-15-

Table 4 shows that free Rae! as obtained by Agno3
giies on the average a high result. This verifies the
statement.made by R. Hall which was quoted previously.
It should be noticed that titrations by AgNOz give some-
what irratic results. To bring out this matter more
clearly, Fig. 2 has been drawn. Titration reactions
assumed for this figure are:

131703 «I» ZlaCl ——> HaAgCGltlz + Balms
CdSO4 1-3.99naClwe> Ha1.9,6d(CN)3.99 f-hazso‘

When we remember that this set of seven solutions
were prepared so that the ratio of Nae] to Cd0 was con-
stant and that titrations by 8580, brings out this
straight line relationship quite accurately, we have
further substantiated the precision possible by this
method.

fart 2

 

Experimental.--L set of nine solutions were made by add-

 

ing various amounts of Cd0 to sodium.cyanide solutions,
each of which contained 200 311. The solutions were
allowed to come to equilibrium for three days and were
then filtered to obtain an absolutely clear solution.

lo sodium carbonate was added.

Titrations were made with standardised Cd804 after
dilution to 100-150 ml.

Calculations were made using equation (5) and results

are tabulated in Table 5.

-15-

Table 5
3/1. n1. Sample 51. x
550 titrated CdSO‘
4.82 2 10.48 8.88
9.54 2 10.05 3.36
14.45 2 9.22 8.58
19.28 2 8.41 8.59
24.05 2 7.59 8.89
48.20 5 18.81 4.17
71.29 5 9.15 4.20
95.89 10 9.92 4.17
120.4 25 7.19 4.05

To draw any conclusions from Table 5, we must re-
member that all eonsiderations must be made in terms of
the solutions which were actually titrated; that is,
after samples have been diluted and the endpoint deter-
mined.

In this series there are at least five factors
which may be changing the coordination number.

(1) Change in sodium hydroxide content

(2) Differences in concentration of the complel
at the end of the titration

(3) Difference in metal content

(4) Change of the cyanide ratio

(5) Radical change in concentration of the indica-
tor ion may be affecting the end point.

Numbers (3) and (4) are listed only because of
statements made by H. R. Tompsong. He makes the fol-
lowing two generalizations;

'(1) Increasing the metal content of the solution

necessarily increases the concentration of both

metal-bearing anions and metal cation. There will

be a tendency for a lowering of the coordination number

when this is possible.---

(2) Addition of alkali cyanide, that is, an in-

crease in the cyanide ratio, decreases the metal

ion concentration and tends to cause a shift to-
ward a higher coordination number, when higher
compounds are pessible.--—'

These two factors do not enter into our considera-
tions because (a) at the time out titration endpoint is
taken, the cyanide ratio is supposedly the same; i.e.,
there is no excess cyanide in any case and (b) for this
reason the metal content is also, supposedly, relatively
the same in each solution.

Numbers (1), (2), and (5) have already been shown
to have an effect. Tables 1 and 2 show that increase
in sodium hydroxide increases the coordination number.
Table 1 shows that increase in the complex concentration
decreases the coordination number. Table 5 and Fig. 3
substantuates these conclusions and also proves that our
'titration endpoint changes with concentration of indica-
tor ion (OHT in this case).

In the first place, a coordination number greater
than four is thought to be unlikely since a coordination
number of five 'does not allow symmetrical space arrange-
ment'l. Therefore some of our results indicate that
Free NaCH by titration with CdSO‘ is not a stoichiometric
titration. This can be shown to be true by calculations

with the extreme values. Points between these will be

ova

anomoan

ONH

acumen oz “aoaz H\m oom -

oeo H\5
can on on

O)
is

.mam

ow

ON

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

o.&

>.n

m.&

o.¢

H.¢

 

 

 

m.v

soqmnn notisutpsooo

-19-

pr0portiona11y more accurate.

Let us assume that at the end of the titration, the
total volume was 150 ml. This represents the maximium
dilution of our titrations. Solution #1 had 4.92 g/l of
Cd0 and since, in any case, one mole of cadmium oxide
reacting with sodium cyanide yields two moles of sodium
hydroxide, we have in this solution 2(4.92 [128.4) or
0.076 mole/l HaOH. Two m1. of this was taken and diluted
to 150 II. (by our assumption). Thus we have 1.5x10"6
soles in 150 ml. or a concentration of 10"5 mole/l.

The solubility product for Cd(0H)2 is given by
Hogness and Johnson11 to be:

Ed‘flfilrjz : 1.221044
Hence, for precipitation to take place the cadmium ion
concentration must be

[05“? . (1.2r10‘14)/(10
Thus in 150 ml. we need l.8xlO"5 mole of Cd .

~5)2 : 1.2x10'4 mole/1.

Now one milliliter of our standard cadmium sulphate
solution contains 0.04958 g. or 2.3110"4 mole. There-
fore approximately 0.10 ml. must be added beyond the
stoichiometric point before precipitation will begin to
take place in this solution. This difference is prac-
tically negligable since calculations were made assuming
maxmium silution. If dilution was less than 150 ml. the
endpoint would be that much more accurate. Readings

within 0.1 ml. are within experimental error.

-20-

£.most interesting phenomenon is brought out by
calculations with solution #3 of Table 5 and solution
#2 of Table 1. In the latter case 5 ml. of 20% 552503
was added as indicator. This is equal to 0.1g. or 10-3
mole or 5x10.4 equivalent. With 5x10"4 equivalent of

razco present in the titration beaker a coordination

3
number of 3:51 was obtained. In solution #3 of Table 5,
14.46g. Cdo was present. Reasoning as before, we find
thus that 0.225 mole or equivalent of Neon per liter is
formed. Taking a two ml. sample of this gives us
4.5x10"4 equivalent of NaOH present in the titration
beaker, a coordination number of 3:53 was obtained.

In view of this, it follows that coordination num-
ber is a function of pH rather than HaOH content. This
follows from the fact that equivalent amounts of either
nozooz or NaOH‘produce the some increase in coordination

number.

By use of solution {9 of Table 5 we can show that
an endpoint appears before stoichiometry is reached.
Once again we shall assume a total of 150 ml. at the and
of the titration.

This solution contains 120.4 g/l CdO and since one
mole of Cd0 yields one mole of complex, in any case, we
have 120.4 4 128.4 or 0.94 mole/l of complex cyanide.
When we take 25 m1. of this solution, we take 2.35x10”4

mole. Upon diluting this to 150 m1., we have a concentra-

tion of 0.16 mole/l. The complex concentration is further

increased by addition of CdSO In this case we used 7.2m1

4.
x0.5 g/ml. which corresponds to 0.0017 mole of CdSO4 or

HaZCd(CN)4. Thus the final total concentration of Cd(Cl)4

is 0.152 mole/l.

The dissociation constant for CdfCN)z’ is given in

1 article to be:

[Cd‘j [Cfl’j‘ - 10-19
Cd(CH)Z’ '

and we know that for each Cd“' formed by dissociation,

Tompson’s

there are 4CN’ ions produced. Therefore:
- [55%] Rod” 4* g 10"18 x 0.152
[Gd“]5 3 255 x 0.152 x 10""8
g 41.5 x 10’18
Em’fl : 5.8 x 10-4 mole/1.

This is the concentration of the cadmium ion at the
stsichiometric point due to dissociation of the complex
ion.

In this same solution the sodium hydroxide concen-
tration is Just twice that of complex, viz., 1.88 mole/1.
Thus a 25 ml. sample diluted to 150 ml. gives us a con-
centration of OH’ of 0.31 mole per liter. For precipi-

tation to take place than, the cadmium concentration

must be:

Ems? g 1.2 x 10’14 z 1.2 x 10-13 mole[1.
0.81

Thus we see that at the stoichiometric point the cadmium

ion concentration due to dissociation is greater than

-22-

that needed for precipitation. For this reason an end-
point will appear too soon.

Although the dissociation constant used in this
calculation was obtained in a dilute solution and there-
fore is most likely very inaccurate in the concentrated
solution used, there can be no doubt that this solution
does act in some measure in the manner shown. This may
account for the values higher than four but cannot ac-
count for the decrease in coordination number obtained
beyond 75 g/l. of odo.

‘Because the free cyanide content of the last two
solution of Table 5 were so small, large samples were
taken for titration, namely 25 and 10 ml. respectively.
These samples were diluted to approximately the same
concentrations as were the other solutions. Therefore,
the concentration of the complex at the end of the tit-
ration is very much larger. We have already shown in
Table 1 that coordination number decreases with increase
in concentration of the complex. This decrease is by no
means negligible and very nicely accounts for the dr0p-
off in coordination number.

From Table 5 and Fig. 3 we may then conclude:

(l) The decrease in coordination number with de-
crease in sodium hydroxide is greater than can be ac-
counted for by solubility calculations.

(2) High coordination numbers at high concentra-

tions of cadmium oxide may be accounted for by dissoc-

iation and solubility calculations.
(3) A decrease in coordination numbers at high
concentrations of CdO was obtained which cannot be ac-

counted for by such calculations.

-24-

EFFECT OF TIME UPON THE COMPLEX

Analyses on the series shown in Table 5, 145 days
later gave the following results:

(1) The total cyanide content had decreased 8.6%

(2) A curve almost identical with that shown in
Fig. 3 was obtained. This verified our previous results.

(3) The coordination number on the average was
about 1.8% smaller.

Analyses on the series in Table 3, 80 days pre-
vious to the results given showed that:

(1) Total cyanide had decreased 3.4% in this time.

(2) The average coordination number in the last
case was about 2% smaller.

Because of the lack of conclusive evidence that
complexes originally formed are not the most stable,
all calculations and conclusions in this paper are made
on the assumption that changes in the coordination num-
ber due to dilution, increase in concentration by titra-
tion, etc. are instantaneous. In view of the above
though, it appears that there might be a slight lower-
ing of coordination number with time.

This decrease may be due to the reactions previ-
ously given under Eggg_§22§ analysis:

shoes 4 21108 + znaos 4 02 anacoa + 25115
NAng‘CB)4 {- m5 od(m3)4(cmz + 25555

These represent a decrease in haOH as well as the mole

ratio CN:Cd.

-25-

IIIECT 0F ELECTROLYSIS
Several plating solutions were analyzed by the
methods given earlier. Resulss are tabulated in Table 6.

10 m1. sample titrated with CdSO in each case. Equa-

4
tion (5) is used.
Table 6
# g/l g/l total m1. 1
CdO NaCN CdSO4
1 23.89 98.34 14.25 3.93
2 42.44 88.78 16.16 2.58
3 25.95 86.53 17.76 2.92
4 20.93 66.09 8.9? 3.68
5 18.77 57.96 8.22 3.56

Numbers 1, 2, and 3 ars commercial plating solup
tions in operation 24 hours per day. Numbers 4 and 5
are solutions from a 20 liter cadmium plating bath.
Number 5 is one month later than number 4. Twenty-four
hour per day electrolysis was carried on during this time.

These solutions are approximately of the same com-
position as those shown in Table 3. That is, at the time
of titration, samples from these plating baths and from
solutions in Table 3 have been diluted to approximately
the same concentration.

It follows from these data that electrolysis causes
a decrease in coordination number. This decrease may be
due to:

(1) Impurities affecting the complex or our endpoint.

(2) Due to different migration rates, the bivalent
complex may be gradually diminished in concentration.

(3) Some mono-valent cadmium may be formed in the

process of electrolysis.

(4) The_aging process already mentioned.

-25-

SUMMARY:

The cadmium cyanide-sodium cyanide complex is dif-
ferent in different solutions.

The coordination number increases with increase in
pH.

The coordination number decreases with increase in
concentration of the complex.

A solution of the type used in cadmium plating baths
(see Table 3) contains a complex which is largely in the
form of NaZCd(CN)‘.

Plating solutions have a coordination number smaller
than 4 but which varies with the plating bath.

There appears to be a slight decrease of coordina-

tion number with time.

-2!-

REFERENCES:

l.-éM.R. Thompson, "The Constitution and Pr0perties of
Cyanide Plating Baths“, Trans. Electrochem. Soc.,
12, 417-37 (1941)

2.--L. R. Westbrook, “The Electr0plating of Cadmium
from Cyanide Baths“, Trans. Electrochem. Soc., 25.
888 (1929) '

3.--S. Glasstone, 'Studies of Electrolytic Polarization.
Part 9. Complex Cyanides", J. Chem. Soc., 1237 (1930)

4.-«A. S. Corbit, 'A.Phase Rule Study of the Zinci-,
Cadmi-, mercurio, and Nickelo-Cyanides of Potassium”
J. Chem. Soc., 8190 (1925)

5.--Hl T. S. Corbet and E. N. Dodd, 'Physicochemical
Studies of Comples Formation Involving Isak Acids.”
Part 5. Solutions of Complex Cyanides of Silver,
Zinc, Cadmium, mercury, and Nickel.“ J. Chem. Soc.,
1940 (1932)

6.--Rathanie1 Hall, "The formula and Analysis of the
Cadmium Cyanide Complex“, metal Ind., 31, 404-406(1939)

7.-S. Wernick, J. Electrodepositor's Tech. Soc., 4, 101
(1929)

8.-.William Blum and George B. Hegaboom, l*Principles of
Electroplating and Electroforming', p. 223 ~‘KcGraw
Hill Book Company, Inc., New York (1930)

9.--Classen Cloeren, and Hall, “Quantitative Analysis
by Electrolysis' - John Wiley 5 Sons, Inc., I!) Ibrk(19l3)

REFERENCES (Cont)

10.--EI. H. Willard and N. H. Furman, "Elementary Quantita-
tive Analysis", p. 188, - D. Van Nostrand Company Inc”
New York, N. Y., (1940)

11.--T. R. Hogness and W. C. Johnson, "Qualitative Analysis
and Chemical Equilibrium" - Henry Holt and Company,
New York, N. Y. (1940)

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