The polarographic behaviour of some elements in concentrated calcium chloride solution

VOL.

8 (w53\

‘1’1~ investigation outlined in this ;md sul~scclucnt papers was undertaken with the object of developing IL J~ol~~rogr~J~l~icmethod for the detcrn~i~~atio~ of a number of elements present in trace quantities :is impurities in calcium metal. A scar& of the litcrnturc rcvealcd that, apart from a rclativcly insensitive method for chromiuml, no polurographic methods for the determination of impurities in calcium were reported. Thcrc arc, however, a large number of methods for the dctcrmination of elements in aluminiurn, magnesium and light nlloys2* 3, although these wcrc considered to lx too insensitive to be ndnptcd for USC with calcium. The general procedure xdoptcd in these methods involves the dissolution of the motai in acid and suitable adjustment of the pir followed by polarogmphy. In view of tllc fact that it WCLSrequired to determine impurities of the order of parts per million it was nccessnry tither to employ chemical concentration procedures, or to use a much more concentrated solution of the base efcctrolytc (calcium salt) than is normally cmploycd. It was considered that the former alternative woulcl entail very complicated chemical separations for each clement imd offer no advantage over those methods at present in use. WC therefore decided to adopt the latter procedure and carry out the determination in a very concentrated calcium chloride solution. 5 molar calcium chloride was sclectcd ns the base electrolyte ZLSthis is sufficiently below saturation point for use in routine work while providing an adequate conccntmtion of the irnpnritics in the final solution. Under these conditions an impurity prcscnt to the cstcnt of xo parts per million in the original metal would yield :l concentration of 2 pg per ml in the final solution, I-Iowcvcr, the use of such a concentrated base clcctrolytc leads to two types of difficulties. The first type is due to the high viscosity of sucll a solution and the second to its high ionic strength and low w;Ltcr free energy. The latter leads to unexpected polarographic behaviour on the p:trt of some clctnents, n subject which will be treated fully in parts XI and JIJ of this scrics. Il’cfcvanccs p* 563.

VOL.

8

(1953)

POLAI~OCI~r\I’II\’

IS

CONC.

CALCIUbI

CIiLOI
I

559

The investigation was confined to the elements copper, chromium, nickel, lead, cadmium, iron and manganese. All others were rejected either bccausc they were not polarog-raphically reducible, or because their half-wave potentials were too close to that of calcium, or because they wcrc likely to be prcscnt in such small amounts that they would not be dctcctablc even under the conditions adopted. The majority of the work was carried out using the Randles type single sweep cathode ray polarograph devclopcd in this Department41 G. This instrument is much more sensitive than conventional instruments and is much spccdicr in opcmtion. The characteristic reduction potential mcasurcd on this instrument is that of the peak of the wave, but for the purpose of discussion all such potentials have been converted to half-wave potentials by subtraction of 0.05 volts K. The term “cliffusion current” when referring to cathode ray polnrograms will be the current flowing at the peak of the wave. Except whcrc otllcrwisc stated all potentials arc given versus the normal hydrogen electrode (vs. N.H.E.) and the American sign convention has been cmployed. On this convention the saturated calomcl clectrocle (S.C.E.) which was used as the standard half-cell for the mcasuremcnt of potentials, was assumed volts vs. N.1-I.E. No corrections for liquid junction potentials have to be -0.245 been attempted. The half-wnvc potcntinls and suitable 1311ranges for the polarography of cnch element esnmincd have been calculated in each USC by means of tllc usual ccluntionso. They are compared with the practical results in the later papers and discrcpancics, where they occur, arc cspluincd. The potential of the mercury pool (which was used as the anode for much of fllc work) was found to be -o.rBG volts vs. N.I-I.E. when in contact with 5111 calcium chloride solution. This agrees well with the calculated value of -0.195 volts. CEItThIX

PROBLEhIS

Prefuwcition

of solrrtiom

ARISING

of

PROhI

THE

USE

OF

5;11

CALCIUhI

CIILOI~IDE

;tnntlnrd ncidity

For the fundamental investigational work a solution prepared from calcium carbonate was used. The following procedure was employed. Calcium carbonate was dissolved in the minimum amount of hydrochloric acid and diluted to give a 2.51I/ calcium chloride solution. 20 ml aliquots of this were evaporated until a crust just formed on the surface of the solution. Water and sufficient hydrochloric acid or sodium hydroside to give the required pry were then added so that the final volume was IO ml, and this solution was polurographcd. E:lemcnts to be studied were added to the aliquots of 2.5AI calcium solution before cvaporation. Nitric acid was not used for the dissolution since the presence of nitrate ion at the dropping mercury electrode would interfere with the determination Jl’cfwemcs

p.

5B3.

SGO

G.

I’.

REYNOLIIS,

I-L. I.

SHALGO.%Y,

T.

J.

WEBBER

VOL.

8

(1953)

of some elements. It is csscntial that the amount of hydrochloric acid used be carefully controlled. If too much is added initially and the excess is removed hy evaporation to dryness, some of the elements present as impurities or by ;Lddition arc rendered insoluble in the final solution. By using the treatment outlined above, however, and making a standard addition of acid or alkali, solutions having a desired per to within rfi 0.75 units can be prepared, even in t11erange MS, Complete experimental details of the above method, together with tllose of the mcthocl for the preparation of solutions from samples of calcium metal will be given in part 1V of this scrics.

‘I’hc measurement of the pr-1 of ;L 5;\1 calcium chloride solution was found to be impossil~lc. The mctcr readings wcrc unsteady and succcssivc readings using the same solution gave widely differing results. The difficulty was overcome by diluting the solution tenfold, This procedure does not enable the absolute value of the per of the Gail solution to bc dctcrmined, but comparisons can quickly and easily be made. Also, if the hydrogen ion activity coefficient in 0.5111calcium chloride solution is assumed to be unity, whilst that in the 5111 solution is at least four’, then the maximum plr of the concentrated solution will bc: masimum

~II( co,,c.) =

mcasurcd

p~(,~,~.)-0.4

All III-L values quoted in these papers will refer to those of the tenfold diluted solution. Due to the uncertain vnluc of the hydrogen ion activity theoretical amounts of acid or alkali to give a clesired p~r to the 5~11 calcium solution could not bc XIcalculated. Graphs showing the change in pr-I of the g + solution with addition of 5 standard acid or alkali so- g lutions wcrc therefore prc- t ‘pared. A typical graph is 2 shown in Fig. I.

Fig. I. Effect of NaOI-I on per of calcium chloride.

the

i?cf~?mrrcrs

p.

563.

1 mr4

IO hbOti

oddrd 3

to Mml 4

ot@~ois chbrhh 5 of mlclum I5 ’

VOL. 8 (X953)

POLAROGRAPHY IN CONC. CALCJLM CIILORIDE I

Solutions of copper, chromium, nickel, lead, cadmium and manganese in 5J/ calcium chloride solution, prepared as described above. were csnmincd using both a Cambridge photograpilic-recording polarograph and the linear sweep cathode-ray polarograph mentioned earlier. The per values of these solutions were adjusted so that the element under examination was fully in solution in an ionic form. The diffusion currents obtained using the conventional polarograph were found to be only about one-half of the values calculated by means of the Ilkovic equation. Those obtained using the cathode-ray polarograph wcrc between one-half and one-sixth of the values calculated using the simplified Randies equation 4* s. For the purposes of these calculations the diffusion coefficients employed were those pertaining to dilute solutions and clearly this data cannot apply to polnrography in a medium of such high viscosity as 5111calcium chloriclc. BRASIIEH AND JONI@ have pointed out that little attention has been paid to the effects of viscosity on polarography and have indicated the inadvis~bi1ity of neglecting these effects in quantitative work. The diffusion cocfficicnt or large, uncharged, spherical molecules is known to be inversely proportional to the viscosity of the medium and this correction has been successfully applied to the diffusion coefficients of many of the small charged ions norm:\tly involved in polarographic reduction+ O. The relative viscosity of 5:\1 c:tlcium chloride solution with respect to water, measured using an Ostwald viscomcter, was 4.85 at 25” C. The calculated diffusion currents mentioned above were accordingly corrcctcd by a factor of (r/4.85)* when the values obtained by mecans of the Ilkovic equation agreed well with those found experimentally using the Cambridge polarograph, The diffusion currents measured using the cathode ray polarograph, however, were still generally lower than the calculated values, viz.: lead zooO/:,, cadmium 5)5(x,, copper 85O/o,manganese So?,, chromium 70(x, ancl nickel 35’y”. This phenomenon of low recoveries was noted by RANULES~ who ascribed it to irreversibility and to the rate of the eicctrode reaction. The effect has also been studied by DELAIIA~~~ using a multi-sweep cathode ray polarograph, which, unlike our apparatus, applies the total potential change a number of times in the lifetime of one mercury drop. I-Ic concluded fi that where the results obtained by the single-sweep and multi-sweep methods are similar, but lower than those predicted theoretically, the discrepancy is mainly due to the irreversibility .of the cathodic process, If the single-sweep method gives approximately the correct result, while the multi-sweep technique gives results which only approach the theoretical value at relatively low rates of change of potential, the discrepancy is due to tltc slow rate of the electrode reaction. It appears, however, that these suggestions may have to be modified in view of our rcsulfs, and his later findings that capacity effects distort the waves and that, in the quiescent periods between RcJerc9accs p. 563.

5Gz

G.

the application :rround For

of successive

the drop

this

given

our

should

wcrc

DELAHAY, has

is not

reason,

below,

technique reported

95, slightly

reduction

of

‘l’his

will

cries

rcportcd

be

be accounted it may

repcatcd present

for by

potential

ray

quantitative

in practice

be

being The

to fncilitatc

the

reduced

at the

future

to

made

quote

become

general,

change the

The

fact that

those

found

of the dropping serve

reaction

of

available

been

calculated

be

the recovus may case,

to the rapidly reductions the

further

mercury

clectrodc that

of potentiitl

of

I)uc Until

current

of such

obtained

bchaviour of the

to

analysis

attempted.

diffusion

at

use

a single-sweep

is constructed.

“non-idcal”

to cmphnsisc

the

when

of the

of cllangc

may

reaction. by

when

equation, not

and The

above. many

of potential has

are,

in each

related

csl~ccially

Rand&

and

measure

rate

discrcpnncics

to the factors

nickel

clcctrodc.

of the clcctroclc

mentioned

of

above

the

the

and second

is reversibly,

mercury

than

of

per second,

per

volts lead

methods,

realised.

0.3

this series.

nnturc

of

perccntagc as some

been should

is difficult

taken

remarks

it is csscntial

rate

were

of the clectrodc

real

more

derivation

nature

is made, may

species

instrument. type

the

the

bccomcs

a variable

of

have

that

two

of IO volts

that

ion

occurslI.

the multi-sweep

lead

dropping

lower

or partly,

rcvcrsiblc

tcclmiqtle

of

rapidity

wholly

which

be

II

the by

(1953)

reducible

cadmium,

the reversibility

considerably

regarding to

having

complcsity

;L treatment ionic

sweep,

considered

instrument a more

either

of

concltldc

it qqcars

in Part

arc

for

rate

nt the

than

an insufficient

information

the cathode the

in &tail

DELAIIAY

\vc

and

rather

cent.

8

species

disadvantages

a sweep

cent.

the

by

obtained

of the order

17 per

rcduccd

is anomalous kinetics

also be due,

Further

35 per

obtained

as those

inherent

using

of

of the reduced

results

of potential

of 83, 70 and

discussed by

the

reserve,

‘I-. J, W13BI3JfII VOL.

the depletion

oxidation

these

irreversibly,

the

swecl~s

of

recoveries,

nickel

itSSOCiZltCCiwith

SIJ[ALGOSKY,

and

of change

100 and

cadmium,

I.

with

before

a rate

recoveries

respectively,

good,

comparison

reported

0ur

II.

potential

made

be treated

using

respcctivclyl(J.

but

RESNOLIX,

I’.

of

the

cathode-ray

in all work employed,

of this in order

comparisons.

Atcltnowlcdgcn~ent is maclc to the Chief Scientist, 13ritish Ministry fur permission to ~~~blish this paper. The au!hors wish to thank Dr. J. I). LAND for his nclv~ce during the worlc.

of

Supply

I-I. STRICIC-

Ccltuin problems associated with the polxrography of metals in 5&f calcium chloride solution and the acidity control of thus base clcctrolyte arc discussed. l’hc general rcsu\ts olAxxincd by the use of a single-sweep cathodc ray polarograph are given and suggestions rcgarcling the fundamental nature of some electrode reactions arc made. References

p.

5G3.

VOL.

8

(q53)

I’OLAROc.RAI’IIY

Ih’

COh’C.

CALCIUM

ClfLORII)H

I

5%

hie dcs rdtaus, en solution Ccrtains probl&n?cs, en relation avec la polarogra tic dam le chtorute de calcium 5.X, et ie contrOle de F riciclitd de cct 6lcctroIytc base, sent discutds. Les rdsultds &ndraux obtenus ;LVCC l’oscilloscol3c & cayon cathodiquc (,,singIe-sweep”) sent dorm& ct des suggestions scmt fnttcs conccrnnnt la nature de quelqucs reactions aux &lectrodcs.