Determination of Cadmium in Zinc Concentrates and Other Zinc-Rich Materials A Cation Procedure SILVE KALLMANN, HANS OBERTHIN, and ROBERT LIU Research Division, Ledoux & Co., Teaneck,
N. I .
b The iodide medium i s well suited for the separation of cadmium and zinc b y ion exchange. While strongly basic anion exchange resin provides a clean-cut separation of the two elements, procedures based on the use of strongly acidic cation exchange resin were found to b e far more rapid and convenient. A method i s described which yields cadmium as the CdlrF2 complex in the first eluate, while all other constituents are retained b y the resin. A cadmium determination can b e carried out in a complex zinc matrix in less than 4 hours.
zinc cation by strongly acidic cation exchange resins. While the stability of the chloro anions of the zinc and cadmium complexes, and hence their adsorption characteristics, differ just sufficiently to allow a separation of the two elements by either cation or anion exchange resins, the iodide medium appears far superior. Measurements of the activity eoefficients of zinc and cadmium halides show that the tendency for complex formation in the case of zinc decreases in the order: zinc chloride > zinc bromide > zinc iodide. I n the case of cadmium, on the other hsnd, the complex formation decreases in the opposite order: cadmium iodide > cadmium bromide > cadmium chloride. Over a range of concentrations, equilibrium constants for cadmium iodide indicate the species CdI+, Cd12, CdI,-, but mainly Cd14+ ( I ) , Kith an additional species, CdTs+, a probability (4). Cd+2and I- ions also are present, especially in very dilute solution, Because of the formation of complex CdId+, the iodide medium has frequently been selected in analyticnl procedures to carry out the separation of cadmium from zinc with the help of various reagents. The difference in the stability of the zinc and cadmium iodide complexes can be strikingly demonstrated by passing hydrogen sulfide into the -A eakly hydriodic a$d solution of the two metals. White zinc sulfide will first be quantitatively precipitated uncontaminated by any cadmium. However, the small quantity of cadmium present as the Cd+2 species gradually is precipitated as the sulfide with postprecipitation leading t o a significant contamination of the zinc sulfide (6). The iodide medium is ideally suited for the separation of zinc from cadmium using strongly basic anion exchange resins. Baggott and Willcocks ( d ) , determining traces of zinc in cadmium, showed that Cd14-2 is quantitatively adsorbed by De-Acidite FF, a strongly basic anion exchange resin, while the less stable zinc iodide is not retained. Hunter and Miller (6) first adsorbed zinc and cadmium as chloro anions on Amberlite
A
aniount of information has been published on the anion exchange properties of elements in chloride media, particularly hydrochloric acid solutions. Conditions have been described for adsorbing some 40 elements, while others, notably the alkali metals, alkaline earths, and rare earths, are not adsorbed (8, 9 ) . Both zinc and cadmium form negatively charged chloride complexes (Zn, CdC14-2) which can be adsorbed on strongly basic anion exchange resin, maximum adsorption occurring a t about 2M hydrochloric acid concentration. The cadmium complex is somewhat more stable than the zinc complex. K h e n the chloride concentration is reduced to 0.0251 the zinc chloro complex is not present any more to an appreciable extent and zinc is desorbed. Cadmium, on the other hand, can be eluted only with 0.001V or rreaker hydrochloric acid (8, 9). The stability of the chloro complex of cadmium is increased by the addition of alcohol (11) to such a n extent as to allow the chromatographic separation of moderate quantities of cadmium and zinc in a 0.01M hydrochloric acid-257, methanol medium (3). Differences in the stabilities of the chloro complexes of cadmium and zinc have been made the basis of a cation exchange procedure (12). The more stable cadmium complex is eluted with 0.5N hydrochloric acid, while the fraction of the negatively charged zinc complex has been reduced sufficiently to cause adsorption of the 58
LARGE
e
ANALYTICAL CHEM!STRY
IRA-400, thus achieving a separation from various other uncomplexed elements; then they eluted the zinc with a hydriodic-nitric acid mixture. I n a method by Kallmann, Oberthin, and Liu (T),only the cadmium iodide complex C C ~ I , -is~ adsorbed, while a11 the other constituents of the sample soiution, including the zinc, arc simultaneously removed. Cadmium is finally eluted with 3 S nitric acid, Khile the anion exchange separation of cadmium from zinc and other cations is quantitative and reasonably rapid, cadmium is obtained only as an end product, after the removal of all other elements. If the primary interest of the analyst lies in the determination of cadmium, a method based on the immediate elution of the cadmium and the simultaneous adsorption of all other constituents of the sample would appear superior. Such a method has been perfected using either Dowex 50 or Amberlite-120, strong acidic cation exchangers. EXPERIMENTAL
I n a preliminary investigation of the elution characteristics of cadmiurn and zinc by strongly acidic cation exchange resin, various concentrations of hydriodic acid were used. Cadmium was not adsorbed over the tested range, 0.007N to 2N of hydriodic acid, n-hil:. zinc was quantitatively retained as long as the normality of the hydriodic acid did not exceed 1.O. This provided additional proof that cadmium in a n iodide medium forms a very stable CdLd2 complex while zinc exists mainly as the cation species (10). Hydriodic acid was preferred to potassium iodide in the present work in order to increase the exchange capacity of the resin for zinc. Because the analysis of metallurgical products containing cadmium usually involves the use of nitric acid and/or hydrochloric acids, both of which can bc expelled by evaporation with sulfuric acid, a mixed sulfuric-hydriodic acid medium mas chosen, rather than straight hydriodic acid. Addition of the sulfuric acid does not affect the elution of the cadmium, but reduces to a minor extent the reten-
tion of the zinc. The mechanism of this decrease in zinc adsorption in the presence of sulfuric acid is not clear. I t does not appear t o be due to an increase in the hydrogen ion concentration, because hydriodic acid of the same normality does not have the same effect. The decrease in the zinc adsorption suggests the formation of a zinc iodide complex in the presence of sulfates. This, however, has been dis, roved by Stokes and b y Baggott and Rillcocks in their n ork n ith anion exchange resin, also by further work in this laboratory v-hich indicates that only free sulfuric acid and not sulfates affects the adsor1 tion of the zinc. I n any event, a 0.3.V hydriodic -0.15A' sulfuric acid medium is ne11 suited to effect the separation of cadmium from large amounts of zinc. A 1-inch inside diameter column containing a settled rcsin bed of 10 inches (about 90 grams of resin dry aeight) will retain more than 4 grams of zinc (and/or other cations), nhile the cadmium can be quantitatively eluted n ith 3 colunin volumes flS0 nil.) of the elutriant APPARATUS AND REAGENTS
Glass columns-1 inch in inside diameter, 15 inches long-to hold a settled resin bed 10 inches deep, are used when up to 4 grams of zinc or 120 meq. of other cations are adsorbed. The columns hold approximately 90 grams of resin, n-ith a liquid capacity of about 60 nil. For milligram and decigram quantities of zinc or other cations, columns 1 cni. or less in inside diameter are used, containing about 10 grams of resin. Donex 50 or Ainberlite IR-120. strongly acidic cation exchange resin, 100- to 200-mesh n ith 8% cross linkage, is treated with n a t e r and a slurry is introduced into the glass columns, n hich h a w a plug of glass TT 001 a t the bottom to ietain the resin. The 1-inch inside diameter columns should then contain a settled resin bed of 10 inches. The resin is then cleaned with 200 nil. of 2Ar hydrochloric acid and finally rinsed n i t h n a t e r Before use, 60 ml. of 0.3K hydriodic acid and 0.15h' sulfuric acid clutriant (sufficient for the void volume) are poured through the resin to prepare it for the reception of the sample. Correspondingly less resin and wash solution are required for smaller columns. ELuTnIkwr, 0.3.Y hydriodic acid and 0 . 1 5 s sulfuric acid. Dilute 50 nil. of hydriodic acid (48y0 to 50%) and 4 nil. of sulfuric acid to 1 liter. ( ~ T H Y L E N I l D I S I T R I L 0 ) T E T R A S C ET A T E
D r s o u r n r SALT (EDTA). Use 3.5 grams per liter for amounts of cadmium less than 50 nig., and correspondingly stronger E D T A solutions for larger quantities of cadmium. ERIOCHROX BLACK T [1-(1-hydroxy2 - naphthylazo) - 5-nit ro-2-naphthol-4sulfonic :icid] sodium salt. Dissolve 0.5 gram of tht. icagent and 4.5 grams of hydroxylamine hydrochloride in 100 nil, of alcohol.
PROCEDURE
The following procedure is specifically written for the determination of cadmium in zinc concentrate and cadmium fumes containing from 0.002 to 20% of cadmium. With slight modifications it can be applied t o the determination of cadmium in a variety of other products. Decompose from 1 to 3 grams of sample b y heating with 10 ml. of sulfuric acid. From time to time add a few drops of nitric acid to hasten decomposition of the sample. Finally, evaporate the solution to complete dryness. Add to the dry salts 90 ml. of 2% (0.7A') sulfuric acid and heat to boiling to dissolve soluble sulfates. Lead sulfate will remain insoluble. Cool to about SO" C., add about 5 grams of iron chips (carbon determination accelerator type). and allo\T- the reaction to proceed for 15 minutes on a steam bath. Cool, filter on a small filter paper containing a few iron chins. and wash with 0.7.V sulfuric acid.' ' Add to the filtrate. which should have a volume of 145 to '150 ml., 7 . 5 ml. of concentrated hydriodic acid ( 6 s ) and pass the solution through the ion exchange column (previously prepared by passing 60 ml. of the hydriodicsulfuric acid elutriant through the resin bed), with the bottom stopcock open, keeping the resin always covered with solution. Receive the eluate in a 600-ml. beaker. Use the full flow rate of the column (about 7 ml. per minute). Rinse the beaker n ith hydriodic-sulfuric acid elutriant and transfer the washings to the column. Wash down the sides of the column and allorr the solution to drain to about 1 ml. above the top of the resin. Wash the column with 3 column volumes of hydriodicsulfuric acid solution. EDTA Procedure (1 to 500 mg. Cd). Add t o t h e eluate containing cadmium and free hydriodic and sulfuric acids 5 grams of ammoniuni chloride a n d neutralize t h e solution with ammonia until red litmus papPr just turns blue. H e a t t h e solution t o about 90" C., a d d 15 ml. of 1 2 5 ammonia and 0.05 ml. of Eriochronie Black T indicator, and titrate the cadmium with E D T A solution. The color change is sharp from purple to blue. When analyzing completely unknown material. expedite the titration by adding a n excess of the reagent and backtitrate with a dilute cadmium solution to a permanent purple end point. Standardize the E D T A eolution with a solution of k n o i ~ i i cadmium content. Prepare the standard cadmium solution by dissolving the metal in hydrochloric. nitric, or sulfuric acid. Avoid hydriodic acid, because the commercially available acid (Baker) \\-as found to coiitain significant amounts of calcium which would be titrated by the EDTA. I n contrast, the calcium introduced with the hydriodic-sulfuric acid reagent into the sample solution is removed by the cation exchange resin and therefore does not interfere with the proposed procedure. To prepare the cadmium standard solution yielding a n identied
titer as a cadmium nitrate, chloride, or sulfate solution, use commercially available hydriodic acid, freed from calcium by passing a dilute solution through the cation exchange resin. Dithizone Procedure ( l o mg. Cd). Evaporate t h e eluate t o complete dryness and continue as in t h e earlier publication (7). Polarographic Determination. T h e iodide medium w s found unsuitable for t h e polarographic determination of cadmium. If a polarographic finish is desired (>0.1 mg. Cd), evaporate the eluate to complete dryness and continue as described ( 7 ) . Regeneration of Resin. T o regenerate t h e resin rapidly, pass 2A' hydrochloric acid through t h e column. Four to fiye column volumes of acid reniore all the cations which have been absorbed in t h e iodide medium. Two column volumes of water remove the free acid and the column is ready for t h e next sample. Interferences (Table I). Cations n i t h which cadmium conceivably may be associated either are removed by filtration in connection with t h e iron reduction step (lead, bismuth, arsenic, copper, silicon dioxide, silver) or are adsorbed as cations b y t h e resin zinc. iron, manganese, aluminum, calcium, molybdenum, nickel, cobalt, tin, indium, germanium, gallium, chromium). Other polyvalent cations not included in the above list presumably are also adsorbed by the resin. If alkali metals are rresent in the sample, they ,zre p r t l y found in the cadmium cluatc, but they do not interfere n ith any of the suggeslcd mrthods. The iron treatment of the sulfate solution is mainlj intended to rrdcce the large amounts of ferric ion, frequently occurring in cudmium-bearing substanccs, to ferrous ion and, similarly, to reduce cupric ion to metallic copper. Both cupric and ferric ions oxidize hydriodic acid to iodine which subsequently would liavc to be reduced n i t h sulfur dioxide or other reducing agents. I n the absence of large amounts VOL. 32, NO. 1, JANUARY 1960
59
of iron(II1) and/or copper, the treatment with iron chips can be omitted. Any free iodine is then reduced to hydriodic acid by careful addition of sulfur dioxide. Any cupric iodide resulting from the reaction of cupric ion with hydriodic acid and other insoluble substances should be filtered off to avoid contamination of the resin. The proposed procedure was applied to the determination of cadmium in the type of samples previously analyzed by the anion exchange method. The results based on duplicate determinations are presented in Table 11. ADVANTAGES OF CATION OVER PREVIOUS ANION PROCEDURE
The cation exchange procedure is far more rapid than the anion exchange method. A cadmium determination in the range of 5 p.p.m. to 50% in a zinc matrix containing a variety of other elements can be obtained in less than 4 hours. The speed of the method can be largely attributed to the fact that, after introduction of the sample into the column, cadmium is obtained in less than 1 hour as the first elution product. I n the anion exchange procedure, on the other hand, all elements but cadmium must be eluted before cadmium can be desorbed. The elution of the cadmium in the anion exchange procedure is more complicated, because the cadmium can be eluted only by destroying the Cd14-? complex with strong oxidizing agents, such as 2 S nitric acid. This causes the formation of considerable free iodine and iodate, nhich must be drstroyed before the determination of the cadmium can be concluded. I n the suggestrd cation procedure, on the othrr
hand, thc first eluate is suited for the direct determination of the cadmium by either the EDTA or the dithizone procedure. Only when the cadmium content is expected to be below 10 p.p.m. is it advisable, because of the volume and acidity of the eluate, t o evaporate the solution to dryness. Because of the large amount of free iodine pres&, the anion procedure requires prior evaporation bcfore cadmium can be detcrmined clcctrolytically. IThilc in tlie electrolytic procedure reported here the solution \vas prepared for electrolysis by prior evaporation to dryness, a f e v expcrimcnts have indicated that this may not be necessary and that tlic only s t q ) required would
Table I. Determination of Cadmium in Presence of Other Elements“
Cadmium Cadmium llethod Present, Found, for Final bIg. lIg. Determination 0.011 Dithizone 0.01 0.005 Dithizone 0.005 0.003 Dithizone 0.003 0 11 Dithizone 0.1 1 98 Polarographic 2.0 49 EDTA 5.0 Polarographic 10 0 10.0 20 3 EDTA 20.0 50 .i EDTA 50.0 99 7 EDTA 100.0 100.4 Electrolytic 100.0 199 7 Electrolytic 200.0 EDTA 301 0 300.0 500 $1 Electrolytic 500.0 a Artificial mixture consisting of zinc 3000 mg.; copper, lead, iron, aluminum manganese,, 100 nig. each; antimony, tin. arsenic, silver, bismuth, nickel, cobalt, calcium, magnesium 20 nig. each; indium, germanium, gallium 10 mg. each.
consist in rendering the solution alkaline with sodium hydroxide, dissolving the cadmium hydroside in a minimum amount of alkali cyanide, and electroplating in the presence of a reducing agent. Not enough work, however, has been carried out t o includc this simplification as a definitc recommendation. One important feature of the suggested cation procedure lies in the ease with TI hich the resin can be rcgeneratcd. This involves only the USE of 2 S hydrochloric acid and 11ater and requircs less than 1 hour. At no point does the procedure lcad to the forniation of frce iodine, because the iodide ion does not enter into the reactions occurring a t the resin site and can be replaced by simple washing procedures. DETERMINATION OF ZINC
This article features the detcrniiiiation of cadmium in zinc matrix substanccs. However, the cation separation of cadmium and zinc is equallJ- suitcd to the determination of zinc in cttdniiuni suhstances. For instance, ultrapure cadmium metal has been rapidly analyzed in this laboratory for a zinc content of 1 p.p.m. or less by passing the iodide solution of 5 to 20 grams of thc sample through a column containing only a few grams of resin and eluting tlic zinc after removal of the cadmium n i t h 2 S hydrochloric acid. Other interesting applications involve thc clution of cadmium in an iodidc medium folloned by elution of the adsorbed cations with hydrochloric acid, followd by trnnsfcr of the 0.5A‘ hydrochloric acid solution to an anion exchange column, n-here only the zinc vi11 be rctained, and finally eluting the zinc n i t h nitric acid. LITERATURE CITED
Table
II.
Accuracy of Proposed Pro(:edure
1 84
1 85
3
Electrolytic
0 98
0 97
3
EDTA
0 98
0 98
3
Electrolytic
0 64
0 63
3
EDTA
0 30
0 29
3
EDTA
0 21
0 21
3
EDT.4
Other Elements Present, Each 0 15% Zn, Pb, -11,Fe, Cu, lh, l l g , S, Si, Bi, As Zn, Pb, .41, Fe, Cu, >In, l l g , S, Si, Bi, As Zn, Pb, Fe, Cu. llg, bIn, -4s, Si, Bi, S Zn, Pb, Fe, Cu, l l g , A h , .h, Si, Bi, Y Zn, Pi), Fe, Cu, Ca, lIg, S, .4g, Sn Zn, Pb, Fe, Cu, Si, IIg, S, 1In Zn, Pb, Fe, Cu, Si, AIg, Pb, Zn, Cu, Bi, .4s, Si, Ph, Zn, Cu, Bi, As, Si,
Cadmium, 54 Present Found 1 84 1 82
-
Material Zinc concentrates
Lead concentrates Cadmium residue Zinc-base alloy Zinc-base alloy Silver solder
60
Sample Wt., G. 3
Final Detn. EDT.4
0 045
0 042
3
Polarographic
0 045
0 041
3
Dithiaone
S,1111 rig,
Ca, Si, Rlg
10 12
10 10
3
Electrolytic
Ag, Ca, Si, RTg Zn, Pb, As, Cu, Fe
10 12 14 0s 13 27
10 18 14 05 13 33 0 0042
2
EDTA Electrolytic EDTA Dithizone Dithizone EDTA
Zn, Pb, As, Cu, Fe Zn, Pb, As, Cu, Fe Zn, Pb, As, Cu, Fe Zn, Al, Cu Zn, Al, Cu Ag, Cu, Zn, Xi
0 004 0 004 16.25
0 0040
16 30
ANALYTICAL CHEMISTRY
3 2 3 1 1
(1) Alberty, R., King, E., J . --lvi. Chem. SOC.73, 517 (1951). (2) Baggott, E. R., Willcocks, R. G. \F7., Analyst 80, 53 (1955). (3) Berg, E. W., Truemper, J. T., -1x.4~. CHEX 30, 1827 (1958). ( 4 ) Cyr! H. )I., in “Comprehensive In-
organic Chemistry,” 11. C. Sneed, R. C. Brasted, eds., Van Nostrand, New York, 1955. (5) Hunter, J. A., Rlillcr, C. S.>.Innlpt 81, 79 (1956). (6) Kallmann, S. H., unpublished expcriment,s, 1957. ( 7 ) Kallmann, S. H., Oberthin, H., Liu, R., ASAL. CHEM.30, 1849 (1958). (8) Kraus, K. .4.,Nelson. F., “Anion change Studies of the Fission Products,” Proc. Internl. Conf. Peaceful Uses Atomic Energy, Vol. 7, p. 113, 19%. (9) Kraus, K. A., Kelson, F., .%ST.\[, Spec. Tech. Publ. 195 (1956). (10) Stokes, R. H., Levien, B. J., J . -4m. Chem. SOC.68, 1852 (1946). (11) Turyan, Y. I., Zhitr. A n d . K h i m . 11, 71 (1956). (12) Yoshino, Y., Kojinia, X., Japan Analyst 4 , 311, (1955). Ilk-
RECEIVEDfor review July 20. 1959. Accepted September 21, 1959.
