Determination of Cadmium and Zinc Separation from Other Elements and Each Other by Anion Exchange SILVE KALLMANN, CLIFFORD G. STEELE, and NORTON Y. CHU1 N. J.
Research Division, Ledoux & Co., 359 Alfred Ave., Teaneck,
APPARATUS A S D REAGEYTS
Zinc and cadmium form negatively charged chloride complexes w-hich are adsorbed by the strongly basic anion exchange resin Dowex 1. Maximum adsorption of zinc and cadmium is obtained in 0.121V hydrochloric acid containing 100 grams of sodium chloride per liter. In this medium most elements with which zinc and cadmium are associated in metals and minerals are not adsorbed by the resin. A 2.V sodium hydroxide solution, containing 20 grams of sodium chloride per liter, quantitatively elutes the zinc while the cadmium is retained by the resin. Finally, cadmium is eluted with 1Y nitric acid.
The apparatus used in the anion separations described belotr is similar t o that used by Hague for his ion exchange work on high-temperature alloys (4). Glass Columns. Columns 1 inch in inside diameter and long enough to hold a settled resin bed 10 inches deep are used when gram portions of elements are t o be separated. These columns hold approximately 90 grams of resin and have a liquid capacity of 40 ml. Columns 1 cm. in inside diameter are used for milligram t o decigram quantities; they should hold 15 grams of resin and have a 7-ml. liquid capacity. A multiple arrangement of columns is used to carry out a number of simultaneous separations. The solutions used for elution are pumped from 5-gallon stock bottles into 21/2-gallon delivery bottles (placed on a top stand t o provide a 6-foot head of liquid), from which they flow by gravity through a system of stopcocks and glass tubes to the columns. The following reagents are needed: Dowex 1, a strongly basic anion exchange resin, 200 to 400 mesh, with 8 t.o 10% cross linkage. The resin is settled in 0.5N hydrochloric acid and the finest and coarsest material removed b y decantation. A slurry of the main portion of the resin is used to fill the columns, which have a plug of glass n.001 in the bottom to retain the resin. The liquid capacity of the resin columns is determined b y mshing with a 10% ammonium chloride solution until the resin darkens, then measuring the amount of 1:V hydrochloric acid required to lighten the color of the resin (4).To extract organic subst'ances which tend t o slow down the flow rate, several hundred milliliters of LV nitric acid should be passed through the resin before the column is used for the first time. Hydrochloric acid, 0.12N containing IO0 grams of sodium chloride per liter (Reagent I). Sodium hydroxide, 2A' containing 20 grams of sodium chloride per liter (Reagent 11). Nitric acid, 1R'. Benzotriazole. Dissolve 2 grams in 100 ml. of water.
B
RTSON and Lenzer recently summarized the complexity. caused by the lack of specific reactions, of the analytical chemistry of zinc and cadmium ( 2 ) . Miller and Hunter cited recent developments in analytical methods for zinc, including two (ethylenedinitri1o)tetraacetic acid (ethylenediaminetetraacetic acid) procedures, two cation exchange procedures, a polarographic method, and a procedure based on cellulose ctiromatogI aphy (10). Kraus and Moore shoxed that the transition elements manganese, cobalt, copper, iron, and zinc form negatively charged chloride complexes which can be adsorbed and selectively eluted from a column of strongly basic anion exchange resin, while nickel and chromium remain in solution as cations. The formation of the various chloro anions depends on the individual element and on the concentration of the chloride ions. Manganese, for instance, is eluted with 9-L' hydrochloric acid and zinc with 0.005S or weaker hydrochloric acid (8). rllthough the exact mechanism of the adsorption of the various chloro anions has not yet been established, several practical and efficient techniques have already been suggested. Hague, Maezkomke, and Bright, using Dowex 1 resin, separated and determined varying amounts of nickel, cobalt, manganese, and iron found in high temperature alloys ( 4 ) . Miller and Hunter, using the strongly basic anion resin Amberlite IRA-400, chloride form, separated 50 mg. of zinc, in the absence of cadmium, from 100 mg. of various elements, forming no chloro anions or cornplexes which are less stable than zinc in 21)- hydrochloric acid. Part of the bismuth, antimony, tin, and indium and a small amount of copper and iron are adsorbed on the column, but a subsequent 8-quinolinol procedure is described which obviates their interference (10). Baggott and Willcocks, taking advantage of the difference in stability between the complex iodides of cadmium and zinc in a mixed sulfate-iodide solution (CdId-is more stable than ZnL--), separated microgram amounts of zinc from as much as 1 gram of cadmium using the strongly basic anion exchanger De-Acidite FF ( 1 ) . The adsorption of cadmium from chloride solutions by anion exchange resins was also observed recently by Jentisch and Frotscher (7') and Kraus, Selson, and Smith (9). However, no anion exchange procedures are described in the literature providing a separation of cadmium from zinc. Because published anion exchange techniques for determining zinc and cadmium are of limited scope, a method was developed t o provide a quantitative separation of cadmium and zinc from other elements and each other. 1
PROCEDURE
Prepare the resin by transferring two 30-ml. portions of Reagent I to a 1-inch column and drain the solution to about 0.5 em. above the to of the resin. If a 1-cm. column is used, two 10-ml. portions of tge reagent are sufficient. Dissolve the dry chloride or sulfate of the sample in a minimum amount of Reagent I. Quantitatively transfer small portions of the concentrated solution of the sample to the column with the bottom stopcock open, keeping the resin covered with solution t o prevent formation of air pockets, and using Reagent I for washing. Finally, wash down the sides of the column with Reagent I. Cap the column tightly t o prevent air leakage and begin t o deliver Reagent I from the reservoir bottle, adjusting the stopcock on the column t o deliver approximately 3 t o 4 ml. per minute. When a 1-inch column is used, 400 ml. of eluent is sufficient to ensure removal of 1-gram portions of elements not adsorbed on the column. Much less eluent is required when milligram or decigram amounts of elements are separated on a 1-cm. column. When sufficient solution has been collected, adjust the stopcocks t o deliver Reagent I1 t o the column (if a sample is known to contain no cadmium, the zinc may be eluted with 1 S nitric acid). As soon as the sodium hydroxide is about halfway down the column (indicated by the change in color of the resin), replace the beaker containing the elements eluted with Reagent I with a beaker to receive the zinc. Continue the elution of the zinc a t the full flow rate of the column. Check the completion of the zinc elution by acidifying a fresh portion of the eluent, then making it slightly ammoniacal and adding benzotriazole solution. If a precipitate forms, indicating incomplete elution of the zinc, and the determination of zinc is required, filter off the benzotriazole precipitate. Dissolve i t by fuming with nitric and sulfuric acide and add t o the main zinc solution.
Present address, Nuclear Developments Aasoc.. White Plains, X. Y .
230
V O L U M E 2 8 , N O . 2, F E B R U A R Y 1 9 5 6 After the elution of zinc is completed, change beakers and adjust the stopcocks to deliver distilled water. Collect about T5 ml. of water ( l l n c h column) to remove most of the sodium hydroxide solution, then adjust stopcocks to deliver l.V nitric acid. After 20 to 30 minutes, the acid will be about halfway down the column, indicated by the lightening of the resin color. Then replace the beaker containing the water and eluted podium hydroxide with a clean beaker. Continue the flow of nitric acid until the color of the resin indicates complete removal of sodium hydroxide, then collect an additional 350 ml. of eluent. Determination of Cadmium in the Eluent. To the nitric acid solution containing the cadmium, add 10 ml. of sulfuric acid and 2 ml. of perchloric acid and evaporate to dryness. At the fuming stage add a few drops of nitric acid to ensure complete destruction of organic compounds. Alternatively, cadmium can be precipitated with benzotriazole in slightly ammoniacal solution and the filtered precipitate fumed with nitric and sulfuric acids. If sufficient cadmium is present, determine it electrolytically ( 5 ) . Determination of Zinc. The final determination of zinc can be performed by a number of procedures. Zinc > 100 mg., potassium ferrocyanide method ( 6 ) Zinc < 100 mg., electrolytic determination ( 3 ) Zinc < 10 mg., dithizone method (11)
231 the resin. Hohever, attempts were unsuccessful to find conditions for the quantitative adsorption of cadmium in the presence of amounts of zinc exceeding the capacity of the columns. Separation of Zinc from Cadmium. Of many reagents tried, only sodium hydroxide solution was found to provide a quantitative separation of zinc from cadmium. The elution of zinc aa sodium zincate is quantitative, but the exact mechanism of the retention of cadmium in a sodium hydroxide medium has not yet been established. It was noted that the addition of sodium chloride to the sodium hydroxide solution not only speeds up the zinc elution considerably, but also helps to retain the cadmium. The cadmium band, which looks gray in a sodium hydroxide-sodium chloride medium, does not noticeably move, even when 5000 ml. of 2 5 sodium hydrokide follon-ed by 500 ml. of water is passed through the columns. Xeutralized portions of the sodium hydrouide eluates (usually 25 ml.) from each 100-ml. fraction gave no positive test for cadmium with dithizone. The tenacity with which the cadmium is held may be partly attributed to the presence of the sodium chloride.
Other procedures could be applied Ignition of the sulfide t o oxide ( 6 ) Determination as zinc pyrophosphate ( 6 ) Precipitation with benaotriazole (13) Regeneration of Resin. iifter cadmium is removed with LY nitric acid, regenerate the resin by passing Reagent I through the column, which is now ready to receive the next sample. DISCUSSION
Adsorption of Zinc and Cadmium. Previous work with chloro anions indicates that the separation of the individual complexes depends largely on the hydrochloric acid concentration. For instance, manganese can be eluted with 9N hydrochloric acid, thus providing a separation from cobalt, copper, iron, and zinc; b u t it is more efficiently eluted with 4N hydrochloric acid which also removes cobalt. Manganese is eluted even more rapidly with 1 S hydrochloric acid which elutes cobalt, iron, and copper in addition. I t is therefore not surprising that Miller and Hunter, by using a rather high acidity to adsorb zinc, encountered difficulties in removing all of the copper and iron with a 2AY hydrochloric acid eluent (IO). I t was found early in this investigation that, in the presence of considerable sodium chloride, the hydrochloric acid concentration can be lowered from LV to 0.12N, without affecting adversely the adsorption of the zinc by the resin. When using Reagent I, maximum adsorption of both zinc and cadmium was obtained (Figure 1). The stability of zinc and cadmium chlorides in a sodium chloride medium is demonstrated in Figure 1. The relative adsorption of zinc :ind cadmium by the resin approximates the ratio of their atomic weights. To obtain these data, 3-gram portions of zinc and cadmium were dissolved in hydrochloric acid and hydrogen peroxide, and these solutions were evaporated t o dryness. The salts were dissolved in 75-ml. portions of dilute hydrochloric acid of varying strength containing from 0 t o 15 grams of sodium chloride per 100 ml. of solvent. The solutions were transferred to 1-cm. columns which had been previously washed viith the solvent used in each particular test. Elution of the excess zinc and cadmium was then continued with the same solvents until exactly GOO ml. of eluate was obtained. The adsorbed zinc or cadmium was then eluted by passing 400 nil. of 1.17 nitric acid through the columns. Zinc was determined by titration with potassium ferrocyanide (6) after evaporating t,he solution several tiines to dryness with intermittent additions of hydrochloric acid. Cadmium was determined electrolytically ( 5 ) . Cadmium is held preferentially by the resin when Reagent I is used. When solutions containing 5 grams of zinc and 50 mg. of cadmium were passed through columns having a capacity t o adsorb 1.5 grams of zinc, T5 to 95% of the cadmium was held by
1
j
2ON
15N
ION
05N
;",
05N
1
ADSORPTiOtv 3F C A D M K
ION
I5N
2ON
H CL CONCENTRATION
ADSORPTION OF ZINC
Figure 1. Adsorption characteristics of zinc and cadmium
Tests indicated that the elution of zinc is independent of the amount of cadmium present, When the 1-inch columns are used, approximately 350 ml. of the sodium hydroxide-sodium chloride solution was sufficient to elute 1 gram of zinc. With a 1-cm. column, only 200 ml. was required to elute the Same amount of zinc. Elution of Cadmium. Early In this investigation, dilute sulfuric acid was used for the elution of cadmium and of zinc in the absence of cadmium. After the chlorides from the sodium chloride-hydrochloric acid medium n ere adsorbed, the acidity of the sulfuric acid had to be raised to 1LY to elute cadmium and zinc. However, it was found that cadmium ran he readily eluted with 1 S nitric acid without adversely affecting the resin. T o avoid the formation of unnecessary sodium nitrate in the eluate, water is passed through the column for a fern minutes prior to the application of the nitric arid. When the 1-inch column is used, 400 ml. of I S nitric acid is sufficient to elute 5 grams of cadmium. The 1-cm. column rcquirei only 1.50 ml. to elute 1 gram of cadmium IYTERFERENCES
Sulfates, Tin, Arsenic, and Antimony. The adsorption of zinc and cadmium is performed in a chloride medium. The sulfates remaining after evaporation of sulfuric acid do not interfere with the proposed method; therefore, decomposition of metals and minerals with nitric and sulfuric acids is possible. antimony,
ANALYTICAL CHEMISTRY
232 -
~-
Table I. Zinc, Grams Used Found 0.0500 0.0606 0.9982 1.0000 1.0000 1,0003 0.5000 0,5009 0.2500 0.2492 0.0500 0.0498 0.0005 0.00042 3.0000 ...
0.5000
...
...
...
Cadmium, Grams Used Found 0,0992 0.1000 0.1000 0.0991 0,2500 0.2607 0.5000 0.5011 1,0004 1.0000 1,0000 1.0001 2,0000 0.66648 0.0005 0.00156 0.0015 0.0247 0.0250
0.2562
0.1000 0.5000 0,2500
0.1003 0.5014 0.2504
0.5006
0,2500
0.2488
Other .. ~ ~ Element Name Grain
..
.. .. ,.
.. ..
..
.. Ca Mgi AI , Fe Cr cu co Ni Mn Ti Zr ' Th t Be Sb Sn t 5
0.5000
0.2482
0.5000
0.5014
0.2500
0.2516
0 5000
0.5011
0.2500
0,2499
0.2 0.3 0.31 0.3
0,5000
0.5000
0.2500
0.2503
0.1
0.2500
0,2500
0.5
0.2500 0.1000
0.2487 0.0990
0.3 0.3
"08
++ HZ016 ++Hz02 HiSOi + HzSOa HzS01
HNOa; HNO HCI HCI HC1 HCI
HC1 HCI
~
~
C
$ H20z
..
++ Hz02 + H202 H20z "03; HC1
..
..
+ HSOa
a
e e
a
e
a
e
a
e
a
e
0.2 0.3)
+ HzSO4; HBi HNOa + HzSOa (filtered PbSOL H S O s + HzSOi (Pb not filtered off) + H&OI t o dryness (excess P b filtered off) + HzS04 HNOI: Zn iCd collected HNOa
"08
"08
a a
.. d a
with benzotriazole
Cadmium e. Electrolytically. f . Collected with benzotriasole in ammoniacal tartrate solution, finally determined as sulfate g. Dithizone color method. indicates reagents added a t the same time; ; indicates reagents added one after the other.
+
Table 11. Determination of Zinc and Cadmium in Concentration Products Zinc, % Cd, % Present Found Present Found 47.32 47.49 2.20 2.20 64.98 64.81 ... ... 84.32 84.49 , . , , , . 87.68 87.55 ... ... 22.00 34.12 65.04 57.13 55.85
"08 "08
Method * for Final Determination' ~ Zn 6h a a a a h
Zinc a. Potassium ferrocyanide titration. b. Electrolytically. c. Dithieone color method. d. Collected with benzotriazole in aminoniacal tartrate solution, finally determined electrolyticall)..
tin, and arsenic are adsorbed t o some extent by the resin] but can be expelled with hydrobromic acid from a sulfuric acid medium. Elements with which zinc and cadmium are often associated in metals and minerals, such as trivalent iron, manganese, aluminum] beryllium] nickel, cobalt, chromium, copper, titanium, the rare earths, and the alkaline earths, are not adsorbed on the resin in the hydrochloric acid-sodium chloride medium; they can be quantitatively determined in the eluent by standard procedures. Many quantitative recoveries of the above elements were carried out in the course of this investigation, but results are not included in this report.
Material Zinc Concentrate Zinc skimming Cadmium fume Cottrelldust Zinc concentrate
HC1 HC1
HCl; HSOa
0.2500
h
Procedure
0.2j
0 2518
a
~
HCI
0.2500
0.1004 0.1014
~
0.51
;:;I
0.2504
0.1000 0.1000
~
HKOs; HC1
0.2500
, . .
.
0.3) 0.3
0.5001
2.0000
~
Determination of Zinc and Cadmium in Synthetic Mixtures
-
3.0000 3.0000 3.0000 3.0000 0.2500
~-
~-
~~~~
.,,
.,, .. .
.,,
...
11.15
14.04 0.23 0.47 0.97
Other Elements Present above 0.5% Fe, Cu, hln, P b , As, Al, S Fe, Cu, hfn, P b , Cd, AI, Si, S AI, F e AI, Fe
11.22 P h . As, Fe 14.14 Zn 4s Ph Fe 0.24 F e ' Cu' MA P b Ca AI, S 0 . 4 7 Fe' Cu' hln' Ph' AI '5 0 . 9 8 Fe: Cu: hln: Ph: Ca', AI, S
mination of cadmium. Amounts of lead exceeding t,he solubilit,y of lead chloride in Reagent I (lead > 100 mg.) should be removed as lead sulfate or chloride before transferring the solution t o the c.olumns. Bismuth. Bismuth chloride is very soluble in Reagent I and is quantitatively adsorbed on the resin. Tests indicate that it is partially extracted by the 2N sodium hydroxide, which dissolves about 10 mg. of bismuth per 100 ml.; the remainder is eluted by the 1N nitric acid solution. I n the determination of cadmium, small amounts of bismuth can therefore be disregarded. Larger amounts of bismuth can be eliminated by precipitating the cadmium, or both cadmium and zinc, with benzotriazole before transferring the solution t o the column. Bismuth may also be separated from zinc and cadmium by its precipitation os the sulfide from a strongly acid solution. Possible interference of other elements was not investigated. Some, forming insoluble compourids in acid solution (silicon, tungsten, tantalum, and columbium), could be filtered off before transferring the chlorides to the columns; others, which are adsorbed by the resin in a chloride medium (indium and germanium), usually occur in zinc-cadmium material in such minor quantities that their possible effect on the suggested method did not justify further investigation. 4CCURACY
Lead. Lead chloride is adsorbed on the resin, but is quantitatively eluted with the zinc by 2 5 sodium hydroxide. Hence, lead chloride soluble in Reagent I does not interfere with the deter-
Experimental data are presented in Table I for determinations of zinc and cadmium by the proposed procedure in svnthetic mixtures of a number of elements. The method was also applied t o the determination of zinc and/or cadmium in various conpen-
~
V O L U M E 2 8 , N O . 2, F E B R U A R Y 1 9 5 6 ti ,ction products which had lieen analyzed by the standard proc d u r e s given hy Scott f121. These data are given in Table 11. LITERATURE CITED
233 Jentisch, D., and Frotscher, I., Z . anal. Chem. 144, 17 (1955). (8) Kraus, K. .I., and Moore, G. E., J . Am. Chem. SOC.75, 1460 (7)
(1953). (9)
Kraus, K. A , , Kelson, F., and Smith, G. W., J . Phys. ChenL. 58,
(10) (11)
Miller, C. C., and Hunter, J. A , A n a l y s t 79, 483-92 (1954). Sandell, E. B., “Colorimetric Determination of Traces of Metals,” pp. 623-6, Interscience, S e w York, 1950. Scott, W.W., “Standard Methods of Chemical Analysis,” pp. 1054-68, Van Kostrand, New York, 1939. Welcher, F. J., “Organic Analytical Reagents,” vol. 3, pp. 555-6, Van Sostrand, S e w York, 1947.
11 (1954).
(1) Baggott, E . R., and Willcocks, R. G . W , , AnaZyst 80, 53-64 (1955). ( 2 ) Bryson, A%., and Lenzrr. S.,Ibid., 78, 299 (1953). ( 3 ) Dakin, H. D., Analyst 39, 273 (1900). (4) Hague, J. L., hlaezkomske. E. E., and Bright, H. A , , J . Research Nutl. Bur. Standards 53, 353-8 (1954).
( 5 ) Hillebrand, W. F., and Lundell, G. E . F., “Applied Inorganic Analysis,” pp. 256-5. Wiley, Yew York, 1953. (1;) Ibid., pp. 422-3i.
(12) (13)
RECEIVED for review M a g 17, 1955.
.-iccepted November 10, 19.5;.
Continuous Analysis of Trace Amounts of Oxygen in Gases F. W. KARASEK, R . J. LOYD, D. E. LUPFER, and E. A. HOUSER Research and Development Department, Phillips Petroleum Co., Bartlesville, O k l a .
In m a n y chemical processes, a knowledge of t h e oxygen c o n t e n t of gases is necessary. Although laboratory methods of a d e q u a t e acciiracy for d e t e r m i n a t i o n of trace a m o u n t s of ox>gen exist, t h e changing characteristics of p l a n t processes m a k e a c o n t i n u o u s measurem e n t desirable. A c o n t i n u o u s oxygen analyzer of t h e colorimetric-differential p h o t o m e t e r type suitable for p l a n t service h a s been devcloped. Oxygen in t h e s a m p l e gas continuously oxidizes a r e a g e n t p u m p e d i n a closed cycle, producing a color change proportional t o t h e a m o u n t of oxygen present. T h e oxygen-sensitive reagent, a n alkaline m l u t i o n of s o d i u m a n t h r a q u i i i o n e 2-snlfonate, is continuoilsly reduced t o a deep red color 1))- passing i t o \ e r a zinc-mercury a m a l g a m . Barrierla>-er photocells i n a direct c u r r e n t bridge circuit nieasiire t h e differential ahsorptivitv between t h e conipletely reduced. red reagent a i d t h e partially oxidized reagent of lighter color. T h e bridge is contini~oiisl>balanced k)?- a motor-driven p o t e n t i o m e t e r in :I null-balance type servosvstem. Angular position of this p o t e n t i o m e t e r is telenietered t o a s t a n d a r d IO-mv. recorder. T h e i n s t r u m e n t m a y be employed t o nionitor trace a m o u n t s of ox?-gen i n a n y gas which does n o t rapidly destroy t h e reagent. T h e i n s t r u m e n t is conLained i n a housing designed t o be explosion-resistant in Class I, G r o u p (7, locations. T h e full scale sensitivity of t h e analyzer is a d j u s t a b l e f r o m 0-50 t o 0-500 p.p.m. by volume of oxygen.
I
has been employed in a continuous osygen analyzer suitable for plant use in ranges below 1000 p.p.m. PRINCIPLE OF OPERATION
Operation of the analyzer is based on the quantitative change in color of the reduced alkaline reagent solution when it is brought in contact with oxygen-bearing sample gas. The red, reduced dihydrory form is oxidized by molecular oxygen t o the colorless diketo form. The oxidation reaction is rapid; hoivever, the gas and liquid must be brought intimately in contact under conditions such that all of the oxygen is reacted. The reaction is reversed Iiy the reducing action of zinc amalgam PO that the reagent returns t o the original red, dihydroxy form (.3). Operation of the analyzer is illustrated schematically in Figure 1.
4 i_iLlQUlO
GAS VENT
I
S CARRYISG out rnciny (.hemica1 reactions, especially those
involving polymerizations, an accurate knowledge of osygen c~oiic~entrationis desired. In cxontinuous plant processes the resiiltx of laboratory control analysis are often obtained long aftm the required operating corrrctions should have been made. -111 instrument which provides continuous information would fac,ilitat,e control of the plant process. Several laboratory methods for the ana lyse^ of trace quantities of oxygen in gases ( 1 , 2, 5-11) were studied i i the ~ basis for design of a continuous instrument. -4chemical eystem first studied by Feiser ( 4 ) and later used by Brady (3) shon-ed the most promise. This system, based on the reversible reactions OH
OH
0 II
t)
___
G A S DISCHARGE_
OVERFLOW
PLIFIER
I
I
GAS INLET C D N fROLLED
FLOW
PY-PAS PUMP
Figure 1. S c h e m a t i c d i a g r a m of oxygen analyzer
Starting a t the top of the regenerator, the reagent passes over the amalgam, becoming completely reduced t o a deep red color. From the bottom of the regenerator, the reagent is accurately metered by a bellows-type positive-displacement pump equipped with check valves. At a flow rate around 30 mi. per minute, the solution passes through the reference cell, where the optical transmittance of the completely reduced reagent is measured by the reference photocell. I n the reactor, the reagent comes in contact
