Liquid-Liquid Extraction of Cadmium with High-Molecular-Weight Amines from Iodide Solutions Curtis W. McDonald’ and Fletcher L. Moore2 Oak Ridge National Laboratory, Oak Ridge, Tenn. 37830
Considerable interest has developed recently in the pollution problems caused by toxic metals. Cadmium and mercury are two major pollutants for which new and better abatement processes and analytical methods must be developed. Liquid-liquid extraction is an attractive technique for meeting these challenges ( I ) . The extraction of cadmium from 6.9N hydriodic acid by diethyl ether has been reported (2). Cadmium iodide has also been extracted with pyridine-benzene (3) and aliphatic alcohols ( 4 ) . The extraction of cadmium iodide with a secondary amine a t p H 2 3 has been investigated ( 5 ) . Because the high-molecular-weight amines are now firmly established as superior extractants (6)in many systems, we have studied the extraction behavior of cadmium from iodide solutions with the four classes of amines. In particular, detailed studies have been made with a quaternary amine, inasmuch as it is an excellent extractant for cadmium from alkaline as well as acidic solution. Solvent extraction of cadmium from alkaline solution with amines has not been described previously.
EXPERIMENTAL A p p a r a t u s . A NaI(T1) well-type scintillation counter, 1.75 X 2 inches, was used for gamma counting. Reagents. Primene J M - T is a mixture of primary amines, principally in the C18-22 range. Primene 81-R is a mixture of primary amines, principally in the C12.14 range. Amberlite LA-1 (N-dodecenyltrialkylmethylamine)is a secondary amine. This secondary amine and the primary amines listed above are available from Rohm and Haas Co., Philadelphia 5, P a . Alamine 336-S (tricaprylamine) is a tertiary amine available from General Mills, Inc., Kankakee, Ill. Aliquat 336-S (tricaprylmethylammonium chloride) is a quaternary amine chloride available from General Mills. The amines were dissolved in reagent grade xylene. The amines were converted to the iodide salts by the following pretreatment: equal volume portions were extracted twice for 5 minutes each with 10% Na2C03 followed by three 5-minute extractions with 6M KI. The solutions were centrifuged for 5 minutes after the last treatment and the organic phase was used in the various studies. Cadmium-SO9 tracer is available from S e w England Nuclear Co., Boston 18, Mass. All other chemicals were reagent grade. Evaluation Procedure. Five milliliters of the indicated aqueous phase containing 1 x IO5 gamma counts per minute per ml of Io9Cd tracer was extracted a t room temperature with a n equal volume portion of the various solvents in 50-ml heavy duty glass centrifuge tubes. Three-minute mixing periods were selected arbitrarily. High speed motor stirrers equipped with glass paddles were used for the extractions. After extraction, the tubes were centrifuged in a clinical centrifuge for 2 minutes. Each phase was then analyzed for cadmium-109 by counting S-ml aliquots in a well-type gamma scintillation counter. ’Present address, Southern University, Baton Rouge, La. 2To w h o m correspondence should be addressed. (1) F. L. Moore, Environ. Sci. Techno/., 6, 525 (1972) (2) S. Kitahara, Bull. fnst. Phys. Chern. Res. (Tokyo), 24, 454 (1948). (3) L . E. Mattison and J . C. Wolford, Anal. Chem., 38, 1675 (1966). (4) E. Gagliardi and P. Tuemrnler, Talanta, 17, 93 (1970). (5) J. R. Knapp, R. E. Van Aman, and J. H. Kanzelmeyer, Anal. Chem., 34, 1374 (1962). (6) F. L. Moore, NAS-NS-3101 (1960).
RESULTS AND DISCUSSION High-molecular-weight primary, secondary, tertiary, and quaternary amines were investigated to determine their ability to extract cadmium from aqueous iodide solution. Both acidic and alkaline solutions were used in the investigation. Each aqueous solution contained 2 mg/ml carrier cadmium as cadmium iodide. The solutions were studied as outlined in the evaluation procedure described above. The results (Table I) show that all the amines investigated extract cadmium essentially quantitatively from acid solution with the exception of the primary amine mixture, Primene JM-T, which extracts 93%. On the other hand, Table I1 shows a wide variation in the extractabilities of the various extractants in alkaline solution, Only the quaternary amine solution, Aliquat 336-SI-xylene, extracted the cadmium quantitatively with Primene 81-R also showing very high extractability. Because of its great versatility, Aliquat 336-S-I was chosen for further investigation. The mechanism of extraction of cadmium from aqueous iodide solutions with a quaternary amine is of the type:
Cd” 2(R4K1)0
+
+
(Cd14J2,
41-
+ (Cd1,)L-
--L
[(R,N)2CdI,],
+
21,
where RINI = Aliquat 3363-1, o = organic phase, a = aqueous phase. The quaternary amine and its salt with the iodo complex of cadmium are essentially insoluble in aqueous solutions but show high solubility in most organic solvents. The pertinent variables of the cadmium-Aliquat 336-S-I extraction system were studied by use of the evaluation procedure previously described. Each aqueous solution contained 2 mg/ml cadmium carrier as cadmium iodide unless otherwise specified. A minimum of approximately 3% Aliquat 336-S-I-xylene is required for quantitative extraction of an equal volume of 2 mg/ml cadmium solution. This concentration corresponds to a slight excess of a 2 : l mole ratio (Aliquat 336-S-I:cadmium), further confirming the proposed formula of the quaternary amine-cadmium iodo complex salt. There was no measurable extraction observed when attempts were made to extract cadmium with pure xylene or xylene which had been pretreated with 1M HI. A 5% Aliquat 336-S-I-xylene solution was used in further investigations, The extraction of cadmium with 5% Aliquat 336-S-Ixylene as a function of HI concentration (Table 111) shows that it is extracted essentially quantitatively from very dilute to highly concentrated HI solutions. Nevertheless, a small quantity of iodide is necessary in the aqueous phase for quantitative extraction. In a 10-4M HI (with no additional iodide) containing 100 ppb cadmium, 86% of the metal is extracted, whereas if the 100 ppb solution is 0.5M in HI, quantitative extraction is achieved. The effect of pH on the extractability of cadmium is shown in Table IV. Quantitative extraction is observed for highly acidic solutions up to a pH of about 7 . Above pH 7 , A N A L Y T I C A L C H E M I S T R Y , VOL. 45. N O . 6, M A Y 1973
983
Table I . Extraction of Cadmium from Acid Solutions as a Function of Amine Classa Amine extractant in xylene
5% Primene 81-R 5% Primene J M - T 5% Amberlite LA-1 5% Alamine 336-S, 5% Aliquat 336-S-1
Class
Primary Primary Secondary Tertiary Quaternary
Initial aqueous solutions-pH
Cadmium extracted, %
7.3
99.8
2.0
93.0
3.0
99.9 99.9 99.9
5.0 7.1 11 .Eb
6.2 6.0 2.6 0.9
0.9, 2 m g / m l Cd, 0.2M iodide.
-
5% Primene 81-R 5% Primene J M - T 5% Amberlite LA-1 5% Alamine 336-S 5% Aliquat 3364-1
Class
Primary Primary Secondary Tertiary Quaternary
initial aqueous solution-pH
Initial pH, aqueousphase
Equilibrium PH
Table I I . Extraction of Cadmium from Alkaline Solutions as a Function of Amine Classa Amine extractant in xylene
Table I V . Extraction of Cadmium as a Function of pHa
Equilibrium PH
Cadmium extracted, %
11.1
92.5 62.8
10.5
9.6 10.5
1.o
0.05 0.1 0.5 1 .o 2.0 3.0 4.9
99.8 99.9 99.9 99.8 99.9 99.9 99.9
Initial aqueous soiution contained 2 m g / m l Cd Solvent. 5 % Aliquat 336-S-I -xylene a
cadmium hydroxide precipitates unless the aqueous solution contains a fairly large excess of iodide. In 2.4M iodide (as KI), cadmium was extracted quantitatively a t a pH of 11.8. Equilibrium is achieved very rapidly; a mixing period of less than 15 seconds proved to be adequate for quantitative extraction. An aqueous-organic ratio of 100: 1 proved satisfactory for quantitative extraction at the 100-ppb level, providing the Aliquat 336-S-I: cadmium mole ratio is greater than 2 : l and the initial aqueous phase contained sufficient iodide (about 10W2rn).No ratios higher than 1OO:l were investigated. Several aqueous reagents (Table V) were evaluated for their ability to strip cadmium from 5% Aliquat 336-S-Ixylene. The organic solvents initially containing 2 mg/ml cadmium were stripped by extracting for 3 minutes with equal volume portions of the various strippants. Nitric acid solutions (55 M ) readily stripped the cadmium from the organic solvent, although some free iodine was liberated. Sodium sulfide (1M) stripped about 70% cadmium with the precipitation of cadmium sulfide. SOdium hydroxide (5 1 M ) stripped the cadmium by forming a cadmium hydroxide precipitate. EDTA was a poor cadmium strippant from either alkaline or acidic solution. Excellent cadmium strippants were the alkaline reagents-NH40H(5 lOM), 5% ethylenediamine, and 0.1M cysteine-1M NaOH. No troublesome precipitates occur in these systems, because cadmium forms soluble ammine, “en.” or sulfhydryl complexes, respectively. 984
ANALYTICAL CHEMISTRY, VOL. 45, N O . 6, M A Y 1973
99.9 99.9
3.5 5.9
99.8 99.9
7.7 11.9
99.9 99.9
Table V. Stripping of Cadmium from 5% Aliquat 336-S-I-Xylene Solutions Strippant
HN03M, 2 5 10 Na2S, 1 M Na2S03,1M NaOH M, 1 5.9 EDTA, 0.1M pH 6.0 EDTA, 0.1 M pH 9.0 N H 4 0 H M. 1 5 10 14.7
Table I l l . Extraction of Cadmium a s a Function of HI Concentrationa Cadmium extracted, %
1 .o 2.1
*
11.0, 2 m g / m l Cd, 2M K I .
HI concentration, M
Cadmium extracted, %
Initial aqueous phase-2 m g / m l Cd, 0.05M Iodide. Solvent, 5% Aliquat 336-S-I-xylene. Contained 2.4M K I .
7.8 1.6 99.9
10.8
Equilibrium pH, aqueousphase
Ethylenediamine, 2.5% 5.0% 0.1M L-cysteine, pH 1.3 0.1M L-cysteine, pH 9 . 3
0.1M L-cysteine-1M NaOH
Cadm‘ium stripped. %
99%) in the 5% Aliquat 336-S-I-xylene. This separation of cadmium from mercury results from the net stability differences of their iodo and ammine or “en” complexes. Thus, the anionic mercuric iodo species in 5% Aliquat 336-S-I-xylene cannot be stripped with 10M NH40H or 5% ethylenediamine because of the very high stability of its iodo complex (7). On the other hand, cadmium with a weaker iodo complex than mercury shows a greater tendency to form aqueous soluble complexes with ammonia or ethylenediamine. If desired, the mercury can readily be stripped from the solvent with alkaline cysteine (7). ( 7 ) F L Moore, Separ S o ,7, 505 (1972)
APPLICATIONS The high-molecular weight amines are excellent extractants for cadmium from iodide solutions. Because they can be diluted with relatively inert solvents, they are safer than the ethers and alcohols currently used. The quaternary amine, Aliquat 336-S, is especially attractive for the extraction of cadmium from alkaline as well as acidic solution. The amine-iodide system is of particular value to the environmental analyst because it offers a simple, rapid cadmium concentration method from aqueous solutions containing very small amounts of the element. This suggests that the organic solution can be used directly in flame photometric determinations, in which one can ben-
efit from both the solvent concentration effect and solvent enhanced sensitivity for cadmium. By taking advantage of the selective stripping technique described, the analytical chemist can separate cadmium from mercury in a simple, rapid manner.
ACKNOWLEDGMENT The authors gratefully acknowledge the assistance of H. A. Parker and W. R. Laing for some of the analyses. Received for review October 2, 1972. Accepted November 22, 1972. Research sponsored by the U. S. Atomic Energy Commission under contract with Southern University and Union Carbide Corporation.
2,4,6-Triphenylpyrylium Chloride. A New Organic Analytical Reagent for the Determination of Certain Anions Thomas C. Chadwick Union Sugar Division/Consolidated Foods Corp., Santa Maria, Calif. 93454
Nitron ( I ) , tetraphenylarsonium (2, 3 ) , -phosphonium, and -stibonium chlorides, methylene blue ( 4 ) , and tetran-pentylammonium bromide ( 5 ) have all been used as precipitants for gravimetric or volumetric determinations of various anions. More specifically, nitron and tetraphenylarsonium chloride have been used to determine tetrafluoroborate (6, 7 ) , perchlorate (8, 9 ) , and hexafluorophosphate (10, 11) gravimetrically and volumetrically, and tetraphenylarsonium, -phosphonium, and -stibonium chlorides have been used for the volumetric determination of chloro complexes of mercury, tin, cadmium, and zinc (12, 13). A brief survey of the literature disclosed a wealth of information regarding water-insoluble salts of pyrylium cations and anions of the type listed above ( 1 4 ) . In spite of this extensive body of information, little use has been made of water-soluble pyrylium salts as precipitants for anion analysis (15). It appeared likely that some readily synthesized, water-soluble pyrylium salt would be useful as an organic precipitant for anions. During the course of this work, a simple synthesis 3f water-soluble 2,4,6-triphenylpyrylium chloride (TPC) was ( 1 ) F. J. Welcher, "Organic Analytical Reagents," Vol. I l l , D. Van Nostrand Company, Inc., New York, N.Y., 1947, p 138.
(2) F. J. Welcher. "Organic Analytical Reagents." Vol. i V , D. Van Nostrand Company, Inc., New York, N . Y . . 1948, p 326. (3) P. W . Carr and J. Jordan, Anal. Chem., 44, 1278 (1972). (4) G . M . Nabar and C. R. Ramachandran, Anal. Chem., 31, 263 (1959). (51 R . G . Dosch,Anal. Chem., 40,829 (1968). H . E. Affsprung and V. S. Archer, Anal. Chem., 36, 2512 (1964). C. A. Lucchesi and D. D. DeFord,Anal. Chem., 29, 1169 (1957). (8) D. J. Glover and J. M. Rosen. Anal. Chem., 37,306 (1965). (9) K . Kodama, "Methods of Quantitative Inorganic Analysis," Interscience, New York, N . Y . , 1963, pp 122, 456. (10) H . E. Affsprung and V. S. Archer,AnaL Chem., 35,976 (1963). (11) H . E . Affsprungand V . S. Archer, Anal. Chem., 35, 1912 (1963). (12) H. H . Willard and G . M . Smith, Ind. Eng. Chern., Anal. Ed., 11, 269 (1939). (13) H . H . Willard and L. R . Perkins, Anal. Chem., 25, 1634 (1953). (14) K . Dimroth and K. H. Wolf in "Newer Methods of Preparative Organic Chemistry," Vol. I l l , W . Foerst, E d . . Academic Press, New York, N . Y . , 1964, p 3 5 7 . (15) H. A . Potratzand J . M . Rosen.AnaL Chem.. 21, 1276 (1949).
devised, and its behavior with a number of anions was studied. In addition, two representative gravimetric procedures were devised. The first, a direct precipitation of perchlorate ion, and the second, a determination of boric acid by conversion to tetrafluoroboric acid and subsequent precipitation, serve to illustrate the utility of the new reagent.
EXPERIMENTAL Reagents. All melting points are corrected. All inorganic chemicals used were reagent grade. Acetophenone (Aldrich Chemical Company, 1nc.j and benzaldehyde (J. T. Baker Chemical Company) were used as received. Distilled, deionized water was used exclusively. Chalcone (1,3-diphenyl-2-propen-l-onej was synthesized by the procedure of Helmkamp and Johnson ( 1 6 ) . 2,4,6-Triphenylpyrylium perchlorate was synthesized from chalcone and acetophenone by the procedure of Balaban ( I 7). 1,3,5-Triphenyl-2-penten-l,5-dione (Pseudobase). 2,4,6-Triphenylpyrylium perchlorate (3.96 grams, 9.7 mmol) was mixed with 125 ml of boiling 95% ethanol. The mixture was stirred vigorously, and a solution of anhydrous sodium acetate (3.21 grams, 39.1 mmol) in 50 ml of boiling distilled water was added in one portion. The solution cleared momentarily but after a short time the product started to crystallize. The reaction mixture was refrigerated overnight and the crude product (3.21 grams, 100% yield) was collected. Recrystallization from ethanol gave 2.48 grams (79% yield) of white needles: m p 119-122 "C; lit. 115-1163 C ' (18), 119°C (19). 2,i,&Triphenylpyryliurn Chloride (TPC). In a typical preparation pseudobase (3.26 grams, 10 mmol) was dissolved in 40 ml of boiling 95% ethanol. After dissolution was complete, 1.6 ml of 37% hydrochloric acid were added with vigorous stirring and the ethanol was evaporated to give the crude chloride in quantitative yield. The yellow solid was recrystallized from 30 mi of water to which 11 ml of 37% hydrochloric acid had been added. Drying to constant weight at 110 "C yielded 3.28 grams (96%) of pure 2,4,6triphenylpyrylium chloride: m p 217-220 "C; lit. 217-219 "C ( 2 0 ) . (16) G . K. Heimkamp and H. W. Johnson, "Selected Experiments in Organic Chemistry," W. H. Freeman, San Francisco, Calif., 1964, p 81. (17) A. T. Balaban, C. R. Acad. Sci., Ser. C., 256, 4239 (1963). (18) A . Williams, J . Amer. Chem. SOC.,93, 2733 (1971). (19) J . A. Berson, J. Arner. Chem. SOC.,74, 358 (1952). (20) K . Kanai, M . Umehara, H . Kitane. and K . F u k u i , Nippon Kagaku Zasshi. 84, 432 (1963);Chem. Abstr., 59, 139349 (1963). A N A L Y T I C A L C H E M I S T R Y , VOL. 45, NO. 6, M A Y 1973
985
