2320
Anal. Chem. 1983, 55, 2320-2323
I t is apparent that PLC be used not only to obtain highly selective separations but also to obtain information about the solutes being separated. Further studies on the use, mechanism, and theory of PLC are in progress.
LITERATURE CITED Armstrong, D. W.; Terrill, R. 0. Anal. Chem. 1979, 57, 2160-2163. Armstrong, D. W.; Henry, S. J. J. Liq. Chromatogr. 1980, 3 , 657-662. Armstrong, D. W.; Nome, F. Anal. Chem. 1981, 53, 1662-1666. Hlnze, W. L.; Armstrong, D. W. Anal. Lett. 1980, 73, 1093-1104. Burkert, W. G.; Owensby, C. N.; Hlnze, W. L. J. Lig. Chromafogr. 1981. .- - ., 4. ., 1065-1085 . - - - . - - -. Hinze, W. L. Sep. Purif. Methods 1981, 70, 159-237. Armstrong, D. W.; Stine, G. Y. J . Am. Chem. Soc. 198% 105, 2962-2964. Yarmchuk, P.; Weinberger, R.; Hlrsch, R. F.; Cline Love, L. J. Anal. Chem. 1982, 5 4 , 2233-2238.
(9) Armstrong, D. W.; Hinze, W. L.; Bui, K. H.; Singh, H. N. Anal. Left. 1981, 74, 1659-1867. Weinberger, R.; Yarmchuk, P.; Cline Love, L. J. Anal. Chem. 1982, 54, 9, 1552-1558. (11) Dorsey, J. G.; De Echegaray, M. T.; Landy, J. S. Anal. Chem. 1983, 55 924-928. (12) Hinze, W. L. “Solution Chemistry of Surfactants”; Mittal, K. L., Ed.; Plenum Press: New York, 1979; Voi. 1. (13) Slngh, H.; Hlnze, W. L. Anal. Left. 1982, 75, 221-243. (14) Singh, H.; Hinze, W. L. Analyst (London) 1982, 707, 1073-1080. (15) Cline Love, L. J.; Skrilec, M.;Habarta, J. G. Anal. Chem. 1980, 52, 754-759. (16) Callahan, J. H.; Cook, K. D. Anal. Chem. 1982, 5 4 , 59-62. Armstrong, D. W. unpublished results, Georgetown University, 1983. (17)
RECEIVED for review May 27,1983. Accepted August 22,1983. The authors thank the NSF (CHE-8119055)for support Of this work.
Solvent Extraction of Beryllium from Malonate Solutions with Liquid Anion Exchangers R. Raghunadha Rao and S. M. Khopkar* Department of Chemistry, Indian Institute of Technology, Bombay 400 076, India
Berylllum was quantltatlvely extracted at pH 5.5-7.0 In mlcrogram amounts wlth 0.06 M Allquat 336s In xylene from 5 X M malonic acld solution, stripped with 0.5 M hydrochlorlc add, and determlned spectrophotometrlcally at 523 nm as its complex with lhorln. Those metals whlch could not form anlonlc complexes wlth malonlc acld and were not extracted wlth berylllum at pH 6.5 were separated from It. Metals formlng weak malonato complexes were scrubbed from the organlc phase with water. The elements llke blsmuth, antimony, Iron, uranlum, gallium, and vanadium whlch form strong malonato complexes were separated by selective strlpplng with hydrochloric, sulfuric, or nitric acld. The method was extended for the analysls of berylllum In beryl and beryllium alloys.
Beryllium has been extracted with fl-diketones ( I ) with no special advantages, and the extraction with diethyl ether or tributyl phosphate (2) from thiocyanate media was not selective. Throughout the extraction studies, the use of liquid anion exchangers from the organic acids is extremely limited. Trioctylamine (3),Aliquat 336 (4),and dipentylamine in amyl acetate (5)were used for beryllium extraction from thiocyanate media at pH 3.0. Beryllium was extracted from sulfuric acid solutions with tributylamine (6) and substituted octylamine (7), and it was extracted with Aliquat 336 (8) or triisooctylamine (9) from oxalate media. Unfortunately all these extractions were not quantitative. The usual problems encountered in these extractions were the hydrolytic precipitation of beryllium, need of longer equilibration periods for quantitative extraction, and the necessity of using sequestering agents for the elimination of interference of associated elements. Systematic work on the solvent extraction of beryllium with liquid anion exchangers is lacking. Therefore, this paper presents such a study and a selective method for the solvent
extraction separation of beryllium with Aliquat 336s in xylene from malonic acid solution. Methods are suggested for the separation of beryllium from associated elements. The method has been extended for the analysis of beryllium from minerals and alloys.
EXPERIMENTAL SECTION Apparatus and Reagents. A digital pH meter, type 822 (ECIL, India), with glass and calomel electrodes, ECIL Spectrophotometer GS 866 C (India),with matched 10-mmCorex glass cells, wrist action flask shaker (Toshniwal and Co., India), and Remi centrifuge with speed of 6000 rpm were used. About 11.4 g of beryllium nitrate tetrahydrate (E. Merck, Germany) was dissolved in 250 mL of distilled water containing 1% nitric acid. The solution was standardized gravimetrically (IO). It contained 2 mg/mL of beryllium. The diluted solution containing 10 Ng/mL of beryllium was prepared by appropriate di1ution. Thorin (4-[ (2-arsonophenyl)azo]-3-hydroxy-2,7-naphthalenedisulfonic acid, disodium salt) (Loba Chemie, Wien) was used as a 0.1% aqueous solution. Amberlite LA-1 [N-dodecyl(trialkylmethyl)amine] , Amberlite LA-2 [N-lauryl(trialkylmethyl)amine],Primene JM-T [mixture of primary amines in the CI8-Cz2range] (Rohm and Haas Co.), Aliquat 3368 [tricaprylmonomethylammoniumchloride] (General Mills Ltd.), TOA [trioctylamine], and TIOA [triisooctylamine] (Riedel-de Haen, Germany) were used without further purification. The exchangers were converted into the malonate form as per the procedure described earlier (11). Buffer solution (pH 10.4)was prepared by mixing 236 mL of 0.1 M sodium hydroxide and 264 mL of 0.05 M sodium borate solutions in a total volume of 500 mL. General Procedure. To an aliquot of solution containing 10 bg of beryllium was added 5 mL of 0.01 M malonic acid. The pH of the resulting solutionwas adjusted to 6.5 with dilute sodium hydroxide or malonic acid by use of a pH meter. The total volume of solution was made up to 10 mL. Then the solution was transferred to a separatory funnel. It was equilibrated with 10 mL of 0.06 M Aliquat 3368 in xylene for 5 min on the wrist action flask shaker. The two layers were allowed to settle and separate. Beryllium was stripped from the organic phase with 10 mL of 0.5
0003-2700/83/0355-2320$01.50/0@ 1983 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 14, DECEMBER 1983 2321
Table I. Effect of Aliquat 3365 Concentrationa
a
Aliquat 3369 ( 1 x lo-*M )
extraction,
0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6 4.0 4.4 4.8 5.2 6.0
45.2 74.4 86.9 92.3 94.6 96.4 97.3 97.9 98.4 98.8 99.4 100.0 100.0 100.0
%E
distribution ratio, D 0.43 2.91 6.63 11.99 17.52 26.78 36.04 46.62 61.50 82.33 165.67 m 0
m
Be = 1 0 p g ; pH 6.5; [malonic acid] = 5
X
lo-’ M.
Table 11. Effect of Malonic Acid Concentrationa [malonic acid], M concn 1.0 x 10-4 2.0 x 10-4 3.0 x 10-4 4.0 x 10-4 5.0 x 10-4 6.0 x 10-4 8.0 x 10-4 10.0 x 10-4 12.0 x 10-4 1.2 x 10-3-1 x 10-2 M 0.02 0.04 0.08 0.10 0.30 0.60
Flgure 1. Extraction of beryllium from malonic acid with various 4 % solutions of llquld anion exchangers in xylene: ( 0 )Amberlie LA-1; (01 Amberlite LA-2; (A)Primene JM-T; ( 0 )Aliquat 3368; (A)TOA; (0)
TIOA.
M hydrochloric acid. Then the pH of the aqueous phase was adjusted to 10.4, 2 mL of 0.1% thorin solution was added, and the volume was made up to 25 mL. The solution was kept for 10 min to develop color and the absorbance of the yellow colored complex was measured at 523 nm against the reagent blank. The concentration of beryllium was computed from the cablibration curve (12). RESULTS AND DISCUSSION Effect of pH a n d Liquid Anion Exchanger Concentration. The optimum pH for the quantitative extraction of beryllium was ascertained by extracting it with 4% liquid exchangers with xylene as diluent over a wide pH range from 1.0 to 9.0. The phase-volume ratio was kept as 1:l. The results indicated that the optimum pH for quantitative extraction was 5.5-7.0 with Aliquat 3368. The extraction was incomplete with Amberlite LA-1 and LA-2, Primene JM-T, and TOA or TIOA in all pH regions. The extraction was 81% complete with Amberlite LA-2 at pH 4.0-5.5 and 69% complete with Primene JM-T at pH 2.0-3.0. The results are depicted in Figure 1,which shows that only Aliquat 3368 can quantitatively extract beryllium. The concentrationof Aliquat 3368 required for quantitative extraction of beryllium was studied at pH 6.5 with the malonic acid concentration maintained constant a t 5 X M. The extractions were made with varying Aliquat 3368 concentration from 0.004 to 0.06 M in xylene (Table I). The extraction commenced at 0.004 M and was quantitative at 0.05 M Aliquat 3368. Therefore, an Aliquat 3368 concentration of 6 X lo-, M was selected as optimum for the quantitative extraction of beryllium. Effect of Malonic Acid Copcentration. The optimum concentration of malonic acid for the complexation of beryllium was ascertained by extracting at pH 5.5-7.0 with 0.06 M Aliquat 3369 in xylene with varying concentrations of malonic acid (Table 11). The extraction commenced with 1 x lo4 M malonic acid, but it was quantitative with malonic acid concentration of 1.2 X M. To ensure complete complexation, 5 X M malonic acid was used throughout this work. It is interesting to note a decreasing trend in extraction efficiency beyond 0.01 M malonic acid. This may
a
extraction, distribution %E ratio, D 2.24 5.45 8.90 13.09 17.52 22.81 32.33 54.56
69.1 84.5 89.9 92.9 94.6 95.8 97.0 98.2 100.0 100.0 95.2 87.5 83.3 77.4 50.0 35.7
Be =10 pg; pH 6.5; [Aliquat 33691 = 6
m m
19.83 7.00 4.99 3.43 1.00 0.56 X
lo-*M.
be attributed to the setting up of a competitive equilibria between the anionic malonato complex of beryllium and the high concentration of free malonate ions and the liquid anion exchanger. An attempt was made to ascertain the composition of the extracted species by extracting beryllium at a fixed pH and Aliquat 3368 concentration (6 X lo-, M) with varying concentration of malonic acid and also at constant pH and malonic acid concentration (5 X M) with varying concentration of Aliquat 3368 with xylene as the diluent. The plots of log D vs. log [malonic acid] and log D vs. log [Aliquat 33681 showed slopes of 1.2 and 1.98, respectively. Since the extraction of malonato complex of beryllium with Aliquat 3368 showed increasing trend with increasing pH with maxima at pH 5.5-7.0, the existence of hydroxy species of the anionic complex in alkaline medium is generally expected. Therefore, the probable extracted species is (R,N),[ (OH),Be(malonate)]. The findings are consistent with those for oxalate species (9). A similar phenomena was observed by Sastry et al. (8). The mechanism of the extraction can be written as (R4N+),(malonate) + [(OH)2Be(malonate)]2(R4N) [ (OH)2Be(malonate)] + (malonate)2Effect of Various Diluents. Various inert diluents such as benzene, toluene, chloroform,carbon tetrachloride, xylene, hexane, cyclohexane, kerosene, and isobutyl methyl ketone were used for the extraction of beryllium with Aliquat 3368 (Table 111). The phase-volume ratio was kept at unity to eliminate the possible problem of emulsification. The studies revealed that hexane, benzene, xylene, and toluene were
-
2322
ANALYTICAL CHEMISTRY, VOL. 55, NO. 14, DECEMBER 1983
Table 111. Effect of Various Diluentsa dielectric constant,
extraction, 7% E
distribution ratio, D
1.89 2.05 2.24
100.0 97.7 88.8
42.48 7.93
2.28 2.30 2.38 4.80 13.10
100.0
m
100.0 100.0 85.3 94.1
m
5.80 15.95
2.13
96.5
27.57
diluent
E
hexane cyclohexane carbon tetrachloride benzene xylene toluene chloroform isobutyl methyl keton e kerosene
a Be = 10 pg; pH 6.5; [Aliquat 33651 = 6 x [malonic acid] = 5 x M.
m
m
M;
suitable diluents. The extraction was incomplete with cyclohexane, kerosene, and isobutyl methyl ketone whereas carbon tetrachloride and chloroform proved to be even less effective. Therefore, in all these investigations xylene was preferred as the diluent on account of its minimum health hazards and rapid settling of the phases.
Selectioli of Stripping Agents and Period of Equilibration. After extraction of beryllium from malonate solutions into the organic phase, it was stripped with 10 mL of various stripping agents at different concentrations (Table IV). It was noted that the stripping was quantitative with 0.1-8 M hydrochloric or nitric acid or 0.05-2 M sulfuric acid. The stripping was incomplete at higher concentrations of sulfuric acid because of reextraction of the negatively charged sulfato complex of the metal (6, 7). Alkalies at lower con-
centrations were found to be poor stripping agents; at higher concentrations, they formed the oxy-anion of beryllium which was extracted into the liquid exchanger inhibiting quantitative stripping of beryllium. In order to ascertain the optimum period of extraction, the equilibrations were performed for various periods of time, 1, 2 , 3 , 4 , and 5 min. The corresponding percentage extraction was 84.6,96,98, 100, and 100, respectively. This showed the optimum period of equilibration as 5 min. The existing methods needed a much higher period of shaking for quantitative extraction (2). Separation from Binary Mixtures. Beryllium was extracted in the presence of various ions in binary mixtures (Table V). The tolerance limit was set as the amount of foreign ion required to cause a &2% error in the recovery of beryllium. Alkali and alkaline earths, thallium(I), iron(II), silver(I), arsenic(III), and yttrium(II1) were not extracted with beryllium because they could not form malonato complexes. Zinc(II), cadmium(II), nickel(II), cobalt(II), manganese(II), lanthanoids, aluminum(III), and chromidm(II1) form weak malonato complexes. Hence, after their extraction along with beryllium they were separated by stripping with water and then beryllium was stripped with 0.5 M hydrochloric acid. Bismuth(III), antimony(III), mercury(II), and thallium(II1) form strong malonato complexes. After their extraction, beryllium was first stripped with 1 M hydrochloric acid. Under these conditions the chloro complexes of the other metals were reextracted into the organic phase. They were then stripped with 0.5 M sodium hydroxide. Beryllium was separated from iron(III), uranium(VI), gallium(III), and VBnadium(V) after their extraction from malonate solutions by first stripping beryllium with 6 M hydrochloric acid followed by stripping of these ions with 0.5 M hydrochloric acid.
Table IV. Effect of Stripping Agentsa % stripping
stripping agent HCl H2S04
HNO, Na,C03 ",OH NaOH a
0.05 M
0.1 M
0.5 M
1.0 M
2.0 M
4.0 M
6.0 M
8.0 M
86.1 100 92.4 4.4 0 68.3
100 100 100 5.1 0 79.7
100 100 100 22.8 3.8 80.4
100 100 100 26.6 15.2 81.0
100 100 100 35.4
100 98.8 100
100 98.6 100
100
Be = 10 pg; Aliquat 3365 concentration in xylene = 6 X
88.8 100
M.
Table V. Effect of Diverse Ionsa foreign ion Li+ Na+
K' Rb' cs+ Mg 2+ Sr
Ca 2 + Ba2+ T1' Fez+ Ag+ As3' Y3+ Sn4+ La3+ Sm3' Nd3+ ZnZ+ CdZ+ a
added as Li,SO,.H,O NaCl KC1 RbCl CSCl MgSO,. 7H,O Sr(N0,),.2H,O CaC1;6H,O Ba(N0,),:4H20 Tl,SO,
FeSO,. 7H,O &NO, AsC1, Y(N03),.2H,0 SnC1,
Be = 1 0 p g ; pH 6.5; [Aliquat 33651 = 6 X
tolerance limit, mg 5.0 4.0 4.8 3.4 3.8 2.1 2.2 2.4 1.8
3.8 4.0
0.8 1.0
2.2 1.0 1.6 0.5 0.6 1.4 1.2 M.
foreign ion Nil+ co2+ Mn2+ ~
1
+
3
Cr 3 + Hg 2 + Bi3+ ~ 1 3 +
Sb3+ Fe3' U6+ Ga3' VS+
zi
+
Hf4+ Mo6+ 1n3+ Th4+ Ce3+
added as Ni(NO ,),.6H ,O Co(N03),*6H,0 MnS0,.7H20 Al( NO ,),.9H20 WSO4)3 HgC4 Bi(NO ,),* 5H ,O TlC1, SbCl; 3H,O
Zr(k0 j,.5H20 Hf(S04)2 (NH4)6M070Z4'4H20
In,(S04),.5H,0 Th(NO3),*4HZO Ce2(S0,),+3H,O
tolerance limit, mg 1.0
0.8 0.8 1.2 1.6 1.7 0.8 1.2 0.5 1.6 1.2 1.0 0.8 0.8 1.2 1.8
0.5 1.2
0.5
ANALYTICAL CHEMISTRY, VOL. 55, NO. 14, DECEMBER 1983
Table VI. Analysis of Beryllium in Tertiary Mixturesa taken, found, recovery, no. mixture
pg
Pg
%
eluent
1000 10 800 1400 10 600 1000 10 1200
992.4 10.0 796.2 1397.8 10.0 598.4 996.7 10.1 1196.8
99.2 100.0 99.5 99.8 100.0 99.7 99.7 101.0 99.7
4
Cr(II1) 1240 Be(I1) 10 Mo(V1) 1000
1234.6 10.0 1001.5
99.6 100.0 100.2
5
Co(I1) Be(J1) Zr(1V) Zn(I1) Be(I1) Hf(1V) Al(II1) Be(I1) Th(1V)
800.0 9.9 598.5 1198.6 10.0 800.0 996.5 9.9 1194.8
100.0 99.0 99.8 99.9 100.0 100.0 99.7 99.0 99.5
water 6 M HCl 0.1M HCl water 6MHC1 0.2M HCl water 0 . 2 M H,SO, 0.5M ",OH water 0 . 2 M H,SO, 0.5M ",OH water 0.2M H,SO, 2MHCl water 0 . 2 M H,SO, 2 M HC1 water 8 M HNO, 0.5MHCl
1 2 3
6
7
a
Al(II1) Be(I1) Fe(II1) Zn(I1) Be(I1) V(V) Ni(I1) Be(I1) Mo(V1)
800 10 600 1200 10 800 1000 10 1200
Aliquat 3368 in xylene = 6
X
M ; pH 6.5.
Beryllium was separated from zirconium, hafnium, and molybdenum in sulfate media. After extraction of these metals as malonato complexes, beryllium was stripped with 0.5 M sulfuric acid while the other metals were reextracted as anionic sdfato complexes. These were stripped with 2 M hydrochloric acid. The separation of beryllium from indium, thorium, and cerium(II1)was accomplished by extracting them as malonato complexes and stripping beryllium first with 8 M nitric acid, followed by stripping the other metals with 0.5 M hydrochloric acid. Separation of Beryllium from Tertiary Mixtures. The separation of beryllium from multicomponent mixtures is shown in Table VI. A tertiary mixture consisting of either aluminum, beryllium, and iron(II1) or zinc, beryllium, and vanadium(V) was extracted with 6 X loe2M Aliquat 3365 in xylene from malonate solution. Aluminum or zinc, which forms weak malonato complexes, was stripped first with water, then beryllium was stripped with 6 M hydrochloric acid, and finally iron(II1) or vanadium(V) was stripped with 0.1 M hydrochloric acid. The separation of nickel(II), beryllium, and molybdenum(VI) or chromium(III), beryllium, and molybdenum(V1)was accomplished after their extraction from malonate media by stripping nickel or chromium with water, followed by stripping beryllium with 0.2 M sulfuric acid, and finally molybdenum with 0.5 M ammonium hydroxide solution. The separation of cobalt(II), beryllium, and zirconium(1V) or zinc, beryllium, and hafnium(1V) was effected by extracting them together as malonato complexes and stripping first cobalt or zinc with water, then beryllium with 0.5 M sulfuric acid, and finally zirconium or hafnium with 2 M hydrochloric acid. Aluminum, beryllium, and thorium(IV) were separated after their extraction from malonato media by stripping aluminum with water, beryllium with 8 M nitric acid, and then thorium with 0.5 M hydrochloric acid.
2323
Application to Analysis of Alloys and Minerals. A 0.10-g sample of an alloy was dissolved in 5 mL of hydrochloric acid. The precipitated tungstic acid was removed by filtration and the solution was made up to 100 mL. An aliquot (1.0 mL) of the diluted solution was treated with malonic acid and extracted with 0.06 M Aliquat 3368 as described under the general procedure. The organic phase was washed with water to remove chromium(III),nickel(II), cobalt(II), manganese(II), zinc(II), magnesium(II), and aluminum(II1). Beryllium was stripped from the organic phase with 6 M hydrochloric acid but the anionic chloro complex of iron remained in the exchanger. In triplicate determinations,the amount of beryllium was found to be 0.80, 0.78, and 0.81 % against the reported value of 0.79%. For the analysis of beryllium in beryl, 0.50 g of finely powdered beryl sample was fused with a mixture of sodium hydroxide and sodium peroxide. The cooled melt was loosened with water. The precipitated hydroxides of beryllium, magnesium, and iron were digested on a steam bath and later filtered. The precipitated mass was dissolved in dilute hydrochloric acid and diluted to 250 mL with distilled water. A portion of the sample containing beryllium, magnesium, and iron was treated with malonic acid and extracted with 0.06 M Aliquat 3365 in xylene at pH 6.5. The organic phase was washed with water and then beryllium was stripped with 6 M hydrochloric acid. Later iron was stripped from the exchanger with 0.1 M hydrochloric acid. The amount of beryllium was found to be 13.46, 13.38, and 13.44% as its oxide against the reported value of 13.42% The important feature of this method is that it permits the separation of beryllium from chromium, nickel, cobalt, zinc, iron, aluminum, and magnesium which are generally associated with it in light alloys and beryl. Since beryllium is used as moderator and source for neutrons in atomic reactors, its separation from fission product elements like zirconium, molybdenum, antimony, thorium, and strontium has special significance in reactor chemistry. From ten replicate determinations the average recovery of beryllium was 99.8 f 2%. The method is simple, rapid, and selective. Registry No. Al, 7429-90-5;Fe, 7439-89-6;Zn, 7440-66-6;V, 7440-62-2; Ni, 7440-02-0; Mo, 7439-98-7; Cr, 7440-47-3; Co, 7440-48-4;Zr, 7440-67-7;Hf, 7440-58-6;Th, 7440-29-1;beryllium, 7440-41-7; malonic acid, 141-82-2; beryl, 1302-52-9.
.
LITERATURE CITED De, A. K.; Khopkar, S. M.; Chalmers, R. A. "Solvent Extraction of Metals"; Van Nostrand Reinhoid: London, 1970. Kalyanraman, S.; Khopkar, S. M. Anal. Chem. 1975, 47, 2041. Novoselova, A. V.; Tamm, N. S.; Pochkaeva, T. I.; Likhanskaya, N. V. Vestn. Mosk. Unlv., Ser. 2 , Khim. 1973, 55. Anal. Abstr. 1973, 2 5 , 2973. El-Yaminl. I. S.; Abd El-Messlch, E. N. Talanta 1978, 2 5 , 704. Novoselova, A. V.; Pochkaeva, T. I.; Tamm, N. S.; Trubacheva, 0. A. Vestn. Mosk. Univ., Ser. 2 , Khim. 1989, 44. Anal. Abstr. 1970, 19, 76. El-Yaminl, I. S.; Farah, M. Y.; Abd El-Messich, E. N. J. Radioanal. Chem. 1978, 4 5 , 365. Elissabeth, E. S. Commls. Energ. At., [Rapp.] CEA-CONF (Fr.) 1988, CEA-3551, 118. Sastrl, M. N.; Prasad, G. V. Indian J. Chem. 1974, 12, 1318. DeBruin, H. J.; Kairaltls, D.; Temple, R. B. Aust. J. Chem. 1962, 15, 457. Vogel, A. I. "The Text-book of Quantitatlve Inorganic Analysis"; Longmans Green: London, 1962; p 518. Rao, R. R.; Khopkar, S. M. Analyst (London) 1983, 108, 346. Elnaga, Hlsahlko; Ishll, HaJlmo Anal. Chlm. Acta 1971, 8 4 , 113.
RECEIVED for review February 14,1983. Accepted August 1, 1983.
