ing that the absorbance increase via reaction 5 is greater than the absorbance decrease cia reaction 9. Effects of Diverse Ions. -4s discussed above, t h e effects of diverse ions are dependent on the cerium (IIT)-.ZC ratio. T h e effects of common anions were investigated by analyzing known levels of fluoride in t h e presence of t h e various anions added as the sodium salt. T h e results of this study are shown in Table 111. With a 1:1 Ce(II1)-AC reagent, high levels of bromide, chloride, nitrate, nitrite, and perchlorate and small amounts of borate and silicate arc without effect. Interference due to phosphate, sulfate, and sulfite are minimized by the use of a 1:2.5Ck(II1).4C rragent. The effects of various cations iiornially found in ground \v.ater are also s1ion.n in Table 111. With a 1 : 1 Ce (III)--iC reagent, harium(IT), calcium
(II), and magnesium(I1) have no effect. Iron(II1) at a n equimolar ratio to fluoride has no effect, but at this level aluminum interferes seriously. The tolerance for these cations is less when the 1: 2.5 reagent is used. hIetals such as zirconium and beryllium which form stable fluoride complexes will interfere also. However, metal ions are separated easily by pyrolysis (9) or pyrohydrolysis (10).
Analysis of Samples. The proposed method has been used t o determine fluoride in samples of natural ground water from the Idaho S a t i o n a l Reactor Testing Station area. Results of these analyses are shown in Table IT’. I n all cases recovery of added fluoride has been quantitative indicating little or no effect from the diverse ions which are present in ground water. Results of t h e analysis of fluoride samples following pyrolysis are also in Table IT’.
LITERATURE CITED
(1) Belcher, R., Leonard, M . A., West, T. S., J . Chem. SOC.1959, 3577.
(2) Belcher, R., Leonard, M. A., West, T. S., Talanta 2, 92 (1959). (3) Ibid., 8, 853 (1961). ( 4 ) Ibid., p. 863. (5) Greenhalgh, R., Riley, J. P., Anal. Chzm. Acta 2 5 , 179 (1961). ( 6 ) Jeffery, P. G., Williams, D., Analyst 86, 590 (1961). (7) Johnson, C. A., Leonard, M. A., Ibid., 86, 101 (1961). (8) Leonard. h l . A , . West. T. S..J . Chem. SOC.1960, 4477. (9) Powell, R. H., Menis, O., ANAL. CHEM.30, 1546 (1958). (10) Warf, J. C., Cline, W. D., Tevebaugh, R. D., Ibzd , 26, 342 (1954). ( I 1) Yamamura, S. S., Kussy, &I. C., Rem, J. E., Ibad., 33, 1655 (1961). RECEIVEDfor review March 22, 1962. Accepted July 6, 1962. Division of Analvtical Chemistry, 141st Meeting ACS; Kashington, D. C. March 1962. Under Contract AT( 10-1)-205 to the Idaho Operations Office, U. S. Atomic Energy Commission.
0,O’-DiaIkyI Phospho rodithio ic Aci ds as Extractants for Metals THOMAS H. HANDLEY Analytical Chemistry Division, Oak Ridge National labordory, Oak Ridge, Jenn. JOHN A. DEAN Department o f Chemistry, University o f Tennessee, Knoxville, Tenn.
b The solvent extractions of 4 2 elements as di-n-butyl phosphorodithioates into CCI, from 0.03 to 9N HCI and H2S04 media are presented as plots of log distribution ratio vs. log normality in two periodic charts. Significant distributions are shown by 22 elements, generally those which react with hydrogen sulfide or diethyldithiocarbamate. Compared to the latter, phosphorodithioic acid, especially when dissolved in the organic phase, is significantly more resistant to decomposition when in contact with strong mineral acid solutions. The effect of chain length and spatial arrangement of the carbon atoms in the alkyl group, of 1 1 organic solvent systems, and of certain masking agents and diverse salt concentrations on the distribution ratio was investigated. The relative order of extraction (displacement series) of 12 metal complexes has been established.
0
RGANOPHOSPHORUS
COMPOUNDS,
both neutral and acid, provide a versatile range of solvent extraction reagents. Systematically, in published reports (11, 12) and investigations in 1312
ANALYTICAL CHEMISTRY
progress, Ive are surveying the solvent extraction characteristics of this group of reagents wherein one or more oxygen atoms have been replaced by other members of periodic group VI-A, in particular sulfur as represented by: tri-n-alkyl phosphine sulfide, RIPS (13) ; O,O,O-trialkyl thionophosphate, (RO)IPS (11); 0,O’-dialkyl phosphorodithioic acid, (RO)*P(S)SH; diary1 phosphinodithioic acid, (Ar)?P(S)SH (13); and 0,O’-dialkyl thionophosphoric acid, (R0)2P(S)OH,or the tautomer 0,O’-dialkyl phosphorothioic acid, (RO)zP(0)SH. The purpose of this paper is t o outline the solvent extraction characteristics of the elements as phosphorodithioates in acid media. Subsequent papers will discuss the nature of the complex formation and characteristics of the reagent. The extraction of certain metal phosphorodithioates had been reported by Busev and coworkers (4-10) and, when this investigation was being prepared for publication, an extensive study by Bode and Arnswald (3) appeared. However, thi,s investigation extends the range of acid concentrations in HCl and HzS04media over that of other Ivorkers,
i t includes several additional elements, and it presents the effect of chain length and spatial arrangement of the carbon atoms in the alkyl group, the effect of different organic solvent systems, and the relative order of extraction of a number of elements. In many respects the dialkyl phosphorodithioic acids resemble sodium diethyldithiocarbamate in extraction characteristics, but with two notable differences. Extractions with the former reagent take place from solutions Jvhich are st,rongly acid, 0.03 to 6 N , which often expedites handling of samples. Second, phosphorodithioio acids are distinctly inore resistant. against rapid decomposition in mineral acid solutions ( 1 . 2, 16). I n 10N HC1 the half-life of the diethyl ester is 4.8 hours (Z), a n d the half-life would be longer when the reagent is dissolved in an organic phase whereby contact with a n aqueous acid phase would be limited t o the period of phase equilibration. dniounts of metal which range from submicrogram t o milligram quantities are extracted quantitatively in each niilliliter of 0.2111 extractant and usually in one equilibration of only a few minut’es duration.
REAGENTS
PhosphorodithioicAcids. Di-n-butyl
EXTRACTION STUDIES OF THE E L E M E N T S NO E.!
ORGANIC PHASE
5 mi CCI.,
0 207M in I&P&),P(S)SH
NO
Eit
AQUEOUS PHASE 5 m l H C i M Shown phosphorodithioic acid, ( ~ Z - C ~ H ~ O ) ~ P(S)SH, mol. u-t. 242, was obtained from Victor Chemical Co., Chicago, Ill. No Ex! As received, the reagent was shown to be 99.6% pure by electrometric titration. Other alkyl esters, commercial grade, were obtained from Lubrizol Corp., Cleveland, Ohio; Shell Oil Co., Wood River, Ill.; and American Cyanamid Co., New York, N. Y. The ammonium salt of the diethyl ester was obtained from the Monsanto Chemical Co., St. Louis, Mo. If necessary, the acids can be synthesized readily by the Figure 1 . Extraction of elements as di-n-butyl phosphorodithioatecomplexes from straightforward reaction of alcohols hydrochloric acid with phosphorus pentasulfide (15,17,19). Short-chain (through butyl) phosphorodithioic acids can be purified by ter for beta emitters when radio-tracers +0.47 volt us. N.H.E. (14). Consevacuum distillation. The esters can were used, or by flame-spectrometric quently, very strong oxidizing agents also be purified by conversion to their should be avoided. Nevertheless, exmethods. Results were rejected if the alkali or ammonium salts, followed by tractions have been conducted successextraction of impurities with benzene sum of the metal concentration in each or CCl, and reformation of the acid fully from an aqueous phase which conphase failed to equal the initial quantity, with addition of mineral acid to the tained 6M or less nitric acid when the within experimental errors considered aqueous layer. The free acid may be to be 2 to 3%. phases were separated immediately after extracted with CCll and the CCll A systematic study was made of the the 10-minute equilibration period. removed by warming under reduced The oxidation product, (di-n-C4HgO)r extraction of 42 elements, for which the pressure. The ammonium salts may P(S)SS(S)P(di - n - C4H90)2,except for distribution data are presented in Figure be recrystallized from ethyl acetate. lowering available reagent concentra1 for HC1 media and in Figure 2 for Standard solutions of the reagent in tion, does not interfere with extractions, H & 0 4 media. Acid concentration varCClg are prepared by appropriate although its strong absorption bands in ied from 0.03 to 9N. All data are for dilution and are standardized either by an alkalimetric two-phase titration or the ultraviolet and near-visible regions one batch contact with an initial conby an iodometric titration in 2 N sulof the spectrum could interfere with centration of di-n-butyl phosphorofuric acid solution using an extractive spectrometric methods. dithioic acid in CCla of 0.207M. The two-phase method. Results by iodoexcess of reagent remaining after the Solutions of copper(I1) salts are parmetric methods are about 3y0 lower tially reduced; however, because copper formation of the metal complex was than those by alkalimetric titration never less than six times the initial (I)is also extracted readily, thereduction (9). metal concentration. is immaterial. Gold(II1) is reduced to Because free phosphorodithioic acids gold(1) but not to metallic gold as has These metals are not extracted are somewhat sensitive to hydrolysis, solutions should be prepared fresh each been reported with the diethyl ester (3). ( 1.5 here. However, mercury(II),
RESULTS
M e t a l Distribution Ratios. Equilibrations were carried out by stirring or shaking equal volumes (5 ml.) of the organic and aqueous phases for 10 minutes a t room temperature in glass centrifuge tubes or separatory funnels of conventional design. Preliminary experiments indicated t h a t the distribution equilibrium was usually achieved within 10 minutes. After separation of the phases, which is hastened by centrifugation for 2 minutes a t 2000 r.p.m., an aliquot of each phase was analyzed for the particular metal concentration by counting in a gamma scintillation counter or by a G-RI coun-
EXTRACTION STUDIES OF T H E E L E M E N T S ORGANIC PHASE AQUEOUS PHASE
}
5 ml C C I 4 , 0 ? 0 7 M n l C & O ) Z P I S ) S H 5 m l H 2 S q N shown
Figure 2. Extraction of elements as di-n-butyl phosphorodithioate complexes from sulfuric acid VOL. 34, NO. 10, SEPTEMBER 1962
0
1313
palladium(II), and silver are extracted completely fromalkalinesolutions (pH = 10). Copper(I1) and zinc extractions are quantitative u p t o p H 8 in ammoniacal medium, although above p H 8 competing ammine formation lowers the distribution ratio. Effect of Alkyl Group. I n their survey of phosphorodithioic acid, Bode and Arnswald (3) used the diethyl ester only. Increasing the chain t o four carbons, as in the di-n-butyl ester, extends the number of elements which can be extracted t o include nickel, thallium (111), and zinc (completely); and cobalt(I1) and chromium(V1) (partially extracted). Also gold(II1) can be completely extracted without concurrent reduction t o metallic gold. A further example of the effect of increasing the length of the hydrocarbon chain is afforded by cesium whose distribution ratio is zero with the di-n-butyl ester but rises to 0.5 at p H 8 t o 9 with the di-(Zethyl) hexyl ester. hlercury(I1) is also extracted better a t lower acidities with thc di-(a-ethyl) hexyl ester, although the distribution ratio never approached as large a value as with the di-n-butyl ester at higher acid concentrations. D a t a are shoxn in Table I.
Table I. Effect of Alkyl Group on Distribution Ratio of Mercury
[Aqueous phas! was 0.002M and 0.0001M, respectively, in mercury( 11); organic phase contained 0.20731 reagent in CClr] Distribution Ratio with (RO)*P(S)SH HC1 DiConcn., (2-ethyl)'I4 n-Butyla hexyl* 6.5 143 0.01 4.3 21 0.05 5.5 46 0.10
0.20 0.30
4.8
50
>18,000 2,500 > 18.000 654 0.40 >18,000 1,380 0.50 > 18,000 460 a Total acid included 0.14Af "Os. * Total acid included 0.10111 "03.
t
-
40OlJ V
z
LL
d i - ( 2 - e t h y l ) hexyi
80-
c-c,
0
z 0 60-
t-
a
a
40-
w
+ 2
y
-
c-c-
-
V
-
20-
a w a
0
I
,
I
,
,
/
The effect of spatial arrangement of the carbon atoms in the alkyl group was investigated for zinc in HC'l medium; the results are shown in Figure 3. Extraction decreases with increased branching of the alkyl chain near the phosphorodithioate group-Le. , chain branching tends to require loner acid concentrations for complete extraction. Branching on an alpha carbon of the ester group requires a lon-er acid concentration for coniplete extraction than does branching on a beta carbon. Lengthening the hydrocarbon chain does not seem to affect the position of the extraction curve with respect t o HC1 concentration. Effect of Organic Solvent. The distribution of zinc a n d mercury between a n aqueous phase, of known mineral acid concentration, and differe n t organic solvents, each containing 0.207.V di-n-butyl phosphorodithioic acid, is given in Table 11. For zinc, n-heptane offers a small improvement in the distribution ratio, as compared with carbon tetrachloride. E t h y l acetate, n-amyl alcohol. and isopropyl ether offer marked improvement in t h e distribution ratio for mercury
Table It. Effect of Solvent on Distribution Ratio (Aqueous phase was 0.35M in HC1; organic phase contained 0 207111 di-n-butyl phosphorodithioic acid) Solvent Zinc Mercury Mercury" +Amyl acetate 5.3 n-Amyl alcohol 4.7 > 10,000 >10,000 Benzene 3.9 1,700 7.4 7 7.7 2,400 Carbon tetrachloride Chloroform 4.2 o-Dichlorobenzene 3.7 1,200 4.2 Ethyl acetate >10,000 3600 n-Heptane 10 4 6,000 ,71 iso-Propyl ether 7.9 3,800 900 Toluene 3.9 7,000 2.1 Xylene 4.0 > 10,000 5.1 Aqueous phase 0.1N HCI.
13 14
i
A L K Y L GROUP, ri
1
I
I
,
I
I
I
I
b
as compared with carbon tetrachloride. This is especially true at low aqueous acidity as shown by results ill the third column; however, a t higher acidity (column 2) this difference is not so great. Those solvents with the highest distribution ratio contain a n oxygen atom in their structure-i.e., acetate, alcohol, and ether. The possibility of interaction between the solvent and solute cannot be ignored: however, confirmation of this must await further study. Back-Extraction Studies. Kashing the organic phase with 0.1M sodium hydroxide removes unrencted reagent and back-extracts these metals: antimony(III), bismuth, cop1)er(II), gallium, iron(III), t i n ( I I ) , zinc, and, slowly, niolybdenum(T'1). -1 colored precipitate, presumably the metal sulfide, often forms in the alkaline system. Copper and zicc, d o n g with nickel and cadmium, can be stripped with aqueous ammonia. Incptction of Figures 1 and 2 will show which elements can he stripped n.ith concentrated acids. Contacting the organic phase with a solution of iodine, bromine. or hydrogen peroxide plus HCl quickly destroys the excess phosphorodithioic acid and the ligand in the metal complex and, consequently, strips the metal from the organic phase. Relative Extraction Constants. T h e relative order in which one metal will displace another from its phosphorodithioate complex \vas established as follows. .4 metal complex was formed and extracted by contacting 5 ml. of 0.207-V di-n-butyl phosphorodithioic acid in CCl, with a stoichiometric excess of the metal ion in a n equal volume of aqueous phase containing a n optimum concentration of acid (Figures 1 and 2). The phases were separated and the organic layer washed, without shaking, with several
ANALYTICALICHEMISTRY
portions of distilled water. Then the organic phase was shaken with an equal volume of an aqueous solution which contained the second metal ion and an appropriate acid concentration. The concentration of one or both metals in each phase wa5 determined by radiochemical means. I n some instances, a definite color change occurred Tvhich could be used to establish the displacement series. From these tests and qualitative observations, the relative order of extract i o n i s : P d f 2 > A u + ~ > C u + > H g +2 * Ag+ > Cut2 > Sbf3 > Bi+3 > Pb+? > Cdf2 > K i t ? > Zn+2. I n addition, qualitative observations reported by Busev and Ivanyutin (4-10) would place tin(II), iron(III), and platinum (IF') between copper(I1) and lead in the series. Effect of Diverse Salts. A thorough study of masking agents has not been made pending completion of studies to determine the formation constants of selected metal phosphorodithioates. These formation constants, the metal displacement series, plus back-estraction information and other qualitative information, will enable one to predict the influence of many masking agents. Some useful information has been accumulated by Bode and Arnswald
(8.
Miscellaneous observations indicate a large value of the over-all extraction constant for certain metal phosphorodithioates. Silver chloride and lead sulfate-even suspensions of bismuth sulfide and mercuric sulfide-disappear within minutes when shaken with the phosphorodithioic acid in CCI4, and the acid concentration in the aqueous phase is adjusted to optimum values for the particular metals. This was true for both freqhly precipitated and aged sulfides. The extraction a t acid concentrations less than about 0.2M is favored by the presence of 0.2 to 0.3V diverse salt concentration. Although the reason is unknown, in the absence of such elec-
trolyte concentration, considerable amounts of certain metal complexes (Bi, Cu, Cd, Hg, Pd, -4g) remain in the aqueous phase as a very fine suspension (or colloidal dispersion). This is independent of reagent concentration. The suspension consisted of stoichiometric combination of metal 1%ith reagent and could be separated by prolonged centrifugation (1 hour). Addition of an oxygen-containing solvent to the extraction system immediately eliminated the suspension. K o r k in H2S04 medium would permit addition of oxygen-containing solvents without concomitant extraction of metal-addition complexes that might extract from HC1 medium.
CONCLUSIONS
This preliminary survey of the distribution of elements as dialkyl phosphorodithioates into cc14 from HC1 and H2S04 media has demonstrated the general utility of this class of extractants. As compared to diethyldithiocarbamates, the reagent is much more resistant to decomposition and extractions may be conducted from relatively strong acid solutions. I n addition, a wide range of metal concentrations can be handled. The maximum amount of metal which can be extracted is set by the concentration of reagent present initially, the phase volume employed, and solubility of the complex in the organic solvent. Here, 0.5 to 1.0 mg. of metal per nil. was commonly employed. Even trace amounts of metal have been quantitatively extracted-as little as 1 x 10-iJI solutions of silver and bismuth. Variation of the distribution ratio with concentration of phosphorodithioic acid has been studied for a number of elements. but the results will be reported later. Here, it will suffice to say that only a small excess (approximately 2%) of reagent over the stoichiometric amount is necessary to achieve quantitative (or maximum) extraction
of bismuth and elements above it in the displacement series. For zinc. however, the distribution ratio varies directly as the square of the reagent concentration and inversely as the square of the hydrogen-ion concentration over a wide range.
LITERATURE CITED
(1) Bode, H., 2. Anal. Chem. 142, 414
(1954). (2) Bode, H., Arnswald, W., Ibid., 185, 99 .. 11962). (3) Ibid., p. 179. (4) Busev, A. I., Z . '4nalit. Khim. 4, 49, 234 (1940). (5) Busev, A. I., Borzenkova, K. P., Zacodsk. Lab. 27, 13 11961). (6) Busev, A. I., Ivanyutin, &I. I., Bull. Moscow Stale Unic. 12 (No. 5), 157 (1957); C. A . 52, 19676c (1958). (7) Ibid., 13 ( S o . 2), 177 (1958). (8) Busev, A. I., Ivanyutin, hI. I., Kauchn. Dokl. Vvsshei Shkolv, Khim. i Khim. Tekhnol. i958, , 73: , Anal. Abstr. 6 , 1268 (1959). (9) Busev. A. I.. Ivanvutin. hl. I.. Tr. \
~
,
Komis. ' Analit. Khih., Akad. ivauk S.S.S.R. Inst. Geokhim. i Analit. Khim. 11, 172 (1960); Anal. Abstr. 8, 4449
(1961).
(10) Busev, A . I., Ivanyutin, 31. I., Zauodsk. Lab. 13, 15 (1958). 111) Handlev. T. H.. Dean. J. A.. ANAL. ' C ~ E M3.2." 1878 Cl960i. ' (12, jbid.. 3'3, io87 (1961).
(13) Handley, T. H., Dean, J. A., Hitchcock, R. B-., unpublished studies. (14) Kakovskii, I. A . , Stepanov, B. A., Ryazantseva, 0. F., Serebryakova, S . Y.. Russian J . Phus. Chem. 33, 178 (1959). (15) Kosolapoff, G. "., "Organic Phosphorus Compounds, p. 236, J. Wiley and Sons, New York, 1950. (16) Martin, A. E., ANAL.CHEM.25, 1260 (1953) (17) hiastin, T. W., Korman, G. R., Keilmuenster, E. A,, J . Am. Chem. Soc. 67, 1662 (1945). (18) Monsanto Chemical Co., St. Louis, Mo., Tech. Data Sheet EBF-850 (1957). (19) Sanin, P., Sher, V. V., Doklady Akad. r a d . S.S.S.R. 107, 551 (1956); C. A . 50, 14514 (1957). RECEIVEDMarch 9, 1962. Accepted July 13, 1962.. Presented at Southwide Regional hleeting, ACS, Sew Orleans, December 1961.
VOL 34, NO. 10, SEPTEMBER 1962
1315
