dium(V). I n this case, the first vanadium wave was not present since the vanadium was already in the plus four oxidation state. Since the second vanadium wave a t - 1.1 volts did not appear when molybdenum was present, it appeared that vanadium(V) could be reduced to V(II1) a t -0.75 volt ( E l i zfor this wave was more negative than when vanadium was present). This may be due to a chemical reduction of vanadium by the Mo(II1) produced a t the electrode surface : Mo(V1) 2 Mo(II1)
3Mo(II1)
+ 3 V(V) -+. 2 Mo(V1) + 3 V(II1)
(1) (2)
If the second reaction were fast compared to the drop time, all of the vanadium(V) reaching the electrode surface could be reduced to vanadium (111) at the potential required for
molybdenum reduction. I n this case, the enhancement as given in Table I would be 100%. When the Mo/V ratio was high at p H 4 and 5 , the enhancement was approximately loo%, see Table I. However, the results were not sufficiently reproducible for analytical use. I n addition, when the Mo/V ratio was low, the enhancement was lower and some vanadium reduction at -1.2 volts could be observed. Boiling the solutions decreased the vanadium wave heights about 2075, but the enhancement effect was unchanged. The same effect was found with vanadium(IV), although the enhancement was somewhat less. This could indicate that reaction 2 was kinetically faster with vanadium(V) than with vanadium(1V). I n conclusion, the presence of molybdenum(V1) causes the second reduction wave of vanadium(V) to shift from - 1.1volts to -0.75 volt and merge with the molybdenum wave. This effect
may be caused by chemical reduction of vanadium to vanadium(II1) by molybdenum(II1). LITERATURE CITED
(1) Feltham, R. D., Martin, E. L., ANAL. CHEM.25, 1935 (1953). Abstract of
paper presented at Regional Conclave of ACS, December 1953, New Orleans, La. ( 2 ) Guibe, L., Souchay, P., J . Chim. Phys. 54, 684 (1957). (3) Pecsok, R . L., Juvet, R. S..Jr.. J . Am. Chem. SOC.75, 1202 (1953). (4) Pecsok, R. L., Sawyer, D. T., Zbid., 78, 5496 (1956). ( 5 ) Siniakova, S . I., Glinkina, XI. I., Zh. Anal. Khim. 13, 186 (1958).
JOHX H. K E N N E D Y ~ KARENJENSEN
Department of Chemistry University of California Santa Barbara, Calif. 1 Present address, General Motors Defense Research Laboratories, Santa Barbara, Calif.
Use of Metal Di-n-butyl Phosphorodithioates as Extractants for Metals SIR: The dialkyl phosphorodithioic acids, (RO)aP(S)SH, are reagents for the solvent extraction of metal ions from aqueous solutions of mineral acids (2, 4 ) . The metal dialkyl phosphorodithioates have greater chemical stability than the corresponding acids and, when used as extractants, improve selectivity ( 4 ) . However, the behavior of the dialkyl phosphorodithioates of certain metal ions-e.g., H g f Z , Ag+, and Pd+2-is more complicated than the simple replacement of one metal ion for another. At high loading, there is formed a complex species which may contain two different metal ions or a quantity of a single metal ion which is greater than that represented by a hydrogen atom-replacement reaction. Mercury(I1) will replace Ag+ from (C4HgO)zP(S)SAg; however, for certain experimental conditions the extracted species will contain some Ag+. Mercuric di-n-butyl phosphorodithioate will therefore extract Ag+ under certain conditions. Silver di-n-butyl phosphorodithioate prepared in a manner to have the stoichiometry shown by the formula (C4HgO)d'(S)SAg will also extract Ag+. Therefore, the use of the dialkyl phosphorodithioates of Hg+, Ag+, or P d f a as extractants may not always improve selectivity. Experimental evidence of the complicated behavior of the alkyl phosphorodithioates of certain metal ions was obtained in studies of the extraction of
recrystallized finally from ethyl alcohol. Small amounts of (C4H90),P(S)SAg"0 and (C4H90)2P(0)SAg110 were also made. The results of the chemical analyses of all these phosphorothioates and -diEXPERIMENTAL thioates agreed with the theoretical values calculated from the formulas Reagents. 1fERCURIC DI-72-BUTYL indicated. A standard solution of each PHOSPHORODITHIOATE [ (C4H00)2P(S)of the compounds was prepared by SIzHg, m.p. 61' to 62' C., was prepared dissolving a known amount of the comas follows. An aqueous phase that was pound in treated CC1,. 0.25M in HzS04 and that contained TREATED CARBONTETRACHLORIDE. H g f 2in known concentration was equiliReagent grade cc14 extracted small brated with an organic phase of namounts of Hg+2 and Ag+ from acid hexane that contained a quantity of diaqueous solutions; therefore, all the n-butyl phosphorodithioic acid, nCC14 used was treated as described (C4H90)zP(S)SH , stoichiometric accordpreviously (1). ing to a reaction in which the hydrogen RADIOACTIVE TRACERS. Radioactive is replaced. The volume of n-hexane was tracers used to determine concentrations reduced by evaporation, and [(CaHgO)z of metal ions were obtained from the P(S)S],Hg was crystallized out by coolIsotopes Division of the Oak Ridge ing the solution in an ice bath. The final National Laboratory. product was recrystallized four times from n-hexane, dried overnight in a Instruments and Apparatus. Welltype, integral gamma scintillation vacuum desiccator, and stored in an amber bottle. Some [(C~H~O)ZP(S)S]Z- counter. Clinical Centrifuge. .I 50ml. centrifuge tube and a mechanical HgZo3was also prepared. stirrer were used to effect all the MERCURICDI-72-BUTYL PHOSPHOROequilibrations between the aqueous THIOATE, [(C4HgO)zP(0)S]zHg, was and organic phases. prepared in a similar manner from diProcedure. The data from which n-butyl phosphorothioic acid nthe DA, values are calculated were (C4H90)2P(0)SH. A smaller quantity obtained as follows. An aqueous of [(C4H90)2P(0)S]2HgZ03 was also phase 0.2M in HCIOl and 0.2M in made. HNOI and of known [Ag+] was preSILVER DI-n-BUTYL PHOSPHOROpared; the Ag+ was tagged with Xg110. DITHIOATE (C4H90)2P (S)SAg, AND ,4n organic phase was prepared that SILVER DI-72-BUTYL PHOSPHOROTHIOATE, contained (C4H90)2P(S)SAg or (CaH90)zP(0)SAg, were prepared in the [(CdHg0)zP(S)S]2Hgin known concensame manner as that described above for the corresponding mercury comtration. Equal volumes of the phases pounds. The silver compounds were were equilibrated for 10 minutes at room Ag+. A series of experiments was made in which a cc14 solution of [(C4Hg0)2P(S)S],Hg or (C4HgO)2P(S)SAg was used to extract Ag+ from a n acid aqueous phase.
VOL. 37, NO. 2, FEBRUARY 1965
e
31 1
temperature (-27" C.). Phase separation was hastened by centrifugation. The total concentration of Ag+ in each phase was determined by counting a suitable aliquot in a gamma scintillation counter. RESULTS AND DISCUSSION
'*;m '0°1
Figure 1 shows the results of a series of three experiments. Line A is a plot of D,, vs. the concentration of [ ( C ~ H Q O ) ~ P(S)S],Hg or of ( C ~ H Q O ) ~ P ( S ) SinA ~ 4, ' O b the organic phase at constant initial [ H + ]and [Ag+]. For a D,, value about 1 or greater, as shown on Line A , the ratio of the concentration of uncombined [(C4H90)~P(S)S]zHg in the organic phase t o initial [Ag+] is large; 0.4 thus, the plot is D,, us. uncombined [ (C4Hg0)2P( S)S]zHg at equilibrium. The straight part of Line ,4 has a slope of , X I , , , , ,,,,1 , I I I ,I(, 2 ; therefore, for D,, < I the experimental points were corrected for a combining EXTRACTANT IN THE ORGANIC PHASE, M to Ag+ of ratio of [(C4Hg0)~P(S)SIzHg 2 to 1. The corrected values lie on an Figure 1. Dag vs. molarity of exextrapolated line, which also has a tractant in the organic phase slope of 2. When (C4H90)2P(S)SAg Extractant: (CaH&)zP(S)SAg or [(CaH$O)zP(S)S]2was used as extractant, the experimental Hg values of D,, were corrected for ex0 [(C4HgO)zP(S)SIzHg; [Ag'l = 4.2 X change of Ag+ in the aqueous phase 10-aM with Ag+ in (C4H90)2P(S)Sdg of the X Corrected values for organic phase. The corrected values of 0 [(CiHgO)zP(S)SIzHg; [Ag'l = 7.2 X D,, lie on a line of slope 2 and are in 10-eM agreement with those values obtained 0 (CiHgOhP(SJSAg; (Ag+] 4.2 X lO-'M when [(C4H90)2P(S)S]2Hg was used as the extractant. plex between the [(C4H90)2P(S)S]zHg Line B shows the results of tests and AgN03. wherein the initial [Ag+] was about A study of the effect of [Ag+] a t 11600 that present when the results equilibrium on DAg was also made. A shown by Line A were taken. The slope plot (not shown) of D,, us. the equiof Line B is also 2. This value shows librium [Ag+] at constant initial conthat D,, is directly proportional to the cent ration of [ (CdHgO)* P (S)S ]zHg in the second power of the concentration of (C4Hg0)2P(S)SAgor [(C~H~O)ZP(S)SIZ-organic phase and [ H + ] of the aqueous phase shows inverse first-power deHg in the organic phase. In other pendency of D,, on [Ag+]. A plot of studies at a much higher initial [ilg+] D,, vs. [KO3-] a t constant initial conand concentration of (C4H90)zP(S)Shg centration of [(CdHg0)2P(S)S]2Hg in the or [(C4Hg0)2P(S)S]2Hg in the organic organic phase has an initial slope of 1 phase, the slope is 1.7. A series of tests was made with [(C4H90)2P(S)S]2HgZo3 and levels off when the [hTo3--]is >4.0M. This fact is interpreted to mean as extractant and with inactive Ag+ that Koa- is incorporated in the final present in the aqueous phase; the other complex and a maximum effect is experimental conditions were identical reached at -4.OM N03. with those under which the results Of the metal ions studied, only Ag+, shown by Line A were obtained. Hg+2,and Pd+2 form the addition-type Negligible quantities of HgZo3 were complex. Because of the reduction of released to the aqueous phase. This Cu+2 and A u + ~ by the phosphorofact is interpreted to mean that Ag+ dithioate anion, the extent to which does not replace Hg+2 from its position Cu+2 and Au+3 form the addition romin [(C4H90)2P(S)S]2Hg and that a new plex was not determined. For the complex is formed in which both Hg+2 conditions studied, Bi+3, Cd+2, In+$, and Ag+ are incorporated. The new and Zn+Z do not form the additioncomplex may be an addition-type com-
'*I
312
ANALYTICAL CHEMISTRY
type complex with metal di-n-butyl phosphorodithioates. Spectrophotometric determination of di-n-butyl phosphorodithioic acid by measurement of the absorbance of the Bii3 complex is additional evidence that Bi + 3 does not form an addition-type complex, a t least one that can be detected spectrophotometrically. The silver probably coordinates through the semipolar sulfur. However, the complex formation is not due entirely to the semipolar sulfur atom. S)S12, When the disulfide, [n-(C~HQO)ZP( which is the oxidized form of the acid, is used as extractant for Ag+ and for identical experimental conditions, the DA, value is about the same as that obtained with triisooctyl phosphorothioate, (CsH,70)3PS, as extractant. The trialkyl phosphorothioates selectively extract Ag+ and Hg+2 from an aqueous "03 medium (3). However, with (CsH170)aPS, as extractant for Ag+ and for conditions identical with those under which the data of Figure 1 were obtained, the DAgfsare -103 smaller than those given in Figure 1. A series of tests in which [(C4H90)*P(O)S],Hg was used to extract Ag+ from an aqueous phase 0.2M in HC10, gave DA, values and 0.2M in "03 4 0.01. Thus, exchanging a semipolar oxygen atom for a semipolar sulfur atom prevents formation of the addition complex. This fact is additional evidence that the silver coordinates through the semipolar sulfur. In fact, the stoichiometry of mercury and silver di-nbutyl phosphorothioate is the basis for an isotope-exchange or displacement method for the determination of mercury (1). Attempts to study the addition complex of Hgt2 with [ ( C ~ H Q O ) Z P ( S ) S ] Z H ~ by solvent-extraction techniques failed, because a third-phase precipitate formed. The precipitate was insoluble in chloroform, ether, benzene, and ethyl acetate. LITERATURE CITED
(1) Handley, T. H., ANAL.CHEM.36, 153
(1964). (2) Handley, T. H., Nucl. Sci. Eng. 16, 440 (1963). (3) Handley, T. H., Dean, J. A., ANAL. CHEM.32,1878 (1960). (4) Ibid., 34, 1312 (1962). THOMAS H. HANDLEY Analytical Chemistry Division Oak Ridge Nat,ional Laboratory Oak Ridge, Tenn.
