( 7 ) Berman, S. S., Ironside, R., Can. J . Chem. 36, 1151 (1958). (8) Berman, S. S., McBryde, W. -4. E.
Analyst 81,566 (1956). (9) Berman, S. S., McBryde, W. A. E., Can. J. Chem. 36, 835 (1958). (10) Currah, J. E., McBryde, W. A. E.,
Cruikshank, A. J., Beamish, F. E., IND. ENQ.CHEM.,ANAL.ED.18, 120 (1946).
(11) Faye, G. H., Inman, W. R., Ibid., 33, 278 (1961). (12) Ibid., 34, 972 (1962). (13) Fraser, J. G., Beamish, F. E., McBryde, W. .4.E., Ibid., 26,495 (1954). (14) Kirkland. J. J.. Yoe. J. H.. Ibid.. ' ~. 1340. ' (15) Sen Gupta, J . G., Beamish, F. E., Ibid., 34, 1761 (1962).
(16) Tertipis, G. G., Beamish, F. E., I b i d . , p . 623. (17) Wilson, R. B., Jacobs, W. D., Ibid., 33, 1650 (1961). (18) Yoe. J. H.. Kirkland. J. J.. Ibid..
RECEIVED for review February 18, 1963. Accepted April 8, 1963.
Behavior of Dialkyl Phosphorodithioic Acids in Liquid Extraction Systems RAQUEL H. ZUCAL' and JOHN A. DEAN Department o f Chemistry, University of Tennessee, Knoxville, Tenn. THOMAS H. HANDLEY Analytical Chemistry Division, Oak Ridge Nafional laboratory, Oak Ridge, Tenn.
b The solubility and acidity of the diethyl, diisopropyl, di-n-butyl, and diisobutyl esters of phosphorodithioic acid in water and the distribution of each between CClc methyl isobutyl ketone, or n-amyl acetate and HCINaCl solutions have been studied. From the data, the acid dissociation constants and the partition coefficients were calculated. No evidence was found for dimerization of dialkyl phosphorodithioic acids in the organic phase. These constants are discussed, together with data for the dialkyl phosphates. The acid dissociation constant i s somewhat larger and the partition coefficient is much larger when comparing di-n-butyl phosphorodithioic acid with di-n-butyl phosphate.
D
phosphorodithioic acids have been screened in acid media for their selectivity in extraction of metal ions ( 7 ) , but little is known about estractants which contain thiono and thiolo phosphorus groups. Hence, the principal objective of this investigation was to establish the general behavior of several individual esters of this class of reagents. The use of liquid-liquid partition to investigate the equilibria of dialkylphosphorodithioic acids in solution seemed most applicable among methods which have been employed to study similar equilibria in view of the strength of these acids and the ultimate employment of the information to metal extraction systems. IALKYL
EXPERIMENTAL
Special Reagents. Dialkyl phosphorodithioic acids. iMethods for preparation and purification have been described, and commercial sources of the reagents listed ( 7 ) . Methyl isobutyl ketone, technical grade, was distilled before use. 988
ANALYTICAL CHEMISTRY
Apparatus. Extractions were performed with a Burrell wrist-action shaker. p H measurements were made with a Beckman Model G p H meter which had been calibrated with a phthalate buffer (pH = 4.01 at 25" C.) and checked against a 0.1N HCl buffer (pH = 1.10) which contained a n appropriate quantity of iYaC1 t o adjust t h e ionic strength. Titrations were performed with microsyringe pipets, 0.5- and 1.0-ml. capacity. Procedure. Equal volumes, 5 ml., of both phases were contacted for a period of 2 hours a t 25" C. Phase separation was assisted by centrifugation at 2000 r.p.m. for 10 minutes. The acid concentration in t h e aqueous phase was determined from a p H measurement or, when [H+] > 0.1M, by a n alkalimetric titration. An extractive two-phase method \vas employed with the organic phase. The concentration of phosphorodithioic acid was determined by an iodometric titration (3, 4). The change in volume due t o mutual solubility or the solvents can be neglected for the system CC'14-water and, although i t is about 5% for methyl isobutyl ketone-water and n-amyl acetate-water, corrections nere not attempted. Reagent Solubility in Aqueous Media. The solubility of di-n-butyl phosphorodithioic acid as a function of HCl or NaCl concentration was obtained by saturating the aqueous phase with the pure reagent during a shaking period of 1 hour a t 30" C. Results are shown in Figure 1. The solubility of the diisobutyl ester was determined also in HCl medium; results superimposed essentially on the curve for the di-n-butyl ester. Dependency of Distribution Ratio on Concentration of Reagent in Organic Phase. T w o methods were suggested by Dyrssen ( 5 ) t o establish whether dimerization of the reagent occurred in the organic phase and, incidentally, to determine the partition coefficient, P, = [HR],/[HR] of the
reagent between the aqueous and organic phases when the acid dissociation constant is known. The subscript o signifies the organic phase. I n one case, the hydrogen ion concentration established by the reagent distributing between a n organic solvent and a n aqueous solution of a univalent salt of a strong acid is plotted as a function of reagent concentration in the organic phase, C,,,. For such a system,
where K, is the dimerization constant of the reagent in the organic phase and K, is the acid dissociation constant in Iyater saturated with the organic solvent. -4 plot of log Cor, us. log [H+][R-] is sho.il-n in Figure 2 for solutions of di-n-butyl phosphorodithioic acid shaken ~ ~ i atnhequal volume of 1-11 SaCl. For each of the organic solvent-water systems the points fell reasonably close to the straight line drawn with a slope equal to one although considerable scatter prevailed. A slope of one implies that the reagent exists only as a monomer in the organic phase and the term involving the dimerization constant may be neglected. Subsequent data will lend further support. I n the second ease, the distribution ratio, D, of the reagent between the particular organic solvent and an aqueous solution of HCl n-as studied as a function of total reagent concentration. For such a system,
+
D = 2 I(* (P,/+)*c, P,/+ nhere d = 1 K a / [ H f ] . On doulile logarithmic coordinates, a plot of D os. the equilibrium concentration of reagent in the aqueous phase, Caq, for a series of fixed HCl concentrations, gives a qe;ies of horizontal lines as shown
+
1 Prcscnt address, Comision Nacional Energia htornica, Bucnos Aires, Argentina.
0 "01
1
1
001
I L
1
1
L
10
01
LLl
Figure 2. Distribution of di-n-butyl phosphorodithioic acid between a n organic solvent and an aqueous solution 1 .OM in NaCl
1odium calt. DISCUSSION
The dialkyl phosphorodithioic acids are quite strong acids. Because of this fact, i t was impossible to maintain a constant, but low, ionic strength in the aqueous phase. At high concentrations of hydrogen ion the experimental points showed some scatter. However, the use of three different solvent systems provided a check upon the experimental value of the acid dissociation constant. From the data shown in Figures 2 through 6, values for K, and P, were selected which provided the most coherent fit with the experimental data. These values are collected in Table 11. Because we are dealing with two phases at equilibrium, the values of the acid dissociation constants refer to a n aqueoils phase which has been saturated with an organic solvent; similarly, the parti-
Table 11.
Alkyl group Ethyl Isopropyl
n-Butyl
Isobutyl
pH,/* = pKa
+ log Pr
Fortunately, i t is the ratio PJK, which is actually required in expressions dealing with metal extraction systems. Perusal of Table I1 indicates only a small change in the acid dissociation constant among the several dialkyl esters, but a very large change in the partition coefficients. Each increases in the order: diethyl < diisopropyl < di-n-butyl < diisobutyl. From the limited number of esters studied, it appears that chain branching among iso-
Characteristic Constants for Dialkyl Phosphorodithioic Acids
(Ionic strength = 1.0 except when [H+]> 1M) Solvent system PHI/Z log P r CCl*-water 0.35 0.45 CCh-wa ter 1.89 1.90 2.70 2.52 CClr-wa ter Methyl is0 butyl ketone-water 2.77 2.54 n-Amyl Acetate- water 2.40 2.23 2.74 2.63 CClrwater --
990
tion coefficient of the reagent refers to the ratio of the concentration of H R in an organic phase saturated with an aqueous phase to the concentration of H R in a n aqueous phase saturated with a n organic phase. This stipulation must be borne in mind when comparing values obtained in different pairs of solvent. The more accurate measurements of the distribution ratio are naturally those for which D equals, or is not far removed from, unity. Thus, values of D = 1 (denoted pHltzin metal extraction systems) are more readily determinable than the individual values of P, or K, to which i t is related by the expression :
ANALYTICAL CHEMISTRY
-
PKa -0.10 0.00
0.22 0.25 0.18 0.10
mers brings about an increase in the value of the partition coefficient and the acid dissociation constant. dniong the solvent pairs employed, CClpwater and methyl isobutyl ketone-water behaved alike; but in n-amyl acetatewater the reagent shows a slightly smaller value for the partition coefficient. The dialkyl phosphorodithioic acids are somewhat stronger than are the dialkyl phosphates. Dyrssen ( 5 ) reported pK, = 1.00 for di-n-butyl phosphate as compared with 0.22 for the dithioic acid ester. On comparing the partition coefficients for the two classes of reagents, wherein Dyrssen and Liem (6) reported for the dibutyl phosphate a value log P, = 0.34 in CHC13 and log P, = 1.36 in methyl isobutyl ketone, the partition coefficients for the dithioic acids are an order of magnitude larger, a major difference between the tmo classes of reagents. No evidence for dimerization in the organic phase was detected in this study, a second major difference. Hydrogen bonding involving the thiol group has been reported from infrared studies in concentrated solutions of dithioic acids in CCli (1). However, frequencies assigned to the P + S group remain constant only if the immediate environment of the phosphorus atom or thiono group does not change. Dyrssen and Liem (6) pointed o u t that an increase in the partition coefficient seems to be combined with a decrease in the dimerization constant for di-n-butyl phosphate. Many solvents conform to the relationship, K2P,2 = 5 X 103. If true here, Kz 5 0.08 (methyl isobutyl ketone) and 5 0.05 (CC14). If association does occur, i t must be slight in the dilute solutions employed. Furthermore, the degree of association of dialkyl phosphate. has been found to evcfcd that of rlixlkyl
phosphorothioates 1:s). According t o cryoscopic data (2, '), the weight of diethyl and di-n-butyl phosphorodithioic acids in benzene does not depend on concentrLtion, in the range ljermitted by the method, and agrees with the theoretical value of the monomer. ACKNOWLEDGMENT
The authors thank James -4.Durhin for his assi-t D ance.
LITERATURE CITED
o., J . Chem.
(1) Allen, G., Calclough, R. SOC. 3912 (1957). ( 2 ) Baldwin, I€.,H i g h s ,
c. E., U. 8. Atomic E1lergy Comm., Rept. ORNL3320, pp. 54-5 (1962). (3) Bode, H., Arnswald, W., Z. Anal. Chem. 185, 99 (1962). (4) Busev, A. I., Ivanyutin, I., J . Anal. Chem. ( R ? ~ s s i a n11, ) 559 (1956). ( 5 ) Dyrssen, D., Acta Chpm. and. 11, 1771 (1957). (6) Dyrssen, I]., Liem, D. H., Ibid., 14, 1091 (1960).
w.
(7) Handley, T. H., Dean, J. A . , ANAL. CHEM.34, 1312 (1962). ( 8 ) popov, E. hf,, Thesis, Mosoow c n i v , , 195s. (9) popov, E. RI., &bachnik, M. I., Mayants, L. S.,Russian Cliem. Revs. 362 (1961).
RECEIVED for review Kovember 16, 1962. Accepted April 8, 1963. Presented at Southeastern Regional Meeting, ACS, Gatlinburg, Tenn., November 1962. Miss R. H. Zucal is indebted t o the International Atomic Energy agency for the award of a fellowship.
Di-n-butyl Phosphorothioic Acid as an Extractant for Metal Ions THOMAS H. HANDI-EY Analyfical Chemistry Division, Oak Ridge National Laborafory, Oak Ridge, Tenn.
b Di-n-butyl phosphorothioic acid (5 w./v. in CCla) w13sevaluated as a reagent for extractilqg metal ions from aqueous solutions o f HCI or H2S04. The results are presented in two periodic table type charts as plots of log distribution ratio (D)vs. log [H+]. Of 32 metal ions studied, 18 extracted in significant amoJnts. In general, metal ions that forni insoluble sulfides have the highest D. The relative order of extraction of 1 1 metal di-nbutyl phosphorothioclte complexes was determined. The effect of di-n-butyl phosphorothioic acid concentration on the D values for Ag +, Hg+2, Zn+2, and ln+3 was investigated. The influence of various organic soivents on the D values for Hg+2 and Z n + 2 was studied. The limits of extractim for Ag+, Hg+2, and Pdf2 were in the parts per billion range. The nature of the zinc di-nbutyl phosphorothioate complex was investigated.
yo
T
dialkyl phosphoric acids, (R0)zP(O)OH, have btxen investigated ex-
HE
tensively and used widely as solvent 6,15,16). More extraction reagents (t, recently, the dialkyl phosphorodithioic acids, (RO)2P(S)SH, have been found to he useful solvent extraction reagents (2,S, 10). The dialkyl phosphorothioic acids, (RO)2P(0)SH,appear t o occupy a position intermediate between the dialkyl phosphorodithioic and the dialkyl phosphoric acils with respect to certain properties (IS 14). The study of such a mixed acid thet contains both an oxygen and a sulfur atom attached to the phosphorus atom is of interest. illthough sulfur and oxvgen atoms bear a formal resemblance t > each other, their tendencies for complm formation differ greatly.
The dialkyl phosphorothioic acids exhibit tautomeric equilibrium (12, I S ) as follows :
s I1
(RO)*P-OH Thiono
SH
1
(R0)2P=O
(1)
Thiol
Because of the similarity of the acid properties of the sulfur and the oxygen acids of phosphorus, the equilibrium does not favor either isomer. Therefore, reaction of di-n-butyl phosphorothioic acid with a metal ion of high electroaffinity would be expected to produce stable complexes with coordination through sulfur. Baldwin (1) has made infrared studies of di-n-butyl phosphorothioic acid and several of its complexes, H e reported that in CC1, solution 90% of the acid is in the thiono form, whereas the silver complex exists mainly as the thiol isomer. Thus, the reaction of di-n-butyl phosphorothioic acid with any one of certain metal ions causes a shift of the equilibrium toward the thiol form and a transfer of the reaction center. Hydrogen bonding has been reported for di-n-butyl phosphorothioic acid (9) and is interpreted as being inter- and not intramolecular (19).
fied by recrystallization from a mixture of benzene and n-hexane. The melting point of the purified ammonium di-nbutyl phosphorothioate was 157-9' C. The free acid was obtained after acidification of the ammonium di-n-butyl phosphorothioate with aqueous mineral acid, extraction of the di-n-butyl phosphorothioic acid into benzene or ether, and removal of the solvent by evaporation. The free acid is a viscous liquid that has a slight odor.
It is not known whether di-n-butyl phosphorothioic acid is toxic; however, i t should not be significantly more toxic than the dialkyl phosphorodithioic acids, which have been reported to he only slightly toxic ( 1 7 ) . Titration of a 5% solution of the acid in ethyl alcohol with a solution of sodium hydroxide showed that the acid is monobasic; the titration curve is indistinguishable from that of a strong monobasic acid. -45 (w./v.) yo solution (0.221M) of di-n-butyl phosphorothioic acid in CC14was used for the extraction studies. This solution is stable for days if kept a t room temperature and away from ultraviolet radiation.
Appropriate radioactive tracers were used to determine the total concentration of each metal in each phase. All other reagents were of reagent EXPERIMENTAL grade. Reagents. DI-TL-BUTYL PHOSPHORO- Instrumentation and Apparatus. A THIOIC ACID, (n-C4H90),P(0)SH, m?Geiger-Muller counter (G-AI) was lecular weight, 226. This compound is used to measure beta-emitting radioisotopes. not available commercially; it was preA gamma scintillation counter was pared as follows. Equimolar quanused to measure gamma-emitting tities of di-n-butyl hydrogen phosphite radioisotopes. and sulfur were reacted in benzene Extraction Apparatus. A 50-ml. cenwhile a n equimolar quantity of ammonia trifuge tube and a mechanical stirrer gas was bubbled into the reaction mixwere used to effect all equilibrations ture during a period of approximately 1 between the aqiieous a n d organic hour (7). The ammonium di-n-butyl phases. phosphorothioate thus formed was puriVQL. 35, NO. 8, JULY 1963
991
