Spectrophotometric Determination of Extraction Constants for Certain Metal 1-Pyrrolidinecarbodithioates Werner Likussar’ and D. F. Boltz Department of Chemistry, Wayne State Unicersity, Detroit, Mich. 48202
A modified spectrophotometric method based on the use of a normalized absorbance scale has been used to determine the extraction constants of six metal chelates. The extraction constants for the l-pyrrolidinecar bodithioate com plexes of copper( I I), cobalt( II), cadmium(ll), zinc(ll), bismuth(lll), and gallium(l1l) have been determined. The formation constant of 1-pyrrolidinecarbodithioic acid and the distribution ratio at various pH values have been evaluated. AMMONIUM 1-PYRROLIDINECARBODITHIOATE(APCDT) is frequently used as a chelating agent in the extraction of many elements prior to atomic absorption spectrometric determination (1-5) or spectrophotometric determination (6-9). Although qualitative data (10, 11) on the extraction of metal 1-pyrrolidinecarbodithioates and some quantitative data (10, 12-14) on the extraction of the reagent have been published, no data on the extraction constants of the metal chelates are available. The relatively high stability of these metal chelates makes it difficult to determine the corresponding extraction constants. In this study the extraction of several metal l-pyrrolidinecarbodithioates into chloroform was investigated in order to determine their extraction constant KE, actually a two-phase formation constant, and the reagent or acid extraction constant, KEacid. These values are useful in the prediction of optimum conditions for separation of various metals and for the substoichiometric determination of traces of these metals. The formation of metal 1-pyrrolidinecarbodithioates and the solvent extraction of the metal complex can be described by the following equation: Mn+
+ nR-
(MR&
(1)
The extractability is then determined by the following expression:
where KE is the extraction constant at a designated temperature. The subscript o denotes the organic phase, whereas 1 Present address, Institut fur anorganische und analytische Chemie der Universitat Graz, Graz, Austria
(1) J. E. Allan, Spectrochim. Acta, 17,459 (1961). (2) J. B. Willis, ANAL.CHEM.,34, 614 (1962). (3) S. Sprague and W. Slavin, At. Absorption Newsleft.,3, 37, 160 (1964). (4) R. E. Mansell and H. W. Emmel, ibid., 4, 365 (1965). (5) E. Lakanen, ibid., 5 , 17 (1966). 40, 1086 (1968). (6) M. B. Kalt and D. F. Boltz, ANAL.CHEM., (7) A. Traub and D. F. Boltz, Mikrochim. Acta, 1969, 149. (8) R. W. Looyenga and D. F. Boltz, Anal. Let?.,2, 491 (1969). (9) W. Likussar, C . Sagan, and D. F. Boltz, Mikrochim. Acta, 1970, 683. (10) A. Hulanicki, Tulanta, 14, 1371 (1967). (11) D. J. Halls, Mikrochim. Acta, 1969, 62. (12) R. Zahradnik and P. Zuman, Collect. Czech. Chem. Commun., 24, 1132 (1959). (13) H. Bode and F. Neumann, Z . Anal. Chem., 169, 410 (1959). (14) K. I. Aspila, V. S. Sastri, and C . L. Chakrabarti, Tulanta, 16, 1099 (1969).
concentrations in the aqueous phase are written without subscript, a notation to be used throughout this paper. The spectrophotometric determination of the extraction constants was done by using a modified continuous variations method as described previously (15). Inasmuch as the experimentally obtained constants are conditional extraction constants, the notation K‘E is used instead of KE. The following equations have been used for the calculation of the conditional extraction constants : log K’E
=
0.3522 - 2 log k
+
log y m ,
- 3 log (1
- ynl,,)
for 1 :2 complexes, and log K’E = 0.3750 - 3 log k
+
log ym,,
- 4 log (1 - ym,,)
for 1 : 3 complexes, where k is the constant sum of metal and ligand concentration (CM CR),and y,,, is the maximum of the continuous variations plot in terms of “normalized absorbance.” The relationship between the conditional and true extraction constants is given by the following equation:
+
log KE
=
log K’E
+ log
CYM
+ n log
(YR
- log
(YNR
(5)
where all side reactions which might occur have been taken into account by introducing the appropriate alpha coefficients, as recommended by Ringbom (16). The overall coefficient for each species (M, R, MR) is approximately the sum of the individual coefficients. Usually one coefficient predominates and the other coefficients can be neglected. The side reactions involving the metal ion in the aqueous phase, such as the effect of hydroxide ions and buffering or masking ligands, are considered as (Y,M(oH), (YM(A), and ( Y M ( A ’ ) . The ligand R undergoes two side reactions. First, in the aqueous phase the formation of 1-pyrrolidinecarbodithioic acid, HR, with the formation constant, KHR, with Equation 6 being applicable (YR(H)
=
1 -f [H’IKHR
(6)
The free reagent also exists as H R molecules in the organic phase and the corresponding alpha coefficient is as follows: (YR,
= 1
+ (vo/V)([H+l/[H+l~/d
(7)
where [HIl/z is the hydrogen ion concentration when the reagent is distributed equally between the two phases. The overall coefficient, CXR, is approximately equal to the sum of the individual coefficients. The calculation of (YR(H) and aR, is dependent on the knowledge of KHR and [H+]I/z, respectively. The value of KHRcan be calculated from the measured pH of an aqueous solution of ammonium l-pyrrolidinecarbo(15) W. Likussar and D. F. Boltz, ANAL.CHEM., 43, 1265 (1971). (16) A. Ringbom, “Complexation in Analytical Chemistry,” Interscience, New York, N. Y . , 1963.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 10, AUGUST 1971
1273
Table I. Data and Values Used to Estimate Formation Constant of 1-PyrrolidinecarbodithioicAcid (HR) log CNH,R PH log KNH~' log K H R ~ log K H R ~ -log fd log K H R ~ 0.00 6.20 4.5 2.90 2.899 0.25 3.03 -1.00 6.22 4.65 3.09 3.086 0.12 3.09 -2.00 6.23 4.70 3.16 3.126 0.05 3.06 -3.00 6.35 4.74 3.44 3.199 0.02 3.10 -4.00 6.66 4.76 4.08 3.265 0.00 3.15 a Formation (stability) constants for ammonium hydroxide used. Values for various ionic strengths corresponding to concentration of reagent correspond to those cited by Ringbom (16). Calculated using Equation 11. Calculated using Equations 8a, 9a, and 10a without any approximations. d Values of KHRare corrected for ionic strength ( p = 0.1) by applying activity coefficient, f.
dithioate. The following equilibria are involved (the notation CNH~R is the total or analytical concentration of the reagent used):
KER
EXPERIMENTAL =
[HR] [H+]-'[R-]-'
= [HR][H+]-' =
(8)
(CNE,R- [HR])-'
(84
[HR][H+]-' CNH,R-'
(8b)
KNH~= [NH3][OH-]-'[NH4+]-'
(9)
= ["dOH-l-'
(CNH~R - ["d)-'
(9a)
= [NHJOH-I-'
CNH,R-~
(9b)
By considering the law of electroneutrality, the following equations may be derived : [H+]
Equation 15 also allows one to calculate K D ( H Rsince ) KHR is known from an experiment described previously.
+ [NH4+] = [OH-] + [R-I
(10)
Apparatus. Measurements of pH were made with a L & N pH meter. The absorbance measurements were made in 1.000-crn cells with a Cary 14 spectrophotometer. A Thomas mechanical shaker was used for the extractions. Reagents. All reagents used were of AR purity and the chloroform was spectroquality. Ammonium l-pyrrolidinecarbodithioate (Fisher A-182) was recrystallized three times from a 1 :1 mixture of ethanol and ether. The distilled water was produced with an apparatus described by Hickman (17). The water obtained was almost completely free of organic and inorganic impurities. The electrical resistance was measured to be >4 megohms. STANDARDZINC SOLUTION(10-?M). Dissolve 0.719 g of zinc sulfate heptahydrate, ZnS04 7H20, in distilled water and dilute to 250 ml. STANDARD CADMIUM SOLUTION (2 X 10-4M). Dissolve 51.3 mg of cadmium sulfate, 3CdS04.8H20, in distilled water and dilute to 1 liter. STANDARD COPPERSOLUTION (4 X 10-4M). Dissolve 79.9 mg of copper(I1) acetate monohydrate, Cu(GH30&. H20, in distilled water and dilute to 1 liter. STANDARD COBALTSOLUTION(2 x 10-4M). Dissolve 58.2 mg of cobalt(I1) nitrate hexahydrate, Co(N03)2 6H20, in distilled water and dilute to 1 liter. STANDARD BISMUTHSOLUTION(4 X 10-4M). Dissolve 194.0 mg of bismuth(II1) nitrate pentahydrate, Bi(N03)3 5H20,in 0.2N HCl solution and dilute to 1 liter with the 0.2N HC1 solution. STANDARD GALLIUMSOLUTION (3 X 10-3M). Dissolve 160.4 mg of gallium sulfate, Gaz(S04)3in 0.01M HC1 solution and dilute to 250 ml with this HC1 solution. AMMONIUM 1-PYRROLIDINECARBODITHIOATE (APCDT) SOLUTION (10+M). Dissolve 1.643 g of purified ammonium 1-pyrrolidinecarbodithioate in distilled water and dilute to 1 liter. Aliquots of this reagent solution were diluted to obtain concentrations corresponding to the different standard metal ion solutions. Solutions of potassium chloride, potassium bromide, potassium dihydrogen phosphate, sodium acetate, acetic acid, hydrochloric acid, sodium hydroxide, sodium oxalate, and disodium dihydrogen-EDTA were prepared at concentrations as indicated in Table 111. These solutions were used as buffers or as weak complexants to adjust pH and/or ionic strength to p = 0.1. Procedure I. DETERMINATION OF DISTRIBUTION OF 1PYRROLIDINECARBODITHIOATE BETWEEN CHLOROFORM AND WATER. Transfer 10 ml of a 3 X 10-2M aqueous solution of APCDT and 10 ml of appropriate buffer solution to a 150-ml separatory funnel. The buffer solution should be 1
[H+l -/- CNH,R- ["I] [HR]
=
["SI
[HRI
=
["d
=
[OH-] f CNH~R - [HR]
+ [OH] - [HI
( 1oa>
(lob)
The approximations made in Equations 8b, 9b, and 10b are justified when CNH,R> The combination of these equations and applying the ionic product of water led to the following expression:
+
log KHR = log KNH* 2 pH
- 14
(11)
The hydrogen ion concentration [H+Il/2 can be calculated from the data of an experiment involving extraction of APCDT into chloroform at various pH values. Besides the formation equilibrium expressed by Equation 8 the following equations are valid:
KD(HR)= [HRIJHRI-'
D
=
[HRIdR-I
=
[HR],[R-]-'
(12)
+ [HRI)-' (13)
where K D ( B R ) denotes the distribution coefficient of H R and D the experimentally determinable distribution ratio of the reagent. The approximation made in Equation 13 is permissible since [R-] >> [HR]. By inserting Equations 12 and 13 into Equation 8, one obtains the following equations: pH
=
+ log KHR+ log KD(HR) log KHR+ log KD(HR)
-log D
pH~/z=
(14) (15 )
since at pHIiz the distribution ratio D = 1. If the pH is plotted against log D, Equation 14 gives a straight line with pH1/2 = log K H R log K D ( H Rwhere ) log D = 0.
+
1274
(17) K. Hickman, ANAL.CHEM.,36, 1404 (1964).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 10, AUGUST 1971
Table 11. Summary of Data Used in Calculation of Distribution Ratio and pH,/* Values for 1-PyrrolidinecarbodithioicAcid and the Chloroform-Water System Absorbance Concn x 10-6M/1. D in CHCla in H 2 0 log D [HRlo [R-I PHiiz PH 1.150 103 ... ... 3.00 1500
