of the Equilibrium Constants of Alkylphosphinic Acids Coupled Plasma-Atomic Emission Spectrometry Determination by Inductively

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1 613 Determination by Inductively of the Equilibrium Constants of Alkylphosphinic Acids Coupled Plasma-Atomic Emission Spectrometry M. MARTINEZ, N. MIRALLESt, A. SASTRE and C. HERRANZ Chemical Engineering Department, Universitat Politecnica de Catalunya, E. T S. E. I. B. Diagonal 647, E Barcelona, Spain The equilibrium constants of some organophosphinic acids have been determined at 25 C by measuring the distribution ratio of the organophosphinic acid between an aqueous solution of ionic strength 0.05 and toluene as a function of ph for different concentrations of organophosphorus acid compounds. The distribution ratio of the organophosphorus compounds has been obtained by determining phosphorus by inductively coupled plasma-atomic emission spectroscopy. The data have been analyzed graphically, and numerically by the program LETAGROP-DISTR, in order to determine the dimerization constants K2, the acidity constants Ka and the distribution constants Ko Keywords Mono(2,4,4-trimethylpentyl)phosphinic acid, dioctyl phosphinic acid, monooctylphosphinic acid, inductively coupled plasma-atomic emission spectrometry, dimerization constant, distribution constant, acidity constant Acidic organophosphorus extractants have been extensively investigated for the extraction and separation of several metals including lanthanides, actinides and transition metals using liquid-liquid extraction.' In particular, Danesi et al.2, Preston et al.3, and Yuan et al.4, have carried out comparative studies of the extraction with dialkylphosphoric, alkylphosphonic and dialkylphosphinic acids. In the last decade, several new organophosphorus compounds have been studied. Among these, much work has been devoted to the study of phosphinic acids. Diaklylphosphinic acids, such as di(2-ethylhexyl)- phosphinic acid, dibutylphosphinic acid, and dioctylphosphinic acids are very important extractants for lanthanides and have been studied in great detail.5'6 On the other hand bis(2,4,4-trimethylpentyl)phosphinic acid (Cyanex 272), provides a higher separation factor of Co(II) from Ni(II) than organophosphoric and organophosphonic acids', and it has shown a great versatility on the extraction and separation of lanthanides, actinides8'9 and divalent metals.10 However, only Peppard et al." have studied the extraction of metals with monoalkylphosphinic acids; no data are available on the equilibrium constants of these acids, except pka values determined in mixtures ethanol-water. Nevertheless, a proper interpretation of solvent extraction of metals with organophosphorus extractants requires a knowledge of the equilibrium constants of these reagents, such as the aggregation constants in the organic phase, Kp, distribution constants between the organic phase and the aqueous phase Ko, and the aqueous acid dissociation t To whom correspondence should be addressed. constants Ka. In this context, the equilibrium constants of monooctylphosphinic acid (H[(0)(H)P]), dioctylphosphinic acid (H[DOP]) and mono(2,4,4-trimethylpentyl)phosphinic acid (H[(TMP)(H)P]) have been determined from distribution data of the organophosphinic acids between toluene and an aqueous phase of 0.05 of ionic strength, in order to ascertain the stoichiometry of the metal extracted species with alkylphosphinic acids12"3, and their equilibrium constants. The distribution ratio of the organophosphinic acids has been obtained by determining phosphorus in aqueous phase by ICP. The distribution data have been analyzed graphically and numerically, using the program LETAGROP-DISTR.14 Experimental Reagents and solutions The organophosphinic compounds used: monooctylphosphinic acid (H[(0)(H)P]), dioctylphosphinic acid (H[DOP]), and mono(2,4,4-trimethylpentyl)phosphinic acid (H[(TMP)(H)P]), were synthesized by reaction of the proper alkene with sodium hypophosphite in the presence of t-butyl perbenzoate as initiator in a waterethanol medium and were purified as described by Herranz et a!.15 the purity of the acids was determined to be 93.7%, 95.3% and 96.7% for H[(0)(H)P], H[DOP] and H[(TMP)(H)P], respectively, by potentiometric titration of an 75% ethanol-water solution of the respective acid with sodium hydroxide. In all the cases only one end point was detected. A further test of the impurity was carried out by

2 614 ANALYTICAL SCIENCES OCTOBER 1992, VOL. 8 examining the 31P-NMR of organophosphinic acids in chloroform with H3P04 as an external standard. The 31p spectrum showed a peak at ppm for monooctylphosphinic acid, ppm for dioctylphosphinic acid and ppm for mono(2,4,4-trimethylpentyl)phosphinic acid. We can conclude that the impurities of the organophosphinic acids were not acidic and did not contain phosphorus. Toluene (Merck AR) was used as solvent without purification. Sodium nitrate (UCB AR) was used as the ionic medium. Phosphoric and nitric acids (Merck AR) were used without further purification. Kn, KD, and Ka values. When an organic solution containing a certain amount of an acidic organophosphorus compound is in contact with an aqueous phase, the equilibrium distribution is determined by the following equations: HA = HA org KD = [HA]org [HA] (2) n HAorg = (HA)norg Kn = [(HA)n]org [HA]org (3) Procedure All experiments were performed at 25 C. The experiments were carried out by shaking mechanically, in special stoppered glass tubes, 10 cm3 of organic phase containing C mol dm 3 of an organophosphinic acid HA, where C was varied from to 0.1 mol dm3, with an equal volume of aqueous phase having composition 0.05 mol dm 3 (Nat, H+)NO3-, until equilibrium was achieved. After phase separation with a high-speed centrifuge, the equilibrium ph was measured using a CRISON Digilab 517 ph meter equipped with a Ingold AG 293 combined glass electrode. The concentration of the organophosphinic compounds was determined in the aqueous phase analyzing the phosphorus concentration by using the nm first order atomic emission line of phosphorus with the aid of a A.R.L 3410 ICP Spectrometer with ultrasonic nebulizer. Ar gas (99.99% pure) was used for the plasma with flow rates of 121/mm n for the plasma, 0.81/ min for the nebulizer and 0.61/ min for the auxiliary flow. The incident plasma power was 700 W. Calibration curves were constructed using H3P04 solutions in the concentration range 1.6X104 M to 9.7X103 M by plotting the emission signal against concentration of phosphorus. The concentrations of the organophosphinic compounds in the organic phase were calculated as differences between the initial concentration in the organic phase and the equilibrium concentration in the aqueous phase after distribution. Results The distribution ratio D of the compounds, defined as: organophosphorus D = Corg C aq (1) The data, plotted as log D vs. concentrations, are given in Fig. phinic acid compounds studied. Treatment of Data ph for different HA 1 for the organophos- Fig. 1 log D plotted as a function of ph for different alkylphosphinic acid concentrations. The full drawn lines were calculated by means of the program HALTAFAL using the constants given in Table 2. [HA]tot M; X 0.05 M; M H[(0)(H)P] and H[(TMP)(H)P], M H[DOP]. The results obtained were treated in order to determine

3 615 HA =A-+H+ Ka = [A-][H+] [HA] (4) Determination of the acidity constants In order to evaluate the acidity constants, the data have also been treated graphically using normalized curves.16 Organophosphorus acid compounds are known to be dimers in aromatic solvents such as toluene.'' In that case, in order to have an estimation of the pka values, as a first approximation, the total concentration of organophosphinic acid in organic and aqueous phases can be written as: Corg = [HA]org + 2[(HA)2]org ~ 2[(HA)2]org (5) Caq = [HA] + [A-]. (6) Then the distribution ratio D can be written as: D = Corg - 2[(HA)2]org C aq -- [HA] + [A-] (7) Introducing Eqs. (2), (3) and (4) into Eq. (7), we can obtain the following equation: D/ Corg1~2 =1.41 K2112 KD (1 + Ka/[H+])-1. (8) The normalized curve log Y=-log (l+u) vs. log u, where log Y= log (D/ Corg1~2) - log K21'2 KD -- log 1.41 log u = log (Ka/[H+]) fits the experimental data plotted as log D-1/ 2 log Corg vs. ph. The intersection of two asymptotes (log Y=0 and log Y=u) gives the value of ph=pka and the acidic constants can be obtained (Fig. 2). From the horizontal asymptote, a relation between K2 and KD can be obtained: Fig. 2 log D-1/2 log Corg plotted as a function of ph for different alkylphosphinic concentrations. The full drawn lines correspond to the theoretical function log Y=log (l+u) plotted vs. log u. log (D/ Corg1/2) -1.41= log (K21'2 KD). The results are given in Table 1. Table 1 Values of pka and K21~2 KD 0 btained using normalized curves graphically Determination of the distribution and dimerization constants The graphical determination of the distribution and the acidity constants requires experimental values of the distribution ratio at values of ph higher than 3.5. In our case, only measurables D values at higher hydrogen ion concentrations were obtained, owing to a micelle formation in the system. For that reason, only numerical treatment can be done. The data obtained in the distribution equilibrium studies were evaluated by the computer program LETAGROP-DISTR. In this program, for a given model, the computer searches for the best set of equilibrium constants, which includes K2, KD and Ka, that would minimize the error squares sum defined by U= (log DeXp - log Dcalc)2 where DeXp is the distribution ratio of organophosphorus acids determined experimentally and Dcalc is the value calculated by the program. This program also calculates the standard deviation a (log D) defined by a (log D)= (U/ Np)"2, where Np is the total number of experimental points.

4 616 ANALYTICAL SCIENCES OCTOBER 1992, VOL. 8 Several models with different aggregation numbers of HA were also tried, in order to investigate the possibility of finding other species which could improve the fit to the experimental data. The results are given in Table 2. As seen in Table 2, values of Ka and K21~2KD found graphically are in good agreement with those obtained numerically. Furthermore, these values were used to calculate the full lines on Fig. 1 by means of the program HALTAFAL.18 The good agreement obtained between experimental and calculated values gives us great confidence in the proposed model. Table 2 Equilibrium constants of some organophosphinic acids determined numerically by means of the program LETAGROP-DISTR Table 3 Values of pka in water for different dialkylphosphinic acids found in the literature Discussion The best model found in the numerical calculations indicates that the organophosphinic acids tend to form dimers in the organic phase in the range of concentration studied, due probably to strong hydrogen bonding between the P=0 and P-OH groups on adjacent molecules, leading to eight-membered ring entities. The formation of dimers in nonpolar organic solvents agrees with the results found by other authors17 for other organophosphinic acids. The values for the dimerization constant confirm the great tendency of organophosphorus acid to associate in the organic phase. The value of the dimerization constants for dioctylphosphinic acid in toluene obtained in this work by distribution measurements is in agreement with the value (K2=3.10±0.18)19 obtained by vapor pressure osmometry (VOP) at 25 C. It can be seen that the substitution of hydrogen attached to phosphorus by an alkyl chain, and the branching in the alkyl chain, do not have a great effect on the values of the dimerization constants. The comparison between organophosphinic acids shows that the pka values of dialkylphosphinic acid are higher than the values obtained for the monoalkylphosphinic acids, due to the inductive effect of the two alkyl groups attached to the phosphorus, and seem to be independent of the branching in the alkyl chain. In Table 3, pka values in water for some dialkylphosphinic acids are summarized. As seen in Table 3, the value calculated in this work for dioctylphosphinic acid is in the range of those found in the literature. Comparing pka values in 75% ethanol-water found in the literature (pka 5.3, 3.7, 4.08, 3.69 for dioctylphosphinic, monooctylphosphinic, mono(2-ethylhexyl)phosphinic and mono(2,4,4-trimethylpentyl)phosphinic acids, respectively11,12) with the values of pka determined in water (Table 3), it seems that the values of pka determined in water are about one and two units less for monoalkylphosphinic and dialkylphosphinic acids, respectively. The values of the distribution constants for the two monoalkylphosphinic acids are similar, but they are lower than the KD value for the dialkylphosphinic acid. This fact shows that the distribution between the two phases increases when hydrogen is substituted by a higher lipophilic group, as expected.22 References 1. Y. Marcus, A. S. Kertes and E. Yanir, "Equilibrium Constants of Liquid Liquid Distribution Reactions", Part I: Organophosphorus Extractants, Butterworths, London, P. R. Danesi, L. Reichley-Yinger, G. Mason, L. Kaplan, E. P. Horwitz and H. Diamond, Solvent Extr. Ion Exch., 3, 435 (1985). 3. J. S. Preston and A. C. Du Preez, "The solvent extraction of cobalt, zinc, copper, calcium, magnesium and rare-earth metals. by organophosphorus acids", Council for Mineral Technology, Ranburg, Report II 387 (1988). 4. C. Y. Yuan, Q. G. Xu, S. G. Yuan, H. Y. Long, D. Z. Shen, Y. T. Jiang, H. Z. Feng, F. B. Wu and W. H. Chen, Solvent Extr. Ion Exch., 6, 393 (1988). 5. D. F. Peppard, G. W. Mason and S. Lewey, J. Inorg. Nucl. Chem., 27, 2065 (1965). 6. T. Cecconie and H. Freiser, Solvent Extr. Ion Exch., 7,15 (1989). 7. W. A. Rickelton, A. J. Robertson and D. R. Burley, U.S. Patent (1982); Chem. Abstr., 98, 93267d. 8. Y. Komatsu and H. Freiser, Anal. Chim. Acta, 227, 397 (1989). 9. K. Li and H. Freiser, Solvent Extr. Ion Exch., 4, 739 (1986). 10. A. Sastre, N. Miralles and E. Figuerola, Solvent Extr. Ion Exch., 8, 597 (1990). 11. D. F. Peppard, G. W. Mason and C. J. Andrejasich, J. Inorg. Nucl. Chem., 28, 2347 (1966). 12. M. Martinez, N. Miralles, A. Sastre and C. Herranz, "Solvent Extraction 1990", ed. T. Sekine, p. 147, Elsevier Amsterdam, N. Miralles, A. Sastre, M. Aguilar and M. Cox, Solvent Extr. Ion Exch., 10, 51 (1992). 14. D. H. Liem, Acta Chem. Scand., 25, 1521 (1971). 15. C. Herranz and M. Martinez, J. Dispersion Sci. Technol., 3, 209 (1988). 16. D. Dyrssen, Acta Chem. Scand., 11, 1771 (1957). 17. Z. Kolarik, Pure Appl. Chem., 54, 2593 (1982).

5 N. Ingri, W. Kakolowicz, L. G. Sillen and B. Warnquist, Talanta,14, 1261 (1967). M. Aguilar, N. Miralles and A. Sastre, Rev. Inorg. Chem., 10,1(1989). P. G. Crofts and G. M. Kosolapoff, J. Am. Chem. Soc., 75, 3379 (1953). L. A. Mamaev, A. N. Kamenskii, V. S. Smelov, E. G. Tetorin, V. G. Timoshev and M. V. Ugryumov, Zh. Neorgan. Khim.,12, 3046 (1967). 22. A. Leo, C. Hansch and D. Elkins, Chem. Rev., 71, 525 (1971). (Received March 14, 1992) (Accepted July 6, 1992)