The solubility product of cadmium sulphide

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1089

Whatman No. 1 paper gave much better results than did Whatman Nos. 41 and 542, or Schleicher & Schtlll Nos. 583, 585 and 589. It gave a sensitivity of 0.3 ,~g of ajmaline and no blank. The estimation of ajmaline ph~~utical preparations showed a variation of 2%. R. A. SHAH NARGIS Huss~nv (MRS.)

Centraf Laboratories P.C.S.I.R., Karachi

Summary_-Ajmaline has been determined semiquantitatively with p-dimethylaminobenzaldehyde as reagent, with an error of 52 %. Zusammenfassung-Ajmalin wurde mit ~Dime~yla~o~dehyd als Reagens mit einem Fehler von +2% halbq~ntitativ bestimmt. Rt%nnGOn a determine l’ajmaline semi-quantitativementu au moyen de p-dim&hylaminobenzaldehyde, avec une erreur de &2 %. REFERENCES 1. M. B. Celap, T. J. Janjie and V. D. JevtiC, Mikrochim. Acta, 1963, 1043. 2. F. Ordoveza and P. W. West, Anal. Chim. Acia, 1964,30,227.

Talanta, 1969.Vol. 16,PP. 1089

to 1093.

PersamonPress. Printed in

Northern

Ireland

The solubility product of cadmium sulphide (Received 13 February 1968. Revised 17 October 1968. Acwpted 12 December 1968) IN THBlast 6Oyears, numerous attempts have been made to evaluate the solubilityproduct of cadmium sulphide, by calculations based on free energy relationships and by analytical measurements on saturated or precipitating systems. The most reliable value1 by the former method is 7.8 x lo-*’ molea.l.-a at 25”. Values obtained by the latter procedures vary considerably,“-* even when the older ones are corrected with the currently accepted acidity constants for hydrogen sulphide. Moreover, few of these values are as large as the free-energy value. Various reasons have been suggested for the wide range of values reported. For instance, the solubility of hydrogen sulphide may be markedly intluenced by changes in acidity. Activity coelhcients may not have been used, and the effect of complexing may have been neglected. Finally, it may not have been established whether true equilibrium conditions existed during the measurements. The present investigation takes these factors into consideration. The apparent solubility product of cadmium sulphide in various acidic media is determined by precipitation and dissolution techniques, and is corrected for the effects of ionic strength and formation of complexes. Precautions are taken to ensure the closest approach to ~~librium in the conditions used. EXPERIMENTAL Precipitation of cadmium sulphide Analytical reagent grade salts and acids were used. Cadmium perchlorate was prepared by treating cadmium sulphate solution with potassium carbonate, washing the precipitate free from sulphate, and dissolving it in hot perchloric acid. The reactants were added from burettes to 50-ml stoppered tubes. Where necessary, water was added to give a total volume of 50ml so that the space above the solution was negligible. The difficulty in obtaining standard conditions for precipitating heavy metal sulphides by bubbling hydrogen sulphide through the solution has been indicated elsewhere;’ in this study, a standardized hydrogen sulphide solution was used. The deaerated solutions were added in the following order, in such a way that as little mixing as possible took place before the tubes were sealed: cadmium salt; acid; water (if any); hydrogen sulphide. Lightly greased glass stoppers were used for sealing. The liquids rarely touched the grease, and precipitation was never associated with it. After sealing, the tubes were shaken thoroughly, and the time for a visible precipitate to occur was noted. In each medium, 15-20 experiments were carried out, covering a range of ion products encompassing the solubility product. The ion product, [Cd%+] x [Sl-1, was calculated for each experiment and corrected for complex formation etc. The largest value of this corrected ionic product at which precipitation did not occur within 14 days was taken to be the solubility product. This value was always only slightly (~20%) smaller than those for some systems in which precipitation occurred.

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Between experiments, the tubes were cleaned with concentrated hydrochloric acid and by standing for some days in concentrated chromic acid. They were washed repeatedly with distilled water and dried before use, Ewoiution

ofcadmium sufphide

Samples of cadmium sulphide were precipitated from 0.3M hydrochloric, sulphuric and perchloric acids; a commercial sample of cadmium sulphide was also used. All the precipitates were washed with water, then with acetone, and were dried at the pump and in a vacuum desiccator before use. Portions of these preparations were aged for 12 months in the acids from which they had been preci itated (m all three acids for the commercial sample) at 25”. The acids were treated with a little hy Brogen sulphide before sealing off in SO-ml tubes. After aging, tie sulphides were filtered off, washed and dried as before, and retained for further examination. The filtrates were analysed immediately after filtration. Analytical methods

Hydrogen sulphide was determined by the arsenite method.* Cadmium was determined spectrophotometrically with dithiione@ after the solution for analysis (containing 2-5 pg of cadmium) had been evaporated to a small volume to remove hydrogen sulphide. Ca~~uiu~io~ ofvolubilityproducts

It can be readily shown that K,, = [Cd”*] [H&S]fad*+/& @+]y$*,where k, is the association constant for the reaction 2H+ + Sa- + H,S and f is the activity coefficient for the species indicated in obtaining the activities of ions by subscript: fE86 is taken to be unity. lo There are impactions forming relatively weak complexesP*lp and activity coefficients for cadmium will not be used here; the probable effect of this wiil be included in the error estimate. At the acidities used, the dissociation of hydrogen sulphide is negligible. The values chosen for kl appear to be the most reliable of those reported’* and are log k%= 20.05 (20°) and 1996 (25”). Chloride media. Cadmium forms a series of weak comalexes with chloride. The values for the stabilities of these complexes under the conditions of ionic.strength prevailing in this investigation, viz., log I& = 1.8, log& = 24, log pa = 22, log f& = l-6, ha;e deen obt&ed by interp;lation from valuesls obtained at ionic strengths of 0 and 3M. The concentration of free cadmium ions can be calculated by the equation given by%eden .%lActivity coefficients for acids are taken from Robinson and Stokes:14 for 0.7 and 0*22M hydrochloric acid, f=+= @77. Perch&ate media. For 0.92Mperchloric acid, fa+ = 0.72, and for 2*lM acid, f& = l-08. It is assumed that complexing between cadmium and perchlorate may be ignored. Sulphute media. Hydrogen sulphate is a relatively weak acid, with an association constant18 kI c-r 10 l.mole-’ in 3M sodium perchlorate. Thus the concentration of free sulphate ions, [SO,*-] produced by a formal concentration C of sulphuric acid is given by solving the quadratic equation, kIISO,e-]P -I- (Ck, + 1)[S0,8-] - C = 0. The concentration of hydrogen ions produced by the sulphuric acid is then given by [H+] = C -+- [SOla-]. The activity coefficients for hydrogen ions at the lower sulphuric acid concentrations can be calculated from the Davies equation, log f = 0~5122 {0*2~- q&(1 + 4~)) where 2 is the ionic charge and p the ionic strength. Thus, in 022M and 0*36M sulphuric acid, fn+ = O-71. The activity of the 32M acid is obtained by interpolation from the data of Robinson and Stokes; fa+ = 0.4, Cadmium forms a weak complex with sulphate;lK the stabiiity constant is --IO I.mole-l, so the concentration of free cadmium ions is calculable. Physical examination oftheprecipitates

Freshly prepared and aged precipitates of cadmium sulphide were compared by means of X-ray analysis and light- and electron-~cro~opy. For X-ray analysis, the samples were powdered and set in collodion. A twin strip was exposed to O-1542nm radiation for 30 min. All the specimens gave powder ~hoto~aphs (non-rotated? _in which the lines corresponded to mixtures of the cubic and hexagona crystal forms. The mtensmes of the hnes were difficult to measure owmg to a conslderable background and the diffuse nature of the lines. They were necessary because strong fines from the cubic (111). (220) and (311) lattice planes all coincided with diffraction from hexagonal lattice planes. Thus the.proportio~s oi cubic c&&tls present were calculated by comparing thgratio of the intensities of the limes cor~s~n~mg to the hexagonal (100) plane and to the combined hexagonal (002) and cubic (111) line, with the calculated ratio for the intensities of diffraction from the hexagonal (100) plane and the hexagonal (002) plane alone. Fresh and aged cadmium sulphate was examined by light microscopy at a magnification of X275. For electron microscopy, samples of sulphide were dusted onto a formvar (polyvinyl formal) sup porting film, shadowed with palladium and examined with a Philips 11980 “Metaltex” electron microscope. Magnifications from x 2,OOOto x 60,000 were used.

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AND

DISCUSSION

The values obtained for the soIubility product of cadmium sulphide in the various media are given in Table I. Both the ap arent values (KJ and the vahtes corrected for comp?exing, efc (ksO)are included. They confirm the E. dmgs of numerous other workers that the apparent solubility product of cadmium sulphide is markedly dependent on the acidic medium in which the sulphide is produced. However, the corrected pKsO values from the precipitation and aging experiments agree well for the The value obtained polarograthree acids, with the exception of sulphuric acid at high concentration. phically for hydrochloric acids (pK,, = 27.8) when recalculated with the ks value used here for TABLE L-DETERMINATION OF Ksa FOR CdS BY PRECIPITATION AND AGING EXPBRIMENTS

K-*1,

Medium

10-&M

fH,Sl, IO-‘M

KS, lO-asmole%.I.-a

ICd”+l, lO+M

pKs0

od Cubic CdS

3.8 77 230

27.4 n-2 26.1

n.d. n.d. n.d.

27.5

10

27.5

24

27.3

93

28.0

n.d.

271

21

26.5 27.1

55 n.d.

Precipitation HCI, @7M HClOa, 2.1M HBSOP, 3.2M

89

7.7 41

3-6 51 66

520 7.2 6.5

Aging, fresh CdS HCl, 0*22M HClO,, 0*92M H,SO,, 0.36M

0.05 O*lO @04 0.03 0.12 0.10

600 210 360 350 82 74

64 44 1‘7 ;:‘6

0.09 0.08

95 93

0.032 0.028 0.9 0.7 ;;

6.9

0,017 0034 0.4 0.3 ;:;

Aging, commercfaI CdS

HCIO&, 0-92&f

0.09 O-07

310 500

18 16 3.4 3.9

H,SO,, 0*36M 0*22M

0.17 0.09

167 64

22 11

HCl, 0*22M

AI1 expen’ments at 25”C, except for the precipitation at 20°C. n d.-not determined.

experiments

in HCl and HCIO& which were

hydrogen sulphide is pI& = 26.9. The authors attempted to calculate an activity coe&ient for cadmium, obtaining a value of 04-O-53. Our assumption that j&+ = 1 would give a m~imum error of about O-2 in pK,,. The values obtained by aging freshly prepared precipitates are in good agreement for all the media (pK,, = 27.4 f O*l), and experiments with the commercial precipitates gave similar, but less precise results. Thus,it would seem likely that the true solubility product is 26.8~ pKsD < 275. After taking account of the approximate nature of the activity coefficients in the media of high ionic strength, the accuracy of this detestation can be no better than pK#,, = 27.3 -& 1. That pK,, values agree with products derived from free energy calculations confirms, to some extent, the estimation of Seeds used in the latterJB It is noteworthy that fong “incubation” times were sometimes required for precipitates to appear. The nucleation of cadmium sulphide crystals was evidently a difficult process, for precipitation from solutions with an ion product approaching the solubility product could take days and, occasionally, 2 or 3 weeks. The X-ray diffmction patterns of the aged sulphides were all much clearer than those of the fresh precipitates, and were spotted, showing an increase in crystallinity during aging. Table I shows that the formation of cubic cadmium sulphide is promoted by precipitation and aging in sulphuric acid; the formation of the hexagonal form is promoted by hydrochIoric acid, in agreement with eariier reports,” and by perchloric acid. It would also appear from Table I that the soIub%ty of cubic cadmium sulphide is slightly greater than that of the hexagonal form. However, this difference should only be small because the free energies of formation of both types of crystal are very similar, and both crystals have very similar molecular volumes (cubic, 0.0493 nma; hexagonal 0*0502 nmsf.18 22

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1092

Tn agreement with Milligan17 and Mtilier and Liiffler ,l” the colour of the precipitates, which varied from yellow to orange-red, does not depend on the crystal form present. The precipitates appeared as agglomerates, 10pm across, with a certain amount of smaller matter. A sample of cadmium sulphideprecipitated very slowly (over 3 weeks) from acidified cadmium chloride solution also consisted of agglomerates, but these were 50 times as big. This precipitate was red, and individual grains were visible to the naked eye. Electron microscopy confirmed that all the samples were agglomerates, and although crystal faces were readily seen at the highest magnification, ~dividuai crystals were rare. After aging, the average size of the agglomerates from sulphuric acid was 4 ,um, and 2 pm from the other media. The maximum diameter was 20 pm, but only occasionally was a single crystal of this size observed. Although quantitive measurements were impractical, the effect of aging was apparently to produce a more uniform size of agglomerate: the smaller particles were still present, whilst the larger aggregates seemed to have diminished. In spite of the increased crystallinity of the sulphide on aging, it seemed unlikely that Ostwald ripening had occurred extensively, and more probable that cementation and agglomeration as described by Kolthoff*O had taken place. ~c~~o~~e~ge~e~~-The authors thank Dr. J. E. B. Randles for his interest and advice, Dr. R. W. H. Small for assistance with the X-ray me~urements and Dr. R. Lehrie for aid with the electron microscopy. J. P. G. F. thanks the D.S.I.R. for the award of a maintenance grant, Chemistry Department The University P.O. Box 363, Birmingham 15, U.K.

R. BELCHER A. TOWNSEEND

Department of Industrial Metalhqy The University, Birmingham 15.

J. P. G. FARR

Summary--The solubility product of Casio sulphide has been measured in three acidic media by pr~ipitation and dissolution techniques. The values of log KS0 after correction for complex formation are -27.3 &- 0.6 in ail the media examined. An X-ray and microscopic examination of the precipitates shows an increase in crystallinity on aging, although Ostwald ripening was not observed. ZusammenfassunS-Das Liislichkeitsprodukt von Cadmiumsulfid wurde in drei sauren Medien durch FIillune und Wiederauflosune gemessen. Die Werte von log I& betrugei nach Korrektur fit Komplexbiidung in alien untersuchten Medien -27,3 & 0,6. R&tgen- und mikroskopi~he Unt~suchung zeigen ein Anwachsen des Kristallinitltsgrades beim Altern, obwohl keine Ostwald-Reifung beobachtet wurde. R&n&--On a measure le produit de solubilite du sulfure de cadmium dans trois milieux acides par des techniques de precipitation et dissolution. Les valeurs de K,, aprbs correction pour la formation de Un examen complexe sont -27,3 & 0,6 dans tous les miiieux &dies. aux rayons X et par microscopic des precipites montre un accroissement de cristallinit~ au vieillissement, bien que le murissement d’Ostwald n’ait pas Cte observe. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.

W. H. Waggoner, J. Chem. Educ., 1958,35,339. L. Bruner and J. Zawadzki, Z. Anorg. Chem., 1910,65,136. W. Manchot, G. Grass1 and A. Schneeberger, Z. Anat. Chem., 1925/26,67, 177. L. Dede and T. Becker, 2. Anorg. Chem., 1926,1S2,185. H. Kat&, Sci. Rept., TGhoku Univ., Ser. I, 1939-40,28,544. P. Kivalo and A. Ringbom, &omen Kemistilehti, 1956,29B, 109. R. Belcher and J. P. G. Farr, Tal~ta, 1959,2,95. A. I. Vogel, A Textbook of ~aantitative Znorganic Analysis, 2nd Ed., p. 355. Loners, 1951. 9. B. E. Saltzman, Anal. Chem., 1953,25,493. 10. T. A. Tumanova, K. P. Mishenko and F. E. Flis, Zh. Neorgan. Khim., 1957,2,1990. 11. I. Leden, Z. Phys. Chem. Leipzig, 1941,188,160.

London,

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12. R. A. Robinson and R. H. Stokes, Electrolyte Solutions, 2nd Ed., pp. 425, 432. Butterworths, London, 1959. 13. L. G. Sillen and A. E. Martell, Stability Constants of Metal-Zon Complexes, Chemical Society, London, 1964. 14. R. A. Robinson and R. H. Stokes, Trans. Faraday Sot., 1949,45,612. 15. I. Leden, Acta Chem. &and., 1952,6,971. 16. K. K. Kelley, U.S. Bur. Mines Bull., No. 406 (1937). 17. W. 0. Mill&n, J. Phys. Chem., 1934, 38, 797. 18. P. P. Ewald and C. Hermann. Structurbericht. DV. 77. 79. 1913-1926. 19. W. J. Mtiller and G. Lbffler, hngew. Chem., lkJ?k3,46, 538. 20. I. M. Kolthoff, Analyst, 1952, 77, 1000.

Talanta, 1969, Vol. 16, pp. 1093 to 1098. Pergamon Press. Printed in Northern Ireland

Some aspects of “chemical” interferences in atomic-absorption spectroscopy (Received 3 December 1968. Accepted 11 February 1969) “CHEMICAL” interferences in emission flame photometry have been reported in the literature,l-s and Alkemade4 has made a systematic classification of such interferences. “Chemical” interferences in atomic-absorption spectroscopy have been reported by numerous workers.6-11 Theoretical treatment of chemical interferences has been rather meagre, largely because of the difficulty of accurately observing experimentally the passage of aerosols through a flame; the necessary experimental set-up for such observation has recently been published, however.12 Studies on sensitivity in the atomic-absorption spectroscopy of nickel, tin, titanium, zirconium, hafnium, niobium, and tantalum, as metallocenes or fluoro-complexes, and as simple salts or oxysalts,‘* have shown that the metal-oxygen bonded species in solution (such as are present in the case of simple salts or oxy-salts or complexes with ligands having oxygen as donor) contribute to the total amount of metal oxide in flames, and hence, to the depopulation of the ground electronic state of atoms in flames, thereby reducing the sensitivity for these elements. In the case of metals which form oxides of high dissociation energy, sensitivity is enhanced if the metals in solution are not bonded to oxygen, as in metallocenes or fluoro-complexes. This paper is an extension of the earlier studies,i8 and was undertaken in order to determine the role of mixed metal polynuclear oxygen-bonded species in solution in depressing the absorption signal of the metal to be determined.

THEORY Some of the chemical interferences reported in the literature, e.g., the suppression of magnesium absorbance by aluminium,B are examples of the formation of mixed oxides in solution. An excellent article has been published on mixed metal oxides by Ward,14 with special emphasis on the structural features of these compounds. In aqueous solution metal ions are co-ordinated to water, and in the case of metal ions with high ionic potentials, it is reasonable to expect that hydrolysed species such as MO. will be predominant in aqueous solution at low acidities. In addition to this, the hydrolysed species might polymerize to form polymeric cationic hydroxy species, as in the case of Ti, Zr, Hf, Al, and Fe.16 In solution, the tendency of metal ions to co-ordinate oxide ions, which is determined largely by the relative sizes of the cations and anions, is responsible for the formation of mixed oxides. In the determination of a metal (Ml) in the presence of another (M,), added as an oxy-salt, a situation may arise in which Ml, in solution, is surrounded by MS, and a loose -M,-0-Ml-O---MIframework is formed, which after dehydration leaves a similar framework in the solid state. Such a three-dimensional network of strong bonds extending throughout a crystal is likely to make it involatile, and if the metal-oxygen bond energy is high, few free metal atoms are likely to be formed in a low-temperature flame, since this inevitably involves breaking of the metal-oxygen bonds. EXPERIMENTAL Reagents Analytical-reagent grade iron chloride, chromium(II1) chloride, strontium chloride, barium chloride, magnesium chloride, calcium chloride, potassium titanyl oxalate, zirconyl chloride, hafnyl chloride, cyclopentadienyl compounds of iron, titanium, zirconium and hafnium; chemically pure acids, freshly distilled organic solvents and doubly-distilled water.