Studies on complexes of tartaric acid—I

,I. inorg, ntt~l. C h e m . , 1972, Vol. 34, pp. 335 •-3356.

STUDIES

Pergamon Press.

Printed in Great Britain

ON COMPLEXES

OF T A R T A R I C

ANTIMONY(III)-TARTARIC

ACID-I

ACID SYSTEM

R. K R I S H N A IYER, S. G. D E S H P A N D E and G. S. RAO Chemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay-85, India

(Received 1 July 1971) Abstract-Published data on potassium antimony(Ill) tartrate, known as tartar emetic, are contradictory. The formation of the complex and its behaviour have been reexamined by potentiometric, ion exchange, thermogravimetric and i.r. studies. The results suggest the formulation:

coo.

-1

/

Ikcoo.o___su_1I INTRODUCTION

DATA on potassium antimony(Ill) tartrate, known as tartar emetic, are contradictory [ 1-9]. From a study of the i.r. spectra Girard and Lecomte [4, 5] concluded that there is no unionized COOH group. However, Grdenic and Kamenar [8], by X-ray diffraction methods, have proposed a structure in which one COOH group is free. Kiosse et al. [7] considered racemic potassium antimony (I I I) tartrate to be a monohydrate while Grdenic and Kamenar[8] showed that the complex is a hemihydrate. In view of these discrepancies, it was considered worthwhile to reexamine this system. Thermogravimetric analysis and infrared spectra were used in the solid state while pH titration, polarimetry and ion exchange were adopted for studies in solution. Samples of the complex prepared in the laboratory, as well as obtained from B.D.H. Ltd., England, were used for these studies. EXPERIMENTAL` Preparation of potassium antimony(Ill) tartrate A modification of the published preparation[10] was used. A solution of 18.5g of potassium hydrogen tartrate in 400 ml of water was heated to boiling and - 29 g of Sb20~ was added little by little with stirring. The mixture was boiled for 10 min more and filtered to remove the unreacted H. Reihlen and E. Hegel, Ann. 487, 213 (193 I). K. C. Samantora, D. V. Raman Rao and S. Pani, J. Ind. Chem. Soc. 32, 165 (1955). Ch. B. Nanda and S. Pani, J. Ind. Chem. Soc. 32, 525 (1955). J. L,ecomte and M. Girard, Compt. rend. 240, 415 (1955). J. Lecomte and M. Girard, Compt. rend. 241,292 (1955). E. Chinoporos and N. Papathanasopoulos, J. phys. Chem. 65, 1643 ( 1961 ). G. A. Kiosse, N. 1. Goiovastikov and N. V. Belov, Kristallogr. 9, 402 (1964); Dokl. Akad. Nauk SSSR 155, 545 (1964). 8. D. Grdenic and B. Kamenar, Acta Crystallogr. 19, 197 (1965). 9. A. K. Banerji and K. V. R. Chaff, J. inorg, nucl. Chem. 31, 2958 (1969). 10. Kirk and Othmer, Encyclopaedia of Chemical Technology, Vol. II, 2rid Ed., p. 578. John Wiley, New York (1963). 1. 2. 3. 4. 5. 6. 7.

3351

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R. K R I S H N A IYER, S. G. D E S H P A N D E and G. S. R A O

oxide. The filtrate on concentration and cooling gave crystals of the complex. The yield was - 95%. The complex was purified by recrystallization from water. Pure antimony trichloride was dissolved in 2.5 N hydrochloric acid to yield an - 0-5 M solution. All other reagents used were of analytical reagent grade.

Analysis The antimony content of the solid complex, as well as of the antimony trichloride solution, were determined by an iodimetric method[11]. The potassium content of the solid complex was determined gravimetrically as K2SO4 in the filtrate, after precipitating antimony as sulphide in an acidified solution and destroying the tartrate by a mixture of sulphuric acid and nitric acids. Water content was determined by Karl-Fischer titration [ 11 ].

Determination of ionic charge An aqueous solution of a known weight of the complex ( - 0.02 M) was passed through a column of a strongly basic anion exchanger (Ionenaustauscher III) in the chloride form and the effluent was collected. The column was washed with water till the effluent was free from chloride. The chloride content of the effluent was determined gravimetrically as silver chloride. The number of moles of silver chloride per mole of the complex gives the average charge on the complex anion (assuming that the species do not change on washing with water). The optical rotations of the solutions were measured using a Perkin-Elmer Model 141 polarimeter. pH measurements were carried out at a temperature of 25 ---+I°C with a Beckman Model G pH meter using glass electrode and calomel reference electrode. The ionic strength of the solutions was maintained at 0.2 M by adding suitable amounts of sodium perchlorate. Thermograms were obtained on a Stanton self-recording thermobalance of 1 nag sensitivity at a heating rate of 4°C/min, using 400 nag of sample ( - 2 0 0 mesh). The i.r. spectra were recorded with a Perkin-Elmer Model 21 recording spectrophotometer equipped with sodium chloride prism using KBr pellet as well as the mull techniques. RESULTS AND DISCUSSION

Analytical data for different samples of the complex are given in Table 1. On drying over anhydrous calcium chloride, the complex has the composition C4H206SbK. H20. The potassium hydrogen tartrate used for preparing the complex had a specific rotation o f + 2.5 while the complex isolated in the laboratory as well as the B.D.H. sample had a specific rotation o f + 14.0. Thermograms for the complexes in air are shown in Fig. 1. It may be seen that Table 1. Analysis of samples of potassium antimony(III) tartrate

Sample

Molecular Sb(%) K(%) H~O(%) composition Observed Calculated Observed Calculated Observed Calculated

Freshly prepared crystals of the complex K S b T . - 1½H20 Sample dried over anhydrous CaC12 K S b T . H20 Sample obtained from B D H K S b T . H20

36.6

36-46

11"75

11.71

7.92

8-09

37.3

37.50

12"10

12.00

5.30

5.55

37.2

37.50

11.90

12.00

5" 18

5.55

11. A. I. Vogel, A Text Book of Quantitative Inorganic Analysis. The English Language Book Society and Longmans Green, London (1961).

Studies on complexes of tartaric acid - I

3353

water is lost only above 160°C and the complex becomes anhydrous at about 200°C. These results confirm those reported earlier by Lecomte and Girard [4, 5]. Decomposition of the complex occurs above 265°C. Analysis of the product obtained at the second step (-330°C) shows that it is a mixture of antimony trioxide, potassium carbonate and carbon. The gain in weight observed at 370°C is due to oxidation of Sb203. The weight loss shows that the final product has the probable composition Sb203½KzCO3. I

IOmg

½ I I00

i .

I 300

~"- Temperc]ture

i

l 50O

J

.(IC

Fig. 1. Thermograms in air. (!) KSbT. H20 [prepared by us); (2) KSbT. H~O (BDH). Initial weight of the sample.= 400 mg.

The i.r. spectrum of both samples of the complex obtained using the potassium bromide disc method shows two peaks at 1680 cm -1 and 1635 cm-' in the 1600-1700cm -1 region, which may be attributed to the antisymmetric C O O stretching. The peaks observed at 1380 cm -1 and 1350 cm -1 may be assigned to the symmetric C O O stretching. The spectrum of the anhydrous complex in a Nujol mull shows a broad peak at 1615 cm-L A peak of medium intensity at about 1725 cm -1 observed in the spectrum of potassium hydrogen tartrate is absent in that of the complex, neither does the complex show bands at 1195 and 800 cm-', which are observed in the spectrum of tartaric acid. The latter have been reported to be due to the bending vibrations of the O H bonds of the carboxyl groups in the free acid[12]. These results indicate that there is no unionized C O O H group in the complex. The frequency separation between the antisymmetric and symmetric COO stretching frequencies is > 250 cm -1 indicating that the metal-oxygen bonds 12. A. N. Ermakov, I. N. Marov and L. P. Kazanskii, Russ. J. inorg. Chem. 12, 1437 (I 967).

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R. K R I S H N A IYER, S. G. D E S H P A N D E and G. S. RAO

have a high degree of covalency. Similar observations have been reported for complexes of antimony(Ill) with aminopolycarboxylic acids [ 13]. With regard to the location of the bands due to O H bending vibrations different views have been reported. The bands at 1450 cm -1 and 1415, 1262 and 1000 cm -1 were attributed to the O H bending vibrations by Kirschner and Kiesling [ 14] and Ermakov et al. [ 12], respectively. It has been shown [ 15] that these bands become weaker or disappear in the spectrum of partly deuterated potassium tartrate. The spectrum of potassium antimony(Ill) tartrate does not show any peak either around 1000cm -1 or in the region of 1415-1450cm -1. From these results, it may be concluded that both the secondary alcoholic groups of tartaric acid take part in complex formation and are deprotonated. Similar observations have been reported in the case of the zirconium tartrate complex[12] where both the hydroxyl groups are coordinated to zirconium with the liberation of protons. The pH titration curves for (i) tartaric acid (H4T), (ii) potassium antimony(Ill) tartrate (KSbT), (iii) 1 : 2 mixture of KSbT and HCIO4, and (iv) 1 : 1.1 and 1 : 2 mixtures of antimony trichloride solution and tartaric acid are given in Fig. 2. The starting point in the titration curve for KSbT (pH - 4.1) is a point of inflection. Precipitation was observed at m ~ 0.2 (m = number of moles of alkali added per mole of metal ion). The titration curve for the 1 : 1.1 mixture of SbCI3

"r

Q.

r

2/

t rn

S

(number of moles of

olkoli

= per

mole of

I I 6 Sb or H4T)

Fig. 2. pH Titration curves. (1) Tartaric acid (H4T), 0.01 M; (2) potassium antimony(III) tartrate (KSbT. H20); (3) K S b T . H 2 0 + H C I O a (1:2); (4) SbCla+H4T (I : l ' l ) ; (5) SbCI3+H4T (1:2); (6) K S b T . H 2 0 + H C i (1:6) (value of m in this case = 4 + value shown in the figure); (7) K S b T . H20 + NaC1 (1 : 10). Value of CM for curves (2)-(7) = 0-01 M. 13. R. Krishna Iyer, Ph.D Thesis, University of Kerala, 1968. 14. S. Kirschner and R. Kiesling, J.Am. chem. Soc. 82, 4174 (1960). 15. Yu. Ya. Kharitonov and Z. M. Alikhanova, Radiokhimiya 6, 702 (1964).

Studies on complexes of tartaric acid - I

335 5

and H4T shows a single inflection at m = 4-2. Deducting 0-2 mole of alkali consumed by the 10% excess of tartaric acid, the number o f protons liberated as a result of complex formation is 4. In the case of the 1 : 2 mixture, the curve shows a single inflection at m ~ 6.0. Subtracting the 2 mole of alkali consumed by the excess tartaric acid, the number of moles of alkali consumed per mole of Sb in the formation of the complex is still 4-0. Precipitation was observed at p H > 8.0. T o determine the influence o f chloride ions, p H titrations of potassium antimo n y ( I l l ) tartrate with alkali were carried out in presence of hydrochloric acid as well as sodium chloride. T h e s e titration curves are shown in Fig. 2. Precipitation was observed on addition of hydrochloric acid. H o w e v e r , the solution became clear when the p H rose to 2 on addition of sodium hydroxide. It may be seen in Fig. 2 that there is a slight shift in the p H to lower values (above p H 4.5) due to the presence o f chloride, but the alkali consumption at the inflection point remains unaltered. Thus the number o f protons liberated as a result of complex formation is not affected by the presence of chloride ions in the solution. T h e formation of complexes with the following structural formulae can result in the liberation of four protons.

coo

rcoo

~ H O H~Sb:==O

[~HO

COO "Sb--OH HO

CHOH

COO /

HO.

Na

I

COONa 1

Sb--OH

OONa

HO

Sb

1.o/

lOONa

III II

IV

O f these four structures, structure I is improbable since acetic acid does not form a stable complex with Sb(III). If the complex has structure II which contains a free C O O H group, the p H of a 0.0! M solution of the complex should be - 3.5 (pK2 for tartaric acid = 4.14) or < 3.5. (In general, the p K value for the liberation of proton from the ligand is lowered as a result of complex formation.) The observed p H is 4.1 and on addition of N a O H there is a sharp rise in pH. If there is a free C O O H group in the complex, an inflection at m = 1 is expected in the titration curve, which is not observed. On the contrary, precipitation sets in at m ~ 0-2. N a n d a and Pani [3] have shown that addition of sodium hydroxide to an aqueous solution of the complex in the mole ratio 1 : ! removes all the antimony from the solution as hydrated oxide. T h e charge on the complex was found to be - I _-+0.02 by anion exchange experiments. I f the complex has structure II, the C O O H group should dissociate and as a result o f this, a value between - 1 and - 2 should be obtained for the average charge. Also, the effluent obtained on passing the solution through the anion exchanger would be e x p e c t e d to be acidic. T h e effluent was neutral indicating that no protons are liberated. Reihlen and Hegel [ 1] explained the acidity of tartar emetic on the basis of the

3356

R. KRISHNA IYER, S. G. DESHPANDE and G. S. RAO

equilibrium COO

COO

(i)

oox If this is the case, the average charge on the complex should again be between 1 and - 2. The fact that the charge is -- 1 suggests that such a hydrolytic reaction does not occur. A solution of potassium antimony(III) tartrate does not react with cupric ions [6]. With tartaric acid Sb(III) forms only the 1:1 complex even with an excess of tartaric acid. The reaction of Sb203 with potassium hydrogen tartrate results in the 1 : 1 complex irrespective of the proportion employed. With lactic and oxalic acids [16, 17] formation of 1:2 complexes has been reported. If the complex has structure III one wolald expect antimony to form 2: 1 and 1 : 2 complexes with tartaric acid. Although the pK values for the liberation of protons from the O H groups in tartaric acid are high[18], it has been shown that in the case of aluminium [ 19] and zirconium [ 12] complexes with tartaric acid the protons from both the alcoholic groups are liberated during complex formation. These observations lend support to the possibility of both the alcoholic oxygens being coordinated to antimony as shown in structure IV. -

A c k n o w l e d g e m e n t s - T h e authors express their thanks to Dr. J. Shankar, Head of Chemistry Division, Bhabha Atomic Research Centre, for his interest in the work and to Drs. M. D. Karkhanavala and V. V. Deshpande for thermogravimetric facilities.

16. 17. 18. 19.

B. C. Mahanty and S. Pani, J. Ind. chem. Soc. 31,593 (1954). A. C. Nanda and S. Pani, J. Ind. chem. Soc. 31,588 (1954). M. Beck, B. Csiszar and P. Szarvas, Nature 188, 846 (1960). A. V. Pavilinova, J. Gen. Chem., USSR 17, 3 (1947); Cf. C.A. 42, 53e (1948).