TABLE 1 - POLAROGRAPHIC BEHAVIOUR OF Cd-BTCOMPLEX

(ligand]M

,-Et

(V VS SeE)(kg)B X 10'

Matsuda KORYTA Gellings

0·010'020·04

4·9582·2261-170

4'4572·0541'019

0·5750·5920'614

4·2561·9711'007

A single well-defined reduction wave is obtainedin each case. The diffusion coefficient (D) ofCd2+ in the medium used is 6·8 X 1O-8cm2 sec'". Usingthis value for D in Ilkovic equation, the number ofelectrons, n, involved in the reduction process isfound out to be 2. The id VS h~~~rplot is linear,indicating the diffusion-controlled nature of thelimiting current.

The complexes Cd-ST and Cd-VT produce rever-sible waves for the series of ligand concentrations.The slopes of the 'log plots' for these complexesare 31 ± 3 mV, which are as per expectation for atwo-electron reversible electrode process. The logplots of Cd-BT complex are curved, indicatingquasi-reversible reduction.

The plots of EI/2 vs log C; were plotted forCd-ST and Cd-VT and Et vs log ex for Cd-BT.All the plots are linear and from their slopes it isinferred that Cd-ST forms 1: 1 complex while Cd-VTand Cd-BT form 1: 2 complexes. The formation of1:1 complex of Cd-ST may be due to OH- groupof ligand ST at ortho-position, because this structureshows the possibility of one more bond withmetal ion.

The overall electrode reaction rate constants ofthe Cd-BT complex have been determined (Table 1)using three different methods, viz. Matsuda-Ayabe+,Koryta 5 and Gellings". The details of the methodsare discussed in our previous work", The value ofnumber of ligands attached to the metal ionin the bulk of the solution (N); and the numberof ligands attached to the metal ion at theelectrode surface (p) are found to be 2 and 1respectively according to Matsuda and Ayabe treat-ment. The results of overall electrode reaction rateconstant as determined by Matsuda-Ayabe and bymodified Koryta and Gellings treatments' using thesame value of Nand p as determined by Matsuda-Ayabe are in fair agreement. Further the resultssuggest that the overall reaction rate constantdecreases with the increase in ligand concentration.This might be due to less positive character of thecomplex at the electrode surface at high ligandconcentration because of the lone pair on thedonor atom N of the ligand.

Cd-BT and Cd-VT form 1: 2 complexes; however,the results show that Cd-VT is more stable thanthe Cd-BT complex which might be due to highersubstitution on benzene ring in the case of VTresulting in higher electron density on donor Natom.

The stability constants of the complexes weredetermined by Lingane relation" in its original form

NOTES

except for Cd-BT where (Ei), of the Linganerelation was replaced by E; of the complex assuggested by Matsuda and Ayabe. The logarithmicstability constants for Cd-ST, Cd-VT and Cd-BTcomplexes are 2·72, 4·90 and 4·41 respectively andtheir free energy changes t:..G are -3·72, -6·70and -6,04 kcaljmole respectively.

The authors thank Saurashtra University, Rajkot,for the laboratory facilities and the CSIR, NewDelhi, for the award of junior research fellowshipsto two of them (M.S.P. and T.T.).

References

1. HEYROVSKY, J. & KUTA, J., Principles of polarography(Academic Press, New York), 1966, 63.

2. PATEL, M. S., TRIVEDI. T. & VVAS, D. x., ]. electrochem,Soc. India, 26 (1977), 21.

3. McBAIN, J. W. & Lru, T. H., J. Am. chem . Soc., 53 (1931),59.

4. MATSUDA, H. & AYABE, Y., Z. Elektrochem., 63 (1959).1164.

5. KORYTA, J., Electrochim . Acta, 6 (1962), 67.6. GELLINGS, P. J., Z. Etektrocbem., 66 (1962), 477.7. PATEL, M. S., TRIVEDI. T. & VYAS, D. N., Indian]. Chem.,

15A (1977), 1051.8. LINGANE, ]. J., Chem. Rev., 29 (1941). 1.

Quantitative Bromination of Some OrganicCompounds Having Activated Phenyl Rings

by Bromine Monochloride

1. C. SHUKLA. & RAVI PRAKASH

Department of Chemistry, University of AllahabadAllahabad 211002

Received 28 N ouember 1977; accepted lOA pril 1978

A simple method is described for the milligramdetermination of azo compounds containing activatedphenyl rings. The method involves quantitative bro-mination of the organic compounds by BrCI followedby the iodometric determination of excess BrCI. Theaccuracy of the method is ±O·5%.

QUANTITATIVE bromination of aromatic com-pounds having activated phenyl rings has

widely been studiedv+, In the present work a quickand convenient method has been developed for themilligram determination of azo dyes using BrCl.The sample is allowed to react with a calculatedexcess of BrCI in a freezing mixture, and the excessbromine is determined by iodometric titration, usingstarch indicator. The method gives fairly accurateand reproducible results with an accuracy of ± O· 5%.The method is superior to many other methods inoperation, sensitivity and accuracy.

All the reagents used were of AR grade. Brominemonochloride solution (0·05N) was prepared asfollows:

Potassium bromate (0'6958 g) and potassiumbromide (0·9917 g) were dissolved in distilled water{125 ml). The solution was cooled in ice, conc.hydrochloric acid (50 ml) added and the solutionmade up to the mark (500 ml) with distilled water.

725

INDIAN J. CHEM., VOL. 16A, AUGUST 1978

The results for the determination of four azocompounds are given in Table 1. The effect ofchange in reaction time, amount of glacial aceticacid and the concentration of BrCI was studiedand the stoichiometry of the reaction was establishedfor all the azo dyes. It was found that theconsumption of BrCI becomes constant after 5 min.in the case .of methyl red (4 moles) and methylorange (3 moles) and 10 min in the case ofchrysoidine (2 moles) and Congo red (2 moles),indicating the formation of corresponding bromoderivatives. In the case of chrysoidine the dibromoderivative is obtained by the bromination of substi-tuted phenyl ring, while the dibromo derivative ofCongo red is obtained by desulphonation. In thecase of methyl orange the tribromo derivative isobtained through bromination and desulphonationof the phenyl rings, while the tribromo product ofmethyl red is due to bromination and decarb-oxylation. The estimation of azobenzene has notbeen possible with the present method beca~seof the absence of a ring-activating group whichmay help the bromination of phenyl rings.

TABLE 1 - DETERMINATION OF Azo Dvss BY Brei

Amounttaken(mg)

Moles BrCIper mole of

azo compound

Error(%)

Amountrecovered

(mg)

CHRYSOIDINE

1·02002·04003·0600

1·03002·05203·0620

2 +0·98+0·58+0·65

CONGO RED

1·02402·04803·0nO

0·03002·05403·0800

2 +0·58+0·29+0·25

METHYL RED

1·01002·02003·0300

1·02002·03103·0400

4 +0·99+0·54+0·33

METHYL ORANGE

1·03002·06003·0900

1·04002·0nO3·1000

+0·97+0·58+0·32

3

The solution was standardized against O·02N sodiumthiosulphate solution.

A stock solution of each sample was preparedby dissolving an accurately weighed amount of thesample in distilled water or glacial acetic acid.All the samples were purified by recrystallizationand their purity checked.

Procedure - To an aliquot containing 1-5 mg ofthe sample was added glacial acetic acid (5 ml).Bromine mono chloride solution (10 ml) was addedto it and the contents shaken thoroughly. Theflask was stoppered, placed in an ice-salt mixtureand allowed to stand for 10 min with occasionalshaking. After the reaction was over the stopperwas washed with distilled water (5 mI) and potas-sium iodide solution (5 ml, 15%) added to it.Contents were shaken thoroughly and kept forone min. The liberated iodine was titrated withstandardized 0·02N sodium thiosulphate solutionusing starch (1% aq. solution) as an indicator.A blank experiment was also run under identicalconditions using all the reagents except thesample.

The amount of azo compound was determinedusing relation (1)

f I (B-A)xMxNmg 0 samp e = 2 X n

where A = ml of sodium thiosulphate solution usedfor sample, B = ml of sodium thiosulphate solutionused for blank, n = moles of bromine monochlorideequivalent to one mole of sample, M = mol. weight01 the sample, and N = normality of sodiumthiosulphate solution.

726

References

1· l\'[LODECKA, Jr, Rec. chim. Bucharest, 10 (1959), 343.2· DE REEDER, P. L., Anal. chim. Acta, 9 (1953), 314.3· PANDE, V.C. & GOPAL, ::'II., T'al anta, 23 (1976), 328·4· TIEWARI, R. D., SHARMA, J. P. & SHUKLA, T· C·, Talanta,

14 (1967), 853.

Phenolphthalin as a Redox Indicator for theDetermination of Hydrazine & Hydroxylamine

SALAH SHAHINE* & SAMIR EL-MEDANY

Faculty of Engineering, Ain Shams University, Abbassia,Cairo, Egypt

Received 15 December 1977; accepted 20 February 1978

Phenolphthafin (reduced form of phenolphthalein)has been succes sfully used as a redox indicator in thetitrimetric determination of hydroxylamine and hydra-zine with potassium ferricyanide.

PHENOLPHTHALEIN (I) is reduced to phenol-phthalin (II) by zinc dust in strong alkaline

medium. The colourless alkaline solution of IIslowly turns pink on standing in contact with airby oxidative regeneration of phenolphthalein.

The solution turns red immediately on addition ofoxidizing agents in alkaline medium. Thus, the

... (1) 6~-°-o~U(II)

colourless

reduction.'Oxidation

(1)

red

*Present address: Faculty of Science, Sana's University,Sana's, Yemen.