The reaction of chromate with diphenylcarbazide. I

VOL. 10 (1954)

THE

ANALYTICA

REACTION

CHIMICA ACTA

OF CHROMATE

WITH

MONISHA

J_k!/wvtrueut

J>lrysical

201

DIPHENYLCARBAZIDE.

I

l3OSE

Chcrnist~y,

College 9

of

&

Tcchtology,

(Imiia)

‘I’hc colour reaction between chromate and diphenylcarbazide in acid medium is the best method yet devised for the detection and calorimetric estimation of very small amounts of chromium. CAZENEUVE~, the pioneer in this field, was of opinion that the intense coloration was clue to an organo-metallic chromium compound. He obtained the compound in the form of a violet amorphous powder but its composition varied with the conditions of preparation. Later l'vlou~1~2 made a more systematic attcmpr at the isolation of the coloured compound but without any ultimate success. The soluble red-violet compound has not yet been isolated in the pure state and FEIGL~ in his book on spot tests has simply mentioned that it is a soluble violet compound of unknown composition. Later he assumes “a heteropolyacid may be formed between chromic acid and diphenyl that carbazidc*” and that the coloration is due to this polyacid. Hc admits, however, that experimental proof for such a formulation is lacking. BABICO AND PALING have very recently reported that the violet coloration when extracted with amyl alcohol from an aqueous solution, contains no chromium and they have attributed the colour to an ordinary oxidation product of carbazide without any complex formation. According to them, the action of chromate is a case Such specificity in a simple oxidation reaction in the of ‘specific oxidation’. absence of complex formation is rather difficult to understand. BABKO’S work, therefore, does not appear tomake any further headway in clarifying the mechanism of this classical reaction. Thus, the course of the chromate carbazide reaction is yet to be explained. The present work is an attempt to eluciclate the mechanism of the reaction in solution from different physicochemical points of view. A very striking feature of the chromate-carbazide reaction is that the chromium must be present in its hexavalent state and the reaction medium should be sufficiently acidic (o&V is the optimum acidity). When rhe solution is alkaline, diphenylcarbazide does never react wirh chromate. This indicates that the reaction since other oxidising agents like nitrate, is primarily a redox one. However, Refc~ewccs

+. 208.

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202

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(1954)

permanganate, ceric salts, iodine, etc. fail to produce the characteristic violet colour with carbazide, it appears probable that the colour produced with chromate is due to some secondary complex formation. Oxidation, of course, plays a primary role in the complete process leading to colour formation. Diphenylcarbazide when oxidised with chromate may give rise to carbazonc or to its still higher oxidation product carbadiazonc or the carbazidc moIeculc’ itself may be complctcly ruptured at C=O linkage The chromate molecule, on the other hand, will bc simultaneously reduced during the oxidation process and chromium in a lower state of valence is expected, which may act as a cation in further complex formation. This is an important factor, since both carbazide and cnrbazonc (but not carbadiazone) arc known” to be typical agents which can combine with a large number of metal ions to produce intcnscly coloured inner complex salts. Reduction of chromate (C@) in acid medium lcads gcncrally to the tcrvalent Crek3 stage. Uut chromic chromium is known to be non-reactive towards either carbazide or carbazone. The obvious conclusion is that Cr in the bivalcnt state (Cr+2) is the ion responsible for the colour reaction. With these premonitions, the cffcct of chromous and chromate salts on carbazide and carbazone were studied separately to determine the causative agents really responsible for the colouration produced, EXPERIMENTAL

Reagents

and ntnteriuls

The reagents used wcrc either of ‘AnalaR grade or were prcparcd and purified in the laboratory. Fresh solutions of the reagents were always employed. The solutions of diphcnylcarbazidc and diphcnylcarbazonc wcrc mixed separately with chromate and chromous salts in acid medium (o.2N). The results are given in Table I . In case of chromous solutions, it is preferable to work in absence of air though tbc reaction dots take place to some extent cvcn if air bc not cxcludcd. TAl3I.E - .--

---

C:oloz~* reaction ---_--

-___-

I

o/ llbe tliffrve~lt syslc111.5 -.. -.----_--__-____-_--

._.

_..___ _

same red violet -C carbazide coloration* (2) + carbazoue -t_ carbazonc 3) no coloration + carbazicle I 4) _- -._.--~__ --_-----_ ----__I *Excess of chromate in case of (I) and (2) and excess of chromous in case of (3) discharge the colour. (I)

Chromate Chromate Chromous Chromous

It will bc found from the above table that Cr+z in reaction (4) fails to produce any coloration, whereas all the other three reactions give rise to similar red violet colour. Though colour itself has long been considered as being characteristic 12cfevonces p.

208.

VOL.

10

(X9++)

RI3i\CTION

OF

CIIROMATE

WITH

DIPHENYLCARBAZIDIS

I

a03

of a definite molecular species, it is prudent not to rely on visual observations alone. So the recognised method of comparing the characteristic absorption spectra was applied for the identification of the compound responsible for the red-violet colour produced in all the three reactions (I), (2) and (3) already mentioned. The optical measurements of these systems wcrc performed with a Beckman cores cells. l’hc tungsten Quartz Spectrophotometer Model DU using I-cm lamp was ernplo~~l as the light source. The results arc shown in Fig. (I).

400

425

GO

475

500

525

550

575

wovelenpth

Fig.

1. Abs;rption

-

spectra of three Chromate-carbazlclc Chromate-carbazone Chromous-carbazonc

600 625 In mp

differcnt

systems

DISCUSSION

All the curves are found to be of similar nature and show a maximum absorption at 540 mp. The spectral results thus indicate that the coloured reaction products in the three systems are identical. It seems probable, therefore, that the oxidation of carbazide by chromate products carbazone which reacts with the Cr+2 produced by the reduction of chromate itself giving the characteristic colour. In case of the chromate-carbazone system, it is reasonable to assume that the chromate oxidises carbazone to its higher oxidation product and the chromous Rcfercncrs

p.

208.

204

M.

BOSE

VOL.

10

(1954)

ion, thus formed, reacts with ~XCCSSof carbazonc present in the medium. Thus, both carbazone and carbazidc may be employed for the reaction with chromate. Carbazidc, however, stems to be more sensitive than carbazone as has also been observed by KRUMIIOLZ ‘. l3ut that carbazonc is the organic molecule involved in the colour formation is borne out from the fact that it is the only common constituent present or produced as a result of reaction in the 3 different systems. In system (3) the formation of carbazide by the reduction of carbazonc molecule with Cr+z ions may bc visualised but the indcpendcnt action of Crf2 ion on carbnzidc (free from carbnzonc) in system (4) disproves such a formulation. On the contrary, cxccss of Cr+2 in system (3) has been found to cause the discharge of colour initially produced. This can only bc cxplaincd as being due to the reduction of carbazone %.ocarbazide thus dcstroyirrg the colour forming capacity. Morcovcr, reaction (z) cannot by any stretch of imagination be considered to product the carbazide molecule. Hcncc, the only conclusion which seems logical is that the components responsible for colour formation is carbazone on the one hand and Cr+2 ions on the other. The reaction may be cxprcsscd as ,/N (-cl

2

+

=I

NC&I,

Crje2 carbazonc complex

CO

‘\NH-NHC,I-I, cliphcnylcnrbazone

1

-J- 21-I +

The discharge of colour wllen csccss of chromate is added may 1.~ cxplaincd as due to the complcie oxidation of the carbazonc involved in the combination.

PRESENCE

OI; A

SINGLE

COLOURED

COMPLEX

‘l’hc reaction systems (I), (2) and (3) have been shown to contain the same colourccl complex. It is still ncccssary to establish unequivocally that the reaction product is a single colourcd complex. This is done by the m&hod of Vosuuncx AND COOPERS who showed that, in the case of a mixture of several coloured complexes, the absorption spectra of solutions containing the reactants in different molar proportions in which they were cxpccted to react, would not show a constant wavelength of m‘aximum absorption. In the present cast, therefore, mixtures of chromate-carbazide solutions were prepared in the molar ratios of I:I, I :2 and I :3 keeping the chromium concentration constant and the spectral curves were determined with a Lumetron Photoelectric Colorimeter Model 402 El?. The results are indicated in Fig. 2. In all the cases, the wavelength of maximum absorption remains unchanged, thereby excluding the possibility of formation of several complexes. Rcfcrcnces

9.

208.

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10

(1954)

REACTION

OF

CHROMATE

WIT11

DIPHENYLCARBAZIDE

1

205

0.7 0.6 05 0.4 0.3

0.2 0.7

COO

&?S

450

475

500

525

575

550

Wavelength

600 625

in rnp

mixtures containing the reactants Fig. 2. Absor tion spectra of chromate-carbaziclc in cl1 *Pfercnt molar proportions (Cd-0 concentration remaining constant = 2*x0-5M) Cr+e : carbazidc = I : I *: Cd0 : carbaziclc = I : 3 TIl Cr+O : cwbazitlc r : 3 NATURE

OF

TIIE

COME’LEX

FORhfED

CAZE~;EUVE’S original idea that the colourcd reaction product is an organometallic chromium complex thus seems to be vindicated. Nevertheless, the above studies are rather insufficient to throw any light on the exact nature of the complex formed. The coloured Cr’+e -complex may either bc a monovalent cation or an inner complex salt. The alternative formulations, however, envisage a great divergence in the properties of the complex and hence with a view to

Refevewces

p.

208.

M.

206

BOSE

VOL.

10 (1954)

or not, decide, once for all, the nature of the complex, whether electrolytic migration studies as well as solubility determinations in non-polar solvents were carried out.

The apparatus cmploycd was similar to that used by DUVAL~. Since concentrated solutions are prcfcrable for transport determinations, almost saturated solutions of cnrbazidc in alcohol was mixed with aqueous acidified chromate ancl this mixture was subjcctcd to migration stucly. o.xN H,SO, was employed as the indifferent clectrolyta for the passage of clcctric current through the solution. Even after 4 hours of electric current, the red colour failed to move within the limbs of the apparatus showing that the compound is non-clcctrolytic in nature. Solivbility

sldics

The solubility 01 a compound in various non-polar solvcntsusuallyinclicntcsits cletcrmination of the inner complex naturelo. In the prcscnt case, solubility compound in the solid state was not possible and hence extraction from aqueous solution by various non-polar solvents was tnkcn rccoursc to for ascertaining the soluhility of the compound in the corresponding solvent. CAZIINEUVI‘:~ was the first to study the extractability of the compound. Accordbenzene failed to extract the colourccl compound, nmyl ing I.0 him, though alcohol was successful in this rcspcct. Very recently, I~ERN~CARDTPC~rL.‘l, in their attempt to estimate minute traces of chromium in uranium, succcccled in cxtracting the compound quantitatively with cyclohexanol and qz-hcxanol. In the present work, the rcagcnt carbaziclc dissolved in different non-polar solvents was acldcd to an acidifiecl chromic acid solution and the mixture was shaken in a stoppcrcd test tube and allowed Lo scttlc. It was rather peculiar to find that the nnturc of the acicl cmploycd to acidify the reaction misturc had a profound effect on the partition of the coloured product between the 2 phases. In organic acid (RcOT-I) medium and with solvents C,& and CT-ICI,, the coloured product was founcl to distribute itself bctwccn the 2 phases, the distribution being greatly in favour of the organic solvent, and with cyclohcsanol, the aqueous layer was completely colourless. Howcvcr, when mineral acicls such as sulphuric acid was used the organic layer was almost colourless except in the case of cyclohcxanol. Even then, the colour in the cyclohcsnnol lnycr is very mnch less in contrast to that produced in organic acid mcdiutn. CONCLUSION

‘The tmnsport cspcriment is non-electrolytic I~CfL’)‘(‘1ICCS20s.

unequivocally

prows

that the coloured compound solvents is also indi-

VOL.

10

REACTION

(1954)

OF

CHROMATE

WITH

DIPHENYLCARBAZIDE

I

207

cative of its inner-complex nature. Moreover, the production of the intense colour by the action of chromate or chromous ion on the respective reagents may also be regarded as due to the formation of an inner-complex compound12. Thus, the reaction of carbazide or carbazone with chromium may be considered to be in conformity with their well-known property of forming inner-complex salts with other bivalent metals.

The author’s Chemistry for arc also tluc to b’c~ticlns.

thanks arc due to Prof I>. I3. Sarkar, l-Icad of the Dcpnrtmcnt of his lcccn lntcrcst and for providing laboratory facilities. l’hanlts I3r. T% C. I-1ALrJAR. lately of this clci)artmcnt, for his helpful sug-

The mcchanlsm of the classical colour reaction between chromate and carbnzidc has IJeen studied in solution from various physicocl~cmical points of VICW. rt has been establishctl from spectral identity that the same red-viol& compound is producccl in tlic 3 cliffcrcnt rcactlon systems. chromate-carba2ziclc, chromatc-carbazonc ant1 chronlous-carbns?onc. ‘I’h~s leads to the conclusion that the chrumatecarbazitlc reaction is a composite (Jnc involving prclirninary oxidation followed subscqucntly b>- coniplcx f(,rmation. Morcovcr, the actual constituents involved in the rCaCti(Jll have been idcntificcl iis Cr+2 011 the one had and carbazonc con the other, giving rise to an intcnscly colourcd inner CcJInplex CcmlpcJll~~d. The inner com}llcx ll:LtUre CJf the CdOUrcd ]~rOdilct \ViIS gauged i~~dC~~CllClcntly from mi&ratiOrl studies and from cStraCtl(Jn of tlic colourcd compouncl by non-polar solvents.

1-e iil&anisme dc la ftiaction dc COltJratioli Cl;iSSiClUC, cntrc ~111chromate ct unc carbaxiclc, a 6t6 dtucliC par diffdrcnts proc&l&s yhysicv-cl~llniclucs. Xl a dt6 dGn~ontr& chromate-carba~idc. c]uC Ic ni&iic composd rouge vlolct cst furmd dans lcs 3 reactions: chromate-carl~a;?one et chromc( 1 I)-carba~.one. Ccci pcrmct clc conclurc quc la rdaCtbl chronlatc-carbazidc cansistc d’abord cn une oxydation suivie clc la formation du de Cr+a complcxe. D’autrc part, le complcxc obtcnu par ccttc r&action cst fork et de carbazonc, c’est uii complcxc lntcrne furtcmcnt CcJlord. La nature de cornplCxC intcrne du procluit form& a dt& dtablic indtipcndamnlcnt par des dtucles dC! migration et par extraction dLi COlilpOd color& par ties solvants non polnircs. %USAMrvIENPASSUKG Der NTcchanisnlu~ dcr klassischcn Farbrealction zwischcn Chromat und CarbaAd in %sungcn wurdc von vcrschicclencn physikalisch-chcmischcn Gcsichtspunlctcn aus untersucht. Durch chc spektrale fdcntltat wurdc festgcstcllt, class die gleichc rotviolcttc Vcrbindung in clrei vcrschicdencn l
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208

M.

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13OSE

10 (1cJgq)

REFERENCES P. CAZl3NIZUVII, Corit/~l. YEW?., 13 I (‘900) 346; &r/C. .sOC. Cibilri., (3) 23 (rgoo) 70 1 ; 25 (rgor) 7Gr. a A. MOULIN, Bull. sot. dim., 31 (1904) 295. 0 I?. I?EIGL, Spol Tcsls, Elsevier, hmsterclam, I 954. Vol. I, p. 153. 4 Sdcctivc und Smtsilivc J?euclro9ts, Academic I?. FEIGL, Chnislvy of Spcczfic, Press Inc., New York, N. Y., (1949) p. 2.4 I. II A. Ii. BA~ICO ANL) L. A. PALI?, %h?~. /Inad. I
IP

k?*k. 1&‘~7~~r~x_~. AND M. CALVIN, Chcruislvy Prentice-Hall, Inc., 1952, Sec. 2.5.

of

litc

klctal

Reccivecl

c/bclolc October

Col~Lfmr?Lds.

rqth,

I 953