N-Substituted phenothiazines as redox indicators in bromatometry

0039-9140/79/0301-0233102.00/0

7-ufunru. VoL 26. pp. 233-235 8 Pergamon Press Ltd 1979. Printed in Great Britain

AS REDOX N-SUBSTITUTED PHENOTHIAZINES INDICATORS IN BROMATOMETRY H. SANKE GOWDA and S. AKHEEL AHMED Department of Post-graduate Studies and Research in Chemistry, University of Mysore, Manasa Gangotri, Mysore, India (Received 13 December

1977. Revised 21 February

1978. Accepted

5 July

1978)

Summary-Diethazine hydrochloride, butaperazine dimaleate, trifluoperazine hydrochloride, promethazine hydrochloride, prochlorperazine maleate and chlorpromazine hydrochloride have been studied as indicators in bromate titration of quinol. metol and ascorbic acid. They give a very sharp reversible colour change at the equivalence point. Their formal potentials have been determined. A simple but accurate method for the estimation of quinol and metol is reported.

About ten reversible organic redox indicators have been proposed for bromate titrations, although numerous organic dyestuffs have been used as irreversible bromometric indicators. Most of the proposed reversible indicators are unsatisfactory for one reason or another. c+Naphthoflavone’ forms a brownish precipitate with free bromine liberated at the end-point. p-Ethoxychrysoidine’ gives a variable blank and its reversal cannot be repeated more than twice. Quinoline Yellow3 gives a very faint colour change and the blank is rather high. Ferroin,4 which requires high temperature (4&50”) and catalysts (osmic acid or sodium vanadate), gives sluggish endpoints. Fuchsine5 acts reversibly only at boiling temperature. The colour of 2,6-dichlorophenolindophenol6 at the end-point is immediately destroyed by the addition of a drop of bromate. We have now found the optimum conditions for the direct titration of quinol, metol and ascorbic acid with potassium bromate, using diethazine hydrochloride (DH), butaperazine dimaleate (BPDM), trifluoperazine hydrochloride (TFP), promethazine hydrochloride (PH), prochlorperazine maleate (PCPM) and chlorpromazine hydrochloride (CPH) as reversible redox indicators.

Determination

of formal

potentials

Ten-ml portions of the potentiopoised vanadate-vanadyl solutions were mixed with 0.5 ml of indicator solution, and the average of the potentials of the two solutions bracketing the colour change was taken as the formal redox potential. Schilt’s method” was also used. The results are presented in Table 1. Titration procedures Quinol and metol. Take 20ml of 0.05401 M quinol, lOm1 of %eo/, potassium bromide solution and enough hydrochloric or sulphuric acid to give a concentration of 0.6 M (0.3 M sulphuric acid for CPH) at the end-point, and dilute to 40 ml. Titrate with 0.025-0.005 M potassium bromate to an orange-red, using 1 ml of 0.2% DH, BPDM, TFP, PH. PCPM or CPH added near the end-point (after 95% titration). For titration of 0.005-0.0025 M quinol, dilute 1Oml of sample, the acid and 3 ml of 10% potassium bromide solution to 25 ml and titrate with 0.00250.00125M potassium bromate, adding 0.5 ml of the indicator towards the end-point. In the titration of metal, use 1M sulphuric or 0.5M hydrochloric acid medium and 1 ml of 0.2% DH, BPDM, TFP or PH near the end-point. Ascorbic &id. Tai% ?i, til of 0.054025 M ascorb;ic acid, 10 ml of 10% potassium bromide solution and enough acid to give a concentration of 1.5 M hydrochloric acid(0.8 M for PCPM and CPH) or 1 M sulnhuric acid at the endpoint, and titrate as,ihe mixture fbr quinol.

EXPERIMENTAL RESULTS AND DISCUSSION Reagents Indicator

solutions. All reaRents were analytical-trade chemicals. Aqueous solutions 10.2%w/v) of CH, BFDM, TFP, PH. PCPM and CPH were DreDared and stored in amber bottles. BPDM and PCPM’ were dissolved in hot water (60”). Potentiopoised solutions. Equimolar vanadate-vanadyl potentiopoised solutions in 0.0062540 M sulphuric acid were prepared.‘.’ Reductants. Approximately 0.05 M solutions of quinol, metol and ascorbic acid were prepared by dissolving the requisite quantity of material in 0.04 M and 1 M sulphuric acid and 0.01% EDTA solution respectively. The quinol and met01 solutions were standardized with ceric sulphateg and the ascorbic acid with potassium iodate.” The solutions were stored in amber bottles and diluted as required. T&I.. 26/3--o

DH, TFP, PH, PCPM and CPH are highly soluble in water, giving colourless solutions. BPDM gives a light yellow solution in water. The aqueous solutions are stable for about 3 days at room temperature (27”) and for about 2 months in an amber bottle at low temperature (8”); they slowly undergo photochemical oxidation to give a pale pink colour, but this does not interfere in their indicator action. They undergo one-electron reversible oxidation to a red intermediate which is believed to be a radical cation.12 _me radical cation is further oxidized irreversibly by excess of oxidant to a colourless sulphoxide, with the. loss of one electron.13*14 The mechanism of oxidation 233

H. SANRE GOWDA and S. AKHEEL AHMED

234

of DH can be represented as follows:

The mechanism of oxidation of BPDM. TFP, PH. PCPM and CPH is similar. The formal potential of PCPM has already been reported to be 0.795 V,r5 that of CPH 0.799 V,16 of PH 0.883 V,” and of DH 0.856 v.s Titrations

Quinol. Kolthoff’s reported that the end-point of the potentiometric titration of quinol with potassium bromate is not sharp because of formation of the addition compound of quinone with the bromine liberated at the equivalence point. Francis and HillI proposed adding excess of bromate-bromide and determining the unreacted bromate iodometrically. No attempts have previously been made to use indicators for titrating quinol directly with bromate. We have found that DH, BPDM. TFP. PH PCPM and CPH can be used for this purpose. All six give no colour change if the hydrochloric acid concentration is below OSM; the useful acidity ranges are 0.51.5M for BPDM and TFP, OS-l.OM for PH and DH and 0.5-0.8 M for PCPM and CPH. At higher acidities the results are too high. The colour change is from light yellow to orange-red in the titration of 0.025-0.05M quinol. The end-point colour is stable for about 90 sec. The end-points are brighter and sharper in the titration of 0.0025-0.005N quinol. The end-point colour (pink) is stable for about 10min. At higher acidities under-titration occurs. BPDM, TFP, PH and DH do not function at sulphuric acid concentrations less than 0.5M. CPH does not give a colour change at acidities below 0.25M sulphuric acid. BPDM and TFP give sharp end-points in 0.551.75M, PH in 0.5-0.75M, DH in OS-l.OM and CPH in 0.2550.4M sulphuric acid. Results are too high at higher acidities. The colour change for 0.025-0.05M quinol is from light yellow to orange-red which is stable for about 70 set and Table

I. Determination of formal potentials (rnY) BPDM, TFP, PH. DH. PCPM and CPH Potentiopoised method

of

Schilt’s method

CWO,lM Indicator 0.075 0.20

0.25

0.5

0.75

1.0

1.25

1.50

BPDM TFP PH DH PCPM CPH

881 893 883 856 807 799

865 881 877 845 799’ 788

838 870 870 834 785 760

828 863 862 825 771 753

812 854 852 810 758 732

793 836 842 797 748 721

\

900 900 900

921 -

the indicator correction is almost negligible. All five indicators give very sharp pink end-points which are stable for about 8 min in the titration of 0.0025XUlO5M quinol. The results compare favourably with those obtained potentiometrically with cerium(IV) sulphate. The minimum potassium bromide concentration required in the titration of 0.025-0.05M quinol is w 1.5%, and 1% for titration of 0.0025-0.005M quinol. Higher concentrations (up to 16%) do not affect the results. At least 1.0 ml of 0.2% indicator solution is necessary in the titration of 0.02~0.05M quinol but >2.5 ml gives premature end-points. In the titration of 0.0025-0.005M quinol 0.5 ml of 0.2% indicator solution is required; > 1.5 ml gives higher titration values. The sharpness of the end-points is in the order BPDM > TFP > DH > PH > CPH > PCPM. Merol. Metol has not previously been estimated with bromate. Its accurate estimation is of importance because of its application in photography. The methods using iodine,” iodine monochloride,” cerium(IV) sulphate’ and sodium vanadate” are not quite satisfactory. The estimation with bromate, using DH. BPDM, TFP and PH as reversible redox indicators, is simple but accurate. Metol is quantitatively oxidized by bromate to N-methyl-p-quinonimine in a two-electron change in hydrochloric or sulphuric acid medium containing bromide. Stoichiometric results are obtained in sulphuric acid medium ranging from 0.75 to 1.5M, with sharp end-point changes from light yellow to an orange-red which is stable for about 70 set in the titration of 0.02550.05M metol. Low results are obtained at higher acidity and the indicators do not function at lower acidities. All four indicators give no colour change at acidities less than 0.4M hydrochloric acid. BPDM and TFP give sharp colour change from light yellow to orange-red in 0.4-0.8M hydrochloric acid and PH and DH in 0.40.6M hydrochloric acid. The endpoint colour is stable for about 90 sec. At higher acidities premature end-points are obtained. All four indicators give very sharp colour change from very light yellow to pink in the titration of 0.0025-0.005M metol. The end-point colour is stable for 2-3 min in hydrochloric or sulphuric acid medium. The indicator correction is almost negligible in the titration of 0.025005 M metol. The results compare favourably with potentiometric values

Phenothiazines Table 2. Titration of quinol, metol and ascorbic acid in presence of DH. BPDM. TFP, PH. PCPM and CPH indicators Reductant taken, ‘Y

Reductant found.* my

Standard deviation, mg

111.3 85.5 55.4 10.43 5.58 2.23

111.4 85.5 55.4 10.45 5.59 2.24

0.09 0.08 0.07 0.07 0.07 0.07

157.2 120.2 40.5 15.12 12.02 4.42 Ascorbic acid 166.3 122.4 88.5 40.6

157.3 120.2 40.5 15.14 12.04 4.43

0.05 0.08 0.09 0.09 0.05 0.01

166.5 122.5 88.5 40.6

0.05 0.01 0.01 0.01

Quinol

Metol

* Average of five determinations.

obtained with cerium(IV) sulphate. The influence of the bromide and indicator concentration is similar to that for titration of quinol. The sharpness of the end-points is in the order BPDM > PH > TFP > DH. Ascorbic acid. pEthoxychrysoidine2 and 2,6-dichlorophenolindopheno16 are the only reversible organic indicators recommended for the titration of ascorbic acid with bromate. DH, BPDM, TFP, PH, PCPM and CPH are all suitable as redox indicators in this titration, giving a sharp reversible colour change from colourless to pink (orange-red for TFP). The end-point colour is more stable in hydrochloric than sulphuric acid medium. BPDM, TFP, PH and DH give no colour change in < 1M hydrochloric acid and PCPM and CPH in
as redox indicators

235

The indicator correction is almost negligible. The effect of indicators and bromide concentration is similar to that for quinol. The sharpness of the end-points is in the order TFP > BPDM > DH > PH > CPH > PCPM in hydrochloric acid medium and DH = BPDM > TFP > PH > CPH in sulphuric acid medium. Oxalic, citric, tartaric, succinic, acetic and malic. acids, glucose, fructose, sucrose, starch and acetone do not interfere in the determination of a tenth of their amount of ascorbic acid. The results compare favourably with those obtained by Ballentine.” Comparison

with other indicators

All six indicators have advantages over p-ethoxychrysoidine in that they.give sharper and brighter end-points which can be reversed several times. They are superior to 2,6-dichlorophenolindophenol because (i) they give sharper and brighter end-points, (ii) the end-point colour is more stable and (iii) a slight excess of bromate after the equivalence point does not destroy the colour. In all the bromate titrations of quinol, metol and ascorbic acid the results are accurate to + 0.2% (Table 2). REFERENCES 1. R. Uzel. Collecrion Czech. Chem. Commun., 1935, 7, 380.

2. E. Schulek, J. Kovacs and P. Rozsa, Z. Anal. Chem., 1941, 121, 17.

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7. G. F. Smith and W. M. Banick, Jr., Talanta. 1959. 2, 348.

8. H. S. Gowda and S. A. Ahmed, Indian J. Chem. 1977, 15A, 907.

9. T. P. Sastri and G. G. Rao, Z. Anal. Chem.. 1958. 163, 263. 10. R. Ballentine, Ind. Eng. Chem. Anal. Ed., 1941, 13, 89. 11. A. A. Schilt, Anal. Chem., 1963, 35, 1599. 12. P. C. Dwivedi, K. G. Rao, S. N. Bhat and C. N. R. Rao, Spectrochim. Acta, 1975, 31A. 129. 13. I. S. Forrest, F. M. Forrest and M. Berger, Biochim. Biophvs. Acta. 1958. 29. 441.

14. D. j. Cavanaugh, Science, 1957, 125, 1040. 15. H. S. Gowda and R. Shakunthala, Indian J. Chem.. 1976, 14A. 431. 16. H. S. Gowda and S. A. Ahmed, J. Indian Chem. SOC., 1977, 55, 352. 17. Idem, Z. Anal. Chem., 1976, UIl, 301. 18. 1. M. Kolthoff, Rec. Trao. Chim., 1926, 45, 745. 19. A. W. Francis and A. J. Hill, J. Am. Chem. Sot., 1924, 46, 2498. 20. H. L. Baumbach, J. Sot. Motion Picture Eny. 1939, 33,_517. 21. J. Cihalik and D. Vavrejnovl, Chem. Listy, 1955, 49, 1176. 22. G. G. Rao and T. P. Sastri, Z. Anal. Chem., 1956, 151,

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