Analyst, January, 1967, Vol. 92, $$. 27-30 27 The Catechol Violet Colour Reaction for Tin(1V) Sensitised by Cetyltrimethylammonium Bromide BY R. M. DAGNALL, T. S. WEST AND P. YOUNG (Chemistry Department, Imperial College, London, S. W. 7) The sensitivity of the colour reaction between tin(1V) and catechol violet has been greatly increased by the sensitising action of cetyltrimethylammonium bromide ( E ~ ~ ~ ~ ~ = 95,600). The sensitisation is also accompanied by a considerable bathochromic shift (107 mp) of the wavelength of maximum absorption. Full colour development occurs within 5 minutes at pH 2.2, and Beer’s law is obeyed down to 0.2 p.p.m. of tin(1V). THE most widely used methods for the spectrophotometric determination of tin are based on reactions with dithiol (4-methyl-l,2-dimercaptobenzene) p 2 y3p4 y 5 and phenylfluorone (2,3,7-trihydroxy-9-pheny1-6-fluorone)l $6 in the presence of dispersing agents such as gelatin. Neither of these reagents has proved entirely satisfactory with respect to specificity and sensi- tivity, and the non-reproducibility of the dispersing agents adds further difficulties.Other spectrophotometric methods, based upon the reducing action of tin(II), include those with cacotheline,* p 9 and heteropoly molybdic acids1 Recently, Ross and Whitelo have described the use of catechol violet as a reagent for tin. These authors report the formation, at pH 2.5, of a red 2 : 1 complex of the reagent and tin(1V) with a high molecular extinction coefficient ( E ~ ~ ~ mp = 65,000). They also produced some evidence for the formation of a 1 : 2 complex (Amax.= 620 mp). Recently, MalAtll has also investigated the use of catechol violet as a reagent for tin and observed that in the presence of gelatin there was a considerable bathochromic shift in the absorption maximum of the complex from 555 to 640mp. He was apparently unable to obtain the sensitivity reported by Ross and White, even in the presence of gelatin, which has a sensitising effect on the reaction, and the results he reports correspond to a E~~~~~ value of 38,000 for the blue complex. These observations prompted us to investigate this phenomenon and to study the effects of other dispersing agents on the catechol violet - tin complex, as the use of gelatin leaves much to be desired. From these investigations, cetyltrimethylammonium bromide was found to produce a more pronounced bathochromic shift of the absorption maximum (555 mp to 662 mp), and to increase greatly the sensitivity of the system (E662mp = 95,600).EXPERIMENTAL REAGENTS- M-Dissolve 0.1187 g of tin metal (analytical-reagent grade) in 20 ml of concentrated sulphuric acid (analytical-reagent grade). After cooling, add the solution carefully to about 250ml of ice-cold water, and add a further 60ml of the sulphuric acid. Dilute to 1 litre. Tin(1V) solution, M-Prepare by appropriate dilution of M tin(1V) solution when required. Catechol violet, M-Dissolve 0.286 g of catechol violet (Hopkin and Williams Ltd.) in distilled water and dilute to 1 litre. Cetyltrimethylammonium bromide solution-Dissolve, with warming, 0.1 g of cetyltrime- thylammonium bromide (obtainable from British Drug Houses Ltd.) in 100 ml of distilled water.Tin(1V) solution, APPARATUS- Spectrophotometer-Beckman DB with 1-cm glass cuvettes.28 DAGNALL, WEST AND YOUNG: CATECHOL VIOLET [Analyst, Vol. 92 PREPARATION OF CALIBRATION GRAPH- Transfer by pipette aliquots increasing by 1 ml each time, from 1 to 10 ml, of 10-4 M tin(1V) solution and 5-ml aliquots of 1 0 - 3 ~ catechol violet solution into a series of 100-ml beakers. Add 2 ml of 0.1 per cent. cetyltrimethylammonium bromide solution and dilute each to about 50 ml. Adjust the pH of each solution to 2.2 with dilute ammonia solution by using a pH meter. Transfer the solutions to 100-ml calibrated flasks and dilute to the mark. After 10 minutes, measure the absorbances in 1-cm cuvettes against a reagent blank at 662 mp.A plot of absorbance against tin concentration is a straight line that passes through the origin. RESULTS AND DISCUSSION SPECTRAL CHARACTERISTICS- Fig. 1 curves A and B show the absorption spectra of catechol violet and its tin complex at pH 2.2, respectively, while curve C shows the effect of the addition of 0.002 per cent. of cetyltrimethylammonium bromide to the complex, as represented in curve B. I t will be noted that the effect is a pronounced one, and that the absorbance of the reagent is small at the wavelength of maximum absorption of the sensitised tin complex (662 mp), thus promoting conditions that are nearly ideal for analytical measurement. 0.2 - 400 500 600 700 Wavelength, my Fig.1. Absorption spectra of tin(1V) - catechol violet system; curve A, 5 ml of M catechol violet solution diluted to 100 ml a t pH 2.2, measured against distilled water: 1-cm cuvette; curve B, as A but with the addition of 5 ml of 10-4 M tin(1V) solution before dilution to 100 ml; and curve C, as B but with the further addition of 2 ml of 0.1 per cent. cetyltrimethyl- ammonium bromide solution OPTIMUM CONDITIONS FOR COLOUR DEVELOPMENT- Buffering experiments over the pH range of 0-5 to 4.0, cf. Fig. 2 curve A, revealed that maximum sensitivity was obtained at pH 2.2. This differs only slightly from the values quoted by the previous authors, vix., pH 2-311 and 2 ~ 5 . l ~ The curve reveals that the reaction is dependent on the pH, but not critically so.The maximum response was obtained with a 3-fold (molar) excess of catechol violet reagent relative to tin, cf. Fig. 2 curve B. The effect of using a greater concentration of reagent was to reduce the sensitivity of the reaction slightly, but this effect was insufficiently marked to place any practical restrictions on the proposed procedure. Variation of the cetyltrimethylammonium bromide concentration in the range of 0.001 to 0-006 per cent. revealed, Fig. 2 curve C, that the maximum absorbance was obtained at about 0.002 per cent., and that a further increase in concentration produced an effect similar to that produced by excess of reagent. The maximum absorbance of the complex was found to develop within 5 minutes and to remain constant for about 2 hours. After 2 hours a gradual decrease in absorbance was observed.January, 19671 COLOUR REACTION FOR TIN(IV) 29 L 1 2 3 4 5 6 0.001 0,002 0.003 0.004 0.005 0,006 1 2 3 4 5 6 PH Cetyltrirnethylammoniurn bromide, per cent.Reagent molar excess Fig. 2. Effects produced by variation of pH or of con- centrations of catechol violet or cetyltrimethylammonium bromide: curve A, 5 ml of M catechol violet and 2 ml of 0.1 per cent. cetyltrimethylammonium bromide solutions buffered at varying pH values, diluted to 100 ml; curve B, 5 ml of M tin(IV), 2 ml of 0.1 per cent. cetyltrimethylammonium bromide buffered a t pH 2.2 and varying excesses of catechol violet solution, diluted to 100 ml; and curve C, 5 ml of M tin(IV), 5 ml of 10-3 M catechol violet solution buffered at pH 2.2 and 1.0 to 6.0 ml of 0.1 per cent cetyltrimethylammonium bromide solution, diluted to 100 ml M tin(IV), 5 ml of With the established optimum conditions, a calibration graph over the range to 10-5 M tin concentration was prepared.Beer’s law was obeyed and a molecular extinction coefficient of 95,600 was obtained at 662mp. NATURE OF THE COMPLEX- A continuous variation plot measured at 662 mp exhibited a clear indication that a 2: 1 catechol violet - tin complex was formed. Measurements at higher and lower wave- lengths indicated that no other species were formed. A mole-ratio plot for the same system also indicated a 2: 1 complex. CONCLUSIONS The bathochromic shift in the absorption spectrum of the catechol violet - tin(1V) complex, which may be caused by short range electrostatic forces operating on the surface of the micelle double layers of the cetyltrimethylammonium bromide,12 provides the basis of a sensitive method of determining traces of tin(1V).The sensitivity of the method (E662mw = 95,000) compares favourably with that reported by Ross and White10 ( E ~ ~ ~ mw = 65,000), and, indeed, it will be seen from Fig. 2 curves A and B, that we have been unable to reproduce their results. The method also compares well with that reported by MalAtll (€640 mw w 38,000) who used gelatin and is much more sensitive than standard methods based on toluene-3,4-dithiol (€530 mv = 5800)13 and phenylfluorone (€520 mw m 4000), calcu- lated from the data of Bennett and Smith.14 The method is precise (with a percentage standard deviation of 0.6) and rapid.The possibility of combining the selective solvent extraction of tin(1V) iodide with the cetyltrimethylammonium bromide sensitised reaction was investigated. The results obtained showed that the small amounts of iodide or iodine, or both, remaining in solution caused precipitation of the ce t yltrimet h ylammonium bromide, thereby giving erroneous results. Although we have not examined the method further, it would appear that the com- bination of this absorptiometric finish with a preliminary separation, such as distillation of the volatile bromide,l or precipitation of the tin as metastannic acid (with the aid of a carrier, if necessary, if the amount is small),l would provide a sensitive method for the determination of tin in commercial products. We thank United Dairies Ltd., and the British Welding Research Association for financial aid in support of this work.30 DAGNALL, WEST AND YOUNG 1.2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. REFERENCES Sandell, E. B., “Colorimetric Determination of Traces of Metals,” Third Edition, Interscience Board, Y. W., and Elbourne, R. G. P., Analyst, 1964, 89, 555. Onishi, H., and Sandell, E. R., Analytica Chim. Acta, 1956, 14, 153. Clarke, R. E. D., Analyst, 1936, 61, 242. Gilbert, D. D., and Sandell, E. B., Microchem. J., 1960, 4, 491. Luke, C. L., Analyt. Chem., 1956, 28, 1276. Fritz, G., and Scheer, H., 2. anorg. allg. Chem., 1964, 331, 151. Newell, I. L., Ficklen, J. B., and Maxfield, L. S., I n d . Engng Chem. AHalyt. Edn, 1935, 7, 26. Ross, W. J., and White, J. C., Analyt. Chem., 1961, 33, 421 and 424. Mal&t, M., 2. analyt. Chem., 1962, 187, 404. Svoboda, V., and Chromy, V., Talanta, 1966, 13, 237. “Tables of Spectrophotometric Absorption Data of Compounds Used for the Colorimetric Deter- mination of Elements,” IUPAC Commission on Spectrochemical and Other Optical Procedures for Analysis, Butterworth & Co. (Publishers) Ltd., London, 1963. Received July 13th, 1966 Publishers Inc., New York, 1959. -, Ibid., 1937, 62, 661. Bennett, R. L., and Smith, H. A., Analyt. Chew., 1959, 31, 1441.