Talanta, Vol. 32, No. 1, pp. 5456, 1985 0039-9140/85 Printed in Great Bntain $3.00 + 0.00 Pergamon Press Ltd 2-[2-(5-CHLOROPYRIDYL)AZO]-5-DIMETHYLAMINOPHENOL AS INDICATOR FOR THE COMPLEXOMETRIC DETERMINATION OF ZINC E. MARCHEVSKY, R. OLSINA and C. MARONE Universidad National de San Luis, Chacabuco y Pedemera, 5700-San Luis, Argentina (Received 13 October 1983. Revised 4 June 1984. Accepted 16 July 1984) Summary-2-[2-(5-Chloropyridyl)azo]-5-dime~hylaminophenol (CIDMPAP) is proposed as a metallochromic indicator for zinc. The end-point colour change is from the violet-red of the zinc complex to the brownish-yellow of the indicator. The colour contrast is markedly greater than that for Erio-T. Eriochrome Black T (Erio-T) is often used as a metallochromic indicator for complexometric determination of zinc,le3 but has some important deficiencies. Its solutions are unstable, and copper(II), cobalt(II), nickel(I1) and aluminium(II1) can interfere by blocking the indicator. Iron(II1) and titanium(IV) also interfere. Cadmium(I1) is cotitrated. Use of PANb6 and PAR’ has therefore been recommended. Other pyridylazo reagents used in zinc Ammonia-ammonium chloride buffer, ca. 0.1 M, pH 8.5. Ammomum chloride (4.55 g) was dissolved in a small volume of water and concentrated ammonia solution was added dropwise until pH 8.5 was reached. Final pH adjustment was made after dilution to nearly 1000 ml. Aqueous Erzochrome Black T solution, ca. 0.05%. Prepared immediately before use. All other reagents, chemicals and standards were of analytical grade. Apparatus titrations include 5-(2-pyridylazo)-2-monoethylamino-p-cresol,’ 2-(2-pyridylazo)-4-methylphenol, 5-(2-pyridylazo)-p-cresol,‘” and 2-[2-(5-bromo-2-pyridylazo)]-5-diethylamino-m-phenol.9 All have in common a high colour contrast between the zinc complex and the free reagent, from violet-red to brownish-yellow, with a change of about 100 nm in the wavelength of the absorption maximum. The molar absorptivities (E) are also high, cu. 1 x 10e5 l.mole-‘.cm-‘. 2-[2-(5-Chloropyridyl)azo]-5-dimethylaminophenol (CIDMPAP) is known” to react instantly with zinc in aqueous solutions buffered at pH 8.2-10.4, producing an intensely violet-red complex (E is much greater than that for the zinc complexes listed above). The complex is stable for at least 5 days. The reagent is readily displaced from its zinc complex by EDTA and thus should be suitable as an indicator for EDTA titration of zinc. EXPERIMENTAL Reagents Spectrophotometric titrations were done in a cell constructed according to Sweetser and Bricker.14 and the titrant was added from a Metrohm piston-burette. Take a 50-ml sample, make it slightly alkaline and finally adjust to pH 8.5 by adding 5 ml of ammonia-ammonium chloride buffer (the buffer capacity of the NH:-NH, solution was enough to maintain this pH in the titration of 50 ml of zinc solution of concentration up to about 800 ppm). Add 60 ml of alcohol and 2 or 3 drops of indicator. Titrate with EDTA added at ca. 0.1 ml/min near the end-point. Typical results are summarized in Table 1. Analysis of zmc insulin In a 25-30 ml centrifuge tube, take 10.0 ml of the liquid sample. Add dilute hvdrochloric acid (1 + 50) until a clear solution is obtained, -and 6 ml of loo/, trichl&oacetic acid solution. Shake, then centrifuge for 5 min. Transfer the supematant liquid to a 25-ml standard flask and dilute to the mark with water. Transfer 20 ml of the solution to the photometric titration cell and add 5 ml of ammonmm chloride-ammonia buffer (pH 8.6), enough ethanol to give a final content of about 50% v/v, and 3 drops of SClDMPAP indicator, and titrate with EDTA. Table 1. Changes in absorbance during the EDTA titration of zinc, with Erio-T and SCIDMPAP as indicators Standard zinc so&ion (0. IOOOM). Prepared by d!ssolving the pure metal (6.537 g) with ca. 20 ml of hydrochloric acid (1 + 1) by gentle warming, cooling and dilution to volume in a standard flask with water. EDTA solution, ca. 0. I M. Standardized potentiometrically with the standard zinc solution’2 and further diluted as required. 5CIDMPAP. The reagent was synthesized according to Johnson and Florence.‘j A 0.05% solution in 95% ethanol was used as indicator. For spectrometric studies, solutions of the required molarity were prepared with 95% ethanol as solvent Error, y0 Zn taken. rnz 0.813 3.250 8.14 16.28 40.69 54 5CIDMPAP I.0 0.06 0.03 0.02 0.006 Erio-T 1.5 0.1 0.1 0.08 0.09 SHORT 55 COMMUNICATIONS RESULTS AND DISCUSSION 12 The spectral differences between Erio-T and SCIDMPAP and their zinc complexes are shown in Fig. 1. SCIDMPAP is clearly a very much more sensitive reagent, and the shift in the wavelength of maximum absorption on complexation is twice as great for SCIDMPAP as for Erio-T, and gives a much bigger contrast in the colour change. The human eye is also more sensitive to the colour change for the SQDMPAP system. Graphical methods”-” were used to choose the optimum pH for the titration. The equilibrium constants required for construction of the graphs were obtained from the literature’* or determined by us.j9 Results obtained by the method of Reiiley and Schmid” are shown in Fig. 2. It can be seen that the pH range 5.5-9 is the most suitable; this was experimentally verified (Fig. 3). The curves in Fig. 4 correspond to spectrometric titrations with Fig. 3. Photometric titration: Effect of pH on the end-point. pH: (1) 4.5; (2) 5.6; (3) 7.2; (4) 8.5; (5) 9.0; (6) 10.8; (7) 12. I 0 400 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 500 600 X fnm ) Fig. I. Spectral curves of SCIDMPAP and Erio-T and their zinc complexes, C,,, = 2.0 x WSM; C,, = 6.2 x 10-4M; (1) Erio-T; (2) Erio T-Zn complex; (3) SCIDMPAP; (4) 5ClDMPAP-Zn complex. Fie. 4. Absorbances at 1,, of the zinc complex, in photom&ic. titration of zinc. [SCIDMPAP] = (1) 5 x 10-6M; (2) zyxwvu 1 x iO-sM; (3) 2 x 10-5M; [Erio-T] = (4). 6 x fO-‘M. 5CIDMPAP. Curves for Erio-T are included for comparison. Both indicators give the same location of the end-point, but the colour change is much sharper with SCIDMPAP than with Erio-T. This is clearly shown by the change in absorbance (AA) on addition of 0.0%ml increments of EDTA in the vicinity of the end-point (Table 2). Table 2. Comparison of results obtained with SCIDMPAP and Erio-T in EDTA titration of zinc 5ClDMPAP Titration, “/, Fig. 2. The pZn data required for the method of Reilley and Schmid? (1) for the system Zn plus NH:-NH, buffer; (2) for 50% transformation of the indicator; (3) for the system with EDTA added in iOOu/,excess. 97.2 98.5 99.7 101.1 102.3 103.5 A,, 0.686 0.655 0.574 0.090 0.088 0.087 Erio-T AA A (m AA 0.03 1 0.081 0.484 0.002 0.001 0.961 0.935 0.869 0.753 0.650 0.647 0.026 0.066 0.116 0.103 0.003 SHORT COMMUNICA’IIONS 56 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Effect of ethanol content and ionic strength Erio-T has the advantage that it can be used in purely aqueous solution, whereas 5ClDMPAP (which is insoluble in water) must be used in aqueous ethanolic medium. Hence the effect of the ethanol concentration was tested over the range 20-60% v/v. With ~40% ethanol the reaction is slow, more time is required to stabilize the absorbance, and the endpoint is sluggish. These effects are basically due to the low solubility of the zinc-SCIDMPAP complex. With 40-60x ethanol present these difficulties are overcome, but the relatively high ethanol content required limits the concentration of inorganic salts in the solution. The effect of ionic strength was tested on zinc solutions in 50% v/v ethanol-water medium by addition of sodium perchlorate, nitrate, chloride, sulphate and acetate. The most serious limitation is imposed by sulphate. At concentrations >0.2M sodium sulphate is not completely soluble in this medium and the dispersed solid makes it difficult to see the colour change at the end-point. For the other sodium salts tested this effect does not arise. With Erio-T in purely aqueous medium, the only effect was a slight diminution in AA at the end-point. Effect of foreign ions The ions which interfere were known from previous work,” the most serious being cadmium, copper, nickel, iron and cobalt, which also interfere in the complexometric titration. Nickel, copper and iron may be effectively dealt with by the cyanide-formaldehyde method (Table 3) but cadmium and cobalt cannot. Sodium, potassium, magnesium, barium, and strontium do not interfere in at least 100: 1 w/w ratio to zinc. Calcium is tolerated at 25 : 1 ratio but iron and mercury are tolerated only in 5: 1 ratio to zinc. Tolerance limits for most common anions were determined when the effect of ionic strength was studied, and found to be dictated by the solubility of the sodium salts in the alcoholic media used. Lead interferes but can be dealt with by the method used to eliminate its interference when Erie-T is used as indicator.*’ Xylenol Orange shows an advantage over 5ClDMPAP in selectivity, mainly in the case of bismuth, thorium, scandium, lanthanum, lead, zinc Table 3. Determination of zmc in binary samples; interferents were masked by the cyanide-formaldehyde method Zinc, mg Interferent Nickel Copper Iron Taken Found 1.63 1.64 1.64 1.65 1.63 1.63 Ratio (w/w) of interferent to Zn 12.5 24.5 1.22 and cadmium,*’ because it is used at pH < 6 (SCIDMPAP can be used in the pH range 5.5-9, with an appropriate buffer). Applications Analysis of a zinc insulin preparation (Lilly) gave a mean of 0.156 mg/ml by the present method (standard deviation 0.002 mg/ml), 0.157 (s.d. 0.0013) by a reference method,** and 0.154 (s.d. 0.0010) by AAS, for six replicates. Synthetic brass samples corresponding to British Standards BCS 385 and BCS 344 were analysed by a modification of the recommended procedure (immediately after the pH was adjusted, 2 ml of 5% potassium cyanide solution followed by 4 ml of 5% formaldehyde solution were added). The zinc results were 38.4% (s.d. 0.02’%) for BCS 385 (certified value 38.5%) and 30.8% (s.d. 0.02%) for BCS 344 (certified value 31.0%) for six replicates in each case. Acknowledgements-The authors are gratefully Indebted to SUBCYT for financial assistance. and Prof. Dr. .I. A. Catoggio for helpful discussion. REFERENCES 1. W. Biedermann and G. Schwarzenbach, Chimia, 1948. 2, 1. 2. H. Flaschka, Mikrochem. Mikrochim. Acia, 1952, 39, 38. 3. E. G. Brown and T. J. Hayes, Anal. Chim. Acta, 1953, 9, 1. 4. H. Flaschka and H. Abdine, Mikrochim. Acta, 1956, 770. 5. K. L. Cheng, Anal. Chem., 1948, 30, 243. 6. I. Crigan and P. Tanase, Rev. Chim. Bucharest, 1968,19, 228. 7. M. HniliEkova and L. Sommer, Collection Czech. Chem. Commun., 1961, 26, 2189. 8. S. I. Gusev and L. M. Shchurova, Zh. Analit. Khim., 1971, 26, 1740. 9. G. Nakawada and H. Wada, Nippon Kagaku Zasshr, 1962, 83, 1098. 10. S. I. Gusev, E. M. Nikolaeva and E. A. Pirozhkova, Zh. Analit. Khim., 1971, 26, 1740. 11. E. Marchevsky, R. Olsina and C. Marone, Anal. Asoc. Quim. Argentina, 1984, 72, 35. 17 C. N. Reilley, R. W. Schmid and D. W. Lamson, Anal. Chem., 1958, 30, 953. 13. D. A. Johnson and T. M. Florence, Anal. Chim. Acta, 1971, 53, 73. 14. P. B. Sweetser and C. E. Bricker, Anal. Chem., 1953,25, 253. 15. C. N. Reilley and R. W. Schmid, ibid., 1959, 31, 887. 16. E. Still and A. Ringbom, Anal. Chim. Acta, 1965.33.50. 17. E. Still, Acta Acad. Aboensis, Math. Phys., 1966, 26, 6. 18. Intersci&ce, A. Ringborn, New Complexation Chemistrv. York, 1963.in Analvtical . I IL. 19. F. Ferretti, R. Olsina and C. Marone, Anal. Asoc. Quim. Argentina, 1978, 70, 501. 20. H. Flaschka, EDTA Titrations, 2nd Ed., p. 102. Pergamon Press, Oxford, 1967. 21. J. Kiirbl, R. PEbil and A. Emr, Collection Czechoslov. Chem. Commun., 1957, 22, 961. 22. U.S. Pharmacopeia, 1975, p. 634.