Mikrochimica Acta [Wien] 1978 II, 191--196 MIKROCHIMICA ACTA 9 by Springer-Verlag 1978 Department of Chemistry, Himachal Pradesh University, Simla-171005, India Thiourea as Reductimetric Standard: Standardisation of Cerium(IV), Dichromate, Bromate, Chloramine-T, and N-Bromosuccinimide Solutions By Balbir Chand Verma, K. Swaminathan, and Swatantar Kumar (Received March 28, 1977. Revised June 14, 1977) The use of thiourea for the standardisation of Ce(IV), potassium dichromate, potassium bromate, chloramine-t and N-bromosuccinimide solutions is reported in the present communication. Singh et al. 1 have studied the oxidimetric determination of thiourea and its organic derivatives with Ce(IV) 2, potassium bromate a, chloramine-t 4 and N-bromosuccinimide (unpublished work) and found that these oxidants oxidise these compounds rapidly and quantitatively in acidic medium in the presence of potassium iodide to the corresponding substituted formamidine disulphides through iodine liberated in situ. The reaction of thiourea with potassium bromate may be represented as: BrOa-+6 H++6 I- --+ Br-+3 H20+3 I2 (i) H2N\ HN//'C-SH+3 HeN\ /NH2 I2 --+3 HNjC-S -S-C-%H N +6H++6I - (H) The oxidation with other oxidants follow the same course as represented with bromate. The appearance and disappearance of iodine (reactions I and II respectively) can be visually detected in the course of titration. With the first drop of oxidant solution in excess, the liberated iodine forms the well known blue colour with starch. 13 Mikrochim. Acra 1978 II/3-4 0026-3672/78/7802/0191/$ 01.20
192 B. Ch. Verma et al.: In the absence of potassium iodide, potassium dichromate s and Ce(IV) 2,6 quantitatively oxidise thiourea (in acidic medium) to the corresponding disulphide: H2N\ H2N\ /NH~ 6 HN//C-SH+Cr~O72-+8H+~3 HN//C-S-S-CGN H + +2 Cr a+ +7 HzO (III) H2N\ H2N\ /NH2 2 HN//C-SH+2Ce4+--~ HN//C-S-S-C~N H + +2 Cea+ +2 H + (~v) These reactions permit the standardisation of potassium dichromate and Ce(IV) solutions against thiourea solution using redox indicators. Experimental Reagents Thiourea, 0.0100 N, was prepared by dissolving 761.2 rag of the compound (BDH), twice crystallised from alcohol, dried at 110 ~ in distilled water and making the volume to 1 1. Ammonium hexanitratocerate, approximately 0.01 N, was prepared by dissolving 5.46 g of the compound (BDH) in 2 N sulphuric acid and making the volume to 1 1 with the acid. Potassium dichromate, approximately 0.01N, was prepared by dissolving 460 rag of the compound (BDH) in water and making the volume to 11. Potassium bromate, approximately 0.01 N, was prepared by dissolving 280 mg of the compound (BDH) in water and making the volume to 1 1. Chloramine-T, approximately 0.01 N, was prepared by dissolving 1.40 g of the compound, prepared by the method described by Vogel 7, in water and making the volume to 1 1. N-Bromosuccinimide, approximately 0.01 N, was prepared by dissolving 900 mg of the recrystallised sample (dried in a vacuum desiccator) of the compound in water and making the volume to 1 1. Ferroin, 0.01 N, was prepared by dissolving 594 mg of o-phenanthroline monohydrate and 278 mg of analytical grade ferrous sulphate heptahydrate in 100 ml of water. Diphenylamine, 0.5% solution in concentrated sulphuric acid. Potassium iodide, 1% aqueous solution. Amylose, 1% aqueous solution.
Thiourea as Reductimetric Standard 193 Potassium iodate, 1.7 mm, was prepared by dissolving 363.8 mg of the compound (BDH, Analar) in water and making the volume to 11. Microburet, 10-ml capacity, graduated in 0.01 ml divisions. Procedure (a) Standardisation of ammonium hexanitratocerate, potassium bromate, chloramine-t and N-bromosuccinimide solutions: Amylose indicator method. A known volume (10--15 ml) of standard (0.0100N) thiourea solution was pipetted into a titration flask. Sufficient water (30--50ml) and enough of sulphuric acid (16--20 ml of ~4. N) were added to keep the normality of the solution at ~ 1 N and its volume at ~70 ml. The solution was mixed with 4.0 ml of 1% potassium iodide solution when standardising potassium bromate, chloramine-t and N-bromosuccinimide, and with 0.2 ml of 1% potassium iodide solution when standardising ammonium hexanitratocerate solution. The mixture was cooled to room temperature (~26 ~ in each case, mixed with amylose (0.2 ml) indicator, and titrated with each of the oxidant solution delivered from a microburet graduated in 0.01 ml divisions. The end-point was the appearance of permanent blue-colour. The normality of each solution was also determined by iodometric titration for comparison. (b) Standardisation o[ ammonium hexanitratocerate solution: Ferroin indicator method. A known volume (10--15 ml) of standard (0.0100 N) thiourea solution was pipetted into a titration flask. Sufficient water (5--10 ml) and enough of sulphuric acid (50--60 ml of,-~4 N) were added to keep the normality of the solution at ~3 N and its volume at ~70 ml. One drop of potassium iodate (1.7 ram) solution was added to catalyse the reaction. One drop of ferroin indicator was also added. The solution was cooled to room temperature (~26 ~ and titrated with ammonium hexanitratocerate solution. The end-point was marked by a sharp colour change from red to light blue. For comparison, the normality of the oxidant solution was also determined by titration against ferrous ammonium sulphate using ferroin as indicator. (c) Standardisation of potassium dichromtae solution: diphenylamine indicator method. A known volume (10--15 ml) of standard (0.0100 N) thiourea solution was pipetted into a titration flask. Sufficient water (5--10 ml) and enough of sulphuric acid (75--85 ml of ~4N) were added to keep the normality of the solution at ~3.5 N and its volume at ~ 100 ml. The solution was mixed with 13"
194 g. Ch. Verma et al.: 1--2 drops of diphenylamine, and after cooling to room temperature (~ 26 ~ was titrated with the dichromate solution. The end-point was marked by a sharp colour change from green to violet or blue black. The normality of the oxidant solution was also determined by titration against ferrous ammonium sulphate using diphenylamine as indicator. Results and Discussion The results given in Table I indicate that 0.01 N thiourea solution can be used as a reductimetric standard for the standardisation of ammonium hexanitratocerate, potassium bromate, chloramine-t and N-bromosuccinimide solutions in sulphuric acid medium in the Table I. Standardisation of Ammonium Hexanitratocerate, Potassium Bromate, Chloramine-T and N-Bromosuccinimide Solutions in Acid Medium in the Presence of Potassium Iodide (Amylose Indicator Method) Oxidant standardised Thiourea Oxidant Normality Normality of the 0.0100 N solution of the oxidant found by taken used oxidant found iodometric method ml ml* N* N* (NH4)~Ce(NOa)o KBrOa Chloramine-T N-Bromosuccinimide 10.00 9.90-- 9.94 0.01006--0.01010 0.01004--0.01009 15.00 14.84--14.92 0.01005--0.01011 0.01003--0.01011 10.00 10.09--10.14 0.00986--0.00991 0.00984---0.00990 15.00 15.15--15.24 0.00984--0.00990 0.00982--0.00989 10.00 9.92-- 9.96 0.01004--0.01008 0.01001--0.01006 15.00 14.90--14.97 0.01002--0.01007 0.01001--0.01009 10.00 10.02--10.05 0.00995--0.00998 0.00994--0.00999 15.00 15.05--15.14 0.00991--0.00997 0.00988--0.00995 * Range of four readings presence of potassium iodide using amylose as an indicator. The standardisation of potassium dichromate and ammonium hexanitratocerate solutions against thiourea in acid medium can also be carried out using redox indicators (Tables II and III). It has been observed in the titration of thiourea with ammonium hexanitratocerate solution in sulphuric acid medium that the reaction between thiourea and Ce(IV) is rapid in the initial stages but becomes slow as the titration proceeds, and that the reaction is accelerated by adding 1 to 2 drops of 1.7 mm potassium iodate solution as catalyst. The oxidants listed in Tables I, II and III have also been standardised separately by well known methods and the values agreed within analytical precision with those obtained by proposed methods.
Thiourea as Reductimctric Standard 195 Iodometric method is the commonly employed method for the standardisation of majority of the oxidants including those reported in the present and earlier communication s. The main drawback in this method is that, although sodium thiosulphate, Na2S2Oa.5H20, Table II. Standardisation of Ammonium Hexanitratocerate Against Thiourea Solution in Acid Medium (Ferroin Indicator Method) Thiourea Oxidant Normality Normality of the 0.0100 N solution of the oxidant found by taken used oxidant found titration with ferrous ammonium sulphate ml ml* N* N "~ 10.00 9.90-- 9.95 0.01005--0.01010 0.01004--0.01010 15.00 14.82--14.93 0.01005--0.01012 0.01003--0.01014 * Range of 4 determinations Table IIL Standardisation of Potassium Dichromate Against Thiourea Solution in Acid Medium (Diphenylamine Indicator Method) Thiourea Oxidant Normality Normality of the 0.0100 N solution of the oxidant found by taken used oxidant found titration with ferrous ammonium sulphate ml ml* N* N* 10.00 9.80-- 9.85 0.01015--0.01020 0.01016--0.01022 15.00 14.68--14.80 0.01013--0.01022 0.01012--0.01023 * Range of 4 readings of high purity is obtainable there is always some uncertainty as to its exact water content because of the efflorescent nature of the salt and for other reasons. This necessitates its solutions to be standardised 9. Furthermore, solutions of sodium thiosulphate in ordinary distilled water undergo slow decomposition 9. In comparison to thiosulphate, thiourea can be made available in a pure state and its solutions are quite stable as reported earlier s. Therefore, the standardisation procedure using thiourea as a standard have distinct advantages over the iodometric method. Moreover, since in the thiourea procedure iodine is liberated in situ during titration, there is no risk of loss of iodine due to volatilisation. Potassium dichromate and Ce(IV) solutions are commonly standardised by titration against ferrous ammonium sulphate in acid medium using diphenyl amine or ferroin as indicator. Ferrous ammonium sulphate is not recommended 9 for accurate work because its
196 B. Ch. Verma et al.: Thiourea as Reductimetric Standard water content and the presence in exactly equimolar proportions of its constituent salt is open to suspicion. The standardisation can be done using thiourea instead (Tables II and III), taking advantage of the stability of thiourea solutions. Summary Thiourea as Reductimetric Standard: Standardisation of Ce(fV), Dichromate, Chloramine-T and N-Bromosuccinimide Solutions The use of 0.01 N thiourea as a reductimetric standard has been described for the standardisation of Ce(IV), potassium dichromate, potassium bromate, chloramine-t and N-bromosuccinimide solutions. Zusammenfassung Die Anwendung yon 0,01 N Thioharnstoffl6sung als reduzierende Standardl6sung fiir Ce(IV), Bichromat, Bromat, Chloramin-T und N-Bromsuccinimid wurde beschrieben. References t B. Singh and B. C. Vercaa, J. Sci. Ind. Res. 24, 536 (1965). e B. Singh and B. C. Verma, J. Ind. Chem. Soc. 40, 39 (1963). a B. Singh, B. C. Verma, and M. S. Saran, Bull. Chem. Soc. Japan 38, 43 (1965). 4 B. Singh, (1963). 5 B. I3. 7 A. London: B. C. Verma, and Y. K. Kalia, J. Ind. Chem. Soc. 40, 697 Singh and B. C. Verma, Z. analyt. Chem. 196, 132 (1963). Singh and B. C. Verma, Mikrochim. Acta [Wien] 1974, 123. Vogel, A Text Book of Practical Organic Chemistry, 3rd Ed., Longman, Green. 1971. p. 822. 8 B. C. Verma, S.M. Ralhan, and N. K. Ralhan, Mikrochim. Acta [Wien] 1976 I, 201. 9 A. I. Vogel, A Text Book of Quantitative Inorganic Analysis, 3rd Ed., London: The English Language Book Society and Longman.' 1975. pp. 281, 319, 348. Correspondence and reprints: Dr. Balbir Chand Verma, Department of Chemistry, Himachal Pradesh University, Simla-171005, India.