650 HARRIS AND LINDSEY: VIBRATING ELECTRODES I N [Vol. 76 Vibrating Electrodes in Amperometric Titrations Part I1 Bromometric Determinations of Antimony and Arsenic BY E. D. HARRIS AND A. J. LINDSEY (Presented at the meeting of the Physical Methods Group on Tuesday, April loth, 1951) The reduction of bromine at a vibrating platinum micro-electrode in N hydrochloric acid containing 0.05 A- potassium bromide commences at +0.75 volt with reference to the saturated calomel electrode, and a region of constant diffusion current is reached at +0*20 volt. Tervalent antimony and arsenic yield no diffusion current and may be determined by amperometric titration with potassium bromate and a vibrating electrode polarised to 0-2 volt. With 0.01 N potassium bromate and 0.001 N solutions of the metals an accuracy of +0-1 per cent.can be attained. TERVALENT arsenic and antimony compounds are readily oxidised by potassium bromate in acid solutions, and when the reaction is complete, additional bromate is converted into bromine by the reaction- BrO,' + 5Br' + 6H' = 3H,O + 3Br, This has been made the basis of a method of determination by using an indicator such as methyl orange to show the liberation of bromine,' but since the appearance of the yellow colour of the end-point is a slow reaction and is not reversible the titration is tedious and it is easy to overshoot the end-point. A recent paper2 refers to quinoline yellow as a reversible indicator in such titrations. The liberation of bromine is, however, readily and immediately detected by a vibrating electrode polarised to +0.2 volt with reference to a saturated calomel electrode. ESTABLISHMENT OF SUITABLE CONDITIONS FOR THE DETERMINATIONS Solutions of tervalent antimony and arsenic (0.001 N ) were titrated with a 0.01 N solution of potassium bromate in a supporting electrolyte of N hydrochloric acid containing 0.05 N potassium bromide.Current - voltage curves for the supporting electrolyte without and with bromate are shown in Fig. 1 as curves A and B, respectively. It will be noted that the bromine wave begins at +0.75 volt with reference to the saturated calomel electrode and that the currents reach a constant value in the region of +0.2 volt. It was therefore decided to polarise the electrode to this voltage in determinations of the metals. To confirm this choice, 60ml of supporting electrolyte were titrated with 0.01 N potassium bromate and the observed current was plotted against the volume added after correcting for volume changes.Fig. 2 shows that there is a direct relationship between the quantity of bromine and the current over the range 2.5 x to 9.2 x N bromine. This agrees with the results of Kolthoff and Laitinen3 for rotating eIectrodes and it was accordingly decided that the end-point of a titration can be found by extrapolating the current - volume curve to zero current. DETERMINATION OF TERVALENT ANTIMONY- A solution of 1O-ml volume containing exactly 0.001 N potassium antimony1 tartrate, N hydrochloric acid and 0.05 N potassium bromide was measured into an open beaker con- taining the vibrating electrode and an agar- potassium chloride bridge connected to a saturated calomel half-cell. The electrode was polarised to +0.2 volt and an exactly 0.01 N solution of potassium bromate was run in from a micro-burette.Current and volume readingsNov., 19511 AMPEROMETRIC TITRATIONS. PART I1 651 were taken and plotted and the result is shown in Fig. 3, from which the end-point was read as 10.0 0.01 ml. This result is typical of a large number of determinations. DETERMINATION OF TERVALENT ARSENIC- curve gave a result of 10.0 & 0.01 ml. The same technique was followed for arsenic as for antimony; a typical current - volume Voltage Fig. 1. Current - voltage curves Curve A, supporting electrolyte alone ( N hydrochloric acid and 0.05 N potassium bromide) ; curve B, supporting electrolyte with added potassium bromate (0.6 x 10dN) Potassium bromate, ml Potassium bromate.ml Fig. 2. Diffusion current - bromine conceatra- Fig. 3. Diffusion currents measured when 100 ml of supporting electrolyte, exactly 0.001 N with respect to antimony, is titrated with 0.01 N potassium bromate tion graph Titrations were also carried out in which the concentrations of tervalent antimony and arsenic were 0.002 N and in which a 50-ml burette graduated to 0.1 ml was used. Direct proportionality was again found between diffusion current and volume. The precision is not so great, as end-points may differ by 0.1 ml, which corresponds to an accuracy of &0.5 per cent.662 CORBETT: THE ESTIMATION O F OXYGEN [Vol. 76 The authors wish to state that the experimental work was approved as part of a thesis for the degree of Master of Science of the University of L ~ n d o n . ~ They also wish t o record their thanks to Imperial Chemical Industries Ltd., for a grant used for the purchase of chemicals. REFERENCES 1. 2. 3. 4. Willard, H. H., and Diehl, H., “Advanced Quantitative Analysis,” D. van Nostrand Company Belcher, R., Anal. Chim. Acta, 1951, 5, 30. Kolthoff, I. M., and Laitinen, H. A., J . Phys. Chew., 1941, 45, 1079. Harris, E. D., M.Sc. Thesis, University of London, 1949. Inc., 1943, p. 345. S I R JOHN CASS COLLEGE LONDON, E.C.3