Stalagmometric titration using surface-active indicator

Abstract

The end point detection in the chelatometric titration of EDTA with copper sulfate using surface-active indicator was achieved by the stalagmometric measurement of the surface tention between mercury and the solution. All the experiments were carried out at a room temperature under the atmospheric conditions. The capillary characteristics at h=60 cm are : m=0.633 mg s-1, in pure water and t=6.23 s in 10-2 M Na2SO4, at open circuit.Chloride and bromide ions were selected as the indicator. The drop time of the polarographic dropping mercury electrode at 0 volt vs. SCE was plotted against the volume of titrant added, and the titration curves as shown in Figs. 3 and 4, were obtained. This method was compared with the ordinary visual method using Murexide indicator. The F- and t-tests showed that there is no significant difference between the variances and the means of these two sets of data. Since the stability constant of copper-EDTA chelate is much greater than those of copper-halide complexes, before the end point copper ion reacts with EDTA, as shown by eqn. (1), where H4Y is EDTA. Since CuY2-is surface-inactive, the drop time does not change. After the end point, however, copper ion reacts with halide ion, according to eqn. (2), and the surface-active halocomplex CuXn(2 -n)+ is formed as shown in Fig. 2. Thus, the drop time decreased remarkably as shown in Figs. 3 and 4.