1978 Volume 27 Issue 11 Pages 737-738
A stalagmometric back titration of chromium(III) with copper (II) was successfully performed in the presence of calcium myristate as a surface-active indicator. The end point indicated by the remarkable increase in the surface tension was in good agreement with that obtained by the visual method with manganese (II) standard solution using BT indicator, in which the chromium (III) solution was previously five times diluted, the blue-violet coloration of the chelate being weakened. The measurement of the surface tension was carried out with a Shimadzu du Nouy surface tensiometer equipped with a platinum ring (0.610 mm in thickness, 19.568 mm in mean diameter). To a portion of 0.05 M chromium(III) nitrate standard solution in a beaker are added 10.0 ml of 0.05 M EDTA·2Na standard solution and 10 ml of 1 M CH3COONH4-HCl buffer solution of pH 3.3. After boiling the solution for 15 min, followed by cooling to room temperature, the pH of the solution is adjusted to 9.7 with ammonia water. As the indicator, 0.005 g of calcium myristate is added and the solution is diluted to about 60 ml with water. After addition of a small increment of the 0.05 M copper (II) standard solution the mixture is mixed well with a magnetic stirrer and subjected to the surface-tension measurements. The sudden increase in the surface tension at the end point is ascribed to the formation of slightly soluble calcium myristate.
