Microtitration of sulfate ions

A procedure for the microtitration of sulfate ions has been devised as follows: Neutralize an acid sample solution containing 5100mg SO42-/l to pH 4 with aqueous ammonia against p-nitrophenol. Add a known volume (25ml) of 0.005M BaCl2 solution exactly and allow the mixture to stand 15 min. to precipitate BaSO4 completely. Then add 12 ml of a mixture of nitrobenzene and iso-amylalcohol (1:5 v/v), together with 0.5 ml of Na-rhodizonate indicator (5mg/5 ml H2O). Titrate the solution with 0.005M (or 0.0025M) standard solution of H2SO4 until the red color of Ba-rhodizonate, which is adsorbed at the intersurface of the liquids, vanishes. (1) Since the Ba-rhodizonate indicator is gathered at the intersurface between organic and aqueous layers, and separated from the BaSO4 precipitate which remains in the aqueous layers, the color change of the indicator is more distinct than that in other methods. Thus, as little as 3μg of Ba ion is detectable. (2) Ba-rhodizonate has a stable color at pH 2, but the Na-salt is decomposed immediately and is colorless under these conditions. When the sample solution under those conditions is titrated with a standard solution of 0.005M H2SO4, the equivalent point is very distinct. These improvements (1) and (2) render it possible to carry out the titration of as little as 5mg/l SO42- with less than about 4% error (Table 4 and 5). To obtain more accurate results, the use of acetone is recommended. This makes BaSO4 less soluble and the adsorption of Ba-rhodizonate more suitable for the end point. Cations such as Ca2+, Mg2+, and Al3+ do not interfere, but Fe3+ and Pb2+ interfere seriously, because the latters react with rhodizonate to make colored compounds. This procedure, together with ion-exchange separation for the removal of interfering cations, can be applied to water analysis (Table 6).