The Micro-estimation of Sulphates in Sea Water and the Body Fluids of Marine Animals

IN a series of methods recently proposed (Robertson & Webb, 1939) for the microestimation of the principal constituents of sea water and of the body fluids of marine animals, that for sulphate was on the whole the least satisfactory. As there described it consists of precipitation by means of barium chloride and direct titration of the excess barium with sodium sulphate, using rhodizonic acid as indicator. The simplicity of the method is to some extent outweighed by the capricious behaviour of the indicator, which renders the judging of the end-point an operation which, even after considerable practice, it is difficult to perform with great confidence. Although the error may usually be kept without difficulty below 2 %, duplicate titrations are necessary if an occasional error somewhat larger than this is to be avoided. In cases where speed is of greater importance than the highest accuracy the method is probably the best that is available, but it was thought that an alternative method, based on that of Van Slyke et al. (1927) for total base in blood, might be made to yield more accurate results and require less practice. The basic principle of this method is the same as that used by Sendroy (1937) for the estimation of chloride; namely, the addition of an insoluble iodate, which reacts with the ion to be estimated so as to form a more insoluble salt and liberate iodate ions'into the solution. For the estimation of chloride silver iodate is used; for sulphate, barium iodate. A few preliminary experiments with lead iodate and lead iodide showed that they were useless except perhaps for very high concentrations of sulphate. The procedure adopted by Van Slyke et al. is first to convert all the bases to sulphates; these are then dissolved in a known volume of very dilute acetic acid and shaken with barium iodate for an hour. After centrifugation the iodate is estimated in an aliquot of the solution by treatment with hydrazine and measurement of the volume of nitrogen liberated. The reactions are: