Inconsistency in the conductivity of standard potassium chloride solutions made from different high-quality reagents

The salinity of seawater has been defined in terms of the ratio of its electrical conductivity to that of a standard potassium chloride solution. The potassium chloride solution is obtained by dissolution of high-purity potassium chloride in ultrapure water. However, even high-purity potassium chloride is not 100% pure, and suppliers do not certify it, as the standard for electrical conductivity. We prepared defined solutions using several kinds of high-purity potassium chloride and examined the difference between the measured electrical conductivity ratio and the value calculated from the experimental equation given in the UNESCO background papers. The differences between the electrical conductivity ratios of the solutions made from the various potassium chloride reagents were equivalent to about 0.0012 in salinity. The solution may not actually become a standard for salinity measurements even if it is prepared exactly according to the present definition. The lot dependency of the electrical conductivity ratio of the potassium chloride solutions may result in a systematic error in the measurement of salinity.