A Critical Study on the Methods of Water Analysis. III

Application and Improvement of Fajans' Method for the Chloride Determination of Sea Water

Abstract

Mohr's argentometry which is the international standard method for the chloride determination of sea water, has many demerits from a point of view of the analytical chemistry of to-day.
I have tried to use Fajans' adsorption indicator method instead of Mohr's, and obtained very satisfactory results. The indicator used is a mixture of 5cc of 0.1% uranine (sodium salt of fluorescein) solution and 100cc of 1% starch solution. 2cc of this indicator is added for each 15cc of sea water, where starch serves as a protectivecolloid.
This modified Fajans' method surpasses Mohr's in the following points:
1. There is no fear that an end point comes too early.
2. There exists no error due to the sensibility of the indicator.
3. The colour change at an end point is quite clear.
4. It is unnecessary to add any distilled water to avoid the coagulation of silver chloride.
5. The time needed for each titration is shorter than half of Mohr's.
I have also made an attempt to use the pure potassium chloride solution in place of the normal sea water, this attempt has proved successful.