Determination of hydroxide in the presence of aluminate using a modified potentiometric titration

Abstract

A procedure for the determination of hydroxide concentration in the presence of aluminate and other interfering ions was developed using the method of standard additions (or more specifically in this case, standard subtractions). The procedure called for titration with a strong acid which was added in equal increments at regular time intervals. The potential was recorded after each addition of acid. The data was plotted on Gran's Plot Paper, which is based on work done originally by Gran, or it was entered into a computer to determine the equivalence volume of the titrant. When used on many different samples in different matrices, the method gave results that were approximately 100 +- 1 percent of the calculated value. This was shown to be true even in systems containing aluminate (Al(OH)/sub 4//sup -/), phosphate (PO/sub 4//sup 3 -/), sulfate (SO/sub 4//sup 2 -/), nitrate (NO/sub 3//sup -/), nitrite (NO/sub 2//sup -/), carbonate (CO/sub 3//sup 2 -/), and other ions. The effect of these ions was shown to be negligible if the initial OH/sup -/ concentration was at least 10/sup -3/ M and barium chloride (BaCl/sub 2/) was added to complex the PO/sub 4//sup 3 -/ and CO/sub 3//sup 2 -/ ions. These ions were checked at levels of up to 3 M for Al(OH)/sub 4//sup -/ and NO/sub 3//sup -/, 2 M for NO/sub 2//sup -/, and 1 M for SO/sub 4//sup 2 -/, PO/sub 4//sup 3 -/, and CO/sub 3//sup 2 -/. The method is applicable to radioactive as well as nonradioactive samples and any heat involved with a radioactive sample is insufficient to cause an error in the determination. On the basis of the work performed it was shown that the method was general enough to be run on a routine basis.