Rapid Titration of Total Silica as Fluosilicate

To determine silica in solutions containing more than about 0.1% SiOj, a convenient and rapid method developed by Kdnig (334) is based on the conversion of SiOj at ordinary temperature to HjSiF, with excess HF the excess is neutralized with NaOH without decomposing the SiF, ion. Then in hot solution, SiF. can be hydrolyzed quantitatively by titration with NaOH. The solution must be free of COj. 

Halfter (335) had already applied a similar but less versatile method for determining silica in soluble silicates. Another variation was adapted to determining silica in riuosilicic acid (336). 

Reagents. (A) Indicator mixture 0.2% bromothymol blue and 0.1% phe-nolphthalein. 

Procedure. (1) In a clear 500 ml wide-mouth plastic bottle place the sampfe, liquid or solid, containing about 0.1-0.3 gram SiOj, and add water to give a total volume of usually no more than about 20 ml. 

Preferably a size of sample should be taken to give a titration of 3-10 ml. Silica content of sample is calculated on the basis that 1 ml 3 A NaOH = 0.045 gram SiOj. 

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