iron(III) hydroxide; hydrochloric acid; sodium hydroxide in water monosilicic acid soln. adjusted to pH 6-12 with HCl and NaOH soln., NaCl concn. adjusted to 0.1 M, iron(III) hydroxide introduced, pH readjusted, suspn. stirred vigorously at 25°C for 30 min; rapidly filtered, solid analyzed by trimethylsilylation-gas chromy.;