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A procedure for silicate rock analysis after acid decomposition is described. Using a special PTFE vessel assembly, the SiF4 evolved during decomposition is sorbed on KNO3 as K2SiF6. After hydrolysis, the HF is titrated with NaOH to determine the SiO2 content in the sample. The sample residue after acid decomposition is dissolved and used for the determination of the remaining elements by chelatometry, spectrophotometry, and AAS.

At the Geological Survey of Prague an expeditious procedure for the determination of silica has been developed and is currently used routinely. The procedure is based on sample decomposition using acids (HF + H2SO4), chemisorption of the evolving H2SiF6 on solid potassium nitrate and acidimetric titration of the HF released by hydrolysis of the resulting K2SiF6. The sample residue from this decomposition is dissolved, transferred to a PTFE beaker and HF is fumed off. The resulting solution is then used for the determination of other sample constituents either by chelatometry (high Al, Ca, Mg), AAS (Li, Na, K, Mn, Ti, low Al, Ca, Mg, Fe), spectrophotometry (P, high Fe) or ICP-ES (all of these). For samples containing minerals not attacked by HF the sample solution is first filtered off, incinerated, and the insoluble residue fused with a mixture of Cs2CO3 and boric acid. After dissolution it is added to the sample solution and its SiO2 content measured by AAS or ICP-ES. This silica amount is then added to that found by titration. The PTFE vessel assembly used for the sample decomposition and the procedure for Si will be described in detail. Results on international standard reference materials will be used to illustrate performance of the procedure. Examples of samples with unusual composition for which the procedure fails to give accurate results will be discussed. The advantage of the procedure presented is that it is independent of reference samples with well established chemical composition and possesses the accuracy of wet-chemical procedures while considerably increasing the sample throughout. It may be applied with advantage particularly in laboratories where XRF spectrometry is not available.

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