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Decades ago my job was to find the silicon content in cast iron gravimetrically. The first step was dissolving the sample in hot Nitric acid. Did this turn all of the constituents in the sample, except the silicon, into water soluble salts? Was the silicon then bound as silicon dioxide?

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  • $\begingroup$ When heated, concentrated nitric acid is a strong oxidizing agent. The Fe is oxidized to Fe3+. I think the Si is oxidized to SiO2, but I'm not entirely sure about that part. $\endgroup$ – Cyclopropane Aug 7 '18 at 2:08
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No, nitric acid is a sufficiently hard acid to dissolve silicon metal without precipitating in low concentrations. In fact, nitric acid is used to dissolve silicon in ICP Standards. The oxide reacts slowly with water and water-based solutions - nitric acid included, but the acid does not increase the kinetics appreciably, otherwise glass would be unsuitable as a vessel for nitric acid. The element in an alloy, however, can be dissolved more easily like that in cast iron.

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