Abstract
The dissolution of an iron-containing substance in orthophosphoric acid either alone or mixed with nitric acid or nitric acid—sulphuric acid mixture at 270–280° has been examined, to determine the composition of the solutions which are used for analytical determinations. The possibility of condensation of H 3PO 4 and formation of polyphosphoric acids during the dissolution has been proved. Solutions which may contain pyrophosphoric acid, tripolyphosphoric acid and tetrapolyphosphoric acid have been obtained. The exact composition of these solutions depends on the initial H 3PO 4 concentration, the heating time, and, in some cases, the presence of nitric acid and sulphuric acid. It is assumed that the nitric and sulphuric acid affect the loss of water and therefore the thermal dehydration of the orthophosphoric acid during the dissolution. No qualitative changes in the phosphoric acid composition have been observed in the presence of Fe(III). Formation of Fe(III) polyphosphates is assumed.
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