Hydrolysis of pyrophosphoric acid in the process of its ion exchange production
Abstract
Pyrophosphoric acid is used in various fields: as a metal corrosion inhibitor, as a buffer agent in an electrolyte solution in electroplating technology, in the production of vanadyl pyrophosphate, which is a selective catalyst in the oxidation of n-butane to maleic anhydride. Attention to pyrophosphoric acid is also due to the development of resorbable inorganic phosphate materials for the regenerative treatment of bone tissue defects. Pyrophosphoric acid can be obtained as a result of the reaction of phosphoric acid with phosphorus oxychloride. But the simplest way to obtain pyrophosphoric acid in an aqueous solution is considered to be cation exchange from sodium pyrophosphate on strongly acidic polystyrene cation exchangers. Such processes are carried out both in columns with cation exchanger and under static conditions by simply mixing excess cation exchanger in hydrogen form with the corresponding salt. In this work, we study the production of pyrophosphoric acid from sodium pyrophosphate in columns with a strongly acidic cation exchanger at temperatures of 25 and 60°C with an emphasis on the extent to which pyrophosphoric acid manages to transform into orthophosphoric acid during its stay in the ion-exchange column and how quickly hydrolysis continues in the resulting filtrate. It is shown that the filtrate immediately after its collection from the column contains, in addition to pyrophosphoric acid, a certain amount of orthophosphoric acid (6% of the total phosphorus content is presented as orthophosphoric acid at 25 °C and 13% at 60 °C). During subsequent storage of the filtrate at 25oC, the hydrolysis of pyrophosphoric acid continues, but relatively slowly. Three hours after obtaining the pyrophosphoric acid solution, the degree of its conversion into orthophosphoric acid increases relatively little, and complete hydrolysis of pyrophosphoric acid does not occur even after 19-21 days. Analysis of the dependence of the degree of hydrolysis of pyrophosphoric acid on time showed that the process occurs according to the kinetic equation of a first-order reaction.
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References
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