Attack of Phosphoric Acid on Oxides

Phosphoric acid does not react with anhydrous crystalline forms of Si02, AI2O3, Cr203, Z1O2 and Ti02 at room temperature. Some reaction occurs with the hydrated or colloidal forms of these oxides or with the anhydrous forms themselves if the temperature rises above about 200°C. Below this temperature silica glass is not appreciably attacked by the acid in moderate concentration. 

Phosphoric acid combines rapidly at room temperatures with the oxides of Be, Mg, Ca, Sr, Ba, Zn, Cu, Mn and Pb producing both acid and neutral salts. Some of the products form hard cohesive masses which can be utilised as cements. These include cements for dental, electrical, refractory and constructional purposes (Section 12.10). 

Phosphate bonding of refractory compositions, particularly those containing alumina, is an important industrial practice (Section 12.10). 

Phosphoric acid will stabilise clay soils against the effects of changes in moisture content and has been used in road construction. If a small quantity is added to clay soil under moist conditions it eventually extracts sufficient iron and aluminium to form iron and aluminium phosphates which are precipitated and then cements the clay particles together. 

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