It is well known that phosphoric acid belongs to medium-strong acid.

Phosphoric acid boiling point: 261 C (decomposition, phosphoric acid gradually dehydrated by heating, so there is no boiling point of its own)

The most common phosphoric acid seen in the world is a viscous concentrated solution containing 85% H3PO4. When crystallized from concentrated solution, the semihydrate 2H3PO4 H2O (melting point 302.3K) is formed.

Its crystallization point (freezing point) is 21 C. When it is below this temperature, hemihydrate crystallization will be precipitated.

Of course, phosphoric acid usually does not crystallize at temperatures above or even lower than 10 C. This is because phosphoric acid has the characteristics of supercooling, that is to say, when the market phosphoric acid is lower than 21 C, it will deviate from its crystallization point and will not crystallize immediately.

However, as long as such a low temperature is maintained for a period of time, phosphoric acid can easily crystallize in a static state. Phosphoric acid crystallization, like other liquid crystallization, belongs to physical rather than chemical changes.

Its chemical properties will not be changed by crystallization, that is to say, the quality of phosphoric acid will not be affected by crystallization, as long as it is melted at temperature or diluted by hot water, it can still be used normally.

Phosphoric acid crystallization (ice) reasons: high concentration of phosphoric acid, high purity, high crystallinity. According to experience, when the temperature is about 4 degrees Celsius and the concentration is more than 85%, the crystallinity of phosphoric acid will increase. If crystallized phosphoric acid is mixed carelessly, it will cause the phosphoric acid which did not crystallize immediately to infect and crystallize. Moreover, the crystallization of phosphoric acid is abnormally fast, and most of the storage containers of phosphoric acid will crystallize directly. After phosphoric acid crystallization, the upper phosphoric acid edge is thin, and the lower part deposits needle crystal pure phosphoric acid. According to experience, 75% phosphoric acid is difficult to crystallize at a lower temperature (near 4 C). Therefore, 75% phosphoric acid is recommended to be used at a lower temperature.

Phosphoric acid crystallization is like water freezing, which is its physical properties. Its inherent properties can not be changed. Only proper preservation and treatment can prevent crystallization.