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Freezing of Phosphoric Acid

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Question
I purchased some 85% phosphoric acid and it arrived partially frozen. Why did this happened and what can I do about it?
Answer

Concentrated phosphoric acid products are typically sold as 85%, 80%, or 75% solutions. The freezing point varies with the concentration:

% H3PO4

Freezing Point

75%

-18°C

0°F

80%

5°C

41°F

85%

21°C

70°F

Because of the nature and properties of this chemical, it is not uncommon for the 80% or 85% acids to partially freeze or crystallize during shipment at cooler times of the year. It can be difficult to get the crystals to redissolve. If orthophosphoric acids start to freeze, the acid must then be warmed to about 30°C before the crystals all dissolve. Heating should be done carefully, in a waterbath or warm area. Before attempting to heat the product be sure to review and understand the safety information on the Material Safety Data Sheet (MSDS). To avoid any pressure build-up, loosen the bottle cap or container opening slightly to allow venting. Take all appropriate safety precautions.

It can be difficult to completely thaw phosphoric acid which has been even partially frozen. As the acid is cooled, crystals of the hemi-hydrate begin to form. The remaining solution will therefore become more dilute and will have a lower freezing point. Therefore the acid will not freeze solid. If the temperature of the system is lowered further, the hemi-hydrate will continue to form until, at around -85°C, the entire system will freeze as a mixture of ice and 2H3PO4•H2O. However, even if that point is not reached, once some of the hemi-hydrate has formed it will not melt until the system is warmed to the hemi-hydrate's melting point of 29.3°C. Thus, if orthophosphoric acid starts to freeze, the acid must then be warmed to about 30°C before the crystals of hemi-hydrate will all dissolve.

Freezing and subsequent thawing of phosphoric acid solutions are not expected to affect the quality or performance of the product as long as all crystals are completely and thoroughly dissolved and the product is mixed prior to sampling.

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Keywords0260, 6908, 0262, 0268, 2788, 2796, frozen,

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