Disodium hydrogen phosphate, disodium hydrogen phosphate dihydrate, and disodium hydrogen phosphate dodecahydrate all contain disodium hydrogen phosphate, but the different number of water-containing molecules in their molecules results in very different properties, uses, preparation methods, and retention periods.
I. Chemical formula and CAS number
Disodium phosphate chemical formula is NaH2PO4, the chemical formula of disodium phosphate dihydrate is Na2H2PO4-2H2O, and the chemical formula of disodium phosphate dodecahydrate is Na2HPO4-12H2O, which can be seen that the main difference in their chemical formulas lies in the number of water molecules contained in the molecule.
Disodium hydrogen phosphate Cas No. is 7558-79-4, while the Cas No. of disodium phosphate dihydrate is 10028-24-7, and the Cas No. of disodium phosphate dodecahydrate is 10039-32-4.
II. Similarities and differences in properties
All three are white crystals or granular powder, soluble in water. However, they differ in solubility and hydrolysis.
Disodium hydrogen phosphate is insoluble in alcohol and stable in water. Density 1.064 g/cm³. Easily absorbs moisture and is easily weathered. Aqueous solution has a slightly alkaline reaction (pH of 0.1-1N solution is about 9.0). Melting point 243 to 245 ℃.
Disodium hydrogen phosphate dihydrate is easily hydrolyzed, with a molecular weight of 177.989 g/mol , approximately equal to 178 g/mol, and a melting point of 92.5 ℃. The pH of the solution is generally in the acidic or neutral range. It decomposes at pH values above 7.2.
Disodium hydrogen phosphate dodecahydrate (Na2HPO4-12H2O): also hygroscopic. Melting point: 35°C. Storage temperature is between 15°C and 25°C. At room temperature, it will gradually lose crystalline water, firstly, it will lose about 5 crystalline water to form disodium phosphate heptahydrate (Na2HPO4-7H2O), and when further heated up to 100°C, it will lose all crystalline water, and finally, it will get anhydrous disodium phosphate (Na2HPO4). At 250℃, anhydrous disodium phosphate will decompose into sodium pyrophosphate (Na4P2O7). At 280-300°C it decomposes to sodium metaphosphate (NaPO3)n. The common form of sodium metaphosphate is sodium hexametaphosphate (NaPO3)6.
Disodium hydrogen phosphate dodecahydrate precipitates in solution over a temperature range of 0 to 35°C, giving disodium hydrogen phosphate heptahydrate between 35 and 38°C, disodium hydrogen phosphate dihydrate and phosphate dihydrate between 48 and 95°C, and anhydrous disodium hydrogen phosphate above 95°C.
III. Uses
Disodium phosphate in food grade is commonly used in the production of food and feed in the expansion agent, as a preservative and buffer, but also can be used in the preparation of drugs, but also can be used for the treatment of gastric acid caused by excessive symptoms of discomfort and hypophosphatemia. It can manufacture phosphoric acid fertilizer, cleaning agent, able to be used as copper nitrate and cobalt chloride electrodeposition agent, metal surface treatment agent, etc..
Disodium hydrogen phosphate dihydrate is mainly used in boiler water treatment, plating buffer, dye additives, pigment precipitants, detergents, mica flakes masonry, baking powder manufacturing and so on.
The main uses of disodium hydrogen phosphate dodecahydrate include the production of citric acid, water softener, fabric weight gainer, fire retardant, etc., and is used in glaze medicine, welding medicine, medicine, pigment, food industry, and the production of other phosphates.
IV. Methods of preparation
A. Disodium hydrogen phosphate dodecahydrate:
Add hot phosphoric acid to a neutralization reactor with stirring and corrosion resistance. Slowly add soda ash solution (e.g. sodium carbonate solution) under stirring to carry out the neutralization reaction, so that the pH value of the reaction solution is maintained at 8.4 to 8.6. Filter the reaction solution and crystallize in the filtrate by cooling, centrifugal separation, and drying to produce the finished product of disodium hydrogen phosphate dodecahydrate.
Besides raw materials mentioned in this method there are also others, please check “what is disodium phosphate made from” for the details.
B. Disodium phosphate:
Put soda ash or sodium hydroxide into the reaction kettle, dilute it with water to a certain concentration, add hot phosphoric acid from the metering tank under stirring, and control the end point of the reaction at pH 4.2-4.6. Filter the neutralized liquid, put it in the storage tank, and send it into the atomizer of the spray drying tower for atomization with a measurable slurry pump, and carry out heat exchange with filtered and heated hot air, and the exhaust temperature is controlled at 95-105℃, and the product can be obtained with less water. 105 ℃, can be less than 0.6% water powder or granular solid sodium dihydrogen phosphate, and then sent to the converter for drying, that is, anhydrous sodium dihydrogen phosphate finished products.
C. Disodium hydrogen phosphate dihydrate:
The industrial preparation method of disodium hydrogen phosphate dihydrate mainly includes the following steps:
1. Dissolution: the mixture material of disodium hydrogen phosphate dodecahydrate and water is dissolved at 5～45℃, and after completely liquefied, it is put into the raw steam heat exchanger for further heating to above 65℃, and then it enters into the forced circulation evaporator.
2. Evaporation, concentration, and crystallization: The mixture will be evaporated in the forced circulation evaporator for concentration and crystallization, evaporation temperature of 90 ~ 95 ℃, evaporation time of 1 ~ 2 hours. Evaporation process produces secondary steam, the steam into the MVR compression system for compression, compressed steam and then into the evaporation chamber for heating the mixture.
3. Crystallization separation: The crystals generated from the crystallization separator are transported to the thickener through the discharge pump to increase the solid content of the crystal concentrate to meet the separation requirements of the centrifuge. Finally, the material at the bottom of the thickener is separated from the disodium hydrogen phosphate dihydrate crystals by centrifuge, controlling the separation temperature between 48 and 95℃.
V. Storage
All three should be stored in a cool, dry, well-ventilated place, away from direct sunlight and high temperatures. Protect from moisture and seal. Avoid contact with other substances to prevent chemical reaction.
Generally speaking, the shelf life of sodium dihydrogen phosphate dihydrate is about half a year. For long unused sodium dihydrogen phosphate dihydrate, it can be properly turned or stirred to promote uniformity and prevent lumping.
Unopened anhydrous disodium hydrogen phosphate can be stored for about 2-3 years. The recommended storage temperature is below 25°C.
The shelf life of disodium phosphate dodecahydrate is generally 2 years
VI. Conclusion
It can be seen that although disodium hydrogen phosphate, disodium hydrogen phosphate dihydrate and disodium hydrogen phosphate dodecahydrate contain disodium hydrogen phosphate, their properties, uses are not exactly the same. Their preparation methods are also different, they can not be simply regarded as the same substance. In practical application, they should be selected and used according to specific applications.
