Hosta

Monosodium phosphate (MSP), also known as monobasic sodium phosphate and sodium dihydrogen phosphate, is an inorganic compound with the chemical formula NaH₂PO₄. It is a sodium salt of phosphoric acid. It consists of sodium cations (Na⁺) and dihydrogen phosphate anions (H₂PO−4). One of many sodium phosphates, it is a common industrial chemical. The salt exists in an anhydrous form, as well as monohydrate and dihydrate (NaH₂PO₄·H₂O and NaH₂PO₄·2H₂O respectively).^[1]

Production and reactions

The salt is obtained by partial neutralization of phosphoric acid. The pK_a of monosodium phosphate is 6.8-7.2 (depending on the physicochemical characteristics during pK_a determination).^[2]

Heating this salt above 169 °C gives disodium pyrophosphate:

2 NaH₂PO₄ → Na₂H₂P₂O₇ + H₂O

When heated at 550 °C, anhydrous trisodium trimetaphosphate is formed:^[3]

3 NaH₂PO₄ → Na₃P₃O₉ + 3 H₂O

Uses

Phosphates are often used in foods and in water treatment. The pH of such formulations is generally adjusted by mixtures of various sodium phosphates, such as this salt.^[1] The sodium chloride equivalent value, or E-Value, is 0.49.^{[clarification needed]} It is soluble in 4.5 parts water.^{[clarification needed]}

Food additive

It is added in animal feed, toothpaste, and evaporated milk. It is used as a thickening agent and emulsifier.

Detection of magnesium

Monosodium phosphate is used to detect the presence of magnesium ions in salts. Formation of a white precipitate on the addition of ammonium chloride, ammonium hydroxide and monosodium phosphate to an aqueous or dilute HCl solution of the salt indicates presence of magnesium ions.

Notes