Monosodium citrate, or sodium dihydrogen citrate, is an acid salt

Acid salt

Acid salt is a somewhat obscure term for a class of salts formed by the partial neutralization of diprotic or polyprotic acids. Because the parent acid is only partially neutralized, one or more replaceable hydrogen atoms remain. Typical acid salts have one or more alkali metal ions as well as one...

 with the chemical formula NaH₂C₆H₅O₇, or C₃H₄OH(COOH)₂COONa. Since it has two remaining open spots on the citrate anion, it is used as a relatively strong sequestrant

Sequestrant

A sequestrant is a food additive whose role is to improve the quality and stability of the food products. Sequestrants form chelate complexes with polyvalent metal ions, especially copper, iron and nickel, which serve as catalysts in the oxidation of the fats in the food...

. It is used to prevent platelet clumping in blood samples. It is one of the 3 citric acid salts.

Preparation

Monosodium citrate can be prepared by the direct reaction of sodium carbonate or bicarbonate with citric acid

Citric acid

Citric acid is a weak organic acid. It is a natural preservative/conservative and is also used to add an acidic, or sour, taste to foods and soft drinks...

:

NaHCO₃ (s) + C₆H₈O₇ (aq) → NaC₆H₇O₇ (aq) + CO₂ (g) + H₂O (l)