Chemical formula for sodium dihydrogen phosphate

Written by vincent summers
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Chemical formula for sodium dihydrogen phosphate
(Public Domain - Wikimedia Commons)

Sodium dihydrogen phosphate, SDP, is also called monosodium phosphate. Phosphoric acid, H₃PO₄, is a triprotic acid (it has three hydrogens that can be replaced). It is possible to form monosodium phosphate or sodium dihydrogen phosphate (NaH₂PO₄), disodium phosphate (Na₂HPO₄), or trisodium phosphate (Na₃PO₄). Partial neutralisation of only one hydrogen takes only one molecule of sodium hydroxide (or some other basic sodium salt), so sodium dihydrogen phosphate is considered monobasic.

Chemical Properties

Sodium dihydrogen phosphate can be purchased anhydrous (no water of crystallisation) or at various levels of hydration (waters of crystallisation per molecule: NaH₂PO₄·XH₂O where X is the number of water molecules). SDP is useful as a pH buffer. A pH buffer added to a solution helps keep that solution at its initial pH. When an acid or a base is added to a solution containing a buffer, the pH doesn't change nearly as much as it would if the buffer was not present.


Sodium dihydrogen phophate was provided in 20cc hardcase Teflon ampules and was used as a buffer in the drinking water supply aboard the Apollo 11 Space Mission. The buffer was added after the initial chlorine disinfectant treatment.


SDP is used as a sequestrant in cheese, milk, fish and meat products. A sequestrant is a kind of food additive that improves food quality and stability by protecting fats from polyvalent metal ion oxidation by forming a complex with any metallic ions that may be present. Thus, a sequestrant can also be defined as a preservative.

Baking Powders

Sodium dihydrogen phosphate is also used in baking powders. Baking powders produce a rising reaction by generating carbon dioxide gas during cooking. They do this by combining sodium bicarbonate, a mild alkali capable of liberating carbon dioxide gas, with a mile acid. The type of baking powder is named after the acid ingredient. The three common baking powders are Cream of Tartar baking powders, Alum baking powders, and Phosphate baking powders. The ingredient in phosphate baking powders is sodium dihydrogen phosphate. The government recognises a fourth form of baking powder, combining sodium bicarbonate with any combination of the three acid ingredients listed above.

Enema Preparations

SDP is currently used in some effective enema preparations, including those used for endoscopy; however sodium phosphates are coming under a degree of fire in relation to kidney (renal) health, in some instances even resulting in complete renal failure.

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