Sodium dihydrogenorthophosphate
[CAS 7558-80-7]

Identification

• Classification: Inorganic chemical industry >> Inorganic salt >> Phosphides, metal phosphoric acid, metaphosphoric acid, hypophosphorous acid and pyrophosphate
• Name: Sodium dihydrogenorthophosphate
• Synonyms: Monosodium dihydrogen phosphate; Monosodium phosphate; Sodium phosphate monobasic
• Molecular Formula: NaH₂PO₄
• Molecular Weight: 119.98
• CAS Registry Number: 7558-80-7
• EC Number: 231-449-2
• SMILES: OP(=O)(O)[O-].[Na+]

Properties

• Density: 2.04 g/mL (Expl.)
• Melting point: 60 °C (Expl.)
• Boiling point: 204 °C (Decomposes) (Expl.)
• Solubility: water 52% w/w (0 °C) (Expl.)

Safety Data

• Hazard Symbols: GHS07 Warning
• Risk Statements: H319
• Safety Statements: P264+P265-P280-P305+P351+P338-P337+P317

chemBlink Chemical Story

Sodium dihydrogen phosphate is the acidic partner of the phosphate buffer pair. In water it supplies H₂PO₄^-, which can donate a proton to added base while remaining in equilibrium with HPO₄^2-. That simple reaction underlies thousands of biochemical, analytical, and pharmaceutical procedures, but the anhydrous salt, CAS 7558-80-7, also illustrates why solid form and exact formula matter in quantitative chemistry.

Monobasic sodium phosphate exists in anhydrous and hydrated forms. The phosphate ion chemistry in solution may be similar after dissolution, but the mass required to prepare a specified molarity is not. Water of crystallization is part of the formula weight. Confusing an anhydrous material with a monohydrate or dihydrate produces a systematic concentration error even if every balance reading is perfect. For buffers used in sensitive biological or analytical work, that apparently small documentation detail can propagate into pH and ionic-strength differences.

As the acidic component of a phosphate buffer, NaH₂PO₄ is commonly mixed with Na₂HPO₄. The ratio, rather than either salt alone, sets pH through the second dissociation equilibrium of phosphoric acid. Increasing monobasic phosphate shifts the mixture toward lower pH; increasing dibasic phosphate shifts it upward. This is a direct laboratory expression of the Henderson-Hasselbalch relationship.

Phosphate formulations become more complicated when temperature changes or water freezes. As ice excludes solutes, concentrations rise in the remaining liquid, and individual salts can crystallize selectively. Pharmaceutical studies have shown large transient pH shifts in frozen sodium phosphate buffers. That matters in lyophilization, because a protein can be exposed to a damaging pH during the frozen stage even when the formulation looks acceptable after reconstitution.

The word "monobasic" can be confusing because it describes the salt's neutralization state rather than saying the solution must always be strongly acidic. Sodium dihydrogen phosphate still participates in equilibria with other phosphate species, and actual pH depends on concentration, temperature, ionic strength, and what other components are present. This nomenclature is useful precisely because phosphoric acid can lose three protons stepwise. Mono-, di-, and tribasic phosphate salts represent different positions along that neutralization sequence, not three unrelated chemicals.

Sodium dihydrogen phosphate matters because it connects elementary acid-base chemistry with the discipline of formulation. Correct hydrate identification, accurate weighing, buffer ratio, ionic strength, metal compatibility, and thermal history all matter. A bottle labeled with a familiar phosphate name is therefore not merely a commodity reagent. It is one defined component in an equilibrium system whose behavior depends on both solution chemistry and solid-state identity.

References:

1. Cold Spring Harbor Protocols. Phosphate buffer. DOI: 10.1101/pdb.rec8543.

2. Gomez G, Pikal MJ, Rodriguez-Hornedo N. Pharmaceutical Research. 2001;18:90-97. DOI: 10.1023/A:1011082911917.

3. Handbook of Pharmaceutical Excipients. Sodium phosphate, monobasic: anhydrous and hydrate forms.

4. Pharmacopeial monographs for sodium dihydrogen phosphate, CAS 7558-80-7.

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