Sodium dihydrogen phosphate dihydrate
[CAS 13472-35-0]

Identification

• Classification: Inorganic chemical industry >> Inorganic salt >> Phosphides, metal phosphoric acid, metaphosphoric acid, hypophosphorous acid and pyrophosphate
• Name: Sodium dihydrogen phosphate dihydrate
• Synonyms: Sodium dihydrogen orthophosphate; Sodium phosphate monobasic dihydrate
• Molecular Formula: NaH₂PO₄.2(H₂O)
• Molecular Weight: 156.01
• CAS Registry Number: 13472-35-0 (53807-02-6)
• EC Number: 603-853-2
• SMILES: O.O.OP(=O)(O)[O-].[Na+]

Properties

• Density: 1.91 g/mL (Expl.)
• Melting point: 60 °C (Expl.)
• Solubility: water soluble (Expl.)

Safety Data

• Hazard Symbols: GHS07 Warning
• Risk Statements: H302-H315-H319-H335
• Safety Statements: P261-P305+P351+P338

chemBlink Chemical Story

Sodium dihydrogen phosphate dihydrate is a useful example of why solid-state form matters even for a familiar inorganic salt. Its formula, NaH₂PO₄·2H₂O, contains the same acidic phosphate ion used in countless buffer solutions, but two water molecules are incorporated into the crystal lattice. Those waters change the formula mass, crystal packing, thermal behavior, and the way the material responds to drying and humidity.

In solution, the chemically important ion is H₂PO₄^-, the acidic member of the phosphate buffer pair. It can donate a proton to become HPO₄^2-, so mixtures of mono- and dibasic phosphate salts provide buffering near neutral pH. Once dissolved, a properly prepared solution does not 'remember' whether the phosphate entered as a monohydrate or dihydrate. Before dissolution, however, the hydration state determines how many moles of phosphate are present in each gram of solid.

That distinction is a routine source of laboratory error. If a protocol calls for a certain molarity and the calculation uses the molecular weight of the monohydrate while the bottle contains the dihydrate, the resulting phosphate concentration will be low. In a robust qualitative experiment the difference may go unnoticed. In analytical calibration, enzyme work, pharmaceutical manufacturing, or formulation development, the same error can change ionic strength, buffer capacity, and sometimes product performance.

Hydrates also reveal that a crystal is not simply a pile of independent ions. Water molecules can bridge sodium and phosphate ions through coordination and hydrogen bonding, stabilizing one lattice under a particular range of temperature and humidity. Heating can drive off water and create another solid form; exposure to moist air can shift hydration in the opposite direction. Manufacturers therefore specify assay, loss on drying, and storage conditions because solid-state composition affects how the reagent should be weighed and handled.

Phosphate buffers become even more complicated during freezing. Ice formation concentrates solutes, and sodium phosphate salts may crystallize selectively. This can move the acid/base ratio and cause transient pH shifts in the remaining liquid. The dihydrate itself is not the only phase involved, but its existence belongs to the same broader lesson: phosphate chemistry in a formulation includes solid-state equilibria as well as acid-base equilibria.

Sodium dihydrogen phosphate dihydrate is memorable because it makes 'water' part of the chemical identity. Two transparent crystals labeled with the same phosphate name can deliver different moles per gram because their lattices contain different amounts of bound water. In precise chemistry, hydration state is not packaging information; it is stoichiometry.

References:
1. PubChem and regulatory identity records for sodium dihydrogen phosphate dihydrate, CAS 13472-35-0.
2. Cold Spring Harbor Protocols. Phosphate buffer. DOI: 10.1101/pdb.rec8543.
3. Gomez G., Pikal M.J., Rodriguez-Hornedo N. Pharmaceutical Research. 2001, 18, 90-97. DOI: 10.1023/A:1011082911917.
4. Standard pharmacopeial and reagent specifications for sodium phosphate hydrates, including assay and loss-on-drying control.

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