Disodium hydrogenorthophosphate
[CAS 7558-79-4]

Identification

• Classification: Inorganic chemical industry >> Inorganic salt >> Phosphides, metal phosphoric acid, metaphosphoric acid, hypophosphorous acid and pyrophosphate
• Name: Disodium hydrogenorthophosphate
• Synonyms: Sodium hydrogen phosphate; Disodium monohydrogen phosphate
• Molecular Formula: Na₂HPO₄
• Molecular Weight: 141.96
• CAS Registry Number: 7558-79-4
• EC Number: 231-448-7
• SMILES: OP(=O)([O-])[O-].[Na+].[Na+]

Properties

• Density: 1.52 g/mL (Expl.)
• Melting point: 250 (Decomposes) °C (Expl.)
• Solubility: water > 10 g/100 mL 20 °C (Expl.)

Safety Data

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

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Eye irritation	Eye Irrit.	2	H319
Skin irritation	Skin Irrit.	2	H315
Specific target organ toxicity - single exposure	STOT SE	3	H335
Specific target organ toxicity - single exposure	STOT SE	2	H371
Acute toxicity	Acute Tox.	4	H332
Specific target organ toxicity - single exposure	STOT SE	3	H336

chemBlink Chemical Story

Disodium hydrogen phosphate is the basic half of one of laboratory chemistry's most familiar buffer pairs. Together with sodium dihydrogen phosphate, it supplies the HPO₄^2-/H₂PO₄^- conjugate acid-base system used near neutral pH. Because phosphate chemistry is so familiar, it is easy to treat this salt as chemically uninteresting. In practice, its buffering, metal-binding, and solid-state behavior make it anything but trivial.

A buffer works because added acid can be accepted by HPO₄^2-, while added base can be consumed by H₂PO₄^-. By mixing defined amounts of mono- and dibasic phosphate, chemists can prepare reproducible pH environments for enzymes, analytical methods, biological samples, and pharmaceutical formulations. The phosphate pair is especially convenient because its second acid dissociation lies near the pH range important to many biochemical systems.

But phosphate is not merely a spectator. It can bind metal ions, precipitate with calcium or magnesium under some conditions, interact with proteins, and change reaction rates. A buffer that maintains pH can therefore simultaneously alter the chemistry being studied. Biochemists learn that "same pH" does not necessarily mean "same environment" when phosphate is exchanged for another buffer.

Freezing provides an even more dramatic lesson. Sodium phosphate buffers can undergo selective crystallization of one phosphate salt or hydrate as ice forms. Removing one member of the buffer pair from the remaining liquid changes the acid-base ratio and can produce large transient pH shifts. Studies of frozen pharmaceutical formulations showed that sodium phosphate systems may become much more acidic during freezing, with consequences for protein stability, even if the thawed solution later appears normal.

Anhydrous versus hydrated identity must also be checked in phosphate salts. Disodium hydrogen phosphate is sold in several hydration states, and each has a different formula mass and sometimes different handling behavior. Once dissolved, the same phosphate equilibria may dominate, but weighing the wrong hydrate produces the wrong molarity. For a reagent used precisely because it should control pH, an unnoticed concentration error is particularly ironic. Good buffer preparation therefore begins with the bottle label and certificate of analysis, not merely with a recipe copied from a protocol.

Disodium hydrogen phosphate matters because it shows that buffers are chemical participants. In a beaker at room temperature it looks like a simple source of HPO₄^2-; during freezing, in the presence of calcium, or near a protein surface, its behavior can change the system substantially. The familiar phosphate buffer is therefore not a chemically invisible background. It is a carefully chosen reagent whose own equilibria must be understood.

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. Biochemical buffer literature on phosphate-metal and phosphate-protein interactions.

4. Pharmacopeial monographs for dibasic sodium phosphate, CAS 7558-79-4.

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