Potassium dihydrogen phosphate (KH2PO4)
Potassium dihydrogen phosphate (KH2PO4, CAS 7778-77-0) is a water-soluble inorganic salt of phosphoric acid, also known as monopotassium phosphate or potassium phosphate monobasic. It is widely used as the acidic component of phosphate buffers in laboratories, HPLC mobile phases and culture media. It is also used in fertilisers and as a food additive (E340(i)), and it is supplied in grades such as ACS, USP, EP and FCC.
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Potassium dihydrogen phosphate
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What is potassium dihydrogen phosphate?
Potassium dihydrogen phosphate is the monopotassium salt of orthophosphoric acid, with the formula KH2PO4 and a molar mass of approximately 136.09 g/mol. It is typically supplied as a white crystalline powder or colourless crystals that dissolve readily in water and give a mildly acidic solution. Its CAS number is 7778-77-0.
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How is it used in the laboratory?
KH2PO4 is most often combined with dipotassium hydrogen phosphate (K2HPO4) to prepare potassium phosphate buffers. These buffers work best in the near-neutral range because the second dissociation constant of phosphoric acid (pKa2) is about 7.2. Typical applications include:
- buffered mobile phases for HPLC
- enzyme assays and protein work
- microbiological and cell culture media
- components of PBS-type formulations
It is also the basis of KDP crystals, which are used in non-linear optics.
Which grades and documentation should buyers consider?
The right grade depends on the application:
- ACS reagent grade for analytical work
- USP, BP or EP (Ph. Eur.) grades for pharmaceutical use
- FCC grade for food applications
- HPLC or molecular biology grades where low UV absorbance or the absence of nucleases matters
Procurement teams should request a CoA for each batch, confirming the assay, pH, loss on drying and heavy-metal limits, together with a current SDS. The substance is commonly not classified as hazardous under GHS, but buyers should check the supplier's SDS and the substance's REACH or TSCA status for their region. The material is somewhat hygroscopic, so containers should be kept tightly closed.
Related concepts
Dipotassium hydrogen phosphate (K2HPO4) is the basic partner of KH2PO4 in phosphate buffers, and sodium dihydrogen phosphate (NaH2PO4) is the sodium analogue. Other related concepts include phosphate-buffered saline (PBS), buffer capacity and pKa. Alternative buffers such as Tris and HEPES are often considered when phosphate interferes with an assay.
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