Potassium Phosphate Monobasic (KH2PO4)

Technical Specifications

Product Description & Scientific Applications

Potassium Phosphate Monobasic (KH₂PO₄) is the acidic member of the phosphate buffer pair. In the standard tables of biological buffers it appears at pKa₂ 7.21, alongside MOPS at 7.20 and HEPES at 7.55, with the dibasic salt entered separately at pKa₃ 12.32.

Buffering. A weak acid and its conjugate base buffer effectively within about one pH unit of the pKa, which places the monobasic and dibasic pair across pH 5.8–8.0 — the range covering most biochemical work. Buffers are prepared either by adding one stock solution to the other until the target pH is reached, or from the tabulated collections of Gomori and of Stoll and Blanchard. A 1 M monobasic stock is 136 g per litre.

Practical behaviour. Phosphate buffer pH is far less temperature-dependent than that of Tris, which is its main advantage in work spanning a temperature range. It is sensitive to dilution, so the pH of a diluted buffer should be measured rather than assumed.

Media and physiological buffers. Phosphate-buffered saline contains 144 mg/L of this salt with 9.0 g/L sodium chloride and 795 mg/L dibasic sodium phosphate. It supplies both phosphate and potassium in defined cell-culture and microbiological media, and is used in protein isolation from bacterial cells. At 100 mM the potassium phosphate pair holds pH against dissolved carbon dioxide.

KDP single crystals. Grown from aqueous solution, KH₂PO₄ is a long-standing nonlinear optical and electro-optical material — frequency doubling and tripling in Nd:YAG systems, electro-optic switches and modulators, and apertures of tens to hundreds of square centimetres in laser fusion facilities. The crystal is ferroelectric below 123 K, the protons occupying either of two sites along each hydrogen bond in the paraelectric phase and ordering below the transition. Optical-grade growth requires purity well beyond reagent grade: trace Cr³⁺, Fe³⁺ and Al³⁺ induce secondary nucleation and narrow the metastable zone.

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