Potassium Phosphate Dibasic, anhydrous

Technical Specifications

Product Description & Scientific Applications

Potassium Phosphate Dibasic (K₂HPO₄) is the basic member of the phosphate buffer pair, entered in the standard buffer tables at pKa₃ 12.32 against the monobasic salt's pKa₂ 7.21. The two are combined across pH 5.8–8.0.

Freeze–thaw stability. The choice of potassium over sodium matters most when the buffer is frozen. On freezing a sodium phosphate buffer, disodium phosphate reaches its solubility limit and crystallises — its eutectic temperature is −0.5 °C against −9.7 °C for the monosodium salt — which strips the basic component from the freeze concentrate and drives the pH down. Falls of 3 to 3.6 units are documented, from pH 7 to as low as 3.4 at −30 °C. Potassium phosphate has no such limit: its pH stays near constant, with at most a small rise from crystallisation of the monobasic salt near −6 °C. Both monomeric and tetrameric β-galactosidase recover markedly less activity after freeze–thawing in sodium phosphate than in potassium phosphate, and recombinant interferon-γ loses helix content and shows aggregation bands on the acidification of PBS that it does not show on the slight alkalinisation of potassium phosphate. Protein-storage protocols name sodium phosphate as the most common mistake in freezing proteins.

Aqueous two-phase extraction. As a water-structuring salt it forms aqueous biphasic systems with poly(ethylene glycol) and with ionic liquids, a polymer-rich upper phase over a salt-rich lower one. In PEG systems it gave a trypsin partition ratio above 25, higher than the other salts tested, and it raises the partition coefficients of amino acids.

Media and fermentation. It supplies phosphate and potassium in defined cell-culture and microbiological media, and serves as a nutrient in antibiotic culture. Its solubility — near 1500 g per litre at 20 °C against 77 for the disodium salt — is what permits the concentrated stocks that buffer preparation and two-phase extraction both require, and it is the same solubility difference that explains the freezing behaviour.

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