Potassium Hydrogen Phthalate

Synonyms : Potassium acid phthalate, biphthalate; Phthalic acid monopotassium salt, potassium hydrogen phthalate, Potassium phthalate monobasic, potassium 2-carboxybenzoate, hydrogen potassium phthalate, phthalic acid potassium salt, potassium biphthalate.

Potassium hydrogen phthalate, often called simply KHP, is an acidic salt compound. It is white powder, colorless crystals, a colorless solution, and an ionic solid that is the monopotassium salt of phthalic acid. The hydrogen is slightly acidic, and it is often used as a primary standard for acid-base titrations because it is solid and air-stable, making it easy to weigh accurately. It is not hygroscopic. It is also used as a primary standard for calibrating pH meters because, besides the properties just mentioned, its pH in solution is very stable.

KHP can be used as a buffering agent (in combination with hydrochloric acid (HCl) or sodium hydroxide (NaOH) depending on which side of pH 4.0 the buffer is to be) but should not be used as a buffer for decarboxylation reactions, as these will degrade the KHP and mop up the conjugation groups. KHP is also a useful standard for total organic carbon (TOC) testing.

Specifications of standard potassium hydrogen phthalate:

Assay: 99.8% min
pH: (50g/L, 25C): 3.8~4.2
Clarify of solution: Conform
Water insoluble: 0.005%
Loss on drying: 0.05%
Chloride (Cl): 0.002%
Sulfate (SO4): 0.006%
Sodium (Na): 0.01%
Iron(Fe): 0.0005%
Heavy Metal (Pb): 0.0005%
Melting Point: 295-300 °C (decomposes)

Specifications of ACS Grade potassium hydrogen phthalate:

Grade: Analytical standard (for titrimetry)
Assay: 99.95-100.05% dry basis
Impurities:
≤0.002% S compounds
≤0.003% Cl compounds
≤0.005% insolubles
≤5 ppm Fe
≤0.005% Na
≤5 ppm heavy metals (as Pb)
pH: 4.00-4.02 (25.0 °C±0.2 °C, 0.05 M)
Melting Point: 295-300 °C (decomposes)