Potassium hydrogen phthalate

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Potassium hydrogen phthalate
IUPAC name	Potassium hydrogen phthalate
Other names	hydrogen potassium phthalate, phthalic acid potassium salt, potassium biphthalate, potassium acid phthalate, 1,2-benzenedicarboxylic acid, monopotassium salt, KHP, KHPh
Identifiers
CAS number	877-24-7 Y
Properties
Molecular formula	KHC8H4O4
Molar mass	204.2212 g/mol
Appearance	White or colourless solid
Density	1.64 g/cm3, solid
Melting point	ca. 295 °C (568 K)--decomposes
Solubility in water	soluble
Structure
Coordination geometry	tetrahedral
Hazards
MSDS	External MSDS
R-phrases	R36, R37, R38
Flash point	Non-flammable
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Potassium hydrogen phthalate, often called simply KHP, is a white or colorless, 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, however, slightly hygroscopic and is generally kept in a desiccator before use.^[1] 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.

In water KHP dissociates completely giving the potassium cation (K⁺) and hydrogen phthalate anion (HP^- or Hphthalate^-). As a weak acid hydrogen phthalate reacts reversibly with water to give hydronium (H₃O⁺) and phthalate ions.

HP^- + H₂O P^2- + H₃O⁺

KHP can be used as a buffering agent (in combination with HCl or 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.

[edit] References

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