Potassium hydrogen phthalate

Potassium hydrogen phthalate

General
Systematic 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
Molecular formula	KHC₈H₄O₄
Molar mass	204.22 g/mol
Appearance	White or colourless solid
CAS number	[877-24-7]
Properties
Density and phase	1.64 g/cm³, solid
Solubility in water	soluble
Melting point	ca. 295 °C (568 K)--decomposes
Boiling point	n/a
Acidity (K_a)	3.9x10^-6
Structure
Coordination geometry	tetrahedral
Crystal structure	?
Hazards
MSDS	External MSDS
Main hazards	Irritant to eyes, skin, and respiratory system
NFPA 704	?
Flash point	Non-flammable
R/S statement	R: R36, R37, R38 S: ?
RTECS number	?
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and 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. The pH of a 0.05 mol kg⁻¹ aqueous solution of KHP at various temperatures is given by the following table:^[2]

Additional recommended knowledge

Weighing the right way

What is the Correct Way to Check Repeatability in Balances?

Daily Sensitivity Test

T (°C)	pH
0	4.000
5	3.998
10	3.997
15	3.998
20	4.000
25	4.005
30	4.011
35	4.018
40	4.022
45	4.027
50	4.050

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.