Potassium hydrogen phthalate

Potassium hydrogen phthalate

Names
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
ChemSpider	12839 Y
Jmol interactive 3D	Image
PubChem	13413
InChI InChI=1S/C8H6O4.K/c9-7(10)5-3-1-2-4-6(5)8(11)12;/h1-4H,(H,9,10)(H,11,12);/q;+1/p-1 Y Key: IWZKICVEHNUQTL-UHFFFAOYSA-M Y InChI=1/C8H6O4.K/c9-7(10)5-3-1-2-4-6(5)8(11)12;/h1-4H,(H,9,10)(H,11,12);/q;+1/p-1 Key: IWZKICVEHNUQTL-REWHXWOFAT
SMILES [K+].O=C(O)c1ccccc1C([O-])=O
Properties
Chemical formula	C8H5KO4
Molar mass	204.22 g·mol−1
Appearance	White or colorless solid
Density	1.636 g/cm3
Melting point	~295 °C (decomposes)
Solubility in water	80 g/L (20 °C)[1]
Solubility	slightly soluble in alcohol
Acidity (pKa)	5.4
Structure
Coordination geometry	tetrahedral
Hazards
Main hazards	Irritant to eyes, skin, and respiratory system
Safety data sheet	External MSDS
R-phrases	R36 R37 R38
Flash point	Non-flammable
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Potassium hydrogen phthalate, often called simply KHP, is an acidic salt compound. It forms white powder, colorless crystals, a colorless solution, and an ionic solid that is the monopotassium salt of phthalic acid. KHP 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.^[2][3][4] 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 dissociates completely in water, 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 P²⁻ + H₃O⁺

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. Most TOC analyzers are based on the oxidation of organics to carbon dioxide and water, with subsequent quantitation of the carbon dioxide. Many TOC analyzers suggest testing their instruments with two standards: one typically easy for the instrument to oxidize (KHP), and one more difficult to oxidize. For the latter, benzoquinone is suggested.

References