Potassium hydrogen phthalate

Names
Preferred IUPAC name Potassium 2-carboxybenzoate
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
3D model (JSmol)	Interactive image
ChEMBL	ChEMBL3187152
ChemSpider	12839 Y
ECHA InfoCard	100.011.718
EC Number	212-889-4
PubChemCID	23676735
UNII	GG9121M623 Y
CompTox Dashboard(EPA)	DTXSID0042167
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	204.222 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[2]
Structure
Coordination geometry	tetrahedral
Hazards[3]
Occupational safety and health (OHS/OSH):
Main hazards	Irritant to eyes, skin, and respiratory system
GHS labelling:
Pictograms
Signal word	Warning
Hazard statements	H315,H319,H335
Flash point	Non-flammable
Safety data sheet (SDS)	External MSDS
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). Y verify (what is YN ?) Infobox references

Potassium hydrogen phthalate, often called simplyKHP, is an acidic salt compound. It forms white powder, colorless crystals, a colorless solution, and an ionicsolid that is themonopotassium salt ofphthalic acid. KHP is slightlyacidic, and it is often used as aprimary standard foracid–base titrations because it is solid and air-stable, making it easy to weigh accurately. It is nothygroscopic.^[4][5][6] It is also used as a primary standard for calibratingpH meters because, besides the properties just mentioned, itspH in solution is very stable. It also serves as a thermal standard inthermogravimetric analysis.^[7]

KHP dissociates completely in water, giving the potassium cation (K⁺) and hydrogen phthalate anion (HP⁻ or Hphthalate⁻)

KHP${\displaystyle \stackrel{{\text{H}}_2^{}\text{O}}{\to }}$ K⁺ + HP⁻

and then, acting as aweak acid, hydrogen phthalate reacts reversibly with water to givehydronium (H₃O⁺) and phthalate ions.

HP⁻ + H₂O ⇌ P²⁻ + H₃O⁺

KHP can be used as abuffering agent in combination withhydrochloric acid (HCl) orsodium hydroxide (NaOH). The buffering region is dependent upon the pKa, and is typically +/- 1.0 pH units of the pKa. The pKa of KHP is 5.4, so its pH buffering range would be 4.4 to 6.4; however, due to the presence of the second acidic group that bears the potassium ion, the first pKa also contributes to the buffering range well below pH 4.0, which is why KHP is a good choice for use as a reference standard for pH 4.00.^[8][9]

KHP is also a useful standard fortotal 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 analysts 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.

• Carboxylic acids
• Phthalates
• Potassium compounds

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