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
Solid is one of the State of matter#Four fundamental states, four fundamental states of matter (the others being liquid, gas, and Plasma (physics), plasma). The molecules in a solid are closely packed together and contain the least amount o ...
that is the
monopotassium salt of
phthalic acid
Phthalic acid is an aromatic dicarboxylic acid, with formula C6H4(CO2H)2. Although phthalic acid is of modest commercial importance, the closely related derivative phthalic anhydride is a commodity chemical produced on a large scale. Phthalic aci ...
. KHP is slightly
acid
In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a sequ ...
ic, and it is often used as a
primary standard A primary standard in metrology is a standard that is sufficiently accurate such that it is not calibrated by or subordinate to other standards. Primary standards are defined via other quantities like length, mass and time. Primary standards are u ...
for
acid–base titration
An acid–base titration is a method of quantitative analysis for determining the concentration of an acid or base by exactly neutralizing it with a standard solution of base or acid having known concentration. A pH indicator is used to monitor ...
s because it is solid and air-stable, making it easy to weigh accurately. It is not
hygroscopic
Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature. If water molecules become suspended among the substance ...
. It is also used as a primary standard for calibrating
pH meter
A pH meter is a scientific instrument that measures the hydrogen-ion activity in water-based solutions, indicating its acidity or alkalinity expressed as pH. The pH meter measures the difference in electrical potential between a pH electro ...
s because, besides the properties just mentioned, its
pH in solution is very stable. It also serves as a thermal standard in
thermogravimetric analysis
Thermogravimetric analysis or thermal gravimetric analysis (TGA) is a method of thermal analysis in which the mass of a sample is measured over time as the temperature changes. This measurement provides information about physical phenomena, such ...
.
KHP dissociates completely in water, giving the potassium cation (K
+) and hydrogen phthalate anion (HP
− or Hphthalate
−)
: KHP + H
2O → K
+ + HP
−
and then, acting as a
weak acid
Acid strength is the tendency of an acid, symbolised by the chemical formula HA, to dissociate into a hydron (chemistry), proton, H+, and an anion, A-. The Dissociation (chemistry), dissociation of a strong acid in solution is effectively comple ...
, hydrogen phthalate reacts reversibly with water to give
hydronium
In chemistry, hydronium (hydroxonium in traditional British English) is the common name for the aqueous cation , the type of oxonium ion produced by protonation of water. It is often viewed as the positive ion present when an Arrhenius acid is d ...
(H
3O
+) and phthalate ions.
: HP
− + H
2O P
2− + H
3O
+
KHP can be used as a
buffering agent
A buffer solution (more precisely, pH buffer or hydrogen ion buffer) is an aqueous solution consisting of a mixture of a weak acid and its conjugate base, or vice versa. Its pH changes very little when a small amount of strong acid or base is a ...
in combination with
hydrochloric acid
Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid
Acid strength is the tendency of an acid, symbol ...
(HCl) or
sodium hydroxide
Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions .
Sodium hydroxide is a highly caustic base and alkali ...
(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.
KHP is also a useful standard for
total organic carbon
Total organic carbon (TOC) is the amount of carbon found in an organic compound and is often used as a non-specific indicator of water quality or cleanliness of pharmaceutical manufacturing equipment. TOC may also refer to the amount of organic c ...
(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 Benzoquinone (C6H4O2) is a quinone with a single benzene ring. There are 2 (out of 3 hypothetical) benzoquinones:
* 1,4-Benzoquinone, most commonly, right image (also ''para''-benzoquinone, ''p''-benzoquinone, ''para''-quinone, or just quinone)
* 1 ...
is suggested.
References
{{reflist
Carboxylic acids
Phthalates
Potassium compounds