Potassium dihydrogen phosphate

Structural formula
General
Surname	Potassium dihydrogen phosphate
other names	Potassium bisphosphate Monopotassium phosphate mono-potassium orthophosphate primary potassium phosphate monobasic potassium phosphate KDP E  340 POTASSIUM PHOSPHATE ( INCI )
Molecular formula	KH 2 PO 4
Brief description	odorless and colorless crystals
External identifiers / databases
CAS number 7778-77-0 EC number 231-913-4 ECHA InfoCard 100.029.012 PubChem 516951 ChemSpider 22914 DrugBank DB09413 Wikidata Q415049
properties
Molar mass	136.09 g mol −1
Physical state	firmly
density	2.34 g cm −3
Melting point	253 ° C (decomposition)
solubility	easily soluble in water: 222 g l −1 (20 ° C)
safety instructions
GHS labeling of hazardous substances no GHS pictograms H and P phrases H: no H-phrases P: no P-phrases
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Potassium dihydrogen phosphate is a colorless and odorless potassium salt of phosphoric acid . It is composed of a potassium cation (K ⁺ ) and a dihydrogen phosphate anion (H ₂ PO ₄ ^- ). It belongs to the dihydrogen phosphates .

properties

KH ₂ PO ₄ single crystal

Chemical properties

Potassium dihydrogen phosphate has an acidic reaction in water : pH 4.4 in a 5% solution. Anhydrous potassium dihydrogen phosphate is hygroscopic .

use

Potassium dihydrogen phosphate

Potassium dihydrogen phosphate is used primarily in biochemistry and related natural sciences for the production of buffer solutions . It is often used together with dipotassium hydrogen phosphate , which, in contrast to potassium dihydrogen phosphate, has a basic reaction. The associated basic phosphate is potassium phosphate . Combinations of the three salts lead to aqueous phosphate solutions with a defined pH value. The potassium hydrogen phosphates can be substituted by the corresponding sodium hydrogen phosphates if potassium ions are not desired.

Potassium dihydrogen phosphate is used in food technology as an acid regulator . Together with secondary dipotassium phosphate (K ₂ HPO ₄ ) and tertiary potassium phosphate (K ₃ PO ₄ ), it is approved in the EU as a food additive under the common number E 340 ("potassium phosphate") for certain foods, each with different maximum quantity restrictions. According to the Additive Admissions Ordinance , these are - largely uniform for most of the approved phosphates - individual specifications for a wide range of numerous different types of food. The maximum permitted amounts vary from 0.5 to 50 grams per kilogram (in coffee whiteners for vending machines) or the lack of a fixed limit ( quantum satis - as required, for food supplements and sometimes for chewing gum). Phosphoric acid and its salts are suspected of causing hyperactivity , allergic reactions and osteoporosis . A permissible daily dose of 70 milligrams per kilogram of body weight was set for the total amount of phosphoric acid and phosphates ingested.

Potassium dihydrogen phosphate is also used as a fertilizer.

In non-linear optics , potassium dihydrogen phosphate crystal plates cut at certain angles are used to double the frequency . In the way of parametric fluorescence and frequency halving is possible. For the experiments to develop laser- ignited inertial fusion in the National Ignition Facility , several potassium dihydrogen phosphate crystals with a mass of 318  kg per crystal were grown.

proof

Potassium cation

Like other potassium salts, potassium dihydrogen phosphate can be detected by the violet color of the flame .

Potassium can be detected very specifically, if not very sensitively, with perchloric acid . In the process, poorly soluble potassium perchlorate precipitates .

${\displaystyle \mathrm {K^{+}+ClO_{4}^{-}\rightarrow KClO_{4}\downarrow } }$

Dihydrogen phosphate anion

If ammonium heptamolybdate solution is added to a nitric acid , phosphate- containing solution, yellow ammonium molybdate phosphate precipitates .

${\displaystyle \mathrm {H_{2}PO_{4}^{-}+22\;H^{+}+3\;NH_{4}^{+}+12\;MoO_{4}^{2-}\rightarrow (NH_{4})_{3}[P(Mo_{3}O_{10})_{4}\cdot aq]\downarrow +12\;H_{2}O} }$

Arsenic acid has a disruptive effect in this detection, as it is partially precipitated as molybdenum arsenate, analogous to phosphate.

If the solution is initially neutralized, there are further possibilities for evidence:

With silver nitrate , a yellow precipitate of silver phosphate forms , which dissolves in weak acids.

${\displaystyle \mathrm {H_{2}PO_{4}^{-}+3\;AgNO_{3}\rightarrow Ag_{3}PO_{4}\downarrow +3\;NO_{3}^{-}+2H^{+}} }$

When barium chloride is added, white barium hydrogen phosphate precipitates, which easily dissolves in acetic acid .

${\displaystyle \mathrm {H_{2}PO_{4}^{-}+BaCl_{2}\rightarrow BaHPO_{4}\downarrow +2\;Cl^{-}+H^{+}} }$

literature

• Gerhart Jander , Ewald Blasius et al .: Introduction to the inorganic-chemical internship . 14th revised edition. Hirzel, Stuttgart, 1995. ISBN 3-7776-0672-3

Web link

• 318 kilogram crystals of potassium dihydrogen phosphate and their uses

swell

1. ↑ Entry on E 340: Potassium phosphates in the European database on food additives, accessed on July 1, 2020.
2. ↑ Entry on POTASSIUM PHOSPHATE in the CosIng database of the EU Commission, accessed on July 1, 2020.
3. ↑ ^{a b c d e} Entry on potassium dihydrogen phosphate in the GESTIS substance database of the IFA , accessed on December 19, 2019 (JavaScript required)