Monopotassium phosphate

Monopotassium phosphate

MKP from a pet shop.
Names
IUPAC name Potassium dihydrogen phosphate
Other names Potassium phosphate monobasic KDP
Identifiers
Jmol-3D images	Image
SMILES [K+].OP(O)([O-])=O
Properties
Chemical formula	KH2PO4
Molar mass	136.086 g/mol
Appearance	White crystalline powder
Odor	Odorless
Density	2.338 g/cm3
Melting point	252.6 °C (486.7 °F; 525.8 K)
Boiling point	400 °C (752 °F; 673 K) (decomposes)
Solubility in water	22.6 g/100 mL (20 °C) 83.5 g/100 mL (90 °C)
Solubility	Slightly soluble in ethanol, methanol Insoluble in benzene, dichloromethane
Acidity (pKa)	6.86
Hazards
Safety data sheet	ScienceLab
Flash point	Non-flammable
Related compounds
Related compounds	Dipotassium phosphate Tripotassium phosphate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Monopotassium phosphate (MKP) or potassium dihydrogen phosphate (KDP) is a soluble phosphoric acid salt of potassium with the chemical formula KH₂PO₄.

Properties

Chemical

Monopotassium phosphate is commonly used as a buffer with a pKa of 6.86, and will react with both acids and bases in solution. Reacting MKP with 1 mole of potassium hydroxide will produce dipotassium phosphate, and further adding phosphoric acid will convert it to tripotassium phosphate:

KH₂PO₄ + KOH → K₂HPO₄ + H₂O

K₂HPO₄ + KOH → K₃PO₄ + H₂O

Using potassium carbonate for the same reaction stops at dipotassium phosphate due to the lower pKa of potassium carbonate.

Physical

Potassium dihydrogen phosphate is a white solid, soluble in water. It decomposes when heated above 400 °C. MPK's density is 2.338 g/cm³ at standard conditions.

Availability

MKP is sold in some agricultural stores as fertilizer. Hydroponics stores and companies sell it as a water-soluble nutrient.

It can also be purchased from some pet stores for use in household aquariums.

Preparation

Potassium dihydrogen phosphate can be made by the partial neutralization of phosphoric acid with potassium hydroxide or potassium carbonate. Excess phosphoric acid is recommended as it will limit the generation of any monohydrogen phosphate species. It can easily be crystallized from solution due to its lower solubility at ambient temperatures.

Projects

• Grow large crystals
• Make elemental phosphorus
• pH buffer

Handling

Safety

MPK is safe to handle and poses little toxicity.

Storage

MKP is highly stable with time and doesn't require any particular storage conditions.

Disposal

MKP is nontoxic and safe to dispose of down a drain or in normal household waste.

References

Relevant Sciencemadness threads

• mono, di, or tribasic Potassium Phosphate