Potassium dihydrogen phosphate

Potassium dihydrogen phosphate (KH₂PO₄) is an inorganic salt with multiple uses. It is also known as monopotassium phosphate, potassium phosphate monobasic, potassium biphosphate, and potassium acid phosphate.

In US Patent 374,201 (1887), Carl. V. Petraeus of Camden, NJ, described a process for making potassium phosphates, specifically KH₂PO₄ with a slight excess of potassium. His starting materials were “bone, bone-black, South Carolina rock, or any compounds mainly composed of lime”. Petraeus’s somewhat convoluted process consisted of five steps:

1. decomposing the starting material with sulfuric or oxalic acid;
2. leaching with water to produce an impure “acid phosphate of lime” (calcium dihydrogen phosphate);
3. treating the calcium dihydrogen phosphate with potassium sulfate;
4. adding sufficient potassium carbonate or hydroxide to form a solution of KH₂PO₄ with excess potassium; and
5. after filtering to remove the calcium salts, evaporating the solution to the point of crystallization.

Today, KH₂PO₄ is manufactured simply via the reaction between widely available phosphoric acid and potassium carbonate.

The uses of KH₂PO₄ include agricultural plant nutrients, buffering agents in pharmaceuticals and other preparations, and applications that take advantage of its nonlinear optical properties. To that end, in 2006 Changshui Fang and colleagues at Shandong University (Jinan, China) reviewed how lasers induce damage in large-aperture KH₂PO₄ crystals and suggested ways to improve the crystals’ optical properties.

Large-aperture crystals are useful in Pockels cells, in which an external electric field is used to control the cell’s refractive index, and in frequency-doubling devices used in inertial-confinement fusion power plants.