Potassium dihydrogen phosphate

Each ml of this solution contains 1 mmol of potassium ions, 1 mmol of phosphate ions, and 2 mmol of hydrogen ions.

Lets dwell on that for a moment.

The compound is composed of K+, 2H+ and PO43-. In fact it very closely resembles phosphoric acid (H3PO4) except one of the hydrogen atoms has been substituted with potassium.

molecule of potassium dihydrogen phosphate The pH to 4.5; so not only is this solution hyperosmolar (2 osmoles per Kg), it is also acidic. One would be very upset to have it splash on their paper cut. The total content of each ampoule is 1.361 g of potassium dihydrogen phosphate per 10mls, which comes to 136.1g of the dry phosphate per litre.

Contents of a potasium dihydrogen phosphate ampoule.

Some textbooks (and specifically the MIMS PI for the DBL potassium dihydrogen phosphate) report authoritatively that at physiological pH the most numerous form of phosphate is the divalent PO42- form, outnumbering the monovalent form 4 to 1.

In other textbooks the reverse is stated.

One assumes that the monovalent form ought to be the dominant in the realm of extracellular fluid, given that the pKa for the monovalent form is roughly 7.5 and the pKa for the divalent form is 6.2.
The region in which phosphoric acids are in equilibrium with their conjugate base are pKa +/-2.