Notes

Lecutre 16:
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For the deprotonation of acids, K is known as Ka, the acid dissociation constant. Stronger acids, for example sulfuric or phosphoric acid, have larger dissociation constants; weaker acids, like acetic acid, have smaller dissociation constants.z

(The symbol {displaystyle K_{a}} K_{a}, used for the acid dissociation constant, can lead to confusion with the association constant and it may be necessary to see the reaction or the equilibrium expression to know which is meant.)

Acid dissociation constants are sometimes expressed by {displaystyle pK_{a}} {displaystyle pK_{a}}, which is defined as:

{displaystyle {text{p}}K_{a}=-log _{10}{K_{a}}} {displaystyle {text{p}}K_{a}=-log _{10}{K_{a}}}
This {displaystyle mathrm {p} K} mathrm{p}K notation is seen in other contexts as well; it is mainly used for covalent dissociations (i.e., reactions in which chemical bonds are made or broken) since such dissociation constants can vary greatly.

A molecule can have several acid dissociation constants. In this regard, that is depending on the number of the protons they can give up, we define monoprotic, diprotic and triprotic acids. The first (e.g., acetic acid or ammonium) have only one dissociable group, the second (carbonic acid, bicarbonate, glycine) have two dissociable groups and the third (e.g., phosphoric acid) have three dissociable groups. In the case of multiple pK values they are designated by indices: pK1, pK2, pK3 and so on. For amino acids, the pK1 constant refers to its carboxyl (-COOH) group, pK2 refers to its amino (-NH3) group and the pK3 is the pK value of its side chain.
Perrin DD, Dempsey B, Serjeant EP (1981). "Chapter 3: Methods of pKa Prediction". pKa Prediction for Organic Acids and Bases. (secondary). London: Chapman & Hall. pp. 21–26. doi:10.1007/978-94-009-5883-8. ISBN 978-0-412-22190-3.