Appendix

1072 Analytical Chemistry 2.0Appendix 1: NormalityNormality expresses concentration in terms of the equivalents of one chemical species reacting stoichiometricallywith another chemical species. Note that this definition makes an equivalent, and thus normality, afunction of the chemical reaction. Although a solution of H 2 SO 4 has a single molarity, its normality dependson its reaction.We define the number of equivalents, n, using a reaction unit, which is the part of a chemical species participatingin the chemical reaction. In a precipitation reaction, for example, the reaction unit is the charge ofthe cation or anion participating in the reaction; thus, for the reactionPb 2+ (aq) + 2I – (aq) PbI 2 (s)n = 2 for Pb 2+ (aq) and n = 1 for 2I – (aq). In an acid–base reaction, the reaction unit is the number of H + ionsthat an acid donates or that a base accepts. For the reaction between sulfuric acid and ammoniaH 2 SO 4 (aq) + 2NH 3 (aq) 2NH 4 + (aq) + SO 4 2– (aq)n = 2 for H 2 SO 4 (aq) because sulfuric acid donates two protons, and n = 1 for NH 3 (aq) because each ammoniaaccepts one proton. For a complexation reaction, the reaction unit is the number of electron pairs that themetal accepts or that the ligand donates. In the reaction between Ag + and NH 3Ag + (aq) + 2NH 3 (aq) Ag(NH 3 ) 2 + (aq)n = 2 for Ag + (aq) because the silver ion accepts two pairs of electrons, and n = 1 for NH 3 because each ammoniahas one pair of electrons to donate. Finally, in an oxidation–reduction reaction the reaction unit is thenumber of electrons released by the reducing agent or accepted by the oxidizing agent; thus, for the reaction2Fe 3+ (aq) + Sn 2+ (aq) Sn 4+ (aq) + 2Fe 2+ (aq)n = 1 for Fe 3+ (aq) and n = 2 for Sn 2+ (aq). Clearly, determining the number of equivalents for a chemical speciesrequires an understanding of how it reacts.Normality is the number of equivalent weights, EW, per unit volume. An equivalent weight is the ratio ofa chemical species’ formula weight, FW, to the number of its equivalents, n.EW=FW nThe following simple relationship exists between normality, N, and molarity, M.N = n×M