A-Level pH, dissociation constants and buffers: worked solution

Question

Isopropylamine, (CH3)2CHNH2, is a weak base. Describe two laboratory tests, together with the results you would expect from each, that would allow you to tell apart a 0.10 mol dm⁻³ aqueous solution of isopropylamine from a 0.10 mol dm⁻³ aqueous solution of sodium hydroxide.

Both are basic, but NaOH is a strong (fully ionised) base and isopropylamine is a weak (partially ionised) base, so at the same 0.10 mol dm⁻³ concentration they differ in the number of free ions. Test 1 - measure the pH (pH meter or universal indicator). NaOH is fully ionised so [OH^-] is high, giving pH about 13. Isopropylamine is only partially ionised so [OH^-] is lower, giving a lower pH of about 11-12. Test 2 - measure the electrical conductivity. NaOH provides a high concentration of ions and so conducts strongly; isopropylamine is only partly ionised, gives far fewer ions and so has a much lower conductivity. (An acceptable alternative test: the amine has a characteristic fishy/ammoniacal smell whereas NaOH is odourless.)