Aqueous silver ions, Ag+(aq), react slowly with aqueous iron(II) ions, Fe2+(aq). An equilibrium is established. Ag+(aq) + Fe2+(aq) ⇋ Ag(s) + Fe3+(aq) The concentration of Ag+(aq) at equilibrium can be determined by titration with a standard solution of aqueous potassium thiocyanate, KSCN(aq). During the titration, the remaining Ag+(aq) ions react with SCN-(aq) ions to form a precipitate of AgSCN(s). Ag+(aq) + SCN¯(aq) → AgSCN(s) When all Ag+(aq) ions have been removed from solution, excess SCN-(aq) ions react with Fe3+(aq) to form a complex ion, FeSCN2+(aq), which has a red colour. Fe3+(aq) + SCN¯(aq) → FeSCN2+(aq) The appearance of the red colour indicates the end-point. A student carries out an experiment to determine the equilibrium constant, Kc. Kc = [Fe3+(aq)]eqm / [Fe2+(aq)]eqm [Ag+(aq)]eqm The student makes 250.0 cm³ of 0.0200 mol dm⁻³ KSCN(aq) to use in the titration.