I2(aq) + 2S2O32-(aq) → 2I-(aq) + S4O62-(aq). The copper(II) ions can also be titrated against a solution of EDTA in a complexiometric titration. The indicator to use is called Fast Sulphon F. An acid-base titration would not at first sight seem to be very helpful.he blue colour of your solution is caused by the Cu2+(aq) ion. The more concentrated the solution the more intense the colour is. You can make up known concentrations of copper(ll) ion solution and use a colorimeter to produce a copper(ll) ion concentration/absorbance calibration curve.