Abstract: There is disclosed a method of measuring formation of a barrier to restrict or reduce movement of an electroactive species. The method comprises providing an electrochemical cell (1, 2, 3) having a working electrode (23, 24) and a counter electrode (23, 24) spaced from the working electrode, providing a subject component, a testing component and at least one electroactive species within the cell, the subject and testing components being intended to cause the formation of a barrier to restrict or reduce movement of an electroactive species, applying a potential between the working electrode and the counter electrode sufficient to produce a current proportional to the concentration of the electroactive species being measured, and measuring the current at the working electrode to obtain a measure of the formation of the barrier to restrict or reduce movement of the electroactive species.