Abstract: A sensing arrangement for sensing the addition of reactants to a solution within a tank which includes a first inlet, a second inlet, and a reactant supply controller for controlling the addition of reactants into the tank through the first and second inlets. The first inlet is configured to supply a first reactant to the tank and the second inlet is configured to supply a second reactant to the tank. The sensing arrangement comprises a detector, such as a conductivity detector or an ion concentration detector, having a probe for measuring the presence of the reactants in the solution within the tank. The probe is positioned substantially adjacent the second inlet. The detector provides a mechanism for measuring the presence of and concentration of the first reactant in the solution while the first reactant is added to and mixed with the solution.