Cation concentration measurements

In a system for measuring ion concentration in a sample liquid, said system including an analyzer cell having an electrode which produces a voltage representing the concentration of ions in contact with said electrode, and a meter having an indicator which moves over a meter scale in accordance with the voltage on said electrode, calibration apparatus comprising:

• means for supplying to said analyzer cell a first sample having unknown concentrations c.sub.a and for supplying a second sample liquid having a concentration which is a known concentration increment c.sub.i above the concentration c.sub.a, and
• an auxiliary scale on said meter movable with respect to said fixed scale, said auxiliary scale having:
• a reference marking,
• scale calibration markings representing concentration, the distance between the reference marking and any scale marking being given by ##EQU9## where B is a known constant and E.sub.a is the voltage on the electrode when said first solution is in contact with it, E.sub.b is the voltage on said electrode when said second voltage is in contact with it,
• said auxiliary scale being movable with respect to said fixed scale so that said reference mark is aligned with said indicator when said first sample liquid is supplied to said analyzer.