Colorimetric coulometers:

They are based on the principle of building a colored species along with a reagent with a metal ion that might be anodically stripped. For instance, the formation of a colored species for a cobalt ion along with nitroso-R-salt and measurement of absorbance within a spectrophotometer. A metal ion might be cathodically decreased and subsequent absorbance might be measured. For instance, Reduction of iron (III) at the cathode and following measurement of absorbance along with l,10-phenanthroline reagent.