The absorbance A is calculated via where I is the light intensity (raw data units) measured with the electrolyte, I ref is the light intensity measured with pure water, used as reference, and l is the path length, chosen as 0.15 mm in this work. As an illustrative example, Table .3 shows the light intensity (in raw data units) obtained by measuring a 1.82 M solution of pure V IV ; the reference intensity was acquired with pure water and the dark signal was measured with no light exposure. The zero values observed in the dark signal confirm that neither external light nor internal noise was detected. For this reason D is not included in equation 1. The total measurement time is ≈ 10 s, including 5 s of integration time.