I have used new distilled water from two different companies in the preparation of two FeSO4·7H2O saturated solutions. The addition of distilled water from manufacturer A produced a distinct brown colour to the solution. When distilled water from manufacturer B was added to FeSO4·7H2O, the solution turned (as expected) into a beautiful greenish colour. I carried out two tests using the galvanic cell. I wonder why distilled water A caused the oxidation of Fe2+ cations to Fe3+? I attach snapshots of the two galvanic cells (for both saturated FeSO4·7H2O solutions I used an iron electrode, for both distilled waters I used a carbon electrode.).