Water intended for use as an ingredient in the production of beer (brewing liquor) or other foods
Oxygen is present in water in varying concentrations. The oxygen content of cold tap water has a significant impact on iron pipes. If the water exhibits an oxygen deficiency (less than 2–3 mg/l O2), iron pitting can occur, since a protective layer between the water and pipe cannot form.
However, for boiler feed water, the absence of oxygen is normally required (depending upon the type of boiler, no more than 0.02–0.5 mg/l should be present) in order to prevent corrosion. The oxygen determination is vitally important (BOD5 value) for testing and evaluating wastewater.
In an alkaline medium, dissolved oxygen reacts with manganese(II) ions forming higher manganese hydroxides of varying composition. In a strongly acidic medium, they form manganese(III) ions which release quantities of iodine equivalent to the dissolved oxygen from iodide solutions. The iodine liberated is titrated with sodium thiosulfate.
2 Mn3+ + 2 I - → 2 Mn2+ + I2
I2 + 2 S2O32- → 2 I - + S4O62-