Oxygen consumed means the oxygen that the oxidizable compounds of sewage and water consume when treated in an acid solution with potassium permanganate. The expression is synonymous with oxygen required, oxygen absorbed, and oxygen-consuming capacity. It should not be confused with biochemical oxygen demand. As the carbon, not the nitrogen, in organic matter is oxidized by potassium permanganate, oxygen consumed is considered by some an indication of the amount of carbonaceous organic matter present. The determination indicates, however, only part of the carbon, the proportion varying in different samples because the carbon in nitrogenous matter is not so readily oxidized as that in carbonaceous organic matter. Furthermore, it does not directly differentiate the carbon present in unstable organic matter from that in fairly stable organic matter, such as is sometimes referred to as residual humus matter. As nitrite nitrogen, ferrous iron, sulfide, and other oxidizable mineral substances reduce potassium permanganate, corrections for them should be made in the determination. RECOMMENDED METHOD. _Reagents._—1. Dilute sulfuric acid. Dilute 1 part of concentrated sulfuric acid with 3 parts of distilled water and free the solution from oxidizable matter by adding potassium permanganate until a faint pink color persists after the solution has stood several hours. 2. Standard ammonium oxalate. Dissolve 0.888 gram of the pure salt in 1 liter of distilled water. One cc. is equivalent to 0.1 mg. of oxygen. An equivalent quantity of oxalic acid or sodium oxalate may be used. 3. Standard potassium permanganate. Dissolve 0.4 gram of the crystallized salt in 1 liter of distilled water. Add 10 cc. of the dilute sulfuric acid and 10 cc. of this solution of potassium permanganate to 100 cc. of distilled water, and digest 30 minutes. Add 10 cc. of the ammonium oxalate solution, and then add potassium permanganate till a pink coloration appears. This destroys the oxygen-consuming capacity of the water used. Now add another 10 cc. of ammonium oxalate solution and titrate with potassium permanganate. Adjust the potassium permanganate solution so that 1 cc. is equivalent to 1 cc. of ammonium oxalate solution or 0.1 mg. of available oxygen.