(a) These methods describe procedures for the determination of dissolved oxygen in industrial waste water in the presence of certain interfering substances. Common interfering substances which may occur in such water are nitrites, ferrous iron, organic material, sulfites, or other oxidizing or reducing substances. Provisions incorporated in this method will, in most cases, lessen the interference of such substances in the determination of dissolved oxygen. Four methods are given, as follows: Sections Alsterberg (Azide) Method 7 to 11 Rideal-Stewart Method 12 to 16 Pomeroy - Kirshman - Alsterberg Method 17 to 21 Polarographic Method 22 to 28 (b) Selection of the method to use for the dissolved oxygen determination will be determined by the nature of the sample and the type or types of interferences present. (1) The Alsterberg (azide) method is the most effective method for removing interference caused by nitrites. Nitrites are the most common interferences found in surface waters. This method can also be used in the absence of other interfering substances. (2) The Rideal-Stewart (permanganate) method is used only in the presence of ferrous iron and not in the presence of organic matter. (3) The Pomeroy-Kirshman-Alsterberg method is used for waters supersaturated with oxygen or those having a high organic content. (4) The polarographic method is suggested for certain industrial wastes, particularly where sulfites are present. This method is in an early stage of development and is provisionally recommended only for wastes after dilution with natural water by a factor of 4 to 100.