ÖNORM EN 25813

This International Standard specifies an iodometric method for the determination of dissolved oxygen in water by the so-called "Winkler procedure" modified in order to make allowance for certain interferences. The iodometric method is the reference method for the determination of dissolved oxygen in water. It is applicable to all types of water having dissolved oxygen concentrations greater than 0,2 rng/l, up to double saturation of oxygen (approximately 20 mg/l), which are free from interfering substances. Readily oxidizable organic substances such as tannins, humic acid and lignins, interfere. Oxidizable sulphur compounds such as sulphides and thiourea also interfere, as do actively respiring Systems which readily consume oxygen. In the presence of such substances, it is preferable to use the electrochemical probe method specified in ISO 5814. Nitrites up to a concentration of 15 mg/l do not interfere with the determination because they are destroyed by the addition of sodium axide. If oxidizing or reducing substances are present, it is necessary to make modifications to the method; these are described in clause 9. If suspended matter, capable of fixing or consuming iodine, is present, the method may be used with the modification described in the annex, but it is preferable to use the electrochemical probe method.