ISO 5814-2012 pdf free download.Water quality – Determination of dissolved oxygen – Electrochemical probe method.
ISO 5814 specifies an electrochemical method for the determination of dissolved oxygen in water by means of an electrochemical cell which is isolated from the sample by a gas permeable membrane.
Measurement can be made either as a concentration of oxygen in milligrams per litre, percentage saturation (% dissolved oxygen) or both. The method measures oxygen in water corresponding to 1 % to 100 % saturation. However, most instruments permit measurement of values higher than 100 %, i.e. supersaturation.
NOTE Supersaturation is possible when the partial pressure of oxygen is higher than in air. Especially when strong algal growth is present, supersaturation of up to 200 % and above can occur.
The method measures oxygen in water with a saturation higher than 100 %, when special arrangements to prevent the outgassing of oxygen during the handling and measurement of the sample are made.
The method is suitable for measurements made in the field and for continuous monitoring of dissolved oxygen, as well as measurements made in the laboratory. It is the preferred method for highly coloured and turbid waters, and also for analysis of waters not suitable for the Winkler titration method because of iron- and iodine- fixing substances, which can interfere in the iodometric method specified in ISO 5813111.
The method is suitable for drinking waters, natural waters, waste waters, and saline waters. If used for saline waters, such as sea or estuarine waters, a correction for salinity is essential.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are indispensable for its application. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 3696, Water for analytical laboratory use — Specification and test methods
3 PrincIple
Immersion of a probe, consisting of a cell enclosed by a selective membrane and containing the electrolyte and at least two metallic electrodes, in the water to be analysed.
NOTE The membrane is effectively impermeable to water and ionic dissolved matter, but is permeable to oxygen and a certain number of other gases.
One of the electrodes is made of a noble metal like gold or platinum. Oxygen is reduced at its surface by an electrochemical process. In order to make this process possible, a suitable electrochemical potential is established at this electrode. For polarographic probes, this is achieved by applying an external voltage related to a second electrode. Galvanic probes are able to build up the potential by themselves.
The current resulting from the reduction of oxygen is directly proportional to the rate of transport of oxygen through the membrane and the layer of electrolyte, and hence to the partial pressure of the oxygen in the sample at a given temperature.
Temperature has two different influences. The first relates to the variation of gas permeability of the membrane with temperature. So the primary signal of the probe has to be compensated with a built-in temperature sensor. Meters manufactured recently are able to do this automatically. The second is the temperature effect on the electrode reactions.
To calculate the percentage of saturation of samples in contact with an atmosphere, it is necessary to include the effective pressure, This can be performed manually or by implementing a pressure sensor for automatic compensation. Salinity can also be an influence.
4 Interferences
Gases and vapors such as chlorine, hydrogen sulfide, amines, ammonia, bromine, and iodine which diffuse through the membrane can interfere, if present, by affecting the measured current.
Other substances present in the sample can interfere with the measured current by causing obstruction. deterioration of the membrane or corrosion of the electrodes. These include solvents, oils, sulfides, carbonates, and biofilms.
5 Reagents
During analysis, use only reagents of recognized analytical grade.
5.1 Water, grade 2. as specified in ISO 3696. optionally from commercial sources.
5.2 Sodium sulfite, anhydrous, Na2SO3 or heptahydrate, Na2SO3’7H20,
5.3 Cobalt(lI) salt, for example cobalt(Il) chloride hexahydrate, CoCI2-6H20.
5.4 Nitrogen gas, N2, purity 99,995 % volume fraction or better.
6 Apparatus
6.1 Measuring instrument, comprising the components specified in 6.1.1 and 6.1.2.
6.1.1 Measuring probe. either of the galvanic type (e.g. lead/silver) or the polarographic type (e.g. silver/gold) with, if required, a temperature-sensitive compensating device.ISO 5814 pdf free download.