⚗️ Dissolved oxygen in water — optical sensor (luminescent, quenching of fluorescence)

Physicochemistry ISO 17289

Overview

ISO 17289:2014 specifies the optical method of determining dissolved oxygen in water with a sensor working on the principle of fluorescence quenching. The measurement is expressed as an oxygen concentration in mg/l, as a percentage saturation, or both; depending on the instrument, detection limits of 0.1 or 0.2 mg/l are achieved according to the manufacturer's instructions, and most instruments also measure values above 100 % (supersaturation — possible during a heavy algal bloom even up to 200 % and more). Above 100 % saturation degassing has to be prevented, and below 100 % the ingress of oxygen into the sample during sampling and measurement. The method is suitable for field measurements, continuous monitoring and laboratory measurements; it is one of the preferred methods for strongly coloured and turbid waters and for waters unsuitable for the Winkler titration because of substances that bind iron and iodine (which interfere with the iodometric method ISO 5813). It applies to drinking water, natural waters, wastewater and saline waters; for sea water and estuarine water a salinity correction is essential when the concentration is measured. In the waters within the scope of application no interferences have been found.

Manufacturers describe optical sensors (e.g. luminescent LDO probes) as using light and a luminescent material; they require less maintenance, hold their calibration better, and hydrogen sulfide and other dissolved gases do not affect their performance; they provide accuracy, stability and repeatability with a short response time — unlike galvanic and Clark electrodes, which require frequent calibration and stirring of the sample. The PKN catalogue does not list a Polish edition of this standard (the Polish title in this entry is a working translation).

Method principle

Optical sensors measuring the lifetime or the phase shift of luminescence/fluorescence consist of a luminophore or fluorescent dye in the sensor cap, a light source (e.g. an LED) and a photodetector. Pulsed or modulated light excites the luminophore, whose luminescence is quenched in the presence of oxygen; the photodetector converts the emission into an electrical signal, from which the phase shift or the lifetime is calculated — and on that basis the concentration of dissolved oxygen. Temperature has a twofold effect: on the quenching process in the membrane (the primary signal is compensated by a built-in temperature sensor — modern meters do this automatically) and on the solubility of oxygen in the sample; salinity also has a significant effect. To calculate the percentage saturation of samples in contact with the atmosphere the atmospheric pressure is needed (entered manually or taken from a built-in pressure sensor). Most sensors have a second LED as an internal reference for compensation. Reagents (only for calibration): grade 2 water (ISO 3696), anhydrous sodium sulfite or its heptahydrate, a cobalt(II) salt (e.g. CoCl₂·6H₂O), ascorbic acid, NaOH 1 mol/l, nitrogen 99.995 %. Apparatus: a measuring instrument with a probe (designs differ in the excitation wavelength and the luminophore) and a meter indicating mg/l and/or % saturation, a thermometer with a scale division of 0.5 °C or finer (usually built in), a barometer with a scale division of 1 hPa (usually built in). The measurement is usually made directly in the water on site; if that is not possible — in a gas-tight flow-through vessel or immediately after sampling in a discrete sample (with a greater uncertainty); when filling the vessel, the uptake or the degassing of oxygen is kept to a minimum by transferring the sample without turbulence (laminar flow): the vessel is immersed slowly, and from a tap an inert tube is led to the bottom of the vessel and at least 3 volumes are allowed to overflow; only pumps that displace the water are used (not air pumps). During the measurement the sample has to flow past the cap at a sufficient velocity (natural flow, movement of the probe or a magnetic stirrer) with no air bubbles. The result in mg/l is given to one decimal place; the saturation w(O₂) = ρ(O₂)/ρ(O₂)_th × 100 is rounded to a whole number, and the temperature, the pressure and, where applicable, the salinity are stated (annex A: table of conductivity vs. salinity, pressure vs. altitude).

Applications

• Field measurements and continuous monitoring of oxygen in rivers, lakes and reservoirs (including supersaturation during blooms)
• Wastewater treatment plants — control of aeration in activated sludge tanks with LDO sensors
• Strongly coloured and turbid waters and waters containing substances that bind iron or iodine, for which the Winkler method fails
• Sea water and estuarine water (with a salinity correction), drinking water, process waters
• Laboratory measurements (e.g. oxygen before and after BOD incubation) — the sensor consumes no oxygen and no stirring is needed

Key parameters

Standard

Standard number

ISO 17289:2014 (EN ISO 17289:2014)

Title (PL)

Jakość wody — Oznaczanie tlenu rozpuszczonego — Metoda z czujnikiem optycznym (tłumaczenie robocze)

Title (EN)

Water quality — Determination of dissolved oxygen — Optical sensor method

Step-by-step procedure

1. Condition of the instrument

   Check the sensor cap (undamaged; small scratches are usually harmless) and calibrate it if necessary according to the manufacturer's instructions; every day and after a change of temperature or pressure check the calibration at air saturation.

2. Zero check

   If necessary immerse the probe in 1 l of water with at least 1 g of sodium sulfite (about 1 mg of a Co(II) salt shortens the time from about 20 to 5 min), or in about 110 ml of alkaline ascorbic acid solution (2 g of the acid + 25 ml of NaOH 1 mol/l + 85 ml of water, stirred slowly, 3 min before use; response after about 30 min), or in pure nitrogen; correct the zero if the instrument allows it. Cobalt(II) salts are toxic.

3. Calibration at saturation

   Calibrate in a suitable container according to the manufacturer — simply and effectively in water-vapour-saturated air. Replace the cap when calibration is impossible, when the software does not accept the cap's response, or when the response is unstable or slow; the values can be checked by Winkler titration (ISO 5813).

4. Sampling / measurement on site

   Measure directly in the water. If that is not possible — in a gas-tight flow-through vessel or immediately after sampling: immerse the vessel slowly; from a tap connect an inert tube to the bottom of the vessel and let at least 3 volumes overflow; use only pumps that displace the water, fill from the bottom with overflow, keeping the flow laminar.

5. Measurement

   Immerse the probe so that the cap and the temperature sensor are in intensive contact with the water; ensure flow past the cap (natural flow, movement of the probe, stirrer) with no bubbles; check the temperature, the atmospheric pressure and the salinity of the sample (for instruments without automatic compensation — corrections according to annex A).

6. Result and test report

   Give the concentration in mg/l to one decimal place and/or the saturation in % (w(O₂) = ρ(O₂)/ρ(O₂)_th × 100, a whole number) together with the water temperature, the atmospheric pressure and the salinity; record the reference to ISO 17289:2014, the identification of the sample and any circumstances affecting the result.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Soluble cobalt(II) salts are toxic to humans and to aquatic organisms — dispense carefully, do not discharge into the environment
• Sodium hydroxide 1 mol/l — goggles and gloves
• Nitrogen from a cylinder — ventilate the room, secure the cylinder
• Field measurements over water — protection against falling in and drowning

Frequently asked questions