How to Measure the Dissolved Oxygen Level of Water

A DO meter measures the amount of oxygen dissolved in an aqueous solution. There are two primary types of dissolved oxygen sensing technologies available: the optical based sensing method which is commonly referred to as luminescent and the Clark electrochemical or membrane-covered electrode. Within these two types of technologies, there are slight variations available. For example, there are two types of optical sensors. Both types of optical sensors measure luminescence as it is affected by the presence of oxygen; however, one sensor measures the lifetime of the luminescence while the other sensor measures the intensity of the luminescence. The two types of electrochemical sensors available are polarographic and galvanic.

Measuring Dissolved Oxygen by the Titrimetric Method

The titrimetric method for dissolved oxygen analysis is known as the Winkler method. This method was developed by L.W. Winkler, a Hungarian chemist, in 1888. Also known as the iodometric method, the Winkler method is a titrimetric procedure based on the oxidizing property of dissolved oxygen. This method has long been the standard for accuracy and precision when measuring dissolved oxygen.

Step 1: 2Mn2+ + O2 + 4OH- → 2MnO(OH)2

Step 2: MnO(OH)2 + 2I- + 4H+ → Mn2+ + I2 + 3H2O

Step 3: I2 + 2S2O32-- + S4O62-

Click here See Hanna titration for Dissolve Oxygen.

Measuring Dissolved Oxygen by the Sensor Method

The most popular method for dissolved oxygen measurements is with a dissolved oxygen meter and sensor. While the general categories of dissolved oxygen sensors are optical and electrochemical, electrochemical sensors can be further broken down into polarographic, pulsed polarographic and galvanic sensors. In addition to the standard analog output, several of these dissolved oxygen sensor technologies are available in a smart sensor platforms with a digital output.

As dissolved oxygen concentrations are affected by temperature, pressure and salinity, these parameters need to be accounted for 7. These compensations can be done manually or automatically with a dissolved oxygen meter or data logging software. Temperature is generally measured by a thermistor within the sensor and is acquired by the meter or data logger without prompting. Many DO meters include an internal barometer, and data logging systems can be set up with an external barometer or water level sensor for pressure measurements. Barometric pressure can also be manually input as altitude, true barometric pressure or corrected barometric pressure.

Calibration and operating procedures can vary between models and manufacturers. An instruction manual should be referenced during the measurement and calibration processes.

Click Here See Hanna Dissolve Oxygen Meter with Polarographic cell sensor.

Click Here See Hanna Dissolve Oxygen Meter with Galvanic cell sensor.

Optical Dissolved Oxygen Sensors

Optical dissolved oxygen sensors measure the interaction between oxygen and certain luminescent dyes. When exposed to blue light, these dyes become excited (electrons gaining energy) and emit light as the electrons return to to their normal energy state. When dissolved oxygen is present, the returned wavelengths are limited or altered due to oxygen molecules interacting with the dye. The measured effect is inversely proportional to the partial pressure of oxygen. While some of these optical DO sensors are called fluorescent sensors, this terminology is technically incorrect. These sensors emit blue light, not ultraviolet light, and are properly known as optical or luminescent DO sensors. Optical dissolved oxygen sensors can measure either the intensity or the lifetime of the luminescence, as oxygen affects both.

An optical DO sensor consists of a semi-permeable membrane, sensing element, light-emitting diode (LED) and photodetector. The sensing element contains a luminescent dye that is immobilized in sol-gel, xerogel or other matrix. The dye reacts when exposed to the blue light emitted by the LED. Some sensors will also emit a red light as a reference to ensure accuracy. This red light will not cause luminescence, but simply be reflected back by the dye. The intensity and luminescence lifetime of the dye when exposed to blue light is dependent on the amount of dissolved oxygen in the water sample. As oxygen crosses the membrane, it interacts with the dye, limiting the intensity and lifetime of the luminescence. The intensity or lifetime of the returned luminescence is measured by a photodetector, and can be used to calculate the dissolved oxygen concentration.

Click Here See Hanna Dissolve Oxygen Meter with optical sensor.