Dissolved Oxygen (DO) Testing

Oxygen is not only present in the air we breathe, it is also an important element in liquids such as water. Fish and plants depend on certain levels of dissolved oxygen to survive. Dissolved oxygen can also be measured as a percentage which refers to the saturation level of oxygen in the water. Dissolved oxygen testing is to measure the number of free oxygen molecules in water, and measuring dissolved oxygen levels is an important indicator for industries such as environmental water quality systems and aquatic ecosystems. In wet chemistry, testing DO in water is achieved by a number of chemical analysis techniques such as titration, optical dissolved oxygen and colorimetric measurements.

Figure 1. Dissolved Oxygen in Water.

Application of DO Testing

DO is a fundamental parameter for monitoring water quality and a key indicator of a healthy aquatic ecosystem. Low DO in water tends to kill most aquatic organisms, shortage of DO is not only a sign of pollution, it is harmful to the fish. While too much oxygen in water is also harmful, which is known as supersaturated oxygen. In wastewater treatment, testing DO levels in water helps us understand biodegradable organic matter and biological oxygen demand (BOD).

• Environmental water health or cleanliness
• The amount and type of biomass that the freshwater system can support
• The amount of decomposition that occurs in the lake or river
• Prevent costly corrosion damage

Our DO Testing Services

At BOC Sciences, we test DO in water by applying the following methods: titrimetric, optical DO and colorimetric methods

Titration Method - Winkler Method

In titration, our chemists use iodine as an indicator, which appears or disappears at the end of the titration. The titration method used to test for dissolved oxygen in water is called the 'Winkler method' and we use the BOD bottle to collect the water sample and perform the titration analysis

• We fill the sample bottle completely with water
• The dissolved oxygen in the sample is then 'fixed' by adding a series of reagents that form an acidic compound
• The final titration is done with a neutralizing compound, which causes a color change
• The point of color change is called the 'end point', and coincides with the concentration of dissolved oxygen in the sample

Figure 2. Range of Tolerance for Dissolved Oxygen in Fish

Optical Dissolved Oxygen Method

We use a probe with a semi-permeable membrane to test DO in the water. If there is DO in the water, it will suppress the red light emitted by the probe because the wavelength is limited/changed. The frequency, intensity and attenuation of the red light depend on the amount of DO in the water. Our optical dissolved oxygen method provides continuous dissolved oxygen measurements.

Figure 3. Range of Tolerance for Dissolved Oxygen in Fish

Colorimetric Method

We mainly use the indigo-carmine method and the Rhodazin D method. In the colorimetric method, different chemical reagents react with the DO in the sample to show a specific color, and the intensity of the color is proportional to the amount of DO in the sample.

• Indigo-carmine method is used to measure DO concentrations between 0.2 and 15 ppm, while Rhodazine D is used to measure much lower DO concentrations (ppb)
• Indocarmine reacts with DO to produce a blue color, while Rhodazine D reagent reacts with DO to produce a rosy or pink solution
• Our colorimetric method is not affected by salinity, temperature or dissolved gases
• We analyze water samples within 30 seconds of adding reagents because this method is time-dependent