What is Dissolved Oxygen or DO means to us?
Dissolved Oxygen or DO means to amount of free DO in water or any other liquid. This is very crucial parameter for water quality, pollution aspect point of view & treatment process in various industrial application.
What are the methods of measuring Dissolved Oxygen ?
Most scientific community measures measures in % saturation, or mg/liter and to accomplish this measurement there are several methos which are mentioned below.
AA) Winkler Method:
This technique has been in use since invented in late 1800 and even today it is relevant. This technique is called titrimetric means involving series of chemical reactions, use of various chemicals, specialized titration sealed bottle called BOD bottle which does not allow air from outside and taps inside air only. The water sample under testing is collected into these BOD bottles followed with addition of Manganese peroxide solution and Sodium Hydroxide in proportion. This creates a precipitate of Manganese Hydroxide and its color is brown if sample contains oxygen or else it will give white precipitate.
Equation: Mn2++2)H-MNOH2 (Brown precipitate)
Then this precipitate is dissolved in an acid in presence of Iodine to release Iodine which is titrated with Sodium thiosulphate to determine exact amount of oxygen.
Although this method remains recognized but it is prone to human error, sample contamination, reagents with aging leads to inaccuracy of results. In the field it is cumbersome to execute. Due to this new technologies came in existence mainly focusing quicker, and easier way to measure DO in lab, in field and even online continuous measurement. These are called Electrochemical methods.
Electrochemical Methods:-
The use of electrode or we call it as DO sensor is becoming popular. These sensors mainly operate on the principal of oxygen consumption at its surface be it galvanic or polarographic ones. In very simple way, these sensors utilize two polarized electrodes called anode and cathode separated with an electrolyte. When electrode is dipped in water, the dissolved oxygen diffuses across the diaphragm @ rate proportional to the partial pressure in the sample. The DO is reduced and consumed at the cathode, thus produces current which is directly proportional to oxygen level.
Difference between Galvanic and Polarographic Sensors:
Polarographic sensors needs to be charged at least 30 minutes before its use & that is achieved by connecting the DO sensor with meter in use whereas the Galvanic sensors is self polarized, do not require any warm up time.
Another type of sensor- Optical sensor.
This type of sensor measures the interaction between DO and certain luminescent dyes. In the process the dye is exposed to incident light which results the molecule of dye excited and it emits light with longer wavelength. If the DO is present, the wavelength of emitted light will be limited or it gets altered due to interaction between DO & dye. The frequency, intensity and decay of emitted light are related to partial pressure of oxygen, this is what determines concentration of DO in water sample. It requires minimum maintenance, no consumption of any electrolyte, does not get affected due to any other presence. Optical sensor can measure lower levels than other electrochemical sensors.
Polarographic DO probe
Galvanic DO probe
Optical DO sensor
********************************************************************************
Comments