DISSOLVE OXYGEN MEASUREMENT

DISSOLVE OXYGEN MEASUREMENT

Different methods are used for measurement of the amount of dissolve oxygen present in a liquid. It measure the dissolve oxygen in mg/L, partial pressure or a % of saturation or ppb.

There are three techniques for DO measurement

1)      Colorimetric method

2)      Titration

3)      Sensor and meter

·       Colorimetric method: It is very simple method mainly used for determination of DO in water sample. Based on the reagent used, this method is divided in two types.

a)      Indigo carmine method: In this method, reduced indigo carmine dye added into the sample that react with the presence of dissolve oxygen and undergo color changes (blue color), and change in color is directly proportional to DO. This color change compare with color comparator and determine the DO. LOD of this method is 0 to 50 ppm.

b)      Rhodazine D method: This method is used to determine the dissolve oxygen in very low concentration in ppb. In this method, rhodazine D reagent react with dissolve oxygen and to form deep rose color which is directly proportional to DO, and determine by using color comparator.

·      Titration method: Traditional method is the winkler titration method, but have under question of human error. This test is mainly use for water quality study.

Principle: In this method manganese hydroxide {Mn(OH)₂} is added to the sample which react with dissolve oxygen molecules in the sample and produce brown precipitate of manganic oxide {MnO(OH)₂}, Which react with concentrated sulfuric acid to liberate nascent oxygen. Potassium iodide (KI) is added which react with nascent oxygen and convert to iodine (I₂) molecules that is then titrated against the sodium thiosulphate using starch as an indicator. Thus, the amount of sodium thiosulphate consumed is directly proportional to dissolve oxygen concentration.

·        Sensor and meter: Two type sensors are available (1) Optical and (2) Electrochemical (ampherometric /Clark type).

1)      Optical sensor:

Working Principle: Detection based on fluorescent quenching, fluorescent lifetime detection (phase shift) and fluorescent intensity detection (through photodiode) as O2 affect both.

It measure the interaction between O2 and luminescent dye, when this dye expose to blue light electron get the energy go to excited stage and come back to normal stage post releasing the energy. When O2 molecules are present, it interact with the dye and alter the return wavelength. The measured effect is inversely proportional to the partial pressure of O2. The intensity of lifetime of returned luminescence is measured by photodetector and can be used to calculate the DO concentration.

Advantages:

o   Fast response time,

o   Doesn’t require calibration, membrane, filling solution/electrolyte, stirring, warm up time

o   Not affected by hydrogen sulfide or other gases that can permeate the electrochemical DO membrane

Disadvantages:

o   Susceptible to external temperature, pressure and salinity

o   Attenuation of light source

o   Drift due to degradation and leaching of the dye

2)      Electrochemical sensor: Electrochemical sensor further break down in two types a) Polarography sensor and b) Galvanic sensor

a)      Polarography sensor: Polarography sensor break down in two types, polarography and pulse polarography. In polarography sample needed to stir while in pulse polarography there is no need to stir the sample. Both the sensor have two polarized electrodes, an anode made up of silver-silver chloride and a cathode made up of gold/platinum in an electrolyte solution and are isolated by thin semipermeable membrane. At a time of measurement, dissolve oxygen diffuse via membrane at a rate of partial pressure of oxygen. This oxygen undergoes the reduction at cathode and produce an electric current; this current is carried out by ions in the electrolyte from cathode to anode. This current is proportional to partial pressure of oxygen.

b)      Galvanic sensor: This sensor having two electrode, cathode and anode immersed in electrolyte. The oxygen from the sample diffuse to the cathode and reduced to hydroxyl ion. The hydroxyl ion moves to anode where it oxidized, and generate a current. This current is directly proportional to concentration of oxygen.

Digital sensor: Digital sensor available in market with a memory storage device between cable and probe so once it calibrated can be used directly to the other fermenter without recalibration.

Analog sensor: There is no memory device in this sensor so needed to recalibrate whenever cable changed or dismissed from the cable.

DO Probe Calibration: Calibration can be carried out at two points, 

(1) zero percent calibration and (2)100% calibration.

●Zero percent calibration can be performed by i) using zero gel/saturated solution of sodium sulfite, ii) by purging the nitrogen into the sample, iii)by dismantling the probe from the cable or iv) at sterilization hold time.    

●100% calibration can be performed i) in air in the open environment, ii) in fermentation post sterilization with working aeration and agitation.

In fermentation process probe calibration should be carried out after sterilization of media because during sterilization there is a chances of tear and wear of membrane and there is a chances of sticking of degraded media component to the membrane which gives false result.

DO Probe Polarization: For new DO probe polarization six hours is required, For routine if probe undergoes depolarization for short time like 5-10 minutes than three time of total depolarization time required for activation of it.

I.e. If probe goes to depolarization for 10min than it required to keep for polarization (10*3=30min) at least 30min.

DO Probe Storage: For long-term storage, stored in dry condition in which remove electrolyte from membrane give water wash to probe and stored in dry condition. For short-term storage stored in water.

Membrane: Teflon membrane (T type membrane available having three layers Teflon-ss-teflon, useful for longtime process), polythene membrane, SS membrane (S type membrane available having two layers ss-teflon, useful for short time process. 

Factor Affecting to DO:

Temperature – Inversely proportional, as temperature increases DO decreases. At ≥ 100 ͦc it shows zero.

Pressure -Directly proportional, as pressure increases DO increases.

Salinity - Inversely proportional, as salinity increases DO will decreases.

Aeration -Directly proportional, as aeration increases DO will increases.

Agitation- Directly proportional, as agitation increases DO will increase.