Shen ChangHong COD analyzer is a kind of chemical oxygen demand analyzer. The COD measurements value will vary with the measuring of reducing substances in water samples and its measuring methods. At present, the most commonly used COD measuring methods are closed digestion Colorimetry method(tube test), (vessel test), (prefabricated tube test). The closed digestion colorimetry is also the instrument principle of COD rapid analyzer.
Measuring principle of closed digestion Colorimetry (prefabricated tube)
Add a certain amount of potassium dichromate into strong acid solution as oxidant. Under the action of special composite catalyst, potassium dichromate was heated at 165 ℃ for 15 minutes, then it was reduced to trivalent chromium by organic matter in water. The content of hexavalent chromium / trivalent chromium ion was measured at wavelength 420nm (low range) or 610nm (medium / high range), and then the mass concentration of oxygen consumption was converted according to the content of hexavalent chromium / trivalent chromium ion.
Operation and analysis steps of COD analyzer
When the concentration of CODCr is 5.0-200mg/l (CODcr_L), 200-1000mg / L (CODcr_M), 1000-2000mg / L (CODcr_H):
Preparation of blank sample: put 2.0ml distilled water into CODcr preformed test tube (select appropriate range), shake well with plug.
Preparation of sample: Take 2.0ml samples to be tested and place them in a CODcr prefabricated test tube (select an appropriate measuring range). Shake well with the plug.
Digestion: When the temperature of the digester reaches 165 ℃, insert the digestion colorimetric tube into the digester hole in turn, cover the protective cover, and digest (see the instructions of dis-16 / 25 / 30 / 36 multi-functional numerical control digester for specific operation and precautions). The buzzer will give a finish alarm after digestion for 15 minutes.
Cooling: Take out the digestion colorimetric tube and put it into a dry beaker. After natural cooling for 2min, cool it to room temperature. Then place the digestion colorimetric tube on the test tube rack until all the precipitates are settled on the bottom of the tube and then test on the machine. (Note: when cooling, it is necessary to prevent the cooling water from entering the digestion colorimetric tube or contaminating the tube plug at the tube mouth, which will affect the measurement results. )
How to eliminate interfering substances
When measuring the concentration of CODCr, chloride ion is the main interfering substance. Mercury sulfate is added to the prefabricated test tube to shield 2000 mg / L of chloride ion. If the solution is turbid after digestion, it means that the concentration of chloride ion has exceeded the masking capacity of mercury sulfate. At this time, dilution of water sample or addition of appropriate amount of mercury sulfate can be selected to eliminate the interference of chloride ion.
