The determination of COD (Chemical Oxygen Demand), Total Phosphorus (TP), and Total Nitrogen (TN) are commonly used indicators in water quality analysis, which can reflect the degree of pollution in water bodies. The digestion tube is mainly used for this type of measurement by high-temperature chemical reactions to decompose organic matter in water samples, releasing measurable substance concentrations. The digestion tube plays a role in heating and reacting in the chemical analysis process, especially in COD determination, where the sample and oxidant are heated together to undergo oxidation reaction.
The following are the operating procedures for COD, total phosphorus, and total nitrogen digestion tubes:
1. COD determination
Chemical Oxygen Demand (COD) refers to the consumption of organic matter in water samples by oxidation under the action of strong oxidants such as potassium permanganate or potassium dichromate. The test requires the use of a digestion tube for digestion reaction.
Required materials and instruments
Dissolving tube: Suitable for COD determination.
Water bath digester: used for heating digestion tubes.
Dissolving reagents: potassium dichromate (K2Cr2O7) solution, sulfuric acid, silver salt catalyst, etc.
COD measuring instruments: such as spectrophotometers, COD colorimeters, etc.
Water sample: Water sample to be tested.
operating steps
Sampling and preparation:
Prepare the water sample to be tested, take a certain volume of water sample, and add an appropriate amount of COD digestion agent (such as potassium dichromate).
Add a catalyst, usually a silver salt catalyst.
Heating digestion:
Place the test tube (digestion tube) into a water bath digestion device for heating. The usual digestion time is 2 hours, and the temperature is controlled at around 150 ° C. Be careful to avoid excessive temperature that may cause the test tube to rupture.
Cooling and Reaction:
After digestion, remove the digestion tube and let it cool at room temperature.
After cooling, take out the sample for subsequent analysis.
Color comparison measurement:
Measure the absorbance of the digested sample using a spectrophotometer or COD colorimeter, and calculate the COD value according to the standard curve.
Precautions
Attention should be paid to protection during operation, avoiding strong acids and high temperatures.
The digestion process requires strict control of time and temperature to ensure complete reaction.
2. Determination of total phosphorus (TP)
Total phosphorus refers to all forms of phosphorus in water samples, including organic phosphorus, inorganic phosphorus, and phosphate. The ammonium molybdate method is usually used for determination.
Required materials and instruments
Dissolving tube: suitable for total phosphorus digestion.
Dissolving reagents: ammonium molybdate, sulfuric acid, etc.
Water bath digester: used for heating digestion tubes.
Colorimeter or spectrophotometer: used for measuring the final concentration.
operating steps
Sample pretreatment:
Take a certain volume of water sample and add digestion reagents (ammonium molybdate solution, concentrated sulfuric acid, etc.).
Dissolve:
Place the water sample with added reagents into a digestion tube and heat it up in a water bath digester. The general heating time is 30 minutes to 1 hour, and the temperature is controlled at around 150 ° C.
Cooling and color response:
After the reaction is complete, remove the digestion tube and cool it to room temperature.
Add reducing agents and other reagents to promote the formation of blue phosphomolybdic acid.
Colorimetric determination:
Measure the absorbance of the digested water sample using a colorimeter or spectrophotometer, and calculate the total phosphorus concentration according to the standard curve.
Precautions
Avoid reagent contamination during the processing.
The amount of reagents added and the heating temperature should be strictly controlled.
3. Total nitrogen (TN) determination
Total nitrogen is the sum of all nitrogen compounds in a water sample, including ammonia nitrogen, nitrite nitrogen, nitrate nitrogen, etc.
Required materials and instruments
Dissolving tube: suitable for total nitrogen determination.
Dissolving reagents: including sulfuric acid, catalysts, etc.
Water bath digester: used for heating samples.
Spectrophotometer or colorimeter: used for final determination.
operating steps
Water sample preparation:
Take water samples and add digestion reagents (such as concentrated sulfuric acid, selenic acid, etc.), and sometimes catalysts (such as copper, selenium, etc.) need to be added.
Dissolution process:
Place the water sample with added reagents into the digestion tube and heat it in the water bath digestion device. The usual heating time is 1 hour, and the temperature should be around 150 ° C.
Cooling and color reaction:
After the reaction is complete, remove the digestion tube and cool it to room temperature.
Add color reagent and let it react to form a color.
Colorimetric determination:
Measure the absorbance of the digested water sample using a spectrophotometer or colorimeter, and calculate the total nitrogen concentration based on the standard curve.
Precautions
In operation, attention should be paid to the sequence and temperature of the operation to prevent incomplete reaction.
Prevent reagent poisoning and contamination.
4. General operating procedures
Instrument calibration: All instruments (colorimeters, spectrophotometers, etc.) need to be calibrated before use to ensure accuracy.
The use of digestion tubes: Digestion tubes are important containers in the digestion process and should be regularly inspected for damage or cracks to ensure safe use.
Safe use of chemicals: When using highly corrosive and toxic chemicals such as concentrated sulfuric acid and potassium dichromate, appropriate personal protective equipment such as gloves, goggles, and lab coats should be worn.
By following the above operating procedures, the COD, total phosphorus, and total nitrogen content in water samples can be accurately measured, and the water pollution situation can be evaluated accordingly. Each detail in these steps is important, so it is important to be careful and follow standard laboratory operating procedures during operation.