What should I pay attention to when sampling for rapid water quality testing?

- Jul 29, 2020-

Water sample collection

 

Before collecting water samples, wash the sampling plastic or glass bottles and bottle caps 2 to 3 times with water samples. When collecting water samples, pay attention to filling with water and tighten the bottle cap. If multiple water samples are collected, they must be marked to prevent confusion.


Collection of surface and groundwater samples

water quality test meter

◆ Collect well water. Allow the pump to run for sufficient time to drain the water in the pipeline before taking a fresh water sample.

◆ Sampling spring water can be sampled directly at the gushing mouth.

◆ Collect surface water Try to collect samples in the center of the water area, and collect water samples 3~5cm below the water surface. If a container with a lid is used, immerse the container under the liquid surface before removing the cap.


Sewage collection


◆ Moderate and lightly polluted wastewater. For example, samples are taken at certain discharge outlets of wastewater after industrial treatment. At the same time, it is necessary to record the sample collection process including time and location for future analysis and research.

◆ Sewage collection in the water area When the water depth is more than 1m, sampling is taken at 1/4 of the surface layer, and when the water depth is ≤1m, sampling is taken at 1/2 of the water depth.


The sampling position is in the center of the sampling section, and the sample container must be rinsed with water three times before sampling. When sampling, care should be taken to remove debris, garbage and other floating objects on the water surface.


Storage of water samples


After the sample is collected, analysis should be carried out as soon as possible to reduce experimental errors and reduce the workload. It is advisable to conduct analysis and determination immediately when using instruments for detection. The following is about the storage of water samples before COD, ammonia nitrogen, total phosphorus, and total nitrogen. (Note: If the water sample is acidified, the pH value of the water sample needs to be adjusted to about neutral before the test! Otherwise it will affect the test!)

multiple parameter water quality test equipment

COD water sample preservation method:

 

The water sample used to determine COD is best measured immediately after the sample is taken, otherwise, due to the long storage time, the reducing organic matter in the water sample will absorb the oxygen in the air under the action of microorganisms, and the oxidation-reduction reaction will proceed slowly. Decomposed, making the measurement result low. Therefore, the water sample should be measured within 2 hours. If it must be stored for a certain period of time, it should be acidified with sulfuric acid to a pH value of <2, and stored at a low temperature (about 4°C) to slow down the activity of microorganisms. And the measurement is completed within 48 hours.

 

Storage method of ammonia nitrogen water sample:

 

It should be stored in a polyethylene bottle or glass bottle and sealed with a cover to prevent the absorption of ammonia in the air and increase the concentration. If the NH3 concentration in the water sample is high, the water sample should be acidified with sulfuric acid to a pH value of <2, and stored at low temperature (about 4°C).

 

Storage method of total nitrogen water sample:


Sulfuric acid should be added to acidify to PH value <2, and stored at low temperature (about 4 ℃). And the measurement is completed within 48 hours.


Storage method of total phosphorus water sample:

 

The water sample used for the determination of total phosphorus is best to be sampled in a glass bottle, and the sample should be measured immediately after the sample is taken. If it must be stored for a certain period of time, it should be acidified with sulfuric acid until the PH value is less than 2, and stored at low temperature (about 4°C) .


Storage method of orthophosphate water samples:

multi-parameter desktop analyzer  

For the determination of orthophosphate water samples, no acid or other preservatives can be added to prevent other substances from turning into orthophosphate, or orthophosphate into other forms of phosphate. It only needs to be stored in a refrigerator at about 4℃. Test within hours.


TIPS:


◆ COD: Shake well before sampling and then sample again. Take water with a depth of about 1/2. (Note that if the water sample itself contains suspended solids, it cannot be filtered first according to GB11914-89; under certain conditions , After potassium dichromate oxidation treatment, the concentration of oxygen corresponding to the dichromate consumed by the dissolved substances and suspended solids in the water).

◆ Ammonia nitrogen: When sampling, you should pay attention to taking 1/2 depth of water for testing after all the suspended matter in the water settles, and filter it if necessary.

◆Total Phosphorus: Shake well when sampling, and then take a water sample of about 1/2 depth for testing. If the solution is turbid, you can choose to dilute it or perform color compensation. For details, refer to the national standard GB11893-89.

◆ Total nitrogen: Shake well before sampling, and then take water samples of about 1/2 depth for testing. Total nitrogen mainly reflects the degree of water pollution by nutrients. High total nitrogen is not suitable for rapid testing. Use other methods.