Common sense of water quality testing

- Dec 13, 2019-

water meter

What are interfering substances?

Q & A:

Interfering substances are substances that exist in water and can interfere with chemical reactions in the test.

Interfering substances can cause measurement results to be higher or lower than the correct results.

How to check if there is interference in the test sample?

Q & A:

The standard addition method is the best way to check whether the sample contains interfering substances.

In this method, the total amount of the test substance is directly added to the sample. If the test substance in the sample is measured before and after the addition, it can be found that the measurement result after the addition is higher than that before the addition by a known amount.

Example: The copper-containing sample is measured, and the copper ion concentration is 1.0 mg / L. Then 0.5 mg / L copper ion was added to an unused portion of the sample. The concentration of copper ions in the sample was measured after the addition. The theoretical measurement result should be 1.5 mg / L.

If the measurement result of the sample after adding copper ion is significantly lower or higher than 1.5 mg / L, there may be interference substances in the sample that will interfere with the determination of copper ion. Make sure that the test operation and the test results obtained with the standard solution are correct.

In order to ensure normal work in most tests, the pH value of the sample is required to be within a certain range. The test reagent contains a buffer to adjust the pH value to within the required range. If the pH of the sample is too low, too high, or very alkaline, the sample may not be adjusted to the correct pH.

If the sample contains interfering substances, it is best to dilute the sample to a concentration where the interfering substances do not cause interference. If this does not work, you need to use another analysis method.

What happened when the sample was tested but did not develop or showed the wrong color?

Q & A:

There may be several reasons why no color is observed when testing a sample with a colorimeter:

1. Test procedures are not performed correctly

Perform detailed inspections one by one according to the test steps, and make sure that all required test reagents are added to the test, and that all steps are performed correctly.

2. After adding the test reagent, the pH value of the sample may change so that it is not within the range required by the test. Find the optimal pH range for your test by examining the sampling and storage stages. If the optimal pH range cannot be found, test reagents can be added to deionized water to determine the pH of the solution. If this pH value is significantly different from the pH value of the sample to be tested after adding the test reagent, the pH of the sample can be adjusted with acid or alkali so that it is within the pH range of the reagent's aqueous solution, and then the test is performed.

3. The concentration of the parameter to be measured may be lower than the lower limit of the instrument.

4. The test sample may need to be digested. Check if the test instructions mention the need for digestion.

5. Interfering substances may be present in the test sample. The use of standard additives is required to verify the presence of interfering substances in the sample to be tested.

If the actual color of the test sample is significantly different from the color that should theoretically appear, you can do the following:

1. As mentioned previously, check the pH of the sample and adjust as necessary.

2. If the test procedure indicates that the test sample requires digestion pretreatment, it is necessary to confirm that the test sample has been digested.

3. Dilute the sample. This will dilute any interference components that may be present in the sample to a concentration that will not cause interference. If the sample dilution is still ineffective, another analysis method is required.