Common sense of water quality testing (2)

- Dec 13, 2019-

bench top meter

How does the standard addition method work?


Q & A:

A detailed description of standard join operations is provided in the accuracy check section of most Hach test procedures. The concentration and exact volume of the standard solution used can be checked from it. Many Hach instruments record the concentration and total amount of the test substance in the standard solution used, and calculate the difference between the theoretical value and the actual measured value.


If no operating instructions are found, the standard solution can be added to the sample to be tested three times, and the amount of the standard solution is increased in increments, such as 0.1, 0.2, and 0.3 mL. For colorimetry, add a standard solution to the sample before adding the test reagent. The test is performed in the usual way, and the concentration value is determined every time the standard solution is added. The reagent measurement results are compared with the theoretical values to check whether there is a difference between the two.


The theoretical concentration value is calculated by multiplying the volume of the standard solution by the concentration of the standard solution, and then dividing by the total sample volume (to ensure high accuracy, the volume of the standard solution added is included in the total sample volume).





When using a standard solution to check the accuracy of a colorimeter / spectrophotometer or test kit, the indicator always shows zero. What's wrong?


Q & A:

Make sure that the test solution is added to the standard solution as the test sample. The standard solution is the same as the sample and needs to be developed in the same way.


Still follow the steps described in the test procedure in order, except that the sample is replaced with a standard solution. For example: add the standard solution to the sample tube, add test reagents, and wait for the set time to end, then read the measurement result of the standard solution on the toolbox or instrument.



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 does not work, you need to use another analysis method



How do I verify accuracy using a standard solution?


Q & A:

The "standard solution method" or using a standard solution instead of the sample to be measured is the simplest method of using the standard solution. When the Hach test procedure reaches the step of "adding a certain amount of sample to be tested to the sample tube", replace the sample to be tested with the standard solution and add it to the test tube, and then perform the test according to the next steps in the procedure book.


If the measurement result is close to the known concentration of the standard solution, you can be sure that the instrument and reagents are working properly and the test operation is correct.