If there is no color change after adding reagent or the wrong color develops when testing a sample, how can this be resolved?
No color develops or the wrong color develops when testing a sample
There are several possible reasons why color does not develop in a colorimetric test:
- The procedure is not being followed correctly. Carefully review the step-by-step procedure to be sure all reagents are added and following the instructions correctly.
- The pH of the sample may not have adjusted to the proper range after reagents were added. Check the sampling and storage section of the procedure to find the optimum pH range for the test. If the method does not provide this information, add the reagents to a sample of deionized water and measure the pH of this solution. If the pH of the sample with reagents is significantly different, adjust the pH of a fresh portion of sample to this pH range using an acid or base and try the test again.
- The concentration of the parameter may be below the measurable limit for the test.
- The sample may need to be digested. Check the instructions for the test to see if digestion is mentioned. There may be an interference in the sample. Follow the standard additions instructions to find if the sample might have an interference.
- If the color that develops in the sample is significantly different from what it should be:
- Check the pH of the sample as described above and adjust if necessary.
- If the procedure mentions sample pretreatment by digestion, make sure to digest the sample.
- Dilute the sample. This can often dilute an interfering component to the point at which it no longer interferes. If this does not help, use a different analysis method.