What are the possible reasons for the sample to turn a faint pink when the bromcresol-green methyl red indicator is added instead of the normal blue-green color?

Document ID

Document ID TE103

Published Date

Published Date 08/01/2019
Question
What are the possible reasons for the sample to turn a faint pink when the bromcresol-green methyl red indicator is added instead of the normal blue-green color?
Summary
Alkalinity indicator has unexpected faint pink color and not blue-green color
Answer
The total alkalinity indicator bromcresol-green methyl red will turn the sample blue-green in color. If the sample turns a faint pink color then the sample contains an interference. The most common interference to the alkalinity titration methods is chlorine above 3.5 mg/L. Chlorine at this concentration will bleach the normal color of the indicator.

To remove the chlorine interference, pretreat the sample by adding 1 drop of the Sodium Thiosulfate Standard Solution, 0.1N, 100 mL MDB (Product # 32332) per 100mL sample, before adding any other reagents.

For more information on common alkalinity interferences, please reference What are some common interferences in the alkalinity titration methods?