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

Version

Version 2.0

Status

Status Published

Published Date

Published Date 27/05/2017
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 a drop of the 0.1 N Sodium Thiosulfate Standard Solution catalog number 32321 per 100mL sample, before adding any other reagents.