My sample is brown or black instead of blue after the addition of starch.

Chlorine Testing

Starch and iodine form a blue complex. The fainter the intensity of the yellow color of your sample before the addition of starch indicator, the more likely you will see a blue color. Often, the remaining amount of iodine in the test will generate a brown or black color after the addition of starch. This does not affect the endpoint. Continue the titration to the test’s colorless endpoint.