What causes the bromocresol green-methyl red indicator to fade?
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To determine waste sulfuric acid (sulfuric acid after drying chlorine gas), bromocresol green-methyl red is used as an indicator. After adding the indicator, before titration begins, the indicator fades – what can be done in this situation? What causes the indicator to fade? The method we’ve been using before didn’t cause any fading.
It is the residual chlorine in the sample that causes the acid-base indicator to fade; free chlorine possesses strong oxidizing properties, which alter the structural composition of the indicator or even destroy its color-changing groups, thereby preventing it from properly indicating the end point of the titration. You can use a pH meter to indicate the end point of the titration, or add an appropriate reducing agent before the titration to react with the free chlorine; however, care must be taken to ensure that the reducing agent added does not affect the reaction between sulfuric acid and the base.