Question

How can you estimate the pH range of an indicator’s color change? Why do some indicators have two separate pH ranges?

Short answer

When the pH of an indicator equals the of pKa the indicator, the colour changes. The colour change range for an indicator is $\text{pKa(indicator) - 1 to pKa(indicator) + 1}$.

This happens because some indicators have more than one ion to lose in their structure, resulting in multiple pKa.

Step-by-step solution

Define indicators

A material that can be added to a reaction mixture to show a titration's equivalence point is known as an indicator. A pH is frequently measured with coloured indicators. In a basic environment, these indications are different in colour than in an acidic environment.

Explanation

When it comes to pH indicators, it is important to note that each one changes colour based on a specific pH value. When the pH of an indicator equals the pKa of the indicator, the colour changes.

The colour change range for an indicator is $\text{pKa(indicator) - 1 to pKa(indicator) + 1}$.

For instance, the indicator methyl red has a $\text{pKa 5.1}$and a colour range of around$\text{pH 4.1}$to role="math" localid="1663331588901" $\text{pH6.1}$. (Under 4.1 it is red and above 6.1 it is yellow).

We can discuss thymol blue when we're talking about pHindicators with two different pH ranges. It has a ${\text{pKa}}_{\text{1}}\text{of 1.65}$and a ${\text{pKa}}_{\text{2}}\text{of 8.9}$. As a result, this indicator changes colour in two different pH ranges: ${\text{pH}}_{\text{1}}\text{= 1.2 - 2.8}$and${\text{pH}}_{\text{2}}\text{= 8 - 9.6.}$

This happens because some indicators have more than one ion to lose in their structure, resulting in multiple $\text{pKa}$.