Indicators are complex molecules that are themselves weak acids (e.g. phenolphthalein) or weak bases (e.g. methyl orange).there are six major types of indicators. they are the organic compounds. They have different colors in acidic and alkaline solutions. Litmus is a common indicator. It is red in the acidic solutions and blue in the alkaline solutions. which are as follows:
- thymol blue
- bromothymol blue
- methyl orange
- bromcresol green
Each indicator has a specific color in the acidic medium which changes at a specific pH to another color in basic medium. For example, phenolphthalein is colorless in the strongly acidic solution and red in the strongly basic solution. It changes color at a pH of about 9. This means phenolphthalein is colorless in a solution with a pH of less than 9. If the pH is above 9, phenolphthalein is red.
Use of indicators
Indicators are commonly used to find out the endpoint of the acid-base titration.
|Indicators||Colour in Acid||Colour in Base||Colour When Neutral|
Choice of Indicator
For strong acid and weak base, methyl orange is used. For a strong base and weak acid, phenolphthalein is commonly chosen.
Explanation of Indicators Behavior
The indicator dissociates in a solution and gives an ion that has a different color from the un-dissociated molecule of the indicator.
Consider the example of the methyl orange, which is of red-colored in unionized form and yellow-colored in unionized form.
If we represent an indicator by the general formula ‘HIn’ then the dissociation of the indicator can be shown as follows:
HIn ====== H+ + In–
The behavior of Indicator (Methyl Orange)
- In Acidic Medium
If an acid is added which increases the concentration of the H+ then equilibrium will move to the left and unionized ‘HIn’ will predominate and the solution will be red.
In Basic Medium
If a base is added which reduces the concentration H+ then equilibrium will move to the right and ionized ‘In’ will predominate and the solution will be yellow.
Measuring the pH of a Solution
Some indicators are used as mixtures. The mixture indicators give different colors at different pH values. Hence, it is used to measure the pH of a solution. Such a mixed indicator is called a universal indicator or simply a pH indicator.
The pH of a solution can be measured by dipping a piece of Universal Indicator paper in the solution. The pH is then found by comparing the color obtained with a color chart.
The pH Meter
The pH of a solution can be measured with a pH meter. It consists of a pH electrode connected to a meter. The electrode is dipped into the solution and the meter shows the pH either on a scale or digitally. It is a much more reliable and accurate method of measuring pH than the Universal Indicator paper, through the latter often more convenient.