What is a pH Indicator?
A pH indicator, also known as an acid-base indicator, is a compound that changes colour in solution over a narrow pH range. To produce a visible colour change, only a small amount of indicator compound is required.
A pH indicator does not affect the acidity or alkalinity of a chemical solution when used in a dilute solution.
pH indicators are available in the form of liquid dyes and dye-infused paper strips. They are mixed into various solutions to determine their pH values. Whereas pH indicators in liquid form are typically added directly to solutions, pH indicators in paper form are dipped into solutions and then removed for comparison against a colour/pH key.
Table of Contents
- How Does the Indicator’s Color Change Occur?
- pH Indicator Ranges
- Universal Indicator
- Uses of pH Indicators
- Frequently Asked Questions – FAQs
How Does the Indicator’s Color Change Occur?
The dissociation of the H+ ion from the indicator itself causes the colour change of a pH indicator. Remember that pH indicators are both natural dyes and weak acids. The solution changes colour due to the dissociation of the weak acid indicator. The equation for the dissociation of the pH indicator’s H+ ion is shown below-
HInd + H2O ⇋ H3O+ + Ind–
with HInd being the acidic pH indicator and Ind– being the pH indicator’s conjugate base
The above equation is in equilibrium, which means that Le Chatelier’s principle applies. As a result, as the concentration of H3O+ (H+) rises or falls, the equilibrium shifts to the left or right.
An increase in the concentration of HInd acid causes the equilibrium to shift to the right (towards products). In contrast, an increase in the concentration of Ind– base causes the equilibrium to shift to the left (towards reactants).
pH Indicator Ranges
pH indicators are specific to the pH range that is desired to be observed.
1. Common indicators, such as phenolphthalein, methyl red, and bromothymol blue, are used to indicate pH ranges of approximately 8 to 10, 4.5 to 6, and 6 to 7.5, respectively.
- Phenolphthalein ranges from colourless to pink.
- Methyl red ranges from red to yellow.
- Bromothymol blue ranges from yellow to blue.
- Bromocresol green is a pH indicator that detects pH values from 3.8 to 5.4. The indicator solution is yellow below pH 3.8. The solution is blue above pH 5.4. The indicator solution is green between pH values of 3.8 and 5.4.
2. The pH range for universal indicators, on the other hand, is much broader, and the number of colour changes is much greater. Typically, universal pH indicators come in the form of a paper strip.
Universal Indicator
A universal indicator is a combination of pH indicator solutions that can be used to determine the pH of a solution over a wide range of values. There are numerous formulas for universal indicators. Thymol blue, methyl red, bromothymol blue, and phenolphthalein are common components of a mixture.
The colour change is used to identify pH values. The following are the most common universal indicator colours:
Red is for 0 ≥ pH ≥ 3
Yellow is for 3 ≥ pH ≥ 6
Green is for pH = 7
Blue is for 8 ≥ pH ≥ 11
Purple is for 11 ≥ pH ≥ 14
Uses of pH Indicators
- pH indicators are used to estimate the pH of a chemical solution. A pH metre is used for precise measurements.
- In titration, indicators are used to indicate the completion of an acid-base reaction.
- pH indicators can be used to measure the pH of farm soil, shampoos, fruit juices, and bodies of water, among other things.
Frequently Asked Questions on pH Colour Change
How does pH affect colour?
The dissociation of the H+ ion from the indicator itself causes the colour change of a pH indicator. Remember that pH indicators are both natural dyes and weak acids. The solution changes colour due to the dissociation of the weak acid indicator.
What effect does a pH change have on a solution?
The charge state of the solute can be changed by varying the pH of the solution. When the pH of a solution is such that a specific molecule carries no net electric charge, the solute has little solubility and precipitates out of the solution.
What is the difference between pH paper and Universal indicator?
A universal indicator is a liquid/solution substance that is dropped into a sample and changes the substance according to the pH.
pH paper is an indicator that changes depending on the substance in which it is dipped (acidic or basic).
What is the range of the pH scale?
The pH scale ranges from 0 to 14, with-
- pH below 7 is considered acidic
- pH above 7 is considered basic
- pH = 7 neutral, depending on the concentration of hydrogen ions present in the substance.
Which is more accurate, pH paper or a pH metre?
While pH metres and pH paper are portable methods for measuring the pH of a solution, pH metres provide more accurate results. Colour blindness is not an issue because the results are displayed on a digitally readable display. However, pH paper is a much less expensive and simpler way to measure pH.
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