Acids and bases are chemicals that have many important uses in science, industry and everyday life. Determining whether a substance is acidic, basic or neutral is very important. One simple way to test for acids and bases is by using acid-base indicators. Indicators are substances that change color in the presence of an acid or base.
How Acid-Base Indicators Work
Acid-base indicators are weak organic acids or bases that have a different color depending on whether they are in their acidic form or basic form. For example, litmus is a commonly used indicator. In its acidic form it is red, while in its basic form it is blue. Other naturally occurring indicators include red cabbage juice, turmeric, daisies, hydrangeas and grapes.
Here is how acid-base indicators work:
- The indicator molecule contains an OH or NH chemical group that can accept or release a hydrogen ion (H+).
- In acidic solutions, the indicator accepts an H+ ion, forming the acidic form (different color).
- In basic solutions, the indicator releases an H+ ion, forming the basic form (different color).
When a few drops of indicator solution are added to a substance, the color change shows whether the substance is acidic, neutral or basic.
Common Acid-Base Indicators
There are many different acid-base indicators used in chemistry. Some common ones are:
| Indicator | Color in Acid | Color in Base |
|---|---|---|
| Litmus | Red | Blue |
| Phenolphthalein | Colorless | Pink |
| Methyl orange | Red | Yellow |
| Bromothymol blue | Yellow | Blue |
As shown in the table, different indicators display different colors in acids and bases. This allows them to be used to distinguish between acidic, neutral and basic substances.
Using Indicators to Test Solutions
Acid-base indicators can be used to visually determine if a solution is acidic, basic or neutral:
- Add a few drops of indicator solution to the test substance.
- Observe the color change (if any).
- Refer to a color chart to determine if the substance is acidic, basic or neutral.
For example, if phenolphthalein indicator turns pink, this signals a basic solution. If methyl orange remains red, the solution is acidic. A green bromothymol blue indicates a neutral pH.
Standard color charts are available for common indicators that correlate the color to approximate pH values. This allows indicators to semi-quantitatively determine the pH.
Using Red Cabbage Indicator
Extracted red cabbage juice is a popular homemade indicator due to its vivid color changes. Here is how to use it:
- Chop up some red cabbage leaves and boil in water for 10-15 minutes.
- Filter the water through a coffee filter or cheesecloth to extract the purple pigment.
- Collect the extracted indicator solution.
- Test by adding a few drops to solutions of known pH like vinegar (acidic) and baking soda (basic).
- Observe the color changes from pink/red in acid to purple in neutral to green/yellow in base.
This cabbage indicator solution can then be used to test an unknown solution. The approximate pH can be estimated by comparing its color to a standard color chart.
Why Indicators Change Color
The color change of acid-base indicators is caused by the protonation or deprotonation of certain chemical groups on the indicator molecule. Here are some details on the chemistry:
- Litmus – Contains a sulfate group that is protonated in acid and deprotonated in base.
- Phenolphthalein – Protonation of the phthalein group in acid disrupts the conjugated system responsible for the pink color.
- Methyl orange – Sulfonate group is protonated in acid, causing a red chromophore to form.
- Bromothymol blue – Protolytic reaction causes shifts between yellow phenol/bromophenol forms and blue phenolate/bromophenolate.
In summary, addition or removal of H+ causes structural changes in the indicator molecule that alter the wavelengths of light absorbed. This creates a visible color change.
Uses and Applications of Indicators
Acid-base indicators have many uses including:
- Testing swimming pool, aquarium or hot tub water
- Checking soil acidity/alkalinity for farming and gardening
- Testing the pH of cosmetics, pharmaceuticals, foods and chemicals
- Science experiments and chemistry demonstrations
- Arts and crafts projects like making homemade litmus paper or rainbow carnations
- Biological research involving pH changes and enzyme function
Some important applications include:
Titration
Indicators are widely used to determine the endpoint in acid-base titrations. As one solution is slowly added to the other, the indicator will suddenly change color when the equivalence point is reached. This allows very accurate titration results to be obtained.
pH probes
Many pH meters and probes use indicator solutions. The probe tip contains an indicator that responds to pH changes. This allows electronic measurement and monitoring of pH in a solution.
Acidosis monitoring
In medicine, blood pH indicators help diagnose and monitor abnormal acidity/alkalinity levels in the blood (acidosis/alkalosis). This can signal lung, kidney or metabolic disorders.
Conclusion
Acid-base indicators are invaluable for testing pH levels and monitoring acidity and alkalinity. The variety of color changes they exhibit allows precise visual detection of acids and bases. Mastering how to use indicators like litmus, phenolphthalein or red cabbage juice allows many important chemical experiments and titrations to be performed.
The vivid color changes of acid-base indicators also make them popular for fun science projects or art experiments. Overall, these colorful chemicals have wide utility both in practical applications and educational settings.