pH and Indicators
4. Natural and Laboratory Indicators
Learning outcomes
- I can identify common indicators used in chemistry.
- I can compare natural indicators and laboratory indicators.
- I can explain how indicators change color.
- I can select appropriate indicators for different investigations.
- I can investigate pH using a variety of indicators.
What Is an Indicator?
A chemical indicator is a substance that changes colour depending on the chemical conditions of its surroundings.
Acid-base indicators respond to changes in pH.
They can therefore help us determine whether a solution is:
- acidic
- approximately neutral
- alkaline
Some indicators occur naturally in plants, while others are prepared specifically for laboratory use.
Why Do Indicators Change Colour?
Indicator molecules can exist in different chemical forms.
The balance between these forms depends on the concentration of H⁺ ions in the solution.
A simplified representation is:
HIn ⇌ H⁺ + In⁻
The two indicator forms can absorb visible light differently and therefore appear as different colours.
Changing the pH shifts the balance between the forms.
Therefore:
change in pH → change in indicator form → change in colour
Indicators Do Not Usually Change at One Exact pH
Most indicators change colour over a range of pH values.
This is called the indicator's transition range.
For example, an indicator might:
- have one colour below its transition range
- show intermediate colours during the transition
- have another colour above its transition range
This is why different indicators are useful for different investigations.
Natural Indicators
A natural indicator is obtained from a naturally occurring source, often a plant.
Many plants contain pigments whose structures respond to changes in pH.
Common examples include:
- red cabbage
- turmeric
- some berries
- hibiscus
- butterfly pea flowers
These materials can be useful for demonstrating acid-base chemistry.
Red Cabbage Indicator
Red cabbage is one of the most useful natural indicators.
It contains pigments called anthocyanins.
These pigments change molecular structure as pH changes.
The exact colours depend on concentration and experimental conditions, but a typical pattern is:
strongly acidic → red/pink
weakly acidic → pink/purple
approximately neutral → purple
weakly alkaline → blue/blue-green
more strongly alkaline → green/yellow-green
This broad range makes red cabbage particularly useful for classroom investigations.
Making Red Cabbage Indicator
A simple classroom preparation involves extracting the coloured pigments from cabbage leaves.
A general method is:
- Cut red cabbage into small pieces.
- Place the pieces in a suitable container.
- Add warm or hot water as directed by the teacher.
- Allow the pigments to move into the water.
- Separate the coloured liquid from the cabbage pieces.
- Use the coloured extract as an indicator.
The resulting purple liquid can then be added to different test solutions.
Testing Substances with Red Cabbage
Suppose red cabbage indicator is added to:
- lemon juice
- vinegar
- water
- baking soda solution
- soap solution
You would expect a range of colours.
The acidic samples shift toward red or pink.
Neutral conditions remain closer to purple.
Alkaline samples shift toward blue, green, or yellow-green.
This produces a natural version of a pH colour scale.
Anthocyanins
Anthocyanins are plant pigments responsible for red, purple, and blue colours in many plants.
They occur in foods such as:
- red cabbage
- blueberries
- blackberries
- purple grapes
- some cherries
Their molecular structures can change depending on pH.
This is why some colourful plant materials can act as natural indicators.
Turmeric
Turmeric contains the yellow pigment curcumin.
It can also behave as an acid-base indicator.
Turmeric generally remains yellow in acidic and approximately neutral conditions.
In sufficiently alkaline conditions, it changes toward:
orange-red or reddish-brown
Therefore, turmeric is particularly useful for identifying alkaline substances, but it is not as useful as universal indicator for estimating a broad range of pH values.
Hibiscus Indicator
Hibiscus flowers contain pH-sensitive plant pigments.
Extracts can show different colours in acidic and alkaline solutions.
Like red cabbage, hibiscus can be used to demonstrate that naturally occurring molecules can respond visibly to changes in chemical conditions.
The exact colour range depends on the type of hibiscus and how the extract is prepared.
Butterfly Pea Indicator
Butterfly pea flowers contain anthocyanin-related pigments that can also respond strongly to pH.
The extract is typically blue under some conditions but can shift toward purple or pink when acid is added.
This is why adding lemon juice to butterfly pea tea can produce a dramatic colour change.
The colour change is an example of acid-base chemistry occurring in an everyday substance.
Advantages of Natural Indicators
Natural indicators can be:
- inexpensive
- readily available
- visually interesting
- useful for classroom demonstrations
- connected to everyday materials
- useful for qualitative comparisons
They are particularly good for introducing the idea that chemical substances can respond to changes in pH.
Limitations of Natural Indicators
Natural indicators also have limitations.
Their colours can depend on:
- plant variety
- age of the material
- extraction method
- concentration
- storage conditions
- lighting
- temperature
Therefore, different batches may not produce exactly the same colours.
Natural indicators are usually better for qualitative observations than precise pH measurements.
Laboratory Indicators
Laboratory indicators are selected or prepared for chemical investigations.
Common examples include:
- litmus
- universal indicator
- phenolphthalein
- methyl orange
- bromothymol blue
Each indicator has a particular colour change and transition range.
Litmus
Litmus is one of the simplest acid-base indicators.
It is commonly used as:
- red litmus paper
- blue litmus paper
Blue litmus turns red in acidic conditions.
Red litmus turns blue in alkaline conditions.
Litmus is useful for quickly determining whether a substance is acidic or alkaline.
However, it does not provide a detailed estimate of pH.
Universal Indicator
Universal indicator is a mixture of several indicators.
It produces a sequence of colours across a broad pH range.
A typical pattern is:
red → orange → yellow → green → blue → purple
Universal indicator is particularly useful when the goal is to:
- estimate pH
- compare several solutions
- arrange substances from acidic to alkaline
Phenolphthalein
Phenolphthalein is a common laboratory indicator.
It is:
colourless in acidic and approximately neutral solutions
and becomes:
pink in alkaline conditions
Its transition occurs approximately between pH 8.2 and 10.
Phenolphthalein is often useful in acid-base titrations where the expected endpoint occurs within this range.
Methyl Orange
Methyl orange changes colour in the acidic region of the pH scale.
It is approximately:
red in strongly acidic conditions
and:
yellow at higher pH
with orange appearing through its transition.
Its transition range is approximately:
pH 3.1–4.4
This makes methyl orange useful when a colour change in the acidic pH region is required.
Bromothymol Blue
Bromothymol blue changes colour near neutral pH.
It is approximately:
yellow in acidic conditions
green around neutral conditions
blue in alkaline conditions
Its transition range is approximately:
pH 6.0–7.6
This makes it particularly useful for investigating changes near neutral pH.
Comparing Common Indicators
Litmus
Best for:
quickly distinguishing acid from alkali
Information provided:
basic classification
Universal indicator
Best for:
estimating pH across a wide range
Information provided:
approximate pH
Phenolphthalein
Best for:
detecting a change into the mildly alkaline region and for suitable titrations
Information provided:
specific transition rather than a full pH measurement
Methyl orange
Best for:
detecting changes in the acidic pH region and for suitable titrations
Bromothymol blue
Best for:
detecting changes near neutral pH
Red cabbage
Best for:
broad visual comparison using a natural indicator
Choosing an Appropriate Indicator
The best indicator depends on the purpose of the investigation.
Ask:
What information do I need?
If you only need to know whether a solution is acidic or alkaline:
→ litmus
If you want an approximate pH:
→ universal indicator
If you want to investigate a broad range using a natural material:
→ red cabbage
If you want to detect a change near neutral pH:
→ bromothymol blue
If you are carrying out a titration:
→ choose an indicator whose transition range matches the rapid pH change near the endpoint.
Indicators in Titrations
A titration is a technique used to determine how much of one solution is required to react with another.
An indicator can help identify the endpoint by producing a visible colour change.
However, not every indicator is appropriate for every titration.
The indicator's transition range should fall within the steep pH change near the equivalence point.
Why Universal Indicator Is Usually Poor for Precise Titrations
Universal indicator is excellent for showing a broad pH range.
However, this can be a disadvantage in precise titrations.
Instead of one sharp colour change, it passes through several colours.
This can make the endpoint harder to identify accurately.
A single indicator such as phenolphthalein or methyl orange may provide a clearer endpoint when appropriately selected.
Example 1: Identifying an Acid
A student places blue litmus paper into an unknown solution.
The paper turns red.
Conclusion:
The solution is acidic.
Can we determine its exact pH?
No.
Litmus does not provide enough information for an exact pH measurement.
Example 2: Estimating pH
A student needs to determine whether five unknown solutions have pH values near 2, 5, 7, 10, or 13.
Which indicator would be useful?
Universal indicator
Why?
Because it produces different colours across a broad range of pH values.
Example 3: Detecting Alkalinity
A student adds phenolphthalein to a solution.
The solution becomes pink.
This indicates that the solution is in the alkaline region where phenolphthalein is pink.
It does not provide an exact pH.
Example 4: Natural Indicator
Red cabbage indicator is added to two unknown solutions.
Solution A becomes pink-red.
Solution B becomes blue-green.
A reasonable conclusion is:
A → acidic
B → alkaline
The exact pH should not be claimed unless a calibrated colour chart has been established for that particular indicator preparation.
Comparing Natural and Laboratory Indicators
Natural indicators have several advantages:
- inexpensive
- accessible
- visually interesting
- useful for demonstrations
- can be prepared from everyday materials
Laboratory indicators often provide:
- more standardized behaviour
- known transition ranges
- more reproducible results
- greater suitability for quantitative laboratory procedures
However, even laboratory indicators usually provide a range, not a perfectly precise pH value.
Investigating Several Indicators
A useful investigation is to test the same substances with different indicators.
Possible test substances:
- lemon juice
- vinegar
- water
- baking soda solution
- soap solution
Possible indicators:
- red litmus
- blue litmus
- universal indicator
- red cabbage indicator
- phenolphthalein
Students can compare what information each indicator provides.
This helps demonstrate that different indicators answer different scientific questions.
Planning the Investigation
For each substance:
- Place a small sample into a clean container.
- Add the selected indicator according to instructions.
- Observe the colour.
- Record the result.
- Repeat using a different indicator and a fresh sample if necessary.
- Compare the results.
- Classify the substance.
- Evaluate which indicator provided the most useful information.
Recording Results
A useful results table could contain:
Substance | Indicator | Initial colour | Final colour | Interpretation
For universal indicator, an additional column could be:
Estimated pH
For a natural indicator, students might instead record:
acidic / approximately neutral / alkaline
unless a suitable calibrated colour scale has been created.
Creating a Natural Indicator Scale
A natural indicator can be made more useful by testing it against solutions of known pH.
For example, red cabbage indicator could be added to reference solutions covering different pH values.
The resulting colours could be photographed or recorded.
Unknown solutions could then be compared with the reference scale.
This is a simple example of calibration.
Reliability of Indicator Results
Indicator results can be affected by:
- amount of indicator added
- concentration of the indicator
- lighting conditions
- original colour of the sample
- contamination
- observer judgement
- temperature
- age of the indicator
Good experimental design tries to control these variables.
Coloured Solutions
Suppose a dark purple drink is being tested.
Its original colour may interfere with the indicator colour.
This could make visual interpretation unreliable.
A better method might be:
a calibrated pH meter
This demonstrates an important scientific principle:
Choose the measuring method to suit the sample.
Using Indicators Safely
The indicator itself is not the only substance that matters.
The test solutions may be hazardous.
Safe practice includes:
- wear appropriate eye protection
- use small quantities
- follow laboratory instructions
- keep chemicals away from eyes and skin
- never taste laboratory chemicals
- do not deliberately smell chemicals directly
- use clean droppers and equipment
- label samples clearly
- clean spills using the correct procedure
- wash hands after practical work
Avoid Cross-Contamination
Imagine using the same dropper for hydrochloric acid and sodium hydroxide.
Some of the first solution may remain in the dropper.
This can:
- contaminate the second solution
- change its pH
- produce incorrect indicator colours
- create unwanted reactions
Use clean equipment or separate droppers where appropriate.
Natural Does Not Mean Harmless
A common mistake is assuming:
natural = safe
This is not scientifically valid.
Natural substances can still:
- irritate skin
- cause allergic reactions
- stain materials
- contain biologically active chemicals
Similarly, a natural indicator does not make an unknown test solution safe.
Always follow appropriate laboratory procedures.
Indicators and Neutralization
Indicators are useful for observing neutralization.
Suppose acid is gradually mixed with an alkali.
As the pH changes, an appropriate indicator changes colour.
Universal indicator may show the entire progression.
A single laboratory indicator may instead show a sharper transition over a narrower pH range.
Indicators and Carbon Dioxide
Bromothymol blue can be used to demonstrate changes associated with dissolved carbon dioxide.
Carbon dioxide dissolves in water and participates in equilibria that can make the solution more acidic.
As pH falls, bromothymol blue can shift:
blue → green → yellow
depending on the starting conditions and amount of carbon dioxide.
This makes it useful in some investigations involving:
- respiration
- photosynthesis
- dissolved carbon dioxide
Indicators in Biology
Indicators can connect chemistry with biology.
For example:
- bromothymol blue can investigate CO₂-related pH changes
- pH indicators can investigate enzyme conditions
- indicators can compare soil conditions
- indicators can test environmental water samples
This demonstrates that acid-base chemistry is important across several sciences.
Indicators in Environmental Science
Environmental scientists measure pH in:
- rivers
- lakes
- soil
- rainwater
- wastewater
Indicators can provide quick estimates.
However, calibrated pH meters are usually preferred when precise measurements are required.
Example 5: Choosing an Indicator
Question:
A scientist only needs to know whether an unknown solution is acidic or alkaline.
Best choice:
litmus
Reason:
It provides a quick acid/alkali classification without unnecessary detail.
Example 6: Comparing Ten Solutions
Question:
A student wants to arrange ten solutions from lowest to highest approximate pH.
Best choice:
universal indicator
Reason:
It produces a broad range of colours that can be compared with a pH chart.
Example 7: Natural Indicator Investigation
Question:
A class wants to demonstrate acid-base chemistry using a plant extract.
Suitable choice:
red cabbage indicator
Reason:
Its anthocyanin pigments produce several different colours across a broad range of pH conditions.
Example 8: Precise pH Measurement
Question:
A researcher needs to distinguish between pH 6.42 and pH 6.58.
Would universal indicator be appropriate?
No.
A calibrated pH meter would be much more appropriate.
Indicators generally cannot provide this level of precision.
Common Mistakes
Mistake 1: Assuming all indicators have the same colours
Different indicators have different colour changes.
Mistake 2: Assuming all indicators change at pH 7
Each indicator has its own transition range.
Mistake 3: Assuming indicators provide exact pH values
Most provide qualitative information or approximate pH ranges.
Mistake 4: Assuming natural indicators are automatically better or safer
Natural and laboratory indicators have different advantages and limitations.
Mistake 5: Using universal indicator for every investigation
The best indicator depends on the question being investigated.
Mistake 6: Using litmus to report an exact pH
Litmus mainly identifies acidic or alkaline conditions.
Mistake 7: Assuming colourless phenolphthalein means the solution must be neutral
Phenolphthalein is also colourless in acidic conditions.
Mistake 8: Assuming a yellow turmeric result proves a solution is acidic
Turmeric can remain yellow across acidic and approximately neutral conditions.
Mistake 9: Ignoring the original colour of the solution
A strongly coloured sample can interfere with the observed indicator colour.
Mistake 10: Forgetting contamination
Dirty equipment can change the pH of the sample and produce misleading results.
Did You Know?
Many familiar colour changes in foods and plants are examples of the same chemistry used in laboratory indicators.
Anthocyanin pigments can change colour because their molecular structures respond to acidity.
This means a colour change in red cabbage or butterfly pea is not simply a visual trick.
It provides evidence that the chemical environment surrounding the pigment molecules has changed.
Key Terms
- Indicator: Substance that changes colour depending on chemical conditions such as pH.
- Acid-base indicator: Indicator that responds to changes in acidity and alkalinity.
- Natural indicator: Indicator obtained from a naturally occurring source.
- Laboratory indicator: Indicator selected or prepared for use in chemical investigations.
- Transition range: Range of pH values over which an indicator changes colour.
- Anthocyanin: Group of plant pigments that can display pH-dependent colour changes.
- Litmus: Indicator commonly used to distinguish acidic from alkaline conditions.
- Universal indicator: Mixture of indicators used to estimate pH over a broad range.
- Phenolphthalein: Indicator that changes from colourless toward pink in its alkaline transition range.
- Methyl orange: Indicator that changes colour in the acidic region.
- Bromothymol blue: Indicator that changes from yellow through green toward blue around the near-neutral region.
- Calibration: Comparing a measuring method with known standards.
- Qualitative: Describing qualities or categories rather than precise numerical values.
- Quantitative: Involving numerical measurements.
- Endpoint: Observed point in a titration associated with completion of the intended reaction.
Indicator Selection Guide
Need a quick acid/alkali test?
→ Litmus
Need an approximate pH across a broad range?
→ Universal indicator
Need a natural classroom indicator?
→ Red cabbage
Need to detect alkaline conditions?
→ Phenolphthalein may be useful
Need a transition in the acidic region?
→ Methyl orange may be useful
Need a colour change near neutral pH?
→ Bromothymol blue
Need a precise numerical pH?
→ Use a calibrated pH meter rather than a colour indicator
Need an indicator for a titration?
→ Select one whose transition range matches the expected rapid pH change near the endpoint
Key Takeaways
- Indicators change colour in response to chemical conditions such as pH.
- Indicator molecules can exist in differently coloured chemical forms.
- Changing H⁺ concentration changes the balance between these forms.
- Natural indicators can be obtained from plants and other natural materials.
- Red cabbage is a useful natural indicator because its anthocyanin pigments produce several colours across different pH conditions.
- Turmeric is particularly useful for detecting alkaline conditions.
- Hibiscus and butterfly pea can also show pH-dependent colour changes.
- Laboratory indicators include litmus, universal indicator, phenolphthalein, methyl orange, and bromothymol blue.
- Different indicators have different transition ranges.
- Litmus is useful for quickly distinguishing acids and alkalis.
- Universal indicator is useful for estimating pH across a broad range.
- Phenolphthalein changes colour in the alkaline region.
- Methyl orange changes colour in the acidic region.
- Bromothymol blue changes colour near neutral pH.
- Natural indicators are useful but can be less standardized than laboratory indicators.
- The best indicator depends on the purpose of the investigation.
- Universal indicator is not usually the best choice for a precise titration endpoint.
- Indicators generally provide less precise measurements than calibrated pH meters.
- Laboratory investigations should control contamination, lighting, sample volume, and indicator quantity.
- A useful decision process is:
scientific question → expected pH range → required precision → choose indicator → observe colour → interpret result → evaluate reliability.