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What is an Acid-Base Indicator?
Did you know you can change the colour of hydrangea flowers by changing the pH of the soil they are growing in?
If the soil is acidic (has a low pH) the hydrangea flowers will be blue.
If the soil is basic or alkaline (has a high pH) the hydrangea flowers will be pink.
soil pH
(at 25°C)(3) |
pH < 7
(acidic soil) |
pH = 7
(neutral) |
pH > 7
(basic soil) |
|---|
| colour of hydrangea flowers |
blue |
|
pink |
The colour of the pigment or dye in hydrangea flowers is giving some indication of whether the soil is acidic or basic and hence is giving some indication of whether the soil has a low pH or a high pH. The hydrangea flowers are acting as a soil "acid-base indicator".
Many coloured pigments, or dyes, extracted from plants such as from certain flower petals or even vegetables, can be used to indicate the pH and therefore the relative acidity or alkalinity (basicity) of a solution and hence are called acid-base indicators.(4)
You may have already heard of litmus, a dye extracted from certain lichens, which changes colour from red in acidic solutions (low pH) to blue in basic or alkaline solutions (high pH).
We can represent these colour changes as shown below:
pH
(at 25°C) |
0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
13 |
14 |
|---|
litmus
solution |
red at low pH
(acidic solution) |
|
blue at high pH
(basic solution) |
Your school laboratory probably has a supply of blue litmus paper (paper impregnated with a solution of litmus in its basic form) and red litmus paper (paper impregnated with a solution of litmus in its acidic form).
When you place red litmus paper in acid the paper stays red, but, if you place red litmus paper in base the paper changes to blue.
Similarly when you place blue litmus paper in base it stays blue, but, when you place blue litmus paper in acid it changes to red.
You could use Litmus paper to test a variety of aqueous solutions around your home and determine whether they are acidic (low pH) or basic (high pH).
The results of your experiments might look like this:
| Substance tested |
Test Results |
Conclusion |
|---|
| Red litmus paper |
Blue litmus paper |
| vinegar |
no change |
turns red |
low pH (acidic) |
| cola |
no change |
turns red |
low pH (acidic) |
| tap water |
no change |
turns red |
low pH (acidic) |
| sodium bicarbonate dissolved in water |
turns blue |
no change |
high pH (basic) |
| washing-up detergent |
turns blue |
no change |
high pH (basic) |
| solution of drain cleaner |
turns blue |
no change |
high pH (basic) |
Litmus is called an acid-base indicator because it indicates whether a solution is an acid or a base by changing colour.
You can make your own acid-base indicator at home or at school:
Experiment: Preparing an Acid-Base Indicator
- Place finely chopped and crushed red cabbage in a steel saucepan (or a beaker) with some water.(5)
- Heat gently to produce a purplish solution.
- Cover and leave the solution to cool.
- Decant the solution into a clean bottle.
This purplish solution contains cyanidin, a substance that changes colour with varying pH (or varying acidity).
Unlike litmus which displays just one colour at low pH and one colour at high pH, cyanidin changes colour with each change in pH.
The various colours of cyanidin indicator for different values of pH are shown below:
pH
(at 25°C) |
0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
13 |
14 |
|---|
cyanidin
(red cabbage water) |
red |
cerise |
purple |
blue |
aqua-
marine |
|
emerald-
green |
|
lime |
|
yellow |
| |
acidic solution |
neutral solution |
basic solution |
An acid (pH < 7) will turn cyanidin red, through to purple and blue, while a base (pH > 7) will turn cyanidin aquamarine through to green and yellow.
Because cyanidin changes colour as the pH of the solution is varied it is referred to as an acid-base indicator.
We can use our cyanidin solution to indicate the approximate pH of a solution and to decide whether a solution is acidic or basic.
If we tested some common household substances with our red cabbage water, cyanidin indicator, we might get results like those given in the table below:
| Substance tested |
Test Results |
Conclusion |
|---|
Cyanidin colour |
Estimated pH |
| vinegar |
cerise |
3 |
acidic |
| cola |
cerise |
3 |
acidic |
| tap water |
blue |
6 |
acidic |
| sodium bicarbonate dissolved in water |
aquamarine |
8 |
basic |
| washing-up detergent |
lime |
10 |
basic |
| solution of drain cleaner |
yellow |
13 |
basic |
We can use these results to decide which substance was the most acidic, that is, vinegar and cola have the lowest pH values so they are the most acidic.
And we can say that the drain cleaner solution is the most basic because it has the highest pH value.
An acid-base indicator is a dye that changes colour when pH changes.
The names of some acid-base indicators you might find in your school laboratory, and their colours in aqueous solutions at varying pH at 25°C, are given in the table below:
| Colour of Some Acid-Base Indicators at Various pH Values |
|---|
aqueous solution
25°C |
low pH
(more acidic) |
→ |
→ |
→ |
→ |
neutral |
→ |
→ |
→ |
→ |
high pH
(more basic) |
|---|
[H+(aq)]
(mol L-1) |
100 |
10-1 |
10-2 |
10-3 |
10-4 |
10-5 |
10-6 |
10-7 |
10-8 |
10-9 |
10-10 |
10-11 |
10-12 |
10-13 |
10-14 |
|---|
| pH |
0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
13 |
14 |
|---|
| alizarine yellow R indicator |
yellow |
|
red |
| phenolphthalein indicator |
colourless |
|
pink |
| cresolphthalein indicator |
colourless |
|
purple |
| thymol blue indicator |
red |
|
yellow |
|
blue |
| phenol red indicator |
yellow |
|
red |
| bromothymol blue indicator |
yellow |
|
blue |
| litmus indicator |
red |
|
blue |
| methyl red indicator |
orange-red |
|
yellow |
| bromocresol green indicator |
yellow |
|
pale blue-
green |
|
blue-green |
| methyl orange indicator |
red-orange |
|
yellow |
| bromophenol blue |
yellow |
|
blue |
| cresol red |
red |
|
yellow |
You've probably noticed that very few of these acid-base indicators change colour at a pH around 7, that is, most acid-base indicators do not allow you to make a definitive judgement about whether an aqueous solution is acidic, neutral or basic.
For example, imagine you have two different aqueous solutions in beakers labelled A and B, both at 25°C. We add a drop of cresol red indicator to each beaker.
Beaker A's solution turns red and we can say with some certainty that solution A is acidic.
Beaker B's solution turns yellow so the solution could be acidic, neutral or basic.
We can say that the solution in Beaker A is more acidic than the solution in Beaker B.
We can say that the pH of solution A is lower than the pH of solution B.
Universal indicator is an acid-base indicator produced by mixing together a selection of other acid-base indicators.
The colours produced by universal indicator in aqueous solutions of different pH at 25°C are shown below:
aqueous solution
25°C |
low pH
(more acidic) |
→ |
→ |
→ |
→ |
neutral |
→ |
→ |
→ |
→ |
high pH
(more basic) |
|---|
[H+(aq)]
(mol L-1) |
100 |
10-1 |
10-2 |
10-3 |
10-4 |
10-5 |
10-6 |
10-7 |
10-8 |
10-9 |
10-10 |
10-11 |
10-12 |
10-13 |
10-14 |
|---|
| pH |
0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
13 |
14 |
|---|
| Universal indicator |
red |
orange-
red |
orange |
pale
orange |
orange-
yellow |
pale
yellow |
green-
yellow |
green |
dark-
green |
|
|
blue |
deep
violet |
Universal indicator is good at giving you a rough approximation of the pH of an aqueous solution at 25°C.
If a drop of this universal indicator is added to a solution and it turns orange, then we would conclude that the pH of the solution is approximately 3, pH ≈ 3, and that the solution is acidic.
Colour-Change Interval, pH Range, of Acid-Base Indicators
I'm sure you will agree that the table of acid-base indicator colours at varying values of pH shown above is very colourful, most attractive, but it occupies a lot of space and is a bit difficult to use.
Take phenolphthalein as an example.
Phenolphthalein is an acid-base indicator found in most chemistry labs.
At low pH it is colourless, but in aqueous solutions with a high pH it is magenta (pink).
Most people would have detected the change in the colour of phenolphthalein from colourless to pale pink at around pH = 9, but some would have noticed it a lot sooner say at pH = 8.5, while others would not detect the colour change until the pH was 9.5.
Different people see colours differently, so when we see tables of acid-base indicators with their associated colours and pH values, we find a pH range over which the colour change occurs, this is called the colour-change interval or just the pH range of the acid-base indicator.
For phenolphthalein the colour change is said to occur over the pH range from 8.3 to 10.0.(6)
pH
(at 25°C) |
0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
13 |
14 |
|---|
| phenolphthalein |
colourless |
pH range
8.3-10 |
pink |
| |
acidic solution |
neutral solution |
basic solution |
This enables us to tabulate the colour-change interval, pH of range, of acid-base indicators in a concise fashion as shown below:
| Acid-Base Indicator pH and Colour |
|---|
| Name |
pH range
(colour-change interval) |
Colour change from lower pH
to higher pH in range |
|---|
| cresol red |
0.2 - 1.8 |
red → yellow |
| thymol blue (1st change) |
1.2 - 2.8 |
red → yellow |
| bromophenol blue |
3.0 - 4.6 |
yellow → blue |
| methyl orange |
3.1 - 4.4 |
red → yellow |
| bromocresol green |
3.8 - 5.4 |
yellow → blue |
| methyl red |
4.4 - 6.2 |
red → yellow |
| litmus |
5.0 - 8.0 |
red → blue |
| bromothymol blue |
6.0 - 7.6 |
yellow → blue |
| phenol red |
6.8 - 8.4 |
yellow → red |
| thymol blue (2nd change) |
8.0 - 9.6 |
yellow → blue |
| cresolphthalein |
8.2 - 9.8 |
colourless → purple |
| phenolphthalein |
8.3 - 10.0 |
colourless → pink |
| alizarine yellow R |
10.2 - 12.0 |
yellow → red |
If, for example, we add a drop of alizarine yellow R to an aqueous solution at 25°C and the solution turns yellow (the colour of the indicator at low pH values), then we can say that the pH is definitely less than 10.2 (pH < 10.2).
Its pH might be a little bit higher, but we could not say that with any degree of certainty.
Its pH most certainly cannot be greater than 12.0!
If the alizarine yellow R indicator turns red (the colour of the indicator at high pH values), then we can say that the pH of the solution is definitely greater than 12.0 (pH > 12.0).
Once again, the pH could be little bit less than 12 but we cannot say that with any level of certainty.
Its pH definitely cannot be less than 10.2!
The table of pH values and colours for different acid-base indicators may be presented in a different format, such as the one below:
| Acid-Base Indicator pH and Colour |
|---|
| Name |
Low pH colour |
pH range
(colour-change interval) |
High pH colour |
|---|
| cresol red |
red |
0.2 - 1.8 |
yellow |
| thymol blue (1st change) |
red |
1.2 - 2.8 |
yellow |
| bromophenol blue |
yellow |
3.0 - 4.6 |
blue |
| methyl orange |
red |
3.1 - 4.4 |
yellow |
| bromocresol green |
yellow |
3.8 - 5.4 |
blue |
| methyl red |
red |
4.4 - 6.2 |
yellow |
| litmus |
red |
5.0 - 8.0 |
blue |
| bromothymol blue |
yellow |
6.0 - 7.6 |
blue |
| phenol red |
yellow |
6.8 - 8.4 |
red |
| thymol blue (2nd change) |
yellow |
8.0 - 9.6 |
blue |
| cresolphthalein |
colourless |
8.2 - 9.8 |
purple |
| phenolphthalein |
colourless |
8.3 - 10.0 |
pink |
| alizarine yellow R |
yellow |
10.2 - 12.0 |
red |
The information in this table is identical to that in the first table, just presented slightly differently.
If, for example, we add a drop of cresol red indicator to an aqueous solution at 25°C it will turn red if the pH less than 0.2 and it will turn yellow if the pH of the solution is greater than 1.8.
Common Uses for Acid-Base Indicators
In general, plants will tolerate growing in soil with pH values between 5.2 and 7.8, that is, slightly acidic to very slightly basic soil, but generally plants will only thrive in soil within a much narrower range of pH values.
- Most fruit, vegetables, flowers, and trees and prefer a sightly acidic to neutral soil, that is, soil pH between 6.0 and 7.0, but there are lots of exceptions:
⚛ Many shrubs and evergreen tress prefer more acidic soil with a pH between 5.0 and 6.0
⚛ Some plants like blueberries, azaleas and rhododendrons prefer even more acidic soil with a pH between 4.5 and 5.2
- Most turf grasses prefer soil pH between 5.5 and 6.5
So it is important to know the pH of the soil we use to grow our plants in.
You can test the pH of soil yourself using an acid-base indicator such as universal indicator:
Experiment: Testing the pH of Soil
- Use a clean disposable plastic spoon to collect a sample of soil and place it on a watchglass.
- Sprinkle a fine layer of a white neutral powder such as barium sulfate or calcium sulfate over the soil sample.
- Leave it for a second to enable water from the soil to penetrate the powder.
- Add a drop of universal indicator to the powder and observe the colour change.
Other living things are also sensitive to small changes in pH. Fish, for example, will die if water becomes too acidic or too basic (alkaline).
So if you keep fish in an aquarium at home you will need to test the pH of the water regularly.
One of the easiest ways to do this is by adding a suitable acid-base indicator to a small sample of aquarium water.
Similarly we need to monitor the pH of waste water we add to rivers and streams so that we are not introducing acidic or basic water that could kill the aquatic organisms present in our waterways.
And ofcourse, we humans require water that is neither too acidic nor too basic to drink.
Acid-base indicators can be used to give a "ball park" figure for the pH of these waters.
The pH of the water in a chlorinated swimming pool needs to be close to 7.2 (between 7.0 and 7.4) otherwise it leads to problems:
- pH < 7.0 : pool water is acidic and you will need eye protection to go swimming .
- pH > 7.4 : pool is too basic (alkaline) which encourages the growth of green algae.
You can buy an acid-base indicator solution, or pH test strips (paper impregnated with the acid-base indicator), where you buy pool supplies.
When you add the acid-base indicator to a sample of pool water, or stick the pH test strip into the sample, the indicator changes colour.
Then you match the colour of the indicator solution, or test strip, to the chart of colours and pH on the packet.
Swimming Pool pH Test
| pH |
6.2 |
6.8 |
7.2 |
7.8 |
8.4 |
Indicator
colour |
|
|
|
|
|
| Conclusion |
very
low |
low |
OK |
high |
very
high |
When you test your sample of pool water, you want the indicator to stay that goldy colour so that the pH = 7.2.
If your pool water sample turns the indicator a different colour, this tells you to not swim in the pool because the pH is either too low (pH < 7.2) or too high (pH > 7.2).
Footnotes:
(1) IUPAC (Compendium of Chemical Terminology, Gold Book, 2014) defines an acid-base indicator as an acid or base which exhibits a colour change on neutralization by the basic or acidic titrant at or near the equivalence point of a titration.
There are other types of indicators used in chemistry, such as redox indicators and adsorption indicators, so it is not a good idea to abbreviate "acid-base indicator" to "indicator" when referring specifically to acid-base indicators.
(2) Historically, substances that change colour with changing pH have been called "acid-base indicators", or "neutralisation indicators". In more recent times the term "pH indicator" has popped up. If you read the IUPAC definition of an acid-base indicator in footnote 1 you might realise why "acid-base indicator" is preferable to "pH indicator".
(3) It will be assumed that all solutions tested are aqueous solutions at 25°C. This is important because an indicator is a substance in which the protonated (acidic) form is in equilibrium with the deprotonated (basic) form, hence the colours you see are determined by the nature of the solution and its temperature.
(4) Plants are not the only source of acid-base indicators. The dried and ground bodies of female cochineal insects, found in Mexico and Central America, can also be used to produce an acid-base indicator that is yellow in acidic solution and deep violet in basic solution.
(5) If you are doing this in the school laboratory, it is easier to extract plant pigments using methanol or ethanol than water, but be aware that alcohols are highly flammable (see Fuel Definitions Tutorial) and you will probably want to use a heating mantle instead of a bunsen burner to reduce the risk of fire.
(6) If you would like to understand why the indicator changes colour, go to the Acid-Base Indicator End-point Tutorial
