Investigation at bottom of page
An Indicator is a substance that can tell us if another substance that it is in contact with is acidic or alkaline.
Litmus is a water-soluble mixture of different dyes extracted from lichens, especially Roccella tinctoria.
The mixture has CAS number 1393-92-6. It is often absorbed onto filter paper. The resulting piece of paper or solution with water becomes a pH indicator (one of the oldest), used to test materials for acidity.
ACID pH < 4.3 NEUTRAL pH > 8.3 BASE
pH is a complicated measure of the amount of H+ ions that are in the mix,
A low pH means a lot of H+ ions, a high value indicates that there is a desire for more H+ ions in the solution.
Naturally occurring pH indicators
Many plants or plant parts contain chemicals from the naturally-colored anthocyanin family of compounds. They are red in acidic solutions and blue in basic. Extracting anthocyanins from red cabbage leaves or the skin of a lemon to form a crude acid-base indicator is a popular introductory chemistry demonstration.
Anthocyanins can be extracted from a multitude of colored plants or plant parts, including from leaves (red cabbage); flowers (geranium, poppy, or rose petals); berries (blueberries, blackcurrant); and stems (rhubarb). An exhaustive list would be beyond the scope of this article -
Why not test some plants to see if they could be used as acid/base indicators
TO MAKE RED CABBAGE INDICATOR
Red Cabbage, Beaker(600ml), water, knife, chopping board; Collection bottle; hot plate; plastic bag; test tubes;
Hydrochloric acid(1M); sodium hydroxide(1M); acetic acid(1M); sodium carbonate(1M).
1) Turn on the hot plate to High.
2) Chop up enough red cabbage to fit into ¾ of a 600ml beaker.
3) Add water until it reaches the 300ml mark.
4) Boil the cabbage for 5 minutes.
5) Half fill four test tubes with the red cabbage liquid.
6) Pour the rest of the liquid into the collection bottle for storage.
7) Place red cabbage scraps into a plastic bag.
8) Add Hydrochloric acid(1M) drop-wise to one of the test tube. Record observation.
9) Add sodium hydroxide(1M) drop-wise to one of the other test tubes. Record observation.
10) Add acetic acid(1M) drop-wise to one of the other test tubes. Record observation.
11) Add sodium carbonate(1M) drop-wise to one of the other test tubes. Record observation.
We could now find the range of turning pH for the red cabbage indicator and also to view the different colours of Universal Indicator
1) Arrange 7 test tubes in a rack
2) Pour 10ml of hydrochloric acid into the first test tube (pH 1).
3) Pour 1ml from the first test tube into the next test tube and add 9ml of water (pH 2).
4) Pour 1ml from the second test tube into the third test tube and add 9ml of water (pH 3).
5) Continue this procedure until all test tubes have 10ml of solution (pH 4 7).
6) Add 2-3 drops of universal indicator to each test tube.
7) Record observations by colouring the appropriate box in the result table.
8) Clean test tubes and repeat steps 1)-7) using the red cabbage indicator.
9) Clean test tubes and repeat steps 1)-7) starting with sodium hydroxide and using universal indicator (pH 14 8).
10) Clean test tubes and repeat steps 1)-7) starting with sodium hydroxide and using red cabbage indicator.
11) Record observations by colouring the appropriate box in the result table.
So Indicators and their colours,
we see that they change different colours in different solutions
but what makes one change might not effect another indicator.
So do we need to learn them all ?
well except to know that
Methyl Orange turns Pink at pH 7
Indicator pH Range Quantity per 10 ml Acid Base
Thymol Blue 1.2-2.8 1-2 drops 0.1% soln. in aq. red yellow
Pentamethoxy red 1.2-2.3 1 drop 0.1% soln. in 70% alc. red-violet colorless
Tropeolin OO 1.3-3.2 1 drop 1% aq. soln. red yellow
2,4-Dinitrophenol 2.4-4.0 1-2 drops 0.1% soln. in 50% alc. colorless yellow
Methyl yellow 2.9-4.0 1 drop 0.1% soln. in 90% alc. red yellow
Methyl orange 3.1-4.4 1 drop 0.1% aq. soln. red orange
Bromphenol blue 3.0-4.6 1 drop 0.1% aq. soln. yellow blue-violet
Tetrabromphenol blue 3.0-4.6 1 drop 0.1% aq. soln. yellow blue
Alizarin sodium sulfonate 3.7-5.2 1 drop 0.1% aq. soln. yellow violet
a-Naphthyl red 3.7-5.0 1 drop 0.1% soln. in 70% alc. red yellow
p-Ethoxychrysoidine 3.5-5.5 1 drop 0.1% aq. soln. red yellow
Bromcresol green 4.0-5.6 1 drop 0.1% aq. soln. yellow blue
Methyl red 4.4-6.2 1 drop 0.1% aq. soln. red yellow
Bromcresol purple 5.2-6.8 1 drop 0.1% aq. soln. yellow purple
Chlorphenol red 5.4-6.8 1 drop 0.1% aq. soln. yellow red
Bromphenol blue 6.2-7.6 1 drop 0.1% aq. soln. yellow blue
p-Nitrophenol 5.0-7.0 1-5 drops 0.1% aq. soln. colorless yellow
Azolitmin 5.0-8.0 5 drops 0.5% aq. soln. red blue
Phenol red 6.4-8.0 1 drop 0.1% aq. soln. yellow red
Neutral red 6.8-8.0 1 drop 0.1% soln. in 70% alc. red yellow
Rosolic acid 6.8-8.0 1 drop 0.1% soln. in 90% alc. yellow red
Cresol red 7.2-8.8 1 drop 0.1% aq. soln. yellow red
a-Naphtholphthalein 7.3-8.7 1-5 drops 0.1% soln. in 70% alc. rose green
Tropeolin OOO 7.6-8.9 1 drop 0.1% aq. soln. yellow rose-red
Thymol blue 8.0-9.6 1-5 drops 0.1% aq. soln. yellow blue
Phenolphthalein 8.0-10.0 1-5 drops 0.1% soln. in 70% alc. colorless red
a-Naphtholbenzein 9.0-11.0 1-5 drops 0.1% soln. in 90% alc. yellow blue
Thymolphthalein 9.4-10.6 1 drop 0.1% soln. in 90% alc. colorless blue
Nile blue 10.1-11.1 1 drop 0.1% aq. soln. blue red
Alizarin yellow 10.0-12.0 1 drop 0.1% aq. soln. yellow lilac
Salicyl yellow 10.0-12.0 1-5 drops 0.1% soln. in 90% alc. yellow orange-brown
Diazo violet 10.1-12.0 1 drop 0.1% aq. soln. yellow violet
Tropeolin O 11.0-13.0 1 drop 0.1% aq. soln. yellow orange-brown
Nitramine 11.0-13.0 1-2 drops 0.1% soln in 70% alc. colorless orange-brown
Poirrier's blue 11.0-13.0 1 drop 0.1% aq. soln. blue violet-pink
Trinitrobenzoic acid 12.0-13.4 1 drop 0.1% aq. soln. colorless orange-red
Universal indicator is a chemical made by combining other indicators, by doing this we have a way of measuring the pH change of a substance continuously as the reaction takes place.
Using Universal Indicator, random substances can have their acidity checked quite easily because the different indicators are combined in the test papers and we do not need many chemicals in our lab or in the field.
There are more exact pH papers, they are Universal papers but they improve their accuracy by having different coloured compounds at different locations along the strip.
Digital pH Meters
There are many type of digital pH meters that can read the pH of substances, one of the problems with these meters is that they require calibration.