Indicators are useful in identifying substances which look-alike.
An acid-base indicator is a substance used to identify whether another substance is alkaline or acidic.
An acid-base indicator works by changing to different colours in neutral, acidic and alkaline solutions/dissolved in waterExperiment:
To prepare simple acid-base indicatorProcedure
(a)Place some flowers petals in a mortar.
Crush them using a pestle.
Add a little sand to assist in crushing.
Add about 5 cm3 of propanone/ethanol and carefully continue grinding.
Add more 5 cm3 of propanone/ethanol and continue until there is enough extract in the mortar.
Filter the extract into a clean 100cm3 beaker.
(b)Place 5 cm3 of filtered wood ash, soap solution, ammonia solution, sodium hydroxide, hydrochloric acid, distilled water, sulphuric(VI)acid, sour milk, sodium chloride, toothpaste and calcium hydroxide into separate test tubes.
(c)Put about three drops of the extract in (a)to each test tube in (b). Record the observations made in each case.Sample observations
The plant extract is able to differentiate between solutions by their nature. It is changing to a similar colour for similar solutions.
(i)Since lemon juice is a known acid, then sulphuric(VI) and hydrochloric acids are similar in nature with lemon juice because the indicator show similar colours. They are acidic in nature.
(ii)Since sodium hydroxide is a known base/alkali, then the green colour of indicator shows an alkaline/basic solution.
(iii) Since pure water is neutral,then the orange colour of indicator shows neutral solutions.
In a school laboratory, commercial indicators are used. A commercial indicator is cheap, readily available and easy to store.
Common indicators include: Litmus, phenolphthalein, methyl orange, screened methyl orange, bromothymol blue.Experiment:
Using commercial indicators to determine acidic, basic/alkaline and neutral solutionsProcedure
Place 5 cm3 of the solutions in the table below.
Add three drops of litmus solution to each solution.
Repeat with phenolphthalein indicator, methyl orange, screened methyl orange and bromothymol blue.Sample results
From the table above, then the colour of indicators in different solution can be summarized
The universal indicator
The universal indicator is a mixture of other indicator dyes.
The indicator uses the pH scale.The pH scale shows the strength of bases and acids.
The pH scale ranges from 1-14.These numbers are called pH values:
(ii) pH values 4,5,6 shows a substance is a weak acid
(iii) pH value 7 shows a substance is a neutral
(iv)pH values 8,9,10,11 shows a substance is a weak base/alkali.
(v)pH values 12,13,14 shows a substance is a strong base/alkali
The pH values are determined from a pH chart.The pH chart is a multicoloured paper with each colour corresponding to a pH value.i.e
(i)Red correspond to pH 1,2,3 showing strongly acidic solutions.
(ii)Orange/ yellow correspond to pH 4,5,6 showing weakly acidic solutions.
(iiiGreen correspond to pH 7 showing neutral solutions.
(iv)Blue correspond to pH 8,9,10,11 showing weakly alkaline solutions.
(v)Purple/dark blue correspond to pH 12,13,14 showing strong alkalis.
The universal indicator is available as:
(i) universal indicator paper/pH paper
(ii) universal indicator solution.
When determining the pH of a unknown solution using
(i)pH paper:pH paper is dipped into the unknown solution.It changes/turn to a certain colour. The new colour is marched/compared to its corresponding one on the pH chart to get the pH value.
(ii) universal indicator solution:about 3 drops of the universal indicator solution is added into about 5 cm3 of the unknown solution in a test tube. It changes/turn to a certain colour.
The new colour is marched/compared to its corresponding one on the pH chart to get the pH value.Experiment:
To determine the pH value of some solutions
(a)Place 5 cm3 of filtered wood ash, soap solution, ammonia solution, sodium hydroxide, hydrochloric acid, distilled water, sulphuric(VI)acid, sour milk, sodium chloride, toothpaste and calcium hydroxide into separate test tubes.
(b)Put about three drops of universal indicator solution or dip a portion of a piece of pH paper into each.
Record the observations made in each case.
(c)Compare the colour in each solution with the colours on the pH chart provided. Determine the pH value of each solution.Sample observations
1.All the mineral acids:Hydrochloric, sulphuric(VI)and nitric(V)acids are strong acids
2.Two alkalis/soluble bases ,sodium hydroxide and potassium hydroxide are strong bases/alkali. Ammonia solution is a weak base/alkali.All other bases are weakly alkaline.
3.Pure/deionized water is a neutral soulution.
4.Common salt/sodium chloride is a neutral salt.
5. When an acid and an alkali/base are mixed, the final product have pH 7 and is neutral.