Acid-base indicators are added to solutions and show specific colour changes as the pH of the solution changes. We’ll start with an example.. Acid-Base Indicators How They Work

It’s called methyl violet indicator. When it is added to a solution with a pH of 0, it’s colour is yellow. As we gradually increase the pH from 0 to 1.6 (click), the colour gradually changes from yellow to green to blue Methyl violet indicator pH

When we gradually increase the pH from 1.6, all the way up to 14 (click), the colour remains blue Methyl violet indicator pH 14

So now we have a line down here showing that methyl violet changes gradually from yellow to green to blue between 0 and 1.6, and stays blue at all pH’s above Methyl violet indicator 1.60 Methyl violet pH 14

Now we’ll start with a new indicator, Orange IV. We see it is red at a pH of 0. As we gradually increase the pH from 0 to 1.4 (click), the colour stays red Orange (IV) indicator Methyl violet 1.40 pH 14

As we increase the pH from 1.4 to 2.8, (click), the colour gradually changes from red to orange to yellow Orange (IV) indicator Methyl violet pH 14

Now we’ll continue raising the pH (click) We see it remains yellow all the way up to Orange (IV) indicator Methyl violet pH 14

We now have a line down here showing the colours of Orange IV from pH 0 up to Orange (IV) indicator Methyl violet Orange (IV) pH 14

Now we’ll start with a new solution to which we’ve added the indictor thymol blue. At a pH of 0, we see thymol blue is red. As we increase the pH up to 1.2 (click), it remains red Thymol Blue Methyl violet Orange (IV) 1.20 pH 14

As we gradually increase the pH from 1.2 to2.8 (click), the colour changes from red to orange to yellow Thymol Blue Methyl violet Orange (IV) pH 14

Now, we’ll keep on increasing the pH. (click) from 2.8 to 8, it stays yellow Thymol Blue Methyl violet Orange (IV) pH 14

Going up from 8 to 9.6 (click), it changes from yellow to green to blue Thymol Blue Methyl violet Orange (IV) pH 14

And going up from 9.6 to 14 (click) it stays blue Thymol Blue Methyl violet Orange (IV) pH 14

So now we have a record of the many colour changes that thymol blue undergoes as we change the pH from 0 up to Thymol Blue 1.60 Methyl violet Orange (IV) Thymol blue pH 14

As well as records for Orange (IV) and Methyl violet Thymol Blue 1.60 Methyl violet Orange (IV) Thymol blue pH 14

The Acid-Base indicator table you are given contains the indicators we’ve looked at and quite a few more. Note how the information on the table corresponds to the colours on the diagrams

Notice, the colours of thymol blue at lower pH’s are near the top,

and those at higher pH’s are further down the table.

Let’s look a little closer at a particular indicator. We’ll use bromthymol blue.

The first colour named is yellow. This means the colour of bromthymol blue is yellow if the pH is less than or equal to 6.0. Yellow when pH ≤ 6.0

The second colour named is blue This means the colour of bromthymol blue is blue if the pH is greater than or equal to 7.6. Yellow when pH ≤ 6.0 Blue when pH ≥ 7.6

The two numbers here give us the pH range in which the colour gradually changes Yellow when pH ≤ 6.0 Blue when pH ≥ 7.6 pH Range in Which colour change occurs

So between 6 and 7.6 the colour gradually changes from yellow at 6 to blue at 7.6. Half way between 6 and 7.6 is 6.8. It is green at 6.8. Yellow when pH ≤ 6.0 Blue when pH ≥ 7.6 pH Range in Which colour change occurs 6.8

Most indicators are organic compounds with complex structures. Here is one form of the indicator bromthymol blue. The Molecular Structure of Bromthymol Blue

Like all indicators, bromthymol blue has two different forms: an acid form and a base form. Acid Form of Bromthymol Blue Base Form of Bromthymol Blue  Gains 1 H + Loses 1 H +

The acid form has one more proton than the base form. This difference is enough to alter the structure of the molecule enough to change its colour. Acid Form of Bromthymol Blue Base Form of Bromthymol Blue  Gains 1 H + Loses 1 H + The Acid form has one more H + than the Base form.

The two different forms are always different colours. Here is the structure of the acid form of bromthymol blue. It is yellow Acid Form of Bromthymol Blue Base Form of Bromthymol Blue  Gains 1 H + Loses 1 H +

Here is the structure for the base form of bromthymol blue. It is blue. Acid Form of Bromthymol Blue Base Form of Bromthymol Blue  Gains 1 H + Loses 1 H +

To save us from writing complex formulas, we can represent the formula of the acid form of any indicator by the simple generic formula HIn. Acid Form of Bromthymol Blue Base Form of Bromthymol Blue  Gains 1 H + Loses 1 H + HIn

And the formula of the base by the simple generic formula In minus. Acid Form of Bromthymol Blue Base Form of Bromthymol Blue  Gains 1 H + Loses 1 H + HIn In –

In all indicators, there is an equilibrium involving a weak acid and a weak base. An Equilibrium Involving a Weak Acid and a Weak Base

The acid form of an indicator, which we’ll call HIn is a weak acid. The acid form of bromthymol blue is yellow, but this is not true for all indicators. Each indicator has its own specific colours. Acid Form is a Weak Acid

The base form of an indicator is the conjugate base of the acid form. The base form of the indicator bromthymol blue is blue. Base Form is Its Conjugate Base

If the concentration of the acid form HIn, is much greater than the concentration of the base form In minus, If the [HIn] is much greater than the [In – ]

Then the acid form, Hin, will predominate and the solution will be the colour of the acid form. If the [HIn] is much greater than the [In – ] HIn will predominate and the solution will be the colour of the acid form.

If the concentration of the base form In minus, is much greater than the concentration of the acid form HIn, If the [In – ] is much greater than the [HIn]

Then the base form, In minus, will predominate and the solution will be the colour of the base form. If the [In – ] is much greater than the [HIn] In – will predominate and the solution will be the colour of the base form.

If the concentration of the acid form, HIn, is equal to or close to the concentration of the base form In minus., If the [HIn] is equal to or close to the [In – ]

Neither form will predominate and the solution will be an equal mixture of the colours of the acid form and base form. For the indicator bromthymol blue, this is an equal mixture of yellow and blue, If the [HIn] is equal to or close to the [In – ] Neither form will predominate and the solution will be an equal mixture of the colours of the acid form and base form.

which is green. If the [HIn] is equal to or close to the [In – ] Neither form will predominate and the solution will be an equal mixture of the colours of the acid form and base form.

The equilibrium position, and the relative amounts of HIn and In – depends on the [H 3 O + ]

If we add an acid to our indicator, which is the same as decreasing the pH.. Add an acid pH is Decreased

The hydronium ion concentration will quickly increase. Add an acid pH is Decreased

The increase in the concentration of H3O + is a stress. The equilibrium will counteract this stress (click) by shifting to the left. Shift to the Left Has Been Increased

The result is, the concentration of the acid form Hin, is higher than it was originally, and the concentration of the base form, In minus is lower than it was originally. [HIn] is Higher [In – ] is Lower

Because the concentration of the acid form HIn is significantly greater than the concentration of the base form, In minus >

The acid form HIn predominates, and the solution will be the colour of the acid form HIn. In the case of bromthymol blue, this is yellow. > The solution will be the colour of the acid form HIn

Now, we’ll add a base to our indicator. That is the same as increasing the pH. Add a base pH is Increased

The OH minus from the base will neutralize some of the H3O+, Add a base pH is Increased OH – + H 3 O +  2H 2 O

And the concentration of H3O+ will decrease (click) Add a base pH is Increased OH – + H 3 O +  2H 2 O [H 3 O + ] will Decrease

The decrease in hydronium ion concentration is a stress to the equilibrium and in order to counteract the stress, (click) it will shift to the right. Shift to the Right Has Been Decreased

As a result of adding a base, and the consequent shift to the right, the concentration of the acid form HIn is lower than it was originally, and the concentration of the base form In minus is higher than it was originally. [HIn] is Lower [In – ] is Higher

Because the concentration of the base form In minus is significantly greater than that of the acid form HIn, the solution will be the colour of the base form, In minus. > The solution will be the colour of the base form In –

Different indicators have different acid strengths and different pairs of colours Methyl violet Orange (IV)

This gives them different pH ranges in which their colours change Methyl violet Orange (IV)

The Acid-Base indicator table shows a wide variety of indicators, with different colours and different pH Ranges. But they all work on the same principle of their equilibria shifting whenever pH is changed.