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.

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 IndicatorsHow 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.

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Methyl violet indicator

0.0 1.6

0 1.6

pH

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

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1.6 14

0 1.6

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 1.6

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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.

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Orange (IV) indicator

1.60

0.0 1.4

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.

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1.60

1.4 2.8

Methyl violet

1.4 2.80

pH

14

Now we’ll continue raising the pH (click) We see it remains yellow all the way up to 14.

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1.60

142.8

Methyl violet

1.40 2.8

pH

14

14

We now have a line down here showing the colours of Orange IV from pH 0 up to 14

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1.60

1.40 2.8

Methyl violet

Orange (IV)

pH

14

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.

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Thymol Blue

1.60

1.20.0

1.40 2.8

Methyl violet

Orange (IV)

1.20

pH

14

14

As we gradually increase the pH from 1.2 to2.8 (click), the colour changes from red to orange to yellow.

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Thymol Blue

1.60

1.2 2.8

1.40 2.8

Methyl violet

Orange (IV)

1.20 2.8

pH

14

14

Now, we’ll keep on increasing the pH. (click) from 2.8 to 8, it stays yellow.

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Thymol Blue

1.60

2.8

1.40 2.8

8.0

Methyl violet

Orange (IV)

1.20 2.8 8.0

pH

14

14

Going up from 8 to 9.6 (click), it changes from yellow to green to blue.

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Thymol Blue

1.60

9.6

1.40 2.8

8.0

Methyl violet

Orange (IV)

1.20 8.0 9.62.8

pH

14

14

And going up from 9.6 to 14 (click) it stays blue.

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Thymol Blue

1.60

9.6

1.40 2.8

14

Methyl violet

Orange (IV)

1.20 9.6 148.02.8

pH

14

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 14.

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Thymol Blue

1.60 Methyl violet

1.40 2.8 Orange (IV)

1.20 2.8 8.0 9.6 14Thymol blue

pH

14

14

As well as records for Orange (IV) and Methyl violet.

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Thymol Blue

1.60 Methyl violet


1.20 2.8 8.0 9.6 14Thymol blue

pH

14

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.


Blue when pH ≥ 7.6

The two numbers here give us the pH range in which the colour gradually changes



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.



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.


Blue


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


Blue


Blue

Gains 1 H+

Loses 1 H+

Here is the structure for the base form of bromthymol blue. It is blue.


Blue


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 .


Blue


Blue

Gains 1 H+

Loses 1 H+

HIn

And the formula of the base by the simple generic formula In minus.


Blue


Blue

Gains 1 H+

Loses 1 H+

HIn In–

In all indicators, there is an equilibrium involving a weak acid and a weak base.

(aq)HIn (aq)In3 (aq)H O

2 ( )H O l

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.


2 ( )H O l

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.


2 ( )H O l

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,

(aq)HIn (aq)In

3 (aq)H O2 ( )H O l

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.

(aq)HIn (aq)In

3 (aq)H O2 ( )H O l

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,

(aq)HIn

(aq)In3 (aq)H O

2 ( )H O l

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.

(aq)HIn

(aq)In3 (aq)H O

2 ( )H O l

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.,


2 ( )H O l

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,


2 ( )H O l


[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.


2 ( )H O l


[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 [H3O+]


2 ( )H O l

The equilibrium position, and the relative amounts

of HIn and In– depends on the [H3O+]

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


2 ( )H O l

Add an acidpH is

Decreased

The hydronium ion concentration will quickly increase.


2 ( )H O l

Add an acidpH is

Decreased

3 (aq)H O

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

(aq)HIn (aq)In2 ( )H O l

Shift to the Left

Has Been Increased

(aq)HIn

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.

2 ( )H O l

3 (aq)H O(aq)In

[HIn] is Higher

[In–] is Lower

(aq)HIn

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

2 ( )H O l

3 (aq)H O(aq)In

(aq)HIn (aq)In >

(aq)HIn

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.

2 ( )H O l

3 (aq)H O(aq)In

(aq)HIn (aq)In >

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.


2 ( )H O l

Add a basepH is

Increased

The OH minus from the base will neutralize some of the H3O+,


2 ( )H O l

Add a basepH is

Increased

OH– + H3O+ 2H2O

And the concentration of H3O+ will decrease (click)


2 ( )H O l

Add a basepH is

Increased

OH– + H3O+ 2H2O

[H3O+] 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.

(aq)HIn (aq)In2 ( )H O l

3 (aq)H O

Shift to the Right

Has Been Decrease

d

(aq)In3 (aq)H O

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.

2 ( )H O l

(aq)HIn

[HIn] is Lower

[In–] is Higher

(aq)In3 (aq)H O

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.

2 ( )H O l

(aq)HIn

(aq)In (aq)HIn >

The solution will be the colour of the base form In–

Different indicators have different acid strengths and different pairs of colours.

1.60 Methyl violet



2 ( )H O l


2 ( )H O l

This gives them different pH ranges in which their colours change.

1.60 Methyl violet



2 ( )H O l


2 ( )H O l

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.

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.

Documents

ph slide

methyl violet ph

methyl violet orange

thymol blue ph

methyl violet changes

orange iv indicator

called methyl violet

colours of orange iv