StarterIn pairs discuss everything you remember about

titrations:1)How they are carried out.2) What are indicators for?3)You carry out a titration of a 25 cm3 acid

solution (c= 0.5 moldm-3) with a 1 mol dm-3 solution of NaOH. Calculate the number of moles of acid after the addition of 10 cm3 of NaOH

Neutralisation

Interpret and sketch acid-base pH curves for strong and weak acid and bases.

Explain the choice of suitable indicator for acid-base titration

Simulation: http://users.skynet.be/eddy/titratie.swfStrong acid/ strong base

pH curvespH curves

Types There are four types of acid-base titration; each has a characteristic curve.

strong acid (HCl) v. strong base (NaOH) weak acid (CH3COOH) v. strong alkali (NaOH)

strong acid (HCl) v. weak base (NH3)

weak acid (CH3COOH) v. weak base (NH3)

In the following examples, alkali (0.1M) is added to 25cm3 of acid (0.1M)

End points need not be “neutral‘ due to the phenomenon of salt hydrolysis

Equivalence point: the point in a titration at which the volume of one solution has reacted exactly with the volume of the second solution

strong acidstrong acid (HC (HCll) ) v. strong base v. strong base (NaOH)(NaOH)

Equivalence point is the centre of the vertical section of the titration curve.

Titration curve for a strong acid–strong base titration

pH 1 at the start due to 0.1M HCl

(strong monoprotic acid)

strong acidstrong acid (HC (HCll) ) v. strong base v. strong base (NaOH)(NaOH)

Very little pH change during the initial 20cm3

pH 1 at the start due to 0.1M HCl

(strong monoprotic acid)

strong acidstrong acid (HC (HCll) ) v. strong base v. strong base (NaOH)(NaOH)

Very little pH change during the initial 20cm3

Very sharp change in pH over the addition of less than half a drop of NaOH

pH 1 at the start due to 0.1M HCl

(strong monoprotic acid)

strong acidstrong acid (HC (HCll) ) v. strong base v. strong base (NaOH)(NaOH)

Very little pH change during the initial 20cm3

Very sharp change in pH over the addition of less than half a drop of NaOH

Curve levels off at pH 13 due to excess 0.1M NaOH

(a strong alkali)

strong acidstrong acid (HC (HCll) ) v. strong base v. strong base (NaOH)(NaOH)

pH 1 at the start due to 0.1M HCl

(strong monoprotic acid)

Very little pH change during the initial 20cm3

Sharp change in pH over the addition of less than

half a drop of NH3

Curve levels off at pH 10 due to excess 0.1M NH3

(a weak alkali)

pH 1 at the start due to 0.1M HCl

strong acidstrong acid (HC (HCll) ) v. weak basev. weak base (NH (NH33))

Steady pH change

Sharp change in pH over the addition of less than

half a drop of NaOH

Curve levels off at pH 13 due to excess 0.1M NaOH

(a strong alkali)

pH 4 due to 0.1M CH3COOH (weak monoprotic acid)

weak acid weak acid (CH(CH33COOH) COOH) v. strong basev. strong base (NaOH) (NaOH)

weak acidweak acid (CH (CH33COOH) COOH) v. weak basev. weak base (NH (NH33))

Types

Steady pH change

pH 4 due to 0.1M CH3COOH (weak monoprotic acid)

NO SHARPCHANGE IN pH

Curve levels off at pH 10 due to excess 0.1M NH3

(a weak alkali)

4.3 exercise 4

Indicator

Hin = H+ + In-

pH colour ranges for some common indicators

Watch demo?Why do you get a different result with a different indicator?

phenolthalein

Methyl orange

COLOUR CHANGES OF SOME COMMON INDICATORS

PHENOLPHTHALEIN

LITMUS

METHYL ORANGE

1 2 3 4 5 6 7 8 9 10 11 12 13 14

CHANGE

CHANGE

CHANGE

pH

Indicator change colours around +- 1 unit of pH from their end point.

An indicator has to be chosen so that their end point is as close as the pH value of the titration’s equivalence point.

So the colour of the indicator changes over the pH range pKIn 1

HIn(aq) H+(aq) + In¯(aq)

Which indicator is the best?

Simulation

What happens when you titrate Na2CO3 with HCl?

What happens when you titrate H3PO4 with NaOH?

4.3 exercise 4

Choice of indicators for titrations

To be useful, an indicator mustchange over the “vertical” section of the curve where there is a large change in pH for the addition of a very small volume of alkali.

The indicator used depends on the pH changes around the end point - the indicator must change during the ‘vertical’ portion of the curve.

In the example, the only suitable indicator is PHENOLPHTHALEIN.

Must have an easily observed colour change.

Must change immediately in the required pH rangeover the addition of ‘half’ a drop of reagent.

Acid-base indicatorsAcid-base indicators

PHENOLPHTHALEIN

LITMUS

METHYL ORANGE

COLOUR CHANGES OF SOME COMMON INDICATORS

Must have an easily observed colour change.

Must change immediately in the required pH rangeover the addition of ‘half’ a drop of reagent.

Acid-base indicatorsAcid-base indicators

PHENOLPHTHALEIN

LITMUS

METHYL ORANGE

1 2 3 4 5 6 7 8 9 10 11 12 13 14

CHANGE

CHANGE

CHANGE

pH

pH curvespH curves

Types There are four types of acid-base titration; each has a characteristic curve.

strong acid (HCl) v. strong base (NaOH) weak acid (CH3COOH) v. strong alkali (NaOH)

strong acid (HCl) v. weak base (NH3)

weak acid (CH3COOH) v. weak base (NH3)

In the following examples, alkali (0.1M) is added to 25cm3 of acid (0.1M)

End points need not be “neutral‘ due to the phenomenon of salt hydrolysis

strong acidstrong acid (HC (HCll) ) v. strong base v. strong base (NaOH)(NaOH)

pH 1 at the start due to 0.1M HCl

(strong monoprotic acid)

strong acidstrong acid (HC (HCll) ) v. strong base v. strong base (NaOH)(NaOH)

Very little pH change during the initial 20cm3

pH 1 at the start due to 0.1M HCl

(strong monoprotic acid)

strong acidstrong acid (HC (HCll) ) v. strong base v. strong base (NaOH)(NaOH)

Very little pH change during the initial 20cm3

Very sharp change in pH over the addition of less than half a drop of NaOH

pH 1 at the start due to 0.1M HCl

(strong monoprotic acid)

strong acidstrong acid (HC (HCll) ) v. strong base v. strong base (NaOH)(NaOH)

Very little pH change during the initial 20cm3

Very sharp change in pH over the addition of less than half a drop of NaOH

Curve levels off at pH 13 due to excess 0.1M NaOH

(a strong alkali)

strong acidstrong acid (HC (HCll) ) v. strong base v. strong base (NaOH)(NaOH)

pH 1 at the start due to 0.1M HCl

(strong monoprotic acid)

strong acidstrong acid (HC (HCll) ) v. strong base v. strong base (NaOH)(NaOH)

Any of the indicators listed will be suitable - they all change in the ‘vertical’ portion

PHENOLPHTHALEIN

LITMUS

METHYL ORANGE

Very little pH change during the initial 20cm3

Sharp change in pH over the addition of less than

half a drop of NH3

Curve levels off at pH 10 due to excess 0.1M NH3

(a weak alkali)

pH 1 at the start due to 0.1M HCl

strong acidstrong acid (HC (HCll) ) v. weak basev. weak base (NH (NH33))

PHENOLPHTHALEIN

LITMUS

METHYL ORANGE

strong acid strong acid (HC(HCll) ) v. weak basev. weak base (NH (NH33))

Only methyl orange is suitable - it is the only one to change in the ‘vertical’ portion

Steady pH change

Sharp change in pH over the addition of less than

half a drop of NaOH

Curve levels off at pH 13 due to excess 0.1M NaOH

(a strong alkali)

pH 4 due to 0.1M CH3COOH (weak monoprotic acid)

weak acid weak acid (CH(CH33COOH) COOH) v. strong basev. strong base (NaOH) (NaOH)

PHENOLPHTHALEIN

LITMUS

METHYL ORANGE

Only phenolphthalein is suitable - it is the only one to change in the ‘vertical’ portion

weak acidweak acid (CH (CH33COOH) COOH) v. strong basev. strong base (NaOH) (NaOH)

weak acidweak acid (CH (CH33COOH) COOH) v. weak basev. weak base (NH (NH33))

Types

Steady pH change

pH 4 due to 0.1M CH3COOH (weak monoprotic acid)

NO SHARPCHANGE IN pH

Curve levels off at pH 10 due to excess 0.1M NH3

(a weak alkali)

PHENOLPHTHALEIN

LITMUS

METHYL ORANGE

NOTHING SUITABLE

There is no suitable indicator- none change in the ‘vertical’ portion.The end point can be detected by plotting a curve using a pH meter.

weak acidweak acid (CH (CH33COOH) COOH) v. weak base v. weak base (NH(NH33))