Indicators

CONTENTS• Acid-base indicators - theory

• Titration curves - introduction

• Titration curve; strong acid - strong base

• Titration curve; strong acid - weak base

• Titration curve; weak acid - strong base

• Titration curve; weak acid - weak base

• Titration curve; acid - carbonate

• Titration curve; phosphoric acid

• Check list

IndicatorsIndicators

Acid-base indicatorsAcid-base indicators

General Many indicators are weak acids and partially dissociate in aqueous solutionHIn(aq) H+

(aq) + In¯(aq)

The un-ionised form (HIn) is a different colour to the anionic form (In¯).



(aq) + In¯(aq)


Apply Le Chatelier’s Principle to predict any colour change

In acid - increase of [H+]- equilibrium moves to the left to give red undissociated form

In alkali - increase of [OH¯]- OH¯ ions remove H+ ions to form water; H+(aq) + OH¯(aq) H2O(l)- equilibrium will move to the right to produce a blue colour



(aq) + In¯(aq)


Apply Le Chatelier’s Principle to predict any colour change

In acid - increase of [H+]- equilibrium moves to the left to give red undissociated form

In alkali - increase of [OH¯]- OH¯ ions remove H+ ions to form water; H+(aq) + OH¯(aq) H2O(l)- equilibrium will move to the right to produce a blue colour

Summary In acidic solution

HIn(aq) H+(aq) + In¯(aq)

In alkaline solution

COLOUR CHANGES OF SOME COMMON INDICATORS

Must have an easily observed colour change. Must change immediately in the required pH range

over the addition of ‘half’ a drop of reagent.


PHENOLPHTHALEIN

LITMUS

METHYL ORANGE

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

CHANGE

CHANGE

CHANGE

pH

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 range

over the addition of ‘half’ a drop of reagent.


PHENOLPHTHALEIN

LITMUS

METHYL ORANGE

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 (HCl) (HCl) 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





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





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)





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

PHENOLPHTHALEIN

LITMUS

METHYL ORANGE


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)


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

PHENOLPHTHALEIN

LITMUS

METHYL ORANGE

strong acid strong acid (HCl) (HCl) 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))

Other pH curves - Other pH curves - acid v. carbonate

Sodium carbonate reacts with hydrochloric acid in two steps...Step 1 Na2CO3 + HCl ——> NaHCO3 + NaCl

Step 2 NaHCO3 + HCl ——> NaCl + H2O + CO2

Overall Na2CO3 + 2HCl ——> 2NaCl + H2O + CO2





There are two sharp pH changes

The second addition of HCl is exactly the same as the first because the number of moles of HCl which react with the NaHCO3 is the same as that reacting with the Na2CO3.

17.50cm3 35.00cm3





First rapid pH change around pH = 8.5 due to the formation of NaHCO3 .Can be detected using phenolphthalein






First rapid pH change around pH = 8.5 due to the formation of NaHCO3 .Can be detected using phenolphthalein

Second rapid pH change around pH = 4 due to the formation of acidic CO2 .Can be detected using methyl orange.


Other pH curves - Other pH curves - polyprotic acids (Hpolyprotic acids (H33POPO44))

Phosphoric acid is triprotic; it reacts with sodium hydroxide in three steps...Step 1 H3PO4 + NaOH ——> NaH2PO4 + H2O

Step 2 NaH2PO4 + NaOH ——> Na2HPO4 + H2O

Step 3 Na2HPO4 + NaOH ——> Na3PO4 + H2O


There are three sharp pH changes

Each successive addition of NaOH is the same as equal

number of moles are involved.





pH of H3PO4 = 1.5





pH of NaH2PO4 = 4.4

pH of H3PO4 = 1.5





pH of Na2HPO4 = 9.6

pH of NaH2PO4 = 4.4

pH of H3PO4 = 1.5





pH of Na3PO4 = 12

pH of Na2HPO4 = 9.6

pH of NaH2PO4 = 4.4

pH of H3PO4 = 1.5




REVISION CHECKREVISION CHECK

What should you be able to do?

Recall the definition of a weak acid

Understand why indicators can be made from weak acids

Understand why indicators must change colour quickly over a small pH range

Recall and explain the shape of titration curves involving acids and bases

Explain why particular indicators are used for certain titrations

Explain the shape of the titration curve for phosphoric acid

Explain the shape of the titration curve for acid -sodium carbonate reactions

CAN YOU DO ALL OF THESE? CAN YOU DO ALL OF THESE? YES YES NONO

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INDICATORS & INDICATORS & pH CURVESpH CURVES

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