IPCORES del SWE

The point at which stoichicometrically equivalent quantities of substances have been brought together is known as equivalence point of titration. To determine when the equivalence point of titration is reached, an indicator is used. In acid base titrations, the indicators such as methyl orange, phenolphthalein and bromothymol blue which are usually organic dyes that change colour according to the hydrogen ion concentration of the solution or liquid to which they added or found themselves, are used.

For example, phenophtalein is an indicatOr that is colorless in acidic solution but changed to pink in basic solution.

Methyl orange (yellow form)

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HO HO

0

C

0

II

Phenophtalein (Colourless form)

These indicators can be regarded as weak acids of which either the undissociated molecule or the dissociated anion, or both are coloured.

If we take methyl orange for example,

HMe H+ + Me"

Red 4—colourless ^yellow

Addition of acid moves the equilibrium to the left so that the solution becomes red. Conversely, when alkali containing hydroxyl ions are added, equilibrium moves to the right and the colour becomes yellow.

It is also said that it is rarely possible to find an indicator that will indicate exact equivalence point, hence the change in colour we observe when we stop the titration indicates the end of the titration.

Although the end point of titration does not coincide with the equivalence point, the error is negligible.

The table below shows the list of indicators commonly used in acid — base titration, the colours they exhibit in various solutions and the pH range where colour changes take place.

3.2.1 ph Scale

The ph of a solution (or a liquid) is a measure of the hydrogen ion, H +, concentration in the solution. The ph scale ranges from 0 to 14. A solution with ph value less than 7 is acidic; exactly 7 is neutral, while the higher the value, the more alkaline the solution. Pure water has a ph of 7, it is neutral.

When the colour of an indicator changes, it is due to the acidity of the change in the environment it has found itself. Near the end — point, both coloured forms will be present in appreciable quantities.

INDICATOR

COLOR in

Acid Neutral ALKALI

pH RANGE

1. Lithmus Red Purple

Orange

Blue Yellow

6.0 — 8.0 3.1 — 4.4 2. Methylorange Pink

3.Phenolphtalein Colorless Colorless Pink 8.3- 10.0

4.Bromothymol Yellow Blue Blue 6.5 — 7.8

3.2.2 Choice of Indicator in Acid — base Titrations

The choice of an indicator in an acid — base titration depends on the strengths (strong or weak) of the acid and base. The classification of the strengths of common acids and bases (alkali) is shown in the table below:

Strength of Acid Strength of Base

STRONG

H2SO4 .HCI;

HNO3

CH3COOH;H2C2 04;H2S0 3

NaOH;

KOH

Na2CO3;

aqNH3 Types of Acid — Base titrations and the choice of indicator.

Acid —Base Titration

Example pH of solution end - point

Suitable Indicator Strong acid

vs strong base

HCI vs NaOH 7 Any indicator:

Strong acid vs

weak base

H2SO4 vs aqNH3

5 - 6 Methylorange Weak vs

strong base

COON vs NaOH

8 — 9 Phenolphthalein Weak acid vs

weak base

CH3COOH vs aq NH 3

variable No suitable Indicator 3.2.3 Wrong use of indicator

The accuracy of a titration depends on the use of the correct indicator.

Wrong choice of indicator will lead to wrong result. For instance, in the titration of a solution of a strong acid (HCI) with that of a weak base (Na 2 CO3 ), methyl orange should be used. If Phenolphthalein in used

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instead, the end point will appear when only half of the weak base, Na2CO3 has been used up, as shown in the following reactions.

HC1 + Na2CO3 NaHCO3 + NaCI

H2SO4 Na2CO3 NaHCO3 NaHSO4

The reason is that Phenolphthalein is sensitive to weak acid such as NaHCO3. The table can easily be explained. For instance if you add say methyl orange indicator the colour you will notice is yellow. As you run in the acid from the burette, the neutralisation reaction occurs.

The colour in the conical flask will still be orange until complete neutralisation when there is stoichiometric equivalence of the acid and base.

The next drop of acid into the conical flask will not have a base to react with. Therefore there will be an extra drop of acid. At the point, the colour in the flask will now have to change because the indicator now finds itself in a new environment i.e. acidic environment. In the particular case of methyl orange, the color will now change to pink thus marking the end point.

SAQ1

1. Can you think of which indicator you would use when a strong acid is titrated against a weak base?

2. State the colour of the indicator before the end point and after the end point.