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TOPIC: ACID-BASE REACTIONS
CONTENT:
PERIOD 1: 1: COMMON ACID–BASE INDICATORS AND THEIR pH RANGE
Acid-base indicator is a weak organic acid or weak organic base (organic dyes) which shows colour change depending on the pH of the solution or organic dye which shows one colour in an acidic medium and another colour in basic medium. It can defined as an organic dye which as colour in an ionized state and another colour in unionized state. Examples of indicators are methyl orange, phenolphthalein, methyl red etc.
CHOICE OF INDICATOR FOR ACID-BASE TITRATION
The choice of an indicator for acid-base titration depends on the pH of the mixture at equivalent point and the pK of the indicator. When a strong acid is titrated against a strong base, at the equivalent point, the salt formed does not under hydrolysis and so any suitable indicator can be used. The salt formed when strong acid is titrated against a weak base undergoes hydrolysis to produce excess hydrogen ions. This makes the solution acidic and any indicator that shows colour change in acidic medium is used in this case. Methyl orange changes colour within a pH of 2.9—4.6 and so suitable for this titration. The salt formed when weak acid is titrated against strong base undergoes hydrolysis to produce excess hydroxide ions making the solution alkaline. Phenolphthalein changes colour in basic medium and so suitable to the kind of titration.
Indicators are used in acid-base titration to show the endpoint when the acid would have materialised the base. Colour changes of the common indicator are shown below
| Indication | pH range | Colour in acid solution | Colour in the alkaline solution | Neutral or end point | Types of acid base titration |
| Litmus | 50-8.0 | Red | Blue | purple | Strong acid and strong base |
| Methyl Orange | 2.9-4.6 | Pink | Yellow | orange | S.A and S.B or Na2Co3 in H2O |
| Phenolphthalein | 8.2 – 10 | Colourless | Pink | colourless | S.A and W.A |
| Methyl red | 4.4 – 6.3 | Red | Yellow | orange | S.A and S.B |
S.A = Strong acid, S.B = Strong base, W.A = weak base
This is volumetric analysis which aim at estimation of the qualities of certain substances in solution
PERIODS 2 AND 3: ACID-BASE TITRATION
This involves titrating a solution (usually the acid) from the burette into a fixed volume (20 or 25cm3) of base until the two solutions have completely reacted. The concentration of one of the solutions is normally known. An indicator is used to determine the point of complete reaction.
For titration, more dilute solutions containing 0.1moldm-3 or 0.05mold-3 are used.
STANDARD SOLUTION
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