Selection of Indicators in Acid-base Titration

Selection of Indicators in Acid-base Titration shows the different levels of combinations of a strong base, strong acid, weak acid, and weak base.

All the indicators may not change their color in the PH range. Phenolpathlin changes its color at the PH range from 8 to 9.5 while the methyl orange changes its color at the PH range from 3 to 4.5. The PH at the equivalence point depends upon the nature of the reactions and the indicator should be selected properly areas:-

1. Strong acid- strong base titration
2. Strong acid- weak base titration
3. Weak acid- strong base titration
4. Weak acid- weak base titration

Explanations:-

1. Strong acid vs strong base titration

During such type of titration PH value changes before and after the equivalence point. PH value changes very slowly to begin, but at the natural point. PH value changes sharply (fastly) on the addition of a small amount of base. Therefore any indicator such as either methyl orange or phemolpathaline can be used.

HCL +NaOH → NaCl + H2O

1. Strong acid VS weak base titration

When strong acid such as HCL is titrated against a weak base such as Na2CO3, NH­4OH, etc, then salt produced at the equivalent point becomes slightly acidic due to hydrolysis.

HCL +Na2CO3 →NaCl + H2O+ CO2

HCL +NH4OH → NH4Cl + H2O

For such reactions, the PH  at the equivalence point becomes less than ‘7’ and for this indicator used should give the color change in the acidic medium. Thus methyl orange is only used as an indicator for such type of titration.

1. Weak acid VS strong base titration

When the titration is carried out between weak acids such as CH3COOH and a strong base such as NaOH, then salt produced in equivalence point becomes slightly basic due to hydrolysis.

CH3COOH + NaOH = CH3COONa + H2O

Sodium acetale

CH3COONa + H2O → CH3COOH + NaOH

For such reactions, the PH at the equivalence point becomes more than ‘7’ and for this indicator used should give the color in basic medium. Thus only phenolphthalein can be used as an indicator for such titration.

1. Weak acid VS weak base titration

When the titration is carried out between weak acids such as CH3COOH and a weak base such as NH4OH, there is no change in PH value takes place i.e. PH value remains practically constant at the endpoint of the reaction due to the formation of buffer solution.

So, no one indicator is used for such type titration.

Buffer solution

The solution which resists the PH on the addition of either acid or base is called a buffer solution. It is of two types:-

1. Acidic buffer solution
2. Basic buffer solution

Acidic buffer solution- The buffer solution which is prepared by mixing weak acid and its salt formed by the strong base is called acidic buffer solution. for eg-  CH3COOH + CH3COONa

Basic buffer solution

The buffer solution which is prepared by mixing a weak base and its salt formed by strong acid is called a Basic buffer solution. For eg- NH4OH + NH4CL

Redox titration:-

The titration involving both the oxidizing agent and reducing agent which causes both oxidation and reduction is called redox titration.

KMnO4 + H2C2O4 + H2SO4 →K2SO4 + MnSO4 + H2O + C2O (oxalic acid)

In this type of titration KMnO4 acts as an oxidizing agent and H2C2O4 oxalic acid acts as a reducing agent. This type of titration is an example of redox titration.

Acid-base titration:-

The titration involving acid and base is called acid-base titration. For eg:-

HCL + NaOH → NaCl + H2O

NH4OH + H2SO4 →  (NH4)2SO4 + H2O

CH3OOH + NaOH→ CH3COONa + H2O

Therefore, the selection of indicators in acid-base titration is explained above.