Question

Which indicator would be the best to use for a titration between 0.30 M NH4Cl with 0.30 M KOH?

1. methyl red, color change at pH 2-4
2. phenol red, color change at pH 6-8
3. thymol blue, color change at pH 8-10
4. alizarin yellow R, color change at pH 10-12
5. bromothymol blue, color change at pH 6-8

Answer 1

Concepts and reason

Titration is a process where a known concentration of solution is used for determining the concentration of an unknown solution. Solution of a known concentration is known as titrant while solution of an unknown concentrated is known as an analyte.

pH is a measure of hydrogen ion present in a solution. It indicates the acidic strength or basic strength of a solution.

of the solution is the negative logarithm of the molar concentration of hydroxide ions present in the solution.

Acid dissociation constant:

Acid dissociation constant of an acid indicates the extent to which the acid dissociates into ion and a conjugate base.

Base dissociation constant:

Base dissociation constant of a base indicates the extent to which the base is protonated to give the corresponding conjugate acid.

Equilibrium constant of water:

Indicator: It is the substance that indicates the end point of a titration or the completion of a titration.

Acid-base indicators are the substances that are used detect the end point in the acid-base titrations. The end point can be noticed by the color change while pH changes. The indicators are weak acids. Acid-base indicators have a specific characteristic features that can detect the endpoint in a particular titration.

Fundamentals

The value is negative logarithm (base) of the Hydrogen ion concentration.

of a basic solution can be calculated by the following equation:

The relationship between can be written as,

Then, the pH of the same solution can be found out by the following expression:

The relationship between and is given by,

Form some base ,

ICE table:

I denotes the initial concentration of each species in the equilibrium reaction.

C denotes the change in the concentration of each species that move towards the equilibrium.

E denotes the concentration of each species at the equilibrium.

Acid-base indicators are the substances whose color changes (change of ion concentration) in specific pH range.

The color change of the indicators at the end point can be explained by considering the indicator to be a weak acid.

When the solution is acidic, the concentration of ions increases and the equilibrium shifts towards the backward direction and the color A appears. When the solution is alkaline, the concentration of ions decreases and the equilibrium shifts towards the forward direction and the color B appears.

The chemical equation for the reaction between and is given by,

On constructing the ICE table for the reaction,

The value of is .

Thus,

Thus the equilibrium constant,

On calculating, the value of is found to be,

Thus,

Thus, at the end point the solution, the product formed will be ammonia. Thus the at the end point will be .

The wrong choices for the indicators are given below:

Methyl red, color change at

Phenol red, color change at

Thymol blue, color change at

Bromothymol blue, color change at

The correct choice of indicator is given below:

Alizarin yellow R, color change at

Ans:

The indicator used for the titration between is given below:

1) Methyl red, color change at

2) Phenol red, color change at

4) Alizarin yellow R, color change at

5) Bromothymol blue, color change at