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Equivalence Point Titration

April 29, 2013

Introduction to Equivalence point titration:

Titration is a volumetric analysis technique.

In this technique, two samples are used.The one with the known concentration is called titrant.The another one whose concentration is unknown is called as analyte.

The titrant is added through calibrated burette, which is marked for volume measurement.

The analyte is kept in a container like a flask or beaker under the burette.

The equivalence point is that point at which the moles of the titrant and analyte are equal.

equivalence point of an acid-base neutralization reaction

Let us consider the titration of 0.1M HCl with unknown concentration of NaOH.

The chemical reaction would be

HCl + NaOH ————–> NaCl  + H2O

The volume of HCl required to neutralize 10ml of 0.5 M NaOH

So the equivalence point would be calculated as under.

No. of moles of acid = number of moles of base

M1V1= M2V2

where 1 stands for acid and 2 for base.

0.1 x  V1 = 0.5 x 10

V1 = 50 ml

So the equivalence point would be when 50 ml of HCl is added.

Since it is a measure of moles, it is also called stoichiometric point.

There could be multiple equivalence points for acids containing more than one hydrogen.

Methods to calculate the equivalence point of titration

Various methods are used to determine the equivalence point, e.g. pH indicator, Potentiometer, pH meter, Conductance, Color change, Precipitation etc.

The most common method used is the pH indicators. The theory behind it is that the indicators change the color when complete neutralization takes place. The pH of the completely neutralized solution is 7, as it is ideally neutral solution. The common indicators are Phenolphthalein, Litmus etc.

Phenolphthalein is colorless when in a acidic solution. However it would acquire pink color in the basic solution.This transition decides the determination of  equivalence point.

Similarly litmus is red when in a acidic solution. However it would aquire blue color in the basic solution.

Titration curve of equivalence point

titration curve of a diprotic acid,

A curve in the plane whose x-coordinate is the volume of titrant , and whose y-coordinate is the concentration of the analyte is called titration curve. In the above mentioned acid-base titration, the y-coordinate is usually the pH of the solution.

The two equivalence points for diprotic acid are seen at pH 5 and 12.

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