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Equivalence Point II

May 20, 2013

Equivalence point II
To define equivalence point, It is a point at which the concentration of added titrant is exactly equal to the number of moles in the analyte. In other words, It is a point of a chemical reaction in titration which is a volumetric analysis technique, at this point the fraction of stoichiometric amount of titrant added exactly equalise or neutralize the titrated substance or analyte substance in given sample. The known concentration of solution is called as titrant and the unknown concentration of solution or whose concentration to be found is called as analyte. Equivalent point is also called as stoichiometric point.

Equivalence-point stoichiometric amount of titrant / titrated substances = 1

Titration Curve Equivalence Point

There are many methods are used to determine the equivalence point. Titration curve is the one of the best way of figure out the equivalence-point and is the plot of the value of pH as a function of added titrant. The titration curve shows inflection point at equivalence-point. A striking fact about equivalence is that in a chemical reaction the equivalence of the reactants as well as products is conserved. The titration curves consist of the volume of titrant as the independent variable and the pH of solution as the dependent variable.

There are totally four possibilities of combination which is depends upon the different strength of acid and base.

1.Strong acid and strong base – for example the reaction between hydrochloric acid and sodium hydroxide

2.Strong acid and weak base – for example the reaction between hydrochloric acid and ammonia

3.Weak acid and strong base – for example the reaction between acetic acid and sodium hydroxide

4.Weak acid and weak base – for example the reaction between acetic acid and ammonia

In above each combination has two titration curves according to the titrant. There are many methods are used to calculate equivalence point. There are as follows,

1. pH indicator
2. Potentiometer
3. pH meter
4. Conductance
5. Colour change
6. Precipitation, etc.,

In above given methods, the pH indicator method is the most commonly used method for calculate the equivalence point. In this method has theory that the indicators change the colour when complete neutralization takes place. The complete neutralized solution has its pH value of about 7, as it is ideally neutral solution. The common indicator used in pH indicators are phenolphthalein, litmus, etc. The phenolphthalein is colourless in acidic solution, however it attains pink colour in the basic solution. This colour transition may decide the determination of equivalence-point.

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