Determination of the neutralization point by conductivity

Determination of the Neutralization Point by Conductivity.—Measurements of the electrical conductance can also be employed in order to determine the point of neutralization of an acid by an alkali, or vice vers and the method is of especial importance when dealing with coloured or turbid solutions, in which the change of colour of an indicator would be more or less masked. 

When a strong acid is added to an alkali, the conductance of the solution will decrease owing to the disappearance of hydroxidion, and its replacement by the less mobile anion of the acid but when all the hydroxidion has been removed, by combination with hydrion from the acid added, then any further addition of acid will cause the conductance to increase. 

Experiment.—Determine the Strength of a Solution of Sodium Hydroxide. 

In order to obtain the strength of the alkali solution, the bridge readings are plotted as ordinates against the number of cubic centimetres of acid added, and curves are drawn through the points so obtained. The point of intersection of the two curves then gives the number of cubic centimetres of acid required to exactly neutralize the solution of alkali (Fig. 65, A). 

The result obtained electrically should be controlled by titration, using phenolphthalein as indicator. 

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