CONDUCTOMETRIC TITRATIONS

Conductometric titration of strong acid with strong base

Consider the conductometric titration of a strong acid, HCl with a strong base, NaOH. Since both the acid HCl and the base NaOH are strong, so complete dissociation occurred. Here neutralization mean replacement of H⁺ ions by an equal number of Na⁺ ions.

H⁺ + Cl^- + [Na⁺ + OH^-] -----> Na⁺ + Cl^- + H₂O

H⁺ ion has much higher ionic conductance than that of Na⁺ ion. So, the conductance of the solution will decrease as more and more NaOH is added. When all the H⁺ ions replaced by the Na⁺ ions, conductance will reach the lowest limit. Any subsequent addition of NaOH means increase in the numbers of both Na⁺ ions and fast moving OH^- ions. These are no longer removed, the conductance therefore increases linearly with addition of NaOH. On plotting the conductance against the volume of alkali added, the point of intersection of two lines, that is the lowest point, corresponds to the minimum conductance, which is the exact neutralization point of this conductometric titration between HCl and NaOH.

Strong acid with strong base

Conductometric titration of weak acid with strong base

Consider the conductometric titration of a weak acid, CH₃COOH with a strong base NaOH. Since the acid is weak, the conductance of the acid will be low on account of its poor dissociation. As NaOH is added the OH^- ion removed the H⁺ ion by producing H₂O. Undissociated CH₃COOH molecules produce fresh H⁺ ions. The conductivity of the solution rises due to presence of some Na⁺ ions. At the starting stage, due to common ion effect, there may be a little fall in conductivity, but later on this fall will be insignificant compared to rise in the conductivity of the solution due to the added new Na⁺ ions.

CH₃COOH+[Na⁺ + OH^-] ----> CH₃COO^- + Na⁺ + H₂O

The total volume of CH₃COOH dissociates slowly in this way and is neutralized. When the acid is completely neutralized, further addition of NaOH will increase the conductivity of the solution, because OH^- ions have large ion-conductance. On plotting the conductance against the volume of alkali added, the point of intersection is obtained by extending the lines, which gives the end point.

Weak acid with strong base

Conductometric titration of mixture of strong and weak acids with strong base

Suppose of a mixture of strong acid HCl and weak acid CH₃COOH, is to be titrated with a strong base NaOH. Since HCl is much stronger acid than CH₃COOH, HCl will get titrated first. When HCl is completely neutralized by NaOH, then titration of CH₃COOH will start. On plotting the conductance against the volume of alkali added, the following type of curve is obtained.

Mixture of strong and weak acids with strong base

Conductometric titration of strong acid with weak base

Consider the conductometric titration of a strong acid, HCl by a weak base, NH₄OH. The conductance of the solution will decrease due to the replacement of fast-moving H⁺ ions by slow moving NH₄⁺ ions. When all the H⁺ ions replaced by the NH₄⁺ ions, conductance will reach the lowest limit and neutralization point comes.

H⁺ + Cl^- + NH₄OH -----> NH₄⁺ + Cl^- + H₂O

Due to weakly ionized nature of NH₄OH, further addition of NH₄OH will not cause any appreciable change in the conductance. On plotting the conductance against the volume of alkali added, the following type of curve is obtained.

Strong acid with weak base

Conductometric titration of weak acid with weak base

Consider the conductometric titration of a weak acid, CH₃COOH by a weak base NH₄OH. Since the acid is weak, the conductance of the acid will be low on account of its poor dissociation. As NH₄OH is added, the OH^- ions removed the H⁺ ions. Undissociated CH₃COOH molecules produced fresh H⁺ ions. Besides the solution has some NH₄⁺ ions.  At the starting stage, due to common ion effect there may be little fall in conductivity. But later on, the conductivity of the solution increases slowly due to the presence of slow moving NH₄⁺.

CH₃COOH + NH₄OH ----> CH₃COO^- + NH₄⁺ + H₂O

The total volume of CH₃COOH dissociates slowly in this way and neutralized. Due to weakly ionized nature of NH₄OH, further addition of NH₄OH will not cause any appreciable change in the conductance. On plotting the conductance against the volume of alkali added, the following type of curve is obtained.

Weak acid with weak base