In acid-base titrations, conductance measurements are utilized to detect the endpoint. This method is grounded on the fact that electrical conductance relies on the number and mobility of ions. For instance, consider titrating strong acid HCl with a strong NaOH base. Initially, the HCl in the conductivity vessel conducts electricity due to the presence of hydrogen ions and chloride ions. As NaOH is gradually added from the burette, the fast-moving hydrogen ions are replaced by slower-moving sodium ions, causing the conductance to decrease. This reduction in conductance continues until all the acid is neutralized. Notably, the concentration of chloride ions remains unchanged throughout the neutralization process. Any further addition of NaOH introduces fast-moving hydroxyl ions, which increase the conductance after it reaches a specific minimum value. When the conductance is plotted against the volume of added NaOH, the plot shows two straight lines. The point where these lines intersect indicates the volume of NaOH required to neutralize the acid.
Similarly, in the conductometric titration of the mixture of HCl and weak acid CH3COOH against NaOH, HCl is neutralized first due to its stronger acidity. The titration process for CH3COOH will commence only once HCl has been fully neutralized. On plotting the conductance against the volume of base added, the plot shows two endpoints corresponding to the neutralization volumes of HCl and CH3COOH.
Furthermore, conductometric titrations come with a plethora of benefits. Solutions that are colored, which traditional volumetric methods using indicators fail to titrate, can effectively be titrated using the conductometric method. This technique also applies to extremely diluted solutions and weak acids or bases. Added to this, there is no need for extra caution near the endpoint as it is graphically determined, simplifying the process.
Conductometric titrations rely on the principle that a solution's conductance depends on the number and mobility of its ions.
For instance, the titration of the strong acid HCl with the strong base NaOH involves the gradual replacement of fast-moving H3O+ ions by slower Na+ ions, decreasing conductance until the acid is neutralized completely.
Post-neutralization, adding NaOH introduces fast-moving OH− ions, increasing conductance. However, the chloride ion concentration remains the same.
A plot of conductance against the volume of base reveals the volume needed for neutralizing acid at the intersection of two straight lines.
In the titration of a mixture of HCl and a weak acid like acetic acid with NaOH, HCl, being a stronger acid, neutralizes first.
As titration continues, acetic acid forms highly ionized sodium acetate, increasing conductance. After complete neutralization, the excess base adds OH⁻ ions, further raising conductance.
The plot shows three linear segments, with points B and C representing HCl and acetic acid neutralization, respectively.
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