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Engineering Chemistry Lab Experiments

Estimation of HCL by Potentiometric Titrations


To determine the strength of the strong acid by titration with strong base Potentiometrically

Apparatus Required:

Burette, pipette, Conical flask, Beakers, Standard flask, Burette stand and funnel etc.,

Chemicals Required :

Sodium hydroxide, Hydrochloric acid and Quinhydrone powder etc.

Principle :

The cell will have certain emf depending upon pH value of the solution, on adding small portions of alkali to an acid. The acid potential changes slowly at first since the change in electrode depends on the fraction of hydrogen ions removed. After addition of certain amount of alkali the titration of hydrogen ions removed by alkali increases, correspondingly there is a rapid decrease in emf on addition of excess of alkali. The emf again shows a flow change. If a graph is plotted by taking volume of alkali added on X- axis and change in emf by point of intersection on Y-axis, a curve is obtained.

HCl + NaOH ————> NaCl + H2O

The cell can be represented as:
H2, Pt/Acid soln // KCl (aq) / Calomel electrode


Calibrate the instrument before starting the experiment. Approximately 0.1N HCl is prepared and standard decinormal solution of NaOH is prepared. Exactly 20 ml of the acid is pipette out into a clean 100ml of beaker and a pinch of Quinhydrone is added which acts as indicator. Platinum electrode and calomel electrodes are dipped in the solution.

The alkali (NaOH) against which the acid is being titrated is taken in burette. The solution is stirred well with a glass rod. The end reading is taken after adding definite amount of alkali. Finally after knowing the range in which the end point can be located, the whole experiment is repeatedly adding in steps of 1 ml in the end point.


Two graphs are plotted of which one is between volume of alkali and observed emf and other is between volume of alkali and ΔE/ ΔV Sigmoid curve.

Estimation of HCL by Potentiometric Titrations


S. No Volume of NaOH (ml) Observed EMF (mv)


N1V1 = N2V2


The normality of strong acid (HCl) determined by titrating against a strong base (NaOH) =________N

Viva Questions:

1. What are the electrodes used in the potentiometric titrations?

2. What is calomel electrode?

3. What is the advantage of potentiometric titrations?

4. What is reference electrode? Give examples?

5. What is the indicator electrode?