13

pH METRIC TITRATION OF ACID VS BASE

AIM

To determine the concentration of a strong acid by a pH-metric titration against a strong base.

pH METRIC TITRATION

The determination of the concentration of acid or a base by titrating against a standard base or acid by monitoring the pH of the titrated solution is called a pH-metric titration. The solution from burette is added to the beaker, the concentration of the titrand in the cell varies linear to the volume of the titrant added. Since the cell potential E_cell, has a logarithmic relationship to the concentration of H⁺, the plot of E_cell versus volume of the titrant added is a sigmoid curve. As a pH in the cell is linear to E_cell gets a similar sigmoid curve for the plot of pH versus volume of the titrant added. The abscissa (volume point) corresponding to the intersection point on the sigmoid is the end point of the titration.

APPARATUS

pH meter with glass electrode and standard calomel electrode (SCE), pipette, micro burette, beaker.

CHEMICALS/SOLUTIONS

Standard sodium hydroxide (0.1 N), hydroxide acid (0.01 N), potassium hydrogen phthalate 0.10 M (pH 4.02), borax 20.00 M (pH 9.15)

PROCEDURE

Step A: Calibration of the pH Meter

Pour about 40 ml of acid buffer (potassium hydrogen phthalate) in a 100 ml beaker. Immerse the glass electrode and SCE in the buffer (a combined glass electrode is a combination of a glass electrode and an Ag/AgCl reference electrode). Gently stir the solution and read the pH of the solution by the pH meter, if it does not show the pH of potassium hydrogen phthalate (4.02), turn on the calibration knob till the display shows 4.02. Clean the beaker and pour about 40 ml the basic borax buffer. Gently stir the solution and read the pH of the solution by the pH meter. If the reading is 9.15 or very close to it, the pH meter is well calibrated.

Step B: pH Metric Titration (Pilot Titration)

Pipette out 40 ml of the acid in a 100 ml beaker. Fill the burette with the standard NaOH solution. Immerse the glass electrode and SCE or the combined electrode in the solution. Note the pH value in the Table against zero volume of the NaOH added. Run 1 ml from the burette into the cell, stir the contents and read the stable pH and continue to add 1 ml each time, read the corresponding pH up to observing the variation in pH. Stop further addition of the NaOH if you observe an abrupt change in pH. Note down the burette readings between which the pH has abruptly changed.

PRECAUTIONS

1. Do not disturb the calibration knob of the pH meter once it is calibrated.
2. Stir the contents in the beaker gently.
   

CALCULATIONS AND GRAPH

1. Plot pH on y-axis and volume of NaOH on x-axis. A Sigmoid graph as shown below is derived.
2. Read the end point from the graph and calculate the concentration of the acid by V₁ N₁ = V₂ N₂.