Most of the chemical and biochemical processes are profoundly affected by the acidity or alkalinity of the medium in which the reaction takes place. All acid dissociate in aqueous solution to yield H+ ions. Some acids like HCl, H2SO4, HNO3 etc. are completely ionized in aqueous medium whereas CH3COOH, HCOOH etc. ionize to a small extent only. The former is known as strong and the later as weak acid. pH of any solution is defined as (–log H+) and has values between 0–14. pH < 7 indicate acidic solution, pH > 7 indicate basic solution and pH = 7 means neutral solution.
The pH of a solution can be measured accurately with the help of a pH meter. Measurement of pH is employed to monitor the cause of acid-base titration. The pH values of the solution at different stage of acid–base neutralization are determined and plotted against the volume of alkali added on adding a base to an acid, the pH rises slowly in the initial stages as the concentration of H+ ion decreases gradually. But, at the equivalence point, it increases rapidly as at the equivalent point H+ ion concentration is very small. Then it flattens out after the end point. The end point of the titration can be detected where the pH value changes most rapidly. However, the shape of the curve depends upon the ionisability of the acid and the base used and also on the acidity of base and basicity of the acid.
Apparatus: pH meter, electrode, beaker, pipette, burette.
Hydrochloric acid (HCl), sodium hydroxide (NaOH), buffer of pH = 4 and 9.2.
- 5(N) NaOH solution is provided.
- HCl solution of unknown strength is provided.
- Switch on the instrument and wait for 10–15 minutes so that machine gets warmed up. Prepare the buffer solution by adding buffer tablets of pH = 4 and pH = 9.2 in 100 mL of water separately. Wash the electrode with distilled water. Then, dip the electrode in the buffer solution (pH = 4) taken in a beaker, so that the electrode immersed to the solution properly. Measure the temperature of the solution and set the temperature compensate control accordingly. Set the pointer to pH = 7 exactly means of set = 0 control. Put the selector switch to proper pH range 0–7 (as the buffer pH = 4). So the pointers to the known pH value of the buffer by burning the set buffer control. Put back the selector at zero position. Wash the electrode with distilled water and standardize the pH meter using basic buffer solution pH = 9.2. Same procedure to be followed except the selector switch is put to range of 7–14.
- pH-metric Titration: Clean the electrode with distilled water and wipe them with tissue paper or filter paper. Take 25 mL of HCl solution in a 100 mL beaker or conical and immerse the electrode in it. Set the burette with NaOH solution. Put the selector at the expected range (0–7). The reading shown on the scale of pH meter is pH value of the HCl solution. Add NaOH solution drop wise from the burette (maximum 10 drops at a time), shake the solution well and note the corresponding pH values. Near the end point, volume of NaOH added should be as small as possible because the acid is neutralized and there will a sharp increase in pH values. Further addition of NaOH(dropwise) , increase the pH value to about 9–10. Put back the selector to zero position after pH measurement, and always keep the selector at zero position when it is not in use.
|No. of drops of NaOH added||Total No. drops of NaOH added||pH|
Plot a graph between pH and volume of NaOH added and find out the volume of NaOH required (V2 mL) for complete neutralization of HCl from the graph. Then find out the strength of HCl (S1).
25 x S1 = V2 x S2
Strength of HCl (S1) = (V2 x S2/25) (N)
The strength of the unknown HCl is __ (N)
- Electrodes must be immersed in the solution properly and sufficient time to be allowed for the electrodes to obtain the temperature of the solution.
- pH meter should be calibrated before the experiment.
- Magnetic stirrer may be used or the solution be stirred mechanically from time to time during pH metric titration.
- Leave the selector in zero position where it is not in use.