Define Procedure for Measurement of pH?
To ensure reliability of the pH readings, you should first calibrate the pH meter. You should adjust the pH meter so that the readings agree with a known standard buffer. The pH value of a solution changes with temperature, hence the instrument must be set to the temperature of the solution. For accurate measurement of the pH, a buffer of appropriate pH must be used for calibration. Now for measurement of pH of a solution, follow the procedure given below in a stepwise manner. But, if the pH meter, which you have in your laboratory, is of a different type, go through its instruction manual and then use it accordingly.
1. Set the selector switch to 'zero' position and adjust the zero position by a screw driver if the pointer does not indicate zero.
2. Mount the electrode assembly - a glass electrode and a saturated calomel electrode or a combination electrode in the clip on the stand. Wash the electrodes well with distilled water.
3. Connect the power cable to a 220 V AC supply. Switch on the instrument and wait for a few minutes till the instrument warms up.
4. Take about 20 ml of the standard buffer solution of pH 7.0 in a beaker and lower the electrode assembly into it. You should make sure that the glass electrode membrane is completely immersed in the solution. The electrodes should not touch each other or the sides or the bottom of the beaker. Swirl solution in the region of the glass electrode surface gently so as to bring it into pH equilibrium.
5. Measure the temperature of the solution and adjust the 'temperature compensation knob' to this temperature.
6. Put the selector switch to a suitable pH range (0-7 for acidic or 7-14 for basic solutions) and adjust 'set buffer knob' in a manner that the pointer reads the pH of the standard buffer solution placed in the beaker, i.e., pH 7.0.
7. Put the selector switch back to the 'zero' position. Remove the electrodes from the buffer solution. Wash the electrodes with distilled water and wipe them dry with a tissue paper. Transfer the standard buffer to the storage bottle.
8. You will need to make adjustments when precise values are required in the acid or basic range. To do this, select the pH 4.0 buffer for an acid reading or the pH 9.18 buffer for a basic reading.
9. Take about 20 ml of second buffer solution (of pH 4.0 or 9.18) in another beaker and place the electrode assembly into it. Set the selector switch in the suitable pH range position (0-7 or 7-14) and read the pH of the solution on the scale. If the meter reading does not agree exactly with the known pH of the buffer, set the 'slope' control until the required reading is obtained. The instrument is now ready for measuring the pH of the unknown solution.
10. Put the selector switch back to zero position and remove the electrodes from the buffer solution. Wash the electrodes with distilled water and wipe them gently with tissue paper. Transfer the buffer solution to the storage bottle.
11. Take about 20 ml of unknown solution in another beaker. Introduce the electrodes into the solution and swirl the solution gently. Put the selector switch back in the suitable range position and read the pH of the solution on the scale.
12. Put the selector switch back to zero position. Remove the electrodes from the solution and keep the electrodes in distilled water when not in use.