pH meter, Prinicple, Components of pH meter,Measurement of pH using pH meter

PH meter, Principle,Components, Measurement

PH meter

 

Principle of pH meter

Components of pH meter

Measurement of pH using pH meter

 

pH meter

pH meter is a very sensitive instrument used to measure the hydrogen ion concentration (pH) of solutions. Generally, pH meter is used in the preparation and quality control of reagents. The measurement of pH is also referred to as potentiometric analysis.

Principle The basis of pH analysis is the development of voltage difference across a thin glass membrane which separates the two solutions with different pH. This difference in voltage is detected by two reference electrodes of known contact voltage. The two reference electrodes are placed in the solution on either side of the glass membrane. The potential thus produced varies with the pH of the two solutions.

Components of pH meter 

There are three basic components of the pH meter. These are:

1. Indicator or glass electrode

2. Reference or calomel electrode

3. Voltmeter 

1. Indicator (glass) electrode The indicator electrode consists of a tip made of special glass sensitive to the conductance of hydrogen ions. The glass electrode is filled with a reference solution of known pH that is resistant to changes in pH. Suspended in this solution is the internal reference electrode, which is usually a Ag-AgCl half cell. Since the pH inside the electrode is constant, any change in voltage is caused by the pH of the exterior test solution (Fig. 4.1). 

2. Reference (calomel) electrode The reference electrode consists of an internal element or reference electrode, a filling solution (saturated potassium chloride), and a liquid junction through which the filling solution can slowly flow. 

This electrode has two basic functions-It supplies a constant reference voltage against which the indicator electrode voltage is measured, and, through the liquid junction the electrical circuit between the pH meter and the test solution is completed. 

The internal element is usually made of Hg-Hg,Cl, half-cell. This is also called calomel (mercurous chloride) electrode. The difference between the ionic strength of the saturated KCl and composition of the test solution has no effect on the potential of the reference electrode. 

3. Voltmeter The two electrodes are immersed in a test solution and connected to a voltmeter. A voltage difference, which is proportional to the pH of the test solution, develops at the surface of the glass membrane. This voltage is amplified and converted to a digital or meter display of pH (Fig. 4.1)

Combination electrodes Some pH meters combine the indicator and reference electrodes in a single glass probe. These are called combination electrodes. This system is compact and useful for measuring pH of small volumes. 

Measurement of pH using pH meter 

The pH meter should usually be left "switched on" and in the "stand by" position. The glass electrodes should be placed in buffer of pH 7.0 when not in use. The calomel electrode should be in saturated KCI. New electrodes should be soaked for a few hours before being put to use.

1. Switch on the instrument.

2. Adjust the meter using the buffer control knob to pH 7.0.

3. Rinse electrodes with distilled water and wipe to remove excess.

4. Place the electrodes in a standard buffer of known pH, e.g., pH 4.0; and adjust meter for correct reading.

5. Rinse the electrodes in distilled water as in step 3.

6. Place the electrodes in the solution to be measured and take the reading. Ensure that the electrode bulbs are immersed in the solution.

7. Rinse again in distilled water and place in buffer, pH 7.0. 

8. Switch instrument to stand by".