1. pH meter

2. A pH Meter is a scientific instrument that measures the hydrogen-ion concentration (or pH) in a solution, indicating its acidity or alkalinity.
 The pH meter measures the difference in electrical potential between a pH electrode and a reference electrode. It usually has a glass electrode plus a calomel reference electrode, or a combination electrode.

3. Types of pH meters
A simple pH meter
pH meters range from simple and inexpensive pen-like devices to complex and expensive laboratory instruments with computer interfaces and several inputs for indicator and temperature measurements to be entered to adjust for the variation in pH caused by temperature.
Specialty meters and probes are available for use in special applications, harsh environments, etc. There are also holographic pH sensors, which allow pH measurement colorimetrically.

4. Calibration and use
For very precise work the pH meter should be calibrated before each measurement.
For normal use calibration should be performed at the beginning of each day. The reason for this is that the glass electrode does not give a reproducible data over longer periods of time

5. Knowledge of pH to greater or lesser accuracy is useful or critical in a great many situations, including of course chemical laboratory work. pH meters of various types and quality can be used for soil measurements in agriculture; water quality for water supply systems, swimming pools, etc.; brewing, industrially or domestically; healthcare, to ensure that solutions are safe when applied to patients or lethal as sterilants and disinfectants; and many other applications.

6. The calibration should be performed with at least two standard buffer solution having the pH of which stretch is calculated. For general purpose buffer to pH 4.00 and pH 10 acceptable. PH meter has one control (calibration) to set the meter reading equal to the first standard buffer and a second control which is used to adjust the meter reading in the value of the second buffer.

7. The Standard Hydrogen Reference Electrode
an electrode must be easy to construct, reversible, and highly reproducible in its behavior. The standard hydrogen electrode (SHE) meets these specifications and has been used throughout the world for many years as a universal reference electrode. It is a typical gas electrode.
The half-reaction responsible for the potential that develops at this electrode is
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8. The hydrogen gas electrode.
By convention, the potential of the standard hydrogen electrode is assigned a value of V at all temperatures.
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