*pH is a quantitative unit of measure which describes the degree of acidity or alkalinity of a solution. It is most commonly measured on a scale of 0 to 14. The term pH is derived from “p” the mathematical symbol of the negative logarithm, and “H” the chemical symbol of Hydrogen.
*pH is a potentiometric measurement where the electrode used produces an electrical signal which is converted to a pH reading The signal is produced and measured as a voltage and since voltage is a potential difference, two potentials are needed to make the measurement.
*Prior to measuring pH, it is recommended that you calibrate (standardize) the pH electrode. It is most common to use pH buffers of 4.01, 7.00, 10.00pH in this procedure.
*Please note that high pH buffers are less stable, as they tend to absorb atmospheric CO₂ which lowers their pH. During  the calibration procedure you should open the bottle only to pour the buffer to the beaker. Never left the bottle open.
*On the pH scale pure water has pH 7, although you will rarely see water pure enough for such pH. Air always contains small amounts of CO2 which dissolves in water making it slightly acidic - with pH of about 5.7. All values on the pH scale lower than 7 denote solutions that are acidic - the lower the pH, the more acidic solution. Conversely, solutions with pH above 7 are basic - the higher the pH the more basic solution becomes.