pH Meter principle of Measurement and Source of Uncertainty.

pH 

pH value is the logarithm of reciprocal of hydrogen iron activity in moles per liter. In water solution, variation in pH value from 7 are mainly due to hydrolysis of salts of strong bases and weak acids or vice ver se. Dissolved gases such as carbon dioxide, hydrogen sulphide and ammonia also affect pH value of water. The overall pH value range of natural water is generally between 6 and 8.In case of alkaline thermal spring water pH value may be more than 9 while for acidic thermal spring waters the pH may be 4 or less than 4.Intrustrial wastes may  be strongly acidic or basic and their effect on pH value of receiving water depends on the buffering capacity of water.The pH value obtained in the laboratory may not be the same as that  of water at the time of collection of samples due to loss or absorption of gases, reaction with sediments, hydrolysis and oxidation or reduction taking place within the sample.pH value should preferably be determined at the time of collection of sample.

pH is calculated by two methods
1.Electrometrically
2.Calorimetrically

Electrometric is more accurate than the calorimetrical method.

Principle of measuring pH by the Electrometric method

The pH value is determined by measurement of the electromotive force of a cell consisting of an indicator electrode ( an electrode responsive to hydrogen ions such as a glass electrode) immersed in the test solution and a reference electrode (usually mercury/calome electrode). Contact between the test solution and the reference electrode is usually achieved by means of a liquid junction, which forms part of the reference electrode. The electromotive force is measured with a pH meter

 Sources of Uncertainty: 

A. Repeatability (Type A) 

B. Uncertainty of Buffer solution. (Type B) 

C. Resolution of pH meter (Type B) 

D. Linearity (Type B) 

E. Temperature (Type B)