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12.8 Hydrogen ion concentration pH

Every aqueous solution whether acidic, alkaline or neutral contains both H+ and OH- ions. The product of their concentrations is always constant equal to 1 ´ 10 -14 at 298 K.

In 1909, Sorensen introduced a convenient scale to find out H+ ion concentration which he called the pH scale.

In the symbol pH, ’p’ stands for protenz (meaning strength) and ’H’ for hydrogen in concentration.

So pH is defined as the negative logarithm to the base 10 of molar concentration of hydrogen ions.

pH = - log10 [ H+ ] = log10

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For pure water and neutral solution [ H+ ] = 1 ´ 10- 7

\ pH = log10 [H+ ] = - log (1 ´ 10- 7) = 7

pH Scale

In most practical cases the concentrations of H+ ions varies from100 to 10 -14 mol dm3 i.e. from

10-14 pH which is the basis of the pH scale. The mid point of the scale i.e. pH = 7 represents a neutral solution. If the pH value is less than 7 the solution is acidic and if it is more than 7, then the solution is alkaline.

Index

12.1 - Lowry and Bronsted Concept
12.2 - Conjugate Acid Base Pairs
12.3 - Amphoteric Substance
12.4 - Lewis Acids and Bases
12.5 - Strong and Weak Acids and Bases
12.6 - Dissociation
12.7 - Ostwald's Dilution Law
12.8 - Hydrogen Ion Concentration : pH
12.9 - Polyprotic Acids
12.10 - Salts
12.11 - Methods of Preparation of Salts
12.12 - Properties of Salts

Chapter 13