Ph definition:The definition of pH is the negative logarithm of

the hydrogen ion (H+) activity in a given solution.

• The range of the ph scale is from 0 to 14.

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Acidity or alkalinity of a solution

• So why are the hydrogen (H+) ions of interest?

The hydrogen ions (H+) are what determines if a solution is acidic, alkaline or neutral. If we can measure the hydrogen ion content of a solution, we can determine the acidity or alkalinity of that solution.

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• Water molecules are in continuous motion. When two water molecules collide, a hydrogen and hydroxide ion are created. This process in which water itself dissociates (splits) into hydrogen and hydroxide ions is called auto dissociation or self ionization.

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pH , pOH and pKwpH : Negative logarithm of

hydrogen ions

concentration

pH= -log[Hᶧ]

pOH : Negative logarithm of hydroxyl ions concentration

pOH= -log[pOH ]ˉ

pH INDICATORpH indicators are substances, usually a weak acid or base, that changes color depending on the pH of the solution it is mixed with.

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Universal Indicator

A universal indicator interprets how acidic or basic a solution is. A universal indicator can be in paper form or present in a form

of a solution

Example of common indicators of pHLITMUS

Color in Acid: Red Color in Base: Blue

A commonly used acid-base indicator is litmus. Litmus is a natural indicator obtained

from certain lichens. It is useful for detecting

acids and bases because it changes color around pH 7.

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• Phenolphthalein

Color in Acid: Colorless Color in Base: Pinkish violet color Approximate pH range for color change: 8.3 -10.0

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• Methyl Orange:

Color in Acid: RedColor in Base: Yellow Approximate pH range for color change: 3.1-4.4

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