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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