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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pH , pOHpH : Negative logarithm of

hydrogen ions

concentration

pH= -log[Hᶧ]

pOH : Negative logarithm of hydroxyl ions concentration

pOH= -log[pOH ]ˉ

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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 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

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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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Ph Scale

• PhenolphthaleinPhenolphthalein, an acid-base indicator used to test the pH of

a solution, turns pink due to the presence of a weak base. The indicator itself is a weak, colorless acid that forms pink anions when it dissolves in water.

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

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Colorless in acids pink in base

• Methyl Orange: Methyl orange is an intensely colored compound used in dyeing and printing textiles. Color in Acid: RedColor in Base: Yellow Approximate pH range for color change: 3.1(red)-4.4The yellow form is basic, the red form is acidic

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