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pH Scale

Describe the use of the pH scale in comparing acids and bases



  • Pure water contains very small amounts of hydrogen and hydroxide ions due to self-ionisation:
  • At 25°C, the concentration of hydrogen ions is 1.0 x 10-7 mol L-1.
  • When an acid is added to water, the above equilibrium shifts in the reverse direction, causing a decrease in the hydroxide concentration.
  • When a base is added to water, the above equilibrium shifts in the reverse direction, causing a decrease in hydrogen ion concentration.
  • Thus, acidic solutions have a higher concentration of hydrogen ions than 1.0 x 10-7 mol L-1, while basic solutions have a lower concentration of hydrogen ions than 1.0 x 10-7 mol L-1.
  • The pH scale is used to compare the concentration of hydrogen ions in solutions of acids and bases.
  • It is a logarithmic scale, as the concentration of hydrogen ions would require a very large linear scale.

The pH Scale
Image taken from http://www.isa.org/Images/InTech/2005/May/20050514sens.gif

Easychem - pH Scale


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