The  concentration of hydrogen ions (H^+,
   or protons) in aqueous solution is measured in pH units.
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   The  H^+  concentration at 25°C ranges from 1 Molar (10^0, or pH=0) to
   10^-14 M (pH=14).
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   These concentrations are plotted on a negative logarithmic scale: each
   unit change in pH is a 10-fold change in H^+ ion concentration.
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   An acid has a high concentration of H^+ ions (low pH).
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   A base has a low H^+ concentration (high pH).
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   Review:  <A ClAsS=ACT Href="http://media.pearsoncmg.com/bc/bc_campbell_biology_7/media/activities/load.html?3&amp;D" oNClick="popact('http://media.pearsoncmg.com/bc/bc_campbell_biology_7/media/activities/load.html?3&amp;D');txtcolor('pH1','gray');return false;" Id=pH1><iMg sRc="/images/tutorial.gif" aLt='' boRDer=0></A>