pH

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Revision as of 14:49, 2 November 2008 by imported>Milton Beychok (→‎Formal definition: Some copy editing of the equations.)
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Potential of hydrogen (pH) is a scale that measures the acidity or alkalinity of a solution. Values for pH range from 0 (strongly acidic) to 14 (strongly alkaline or basic). The pH of a neutral solution, such as pure water, is 7, whereas the pH of an acidic solution is less than 7 and the pH of a basic solution is greater than 7. The pH scale is logarithmic, meaning that each value is 10 fold the next (for example, a pH of 12 is 10 times more basic than a pH value of 11).


Formal definition

pH is defined by

Because of concentration of hydronium ions times the concentration of hydroxide ions is constant, namely



where pOH is defined in a manner similar to pH, as shown below.

pH of common items

SubstancespH range
Human gastric juice1 - 3
Limes1.8 - 2.0
Soft drinks2.0 - 4.0
Lemons2.2 - 2.4
Vinegar2.4 - 3.4
Apples2.9 - 3.3
Tomatoes4.0 - 4.4
Beer4.0 - 5.0
Bananas4.5 - 4.7
Human urine4.8 - 8.4
Cow's milk6.3 - 6.6
Human saliva6.5 - 7.5
Human blood plasma7.3 - 7.5
Egg white7.6 - 8.0
Milk of magnesia10.5
Household ammonia11 - 12

References

  • "General Chemistry, 2nd Ed.", pp 103-117, D. D. Ebbing & M. S. Wrighton, Houghton Mifflin, Boston, 1987.
  • "General Chemistry with Qualitative Analysis, 2nd Ed.", pp. 263-278, Saunders College Publishing, Philadelphia, 1984.