PH: Difference between revisions
imported>Milton Beychok m (Added a bit about the origin of the pH notation.) |
imported>Milton Beychok m (Added a few references) |
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The traditional way to determine whether a solution is acidic or basic is by wetting [[litmus paper]] with the solution. If the wet litmus paper turns red, the solution has a pH less than 7 and is acidic. If it turns blue, the solution has a pH greater than 7 and is acidic. Measuring the actual pH value of a solution is done with a [[pH meter]]. | The traditional way to determine whether a solution is acidic or basic is by wetting [[litmus paper]] with the solution. If the wet litmus paper turns red, the solution has a pH less than 7 and is acidic. If it turns blue, the solution has a pH greater than 7 and is acidic. Measuring the actual pH value of a solution is done with a [[pH meter]]. | ||
The pH scale was originally defined by [[Denmark|Danish]] biochemist [[Søren Peter Lauritz Sørensen]] in 1909, who wrote it as P<sub>H</sub>. It was subsequently changed to the modern notation of pH in 1920 by [[William Mansfield Clark]], an [[United States|American]] biochemist, for typographical convenience in his book ''The Determination of Hydrogen Ions''.<ref>[http://science.jrank.org/pages/49372/pH.html pH: Potenz, The Determination of Hydrogen Ions, History of Analytical Chemistry, Electrochemistry, Past and Present] From the JRank Science & Philosophy website</ref> Some sources refer to the pH as the '''''power of hydrogen''''' or the '''''potential of hydrogen'''''. | The pH scale was originally defined by [[Denmark|Danish]] biochemist [[Søren Peter Lauritz Sørensen]] in 1909, who wrote it as P<sub>H</sub>. It was subsequently changed to the modern notation of pH in 1920 by [[William Mansfield Clark]], an [[United States|American]] biochemist, for typographical convenience in his book ''The Determination of Hydrogen Ions''.<ref>{{cite book|author=Willam Mansfield Clark|title=The Determination of Hydrogen Ions|edition=1st Edition|publisher=William & Wilkins Company|year=1920|pages=page 35|id=}} Available in Google Books [http://books.google.com/books?id=spsLAQAAIAAJ&printsec=frontcover#v=onepage&q=&f=false here]</ref><ref>[http://science.jrank.org/pages/49372/pH.html pH: Potenz, The Determination of Hydrogen Ions, History of Analytical Chemistry, Electrochemistry, Past and Present] From the JRank Science & Philosophy website</ref> Some sources refer to the pH as the '''''power of hydrogen''''' or the '''''potential of hydrogen'''''. | ||
==Formal definition== | ==Formal definition== | ||
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==References== | ==References== | ||
{{reflist}} | |||
*"General Chemistry, 2nd Ed.", pp 103-117, D. D. Ebbing & M. S. Wrighton, Houghton Mifflin, Boston, 1987. | *"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. | *"General Chemistry with Qualitative Analysis, 2nd Ed.", pp. 263-278, Saunders College Publishing, Philadelphia, 1984. | ||
Revision as of 02:45, 10 February 2010
The pH scale 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 (neither acid or basic), such as pure water at room temperature and atmospheric pressure 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 which means that a difference of one pH unit is equivalent to a ten-fold difference in hydrogen ion concentration.
The traditional way to determine whether a solution is acidic or basic is by wetting litmus paper with the solution. If the wet litmus paper turns red, the solution has a pH less than 7 and is acidic. If it turns blue, the solution has a pH greater than 7 and is acidic. Measuring the actual pH value of a solution is done with a pH meter.
The pH scale was originally defined by Danish biochemist Søren Peter Lauritz Sørensen in 1909, who wrote it as PH. It was subsequently changed to the modern notation of pH in 1920 by William Mansfield Clark, an American biochemist, for typographical convenience in his book The Determination of Hydrogen Ions.[1][2] Some sources refer to the pH as the power of hydrogen or the potential of hydrogen.
Formal definition
Hydrogen ions in solution can be written simply as [H+] or as hydronium [H3O+] and both describe the same entity. The mathematical definition of pH is:
![{\displaystyle \mathop {\rm {pH}} =-\log _{10}\left[{\rm {H_{3}O^{+}}}\right]=\log _{10}{\frac {1}{\left[{\rm {H_{3}O^{+}}}\right]}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/fa5bac61469877eb81acca1a88b271a5458614ab)
Because of concentration of hydronium ions times the concentration of hydroxide ions is constant, namely
![{\displaystyle \left[{\rm {H_{3}O^{+}}}\right]\left[{\rm {OH^{-}}}\right]=1.0\cdot 10^{-14}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/26737c368f1e581d3af342fc86472bc21535b63e)
where pOH is defined in a manner similar to pH, as shown below.
![{\displaystyle \mathop {\rm {pOH}} =-\log _{10}\left[{\rm {OH^{-}}}\right]=\log _{10}{\frac {1}{\left[{\rm {OH^{-}}}\right]}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/8a32f6bd6cbbbafe18f7aeeca7a44743236c46d0)
pH of common items
| Substances | pH range |
|---|---|
| Human gastric juice | 1 - 3 |
| Limes | 1.8 - 2.0 |
| Soft drinks | 2.0 - 4.0 |
| Lemons | 2.2 - 2.4 |
| Vinegar | 2.4 - 3.4 |
| Apples | 2.9 - 3.3 |
| Tomatoes | 4.0 - 4.4 |
| Beer | 4.0 - 5.0 |
| Bananas | 4.5 - 4.7 |
| Human urine | 4.8 - 8.4 |
| Cow's milk | 6.3 - 6.6 |
| Human saliva | 6.5 - 7.5 |
| Human blood plasma | 7.3 - 7.5 |
| Egg white | 7.6 - 8.0 |
| Milk of magnesia | 10.5 |
| Household ammonia | 11 - 12 |
References
- ↑ Willam Mansfield Clark (1920). The Determination of Hydrogen Ions, 1st Edition. William & Wilkins Company, page 35. Available in Google Books here
- ↑ pH: Potenz, The Determination of Hydrogen Ions, History of Analytical Chemistry, Electrochemistry, Past and Present From the JRank Science & Philosophy website
- "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.