pH
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 (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.
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
- "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.