pH
From Citizendium, the Citizens' Compendium
Potential of hydrogen (pH) is a scale that measures the acidity or alkalinity of a solution. Values for pH range usually from 0 (strongly acidic) to 14 (strongly alkaline or basic). The pH of a neutral solution, 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, 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). The traditional way to determine the pH value of a liquid is by the change of the colour of litmus paper.
Formal definition
pH is defined by
![\mathop{\rm pH} = -\log_{10} \left[\rm H_3O^+ \right] = \log_{10} \frac{1}{\left[\rm H_3 O^+ \right]}](/images/math/b/f/2/bf207b82e879ca80c5e8570858b9761f.png)
Because of concentration of hydronium ions times the concentration of hydroxide ions is constant, namely
![\left[\rm H_3 O^+ \right] \left[\rm OH^- \right] = 1.0 \cdot 10^{-14}](/images/math/8/7/9/879fe9eec288f4f052c321592011b3c5.png)
where pOH is defined in a manner similar to pH, as shown below.
![\mathop{\rm pOH} = -\log_{10} \left[\rm OH^- \right] = \log_{10} \frac{1}{\left[\rm OH^- \right]}](/images/math/3/5/0/350fe563e843c54b1a8d9aceec83f4c0.png)
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.
