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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 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
![pH = -log \left[H_\mathrm{3}O^+\right] = log \frac{1}{\left[H_\mathrm{3}O^+\right]}](/images/math/9/8/4/9845d6bd2885e54e566aebf869e8f0c6.png)
Because of concentration of hydronium ions times the concentration of hydroxide ions is constant, namely
![\left[H_\mathrm{3}O^+\right] \left[OH^-\right] = 1.0 x 10^{-14}](/images/math/0/3/3/033f402bcf5880cba660072f114b421e.png)
where pOH is defined in a manner similar to pH, as shown below.
![pOH = -log \left[OH^-\right] = log \frac{1}{\left[OH^-\right]}](/images/math/4/5/f/45f85f8389d08a20fe7b17cc83ef9c52.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.
