PH

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

$\mathop {\rm {pH}} =-\log _{10}\left[{\rm {H_{3}O^{+}}}\right]=\log _{10}{\frac {1}{\left[{\rm {H_{3}O^{+}}}\right]}}$

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

$\left[{\rm {H_{3}O^{+}}}\right]\left[{\rm {OH^{-}}}\right]=1.0\cdot 10^{-14}$

$\left(\mathop {\rm {pH}} +\mathop {\rm {pOH}} \right)=14$

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

$\mathop {\rm {pOH}} =-\log _{10}\left[{\rm {OH^{-}}}\right]=\log _{10}{\frac {1}{\left[{\rm {OH^{-}}}\right]}}$

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.