pH is a scale of acidity from 0 to 14. It tells how acidic or alkaline a substance is. More acidic solutions have lower pH. More alkaline solutions have higher pH. Substances that aren't acidic or alkaline (that is, neutral solutions) usually have a pH of 7. Acids have a pH that is less than 7. Alkalis have a pH that is greater than 7.
pH is a measure of the concentration of protons (H+) in a solution. S.P.L. Sørensen introduced this concept in the year 1909. The p stands for the German potenz, meaning power or concentration, and the H for the hydrogen ion (H+).
The most common formula for calculating pH is:
However, the correct equation is actually:
where indicates the activity of H+ ions. But, this equation in most cases provides the same value as the more common formula, so in introductory chemistry the previous equation is given as the definition of pH.
Most substances have a pH in the range of 0 to 14, although extremely acidic or alkaline substances may have pH < 0, or pH > 14.
Alkaline substances have, instead of hydrogen ions, a concentration of hydroxide ions (OH-).
pH indicators[change | change source]
Certain dyes change colour depending on whether they are in an acid solution or an alkaline solution . pH indicator is a chemical compound added in small amounts to a solution so the pH (acidity or basicity) of the solution can be seen. The pH indicator is a chemical detector for hydronium ions (H3O+) or hydrogen ions (H+). Normally, the indicator causes the colour of the solution to change depending on the pH.
Typical indicators are phenolphthalein, methyl orange, methyl red, bromothymol blue, and thymol blue. They each change colour at different points on the pH scale, and can be used together as a universal indicator.
Some common pH values[change | change source]
|Oxygenated water||2.5 - 3.0|
|Orange or apple juice||3.0|
|Blood||7.35 - 7.45|
|Permanent wave||8.5 - 9.2|
|Hand soap||9.0 - 10.0|
|Hair dye||9.5 - 10.5|
Neutralization[change | change source]
Neutralization can be summed up by the equation:
Related pages[change | change source]
Notes[change | change source]
- These are two different ways of representing hydrogen ion concentration.
- Hawkes, Stephen J. (2000-09-01). "Easy Derivation of pH Is Approximately Equal To (pKa1 + pKa2) / 2 Using Autoprotolysis of HA-: Doubtful Value of the Supposedly More Rigorous Equation". Journal of Chemical Education. 77 (9): 1183. doi:10.1021/ed077p1183. ISSN 0021-9584.
- "Universal Indicator". ISCID Encyclopedia of Science and Philosophy. Archived from the original on September 25, 2006.