pH (potential of hydrogen) 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 formula for calculating pH is:
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-).
A 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
|Oxygenated water||2.5 - 3.0|
|Orange or apple juice||3.5|
|Blood||7.35 - 7.45|
|Permanent wave||8.5 - 9.2|
|Hand soap||9.0 - 10.0|
|Hair dye||9.5 - 10.5|
Neutralisation can be summed up by the equation:
- H+ + OH− → H2O
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PH Facts for Kids. Kiddle Encyclopedia.