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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 (called 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:

[H+] indicates the concentration of H+ ions (also written [H3O+],[1] the equal concentration of hydronium ions), measured in moles per litre (also known as molarity).

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]

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+).[1] 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.[2]

Another way is to use litmus paper, which is based on a natural pH indicators. The paper can tell you how strong the chemical is, whether it is a stronger acid or a stronger base.

Some common pH values[change | change source]

pH values of some common substances
Battery acid 1.0
Gastric acid 2.0
Lemon juice 2.4
Cola 2.5
Oxygenated water 2.5 - 3.0
Vinegar 3.0
Orange or apple juice 3.5
Beer 4.5
Coffee 5.0
Milk 6.6
Pure water 7.0
Blood 7.35 - 7.45
Plain shampoo 8.0
Sea water 8.0
Permanent wave 8.5 - 9.2
Hand soap 9.0 - 10.0
Hair dye 9.5 - 10.5
Magic straight 11.5
Household ammonia 11.5
Bleach 12.3
Caustic soda 12.7
Household lye 13.5

Neutralisation[change | change source]

Neutralisation can be summed up by the equation:

H+ + OHH2O

(acid + basewater)

Related pages[change | change source]

Notes[change | change source]

  1. 1.0 1.1 These are two different ways of representing hydrogen ion concentration.
  2. "Universal Indicator". ISCID Encyclopedia of Science and Philosophy. Archived from the original on September 25, 2006.

Other websites[change | change source]