# pH

measure of the acidity or basicity of an aqueous solution

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:

${\displaystyle {\mbox{pH}}=-\log _{10}\left[{\mbox{H}}^{+}\right]}$

[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).

However, the correct equation is actually:

${\displaystyle {\mbox{pH}}=-\log _{10}\left[a_{\mathrm {H^{+}} }\right]}$

where ${\displaystyle a_{\mathrm {H^{+}} }}$ indicates the activity of H+ ions.[2] 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

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+).[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.[3]

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

pH values of some common substances
 pH 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 3.0 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 Drain Cleaner 14

## Neutralization

Neutralization can be summed up by the equation:

H+
+ OH
H
2
O

(acid + basewater)

## Notes

1. These are two different ways of representing hydrogen ion concentration.
2. 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.
3. "Universal Indicator". ISCID Encyclopedia of Science and Philosophy. Archived from the original on September 25, 2006. Cite uses deprecated parameter |deadurl= (help)