Wonkee Donkee’s lesson on Acidity and Alkalinity (pH)

3rd March 2020

	Wonkee Donkee’s lesson on acidity and alkalinity (pH)


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			Before you use a pH meter, you may wish to know a bit more about what acids and alkalis are.

			The word “acid” comes from a Latin word meaning sour. The sour taste of lemons and other foods is something we have been aware of for centuries, Aside from the sour taste, other characteristics we associate with acids are their ability to preserve or chemically burn. Acids can be toxic, yet we happily consume citrus fruit and vinegar which are both acidic.

			The definition of an acid has slightly varied over the years, but it is accepted that acids release hydrogen ions, which are positively charged, when in a solution. The difference between a toxic acid and one that is safe to ingest depends on the concentration of these ions: the higher the concentration, the more toxic the acid.

			Common things associated with alkalis are oven cleaners and a general bitterness. Like acids, there are alkalis which are edible and some that are toxic. Alkalis are also called ‘bases’, and an alkali is actually a soluble base. In the same way an acid is acidic, a base is “basic”, so this term is often used in place of alkaline.

			The toxicity is again dependent on the concentration of ions. However, with alkalis it is the release of hydroxide ions, which are negatively charged, when in a solution.

			Because alkalis and acids have oppositely charged ions, they have the ability to neutralise each other. Neutral substances have an equal amount of hydrogen and hydroxide ions.

			The pH scale was created to categorise acid, alkaline and neutral substances. The capital H represents the chemical symbol for hydrogen, as it is the ions of this element that determine the acidity or alkalinity of a substance.

			What the p stands for is disputed: some say power or potential, but it actually refers to a mathematical operation which represents the calculation for determining hydrogen ions present.

			The pH scale itself is represented by numerical and colour-coded values. The numbers run from zero to fourteen and the colours start red, then transition through orange into green and then through blue to purple. The lower the number, the more acidic the substance and vice versa. Red represents the acidic end of the scale and purple, the alkaline. Green, number seven substances are neutral.