pH - Potential Hydrogen

Chemical change results in the production of new substances; these substances may be helpful or harmful (and sometimes both!). Two types of substances which are very important in everyday life are acids and bases.

Acids are sour, water-soluble substances which are very useful in industry, household cleaning agents, and cooking; some examples are vinegar, vitamin C tablets, club soda, Aspirin, lemon juice and cream of tartar. Vinegar is a solution of about one part acetic acid to 20 parts water - such a mixture of acid and water is called an acidic solution. Lemons and grapefruit have a tangy flavour, because they contain an acid called citric acid. Baking powder contains a dry acid called tartaric acid. Another very important acid is stomach acid (dilute hydrochloric acid) which aids in the digestion of our food. Acids which are not diluted in water are dangerous - they undergo chemical change so readily that they can react with skin and cause burns.

Bases are bitter, water-soluble substances which are also very useful. Examples of bases are ammonia, baking soda, and drain cleaner. Bases are also used in some batteries. The solutions they form with water are called basic or alkaline solutions. Bases are also highly reactive and must be treated with extreme caution, as they also react readily with skin.

You have probably heard of antacids. These substances are bases which are safe to ingest and with react with stomach acid. A chemical change in which an acid reacts with a base is called neutralization. It is called neutralization because equal amounts of acid and base produce a solution which is neutral - neutral acidic nor basic. Antacids are used when the stomach contains too much acid, which irritates the stomach lining.

The pH scale is a measure of how acidic or basic a solution is. This scale ranges from 0 for an extremely acidic solution to 14 for an extremely basic solution. A neutral solution has pH 7. A decrease of one unit on this scale represents multiplying acidity ten times. Most plants prefer a slightly acidic pH 6.0 - 6.5. Correct pH levels are important for the plant to be able to take up all the nutrient supplied in the solution. A too high or low pH is one of the most common problems associated with home hobby growers. These problems show quickly and can be countered quickly and easily!

Most city tap water has a slightly basic pH 7 - 8, the nutrient we mix into a solution is acidic based and will adjust the pH a point or so lower. We may however have to adjust further down using a stable, usable acid such as a dilute phosphoric acid. This is the most common scenario.

Perhaps the source of water we use is acidic (eg. some well and ground waters) and after mixing the nutrient we need to adjust the pH higher. In this case we would use a stable, usable alkali such as a dilute di-potassium phosphate.

A simple method can be used to detect whether a solution is acidic or basic. An indicator is a substance which changes colour, depending on whether it is placed in a an acidic solution or a basic solution. There are also electronic meters available to easily check pH levels in solution. They are simply dipped into the solution and give a digital read out.