pH Measurement

pH is the abbreviation of the Latin phrase “pondus hydro genii”. Pondus = Pressure and Hydro genii = Hydrogen. Thus pH is the measurement of hydrogen ion concentration in a liquid solution. This activity is directly connected to its acidic, neutral or alkaline characteristics.

If a solution has a pH of less than 7 (a large number of Hydrogen ions) it is considered acidic. Those solutions with a pH of 7 are considered neutral as is the pH of pure water at 25°C. Anything with a pH of above 7 (a small number of hydrogen ions) is considered alkaline. The pH scales reads from 1 (acidic) to 14 (alkaline) Sometimes it is possible to come across solutions that may have a pH of below 0 or above 14 however this is very rare and any solutions that are, are concentrated and highly reactive.

Concentration of Hydrogen ions compared to distilled water	pH Level	Examples of Solutions at this pH
10,000,000	0	Battery acid, Strong Hydrofluric acid
1,000,000	1	Hydrochloric acid secreted by stomach lining
100,000	2	Lemon juice, Gastric acid, Vinegar
10,000	3	Grapefruit, Orange juice, Soda
1,000	4	Acid rain, tomato juice
100	5	Soft drinking water, Black coffee
10	6	Urine, Saliva
1	7	Pure water
1/10	8	Sea water
1/100	9	Baking soda
1/1000	10	Great Salt Lake, Milk of Magnesia
1/10,000	11	Ammonia solution
1/100,000	12	Soapy water
1/1,000,000	13	Bleach, Oven cleaner
1/10,000,000	14	Liquid drain cleaner

pH can be measured simply by colour changes of some chemical powder e.g. litmus paper but if you are looking at continuous monitoring and control of pH more sophisticated equipment may be needed.

The most popular method is to use an electrode designed to allow hydrogen ions in a solution to pass through a selective barrier which produces a measurable potential difference proportional to the solutions pH.