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 LevelExamples of Solutions at this pH
10,000,0000Battery acid, Strong Hydrofluric acid
1,000,0001Hydrochloric acid secreted by stomach lining
100,0002Lemon juice, Gastric acid, Vinegar
10,0003Grapefruit, Orange juice, Soda
1,0004Acid rain, tomato juice
1005Soft drinking water, Black coffee
106Urine, Saliva
17Pure water
1/108Sea water
1/1009Baking soda
1/100010Great Salt Lake, Milk of Magnesia
1/10,00011Ammonia solution
1/100,00012Soapy water
1/1,000,00013Bleach, Oven cleaner
1/10,000,00014Liquid 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.

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