The word acid and alkaline (i.e. bases) are derived from direct sensory experience. The word acid comes from the Latin word acere, which means "sour." All acids taste sour. Well known from ancient times were vinegar, sour milk and lemon juice. Early in the 1200s, the strong mineral acids were first isolated. Sulphuric acid was made by heating green vitriol [iron(II) sulphate] and condensing the vapour into water. Other vitriol gave the same product. Mixing a vitriol with nitre (potassium nitrate) and heating produced vapours which gave nitric acid. Adding sal ammoniac (ammonium chloride) to nitric acid gave aqua regia, so named for its ability to dissolve gold. Hydrochloric acid ("spirit(s) of salt" - a name still used in commerce/pharmacy as late as the early 1970s) also was known to the Middle Ages; certainly it was known to Paracelsus (early 1500s). The word alkaline comes from the Arabic al-qily, which means "to roast in a pan" or "the calcinated ashes of plants". By leaching the ashes with water, one can obtain a solution of sodium or potassium carbonate (to use the modern terms). This is then mixed with slaked lime (calcium hydroxide) and you get a solution of NaOH or KOH. This technique was described in writing in the 900s, but may have existed for many years prior. The measurement called pH was originally used by the Danish biochemist S.P.L. Sørensen while working on beer quality, to represent the hydrogen ion concentration of an aqueous solution.

Physics and chemistry

pH is the unit of measure that defines the degree of acidity of a solution. A pH of 1.5 describes the acidity of a solution as 5 meters describes the length of an object. Acidity is a function of the amount of hydrogen ions (H+) present in a solution. pH equals the negative logarithm of the hydrogen ion activity (see also detailed description). Most pH measurements are made in solutions where the pH falls between 0 - 14 pH. The pH of a solution is a very important characteristic. It determines which chemical reactions can occur and how efficiently they will occur. The only other parameter with such importance is temperature.


Applications of pH measurement are endless. Just about everything made in or from or of water will have its pH measured at some stage. This could range from the Pacific Ocean to a can of Coke. Living organisms are "designed" to operate between various limits (of temperature for example) and attempts to interact with any liquid outside these limits (say between pH 6 and 9) could give rise to severe problems. Not wonder oven cleaners have strong warnings about skin contact!

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