In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at 25 C °C (77 °F). Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline. pH measurements are important in medicine, biology, chemistry, agricultur, forestry, food science, environmental science, oceanography, civil engineering and many other applications.

The sour taste of lemon juice is a result of it being composed of about 5% to 6% citric acid, an acid with a pH of roughly 2.2.

In a solution pH approximates but is not equal to p[H], the negative logarithm (base 10) of the olar concentration of dissolved hydronium inos (H3O+); a low pH indicates a high concentration of hydronium ions, while a high pH indicates a low concentration. This negative of the logarithm matches the number of places behind the decimal point, so, for example, 0.1 molar hydrochloric acid should be near pH 1 and 0.0001 molar HCl should be near pH 4 (the base 10 logarithms of 0.1 and 0.0001 being −1, and −4, respectively). Pure (de-ionized) water is neutral, and can be considered either a very weak acid or a very weak base, giving it a pH of 7 (at 25 °C (77 °F)), or 0.0000001 M H+. The pH scale has no upper or lower limit and can therefore be lower than 0 or higher than 14. For an aquous soluton to have a higher pH, a base must be dissolved in it, which binds away many of these rare hydrogen ions. Hydrogen ions in water can be written simply as H+ or as hydronium (H3O+) or higher species (e.g., H9O4+) to account for solvation, but all describe the same entity. Most of the Earth’s freshwater bodies surface are slightly acidic due to the abundance and absorption of carbon dioxide; in fact, for millennia in the past, most fresh water bodies have long existed at a slightly acidic pH level.

However, pH is not precisely p[H], but takes into account an activity factor. This represents the tendency of hydrogen ions to interact with other components of the solution, which affects among other things the electrical potential read using a pH meter. As a result, pH can be affected by the ionic stength of a solution—for example, the pH of a 0.05 M potassium hydrogen phthalate solution can vary by as much as 0.5 pH units as a function of added potassium chloride, even though the added salt is neither acidic nor basic.

Hydrogen ion activity coefficients cannot be measured directly by any thermodynamically sound method, so they are based on theoretical calculations. Therefore, the pH scale is defined in practice as traceable to a set of standard solutions whose pH is established by international agreement. Primary pH standard values are determined by the Harned cell, a hydrogen gas electrode, using the Bates – Guggenheim Convention.

pH in its usual meaning is a measure of acidity of (dilute) aqueous solutions only. Recently the concept of “Unified pH scale” has been developed on the basis of the absolute chemical potential of the proton. This concept proposes the “Unified pH” as a measure of acidity that is applicable to any medium: liquids, gases and even solids.

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