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The meaning of «ph»

In chemistry, pH (/piːˈeɪtʃ/, denoting 'potential of hydrogen' or 'power of hydrogen'[1]) is a scale used to specify the acidity or basicity of an aqueous solution. Acidic solutions (solutions with higher concentrations of H+ ions) are measured to have lower pH values than basic or alkaline solutions.

The pH scale is logarithmic and inversely indicates the concentration of hydrogen ions in the solution. This is because the formula used to calculate pH approximates the negative of the base 10 logarithm of the molar concentration[a] of hydrogen ions in the solution. More precisely, pH is the negative of the base 10 logarithm of the activity of the H+ ion.[2]

At 25 °C, solutions with a pH less than 7 are acidic, and solutions with a pH greater than 7 are basic. Solutions with a pH of 7 at this temperature are neutral (e.g. pure water). The neutral value of the pH depends on the temperature, being lower than 7 if the temperature increases. The pH value can be less than 0 for very strong acids, or greater than 14 for very strong bases.[3]

The pH scale is traceable to a set of standard solutions whose pH is established by international agreement.[4] Primary pH standard values are determined using a concentration cell with transference, by measuring the potential difference between a hydrogen electrode and a standard electrode such as the silver chloride electrode. The pH of aqueous solutions can be measured with a glass electrode and a pH meter, or a color-changing indicator. Measurements of pH are important in chemistry, agronomy, medicine, water treatment, and many other applications.

The concept of pH was first introduced by the Danish chemist Søren Peder Lauritz Sørensen at the Carlsberg Laboratory in 1909[5] and revised to the modern pH in 1924 to accommodate definitions and measurements in terms of electrochemical cells. In the first papers, the notation had H• as a subscript to the lowercase p, as so: pH•.

For the sign p I propose the name ‘hydrogen ion exponent’ and the symbol pH•. Then, for the hydrogen ion exponent (pH•) of a solution, the negative value of the Briggsian logarithm of the related hydrogen ion normality factor is to be understood.[5]

The exact meaning of the p in pH is disputed, as Sørensen did not explain why he used it.[6] He describes a way of measuring it using potential differences, and it represents the negative power of 10 in the concentration of hydrogen ions. All the words for these start with p in French, German and Danish, all languages Sørensen published in: Carlsberg Laboratory was French-speaking, German was the dominant language of scientific publishing, and Sørensen was Danish. He also used "q" in much the same way elsewhere in the paper. So the "p" could stand for the French puissance, German Potenz, or Danish potens, meaning "power", or it could mean "potential". He might also just have labelled the test solution "p" and the reference solution "q" arbitrarily; these letters are often paired.[7] There is little to support the suggestion that "pH" stands for the Latin terms pondus hydrogenii (quantity of hydrogen) or potentia hydrogenii (power of hydrogen).

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