In chemistry, pH (), historically denoting "potential of hydrogen" (or "power of hydrogen"), is a scale used to specify the acidity or basicity of an aqueous solution. Acidic solutions (solutions with higher concentrations of ions) are measured to have lower pH values than basic or alkaline solutions. The pH scale is logarithmic and inversely indicates the activity of hydrogen ions in the solution. where M = mol dm−3. At 25 °C (77°F), 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 (i.e. have the same concentration of H+ ions as OH− ions, i.e. the same as pure water). The neutral value of the pH depends on the temperaturebeing lower than 7 if the temperature increases above 25 °C. The pH value can be less than 0 for very concentrated strong acids, or greater than 14 for very concentrated strong bases. The pH scale is traceable to a set of standard solutions whose pH is established by international agreement. 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. History The concept of pH was first introduced by the Danish chemist Søren Peter Lauritz Sørensen at the Carlsberg Laboratory in 1909 and was 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, thus: 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. The exact meaning of the letter p in "pH" is disputed, as Sørensen did not explain why he used it. Sørensen describes a way of measuring pH using potential differences, and it represents the negative power of 10 in the concentration of hydrogen ions. The letter p could stand for the French puissance, German