E = E 0 + R T F ln ⁡ ( a H + ) = E 0 − 2.303 R T F pH {\displaystyle E=E In 1909, the Danish chemist Søren Peter Lauritz Sørensen introduced the concept of pH at the Carlsberg Laboratory, [4] originally using the notation "p H•", with H• as a subscript to the lowercase p. The concept was later revised in 1924 to the modern pH to accommodate definitions and measurements in terms of electrochemical cells. The concept of pH was developed because ion-selective electrodes, which are used to measure pH, respond to activity. The electrode potential, E, follows the Nernst equation for the hydrogen ion, which can be expressed as:

The pH of a solution is defined as the decimal logarithm of the reciprocal of the hydrogen ion activity, a H+. [14] Mathematically, pH is expressed as: A one and a half storey detached house of traditional stone and lime construction under a pitched timber framed slate covered roof, with dormer windows on the first floor and attractive porch/sunroom set atop a dwarf wall. For example, for a solution with a hydrogen ion activity of 5×10 −6 (i.e., the concentration of hydrogen ions in moles per litre), the pH of the solution can be calculated as follows: Sørensen did not explain why he used the letter p, and the exact meaning of the letter is still disputed. [5] Sørensen described 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 Potenz, or Danish potens, all meaning "power", or it could mean "potential". All of these words start with the letter p in French, German, and Danish, which were the languages in which Sørensen published: Carlsberg Laboratory was French-speaking; German was the dominant language of scientific publishing; Sørensen was Danish. He also used the letter q in much the same way elsewhere in the paper, and he might have arbitrarily labelled the test solution "p" and the reference solution "q"; these letters are often paired. [6] Some literature sources suggest that "pH" stands for the Latin term pondus hydrogenii (quantity of hydrogen) or potentia hydrogenii (power of hydrogen), although this is not supported by Sørensen's writings. [7] [8] [9] The pH scale is traceable to a set of standard solutions whose pH is established by international agreement. [3] 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.In modern chemistry, the p stands for "the negative decimal logarithm of", and is used in the term p K a for acid dissociation constants, [10] so pH is "the negative decimal logarithm of H + ion concentration", while pOH is "the negative decimal logarithm of OH- ion concentration". Externally the property benefits from a large, enclosed garden extending to approximately 0.33 Acres in all. Bacteriologist Alice Catherine Evans, who influenced dairying and food safety, credited William Mansfield Clark and colleagues, including herself, with developing pH measuring methods in the 1910s, which had a wide influence on laboratory and industrial use thereafter. In her memoir, she does not mention how much, or how little, Clark and colleagues knew about Sørensen's work a few years prior. [11] She said: pH = − log 10 ⁡ ( a H + ) = log 10 ⁡ ( 1 a H + ) {\displaystyle {\ce {pH}}=-\log _{10}(a_{{\ce {H+}}})=\log _{10}\left({\frac {1}{a_{{\ce {H+}}}}}\right)}

In chemistry, pH ( / p iː ˈ eɪ tʃ/ pee- AYCH), also referred to as acidity or basicity, historically denotes " potential of hydrogen" (or "power of hydrogen"). [1] It is a scale used to specify the acidity or basicity of an aqueous solution. Acidic solutions (solutions with higher concentrations of hydrogen ( H +) ions) are measured to have lower pH values than basic or alkaline solutions. Enjoying a primarily southerly aspect, Loch Cottage provides comfortable and modern accommodation that, in its current configuration, includes three bedrooms, a bathroom on each floor, kitchen with adjoining dining room and generously proportioned living room. In these studies [of bacterial metabolism] Dr. Clark's attention was directed to the effect of acid on the growth of bacteria. He found that it is the intensity of the acid in terms of hydrogen-ion concentration that affects their growth. But existing methods of measuring acidity determined the quantity, not the intensity, of the acid. Next, with his collaborators, Dr. Clark developed accurate methods for measuring hydrogen-ion concentration. These methods replaced the inaccurate titration method of determining the acid content in use in biologic laboratories throughout the world. Also they were found to be applicable in many industrial and other processes in which they came into wide usage. [11] For the sign p, I propose the name 'hydrogen ion exponent' and the symbol p H•. Then, for the hydrogen ion exponent (p H•) of a solution, the negative value of the Briggsian logarithm of the related hydrogen ion normality factor is to be understood. [4]

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where [H +] is the equilibrium molar concentration (mol/L) of H + in the solution. 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 25 °C 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 temperature and is lower than 7 if the temperature increases above 25°C. The pH range is commonly given as zero to 14, but a pH value can be less than 0 for very concentrated strong acids or greater than 14 for very concentrated strong bases. [2]