Glossary of Water Resource Terms. A B C D E F G H I J K L M N O P Q R S T U V W X Y Z A abandoned water right a water right which was not put to beneficial use for a. H values of some common substances. In chemistry, p. H potential of hydrogen is a numeric scale used to specify the acidity or basicity of an aqueous solution. The HERA project is a European voluntary initiative launched by the suppliers and manufacturers, AISE and CEFIC, of household cleaning products to provide a common. It is approximately the negative of the base 1. More precisely it is the negative of the base 1. Solutions with a p. H less than 7 are acidic and solutions with a p. H greater than 7 are basic. Pure water is neutral, at p. H 7 2. 5 C, being neither an acid nor a base. Contrary to popular belief, the p. H value can be less than 0 or greater than 1. H measurements are important in agronomy, medicine, biology, chemistry, agriculture, forestry, food science, environmental science, oceanography, civil engineering, chemical engineering, nutrition, water treatment and water purification, as well as in many other applications. The p. H scale is traceable to a set of standard solutions whose p. H is established by international agreement. Primary p. H 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 p. H of aqueous solutions can be measured with a glass electrode and a p. H meter, or an indicator. History. The concept of p. H was first introduced by the Danish chemist Sren Peder Lauritz Srensen at the Carlsberg Laboratory in 1. H in 1. 92. 4 to accommodate definitions and measurements in terms of electrochemical cells. Difference Between Trickling Filter And Activated Sludge Definition' title='Difference Between Trickling Filter And Activated Sludge Definition' />In the first papers, the notation had the H as a subscript to the lowercase p, as so p. H. The exact meaning of the p in p. H is disputed, but according to the Carlsberg Foundation, p. H stands for power of hydrogen. It has also been suggested that the p stands for the German. Potenz meaning power, others refer to Frenchpuissance also meaning power, based on the fact that the Carlsberg Laboratory was French speaking. Another suggestion is that the p stands for the Latin termspondus hydrogenii quantity of hydrogen, potentia hydrogenii capacity of hydrogen, or potential hydrogen. It is also suggested that Srensen used the letters p and q commonly paired letters in mathematics simply to label the test solution p and the reference solution q. Currently in chemistry, the p stands for decimal cologarithm of, and is also used in the term p. Ka, used for acid dissociation constants. Definition and measurementp. Hp. H is defined as the decimal logarithm of the reciprocal of the hydrogen ionactivity, a. H, in a solution. Hlog. 10a. Hlog. Hdisplaystyle mathrm p. H log 1. 0atextrm Hlog 1. HrightFor example, a solution with a hydrogen ion activity of 51. H of log. 1021. For a commonplace example based on the facts that the masses of a mole of water, a mole of hydrogen ions, and a mole of hydroxide ions are respectively 1. H 7 water, or 1. H3. Ohydronium ions and 1. Note that p. H depends on temperature. For instance at 0 C the p. Instructions For Monopoly World Edition Download more. H of pure water is 7. At 2. 5 C its 7. C its 6. This definition was adopted because ion selective electrodes, which are used to measure p. H, respond to activity. Ideally, electrode potential, E, follows the Nernst equation, which, for the hydrogen ion can be written as. EE0RTFlna. HE02. RTFp. Hdisplaystyle EE0frac RTFlnatextrm HE0 frac 2. RTFmathrm p. H where E is a measured potential, E0 is the standard electrode potential, R is the gas constant, T is the temperature in kelvins, F is the Faraday constant. For H number of electrons transferred is one. It follows that electrode potential is proportional to p. H when p. H is defined in terms of activity. Precise measurement of p. H is presented in International Standard ISO 3. A galvanic cell is set up to measure the electromotive force e. The reference electrode may be a silver chloride electrode or a calomel electrode. The hydrogen ion selective electrode is a standard hydrogen electrode. Reference electrode concentrated solution of KCl test solution H2 Ptclarification neededFirstly, the cell is filled with a solution of known hydrogen ion activity and the emf, ES, is measured. Then the emf, EX, of the same cell containing the solution of unknown p. H is measured. p. HXp. HSESEXzdisplaystyle textp. HXtextp. HSfrac EtextS EtextXzThe difference between the two measured emf values is proportional to p. H. This method of calibration avoids the need to know the standard electrode potential. The proportionality constant, 1z is ideally equal to 1. RTF displaystyle frac 12. RTF the Nernstian slope. To apply this process in practice, a glass electrode is used rather than the cumbersome hydrogen electrode. A combined glass electrode has an in built reference electrode. It is calibrated against buffer solutions of known hydrogen ion activity. IUPAC has proposed the use of a set of buffer solutions of known H activity. Two or more buffer solutions are used in order to accommodate the fact that the slope may differ slightly from ideal. To implement this approach to calibration, the electrode is first immersed in a standard solution and the reading on a p. H meter is adjusted to be equal to the standard buffers value. The reading from a second standard buffer solution is then adjusted, using the slope control, to be equal to the p. H for that solution. Further details, are given in the IUPAC recommendations. When more than two buffer solutions are used the electrode is calibrated by fitting observed p. H values to a straight line with respect to standard buffer values. Commercial standard buffer solutions usually come with information on the value at 2. C and a correction factor to be applied for other temperatures. The p. H scale is logarithmic and therefore p. H is a dimensionless quantity. HThis was the original definition of Srensen,5 which was superseded in favor of p. H in 1. 90. 9. However, it is possible to measure the concentration of hydrogen ions directly, if the electrode is calibrated in terms of hydrogen ion concentrations. One way to do this, which has been used extensively, is to titrate a solution of known concentration of a strong acid with a solution of known concentration of strong alkaline in the presence of a relatively high concentration of background electrolyte. Since the concentrations of acid and alkaline are known, it is easy to calculate the concentration of hydrogen ions so that the measured potential can be correlated with concentrations. The calibration is usually carried out using a Gran plot. The calibration yields a value for the standard electrode potential, E0, and a slope factor, f, so that the Nernst equation in the form. EE0f. 2. 3. 03. RTFlogHdisplaystyle EE0ffrac 2. RTFlogmboxHcan be used to derive hydrogen ion concentrations from experimental measurements of E. The slope factor, f, is usually slightly less than one. A slope factor of less than 0. The presence of background electrolyte ensures that the hydrogen ion activity coefficient is effectively constant during the titration. As it is constant, its value can be set to one by defining the standard state as being the solution containing the background electrolyte. Thus, the effect of using this procedure is to make activity equal to the numerical value of concentration.