PH value

Johann Maier, winter semester 2015/16

In chemistry, pH is a numeric scale used to specify the acidity or basicity (alkalinity) of an aqueous solution. It is roughly the negative of the logarithm to base 10 of the concentration, measured in units of moles per liter, of hydrogen ions or H+. More precisely it is the negative of the logarithm to base 10 of the activity of the hydrogen ion. Solutions with a pH less than 7 are acidic and solutions with a pH greater than 7 are basic. Pure water is neutral, being neither an acid nor a base. ^[1]

pH measurements are important in many fields of science and also daily life: e.g. in medicine, biology, chemistry, agriculture, civil engineering, chemical engineering, nutrition, as well as many other applications. ^[1]

The pH scale is traceable to a set of standard solutions whose pH is established by international agreement. ^[2] 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 indicator. ^[1]

Definition and measurement

pH is defined as the decimal logarithm of the reciprocal of the hydrogen ion activity, $//<![CDATA[ \begin{array}{l}a_{H+}\end{array} //]]>$, in a solution. ^[2]

^{$//<![CDATA[ \begin{array}{l}pH = -log_{10}(a_{H+}) = log_{10}(\frac{1}{a_{H+}})\end{array} //]]>$}

pH indicators

The most common kind of pH measurement is the use of indicators. Indicators may be used to measure pH, by making use of the fact that their color changes with pH. Visual comparison of the color of a test solution with a standard color chart provides a means to measure pH accurate to the nearest whole number. More precise measurements are possible if the color is measured spectrophotometrically, using a colorimeter of spectrophotometer. Universal indicator consists of a mixture of indicators such that there is a continuous color change from about pH 2 to pH 10. Universal indicator paper is made from absorbent paper that has been impregnated with universal indicator. ^[3]

Literature

1. pH. Wikipedia. January 27th, 2016.
2. Covington, A. K.; Bates, R. G.; Durst, R. A.: Definitions of pH scales, standard reference values, measurement of pH, and related terminology. PDF. 1985.
3. pH indicator. Wikipedia. January 27th, 2016.