Titration and pH Measurement Patricia A Jennings, University of California, San Diego, California, USA Christine A Mullen, Texas A&M University, College Station, Texas, USA Melinda Roy, University of California, San Diego, California, USA Based in part on the previous version of this Encyclopedia of Life Sciences (ELS) article, Titration and pH Measurement by Patricia A Jennings and Christine A Mullen. Titration is the quantitative addition of a solution of known concentration to a solution of unknown concen- tration until the reaction between them is complete to determine the concentration of the second solution. An acid–base titration is the quantitative determination of the concentration of an acid or a base. Titration of an acid with a base requires that the pH, or relative concen- trations of the two reactants, be monitored. pH can be assessed by litmus paper or by indicators, for example, phenolphthalein, but these methods lack precision. Typically, pH measurement in the laboratory is done by measuring the cell potential of that sample in reference to a standard hydrogen electrode. A plot of the pH of an acidic (or basic) solution as a function of the amount of added base (or acid) is a titration curve. From this, the endpoint or equivalent points can be determined. Introduction Acid–base titration is one of the oldest tools of analytical chemistry. It is the determination of the concentration of an acid or base by neutralising the analyte with an acid or base of known concentration. The method was first described by Glauber (1658). It called for potash – potassium carbonate from wood ashes – to be added to nitric acid until gas no longer evolved. We now know that this simple method was a reliable way to produce a solution with a hydronium (H 3 O + ) ion concentration of 5.0  10 25 mol L 21 . Since that time, much effort has been directed at expanding the usefulness of acid–base titrations as well as the methods for H 3 O + ion measurement. A convenient scale, the pH scale, is now used to report the relative acidity and basicity of aqueous solutions. Briefly, pH is defined as pH ¼ log 10 1 ½H 3 O þ  ¼ log 10 ½H 3 O þ . See also: pH and Buffers For a neutral solution at the standard state temperature of 258C, the H 3 O + ion concentration is 1.0  10 27 and the pH equals 7.0. Solutions with a pH less than 7 are acidic and solutions with a pH greater than 7 are basic or alkaline. Many biochemical processes are pH-dependent and many enzymes perform proton transfers, that is, acid–base catalysis. Hence, accurate and precise measurement and control of pH is important. Many of the chemical reactions that occur in aqueous solutions either release or use pro- tons, which then alter the pH value. For example, protein degradation increases basicity and carbohydrate metab- olism increases acidity. With few exceptions, however, chemical reactions of living organisms must occur within a pH range of 6.9–7.5. Measurement of pH is therefore one of the most crucial and frequently used procedures in the life sciences. See also: Acid–Base Catalysis by Enzymes; Cell Biophysics; Organic Reaction Mechanisms Outline of Methods The pH value of a solution can be measured with a strip of litmus paper, or a chemical called an indicator or an instrument called a pH meter. Litmus paper indicates the presence of acids or bases by changing its colour to red or blue, respectively (Figure 1). Unfortunately, the red and blue coloration does not give any indication of the strength of the acid or base. Other, more sensitive types of test strips are impregnated with combinations of acid–base indi- cators that turn various colours as the pH changes. Although these ‘universal’ test strips give only approximate values, they also provide fast, convenient, portable and inexpensive indications of pH. We are concerned with acid–base titrations, in which the concentration of an acidic (or basic) solution (the analyte) is determined by the delivery of a measured volume of base (or acid) of known concentration (the titrant; see Figure 1). The titrant must react rapidly and completely with the analyte and therefore is usually a strong acid or base. In Introductory article Article Contents . Introduction . Outline of Methods . Applications . Future Developments . Summary Online posting date: 15 th December 2010 ELS subject area: Biochemistry How to cite: Jennings, Patricia A; Mullen, Christine A; and Roy, Melinda (December 2010) Titration and pH Measurement. In: Encyclopedia of Life Sciences (ELS). John Wiley & Sons, Ltd: Chichester. DOI: 10.1002/9780470015902.a0002700.pub2 ENCYCLOPEDIA OF LIFE SCIENCES & 2010, John Wiley & Sons, Ltd. www.els.net 1