Acid concentration, relation indicator color

These relations show that the acid and base concentrations ratio depends on the pH through the indicator pKa value. The solution color depends on the pH value. Some indicators exhibit only one colored form they are called unicolor indicators. For example, this is the case of phenolphthalein. The theory concerning them is the same as that concerning bicolor indicators except for the influence of their concentration on the color-change interval (see Sect. 8.4). 

Sorensen is usually considered to be the first to have realized the importance of hydrogen ion concentration in cells and in the solutions in which the properties of cell components were to be studied. He is also credited with the introduction of the pH scale. Electrochemistry started at the end of the nineteenth century. By 1909, Sorensen had introduced a series of dyes whose color changes were related to the pH of the solution, which was determined by the H+ electrode. The dyes were salts of weak acids or weak bases. He also devised simple methods for preparing phosphate buffer solutions covering the pH range 6-8. Eventually buffers and indicators were provided covering virtually the whole pH range. 

The general principles of chemical equilibrium can be similarly used in the discussion of a weak base, such as ammonium hydroxide, and also of salts formed by weak acids and weak bases. In addition, these principles are important in providing an understanding of the behavior of indicators, the colored substances that were described in Chapter 6 as useful for determining whether a solution is acidic, neutral, or basic. These principles are of further importance in permitting a discussion of the relation between the concentrations of hydrogen ion and hydroxide ion in the same solution. 

Take a look at the first titration plot (Figure 9-1) given. You will see a steep region in the plot. The center of this steep region is called the equivalence point. The equivalence point denotes the point at which equivalent amounts of aeid and base have reaeted. To know the equivalence point, we usually add an indicator which will change its color close to the equivalence point. We can use the following relation to equate the amount of acid or base present in a solution against the volume of aeid or base added whose concentration is known. 

The method of Kotrly and Vytras has been used to obtain the optimum concentrations of the indicators for the titration arrangement used. They pointed out that the spatial transition curve of a color change can be divided into small segments related to a suitable variable, usually pH or the concentration of free perchloric acid, Chcio4j in acetic acid medium. The ratio ApH/AE or APCHCIO4/AE was taken as an index of color change perceptibility. 

We first examined acid-base titrations in Section 4.8. In an acid-base titration, a basic (or acidic) solution of unknown concentration reacts with an acidic (or basic) solution of known concentration. The known solution is slowly added to the unknown one while the pH is monitored with either a pH meter or an indicator (a substance whose color depends on the pH). As the acid and base combine, they neutralize each other. At the equivalence point—the point in the titration when the number of moles of base is stoichiometrically equal to the number of moles of acid—the titration is complete. When the equivalence point is reached, neither reactant is in excess and the number of moles of the reactants are related by the reaction stoichiometry (Figure 16.5 t). 

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