Acids, Bases, and Buffers 2: Unknown Solutions Overview Getting Started Techniques Procedure FAQ Full Lab Manual Introduction & Goals Chemistry & Background Key Questions Prelab Problems Safety Procedure In Your Write-up Appendix Experiments Index ChemLab Home		Appendix: Introduction to Acid-Base Indicators Dye molecules whose color depends on the concentration of H3O+ provide a simple means of estimating the pH of a solution. These so-called indicators are weak acids or weak bases whose conjugate acid-base forms are of different color. The equilibrium describing the behavior of the indicator phenol red can be shown by the following simplified equation: HIn + H2O H3O+ + I- red                           yellow This equilibrium is characterized by an acid dissociation constant, Ka which can be expressed as follows: Since phenol red is a weak acid, Ka<<1. If only a small amount of this indicator is added to a solution, the dissociation of the indicator will make a negligible contribution to the concentration of H3O+ in solution. In fact, the concentration of H3O+ in the solution determines the ratio of In- to HIn as shown by dividing both sides of the Ka equation by [H3O+] to obtain: That the color of the solution will depend on [H3O+] is easily seen by considering the following two cases:       (i)      Ka<<[ H3O+] (i.e. [H3O+] is large) Then [HIn]>>[ In-] and the solution is RED.       (ii)      Ka>>[ H3O+] (i.e. [H3O+] is small, [OH-] is large) Then [In-]>>[HIn] and the solution is YELLOW. Since the eye can detect changes in color only when the ratio of the concentrations of the two colored forms falls in the range 0.1 to 10, the range of pH values over which a given indicator is useful is somewhat limited. Concentration ratios falling in the 0.1 to 10 range are considered below for phenol red. This analysis reveals that the indicator is sensitive to a change of pH only in a 100-fold range of H3O+ concentration which is centered on the value [H3O+] = Ka. Thus, a measurement of pH in the range 7±1, requires an indicator whose dissociation constant, Ka is approximately 10-7; a similar condition applies in other pH ranges. The table below gives a list of common indicators, and the ranges in which they are effective. Table 1: pH-ranges of some Acid-Base Indicators Indicator pH range Color change acid to base Thymol blue 1.2 - 2.8 red-yellow Methyl orange 2.1 - 4.4 orange-yellow Methyl red 4.2 - 6.3 red-yellow Bromthymol blue 6.0 - 7.6 yellow-blue Cresol red 7.2 - 8.8 yellow-red Phenolphthalein 8.3 - 10.0 colorless-red Alizarin yellow 10.1 - 12.0 yellow-red From B. H. Mahan, "College Chemistry," Addison-Wesley, Reading, Massachusetts (1966).
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