The Foundations of Chemistry

7.9 10
^8. We can represent its ionization in dilute aqueous solution and its ionization
constant expression as

HIn H 2 O 34 H 3 O In
 Ka7.9 10 
^8

ocolor 1oomfor bromthymol bluen color 2o

oyellowooblueo

HIn represents nonionized acid molecules, and In
represents the anion (conjugate base)
of HIn. The essential characteristic of an acid–base indicator is that HIn and In
must
have quite different colors. The relative amounts of the two species determine the color
of the solution. Adding an acid favors the reaction to the left and gives more HIn mole-
cules (color 1). Adding a base favors the reaction to the right and gives more In
ions
(color 2). The ionization constant expression can be rearranged.

Ka so 

This shows clearly how the [In
]/[HIn] ratio depends on [H 3 O] (or on pH) and the Ka
value for the indicator. As a rule of thumb, when [In
]/[HIn] 10, color 2 is observed;

conversely, when [In
]/[HIn]^  110 , color 1 is observed.
Universal indicatorsare mixtures of several acid–base indicators that display a contin-
uous range of colors over a wide range of pH values. Figure 18-2 shows concentrated
solutions of a universal indicator in flat dishes so that the colors are very intense. The
juice of red (purple) cabbage is a universal indicator. Figure 19-2 shows the color of red
cabbage juice in solutions within the pH range 1 to 13.
One important use of universal indicators is in commercial indicator papers, which are
small strips of paper impregnated with solutions of universal indicators. A strip of the
paper is dipped into the solution of interest, and the color of the indicator on the paper
indicates the pH of the solution. The photographs on page 757 (solutions of universal
indicators) and page 763 (an indicator paper) illustrate the use of universal indicators to
estimate pH. We shall describe the use of indicators in titrations more fully in Sections
19-5 and 19-6.

Ka



[H 3 O]

[In
]



[HIn]

[H 3 O][In
]



[HIn]

[H 3 O][In
]



[HIn]

19-4 Acid–Base Indicators 809

Bromthymol blue indicator is yellow

in acidic solutions and blue in basic

solutions.

Figure 19-2 The juice of the red

(purple) cabbage is a naturally

occurring universal indicator. From

left to right are solutions of pH 1, 4,

7, 10, and 13.