# Acid-Base Indicators

## An indicator is a weak acid (or a weak base) that has sharply different colors in its dissociated and undissociated states.

#### Key Points

• In the reaction $HIn\quad \rightleftharpoons { \quad H }^{ + }\quad +\quad { In }^{ - }$ adding acid shifts the indicator equilibrium to the left.

• In the reaction $HIn\quad\rightleftharpoons { \quad H }^{ + }\quad +\quad { In }^{ - }$ adding base shifts it to the right.

• For optimal accuracy, the color difference between the two colored species should be as clear as possible, and the narrower the pH range of the color change the better.

#### Terms

• An acid-base indicator.

• the negative of the logarithm to base 10 of the concentration of hydrogen ions, measured in moles per liter; a measure of acidity or alkalinity of a substance, which takes numerical values from 0 (maximum acidity) through 7 (neutral) to 14 (maximum alkalinity)

• the determination of the concentration of some substance in a solution by slowly adding measured amounts of some other substance (normally using a burette) until a reaction is shown to be complete -- for instance, by the color change of an indicator

• a halochromic chemical compound that is added in small amounts to a solution so that the pH (acidity or basicity) of the solution can be determined visually

#### Figures

1. ##### Common Acid-Base Indicators

Common indicators for pH indication or titration endpoints is given, with high, low and transition pH colors.

2. ##### pH Measuring Strips

pH can be determined to a reasonable level of accuracy by treating a strip with the solution to be tested, and then observing the color sequence on the treated area.

3. ##### Methyl Orange

The molecule methyl orange is commonly used as an indicator in acid-base equilibrium reactions. In base form, on the left in the figure, the color is yellow. Adding a proton yields the structure on the right, colored red. Note that this color change occurs over the pH range from approximately 3-4.

## Acid Base Indicators

In our study of acids and bases, we use many methods to determine the pH of a solution, and to determine the point of equivalence when mixing acids and bases. These methods range from the use of litmus paper, indicator paper Figure 2, specifically designed electrodes, and the use of colored molecules in solution. Other than the electrodes, all of the methods are visual, and rely on some fundamental changes that occur in a molecule when the pH of its environment changes.

In general, a molecule that changes color with the pH of the environment it is in can be used as an indicator. In the equation

$HIn\quad \rightleftharpoons { \quad H }^{ + }\quad +\quad { In }^{ - }$

the weak acid HIn is shown in equilibrium with its ionized anion In- -- in the case of the indicator methyl orange, the HIn is colored red and the ionized In- form is yellow. The structures are shown in Figure 3.

In this example, ${ K }_{ a }\quad =\quad \frac { \left[ { H }^{ + } \right] \left[ { In }^{ - } \right] }{ \left[ HIn \right] }$

For methyl orange, Ka = 1.6 X 10-4 and pKa = 3.8. The neutral (red) and dissociated (yellow) forms of the indicator are present at equal concentrations when the pH = 3.8. The eye is sensitive to color changes over a range of concentration ratios of approximately 100 or over two pH units. Below pH 2.8 is a solution containing methyl orange is red, and above approximately 4.8 it is clearly yellow.

pH indicators are frequently employed in titrations in analytical chemistry and biology to determine the extent of a chemical reaction. Because of the subjective choice (determination) of color, pH indicators are susceptible to imprecise readings. For applications requiring precise measurement of pH, a pH meter is frequently used. Sometimes a blend of different indicators is used to achieve several smooth color changes over a wide range of pH values. These commercial indicators (e.g., universal indicator and Hydrion papers) are used when only rough knowledge of pH is necessary. Tabulated below Figure 1 are several common laboratory pH indicators. Indicators usually exhibit intermediate colors at pH values inside the listed transition range. For example, phenol red exhibits an orange color between pH 6.8 and pH 8.4. The transition range may shift slightly depending on the concentration of the indicator in the solution and on the temperature at which it is used.

When viewed on the pH scale itself, the color transitions as determined by their transition ranges becomes clearer, and the context of the indicator sensitivity over ranges of pH is laid out more informatively.

#### Key Term Glossary

acid
an electron pair acceptor; generally capable of donating hydrogen ions
##### Appears in these related concepts:
anion
A negatively charged ion, as opposed to a cation
##### Appears in these related concepts:
base
A proton acceptor, or an electron pair donor.
##### Appears in these related concepts:
chemical reaction
A process involving the breaking or making of interatomic bonds, in which one or more substances are changed into others.
##### Appears in these related concepts:
chemistry
The branch of natural science that deals with the composition and constitution of substances and the changes that they undergo as a consequence of alterations in the constitution of their molecules.
##### Appears in these related concepts:
concentration
the proportion of a substance in a mixture
##### Appears in these related concepts:
electrode
the terminal through which electric current passes between metallic and nonmetallic parts of an electric circuit. In electrolysis, the electrodes (cathode and anode) are placed in the solution separately.
##### Appears in these related concepts:
equilibrium
the state of a reaction in which the rates of the forward and reverse reactions are the same
##### Appears in these related concepts:
indicator
a halochromic chemical compound that is added in small amounts to a solution so that the pH (acidity or basicity) of the solution can be determined visually
##### Appears in these related concepts:
measurement
magnitude (or extent or amount) determined by measurement
##### Appears in these related concepts:
molecule
the smallest particle of a specific element or compound that retains the chemical properties of that element or compound; two or more atoms held together by chemical bonds
##### Appears in these related concepts:
pH
the negative of the logarithm to base 10 of the concentration of hydrogen ions, measured in moles per liter; a measure of acidity or alkalinity of a substance, which takes numerical values from 0 (maximum acidity) through 7 (neutral) to 14 (maximum alkalinity)
##### Appears in these related concepts:
pH indicator
An acid-base indicator.
##### Appears in these related concepts:
pKa
A quantitative measure of the strength of an acid in solution; a weak acid has a pKa value in the approximate range −2 to 12 in water and a strong acid has a pKa value of less than about −2.
##### Appears in these related concepts:
ratio
The relative magnitudes of two quantities (usually expressed as a quotient).
##### Appears in these related concepts:
solution
A homogeneous mixture, which may be liquid, gas or solid, formed by dissolving one or more substances.
##### Appears in these related concepts:
Solution
A homogeneous mixture, which may be liquid, gas or solid, formed by dissolving one or more substances.
##### Appears in these related concepts:
state
The physical property of matter as solid, liquid, gas or plasma
##### Appears in these related concepts:
temperature
A measure of cold or heat, often measurable with a thermometer.
##### Appears in these related concepts:
titration
the determination of the concentration of some substance in a solution by slowly adding measured amounts of some other substance (normally using a burette) until a reaction is shown to be complete -- for instance, by the color change of an indicator
##### Appears in these related concepts:
weak acid
one that dissociates incompletely, releasing only some of its hydrogen atoms into solution
##### Appears in these related concepts:
weak base
a proton acceptor that does not ionize fully in an aqueous solution