Titration is a laboratory technique used to determine the concentration of an unknown substance by reacting it with a solution of known concentration. A visual signal is necessary to indicate when the reaction has reached completion. This role is performed by a chemical indicator, a substance added to the reaction mixture to identify the moment the titration is finished.
Understanding Indicators
A chemical indicator serves as a visual alert in a chemical reaction, typically by undergoing a distinct color change. Its purpose is to signal that a particular point in the reaction has been reached. Indicators are substances that respond to alterations in their surrounding chemical environment. They do not permanently react with acids or bases, but change form to display different colors. This color change provides a clear, observable cue for the person performing the titration.
How Indicators Signal Change
Indicators function by changing their chemical structure in response to changes in the solution’s pH. Many common indicators are weak acids or bases that exist in two different forms, each with a distinct color. For example, in an acidic solution, an indicator might be in one colored form, but as the solution becomes more basic, it loses a proton and transforms into a different colored form. This shift is governed by the equilibrium between the two forms, sensitive to the concentration of hydrogen ions.
The point at which the indicator changes color is called the “endpoint” of the titration. This is distinct from the “equivalence point,” the theoretical point where the reactants have completely neutralized each other. Ideally, a suitable indicator’s endpoint should occur very close to the reaction’s equivalence point to ensure accurate results. The color transition occurs over a specific, narrow pH range, typically within about two pH units.
Choosing the Right Indicator
Selecting the appropriate indicator for a titration depends on the pH at which the equivalence point occurs. Each indicator has a characteristic pH range over which its color change is visible. This range must closely align with the pH of the solution at the equivalence point. For instance, if the equivalence point is expected to be acidic, an indicator that changes color in an acidic pH range should be chosen.
Choosing an indicator with a color change range far from the equivalence point would lead to inaccurate results, as the visual signal would appear either too early or too late. While indicators provide a visual approximation, they are precise enough for many laboratory applications. They function optimally in colorless solutions where the color change can be clearly observed.
Common Indicators You Might Encounter
Several indicators are commonly used in laboratory settings due to their reliable and distinct color changes. Phenolphthalein is a widely used indicator that remains colorless in acidic or neutral solutions and turns pink to fuchsia in basic conditions, typically changing color between pH 8.3 and 10.0. Methyl orange appears red in acidic solutions and changes to yellow in basic solutions, with its color transition occurring in the pH range of 3.1 to 4.4. Litmus, often encountered as litmus paper, turns red under acidic conditions and blue under basic conditions, showing a color change across a broader pH range of 4.5 to 8.3.