Titration is a fundamental laboratory procedure used in chemistry to determine the precise concentration of a substance dissolved in a solution. This analytical technique involves gradually adding a solution of known concentration, called the titrant, to a measured volume of the unknown solution, referred to as the analyte. The process relies on a rapid, complete chemical reaction occurring between the two substances. The goal is to accurately measure the volume of the known solution required to completely react with the unknown substance, allowing chemists to calculate the analyte’s concentration.
The Theoretical Basis of Neutralization
The equivalence point (EP) is a theoretical concept representing the moment in a titration when the amount of titrant added is chemically equivalent to the amount of analyte initially present. This point is determined purely by the stoichiometry of the specific chemical reaction. In an acid-base reaction, this is the point where the moles of the added base perfectly match the moles of the acid being analyzed, according to the balanced chemical equation.
The EP signifies the completion of the reaction based on mole ratios. If the reaction is a simple one-to-one interaction, the moles of each reactant are equal at this point. If the mole ratio is different (e.g., one mole of a diprotic acid reacting with two moles of a base), the theoretical equivalence point reflects that specific ratio.
The solution at the equivalence point is not always neutral (pH 7). The pH at the EP depends on the strength of the acid and base involved. For a strong acid and strong base titration, the pH at the EP is 7.0 because the resulting salt does not react further with water.
When a weak acid is titrated with a strong base, the resulting salt reacts with water, causing the equivalence point to occur at a pH greater than 7. Conversely, titrating a weak base with a strong acid results in an equivalence point with a pH below 7. The EP is defined by the stoichiometric ratio, not by a specific pH value.
Identifying the Change in the Lab
The theoretical equivalence point cannot be directly observed in the laboratory. Chemists must rely on a physical signal called the endpoint to approximate its occurrence. The endpoint is the observable change in the solution that indicates the reaction is near completion, signaling when to stop adding the titrant. The goal is to choose a method that makes the endpoint occur as close as possible to the true equivalence point.
One common method uses a chemical pH indicator, a substance that changes color in response to a sudden shift in acidity or basicity. As the titration approaches the equivalence point, the pH changes very rapidly. The indicator is selected so its color transition range falls within this rapid pH change. For instance, phenolphthalein changes from colorless to pink at a pH slightly above 8, signaling the endpoint volume.
A more precise method involves using a pH meter to create a titration curve. This instrument measures the pH of the solution continuously as the titrant is added. Plotting the pH versus the volume of titrant yields a curve showing the region of the most rapid pH change.
The equivalence point is determined from the titration curve by finding the inflection point, the exact center of the steepest segment of the curve. This graphical method provides a more accurate determination than a visual indicator, as it is not dependent on a subjective color change. The data gathered from the pH meter allows for a highly accurate measurement of the volume required to reach the true stoichiometric point.
Determining Unknown Concentrations
The practical utility of the equivalence point lies in its role as a bridge between the measured experimental volume and the final calculated concentration of the unknown substance. Once the volume of titrant required to reach the endpoint is measured, this value is used in a series of steps to quantify the analyte. The first step involves using the known concentration and the measured volume of the titrant to calculate the total moles of titrant added.
Multiplying the titrant’s molarity by the measured volume in liters yields the moles of the known substance. At the equivalence point, the moles of the known titrant are stoichiometrically equivalent to the moles of the unknown analyte. This relationship is defined by the balanced chemical equation for the reaction.
The mole ratio from the balanced equation is then applied as a conversion factor to determine the moles of the analyte originally present. This ratio must be carefully incorporated, especially for reactions that are not one-to-one.
Finally, the concentration of the unknown substance is calculated by dividing the determined moles of the analyte by the initial, measured volume of the analyte solution. This process transforms the laboratory observation of the endpoint into a precise, quantitative measure of the unknown solution’s concentration.