Titration is a fundamental method in analytical chemistry used to determine the concentration of an unknown solution, known as the analyte. This technique relies on a controlled chemical reaction between the analyte and a solution of precisely known concentration. The substance with the known concentration is called the standard solution, and it acts as the essential reference point for the measurement. The accuracy of the final result is directly dependent on how accurately the concentration of the standard solution is known.
Defining the Standard Solution
A standard solution is a chemical solution whose concentration has been determined with a high degree of certainty and precision. Chemists prepare this solution by dissolving a measured mass of a very pure substance, the solute, into a specific, known volume of a solvent, typically water, using specialized glassware like a volumetric flask. The exact concentration is usually expressed in molarity, which is the number of moles of the solute present per liter of solution.
In the context of titration, the standard solution is often referred to as the titrant. The titrant is the solution slowly added from a glass tube called a burette to the solution of unknown concentration, the analyte, until the reaction between the two is complete. Because the concentration of the titrant is already known, measuring the exact volume required to complete the reaction provides the data needed for calculation.
Essential Properties of a Good Standard
The accuracy of a titration hinges on the quality of the substance used to prepare the standard solution. A suitable compound must possess a high level of chemical purity, ideally 99.9% or greater, to ensure that the weighed mass accurately reflects the amount of reactive substance. This purity is necessary because any contaminants would introduce error into the initial concentration calculation.
The compound must also exhibit a high degree of stability, meaning it should not readily react with components in the air or absorb moisture, a property known as hygroscopicity. If the substance absorbs water from the air, its mass changes, making the initial weight measurement inaccurate. Furthermore, a good standard compound should have a relatively high equivalent weight, which minimizes the percentage error that may arise from small inaccuracies in weighing.
Primary and Secondary Standard Solutions
Standard solutions are categorized into two types based on their preparation and stability. A primary standard solution is prepared directly by weighing a highly pure, stable chemical compound and dissolving it in a known volume of solvent. Substances like potassium hydrogen phthalate or anhydrous sodium carbonate are examples of primary standards because they are non-hygroscopic and very stable. The concentration of a primary standard is known immediately upon preparation because the initial mass is considered completely accurate.
A secondary standard solution is one whose concentration cannot be determined directly from the mass of the solute due to the substance being less pure, less stable, or highly reactive, such as sodium hydroxide. These solutions are often used because they are more practical for certain titrations. To accurately determine its concentration, the secondary standard must undergo a process called standardization, which involves titrating it against a previously prepared primary standard solution.
How Standard Solutions Are Used in Titration Calculations
The standard solution’s role is to provide a known quantity of reactant for the chemical reaction with the unknown analyte. As the standard solution is slowly added to the analyte, the reaction proceeds until it reaches the equivalence point. This point is where the exact stoichiometric amount of titrant has been added to fully react with the analyte. This point is typically identified visually by a color change from an added indicator.
The measured volume of the standard solution required to reach the equivalence point, known as the titre, is the most important piece of experimental data. By multiplying the known concentration (Molarity, M) of the standard solution by the measured volume (V), chemists calculate the moles of the standard solution used. Using the balanced chemical equation, this mole count is then directly converted to the moles of the unknown analyte present in the sample. Finally, dividing the calculated moles of the analyte by the initial volume of the analyte solution yields the unknown concentration.