Titration is a fundamental laboratory technique used in quantitative chemical analysis to precisely determine the concentration of an unknown substance, known as the analyte. This method involves reacting the analyte with a titrant, a reagent of precisely known concentration. By measuring the exact volume of the titrant required to complete the reaction, scientists can calculate the concentration of the unknown solution. This process is a common and accurate way to perform chemical measurements, such as determining the concentration of an acid or a base.
Essential Equipment and Preparation
A standard acid-base titration requires several specific pieces of glassware to ensure accurate volume measurements. The titrant is dispensed from a long, graduated glass tube called a burette, which is held vertically by a specialized clamp and stand. The analyte is held in an Erlenmeyer flask, which allows for swirling without spillage. A pipette is used to measure a precise, fixed volume of the analyte into the reaction flask before the titration begins.
Preparation for obtaining reliable results begins with cleaning all glassware using deionized water. The burette must be rinsed with a small amount of the titrant solution itself, ensuring that any residual water or contaminants are removed from the inner surfaces and the tip. After rinsing, the burette is filled with the titrant, usually with the aid of a small funnel, and secured to the stand. The stopcock must be opened briefly to allow a small amount of liquid to drain, which removes any trapped air bubbles from the tip of the burette.
Air bubbles must be removed because their volume would be incorrectly measured as dispensed titrant, introducing error into the calculation. Once the burette is properly filled and the tip is clear, the initial volume of the titrant is read at the bottom of the meniscus. This reading must be recorded to a high degree of precision, typically to two decimal places, before moving the flask underneath the burette. Finally, a few drops of a suitable chemical indicator are added to the analyte solution.
The Step-by-Step Titration Procedure
Once the initial volume has been recorded, the titration begins by carefully opening the stopcock to allow the titrant to flow into the analyte solution. Initially, the titrant can be added relatively quickly, provided the solution is constantly swirled to ensure thorough mixing of the two reactants. The chemical reaction occurs immediately upon contact. As the titrant is added, a temporary color change may be observed in the area where the drops land, which quickly disappears upon swirling.
The rate of addition must be slowed when the volume of titrant approaches the estimated amount needed to complete the reaction. This is often determined by a preliminary rough trial or through prior knowledge of the solutions. As the reaction nears completion, the temporary color change persists for longer periods before fading, signaling the approach of the endpoint. At this point, the stopcock should be adjusted so the titrant is added only drop by drop.
Control is required in this final stage to achieve high accuracy, often requiring the addition of half-drops. A half-drop is achieved by partially opening the stopcock to form a hanging drop on the tip of the burette. This hanging drop is then washed into the flask using a wash bottle containing deionized water, ensuring the drop is fully incorporated. The flask is continually swirled and monitored until the indicator’s color change is permanent and uniform throughout the solution.
Determining the Endpoint and Necessary Calculations
The moment the indicator undergoes a permanent color change marks the visual endpoint of the titration. The indicator is a substance chosen to change color over a narrow pH range that closely matches the theoretical equivalence point of the reaction. The equivalence point is the exact stage where the moles of titrant added are chemically equal to the moles of analyte present. The endpoint is an observable physical signal, while the equivalence point is a chemical condition.
Although the endpoint is what is observed, it is close to the equivalence point if the correct indicator is used. Once the permanent color change is achieved, the stopcock is closed, and the final volume reading from the burette is recorded with the same precision as the initial reading. The total volume of titrant delivered, known as the titre, is calculated by subtracting the initial volume reading from the final volume reading. This volume represents the amount of known concentration solution required to completely react with the unknown analyte.
This measured volume is used to calculate the concentration of the analyte. This calculation relies on the principles of stoichiometry and the known concentration of the titrant. For a simple acid-base reaction, the relationship is often expressed using the formula M1V1 = M2V2, where M is the molar concentration and V is the volume. By knowing the molarity and volume of the titrant and the initial volume of the analyte, the unknown molarity of the analyte can be determined.