Defining Equivalence Point vs. Endpoint
In a chemical titration, the equivalence point is the theoretical point where the moles of the added titrant are stoichiometrically equal to the moles of the analyte in the sample. In contrast, the endpoint is the point in the titration where a physical change, such as a color change from an indicator, is observed, signaling that the reaction is complete.
Section 2: Theoretical Goal vs. Practical Observation
The equivalence point is a calculated, ideal value based on the chemical equation's stoichiometry; it is the true point of neutralization. The endpoint is what is physically measured in the lab. The primary goal of a successful titration is to select an indicator or method where the observed endpoint is as close as possible to the theoretical equivalence point.
Section 3: A Practical Example
Consider titrating a strong acid like hydrochloric acid (HCl) with a strong base like sodium hydroxide (NaOH), using phenolphthalein as an indicator. The equivalence point occurs precisely at pH 7. However, the phenolphthalein indicator changes color from colorless to pink at a pH of about 8.2. This color change is the endpoint. The small volume of extra NaOH needed to go from pH 7 to pH 8.2 represents the difference between the equivalence point and the endpoint.
Section 4: Why the Distinction Matters
Understanding this difference is crucial for accuracy in analytical chemistry. The discrepancy between the endpoint and the equivalence point leads to a 'titration error'. Choosing the correct indicator for a specific reaction is essential to minimize this error and ensure that the experimental results accurately reflect the concentration of the analyte.