Equivalence Point vs Endpoint
Titration is a technique widely used in analytical chemistry to determine acids, bases, oxidants, reductants, metal ions and many other species. In titration, a known chemical reaction takes place. Here, an analyte reacts with a standard reagent known as a titrant. An ideal standard solution used in titrations should have several properties such as,
• React rapidly with the analyte
• React completely with the analyte
• Undergo a selective stoichiometric reaction with the analyte
Sometimes a primary standard, which is a highly purified and stable solution, is used as a reference material in titrimetric methods. The quantity of the analyte can be determined if the volume or the mass of titrant, which is needed to react completely with the analyte is known. Experimentally, the titrant is in the burette and the analyte is added to the titration flask using a pipette. The reaction takes place in the titration flask. An indicator is used to detect the end point. Sometimes change in an instrumental response can be used to identify the end point. The instruments record responses of the solution that vary in a characteristic way throughout the titration. Such instruments are colorimeters, turbidimeters, conductivity meters, temperature monitors, etc. There are different types of titrations. “Volumetric titrimetry involves measuring the volume of a solution of known concentration that is needed to react essentially completely with the analyte.” In gravimetric titrimetry, mass of the reagent is measured instead of the volume. In coulometric titrimetry, the time required to complete the electrochemical reaction is measured.
In any titration, end point is the point where the indicator changes its color. Or else a change in an instrumental response can also be used to identify the end point. For example, HCl and NaOH reacts 1:1 and produce NaCl and water. For this titration, we can use phenolphthalein indicator, which has a pink color in the basic medium and turn to colorless in the acidic medium. If we put HCl in the titration flask and to it, if we add a drop of phenolphthalein, it will be colorless. During the titration, we can add NaOH from the burette and gradually, HCl and NaOH will react in the flask. If we take the same concentration of the two solutions, when we add an equal amount of NaOH to the flask, the solution in the flask will turn to a light pink color. This is the point where we stop the titration (end point). We consider, at this point the reaction is completed.
The equivalence point in a titration is the point at which the added titrant is chemically equivalent completely to the analyte in the sample. This is the point where the chemical reaction is completed stoichiometrically. Although we determine the end point from the indicator color change, it may not be the actual end point of the reaction. That is reaction is completed slightly before that point. At this equivalence point, medium is neutral. After adding an extra NaOH drop, medium will show the basic color of phenolphthalein, which we take as the end point.
What is the difference between Equivalence Point and End Point?
• The equivalence point in a titration is the point at which the added titrant is chemically equivalent completely to the analyte in the sample. End point is the point where the indicator changes its color.
• Equivalence point comes before the end point.
• To get the same equivalent point as the end point, pH of the indicator should match the pH at the equivalence.