Titration vs Back Titration
Titrations are technique widely used in analytical chemistry to determine acids, bases, oxidants, reductants, metal ions, and many other species.
What is Titration?
In a titration, a known chemical reaction takes place. Here, an analyte is reacted 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. In any titration, the point where the reaction is completed (the point of chemical equivalence) is called the end-point. End-point is detected by an indicator, which changes its color at the end-point. Or else a change in an instrumental response can also be used to identify the end-point. The instruments record responses of the solution, which 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.
There are some errors associated with titrations. The equivalence point in a titration is the point at which the added titrant is chemically equivalent completely to the analyte in the sample. However, this is a theoretical point, and we cannot exactly measure this experimentally. We experimentally observe the end-point. Ideally the end-point is not exactly equal to the equivalence point (titration error), but we try to minimize the gap between the two as much as possible. There can be human errors associated with this method. Therefore, to minimize these, often a titration is repeated at least thrice.
What is Back-Titration?
In a back-titration, excess amount of the standard titrant is added to the analyte. Then some amount of the standard titrant will react with the analyte and the excess of it can be determined by a back-titration. For example, the amount of phosphate in a sample can be determined by this method. When excess amount of silver nitrate is added to a phosphate sample, both will react to give silver phosphate solid. Then the excess of silver nitrate is titrated with potassium thiocyanate. Therefore, the total amount of added silver nitrate is equal to the amount of phosphate ion and the amount of thiocyanate used for the back-titration.
What is the difference between Titration and Back-Titration?
• In titration, amount of standard solution, which is chemically equivalent to the analyte amount, is added. In back titration, excess of the standard titrant is added to determine the analyte amount.
• Normally, in a titration, only one direct reaction is taking place, which is between the standard titrant and the analyte. In a back-titration, two chemical reactions are taking place. One is with the standard and analyte, and the other is with the excess-standard titrant and a standard solution.