Back titration is an analytical chemistry technique which allows the user to find the concentration of a reactant of unknown concentration by reacting it with an excess volume of another reactant of known concentration.
The resulting mixture is then titrated back, taking into account the molarity of the excess which was added. Back titrations can be used for many reasons, including: when the sample is not soluble in water, when the sample contains impurities that interfere with forward titration, or when the end-point is more easily identified than in forward titration.
Consider using titration to measure the amount of aspirin in a solution. Using titration it would be difficult to identify the end point because aspirin is a weak acid and reactions may proceed slowly. Using back titration the end-point is more easily recognised in this reaction, as it is a reaction between a strong base and a strong acid.
This type of reaction occurs at a high rate and thus produces an end-point which is abrupt and easily seen. The titration curve for a strong acid with a strong alkali shows that the equivalence point occurs at pH 7.
This means that the indicator phenolphthalein can be used. The end-point will be seen when the pink solution produced by the adding of phenolphthalein fades to colourless. The first stage of this reaction is that of alkaline hydrolysis. This involves reacting the aspirin solution with a measured amount of sodium hydroxide ; an amount that will exceed the amount of aspirin present. Because the hydrolysis reaction occurs at a very low rate at room temperature it will be heated to increase the reaction rate.
The second stage then involves back titration of the hydrolysed sodium hydroxide solution with hydrochloric acid. Monograph: Complexometric chelatometric Titrations. AB Analysis of edible fats and oils. How is a back-titration performed? A back titration is performed according to the following general principle: Add reagent B in excess to analyte A.
Allow reagent B to react with analyte A. This might require a certain waiting time or even refluxing e. Titration of remaining excess of reagent B with titrant T. Figure 2. Example of a Titrator equipped with an additional buret for the addition of reagent B. Back-titration with blank Titer determination of titrant T Blank determination back-titration omitting sample Back-titration of sample.
Back-titration with standardizations Titer determination of titrant T Titer determination of reagent B Back-titration of sample. What to consider when standardizing titrant. How to calculate the result for a back-titration? The reaction ratio can be determined in the following manner: Reaction equation between A and B Reaction equation between B and T Multiplication of the two reaction quotients.
Example 1. Reaction ratio:. Example 2. To summarize: Back-titrations are not so different from regular titrations, and the same general principles apply. The following points are necessary for a back-titration: Know the stoichiometric reactions between your analyte and reagent B , as well as between reagent B and titrant T.
Know the exact concentration of your titrant T. Know the exact concentration of your reagent B , or carry out a blank determination. Use appropriate titration parameters depending on your analysis. How to transfer manual titration to autotitration. Share this: Twitter Facebook LinkedIn. Like this: Like Loading Leave a Reply Cancel reply. Subscribe via email: Email Address Subscribe now. Looking for Applications? Subjects: Science and technology — Chemistry.
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