Back titration is performed if the analyte to be determined does not appreciably react with the standard solution of known concentration. It is also considered valuable in case the analyte is an insoluble salt or if it is volatile in nature. It is a type of indirect titration, the reverse of a direct titration experiment. Continue reading to learn more about back titrations, how they are performed and where are they important.
What is back titration- Definition and Working Principle
In the back titration method, the unknown analyte concentration is determined by reacting it with the excess of reagent A of known concentration. The reagent left unreacted after all the analyte gets consumed is consequently titrated with reagent B. Once the excess concentration of the first reagent is determined, it can therefore be used to find the unknown analyte concentration in the first instance via stoichiometric calculations.
You can better understand this concept with the help of an example given below.
How to perform a back titration
The magnesium (Mg) metal does not react with an aqueous solution of sodium hydroxide (NaOH) directly. Therefore, the Mg concentration present in an unknown sample cannot be determined via direct titration with NaOH. So, the following step-by-step guide can be used to find this Mg concentration using hydrochloric acid (HCl) en route back titration.
In this example:
- Analyte or sample (S): Mg
- Reagent A: HCl
- Reagent B: NaOH
Step I: A known mass of the sample is reacted with an excess of HCl solution of known concentration.
- 1.5 grams of Mg sample is accurately weighed using an analytical mass balance.
- 100 mL of 0.5 M HCl solution is added to the beaker containing the sample.
- An exothermic reaction occurs between Mg and HCl to yield MgCl2 and H2.
- Hydrogen gas is liberated from the solution.
- As an acidic pH is obtained in the reaction mixture, the addition of HCl is stopped.
- This ensures that HCl is present in an excess amount in the reaction mixture at this point.
Step II: Unreacted HCl is titrated with a standard NaOH solution.
- 10 mL from the above reaction mixture is pipetted into a conical titration flask.
- A few drops of an acid-base indicator, such as phenolphthalein, are added.
- This reaction mixture is then gently swirled and titrated with 0.25 M NaOH solution from the burette.
- This stage is an example of the regular acid-base titration experiment.
- The dropwise addition of NaOH from the burette into the titration mixture leads to an acid-base neutralization reaction between HCl and NaOH.
- The indicator changes color from colorless to light pink as the endpoint is reached.
- The volume of NaOH used is recorded at this point.
Step III: Stoichiometric calculations are performed to determine the amount of HCl present in the titration flask (i.e., HCl left unreacted from step I).
Step IV: The amount of HCl unreacted is subtracted from the original amount of HCl added in step I to determine how much HCl reacted with magnesium.
Step V: The concentration and amount of Mg present in the sample are determined.
Why are back titrations important – Applications
- Back titrations are specifically helpful in Iodometry.
- Back titration helps eliminate the impurities present in a sample that may otherwise lead to misleading results in a direct titration experiment.
- A weak acid cannot be titrated with a weak base directly; therefore, back titrations can be used to make that possible.
- The unknown concentration of solid analytes (such as that in the example above) can be more conveniently determined using back titration.
- The back titration principle is also sometimes used in complexometric titration and in precipitation titration.
Revise your concepts on the main difference between direct and back titrations.