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What to consider during back-titration

May 18, 2020

Article

Titrations can be classified in various ways: by their chemical reaction (e.g., acid-base titration or redox titration), the indication method (e.g., potentiometric titration or photometric titration), and last but not least by their titration principle (direct titration or indirect titration). In this article, I want to elaborate on a specific titration principle – the back-titration – which is also called «residual titration». Learn more about when it is used and how you should calculate results when using the back-titration principle.

Figure 1. Reaction principle of a back-titration: Reagent B is added in excess to analyte A. After a defined waiting period which allows for the reaction between A and B, the excess of reagent B is titrated with titrant T.
Figure 1. Reaction principle of a back-titration: Reagent B is added in excess to analyte A. After a defined waiting period which allows for the reaction between A and B, the excess of reagent B is titrated with titrant T.

What is a back-titration?

In contrast to direct titrations, where analyte A directly reacts with titrant T, back-titrations are a subcategory of indirect titrations. Indirect titrations are used when, for example, no suitable sensor is available or the reaction is too slow for a practical direct titration.

During a back-titration, an exact volume of reagent B is added to the analyte A. Reagent B is usually a common titrant itself. The amount of reagent B is chosen in such a way that an excess remains after its interaction with analyte A. This excess is then titrated with titrant T. The amount of analyte A can then be determined from the difference between the added amount of reagent B and the remaining excess of reagent B.

As with any titration, both involved reactions must be quantitative, and stoichiometric factors involved for both reactions must be known.

When are back-titrations used?

Back titrations are mainly used in the following cases:

  • if the analyte is volatile (e.g., NH3) or an insoluble salt (e.g., Li2CO3)
  • if the reaction between analyte A and titrant T is too slow for a practical direct titration
  • if weak acid – weak base reactions are involved
  • when no suitable indication method is available for a direct titration