How do you perform a back titration?

To perform a back titration, you first add an excess of a standard solution to the analyte, then titrate the excess.

In more detail, back titration is a method used when the analyte is volatile or insoluble, or when a direct titration would result in a slow reaction. It involves two steps: the addition of an excess of a standard solution (also known as the reagent) to the analyte, and the titration of the remaining excess reagent.

Firstly, you need to add an excess of a known reagent to the analyte. This reagent should react with the analyte to produce a product. The amount of reagent you add should be more than enough to react completely with the analyte. This is why it's called an 'excess'. The reaction between the analyte and the reagent should be fast and complete.

Next, you need to determine how much of the reagent was left unreacted. This is done by titrating the remaining reagent with another reagent. This second reagent should be of a known concentration and should react with the first reagent, but not with the analyte or the product of the first reaction. The titration should be done carefully, with the solution being stirred constantly. The point at which all the excess reagent has reacted with the second reagent is called the end point. This is usually indicated by a colour change, due to the presence of an indicator.

By knowing the volume of the second reagent used, you can calculate the amount of the first reagent that was left unreacted after the first step. From this, you can then calculate the amount of the first reagent that reacted with the analyte, and hence the amount of analyte present. This is why it's called a 'back' titration - you're working backwards from the end point to find the amount of analyte.