Answer to Question #343399 in General Chemistry for Jiin

Question

Any remaining solid impurities are removed by filtration before continuing with the analysis. In a typical total analysis method, the procedure might read

After digesting the sample in a beaker, remove any solid impurities by passing the solution containing the analyte through filter paper, collecting the filtrate in a clean Erlenmeyer flask. Rinse the beaker with several small portions of solvent, passing these rinsings through the filter paper and collecting them in the same Erlenmeyer flask. Finally, rinse the filter paper with several portions of solvent, collecting the rinsings in the same Erlenmeyer flask.

For a typical concentration method, however, the procedure might state

After digesting the sample in a beaker, remove any solid impurities by filtering a portion of the solution containing the analyte. Collect and discard the first several mL of filtrate before collecting a sample of approximately 5 mL for further analysis.

Explain why these two procedures are different.

Answer 1

In the first case, we collect the entire filtrate in order to transfer the solute quantitatively. In the second case, we need to investigate only the concentration of the substance, for which a small sample is sufficient. Therefore, we wash the filter with 5 ml of solution so that no substances that may be present in the filter get into the sample.