Question

During the spectrophotometric analysis of an unknown, the absorbance of a solution of unknown concentration is measured, and the value is higher than the highest standard used to make the standard curve. What should be done to make sure that the standard curve can be used to determine the concentration of the unknown?

A) Dilute the unknown so that it will have an absorbance within the standard curve. Once the diluted unknown concentration is determined, the full strength concentration can be calculated if the dilution process is recorded. Beer's law only applies to dilute solutions, so diluting the unknown is better than making new standards.

B) A new set of standard solutions should be made to accommodate the unknown solution concentration. Beer's law is linear even at high concentrations, but a new concentration curve should be made to illustrate this point. Even though it is more work, making new standard solutions is a better method of obtaining correct results.

C) There is no need to do anything to the unknown solution. Beer's law describes the relationship between absorbance and concentration. This relationship is linear even at high concentrations, so the standard curve equation will apply for any concentration of that specific solution.

D) Pour a specified amount of the unknown solution into each of the standard curve solutions. This process will increase the absorbance of the solutions, to the point that the unknown solution's absorbance will fit within the standard curve appropriately.

Answer 1

Here, the concentration of the unknown is higher than the highest standard used to make the standard curve. Therefore, the unknown sample can be diluted so that it’ll fit the standard curve. Once the concentration of the unknown is determined, the concentration of the original can be determined by using the formula, C_iV_i = C_fV_f. Where C_i and V_i are the initial concentration and volume and C_f and V_f are the concentration and volume of the diluted sample. Also, Beer’s law holds good for dilute samples only.

Therefore, the correct answer is

Option A) Dilute the unknown so that it will have an absorbance within the standard curve. Once the diluted unknown concentration is determined, the full strength concentration can be calculated if the dilution process is recorded. Beer's law only applies to dilute solutions, so diluting the unknown is better than making new standards.