Do not worry about the portion that says (show work). Only the section before it

Question

Do not worry about the portion that says (show work). Only the section before it. Thank you for the help! Consider how you will prepare all the caffeine standard solutions. Think about the glassware, balances, and sources of errors. Weigh out 5 different caffeine amounts and dilute to the appropriate volume? a. b. Make a standard stock solution and use a serial dilution technique? Make a standard stock solution and use systematic (parallel) dilutions? Identify one of the above options and explain why it will give the best accuracy/precision, minimize materials/waste, etc. Show explicit work for how to generate all five solutions you c.

Explanation / Answer

Ans. Correct option- c. Make a standard stock solution and use systemic (parallel) dilution.

For spectrophotometric estimation, the standard solutions are prepared using systematic dilutions. In the procedure, a standard stock solution is first prepared using standard volumetric flask.

Then certain gradual volumes of the stock solution (say, 0.00, 0.04, 0.08, 0.12, 0.16, 0.20 mL) are transferred to different tubes. The final volume in all aliquots is made up to a fixed volume using distilled water and reagents as applicable. These solutions prepared from standard solution are called standard aliquots.

The concertation of aliquots and their respective absorbance are used to plot the standard graph.

# Advantage of systematic dilution method:

I. The standard solution is needed to be prepared only once.

II. The concertation of aliquots usually increase or decrease in a small, gradual fashion- say, 2 times, 4 times, 6 times, 8 times, etc.

III. The gradual change in concertation in small factors (2times, 4times, etc.) is useful because most solutes follow Beer-Lambert’s law up to a certain range of concentrations.

IV. The volume of standard aliquots can be kept as small as required minimal to be present in the cuvette to be analyzed by spectrophotometer. That is, if the instrument requires minimum 0.5 mL solution to be present in the cuvette for the absorption to be recorded, a volume of 1.0 mL for the aliquots sufficient. So, there is no need to making more than 1.0 mL aliquots, if feasible.

It thus minimizes the need of preparing more volume of solutions and wasting all the resources (deionized water, glassware, etc.) required to make standard aliquots.

# Option B. Serial dilution method: Serial dilution method is preferred to prepare successively, more diluted solutions- say, 10 times dilutes, 100 times dilutes, 1000 times dilutes, etc.

Note that “most solutes follow Beer-Lambert’s law up to a certain range of concentrations”, serial dilution method generally is not suitable because it decrease the concertation very rapidly in the successive dilutions. Such rapid dilutions will cause the graph deviate from Beer-Lambert’s law.

# Option A. Weighing 5 different amounts of caffeine to make respective standard aliquots would increase the relative % error. So, it’s also not suitable. Moreover, the process is extremely tedious and error prone due to five measuring events. This method is never preferred for spectrophometric analysis.

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