Why is it not a good idea to use a solution with very high OD in spectrophotomet

Question

Why is it not a good idea to use a solution with very high OD in spectrophotometric determinations? Since the OD is directly proportional to the concentration of the substance that is responsible for the absorbency, what should be the OD of the most dilute solution in this is responsible for the absorbance, what should be the OD of the most dilute solution in this exercise, based on the OD of the original stock solution? Which appears to be more accurate, the diluted solution obtained serially or the diluted solution prepared directly from the stock solution in one dilution? Explain in detail how you would prepare only 25 ml of 0.020 M NaCl(aq) from pure solid sodium chloride and water? In detail, explain how you would prepare only 25 ml of 0.020 M NaCI(aq) from a 0.10 M stock solution and water? Which method of preparing the 0.020 M NaCI(aq) is likely to be more accurate? Why?

Explanation / Answer

3. Solution with very high OD will not follow the beer's law as all of the light will be absorbed by the sample and none will be transmitted. Thus this solution is not good for experiment.

5. The diluted solution obtained serially would give more accurate result than the solution prepared directly from the stock solution. the error in preparaing the dilute solution from stock is higher.

6. To prepare a 25 ml solution of 0.02 M NaCl in water

mass of NaCl needed = 0.02 M x 58.44 g/mol x 0.025 L

                                    = 0.03 g

So dissolve 0.03 g NaCl in 25 ml water to get a 0.02 M solution

7. Dilution method

stock = 0.1 M

Volume of stock solution needed to prepare 25 ml of 0.02 M NaCl solution = 0.02 M x 25 ml/0.1 M = 5 ml

So 5 ml of 0.1 M stock solution diluted with 20 ml water would give 25 ml of 0.02 M NaCl solution

8. The method described in 7. from stock solution dilution would be more accurate to prepare 0.02 M NaCl solution. As measuring accurately the solid NaCl and dissolution would have more error than the dilution method described in 7.

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