Calibration Standards

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 Preparing solutons for the calibration of an AA Spectrophotometer

You are required to use an Atomic Absorption Spectrophotometer (AAS) to measure the concentration of copper within a test solution. Previous measurements suggest that the concentration of this test solution is approximately 25 ppm. 

You will calibrate the AAS by preparing five standard 100 mL (cm3) solutions of known concentrations of copper. The solvent for the solution will be 5% nitric acid to match the acidified matrix of the test solution. Note that you do not want to produce larger volumes of your final standards because these will be in acidified solutions, and you do not want to prepare an unnecessary volume of acidified solvent. 

As part of the process for producing the standard solutions, you will prepare 1000 mL (= 1.0 dm3) of an initial ‘stock’ solution with an approximate copper concentration of about 1000 ppm which you will keep for later use. The reason for preparing a relatively strong stock solution is that a small amount of copper will leach into the container wall over time, but the effect on the concentration will be proportionally less for a more concentrated solution. 

You will use copper sulphate as an easy source of copper in this example, although an alternative method would be to dissolve copper metal directly in nitric acid. 

You will need to know the following performance specifications of the instruments being used.

  • Linear range of the AA Spectrophotometer for measuring copper: 0 to 5 ppm 
  • Accuracy of balance: +/- 0.5 mg

The following analysis leads you through the types of decisions/calculations that you will have to make, although the exact details of preferred options may depend on the exact situation being considered, available glassware, etc.

 

Decision / Calculation 1    Well done - you have correctly answered this question.

You must first calculate the mass of copper that must be present in 1000 mL (= 1.0 dm3) of solution to produce a copper concentration of 1000 ppm. Select the correct mass from the following options:

(A)  10.0 g
(B)    1.00 g
(C)    0.01 g
(D)    0.001 g 

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Option (B) is correct 

A concentration of1.0 ppm can be produced as follows:

1.0 µg of solute in each 1.0 mL (cm3) of solution gives a concentration of 1.0 ppm, or

1.0 mg of solute in 1000 mL (= 1.0 dm3) also gives a concentration of 1.0 ppm.

Hence

1.0 g (= 1000 mg) in 1000 mL gives a concentration of 1000 ppm. 

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You have calculated that it requires 1.0 g of copper in 1000 mL of solution to obtain a concentration of 1000 ppm. 

 

Decision / Calculation 2 

It is now necessary to calculate the mass of copper sulphate that will contain 1.0 g of copper (use the following relative atomic masses Cu:63.5, S:32.1, O:16.0 and H:1.0). Select the correct mass from the following options:

(A)   249.6 g
(B)  159.6 g
(C)     3.931 g
(D)    2.513 g

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