Department of Chemistry

Stage 2 Chemistry Social Relevance Projects.

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Determination of metal cation levels in water and/or soil samples by Atomic Absorption Spectroscopy.


Introduction

 

Atomic line absorption spectra are used for analysing various metals. Atomic absorption spectrophotometry ("AAS") offers sensitivity, selectivity and simplicity and was developed by CSIRO Melbourne scientists around 1955. "AAS" is an important tool in analytical laboratories. Using this technique, samples of water and soil can be tested for various metal ion levels. Metals commonly analysed include sodium, potassium, magnesium, zinc, chromium, mercury, cobalt, copper, cadmium, iron and arsenic.


Standard solutions

Concentrations of the metal ion standard solutions need to cover a range of 0 to 25 mg/L (ie. 0 to 25 ppm). Some metal ion stock solutions (250 mg/L) will already have been prepared for you and simply require dilution to the appropriate final concentrations. Other metal ion stock solutions you will need to prepare yourself.

Think about...

How many solutions in the 0 to 25 ppm range do you think you will need in order to construct a meaningful calibration curve?

How will you prepare these solutions given a 250 ppm stock solution and standard volumetric glassware?


Determining metal ion concentration in water samples

The atomic absorption spectrophotometer you will be using is semi-automated. Consult the manual for settings and instructions on using the apparatus. Before proceeding with your analysis consult a laboratory demonstrator. All data should be reported as ppm metal (ie. milligrams metal per litre). Samples should be filtered prior to use. Aspirate the sample into the flame. Some samples may report a metal concentration "OVER RANGE"; these should be diluted appropriately and re-measured.

The spectrometer uses a different lamp for each metal to be analysed, so complete the analysis of one metal (calibration curve and duplicate sample measurements) before commencing analysis of another metal. The same solution can be used to analyse for a number of metal ions.


Determining metal ion concentration in soil samples

The atomic absorption spectrophotometer you will be using is semi-automated. Consult the manual for settings and instructions on using the apparatus. Before proceeding with your analysis consult a laboratory demonstrator. All data should be reported as ppm metal per dry weight of material. Soil samples must be dried in an oven at 105° prior to analysis. A sample size of 2 grams is generally appropriate. Samples should be well mixed before weighing to make sure they are homogenous. Extract each soil sample with 20 mL of 0.01 M ethylenediaminetetraacetic acid (EDTA) solution by shaking in plastic centrifuge tubes for 15-20 minutes. Centrifuge the samples then decant the supernatant into a clean test tube. Aspirate the sample into the flame. Some samples may report a metal concentration "OVER RANGE"; these should be diluted appropriately and re-measured.

The spectrometer uses a different lamp for each metal to be analysed, so complete the analysis of one metal (calibration curve and duplicate sample measurements) before commencing analysis of another metal. The same solution can be used to analyse for a number of metal ions.


References

Harris, D.C. "Quantitative Chemical Analysis; 5th Edition"; W.H. Freeman and Company: New York..


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