An environmental chemist working for the Environmental Protection Agency (EPA) w

Question

An environmental chemist working for the Environmental Protection Agency (EPA) was directed to collect razor clams from a heavily-contaminated river superfund site and analyze them for their Cd^2+ content using graphite furnace atomic absorption spectrometry (GFAAS). The chemist dried the dams at 95 degree C overnight and ground them in a scientific blender, resulting in approximately 50 g of homogenized dry weight. A representative 72.55 mg sample was taken from the approximately 50 g of dry material and dissolved in 100.0 mL of 0.1 M HCI to create a sample solution. Using the method of standard additions, the chemist prepared five standard solutions in 100.0-mL volumetric flasks, each containing 5.00-mL aliquots of the sample solution. Varying amounts of a 90.0 ppb (mu g/L) Cd^2+ standard were added to each of the flasks, which were then brought to volume with 0.1 M HCI. The Cd^2+ content of the solutions was then analyzed using GFAAS, resulting in the following absorbance data. Determine the amount of Cd^2+ per gram of dry clam. Express your final result as milligrams of Cd^+ per gram of dry clam.

Explanation / Answer

concentration of standard in solution

with 2.5 ml standard

= 2.5 ml x 90 ug/L/100 ml

= 2.25 ug/L

Using,

Cx/(Cx + Cs) = Ax/(Ax + As)

Cx = Cd2+ concentration of unknown sample of clam

Cx/(Cx + 2.25) = 0.08/0.119

0.119Cx = 0.08Cx + 0.18

Cx = 4.615 ug/L

concentration of Cd2+ in 5 ml sample = 4.615 ug

So, in 100 ml diluted clam sample = 92.3 ug

Cd2+ in 50 g sample = 92.3 ug x 50/0.07255 = 63611.30 ug = 0.064 g

mg Cd2+/g clam = 64 mg/50 g = 1.28 mg Cd2+/g clam

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