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 cadmium (Cd2 ) content using graphite furnace atomic absorption spectrometry (GFAAS). The chemist dried the clams at 95° C overnight and ground them in a scientific blender, resulting in ~ 50 g of homogenized dry weight. A representative 91.65 mg sample was taken from the ~ 50 g of dry material and dissolved in 100.00 mL of 0.1 M HCl to create a sample solution. Using the method of standard additions, the chemist prepared five standard solutions in 100.00-mL volumetric flasks, each containing 5.00-mL aliquots of the sample solution. Varying amounts of a 96.0 ppb (μg ·L-1) cadmium standard were added to each of the flasks, which were then brought to volume with 0.1 M HCl. The solutions were then examined for their Cd2 content using GFAAS, resulting in the following absorbance data.

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Determine the amount of Cd2 per gram of dry clam. Express your final result as milligrams of Cd2 per gram of dry clam

samp vol (ml)       Cd2+ standard vol (ml)                absorbance

5	0	0.080
5	2.50	0.119
5	5	0.163
5	7.50	0.200
5	10	0.241

Answer 1

Find out the concentration of the added standard using the dilution equation. Varying volumes of the standard were added and the total volume of the prepared solutions were 100.0 mL. Let us take 7.50 mL of the standard addition as an example.

Use the dilution equation.

(7.50 mL)*(96.0 ppb) = (100.0 mL)*C_S where C_S denotes the concentration of the added standard in the solution.

===> C_S = 7.2 ppb.

Fill in the table as below.

Volume of sample added (mL)	Volume of standard added (mL)	CS (ppb)	Absorbance
5.00	0.00	0.0	0.080
5.00	2.50	2.4	0.119
5.00	5.00	4.8	0.163
5.00	7.50	7.2	0.200
5.00	10.00	9.6	0.241

Plot the absorbance vs concentration of the added standard and extrapolate to zero.

Extrapolate to zero. Put y = 0 and get

0 = 59.523x – 4.7594

===> x = 4.7594/59.523 = 0.0799 ≈ 0.08

The concentration of original Cd²⁺ in the solution is 0.08 ppb. Note that 5.00 mL of the solution was used to prepare the solution for GFAAS. The dilution factor is (100 mL)/(5.0 mL) = 20. Therefore, the concentration of original Cd²⁺ = (0.08 ppb)*(20) = 1.6 ppb = 1.6µg/L.

We prepared a 100 mL solution so that the mass of Cd²⁺ so that the mass of Cd²⁺ present is (100 mL)*(1 L/1000 mL)*(1.6 µg/1 L) = 0.16 µg.

The total mass of the dry clam was 50 g; therefore, the mass of Cd²⁺ per gram of the clam =

(0.16 µg)/(50 g) = 0.0032 µg/g (ans).