As part of an investigation of Pb in biological ecosystems, some weeds are gathe

Question

As part of an investigation of Pb in biological ecosystems, some weeds are gathered along a highway where they have been exposed to emission from gasoline engines. A representative sample of the weeds weighing 6.250 g is digested. The final volume of the solution is 100.0 mL. A portion of this solution is aspirated into an air-acetylene flame, and a blank-subtracted absorbance of 0.125 plusminus 0.008 is measured at 283.31 nm. Next, 0.100 mL of a 55.0 mg/L Pb standard solution is added to 50.00 mL of the sample solution, which yields a blank-subtracted absorbance of 0.180 plusminus 0.005 upon aspiration. Which calibration strategy was used? What is this strategy recommended for? What is the concentration (and associated standard deviation) of Pb in the weed sample? What is the sensitivity (and associated standard deviation) of the method for Pb?

Explanation / Answer

a) The method of analysis used is standard addition method.

b) Concentration of Pb in the weed sample

moles of Pb standard = molarity x volume = 0.055 x 207.2 x 0.0001 = 1.14 x 10^-3 mols

Molarity of Pb standard in 50.1 ml (0.0501 L) = 0.0227 M

Absorbance = 0.180 +/- 0.005

weed sample absorbance = 0.125 +/- 0.008

absorbance due to standard Pb = 0.180 - 0.125 = 0.055

concentration of Pb is weed sample = 0.0227 x 0.125/0.055 = 0.0516 M

c) Sensitivity of the method

absorbance (std) + sample

maxima : 0.185

minima : 0.175

absorbance (sample)

maxima : 0.133

minima : 0.117

Abrobance due to std

maxima : 0.052

minima : 0.058

Concentration of weed sample

maxima : 0.058

minima : 0.046

so, the concentration of Pb in weed = 0.055 +/- 0.006

Sensitivity = +/- 0.006

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