Chloroform is an internal standard in the determination of the pesticide DDT in

Question

Chloroform is an internal standard in the determination of the pesticide DDT in a polarographic analysis in which each compound is reduced at an electrode surface. A mixture containing 0.350 mM chloroform and 0.650 mM DDT gave signals of 12.5 mu A for chloroform and 9.64 mu A for DDT. An unknown solution (10.0 mL) containing DDT was placed in a 100-mL Volumetric flask and 9 55 mu L of chloroform (FM 119.39, density = 484 g/mL) were added. After dilution to the mark with solvent, polarographic signals of 19.8 mu A and 5.3 mu A were observed for the chloroform and DDT, respectively. Find the concentration of DDT (mM) in the unknown. Report the final answer to 3 significant figures.

Explanation / Answer

Response factor (F) = (area/concentration)chloroform/(area/concentration)DDT

feeding the standard values,

response factor (F) = (12.5/0.350)/(9.64/0.650)

                               = 2.41

For the unknown solution,

concentration of CHCl3 in solution = 9.55 x 1.484/1000 x 0.1 x 119.39

                                                       = 0.0012 mM

Thus,

concentration of DDT in 100 ml solution = 5.3/[(19.8/0.0012)/2.41]

                                                                = 0.000774 mM

Concentration of DDT in the original 10 ml sample = 0.000774 x 10

                                                                                 = 0.00774 mM

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