The selenium content in soil can be determined fluorometrically upon conversion

Question

The selenium content in soil can be determined fluorometrically upon conversion of the selenium to the fluorescent product shown below, followed by extraction in cyclohexane. Emission from the fluorescent product is observed at 518 nm. The selenium content in a 0.141 g soil sample was converted to the fluorescent product and extracted in 50.0 mL of cyclohexane. The method of standard additions was used to determine the selenium content in the soil sample. 3.00 mL of the cyclohexane solution were placed in a cuvette for fluorescence determination. Several small additions of a 2.72 mu g Se/mL standard solution containing the fluorescent product were added to the 3.00 mL solution in the cuvette as shown in the table below. The fluorescence was measured at 518 nm Determine the weight percent (wt percentage) of selenium in the soil sample and find the absolute uncertainty in the wt percentage Se. Report the weight percent and absolute uncertainty with three significant figures. Se wt percentage = Number percentage plusminus Number percentage

Explanation / Answer

Using,

Cx/(Cx + Cs) = Fx/(Fx + Fs)

with,

Cx = concentraton of unknown

Cs = concentration of standard = 2.72 ug/ml x 0.02 ml = 0.0544 ug

Fx = florescence intensity of unknown = 54.3

Fs = fuorescence intensity of standard

Taking values from the chart,

Cx/(Cx + 0.0544) = 54.3/84.5

84.5Cx = 54.3Cx + 2.954

Cx = 0.098 ug = 0.098 x 50 = 4.9 ug in 50 ml

wt% of Se in sample = (0.005/0.141) x 100 = 3.54%

Absolute uncertainty = 0.125%

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