Question 5 of 7 >

The selenium content in soil can be determined fluorometrically upon conversion of

the selenium to the fluorescent product shown, followed by extraction in cyclohexane.

Emission from the fluorescent product is observed at 518 nm.

D Macmillan Learning

The selenium content in a 0.175 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. Initially, 3.00 mL of the cyclohexane solution were placed in a cuvette for fluorescence determination. Several small

additions of a 2.01 µg Se/mL standard solution containing the fluorescent product were added to the 3.00 mL solution in the

cuvette. The fluorescence was measured at 518 nm. The results are shown in the table.

Total added Se standard

volume (µL)

0.00

20.0

40.0

60.0

80.0

Se wt% =

Fluorescence intensity

(arbitrary units)

54.3

64.5

76.7

91.1

108.3

Determine the weight percent (wt%) of selenium in the soil sample and find the absolute uncertainty in the weight percent of

selenium. Report the weight percent and absolute uncertainty with three significant figures.

%

±

Se

%/n