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7. (20 points) Selenium (IV) in natural waters is determined by forming a complex with 2,3- diaminonaphthalene that can be extracted into toluene. Fluorescence is measured at 520 nm following its excitation at 380 nm. Calibration is achieved by the standard addition method. A 5 mL aliquot of a water sample is added to four 50.00 mL volumetric flasks. Various volumes (in mL) of a 50.0 nM selenium standard are then added to the four volumetric flasks containing the sample aliquots. Then 10.00 mL of a 0.1% (w/v) solution of 2,3-diaminonaphthalene in 0.1M HCl is added to each flask. The flasks are then diluted to volume with water. After allowing 2 hours for the solutions to react the contents of the volumetric flasks are transferred to an extraction funnel and extracted three times with 10.00 mL of toluene. The toluene extracts are adjusted to 50.00 mL and the fluorescence is measured. Given the following results what is the concentration of Se(IV) in the sample? (Yes, you need to fit the data. You can use excel or your calculator). Vs (ml) Fluorescence (Arbitrary unit) 2.0 812.2 4.0 1256.3 6.0 1700.5 8.0 2144.7