Question

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. The selenium content in a 0.194 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 1.76 ug 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. Fluorescence intensity (arbitrary units) 54.3 Total added Se standard volume (uL) 0.00 20.0 40.0 60.0 80.0 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 wt% = Se wt% = % +

Answer 1

Solulion: OL Sample vol Vol- std se Std. [se] Total vol,me [se] in aliqua El S.NO VOCAL) VICOL) c. reg mL va = (votvi) C2=Cavilva) ug 3.00 0.0000 | 2.78 3.0000 0.0000 54.300 3.00 0.0200 1:51 3.0200 00100 64.500 3 3.00 10.0400 1.51 3.0400 0.0199_ 76.700 3.00 0.0600 1.51 3.0600 0.0296 91.100 5 ann 300ith 0.0800 1.51 3.0800 0.0392 108-300 Cascanner

120.000 100.000 80.000 =1371.62+51.905 60.000 R = 0.9881 40.000 20.000 0.0004 b.oooo oooo 0.0100 CS Scanned witc 0.0150 0.0200 0.250 0.0300 0.0B50 0.0400 0.04500 ). [se], leglo CamScannel

stepl. Determine [Sej in 3.0mL ON - Spiked aliquot Trendine equalion is: Slope 1371.61 51.9053 intercept sa slope be 86.9152 86 2.0967 Ay intercept given: the final volume in un - Skiped aliquots 3 30 mL intercept regmi Now Slope [se] in 3.0mL = OF UN- Skiped aliquot 51.90533 2.0967 1371.6076 86-91520 15 lg ml - 0.0378 + 0.0028 elgoiz! Stepa Deleomine total se content in 50.0 mL extract Total se content in 500 ml eatoact-- [se] in cen- Spiked aliquot xVOJ. OF extract [0.03785 0.0028] legmi'x60.com - [1.8921 + 0.1422] leg Scanner CamScanner

Step 30 % wt of se giveon, Mass Of Soil Sample = 0.2009 = 200000. Olla % (w/w) se mass of Se in ug T x 100 mass of soil in ug 11.8921 – 0-1422 llg Sell 200000.0 XOO lg [0.0009 + 0.0001] 0.00095 % +0.0000 71%. CS Scanned with CamScanner