The selenium content in a 0.145 g soil sample was converted to the fluorescent p
ID: 854480 • Letter: T
Question
The selenium content in a 0.145 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 2.76 ?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%) of selenium in the soil sample and find the absolute uncertainty in the wt% Se. Report the weight percent and absolute uncertainty with three significant figures.
Hints:
Plot the corrected fluorescence intensity on the y-axis as a function of the final Se standard concentration after each addition, to construct a standard addition plot. The corrected fluorescence intensity is calculated using the following equation,
where Is+x is the measured intensity, V is the final volume of the solution, and V0 is the initial volume of the solution. Remember to convert the volume of Se standard added to the solution from microliters to milliliters. To determine the final concentration, [Se]f, of the Se standard after each addition use the following equation,
where [Se]i is the initial concentration of the standard Se solution (2.76 ?g/mL) and Vs is the volume of standard added. Perform linear least squares analysis to determine the equation of the best fit line and use this to determine the x-intercept by setting y=0. The x-intercept is equal to the concentration of Se in the unknown solution.
The absolute uncertainty of the weight percent is the product of the weight percent and the relative uncertainty of the x-intercept. The relative uncertainty of the x-intercept is determined by dividing the standard deviation of the x-intercept (Sx-int) by the x-intercept. To determine Sx-int use the following equation,
where Sy is the standard deviation of the y values, m is the slope, n is the number of measurements, y-bar is mean value of the y values, Xi is an individual x value, and x-bar is the mean of the x values.
The selenium content in a 0.145 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 2.76 ?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. where Sy is the standard deviation of the y values, m is the slope, n is the number of measurements, y-bar is mean value of the y values, Xiis an individual x value, and x-bar is the mean of the x values. where [Se]i is the initial concentration of the standard Se solution (2.76 ?g/mL) and Vs is the volume of standard added. Perform linear least squares analysis to determine the equation of the best fit line and use this to determine the x-intercept by setting y=0. The x-intercept is equal to the concentration of Se in the unknown solution. The absolute uncertainty of the weight percent is the product of the weight percent and the relative uncertainty of the x-intercept. The relative uncertainty of the x-intercept is determined by dividing the standard deviation of the x-intercept (Sx-int) by the x-intercept. To determine Sx-intuse the following equation, where Is+x is the measured intensity, V is the final volume of the solution, and V0 is the initial volume of the solution. Remember to convert the volume of Se standard added to the solution from microliters to milliliters. To determine the final concentration, [Se]f, of the Se standard after each addition use the following equation, Determine the weight percent (wt%) of selenium in the soil sample and find the absolute uncertainty in the wt% Se. Report the weight percent and absolute uncertainty with three significant figures. Hints: Plot the corrected fluorescence intensity on the y-axis as a function of the final Se standard concentration after each addition, to construct a standard addition plot. The corrected fluorescence intensity is calculated using the following equation,Explanation / Answer
0.0015%