Question

Trace amounts of rare elements are found within groundwater and are of interest to geochemists. Europium and terbium are lanthanide-series elements that can be measured from the intensity of their fluorescence emitted when a solution is illuminated with ultraviolet radiation. Certain organic compounds that bind Eu(III) and Tb(III) enhance the emission, and substances found in natural waters can decrease the emission. For that reason it is necessary to use standard additions to the sample to correct for such interference. The graph at the right shows the result of such an experiment in which the concentration of Eu(III) and Tb(III) was measured in a sample of groundwater. In each case 10.00 mL of sample solution and 15.00 mL of of organic additive were placed in 50-mL volumetric flasks. Eu(III) standards (0, 5.00, 10.00, 15.00, and 20.00 mL) were added and the flasks were diluted to 50.0 mL with water. Eu(III) analysis The Eu(III) standards added to the groundwater sample contained 0.144 ng/mL (ppb) of Eu(III). Calculate the concentration of Eu(III) in the groundwater sample. Tb(III) analysis The Tb(III) standards added to the groundwater sample contained 14.4 ng/mL (ppb) of Tb(III). Calculate the concentration of Tb(III) in the groundwater sample.

Answer 1

The standard additi on equati on is as follows Here. [X] is the initial concentration of analyte, [S, is the initial concentration of standard, I is signal for analyte,Itx signal for standard and analyte,Vo is the initi al volume,V, is the added stan dard volume, and Vis the total volume The given straight line is a graph of Ion the y-axis versus S4 on the x-axis The right side of the above equation is zero when Si | 「一1 Added volume of standard (V,) is -23 mL(from graph) Initial volume of the sample (%) is 1000mL Initi al concentrati on of st an dard ([S],)0.144 ng/mL =-0. 144 ng/mL ×1 23.0 mL 10.00mL -0.33 ng/mL Hence the concentration of Eu (Ш) is 0.33 ng/ml. Added volume of standard (V,) is -9.85 mL(from graph) Initial volume of the sample (%) is 1000mL initial concentrati on of standard (S) = 144 ng/mL -9.85 mL 风=- 14.4 ng/mL × 1100RE = 1418ng/mL Hence the concentration of Tb II)is 14.18 ng/mL