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 both 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.

The Eu(III) standards added to the groundwater sample contained 0.181 ng/mL (ppb) of Eu(III). Calculate the concentration of Eu(III) in the groundwater sample.

The Tb(III) standards added to the groundwater sample contained 18.1 ng/mL (ppb) of Tb(III). Calculate the concentration of Tb(III) in the pond water sample.