You analyze a sample of rice for traces of arsenic. The As content can help you

Question

You analyze a sample of rice for traces of arsenic. The As content can help you find the origin of the rice. You’re given a 155 g sample of rice and you remove 2.50 g, pulverize it with a mortal and pestle, and dissolve it in 10 mL of a 50/50 mixture concentrated nitric acid and sulfuric acids. After dissolution, you take the sample to dryness on a hot plate and then add 10% HNO3 to a volume of 25.0 mL. This sample is stored at 4°C until analysis. The stored sample is removed from the refrigerator and 150 L are removed and diluted to 10.0 mL, again with 10% HNO3. When that diluted sample is analyzed by AAS the As absorbance is 0.351. If LLS line for your calibration, plotting ppm As versus absorbance, was y = 98.5x + 0.022, what was the concentration of As in the rice in ppb?

Explanation / Answer

Linear least square(LLs) line for As sample is given by y = 98.5x + 0.022 for absorbance vs ppm(concentration)

AAS(atomic absorption spectrometry) analyis of the sample gives Absorbance=0.351

so Conc of As can be calculated by putting this value in the LLs equation

0.351=98.5x+0.022

x=0.351-0.022/98.5=0.00334 1mg/L or ppm =0.00334 *10^3 ug/L=3.34ug/L=3.3 ppb

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