A sample of vegetation weighing 6.250 g was ashed to destroy organic material, a

Question

A sample of vegetation weighing 6.250 g was ashed to destroy organic material, and the inorganic redidue was acid digested and diluted to 100mL in a vlumetric flask. A portion of this solution was aspirated into an air-acetylene flame and an absorbance value of 0.124 was obtained using the 283.1nm Pb line. Next, 0.100mL of a standard solution containing 55.0 micrograms of Pb per mL was added to 50mL of the sample solution. When this spiked sample was aspirated into the flame, the absorabance was found to be 0.180. Calculate the Pb content of the vegation sample in parts per million.

Explanation / Answer

Concentration of Pb std dilute solution = 55 ug/ml x 0.1 ml/50 ml = 0.11 ug/ml

The dilute sample of std. Pb gave an absorbance = 0.180

concentration of dilute vegetation sample = 0.11 x 0.124/0.180 = 0.076 ug/ml

Concentraion of Pb in 100 ml sample vegetation = 0.076 x 100 = 76 ppm

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