Question

Atomic absorption spectrometry was used with the method of standard additions to determine the concentration of cadmium in a sample of an industrial waste stream. For the addition, 10.0 μL of a 1000.0 μg/mL Cd standard was added to 10.0 mL of solution. The following data were obtained: Absorbance of reagent blank = 0.044 Absorbance of sample = 0.478 Absorbance of sample plus addition = 0.878 What was the concentration of the cadmium in the waste stream sample?Later, the analyst learned that the blank was not truly a reagent blank, but water. The absorbance of the actual reagent blank, was 0.097. Calculate the cadmium concentration using the new information of the blank.Calculate the percent error caused by using water instead of the reagent blank.

Answer 1

1) Absorbance after blanck correction for Sample = 0.478 - 0.044 = 0.434

Absorbonce afer blanck correction for sample + addition = 0.878 - 0.044 = 0.834

Difference in Absorbance = 0.400

Absorbance due to standard = 0.400

Mass of Cd in standard = (1000ug/1ml)×0.01ml= 10ug

Concentration of added Cd = (10ug/10.01ml)×1ml = 0.9990ug/ml

Absorbance due to added Concentration of Cd = 0.400

Therefore,

Concentration Cd in sample = (0.9990(ug/ml)/0.400)×0.434= 1.084ug/ml

2) New , reagent blank absorbance = 0.097

Sample absorbance after blanck correction = 0.478-0.097=0.381

Sample + standard absorbance after blank correction = 0.781

Difference = 0.400

Concentration due to Cd addition = 0.9990ug/ml

Concentration of Cd in sample = (0.9990(ug/ml)/0.400)×0.381 = 0.9515ug/ml

3) Percentage error

% error = |( Accepted value - experimental value)/Accepted value)|×100

= (0.1325/0.9515)×100

= 13.93%