Atomic absorption spectrometry was used with the method of standard additions to
ID: 887160 • Letter: A
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 L of a 1000.0 g/mL Cd standard was added to 10.0 mL of solution. The following data were obtained:
Absorbance of reagent blank = 0.028
Absorbance of sample = 0.359
Absorbance of sample plus addition = 0.824
[1] What was the concentration of the cadmium in the waste stream sample?
[2] Later, the analyst learned that the blank was not truly a reagent blank, but water. The absorbance of the actual reagent blank, was 0.084. Calculate the cadmium concentration using the new information of the blank.
[3] Calculate the percent error caused by using water instead of the reagent blank.
Explanation / Answer
1)
Absorbance of reagent blank = 0.028
Absorbance of sample = 0.359
Actual absorbance of sample = 0.359-0.028 = 0.303
Absorbance of sample plus addition = 0.824
Actual absorbance of sample plus addition = 0.824-0.028 = 0.796
Actual absorbance of standard addition = 0.796-0.303 = 0.493
Concentartion = Absorbance of sample x standard volume x standard concentartion /Absorbance of standard x
sample volum
= 0.303 x 10x 10-6 L x 1000 g/mL / 0.493 x 10 x 10-3L
= 0.614 g/mL
2) If the absorbance of the actual reagent blank, was 0.084
Then
Actual absorbance of sample = 0.359-0.084 = 0.275
Actual absorbance of sample plus addition = 0.824-0.084 = 0.74
Actual absorbance of standard addition = 0.74-0.275 = 0.465
Concentartion = 0.275 x 10x 10-6 L x 1000 g/mL / 0.465 x 10 x 10-3L
= 0.591 g/mL
3) percent error caused by using water instead of the reagent blank
(0.591-0.614)/0.591 = -0.039%
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