Atomic absorption spectrometry was used with the method of standard additions to

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 = 0.042 Absorbance of sample = 0.350 Absorbance of sample plus addition = 0.752

A) What was the concentration of the cadmium in the waste stream sample?

B) 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.

C) Calculate the percent error caused by using water instead of the reagent blank.

Explanation / Answer

A) Given,

1000 microgram/mL of 10 microliter solution

10 microliter = 0.01 mL

So, If 1 mL of standard solution has 1000 microgram of Cd

then, 0.01 mL will have = 1000 x 0.01 / 1 = 10 microgram of Cd in 0.01 mL

this was added to 10 mL of solution

So, the total volume becomes = 10 + 0.01 = 10.01 mL

Now, since 0.01 mL of Cd standard solution has 10 microgram of Cd in it

then, 10.01 mL will have = 10/10.01 = 0.999 microgram of Cd from standard

  Absorbance recorded for,

Blank = 0.042               

Sample = 0.350       

So absorbance for sample = sample - blank = 0.350 - 0.042 = 0.308

sample + standard = 0.752   

So absorbance of sample + standard = 0.750 - 0.042 = 0.710

Absorbance for standard = (sample + standard) - sample = 0.71 - 0.308 = 0.402

Thus, 0.402 is the absorbance reading for 0.999 micrograms of Cd/mL

So, 0.308 will have = 0.308 x 0.999/0.402 = 0.7654 microgram/mL sample concentration

Thus the concentration of sample is 0.7654/mL

B) Now If the true blank has an absorbance reading of 0.084

the new corrected absorbance readings for sample and sample + standard becomes,

sample absorbance = 0.350 - 0.084 = 0.266

sample + standard absorbance = 0.752 - 0.084 = 0.668

So, the absorbance for 0.999 microgram/mL of Cd becomes = 0.668 - 0.266 = 0.402

that is, the sample with absorbance of 0.266 corresponds to = 0.266 x 0.999/0.402 = 0.6610 microgram/mL of Cd

thus, the new concentration of Cd in the sample becomes = 0.6610 micrograms/mL

C) % error of the concentrations of Cd in the two samples

= (0.7654 - 0.6610/0.6610) x 100 = 15.80 %

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