Chem 275Dr. Ng
POST LAB QUESTIONS ( 10 possible points)
Please show all your calculations. Answers without calculations will not be given any credit.
Determination of Iron in an Unknown Sample by Atomic Absorption Spectrometry
1. Explain why a continuous light source is not used in an atomic absorptionspectrophotometer. (1 pt)
2. .Describe how a hollow cathode lamp works. Why is a different lamp generally needed for each element being measured? (2 pts)
3. Identify two assumptions usually made when making quantitative atomic absorption measurements. (1 pt)
4. A 5.00-mL sample of blood was treated with trichloroacetic acid to precipitate proteins. After centrifugation, the resulting solution was brought to a pH of 3 and was extracted with two 5-mL portions of methyl isobutyl ketone containing the organic lead complexing agent APCD. The extract was aspirated directly into an air-acetylene flame yielding an absorbance of 0.444 at 283.3 nm. Five-milliliter aliquots of standard solutions containing 0.250 and 0.450 ppm Pb were treated in the same way and yielded absorbances of 0.396 and 0.599. Calculate the concentration Pb (ppm) in the sample assuming that Beer’s law is followed.
Hint:
(i) For the 2 points calibration ( based on the 2 standards), find the slope of the standard plot ( 2 pts) DO NOT USE GRAPHING PROGRAM ( e.g. excel).
Show your calculation below to find the slope.
Answer: Slope = ______________ ( 2 pts)
(ii) Let the best-fit line be y = mx +b
Find b by plugging in:
– the slope ( m) found in (i)
– concentration of one of the standard solution (x) and the corresponding absorbance (y)
Answer: Equation for the best fit line is: ____________________________ ( 2 pts)
(iii) Find the unknown concentration by using best fit line equation found in (ii)
Answer: Unknown concentration = ____________________ ( 2 pts)
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