Question

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 0.454 at 283.3 nm. Five-milliliter aliquots of standard solutions containing 0.240 and 0.475 ppm Pb were treated in the same way and yielded absorbances of 0.412 and 0.642. Calculate the concentration Pb (ppm) in the sample assuming that Beer’s law is followed.

Answer 1

Explanation:

According to the Beer-lambert' s law,

        Absrobance = absrobptitvity × lenght × concentration

So, if we will plot a graph between absrobance and concentration then we will obtain a straight line

Hence, formula to calculate slope of the graph is as follows.

   The slope of graph = $\frac{\text{difference in absrobance at two points}}{\text{difference in concentrations at those points}}$

The given data is as follows.

     $A_{1}$ = 0.412 (first point)

     $A_{2}$ = 0.642 (second point)

     $C_{1}$ = 0.240 ppm (first point)

     $C_{2}$ = 0.475 ppm (second point)

Hence, putting these values into the above formula we will calculate the value of slope as follows.

           Slope = $\frac{\text{difference in absrobance at two points}}{\text{difference in concentrations at those points}}$

                     = $\frac{0.642 - 0.412}{0.475 - 0.240}$

                      = $\frac{0.23}{0.235}$

                     = 0.978

It is known that for the graph, line equation can be written as follows.

                       y = mx + c  ........... (1)

where,      C = intercept

                 m = slope

Hence, calculate the value of intercept as follows.

           0.412 = $0.24 \times 0.978$ + c

               c = 0.178

As it is given that the absorbance values 0.454 (the y-axis value). Therefore, putting this value into equation (1) we get the following.

                     y = mx + c

                  0.454 = $x \times 0.978$ + 0.178

                   0.276 = 0.978x

                         x = 0.282 ppm

Thus, we can conclude that the concentration of Pb in the given sample is 0.282 ppm.