Question

A 5mL blood sample is treated with trichloroacetic acid for protein precipitation. After centrifugation, the remaining solution is taken to pH 3 and extracted with two portions of 5.00 mL methyl isobutyl ketone, containing APCD. The extract is aspirated directly into a air/acetylene flame and an absorbance of 0.502 at 283.3 nm is obtained. 5.00 mL aliquots of primary standard containing 0.400 y 0.600 ppm of lead are treated the same way, and absorbances of 0.396 y 0.599, respectively, are obtained. Determine the concentration of lead in the sample (in ppm) assuming it follows Beer's law.

Answer 1

According to Beer-lambert's law ,

Absorbance () = Molar absorption coefficent () *Optical path length () *Molar Concentration()

Molar absorption coefficient is measured in M^-1Cm^-1 , Optical path legth in Cm and Molar Concentration in M.

The plot of Absorbance() versus Concentration () gives a straight line .

The slope of the line

[ is the difference in the absorbance at any two points in the plot and   is the difference in the concentration of lead (Pb) at those points in the plot ]

Given ,

at    .....(1)

at .....(2)

Slope of the line ,  

  

The equation of a straight line is given by   ... (3)

[ is the absorbance (Y-axis) , is the slope of the line , is the concentration of lead (X-axis) and is the intercept of the sttraight line on the Y-axis ]

Using the data (1) given in the problem in the above equation (3) to determine  

[    , and    ]

  

   

Using the slope and the intercept the straight line equation is

   ....(4)

We are to determine the concentration of lead in the sample (in ppm) when absorbance is 0.502 at 283.3 nm .

  

     

Concentration of lead in the sample