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Instrumental analysis finding the concenration

  1. Sep 19, 2010 #1
    1. The problem statement, all variables and given/known data

    The analysis for method of standard addition can be simplified significantly if the volume of the test solution is held constant. However, in some cases, it is difficult to avoid significant volume change after the sample is spiked with known concentrations of the analyte. The change in volume needs to be considered. A sample of coffee is analyzed to determine its caffeine concentration. Two portions of the sample are prepared for analysis. The first portion contains 50.0 ML of brewed coffee, which is added 10.0 ml of water. The second portion contains 50.0 ml of brewed coffee that has been spiked with 10.ml of an aqueous solution that contains 1.0 x10‐2 M caffeine. The first portion of the sample is found to give a measured absorbance of 243 units. And the second portion gives an absorbance of 387 units. What is the concentration of caffeine in the brewed coffee?

    2. Relevant equations

    We need to use standard addition. a background and intensity for each. I am not sure if beer's law is applicable

    3. The attempt at a solution

    So I understand how standard addition works. But I cannot think of a way to find the concentration without the background intensity and the intensity for each portion.
  2. jcsd
  3. Sep 19, 2010 #2


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    Staff: Mentor

    As I told you in other thread - you don't need intensity, if absorbance is given. And yes, Beer–Lambert law is applicable.
    Last edited by a moderator: Aug 13, 2013
  4. Sep 19, 2010 #3
    Ok, I will try beer's law
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