Understanding Beer's Law and Calculating Absorbance

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SUMMARY

This discussion focuses on the application of Beer's Law, specifically the calculation of absorbance (A) from transmittance (%T) and the construction of a calibration curve. The key formula used is A = log(1/%T), which indicates that absorbance increases as transmittance decreases. Participants emphasize the importance of plotting concentration (C) against absorbance (A) to establish a linear relationship, with the expectation that the slope should be positive. Issues arise when the plotted data shows a negative slope, indicating potential errors in data collection or analysis.

PREREQUISITES
  • Understanding of Beer's Law (Lambert-Beer's Law)
  • Familiarity with absorbance and transmittance concepts
  • Basic graphing skills using a graphics program
  • Knowledge of logarithmic functions
NEXT STEPS
  • Learn how to construct calibration curves using Beer's Law
  • Explore the implications of negative slopes in absorbance plots
  • Investigate the role of ε (molar absorptivity) in absorbance calculations
  • Study the effects of concentration on absorbance and transmittance
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Chemistry students, laboratory technicians, and educators involved in analytical chemistry and spectrophotometry.

Feodalherren
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Homework Statement



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The Attempt at a Solution



So I'm at a loss here. I'm not given any units... I know the concentration is C in Beer's law (awesome name btw). But what is my εb? Is it % transmittance?

So then for A

A= (.055)(.0001)
What does this even mean?Ops this was supposed to go to the chemistry forums.
 
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What is the question they are asking? Incidentally, Beer is the guy's name.

Let me guess the question. They give you the transmittance of an unknown solution, and they want you to find its concentration.

Chet
 
εb doesn't matter here.

This is about constructing calibration curve, C vs A. Form the Beer's law (AKA Lambert-Beer's law) you know the dependence is linear (A=kC), so it is enough to determine the value of k - and it doesn't matter what are its components.
 
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Have your graphics program plot the concentration (y axis) as a function of the transmissivity (x axis). Have your graphics program use a semi-log scale for transmissivity). The graph will come out to be a straight line. Have your graphics program fit an equation to this straight line. This will express the concentration as a linear function of log of transmissivity. This will constitute your calibration.

Chet
 
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That's where I kind of get lost.
So my Cs are obviously given. Now I found a formula for A which was
A=log(1/%T) so I found my absorbance numbers.

Thing is, they increase with a lower T as shown by the above equation. I should get a line that has a y-intercept of approximately 0, my line goes the wrong way, it has negative slope :/.
 
Feodalherren said:
That's where I kind of get lost.
So my Cs are obviously given. Now I found a formula for A which was
A=log(1/%T) so I found my absorbance numbers.

Thing is, they increase with a lower T as shown by the above equation. I should get a line that has a y-intercept of approximately 0, my line goes the wrong way, it has negative slope :/.
What does your plot of log T as a function of C look like? It should be a straight line.

Chet
 
It is a straight line but the slope is negative and it has a big y-intercept...

I get absorbances to be the following
A 1.3
B .646
C .335
D .165
E .082
 
Feodalherren said:
It is a straight line but the slope is negative and it has a big y-intercept...

I get absorbances to be the following
A 1.3
B .646
C .335
D .165
E .082
Is it possible to show us your graph?

Chet
 

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