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BeerLambert equation 
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#1
Jan1909, 02:45 PM

P: 501

1. The problem statement, all variables and given/known data
I need to use the LambertBeer equation to plot optical density of a lactate solution. 2. Relevant equations [tex]A=\varepsilon\ell C[/tex] (The path length is 1 cm) 3. The attempt at a solution This is one of my four samples (if you can show me where I'm going wrong with this then I can apply the correction to the others): 2.22 [tex]\mu[/tex]mol in 1.5ml, therefore there is 0.296 [tex]\mu[/tex]mol in 0.2ml This info seems to match to the example graph I have. But using the formula above, where [tex]\varepsilon[/tex]=6.22 for micromol/ml, I get: 6.22 x (0.296/0.2) micromol/ml = 9.2056 This doesn't match up with the graph I have! Any help appreciated! Please! 


#2
Jan1909, 03:54 PM

Other Sci
Sci Advisor
P: 1,385

Beer's law is accurate only for values of A from about 0.1 to 1.0. Experimentally, it is difficult to accurately measure absorption values outside this range.



#3
Jan1909, 05:50 PM

P: 501

That's the main problem, the results from the example graph are at a maximum of 0.4 for the absorption at 340nm (yaxis) when plotted against amount in micromoles up to 0.4 on the xaxis.
I can get a fairly close approximation (the graph itself is supposed to be an estimate) if I use the following formula: (amount in micromoles x amount in ml) x 6.22. But this equation makes no sense; or at least I can't see the reasoning behind it. 


#4
Jan1909, 09:28 PM

Other Sci
Sci Advisor
P: 1,385

BeerLambert equation
The slope of the best fit line to the graph should give you ε.



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