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beer-lambert law

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beer-lambert law

Postby scarletemo » Thu Feb 26, 2009 3:55 pm

hi, im tryin to find out the concentration in micro mol/min
I possess the absorbance (0.106) and the extinction coefficient is 6.22x10 to the power 3 l.mol -1 . cm-1
the distance of travel is 1 cm
i beleive the equation to be C = absorbtion /(epsilon x l)
Is it as simple as putting the results into the equation, or will i have to change units
i couldnt find any examples that found the concentration for micromol/min
any tips would be greatly appreciated xx
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Re: beer-lambert law

Postby jonmoulton » Thu Feb 26, 2009 4:00 pm

micro mol/min is not a unit of concentration -- it is a unit of concentration per time. You can use the Beer-lambert law to get the concentration, but you will need to consider the design of the experiment to figure out how to combine your concentration data with a per-minute factor. You are likely looking at several measurements of concentration performed at different time points. Convert each absorbance to a concentration, then think about the time factor.
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Re: beer-lambert law

Postby scarletemo » Thu Feb 26, 2009 4:27 pm

The reading i have for absorbance is the change in absorbance over 1 minute, so do i just simply find the concentration, and becasue the absorbtion is already expressed as per minute.
are all the unit i have compatible to enter straight into the equation? thanks
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Re: beer-lambert law

Postby jonmoulton » Fri Feb 27, 2009 4:53 pm

I expect the units of the molar absorptivity to be M^-1 * cm^-1. This will give you an answer in molarity. You'll need to convert to micromoles.

Watch out for the unit mol -- I interpret that as moles. Concentration is mol/l or M. You gave the units of the molar absorptivity as l.mol -1 . cm-1, which I would choose to write M^-1 * cm^-1, but these are the same.

I made a mistake in my last post when I wrote "micro mol/min is not a unit of concentration -- it is a unit of concentration per time". I should have identified microM/min as the unit of concentration over time. You must associate a unit of volume with the moles to make it a unit of concentration (and M is defined as mol/l).

Your units on the molar absorptivity do correctly have the volume unit (l * mol^-1 * cm^-1 = M^-1 * cm^-1) and applying this in the Beer-Lambert equation will give units of molarity (M):
C = absorbtion /(epsilon x l) with units 1/((M^-1 * cm^-1)*cm) = M

Since you have readings 1 minute apart, you can take the difference in the concentrations (M) and the difference will be in M/min. If your reaction is occurring in 1 liter, you can treat this as mol/min. You'll need to convert to microM.
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Re: beer-lambert law

Postby scarletemo » Sat Feb 28, 2009 1:18 pm

thankyou for your help. ive manages to complete the question now :)
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