# Chemical Forums

## Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Aetos on November 05, 2011, 02:19:55 PM

Title: how do I use gradient from graph and absorbance to work out concentration
Post by: Aetos on November 05, 2011, 02:19:55 PM
Use the gradient of the plot to convert the absorbance value (0.32) into a concentration of drug present in the test volume.

the line equation I got was y = 30.193x

im guessing  30.193 is the gradient.

do I just make an estimate using the graph or is there an actualy way of working out a answer?
Title: Re: how do I use gradient from graph and absorbance to work out concentration
Post by: Aetos on November 05, 2011, 02:26:39 PM
is this right?

A/E=c
A=Absorbance
C=concentration
A=0.32
E=30.193

0.32/30.193=0.0106
C=0.0106mM
Title: Re: how do I use gradient from graph and absorbance to work out concentration
Post by: TheUnfocusedOne on November 06, 2011, 07:47:25 AM
I'm honestly confused about what you're trying to say. I'll give some generally information and you can tell me if it helped.

You're trying to use Beer's law, which relates absorbance to concentration using the equation:

A=ebc

-A =Absorbance
-e = Molar absorptivity
-b = Path length
-c = Concentration

You want to use a linear relationship (which I'll make apparent in a second) to find e, which works as a slope. When you're trying to use a graph to show a relationship, your basically taking a derivative of this equation (here comes some math). Basically, you're plotting the changes in A (dA, which you can't control so it's your independent or "y" axis) versus the change in concentration (dc, which you can control so it's your x). Therefor, if you look at your fun linear equation (y = mx + b), you can see that e (assuming pathlength is 1, or not a factor) is the slope. Once you measure the slope of the line, you can put in any concentration and get an absorbance and vice versa.

Does that help?

I'm not sure where your numbers are coming from since you're saying you "guessed"
Title: Re: how do I use gradient from graph and absorbance to work out concentration
Post by: Aetos on November 06, 2011, 02:46:08 PM
the numbers came from a graph i did on excel. assuming that the graph is correct, i gave a line equation(y = 30.193x
) and from the line equation i derived 30.193 as the gradient.

thanks for your help but i will stick with my answer as i havent come across the equation you provided during my lectures