beers law & colorimetry. absorbance absorbance is the amount of light that gets stopped by a...
TRANSCRIPT
![Page 1: Beers Law & Colorimetry. Absorbance ABSORBANCE is the amount of light that gets stopped by a material Zero = a perfectly transparent material that lets](https://reader033.vdocuments.site/reader033/viewer/2022042623/55192a1555034642428b4d1a/html5/thumbnails/1.jpg)
Beer’s Law & Colorimetry
![Page 2: Beers Law & Colorimetry. Absorbance ABSORBANCE is the amount of light that gets stopped by a material Zero = a perfectly transparent material that lets](https://reader033.vdocuments.site/reader033/viewer/2022042623/55192a1555034642428b4d1a/html5/thumbnails/2.jpg)
AbsorbanceABSORBANCE is the amount of light that gets “stopped” by a material• “Zero” = a perfectly transparent material
that lets all light through.• “Infinity” = a completely opaque material
that does not let any light through.
Absorbance (A) is directly proportional to concentration (c) : A = kc.This is a mathematical model for something you already know: a darker solution is a more concentrated one.
![Page 3: Beers Law & Colorimetry. Absorbance ABSORBANCE is the amount of light that gets stopped by a material Zero = a perfectly transparent material that lets](https://reader033.vdocuments.site/reader033/viewer/2022042623/55192a1555034642428b4d1a/html5/thumbnails/3.jpg)
Path LengthPATH LENGTH is the distance light travels through a solution.
Note how the solution in the “belly” of this volumetric flask is darker than the solution in the neck.
PATH LENGTH (b) is directly proportional to absorbance (A) : A = kb.
less dark “neck”
darker “belly”
![Page 4: Beers Law & Colorimetry. Absorbance ABSORBANCE is the amount of light that gets stopped by a material Zero = a perfectly transparent material that lets](https://reader033.vdocuments.site/reader033/viewer/2022042623/55192a1555034642428b4d1a/html5/thumbnails/4.jpg)
Beer’s Law
A = abc
absorbance
path length
concentration
constant (nature of solute)
Beer’s Law puts all the factors that affect absorbance together in one equation.
![Page 5: Beers Law & Colorimetry. Absorbance ABSORBANCE is the amount of light that gets stopped by a material Zero = a perfectly transparent material that lets](https://reader033.vdocuments.site/reader033/viewer/2022042623/55192a1555034642428b4d1a/html5/thumbnails/5.jpg)
If we are using only one solute, then “a” is a constant. If we are are careful to always use the same path length, then “b” is a constant, too. This simplifies Beer’s Law to: A = kc.
Beer’s Law Graphs
concentration
ab
sorb
an
ce
![Page 6: Beers Law & Colorimetry. Absorbance ABSORBANCE is the amount of light that gets stopped by a material Zero = a perfectly transparent material that lets](https://reader033.vdocuments.site/reader033/viewer/2022042623/55192a1555034642428b4d1a/html5/thumbnails/6.jpg)
Then, we can find the concentration of any “unknown” by measuring it’s absorbance and interpolating the concentration.
Using Graphs
concentration
ab
sorb
an
ce
If we can measure the absorbance of several known concentrations of a solution, we can make a straight line graph.
![Page 7: Beers Law & Colorimetry. Absorbance ABSORBANCE is the amount of light that gets stopped by a material Zero = a perfectly transparent material that lets](https://reader033.vdocuments.site/reader033/viewer/2022042623/55192a1555034642428b4d1a/html5/thumbnails/7.jpg)
Colorimeters
![Page 8: Beers Law & Colorimetry. Absorbance ABSORBANCE is the amount of light that gets stopped by a material Zero = a perfectly transparent material that lets](https://reader033.vdocuments.site/reader033/viewer/2022042623/55192a1555034642428b4d1a/html5/thumbnails/8.jpg)
Transmittance
• “100%” = a perfectly transparent material that lets all light through.
• “0%” = a completely opaque material that does not let any light through
Colorimeters actually measure TRANSMITTANCE: the amount of light that goes through a solution.
![Page 9: Beers Law & Colorimetry. Absorbance ABSORBANCE is the amount of light that gets stopped by a material Zero = a perfectly transparent material that lets](https://reader033.vdocuments.site/reader033/viewer/2022042623/55192a1555034642428b4d1a/html5/thumbnails/9.jpg)
A Comparison
concentration
ab
sorb
an
ce
concentration
%Tr
ansm
itta
nce
At c =0, A = 0.
At c = ∞, A = ∞.A and c are directly proportional.
At c =0, %T =100.
At c = ∞, A = 0.A and c are exponentially related.
![Page 10: Beers Law & Colorimetry. Absorbance ABSORBANCE is the amount of light that gets stopped by a material Zero = a perfectly transparent material that lets](https://reader033.vdocuments.site/reader033/viewer/2022042623/55192a1555034642428b4d1a/html5/thumbnails/10.jpg)
A %TAbsorbance and transmittance are related exponentially.
10-A = %T/100
so if A = 1: 10-1 = 0.1 = T, or %T = 10% if A = 2, 10-2 = 0.01 = T or %T = 1%
We will usually deal with A < 1. if A = 0.5, 10-0.5 = 0.316 = T or %T = 31.6% if A = 0.1, 10-0.1 = 0.794 = T or %T = 79.4%
Make sure you can duplicate these calculations on YOUR calculator!
![Page 11: Beers Law & Colorimetry. Absorbance ABSORBANCE is the amount of light that gets stopped by a material Zero = a perfectly transparent material that lets](https://reader033.vdocuments.site/reader033/viewer/2022042623/55192a1555034642428b4d1a/html5/thumbnails/11.jpg)
%T A
Most of the time, we need to convert %T (from the colorimeter) to A (so we can plot the direct relationship between A and c.
A = -log(%T/100)
so if %T = 90%, A = -log (90/100) = -log(.90) = 0.045 if %T = 45%, A = -log (45/100) = 0.347
Make sure you can duplicate these calculations on YOUR calculator!
![Page 12: Beers Law & Colorimetry. Absorbance ABSORBANCE is the amount of light that gets stopped by a material Zero = a perfectly transparent material that lets](https://reader033.vdocuments.site/reader033/viewer/2022042623/55192a1555034642428b4d1a/html5/thumbnails/12.jpg)
Sample Problem
1. Calculate “A” for the transmittances in this data table.
2. Graph “c” vs. “A” and get a best fit straight line.
3. If an unknown K2CrO4 (aq) solution was measured at 53.7%T, what would be it’s concentration?
K2CrO4 (aq) Concentration (M)
%Transmittance
0.000 1000.125 79.40.250 63.10.375 50.1
![Page 13: Beers Law & Colorimetry. Absorbance ABSORBANCE is the amount of light that gets stopped by a material Zero = a perfectly transparent material that lets](https://reader033.vdocuments.site/reader033/viewer/2022042623/55192a1555034642428b4d1a/html5/thumbnails/13.jpg)
Answer
At 53.7% T,A = -log(0.537) = 0.270
From the graph, @ 0.270 for “A”, c = 0.338M