So the question is: what is the optical density of the original sample?

In summary: if the sample is not dilute enough, then the absorbance will not be a linear function of concentration.
  • #1
Arjan.
3
0
The problem: how to determine the optical density of a diluted sample
Hi, i work on algae and bioenergy and i have a question about the OD.

I have to determine the optical density of about 400 samples. Some samples however are too "dense" for the photospectrometer, that is they have an optical density greater than the range of the spectrometer (>4,5 OD)

In order to determine the optical density of these samples I diluted them.

So for example:
The original sample has an OD > 4,5, which is too high for the machine
So it was diluted.
0,5 ml of the sample were taken and 3,0 ml distilled H2O were added.
The OD of the diluted sample was measured
The OD of this sample is 1,58.

Question
What is the optical density of the original sample?

Homework Equations



http://upload.wikimedia.org/wikipedia/commons/0/04/Beer_lambert.png

The Lamber-Beer equation says:

Absorbance = extinctioncoefficient x length of path x concentration

Optical density = log ( I0/I1)
Absorbance = log ( I1/I0)

Links: http://en.wikipedia.org/wiki/Optical_density
http://en.wikipedia.org/wiki/Beer-Lambert_law

The Attempt at a Solution



In the example above i diluted 7 times, so the optical density is 7 x 1,58 = 11,06
 
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  • #2
Your answer is correct. The OD of the original sample is the OD of the diluted sample * the dilution factor.
 
  • #3
This doesn't make sense to me. OD is a base-10 logarithm scale. So for example, if you dilute by 10x then the diluted OD will be 1 less than (not 1/10th) of the undiluted OD.

The original OD should be
1.58 + log(7)
= 1.58 + 0.85
=2.43

However, you should have been able to measure OD 2.43 in the original sample.
 
  • #4
According to the Beer Lambert Law, the absorbance of a sample (aka optical density) varies linearly with the concentration for sufficiently dilute samples.
 
  • #5
Deleted post ... just realized an error in how I was thinking about all this.
 
Last edited:
  • #6
Ygggdrasil said:
varies linearly with the concentration for sufficiently dilute samples.

And this word "sufficiently" is the only reason why I have not posted earlier that Arjan procedure was correct...
 

FAQ: So the question is: what is the optical density of the original sample?

1. What is optical density?

Optical density is a measure of how much light is absorbed by a substance. It is typically represented by the symbol OD and is commonly used in chemistry and biology to quantify the concentration of a solution.

2. How is optical density related to concentration?

Optical density is directly proportional to the concentration of a solution. As the concentration increases, so does the amount of light absorbed, resulting in a higher optical density.

3. What is the equation for calculating optical density?

The equation for calculating optical density is OD = log(I0/I), where I0 is the intensity of the incident light and I is the intensity of the transmitted light.

4. How is dilution related to optical density?

Dilution is the process of reducing the concentration of a solution by adding more solvent. This results in a decrease in optical density, as there are now fewer particles to absorb the light.

5. Why is it important to measure optical density in experiments?

Optical density is an important measurement in experiments as it allows for the quantification of the concentration of a solution. This is particularly useful in biological and chemical experiments where precise concentrations are needed for accurate results.

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