Using the CLEAPSS DIY colorimeter

Nick Mitchener

Nick Mitchener

I am not a biologist but I make kit for Biology.

I have a question about using the colorimeter.

I am making a set of Red colorimeters. The initial setup is the put in a cuvette of distilled water and tak a reading from a voltmeter. This will be taken as 100% transmission/zero absorption. Subsequent reading can be ratioed with this to get a percentage of transmission.

However I believe biologists prefer absorption, so in my head 70% transmission is 30% absorption but don't biologists want a number which is a log. How best to help them use the results from the colorimeter?
 
The couple of them in Edexcel A-Level just want them to plot the graph straight numbers whatever they changed versus whichever they measured on the colorimeter

Also they need to zero it with whatever the suspension medium is, not always d. water

So if its pigment in ethanol, the zero would be base ethanol
 
Nick Mitchener said:
However I believe biologists prefer absorption, so in my head 70% transmission is 30% absorption but don't biologists want a number which is a log. How best to help them use the results from the colorimeter?
Click to expand...
Absorption is Logarithmic (0–infinite) and transmision Linear (0–100%).

there are some formulas on mystrica's website A = 2 – log T%

Mystrica | Display

Home of the Mystrica Colorimeter, Practical investigations for students of biology developed and tested by experienced teachers.
www.mystrica.com
 
Paul said:
there are some formulas on mystrica's website A = 2 – log T%
Click to expand...

That's the conclusion I came to. 2= log10(100)

so distilled water in measure voltage , sample in measure voltage. Divide sample voltage by distilled voltage multiply by 100 to get T%

A=2-log10 (T%)
 
Also they need to zero it with whatever the suspension medium is, not always d. water

So if its pigment in ethanol, the zero would be base ethanol



ok will note that, thanks
 
Nick Mitchener said:
Also they need to zero it with whatever the suspension medium is, not always d. water

So if its pigment in ethanol, the zero would be base ethanol



ok will note that, thanks
Click to expand...
is this the beetroot membrane permeability one? for that they need to make a calibration graph for 100-0% extract and then test their samples and look them up on the graph.
 
cephid said:
The equation \(\log(I_0 / I) = \varepsilon c l\) is the Beer-Lambert Law
Click to expand...
is the spreadsheet ok though? Our bio dept said the maths was too complicated for Bio A level students so I am trying a work round so they will use them. Doesn't seem that difficult to me and I'm no good at maths. I would have thought it was better to understand where the figures come from.
 
Nick Mitchener said:
is the spreadsheet ok though? Our bio dept said the maths was too complicated for Bio A level students so I am trying a work round so they will use them. Doesn't seem that difficult to me and I'm no good at maths. I would have thought it was better to understand where the figures come from.
Click to expand...
The equation is just log of the ratio of the intensity of the blank divided by the intensity of the material = absorption coefficient x concentration x path length. I would have thought bio students should be able to do the calculations with a calculator if they can't manage a spreadsheet, might need to be shown if they have not used a spreadsheet before.
 
cephid said:
The equation is just log of the ratio of the intensity of the blank divided by the intensity of the material = absorption coefficient x concentration x path length. I would have thought bio students should be able to do the calculations with a calculator if they can't manage a spreadsheet, might need to be shown if they have not used a spreadsheet before.
Click to expand...
I'm not a teacher so have to take what they say. They don't need to understand spreadsheets to input data.

The equation used here is A=2-log10(T%)
 
Nick Mitchener said:
I'm not a teacher so have to take what they say. They don't need to understand spreadsheets to input data.

The equation used here is A=2-log10(T%)
Click to expand...
That's the same since log(Io/I) is the same as log(Io) - log(I). and Io the blank measurement is set to be 100% and log(100) is 2 - useing logs base 10.
 
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