ATAR Notes: Forum

VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: Andiio on April 25, 2011, 03:28:04 pm

Title: Absorbance?
Post by: Andiio on April 25, 2011, 03:28:04 pm: How is absorbance defined? (all the finer details :P)

To put it in 'informal' terms, would absorbance simply be the amount of EMR (Electromagnetic radiation) absorbed or is it the intensity of EMR detected?

What units is absorbance measured in as well, if any? Is it a ratio?

Thanks!
Title: Re: Absorbance?
Post by: Zien on April 25, 2011, 03:35:04 pm: If I0 = original beam (incident beam) intensity
and It = transmitted beam intensity

Therefore, the transmittance (T) = It / I0

Absorbance (A) = log 1/T

Absorbance doesn't have any units because they cancel when calculating the value of T.

Hope this helps. :p
Title: Re: Absorbance?
Post by: Andiio on April 25, 2011, 03:46:29 pm: Weird, I read somewhere (probably a simplified version without logs and such) that absorbance = initial radiation intensity - final radiation intensity
But I also read somewhere that Abs = -log(T)

What do you mean by log 1/T? O_O that'd just = 0 wouldn't it?

Thanks!
Title: Re: Absorbance?
Post by: Aurelian on April 25, 2011, 03:54:27 pm: Quote from: Andiio on April 25, 2011, 03:46:29 pm
But I also read somewhere that Abs = -log(T)

What do you mean by log 1/T? O_O that'd just = 0 wouldn't it?

log1/T = logT^-1=-logT

Same thing =)
Title: Re: Absorbance?
Post by: pi on April 25, 2011, 03:57:54 pm: Quote from: Andiio on April 25, 2011, 03:46:29 pm
What do you mean by log 1/T? O_O that'd just = 0 wouldn't it?

$A=\log{(\frac{1}{T})}$

$A=\log{(T^{-1})}$

$A=-\log{(T)}$

EDIT: beaten :(
Title: Re: Absorbance?
Post by: Andiio on April 25, 2011, 04:00:50 pm: Woops I read that as (log 1)/T haha, my bad.
Title: Re: Absorbance?
Post by: Andiio on April 25, 2011, 04:45:11 pm: Just a Q about UV-Vis - is one lamp used or are two lamps used as a light/radiation source in a simple UV-Vis analysis? I thought you could just utilise a source of white light? (not talking about double beam spec)
Title: Re: Absorbance?
Post by: Mao on April 26, 2011, 07:01:36 pm: $A=\log (T_0/T)$ , where T0 is the amount of light at the source.

Thus absorbance is the log of the ratio of transmittance. It is unitless, and it also has an arbitrary zero due to the log scale. At least two absorbances must be measured to use absorbance for any calculations (generally, this is the sample + blank), this is because of the arbitrary zero.