ATAR Notes: Forum

VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: DNAngel on February 10, 2011, 07:20:22 pm

Title: Spectroscopy question
Post by: DNAngel on February 10, 2011, 07:20:22 pm
I was just wondering, why do absorbance vs concentration graphs require the line to be drawn above the origin (i.e 0)?

Title: Re: Spectroscopy question
Post by: zibb3r on February 10, 2011, 09:13:38 pm
Not 100% sure, but is it because they cant be of negative value??? :o

Please correct me if im wrong ???
Title: Re: Spectroscopy question
Post by: pHysiX on February 11, 2011, 12:49:38 pm
hmmmm this is only a guess from what i remember:

in a perfect world, we actually want to have the absorbance of 0 for a zero-concentration.
in reality, the fact that there can be contaminants or other error-factors that cause our graph to be started above the origin instead.

very interesting question. maybe the Chairman can shed some light on this :D
Title: Re: Spectroscopy question
Post by: Mao on February 11, 2011, 11:01:46 pm
In an ideal world, our photospectrometer would be a simple instrument: light source, sample, detector. The detector would measure absorbance as , where I is the intensity at the detector and I0 is the intensity at the source. Naturally, as light is absorbed, I decreases, I/I0 decreases and absorbance is greater. In the ideal world, A = 0 for a blank sample as intensity at detector would be the same as the source.

In real life however, many things (diffusion, scattering, absorption by glass) can reduce the intensity at the detector. This means as intensity at the detector is always less than the source. Also due to power fluctuations in the source, we cannot know for sure what I0 is. Therefore usually at least two measurements are conducted at any given time, one measuring a blank and another measuring the sample (i.e. light travel down two paths in a UV vis machine, see your textbook). This means I0 is really arbitrary, and absorbance can be anything since it is relative, the computer inside the spectrometer take care of that for you (and you can set the reference absorbance to any arbitrary number). For simplicity sake we generally calibrate the the blank to be approximately zero. I have worked with both negative and positive absorbances, it does't make a difference.
Title: Re: Spectroscopy question
Post by: Mao on February 11, 2011, 11:02:24 pm
Quote from: pHysiX on February 11, 2011, 12:49:38 pm
maybe the Chairman can shed some light on this :D
"shed some light" in a spectroscopy thread? :P pun intended?
Title: Re: Spectroscopy question
Post by: DNAngel on February 11, 2011, 11:21:37 pm
^ lol how ironic?  ;D