ATAR Notes: Forum

VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: Chemkid on March 15, 2009, 07:31:22 pm

Title: Atomic Absorption Spec
Post by: Chemkid on March 15, 2009, 07:31:22 pm: What happens to the intensity of light transmitted as the concentration of a CuSO₄ solution increases?

Also, in shinny's notes, it says that AAS is qualitative and quantitative, and can use UV light. My teacher said AAs was quantitative only and used only visivle light. Can someone explain this apparent contradiction to me? (lol, I bet shinny's notes are right!)

Thanks in advance!
Title: Re: Atomic Absorption Spec
Post by: shinny on March 15, 2009, 07:55:54 pm: Quote from: Chemkid on March 15, 2009, 07:31:22 pm
What happens to the intensity of light transmitted as the concentration of a CuSO₄ solution increases?
The intensity should decrease as more light will be absorbed by the ions.

Quote from: Chemkid on March 15, 2009, 07:31:22 pm
Also, in shinny's notes, it says that AAS is qualitative and quantitative
No they don't D:

Quote from: Chemkid on March 15, 2009, 07:31:22 pm
and can use UV light
Can't remember enough about AAS to answer this. Anyone else know why I wrote that/if it's wrong?
Title: Re: Atomic Absorption Spec
Post by: Chemkid on March 15, 2009, 08:15:58 pm: lol, sorry about that, thanks =) Could someone explain to me why the intensity of light transmitted decreases though? That would be awesome!
Title: Re: Atomic Absorption Spec
Post by: TrueTears on March 15, 2009, 08:24:24 pm: Quote from: Chemkid on March 15, 2009, 08:15:58 pm
lol, sorry about that, thanks =) Could someone explain to me why the intensity of light transmitted decreases though? That would be awesome!
Because the $CuSO_4$ solution "absorbs" the light, hence the name Atomic Absorption Spectroscopy, it measures the absorption of the solution :P

so the intensity of light transmitted through decreases
Title: Re: Atomic Absorption Spec
Post by: Chemkid on March 15, 2009, 10:58:59 pm: Thanks Truetears! so when they say "transmitted", they dont mean the colour we see the solution, but the amount of light that passes through. Is that right?
Title: Re: Atomic Absorption Spec
Post by: TrueTears on March 15, 2009, 11:01:40 pm: Do you mean transmitted light? If so, then that's the light that is transmitted through, or as you say it, passes through.
Title: Re: Atomic Absorption Spec
Post by: khalil on March 17, 2009, 07:27:49 pm: On another note, what is the point of chopping pulses of light in AAS?
Title: Re: Atomic Absorption Spec
Post by: TrueTears on March 17, 2009, 07:43:11 pm: Ok, basically when you shoot the light at the sample will emit that same light.

If you just have a continuous ray of light, then there will be interference due to the emission of light by the sample.

Thus by pulsing the light, it will avoid the interference.
Title: Re: Atomic Absorption Spec
Post by: khalil on March 17, 2009, 08:22:52 pm: srry i still dont understand!
Title: Re: Atomic Absorption Spec
Post by: TrueTears on March 17, 2009, 08:31:59 pm: AES (atomic EMISSION spectrum) works by shooting light which have the EXACT energy level required to "push" an electron to a higher energy level which causes it to emit light
AAS works by the fact that if that sample emits that certain amount of energy then it must have RECEIVED exactly the same amount of energy(same as in the same energy it emitted) in order to push it to a higher energy level.

So therefore if you do not shoot pulses of light at that sample, then the light that is emitted will interfere with the light that is absorbed.

By chopping the light into pulses, you avoid these interferences.
Title: Re: Atomic Absorption Spec
Post by: shinny on March 17, 2009, 08:37:01 pm: To elaborate on what TrueTears has said, basically the fundamental basis of AAS is that metals will absorb light of a particular wavelength, and that the amount transmitted or absorbed, would be proportional to the concentration of that metal in the sample. However, do remember that when these metals absorb this light, they're in an unstable state and will eventually emit this exact same wavelength of light again to reach a stable 'ground' state again. This causes problems for the AAS technique as you're essentially having the light which was previously absorbed being re-emitted and reaching the detector. This would cause the detector to register this light as well, and would cause the detector to think that the amount of light transmitted is greater than it actually is. Hence, the light from the lamp is pulsed, and the detector is designed to only count light which is pulsed to avoid confusing light from the lamp, and light which is re-emitted.

EDIT: TT beat me to it D:
Title: Re: Atomic Absorption Spec
Post by: /0 on March 17, 2009, 08:56:27 pm: Great explanations TT and shinny! Just wondering, is AES part of the course? My class seems to only be covering AAS o_O
Title: Re: Atomic Absorption Spec
Post by: TrueTears on March 17, 2009, 08:58:33 pm: Great explanation shinny, owns mine :)

AES is part of the course, it's not that hard, should be able to just remember it easily
Title: Re: Atomic Absorption Spec
Post by: shinny on March 17, 2009, 09:03:43 pm: I think you just need to understand AES and that's about it. At most, I think it'll just come as a multiple choice question in the exam since there's not really that much to it...
Title: Re: Atomic Absorption Spec
Post by: khalil on March 18, 2009, 03:36:14 pm: Thank you. Btw, how do u know this stuff? It doesnt explain it in the textbook or any lecture notes that I've read.
Title: Re: Atomic Absorption Spec
Post by: shinny on March 18, 2009, 03:37:57 pm: It should be in one of the textbooks...I used Heinemann and Nelson so it's in either of those (most likely Heinemann; *stabs Nelson*).
Title: Re: Atomic Absorption Spec
Post by: khalil on March 18, 2009, 03:42:06 pm: Yeh i've got heinemann and it doesnt mention it!
Title: Re: Atomic Absorption Spec
Post by: d0minicz on March 18, 2009, 03:43:31 pm: What doesnt it mention?
Title: Re: Atomic Absorption Spec
Post by: khalil on March 18, 2009, 04:47:46 pm: Refer to page 1
Title: Re: Atomic Absorption Spec
Post by: d0minicz on March 18, 2009, 05:01:53 pm: yehhh doesnt mention it ...
which sources do you guys get this information from
Title: Re: Atomic Absorption Spec
Post by: shinny on March 18, 2009, 06:04:19 pm: Maybe it was just from my teacher...I can't remember D:
Title: Re: Atomic Absorption Spec
Post by: khalil on March 18, 2009, 06:39:06 pm: Do calibration graphs always start at zero? What if they are negatively sloped? Also, when they are negatively sloped and cross the horizaontal axis, what does it represent?
These questions refer to UV-visible spectroscopy!
Title: Re: Atomic Absorption Spec
Post by: Chemkid on March 18, 2009, 10:00:29 pm: Calibration graphs usually start at 0, because when the blank solution is introduced to the instrument, it is zeroed to account for the presence of the dissolved substance which is under analysis. Some calibration graphs do not start from 0, as the instrument has not been zeroed. The calibration graph is negatively sloped if we are talking about the intensity of light transmitted (what is not absorbed, and passes throguh the solution). This works logically, as when the concentration, more of the selected wavelength is absorbed, and hence, less light goes through. There are other situations where the calibration curve is negative, but you have to use logic in them. :)