Re: Spectrophotometer Help

From: Farooq W (farooq.w_at_gmail.com)
Date: 03/28/05 

Date: 27 Mar 2005 19:04:29 -0800

Bob wrote:
> On 27 Mar 2005 13:36:25 -0800, "Amemoto" <hateame@yahoo.com> wrote:
>
> >Hi there,
> >I am working on a mini-chemistry project which involves the use of a
> >spectophotometer to measure the absorbance of certain solutions. The
> >spectophotometer is a single beam, 300-900nm range unknown
> >brand/manufacturer. This is how i'd been working with the
instrument:
> >1. Warm Up Instrument - 15 minutes
> >2. Set wavelength (starting at 340 and worked my way up to 900nm, at
> >intervals of 20 nm.)
> >3. I first calibrated the instument by inserting a cuvette filled
with
> >distilled water and then examined my sample(KMnO4).
> >Anyway, i followed every step as labelled on the machine. Things
worked
> >fine until I got to 380nm, where i got an error which read -1.9999
> >(blinking). This problem continued up until i reached 500nm, but
then
> >the machine started reading again. Here's the reading:
> >wavelength(nm) absorbance
> >340 0.706
> >360 1.070
> >380 error(-1.999)
> >400 error(-1.999)
> >480 error(-1.999)
> >500 1.900
> >520 1.230
> >
> >I don't know whether the problem is with the old instrument or not,
but
> >i suspect it has to do with the concentration of my solution.
>
> Then why don't you test that idea? [You did all that work to set this
> expt up, then got an error which you think you understand. By the
> time you posted this and got answers, you could have tested your own
> prediction!]
>
> bob

Bob,

This is a very interesting problem on two accounts:
(i) This is quite similar to the one I observed with a single beam
spectrophotometer. Some students complained that they can not adjust
100 % transmittance at 380 nm using distilled water blank for preparing
a graph of KMnO4 absorbance vs. wavelength. The needle would stay in
the middle. From 390nm on wards, it was fine; someone said that this is
due the insufficient power of the 21-W bulb, which is not providing
adequate energy near the beginning of the visible range. I don't know
what kind of source if this _unbranded_ instrument has inside, but
ordinary single beam spectrophotometers imply use a low wattage
tungsten bulb. How can this source provide sufficient energy near ~ 300
to 380 nm range for a reliable measurement?

(ii) on the possible physical significance of negative absorbance and
transmittance values.

For the OP: Are you trying to locate the wavelength for maximum
absorbance, then decrease the measurement interval ( say 5 nm) near
500- 550 nm. Also note that your KMnO4 appears quite concentrated,
absorbance values should remain less than 1 (photometric error is
minimum in certain range of abs. values), if accuracy if your concern.
Dilute it at least ten times.

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