Re: Spectrophotometer Help

From: Bob (bbx107_at_excite.com)
Date: 03/29/05 

Date: Mon, 28 Mar 2005 17:52:28 -0800

On 27 Mar 2005 19:04:29 -0800, "Farooq W" <farooq.w@gmail.com> wrote:

>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?
>

I don't have any data on the W bulb spectrum at hand.

But in his case, he seems to be getting good data below 380, and the
problem is with the sample not the DI blank (as I read what he posted
above).

Maybe your bulb is dirty. Try cleaning/replacing it.

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

I am guessing that his machine is not reporting a negative A. Instead,
it is reporting an error. But one would have to look at the manual to
be sure. It's also fun to switch standard and sample, to see what the
machine reports when A really is negative.

Why all this interest in measuring KMnO4 spectra? Give the kids
something with a more modest extinction coefficient. (We used it for
an expt where the kids were supposed to watch the dissolving process.
I would dole out the crystals almost one at a time!)

bob