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blogfast25
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| Quote: | Originally posted by Klute
Keep the dichromate in a custom dessicator afetr keeping it inthe oven a while, to insure it's anhydrous; I often use a tupperware with a bed of
CaCl2 covered by a thick layer of toilet paper as a cheap dessicator for general purposes, or a small jar can with a layer of NaOH/KOH or P2O5 mounted
by a plastic screen, avoiding any contact with those agressive materials, for more thorough drying conditions.
Glad to hear it worked out nicely! |
OK, dichromate it is then! Good idea about the homemade desiccators too: I've got plenty NaOH and tupperware and stuff for perforated plastic support
screens: way to go!
As regards it working out, it's going that way but I'm not out of the woods yet: there's of plenty practicing, determination of repeatability,
standardising etc before I can call it a success. But progress is being made and that's what matters the most...
I'll certainly report in full here. 
[Edited on 17-5-2008 by blogfast25]
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Klute
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exactly! Titrations are a matter of practice, you get your ways of doing as you do some, especially when it comes to end -point determination,
everyone had got his own little thing 
There quite alot of information on different titrations, regeants and indicators to use to titrate such and such compound etc etc
This can be very helpfull when trying to determine kinetics of a reaction for example, preparing stock solution of salts, or determining yields of a
product in dilute solution etc
\"You can battle with a demon, you can embrace a demon; what the hell can you do with a fucking spiritual computer?\"
-Alice Parr
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blogfast25
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Klute,
Yes, practice is essential, especially when the end-point is a bit 'spongy', as is the case here.
One of the problems here is that I use kitchen foil as a source of Al (with HCl the nascent hydrogen reduces any TiO<sup>2+</sup> back to
Ti<sup>3+</sup> which doesn't dissolve completely (due to Si and/or C,
I believe), leaving a clouded sample to be titrated and end-point more difficult to establish.
W/o the Al foil, end-point is much sharper, but the titration reads about 2 ml less, so presumably a small amount of Ti is present in the sample
solution as TiO<sup>2+</sup>, rather than Ti<sup>3+</sup>. A blank titration (no Ti, but Al nonetheless) consumes about 0.5 ml
to get the pale orangy colour from diluted FeSCN<sup>2+</sup>, against the turbid background...
I may have to invest in p.a. Al ribbon or try blank Mg ribbon. Zn granules would do it too but take too long to dissolve.
I've taken your advice on desiccation and now I've got a neat little CaCl<sub>2</sub> desiccator from an old pickling jar (you know the
ones with a neat red rubber seal?) My anh. thiosulfate and dried dichromate are drying in there.
And I'm waiting receipt of a more accurate jeweller's pocket scale, so I can get up to four significant digits instead of just three.
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blogfast25
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And now I'm having a little trouble back-titrating the I<sub>2</sub> from Fe<sup>3+</sup> + 1/2 I<sup>-</sup> with
thiosulphate. Here too, the end-point appears more spongy that I seem to remember. I'm using homemade potato starch solution as an indicator, added
near the end-point.
Problem is that after the blue iodine starch complex has cleared, it returns after a few seconds. Adding more thiosulphate, the blue disappears again,
only to reappear in seconds. This makes end-point determination quite problematic.
I thought the problem was the starch, so I titrated the next one without it, only to find that the pale yellow iodine solution near end-point regains
in colour quickly. This way about a ml of titrant can be added going from almost clear to slightly yellow and so on.
Then I tested the KI. The silverprint.co.uk grade is slightly yellow so may contain some free iodine (will test that later), so I replaced it with a
chemset grade that is perfectly white.
To no avail. The iodine keeps reappearing. It's as if the iodide formed during titration is being re-oxidised slightly, back to
I<sub>2</sub>. I'm using distilled water for the thiosulphate solution.
Any ideas, anyone?
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Klute
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Did you leave the iodine at least 10-15min to form? The reaction is autocatalytic IIRC, so it should go fast once all is donbe. 10-15min covered in
the black is essential..
\"You can battle with a demon, you can embrace a demon; what the hell can you do with a fucking spiritual computer?\"
-Alice Parr
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blogfast25
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I left it for 5 minutes but not longer. There are some reputable resources that recommend titrating the iodine immediately, to avoid evaporation.
But I will try 15 minutes (timed) tomorrow, inside a cupboard.
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Klute
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Just covered or stopper the erlen you use. We always use tappered soints erlenmeyers for theses titrations, stoppered and at least 15min in the dark.
If you get the same problem, it could be the starch although I doubt it.
\"You can battle with a demon, you can embrace a demon; what the hell can you do with a fucking spiritual computer?\"
-Alice Parr
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blogfast25
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Hi Klute,
Based on just one single titration I would definitely say the waiting period of 15 mins has improved things quite a lot. There is a marked difference
in terms of how quickly the colour returns: the end-point solution remains clear for much longer. I think this is workable now. I'll get a few
suitably sized bungs for the beer and wine making shop to properly stopper the EM too...
From a previous test (see higher) I know the starch is not at fault.
I also noticed that when I left a half empty burette (stoppered) overnight (I know, I know, shouldn't do that) that the bottom of the thiosulphate
solution had gone cloudy due to S formation. That surprises me a little because my distilled water has a pH of about 8. I call it "distilled" but it's
really conderser water from a domestic tumble dryer! It's generally speaking much purer than tap water in terms of salt content (almost zero) but the
pH does indicate that something has been carried over.
I may have to try commercial deionised water instead but the commercial grades usually contain some anti-oxidant or other...
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blogfast25
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The back titration of iodine with thiosulfate continues to cause me headaches. End-point determination has improved somewhat by allowing the
I<sub>2</sub> to form for a specific amount of time (15 min) but end-point is still too spongy for my liking.
I've replaced the potato starch with laundry starch (1 g in 100 ml of boiling AD) and it changes little, if anything. The only thing I can now think
of is the AD as a source of problems, as the KI has also been checked. Last two titrations differed by 1.1 ml! 
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Klute
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Hum, indeed that might be the problem, becasue thiosulfate solution are stable for months IME, never seen any cloudiness. How concentrated did you
make it? 0.1mol/L?
1.1mL difference is indeed very large... Mayeb try deionised water, even if there is a little antioxidants, it might be less problematic than
metaliic salts or something...
\"You can battle with a demon, you can embrace a demon; what the hell can you do with a fucking spiritual computer?\"
-Alice Parr
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