Sciencemadness Discussion Board

Maximum temperature for a Volumetric Flask

Sulaiman - 30-8-2020 at 19:44

I have a Pyrex 100ml Class-A / USP volumetric flask,
the basic accuracy is +/-0.08ml
but the batch certificate is 100.02ml +/-0.014ml (1 std.Dev.)

Getting it thoroughly dry, to return it to its tare weight of 68.8072g (with stopper) is tedious
(wash and rinse with water, rinse with acetone, wait for residual acetone to evaporate, blow air to remove acetone vapour)

Would heating the flask to e.g. 120oC in an oven to dry it, significantly affect its calibration ?

ArbuzToWoda - 31-8-2020 at 00:54

These sites explain why it's best not to heat volumetric flasks:
https://chemistry.stackexchange.com/questions/9665/why-are-w...
http://faculty.sites.uci.edu/chem1l/files/2013/11/RDGvolflas...

If you're concerned about the precision so much that you calibrate your flasks to the 3rd (edit: 4th) decimal place then you shouldn't heat it up... although I do doubt that it could change it's volume that significantly.

[Edited on 31-8-2020 by ArbuzToWoda]

unionised - 31-8-2020 at 01:32

Quote: Originally posted by ArbuzToWoda  


If you're concerned about the precision so much that you calibrate your flasks to the 3rd (edit: 4th) decimal place ...

[Edited on 31-8-2020 by ArbuzToWoda]

All the volumetric flasks I have seen have been calibrated to the 6th decimal place* or better, some to the 9th place
Very few were calibrated to more than 3 significant figures.

*if one works in tonnes.

Sulaiman - 31-8-2020 at 04:33

Quote: Originally posted by unionised  
... Very few were calibrated to more than 3 significant figures.

Its my current mini-obsession - trying to get textbook values for the density of water to 4 significant digits.
So far I always get results less than textbook.
(after balance re-cal, temperature and buoyancy compensations and using freshly boiled distilled water)

Complete overkill for general chemistry, just a personal challenge.
Even filling the volumetric flask to the calibration line and getting consistent results is challenging me.

unionised - 31-8-2020 at 08:00

Are you remembering to account for air buoyancy?

Herr Haber - 31-8-2020 at 11:41

If it can be autoclaved it can certainly withstand 120 degrees without losing accuracy.

Sulaiman - 31-8-2020 at 13:03

Quote: Originally posted by unionised  
Are you remembering to account for air buoyancy?

Yes, but so far only at nominal 1.2 mg/cm3,
Next is to compensate the buoyancy compensation for air temperature and pressure.
My measurements now generally match textbook values to 3 significant figures,
Its the fourth digit that eludes me.
Theoretically my limit to accuracy should be the volumetric flask uncertainty = fourth digit +/-1.4

Not vitally important for me to succeed, just trying has improved my techniques.

arkoma - 31-8-2020 at 19:11

Quote:
(Volumetric apparatus should not be heated: after washing
it is rinsed with acetone, then pure diethyl ether, and air-dried. Prior to use, equipment can be rinsed with acetone,
then with petroleum ether or pure diethyl ether, to remove the last traces of contaminants.)


From "purification of laboratory Chemicals",6th Edition,page 3