Sulaiman
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Meniscus in volumetric measurement of mercury
I have about 110 ml Hg that I intend to purify so that I can use it in a barometer,
so at the end of my purification processes
(I'll probably start a new thread when I begin, brief description below)
I want to measure it's density as accurately as I can.
I will probably use two methods to determine density;
. Weight vs buoyancy, (details not worked out yet)
. Weight vs volume, which I now describe.
I can measure temperature to 0.3C absolute which is equivalent to a density error of <60 ppm
Absolute weight in this range I can measure to <70 ppm
My best volumetric flask is 100.02 +/-0.014 ml = 140 ppm
So overall best accuracy (1 std dev) will be about 166 ppm.
In my manometer/barometer the combined errors are equivalent to 0.125 mm error at 760 mmHg...... Good enough.
Question:
The volumetric flask is calibrated for water,
which forms a concave meniscus.
Mercury forms a convex meniscus.
How do I compensate for this?
Purification =
digestion of base metals and oxides with dilute nitric acid and air bubbler, 2x 24 hours, then water rinses.
Mechanical filtering with glass fritted funnel and vacuum,
Multiple distillations to remove silver and PGMs etc.
I actually expect the density to be more accurate than my measurements after this process, but I'd like to check it.
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bnull
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Quote: Originally posted by Sulaiman  | Question:
The volumetric flask is calibrated for water,
which forms a concave meniscus.
Mercury forms a convex meniscus.
How do I compensate for this? |
You don't compensate. Just read the meniscus the normal way: by the center. In water, it is the bottom of the meniscus; in mercury, it is the top.
Quod scripsi, scripsi.
B. N. Ull
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Sulaiman
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Then there would be a greater volume of water compared to mercury.
EDIT: by the two meniscus volumes.
Volume measurement is my primary source of error,
currently assumed to be +/-14 ul.
So if two meniscus volume errors <<14 ul I can ignore the meniscus volumes.
Anyone good at estimating meniscus volumes?
[Edited on 10-5-2024 by Sulaiman]
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j_sum1
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You could use a burette and measure the difference between two readings. Of course this would not work for measuring a total volume, but to get a
sample for a density calculation, it should be fine.
Just be aware that you are using the difference between two values when you do your error calculations.
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Sulaiman
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My 100ml volumetric flask (+/-0.014ml)
is more accurate than a Class-AS burette (eg 50ml +/-0.05ml)
which, as you wrote, needs two error-prone readings.
I won't pay for a calibrated pycnometer or volumetric pipette, so
I'll just use the top of the mercury meniscus and ignore this trivia for now.
Unless someone wants to guesstimate the missing meniscus volume for me 
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RU_KLO
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Maybe:
Add water till you transform the negative mercury meniscus to positive water meniscus.
then add the volume of water to the volume of mercury?
Go SAFE, because stupidity and bad Luck exist.
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Sulaiman
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Good idea, I may try just out of curiosity.
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bnull
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You wouldn't need to compensate if the liquid were water, which is not the case. Oh, well.
Post a picture of the meniscus and I'll see how it goes. It must be a close, well focused shot, with the inner diameter of the flask and height of the
meniscus indicated, please.
There's a (rather long) paper by K. C. D. Hickman, "The Mercury Meniscus", published in 1929 in the Journal of the Optical society of America (https://doi.org/10.1364/JOSA.19.000190). It deals with the meniscus, lubrication, and flow of mercury in a tube, among other things.
Quod scripsi, scripsi.
B. N. Ull
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unionised
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Weigh the empty flask. W1
Add about 90 ml of mercury + reweigh it. W2
Top it up to 100 ml with water and reweigh. W3
The difference ( W3- W2) will tell you the weight of the water needed to top up the flask and that effectively tells you the volume of the water.
W2-W1 tells you the weight of the mercury.
The volume of the flask minus the volume of water is the volume of the mercury.
And that lets you calculate the density of the mercury.
You can repeat it a few times with different masses of mercury and take an average.
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Sulaiman
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@bnull: thanks for the offer
- I may take you up on it if the RU-CLO/unionised idea doesn't work out.
Thanks
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Sulaiman
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Found a table of Hg meniscus volumes on page 716 of Mellor_ACTITC_04.pdf in the SM library.
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