Why read bottom/top of menicsus?

[quantum123]

Who established the rule that we must read the top/bottom of the meniscus?
How is the rule's accuracies justified in both the top and bottom cases?

 

[Andy Resnick]

Excellent question! That's how accurate volumetric flasks are calibrated- in other words, where the marks go. Does that help?

I don't think I could use standard volumetric glassware to accurately meter out Hg, but I've never needed to work with volumes of the stuff.

 

[AJ Bentley]

The meniscus is caused by surface tension effects at the point where the meniscus meets the glass (usually it's glass)

So you get a small distortion there and the level of the liquid is higher or lower than the rest of the surface. (depends on the liquid involved)

Basically the rule is just saying read it where the meniscus is flat so you get a good measure of the actual level - don't read it at the distortion.

 

[quantum123]

But how can both the bottom rule and top rule apply for the same flask?

 

[Andy Resnick]

But how can both the bottom rule and top rule apply for the same flask?

If I understand your question, it's an issue of accuracy. To see this, calculate how much fluid is in the meniscus for water on glass (and Hg on glass), and compare that to the overall volume.

For highly precise measurements, the issue becomes more critical. The glassware you are thinking of is not used for metrologic type measurements.

http://ts.nist.gov/MeasurementServices/Calibrations/upload/SP250_72.pdf

I'm not sure if in the end, the most precise measurements of fluid volume are actually performed by weighing and density measurements using things like:

http://www.nist.gov/cstl/process/fluid/upload/250_78.pdf

Page 11/12 has information you may find interesting. A Hg-specific paper is here:

http://iopscience.iop.org/0026-1394/41/2/S02/pdf/0026-1394_41_2_S02.pdf