Picnometers

The use of picnometers came up in one of the threads in the Mineral ID forum. A picnometer has never been one of my favorite devices. I seem always to have difficulty seating the ground glass to a consistent depth and removing the excess water from the tip (and perhaps the inner rim of the flask). In short, my precision is terrible.

Does anyone have tips for consistently seating the top and removing the excess water to get good precision?

Hi Don,
Yes I do. I use one often with good, consistent results. I thought I was probably the last person in the universe to use one, too!

The working fluid (usually distilled water) should be kept in the same place as the pycnometer and should be at the same temperature. Ideally, 20 deg C.

Coarsely-ground material is better than fine (surface effects are significant) and make sure that the material is properly wetted. It may be worth considering a single drop of Teepol (TM) per litre to enable proper wetting, if using water.

The stopper should be very lightly greased with silicone stopcock grease to prevent joint freeze. The ground surface should look foggy rather than opaque.

The stopper should be pressed home with consistent pressure. A small spring or a small lead weight is perfect.

Make sure that you blot the bottle and the neck with tissues, and blot the vent with a piece of blotting paper (buy a big sheet and use squares of it for the most consistent results.) Blot for a slow count of ten.

Consistency of technique is more important than actual technique with this temperamental piece of kit.

If you have a supply of ground-glass stoppered bottles and jars, it is possible to modify them for density determination.

If it helps, I have an article (not quite complete yet) on a range of SG determination techniques and apperatus.

Links here:
Alysson's Website
The Specific Gravity Article (PDF File)

When you invited me to see your etchings, I didn't expect to see so much degraded quartz. Really.



Edited 1 time(s). Last edit at 05/08/2009 03:47PM by Alysson Rowan.

In my experience, the biggest problem in using a pycnometer is the formation of bubbles. One can use tap water if it is not highly mineralized. It should be boiled and allowed to cool to ambient temperature. Use a dropper rather than pouring the water in. If you see air bubbles on the sample try to dislodge them. For minerals I always use the largest piece that will fit. A tiny bubble will be relatively small compared to the sample so the error will be relatively small. There are wide mouth pycnometers available with an opening over a 1 cm. If there is no small "test piece" available it is possible to put the whole specimen in a wide mouth bottle if it appears to be one mineral.

Alysson and Howard,

Thank you for your responses and tips. My problem (I believe) has always been seating the top with uniform pressure and then removing the excess water. I think your suggestions will help a lot. Alysson, I will look forward to seeing your paper when it is published. Please let us know where and when.

Don

Don, the addresses for the (preview) version are as follows:-

Alysson's Website
S.G. Article Download (PDF File)

When you invited me to see your etchings, I didn't expect to see so much degraded quartz. Really.

Allyson, Thank you for the Links. I have downloaded your paper and will enjoy studying it,

Don

The last time I used a pycnometer, it was a very small one for tiny specimens. I noticed a couple things. One is to avoid holding the pycnometer longer than absolutely necessary - your fingers can warm it up and introduce variables. The other is that when I inserted the stopper, I tilted the bottle slightly so the excess tended to squirt away from the pycnometer, reducing the amount of water drops to blot away.

Good tips, Fred ;~}

Fred, your point about holding the vessel is very important. I tend to use a test-tube clip to hold the bottle.

The largest source of variation, though, is the size of the specimen sample. Properly-wetted powder tends to read a higher SG than, say, millet-grain sample, and highly irregular sample tends to read lower than polished.
The first is a surface-tension effect and the second is due to entrapped gas bubbles in the irregular surface.

Most beginners with wet SG measuring techniques tend to forget to ensure that the sample is properly wetted before taking measurements. Try wetting a sample of talcum powder to illustrate the problem.

When you invited me to see your etchings, I didn't expect to see so much degraded quartz. Really.