Glycerin Specific Gravity

Today I used my pycnometer and precision scale to measure the specific gravity of the glycerin which we sell and which I use when I’m trying out recipes for home-made composition rollers.

I started taking a video of this procedure but my camera battery ran out of juice before I was done, and the spare batteries were hiding in some Wii controllers and also pretty much drained. I didn’t want to wash up the equipment and restart another day, and I didn’t want to leave everything out either, so I will have an incomplete video of the process posted later. It turns out that you can only watch someone filling a pycnometer, drying its outside, and weighing it two or three times before it gets tiresome anyway.

The only thing missing from the video is the extra step, when filling the pycnometer with glycerin, of rinsing the overflow off the outside of the bottle before trying to wipe it dry. This ensures there is no film of glycerin on the outside of the bottle adding to its weight.

In any case, the pycnometer held 9.934g of water or 12.505g of glycerin (these are weights, not mass, because there has been no compensation for the buoyancy from the atmosphere), yielding an apparent specific gravity of 1.2588 for the glycerin. Interpolating into a table of apparent specific gravities of water/glycerin mixtures gives the value of 98.1% glycerin, and 1.9% water by weight. Contrary to what I had previously stated, the glycerin bottle does state that it is 99.7% pure. We’ve had this bottle for a while, and it is now only half full, so some water has been absorbed from the air over the years, explaining why I found more than 0.3% water. Further evidence of this is that the bottle was slightly collapsed from the water being drawn out of the air in the bottle.

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