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I think our polypropylene measuring cylinders must shrink with age or autoclaving. The black lines mark the height of the water when this '500 ml' cylinder is filled with 500 grams of water. (Well yes, the temperature is only ~20°C, not 25°C). The markings are off by about 50 ml!
Well the cylinder is marked TD (or to deliver), not TC (or to contain), but I doubt there would be that much of a difference. Nevertheless, the correct way to do this would be to determine the mass of the water that you poured out of the container when it is full.
ReplyDeleteI think our polypropylene measuring cylinders must shrink with age or autoclaving.
DeleteProbably - polypropylene is a thermoplastic polymer, meaning it softens when heated. If I understand correctly, the way they make PP containers is to take a slug of plastic, melt it, inflate it against the walls of a mould, and cool. This results in a product that's slightly under tension. Heat it often enough, and the plastic will contract back toward where it started from, causing the cylinder to narrow and shorten.
Why are you autoclaving your graduated cylinders in the first place?
Well the cylinder is marked TD (or to deliver), not TC (or to contain)
That's certainly not the cause of the discrepancy - it's the wrong way 'round. A cylinder that contains 450 mL of water when filled to a given graduation will be unable to deliver 500 mL, no matter how much sticks to the container.
At any rate, plastic cylinders aren't the most accurate measuring devices - their intrinsic error is probably greater than the TC/TD difference. They're good for rough estimates (though +/- 10% is a little too rough), but if you're worried about TC/TD differences, you'll want to use an accurate glass cylinder, or even a volumetric flask.
We autoclaving (some of) our cylinders to sterilize them, of course, so they can be used to measure sterile solutions.
ReplyDeleteWe use polypropylene cylinders because they're MUCH less likely to break when knocked over than glass cylinders.
We're certainly not working at a resolution where we'd worry about TC/TD differences, but a 10% error in 500 ml is pretty outrageous. If PP cylinders are going to shrink this much then they shouldn't be marketed as 'autoclavable'.
How do you know it's the cylinder that's wrong and not the balance? I agree it's *probably* the cylinder, but you might want to confirm that somehow. Maybe compare to a glass cylinder that's more likely to be accurate?
ReplyDelete(I'm assuming you don't have any calibrated weights to check your balance. They exist, of course, but I don't usually see them except in places like labs operating under GMP.)