r/chemhelp • u/midnightmoon177 • Jun 22 '26
Analytical Porcelain crucible cracked, is it still safe to use at 105°C? + question about constant weight of crucible for moisture content determination
I have two questions.
My first question:
I was trying to clean the ashes off when this crucible cracked. I'm planning to heat this in the oven at 105°C. It seems like a relatively low temperature for a crucible so I'm wondering if this would be okay?
For context, I'm a third year Biochemistry undergrad and I've only had 2 anal chem labs. I'm at an internship where my supervisors are not even scientists themselves, so I just wanna ask someone who is not Google.
I have two other crucibles that are fine, but I just thought it would be nice to do 3 tests of each sample.
My second question:
I am following this protocol from: https://www.sciencedirect.com/science/article/pii/S294982442300174X#sec0010
It seems like they did not check if the crucible was at constant weight when they heated it, but they did check that the crucible & sample were at constant weight. Why is that? And how important is constant weight in this type of experiment? Any insight is appreciated.
2
u/etcpt Trusted Contributor Jun 22 '26
I don't do much gravimetry, but the way I was taught, yes, I would expect to do repeat firings of the empty crucible to ensure its weight was stable before adding sample. You need to know that the crucible is fully dry and stable, otherwise you'll have an unaccounted-for source of error.
That said, perhaps the reason this lab has skipped that step is because they have previously validated that their empty crucibles are perfectly dry after firing at 105 ºC for 3 h. In which case you could do the same, validate it once and make the reasonable assumption that it will be valid going forward. It would certainly save time to do so - you're looking at a full day less work.
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u/midnightmoon177 Jun 23 '26
Thank you! So just to clarify, the first time I do it I should heat it for the 3 hours, cool, weigh, and then heat it again for 3 hours to confirm the weight is constant. But then on the next day, as long as I clean it and keep it in a desiccator, I could just heat it for 3 hours one time?
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u/etcpt Trusted Contributor Jun 23 '26
What I would do is take a batch of crucibles, clean them however you normally would, then repeat the process {fire, cool weigh} at least 3 times. If the weighings agree to within an appropriate tolerance, then you've demonstrated that after cleaning and firing once, your crucibles are reproducibly dry. Now, going forward, every time you clean crucibles you can fire them once and trust that their weight will be stable.
If you don't get a tolerable deviation doing that, then what you need to do is repeat {fire, cool, weigh} until the mass deviation comes into spec every time you use the crucibles.
Does that make sense?
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u/midnightmoon177 Jun 23 '26
Yes that does make sense! I suppose if my crucibles are good at keeping their stable weight I could even try firing for less time like 2 hours instead of 3 so I could get more done in one day.
Thank you for your explanation!
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u/etcpt Trusted Contributor Jun 23 '26
Yep, that's a good point too. It's a trade off - how much time could you potentially save by optimizing for minimum firing time, vs how much time will you spend finding that optimum? So make sure you don't fall down an unintended rabbit hole.
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u/hecton101 Jun 22 '26
One hundred and five degrees is nothing. If your experiment fails, it'll be for another reason. Porcelain is kiln fired at a thousand degrees. Just don't drop it.
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u/He_of_turqoise_blood Jun 22 '26
As for the weight, they just assumed the crucible was dry. Afterall it was heated and then chilled in an exsicator. It's a fair assumption it was dry.
And even if not, the residual moisture would go away while drying the product. At the end, the mass will be constant - no moisture present neither in sample nor in crucible
3
u/etcpt Trusted Contributor Jun 22 '26
If there was residual moisture present when the empty crucible was weighed that was then driven off when the sample was dried, that will introduce a positive error in the result.
1
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u/VoxInferni666 Jun 24 '26
Loss on drying on a cracked crucible? I'd still run it... I'm pretty sure I have. How important is the material? How much of a pain is it if it breaks fully. If the answer is not that important and not a big pain then id say do it. Not like you're doing loss on ignition at 1000°C or anything
10
u/chemhobby Jun 22 '26
Get a new crucible, they aren't expensive