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Author: Subject: watch glasses, need pyrex?
jsc
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[*] posted on 17-8-2014 at 18:22
watch glasses, need pyrex?


I sometimes want to evaporate a fluid to obtain film residue, but when I use a watch glass to do this, they often break, even if I try to direct the flame at the solvent.

Should I try to get boro/pyrex watch glasses for this, or am I just using the wrong tool altogether?

Typically I am doing this to see how pure my solvent is. For example, if I have water, I can estimate the purity of the water by examining the residue. For example, lets say I am washing a precipitate of a salt. After each wash I evaporate a quantity on the watch glass. By looking at the residue I can judge the amount of salt left and tell roughly how effective the last wash was. Also, lets say I say I do a distillation and expect the distillate to be pure. By evaporating it in a watch glass I can tell if it has any dissolved solids in it.
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TheChemiKid
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[*] posted on 17-8-2014 at 18:30


What solvent are you trying to burn off? If it burns with a cool flame, it is ok, but if there is a high temp flame, you need nice glass.
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jsc
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[*] posted on 17-8-2014 at 18:43


Usually water or alcohol. If I do it really slow, I generally succeed, but if I get impatient and heat the glass more aggressively then it breaks.

I actually just ordered some borosilicate watch glasses. I guess thats the only solution.
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Oscilllator
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[*] posted on 17-8-2014 at 21:45


Are you using a butane/propane lamp? If so, you should consider switching to an alcohol one. much more gentle.
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[*] posted on 18-8-2014 at 03:56


Yeah, alcohol flames are much cooler than alkanes, but they are still hot enough to easily evaporate/boil most solvents.
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[*] posted on 19-8-2014 at 18:43


Perhaps you could heat the watch glass over steam, from boiling water in an Erlenmeyer flask. I don't think it would noticeably hinder evaporation. You could also use a sand bath, like a metal can with ~2cm of sand in it, and heat that over a flame.



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