I have a question about distillation.
Or heating in general. When something says heat for 1hr or 1/2 hr do you start counting when you turn on the hotplate? Or do you start counting when the mixture is boiling?
I have a feeling that this issue is affecting my yields as I usually think, "oh this didn't start to actually boil or reflux and so I need to heat it for awhile longer or, maybe it would be better to not heat it for so long even though it wasn't boiling or refluxing untill 15min after I started heating the mixture"
which is correct?
Thanks
5 answers
If it is to boil, start counting when the solution starts to boil. If it says heat for 1/2 hour and some temperature is specified, (such as a real T as in 50 C, then start counting when the T reaches 50 or so but a moderate T heat up to about 40 or 50 and count about 1/2 hour).
what if no temperature is specified?
For example the reaction that I'm doing it says:
"For compound A, the alluminum block temperature should be about 140C, and for compound B, the temperature should be about 120C. Be careful to adjust the reflux ring, if one is visible so that it remains in the lower fourth of the condenser. Violent boiling will cause a loss of product. Continue heating the reaction mixture containing A for 75 minutes.Heat the mixture containing B for 60min."
1-For measuring the temperature of the alluminum block I think that I have to have the thermometer in the aluminum block hole that is exposed to the hotplate itself right?
2- So for this I guess assuming I use compound A, I would heat the reaction mixture for 75 min starting when the alluminum block reaches 140 and the mixture starts boiling? But what if the hotplate reaches the temperature and the mixture is not boiling or vice versa?
Thanks Dr.Bob
For example the reaction that I'm doing it says:
"For compound A, the alluminum block temperature should be about 140C, and for compound B, the temperature should be about 120C. Be careful to adjust the reflux ring, if one is visible so that it remains in the lower fourth of the condenser. Violent boiling will cause a loss of product. Continue heating the reaction mixture containing A for 75 minutes.Heat the mixture containing B for 60min."
1-For measuring the temperature of the alluminum block I think that I have to have the thermometer in the aluminum block hole that is exposed to the hotplate itself right?
2- So for this I guess assuming I use compound A, I would heat the reaction mixture for 75 min starting when the alluminum block reaches 140 and the mixture starts boiling? But what if the hotplate reaches the temperature and the mixture is not boiling or vice versa?
Thanks Dr.Bob
The first statement, what if no temperature is specified is another question. (not supposed to be refering to the bottom example which has temperatures given)
If no T is specified I would count from the time the hot plate reaches its approximate T and the mixture in the flask has been heated somewhat. At least I wouldn't count from a cold start. For #1 above, yes. For #2 yes. Assuming the mixture is to boil, I would count from the time it starts boiling.
Thanks very much Dr.Bob
3 answers