Sciencemadness Discussion Board - Are ground glass thermometer worth it?

Are ground glass thermometer worth it?

Romain - 20-11-2014 at 08:09

Hi,

I'm planning on distilling nitric acid (69% and maybe fuming) sometime in the future and was wondering if I should bother buying a ground glass one or not as it's VERY expensive.

Basically I can either buy an o-ring thermometer adapter and a thermometer for 30$, or a 14/23 ground glass thermometer for 60$. Both go from 0 to 200°C.

Advice?

Dr.Bob - 20-11-2014 at 08:34

If you want to do a real vacuum distillation, then a ground glass joint is essential, as (in my experience) all o-rings will leak to some degree, but a tapered joint will seal better under vacuum. Also, glass is resistant to nearly everything, o-rings will resists some things well, others not so good. Real rubber will not last long in nitric acid or much else, viton or fluoropolymer might hold up a while. Make sure what the o-ring in the adapter is made of, if you go that route.

If you do buy a ground glass therm. for general use, I would get a 10/30 (or 10/18) one, as those are the lab standard joint therm size (at least in the US), and cheaper. This assumes that you have standard joint glassware already. I use a 10/30 one and it works with standard glass lab distillation apparatus, most of which has a 10/30 joint for 24/40 and a 10/18 or 10/30 joint for 14/20 (short paths mostly use 10/18).

jamit - 20-11-2014 at 09:12

I agree with Bob strongly... I have both 10/30 and 10/18 and they are indispensable when doing vacuum distillation.
So, yes, they are worth paying the extra.

Little_Ghost_again - 20-11-2014 at 09:24

Some times if you look really carefully at job lots of glass ware on ebay you can spot one or two in the lots. I have won joblots with loads of glass ware in and a thermometer for less than the thermometer would cost on its own.
I always examine the pics on a large screen and look really closely

Metacelsus - 20-11-2014 at 09:28

Get the ground glass.

Romain - 20-11-2014 at 09:52

Thanks for your replies.

I live in Europe so the standard here is 14/23.
You all seem to agree that it's preferable to get the ground glass one if I want to do vacuum distillation. I don't have a vaccum pump yet but I don't want to limit my possibilities so I'll buy the ground glass one. I'll check out eBay for a cheaper one too!

Lambda-Eyde - 22-11-2014 at 06:10

Get both. They have separate uses. A ground glass thermometer is nice to have for distillations, especially under vacuum as others have noted. However, it is in most cases useless for when you want to stick a thermometer into an angled neck of a flask, because of the fixed immersion. If you get a thermometer adapter with O-ring seal, you can also use it for bubblers, tubes etc. Another alternative is using a thermometer pocket, in which the thermometer rests in a glass well with a fluid to aid heat conduction. This way, the inside is only exposed to glass, as with a ground glass thermometer. And they're much cheaper. If you decide to get a ground glass thermometer, make sure you get the right immersion length. 76 mm is standard on most distillation adapters. And steer clear of mercury ones.

[Edited on 22-11-2014 by Lambda-Eyde]

DJF90 - 22-11-2014 at 06:43

Whats do you think is wrong with mercury ones? I have nine of them and they're all fine. I've used a spirit one too and that was also ok.

Lambda-Eyde - 22-11-2014 at 10:18

Quote: Originally posted by DJF90

Whats do you think is wrong with mercury ones? I have nine of them and they're all fine. I've used a spirit one too and that was also ok.

They work just fine. It's just that handling mercury in a long, fragile glass tube very frequently obviously raises the risk of breakage.

DJF90 - 22-11-2014 at 10:58

Obviously a certain degree of care is required, but thats true of all scientific glassware.

Romain - 23-11-2014 at 06:00

The mercury ones are more expensive anyway... Though they have some kind of advantage over the spirit one don't they? If not why would anyone buy them?

Also thanks for your input about the immersion length, Lambda-Eyde. I had chosen the 55 mm one, so I switched for the 75 mm one.

Lambda-Eyde - 23-11-2014 at 06:38

Quote: Originally posted by Romain

The mercury ones are more expensive anyway... Though they have some kind of advantage over the spirit one don't they? If not why would anyone buy them?

Also thanks for your input about the immersion length, Lambda-Eyde. I had chosen the 55 mm one, so I switched for the 75 mm one.

You should double check this with your distillation adapter to be sure.

Oscilllator - 23-11-2014 at 18:13

In the past I have made an O-ring around my thermometer using PTFE tape. The seal this made was strong enough to hold up to the aspirator vacuum I used to distill nitric acid.

Dr.Bob - 23-11-2014 at 19:24

Quote: Originally posted by DJF90

Whats do you think is wrong with mercury ones? I have nine of them and they're all fine. I've used a spirit one too and that was also ok.

I love mercury ones, they work well, are accurate, and work well with older Hg sensing temp controllers, other than the few times I have broken one. But they are difficult to ship legally, and many work places have banned them due to OSHA/EPA and other regulatory issues. Lots of schools have also stopped allowing them, due to the clean up issues WHEN they are broken (not if).

kjpmi - 26-11-2014 at 14:06

Quote: Originally posted by Dr.Bob

Quote: Originally posted by DJF90

Whats do you think is wrong with mercury ones? I have nine of them and they're all fine. I've used a spirit one too and that was also ok.

Just chiming in about mercury thermometers. I really prefer them over the alcohol based thermometers. But like everything else, it depends on the situation.

Mercury thermometers have a wider range. I've noticed they're accurate at relatively high temps where alcohol based ones aren't as accurate. Mercury has a uniform coefficient of expansion even at high temps.

Alcohol based thermometers can measure below -40 where mercury can't. But I don't know if I've ever had to actually measure a temp that low.