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Author: Subject: Melting point apparatus.
axehandle

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Utter confusion.

No dispute, just me forgetting what thread I was in, confusing me into a sidetrack that had no relevance at all.

Interesting, the entry in the dictionary. "Vulgar latin".... I can't really picture vulgar latin. "Coitus"?

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Magpie
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hate capillary tubes

Filling capillary tubes with crystals has always been a real pain for me. Your other advantageous stated above are compelling also.

Do you have any data on the precision of your device? I'm just wondering how well a glass immersion thermometer works as a contact thermometer. Perhaps because you have essentially inserted it into a block of aluminum with a snug fit it is fairly precise. Let us know please.
Tacho
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As you said, it's a thermometer inserted in a block of aluminum close to the sample, not much can go wrong here.

However, as stated by Murphy's law, there is a catch. If you heat too fast, and I usually do, when the sample melts and I turn off the power, the temperature in the thermometer keeps going up, showing the equilibrium between aluminum and mercury bulb was too slow for my heating.

Therefore, as any book on experimental chemistry should tell you, you must heat slowly when you get close to the mp temperature. A dimmer is necessary if you want precision.

Since the device has recently drawn so much attention, I will do some carefull controled tests with pure compounds to check it's precision.
Tacho
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I did tests using resorcinol and vanillin. Resorcinol melted at 106-108 when Aldrich catalog says it’s mp is 110-113 and the Merck index says its 109-111. Vanillin melted at 79-80, Aldrich says 81-83, merck says “80-81 (81-83)” (sic!).

Just in case you are interested:
Starting at 40 degrees:

5minutes-83 degrees
7minutes-100 degrees
10-120
15-145
20-162
25-173
It stabilizes at 180 degrees in 30 minutes. I use a hotplate or blowtorch to reach higher temperatures.
Magpie
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close, but close enough?

Thanks Tacho for all your work. I don't have enough experience to comment on these results. Perhaps someone else does.

It would be valuable to compare those results to what you would get with a Thiele tube or one of the modern commercial devices. But I'm not asking for you to do more work (while I just sit and come up with more questions).
Hermes_Trismegistus
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Is experience neccessary?

Slow heating, accurate results.

Good job!

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Tacho
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 Quote: Originally posted by Magpie Thanks Tacho for all your work. I don't have enough experience to comment on these results. Perhaps someone else does. It would be valuable to compare those results to what you would get with a Thiele tube or one of the modern commercial devices. But I'm not asking for you to do more work (while I just sit and come up with more questions).

It depends, Magpie. If you are testing something for PURITY, you have to count on precision. Even then, notice that two respectable books give different mp for very common compounds. This happens a lot.

Sometimes the problem is “did the expected reaction happen or not?”. If I the starting compound has a mp of 180ºC, and I am trying to get a compound that has a mp of 90ºC and my resulting compound has the right color and a mp of 80ºC, it’s very likely that I did something right. Recrystallise in the proper solvent and mp goes to 85ºC: very, very likely that I did something right.

Usually, impurities bring the mp down, but if you quote me on this, I’ll deny it and call you a lier. Unless I’m right.
axehandle

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Thought... or lack thereof....

Hmmm. Thinking that perhaps replacing the Al with Ag, and using a Pt thermocouple... hmm. No, that will have to wait.

Brilliant work, Tacho! Me, I'm going to play with sand now.

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Harmless

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MP apparatus

Wow... didnt expect to fuel such a discussion. Thanks tacho for the hard work. I really appreciate it. now to work building one.
Magpie
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good discussion

In my background (academic) I was testing for purity - so precision was important. But I see your point about differences among the experts in the references.

I like axehandle's idea about trying a thermocouple.

I believe you are right about contaminants lowering the melting point. Remember "freezing point depression." It is quite sensitive to this.
axehandle

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Actually my mentioning of both silver and platinum was a hard-to-catch joke referring to my metal fetishism. But there are some valid points as well: 1) Silver has a higher thermal conductivity than Al. 2) A Pt resistivity metre is more accurate. However, there "improvements" would rise the cost of the device by at least a factor of 10, and they would only (guesstimate) lead to a resolution improvement of 2C. I think Tacho's device is fine as it is. Which is proven by the fact that's it's efficient, cheap, and accurate, as well as easy to build.

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Magpie
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metal fetish

Axehandle: at least you did not suggest the use of Rheingold. That would have increased the cost factor infinitely!
axehandle

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 Quote: Axehandle: at least you did not suggest the use of Rheingold. That would have increased the cost factor infinitely!

No, no, no, no, no! We all know that Pt has better thermal conductivity than Au! Platinum is clearly the way to go. Or... perhaps iridium, but I'm not sure about that one.

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mycoguy
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an ex buddy of mine had one of those wooden hot boxes. He would use it to check purity of substances he was aquiring. How many dealers do you know that go through that much trouble?

Anyways. I just came up on an adjustable radioshack soldering station. Unfortunately, the thing goes up to something like 700F.

I'm planning on making one of these MP apparatii out of it, however, I just need to figure out how to drop the temp a bit more, in order to put it in a more controllable range. It even has a built-in digital thermometer. Pretty cool stuff.
mick
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The benefits of a good quality melting point set up must be to check what you have through the mixed melting point method and it will also to to give you a good idea as to how pure the compound is. The closer the shrink temp. is to the melting point range (and the shorter the melting point range) the purer the compound is.
mick
armo
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Melting Point Controller

Hi Tacho,

Nice piece of equipment. However I would use a electronic variac to control the rise of temperature. This kind of controller can be buyed in electronic supply house or built in a DIY basis. I have several circuits that I´ve built in the past. I use then mostly to control my iron solder temperature.

If you build it its going to cost less than US$5.00 armo armo Harmless Posts: 11 Registered: 23-8-2005 Location: Brasil Member Is Offline Mood: Cool.... MP higher temp To reach higher temperatures you can use resistive elements for 100W welder. armo armo Harmless Posts: 11 Registered: 23-8-2005 Location: Brasil Member Is Offline Mood: Cool.... Melting Point again Hi, Tacho Have you calibrated your thermometer? If you take 10 thermometers at the same temperature you will end up with 10 differents readings. Unless you use a precision thermometer all results are not true. You can not compare with literature... armo Tacho International Hazard Posts: 582 Registered: 5-12-2003 Member Is Offline Mood: No Mood Sorry armo, I haven't seen your posts before. I have 1 thermometer that is very acurate. It reads 0°C in water-ice mix and 100°C in boiling water at sea level. My other thermometers have a piece of tape glued to them saying how many degrees I must add or subtract to find the right temperature. If I were to build another device, I would make it out of plaster (gypsum) and only heat the small stage under the sample. A dimmer is a good idea, but maybe just a diode, cutting the power in half, would be enough to ensure a slower temperature rise when needed. cnidocyte Hazard to Others Posts: 214 Registered: 7-7-2010 Member Is Offline Mood: No Mood I'm glad I came across this thread. I was planning on clamping a boiling tube and thermometer into an oil bath but that would be useless for higher MP compounds. I'm definitely gonna build one of these. Amy Winehouse Harmless Posts: 44 Registered: 4-10-2011 Member Is Offline Mood: Saucy DIY Melting point apparatus Im sorry if you opened this expecting me to present an amazing DIY melting point apparatus, but the purpose was more to hear about other's homemade melting point tester experiments. Most melting point apparatuses(sp?) are non-garage chemist friendly i.e. expensive as f$#%. I know there are other cheaper ones like the Theile tube that require an open flame or w/e, but I was actually thinking of something more along the lines of completely homemade, even if it has a small margin of error. Beyond visual observation, I've been pretty blind as to the purity of my solids, has anyone successfully created one? Seems like some inert oil and a heat source is all you need, maybe im completely wrong.

What are some of your successes/experiments?
watson.fawkes
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 Quote: Originally posted by Amy Winehouse Beyond visual observation, I've been pretty blind as to the purity of my solids, has anyone successfully created one?
I haven't made one myself, but I've been thinking about it. One points of the design I'd build is to dedicate a small webcam into the device. They are, relatively speaking, dirt cheap nowadays. With the help of a bit of software, detecting the visual changes could be completely automated. Furthermore, at the cost of some even-cheaper storage, the video record of the changes could be recorded.

With a second version of the control software, the system could cycle the temperature of the sample both up and down, and thereby also measure liquidus and solidus points, important for getting a handle on impure materials and assessing purity.
Nicodem
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19-4-2012 at 08:13
Dr.Bob
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There was a company out of Cal. that advertised an automated system using a small camera for MP that was under \$1000. Haven't seen them for a while, but the big instrument companies all have there versions for much more. Likely, the big companies bought the little one and closed them...

It would be pretty easy to do, and if you automated it, it would not matter if you only did a degree a minute, so you could ramp it very slowly and get good data, or even do multiple samples in parallel that way. The hardware is trivial, as shown above, and the video analysis just needs to look for a change in pixels at certain spots to detect a change, the operator should be able to look at the video and then confirm that is was a melting point. Or the system could get crude values, then ramp back near the MP on the 2nd run and have an operator watch it on a screen for confirmation.
S.C. Wack
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Before going to extremes unless you're looking for a Project, I strongly suggest trying a thermocouple, candle and capillary, twine, oil, and test tube first. A heat gun or electric Bunsen for not-flame, live steam, spent fuel rods...

Duh...and a magnifying glass.

[Edited on 19-4-2012 by S.C. Wack]

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RonPaul2012
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How accurate is a thiele tube anyways ?

I just tried using a test tube suspended over a hotplate , it seems to work ok but it is kind of a hassle to set up .

I really wouldn't want to build a MP apparatus though.

I just want to test the MP of my products in a tidy fashion.
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 Sciencemadness Discussion Board » Fundamentals » Reagents and Apparatus Acquisition » Melting point apparatus. Select A Forum Fundamentals   » Chemistry in General   » Organic Chemistry   » Reagents and Apparatus Acquisition   » Beginnings   » Responsible Practices   » Miscellaneous   » The Wiki Special topics   » Technochemistry   » Energetic Materials   » Biochemistry   » Radiochemistry   » Computational Models and Techniques   » Prepublication Non-chemistry   » Forum Matters   » Legal and Societal Issues