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semiconductive
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OK, I'm going to change one variable; and retry the melting experiment.
The variables I can think of are pre-drying by baking, kerosene as a reactant, urea brand (fertilizer) has contaminant, air may be necessary to melt
urea normally; for it may interact either with oxygen or nitrogen during the melting process.
Did I overlook anything? ( suggestions welcome. )
I don't think kerosene ought to react with urea, but ...
Instead of kerosene, I'm going to try silicone oil, cosmetic grade.
I'll do the same thing, wire the soldering iron to the tube and make good contact; then bake about 1/2CC of urea at just above the boiling point of
water for several hours to dry it; finally do a temperature sweep up to 155 [°C] and find the melting point of dry urea (if any.) I'll keep the
picture count low, if I don't see a reaction and I'll just sumarize. If you want more pictures, just ask...
Setting the soldering iron to 50% power, I get a nice 105 [°C] temperature.
I don't see any steam or bubbling... but only a slow swirling of the silicone due to convection of heat. This suggests that the urea may be dry,
already, in the bag. I am probably only driving off surface moisture, then, from the air.
This is how the drying begins:
I baked it dry for four hours.
Then, I did a temperature sweep all the way up to 100% power over 3 hours.
Urea did not melt under silicone, either!.
1CST silicone oil, on the data sheet says it boils at 151 C.
Picutre says 154C, with rolling boil and a significant amount (10 to 20%) of silicone refluxing. Little bubbles began happening around 143C.
So, my thermometer is giving reasonable values.
I don't see any discoloration, which I expect silicone would cause if it chemically reacted. I'm more inclined to believe urea might react with air
when melting, or perhaps I have an impurity which strangely raises the melting point rather than lowers it.
Not sure what to try next.... but this is repeatable. So something unusual is definitely going on. I've ordered reagant grade urea prills, from
Loud Wolf™ to check for contamination issues.
Edit: Note, allowed last experiment to reflux silicone at 154 [ °C ] for 8 hours. No melting or fusing was observed. After cooling, there was no
residue on the walls of the test-tube. Urea either doesn't dissolve in silicone, or what little does dissolve is temperature independent. However, a
slight yellow color showed up on the urea prills during cooling.
I'm not sure if it's just inconsistency in the lighting. The color went back to white, slowly, while at room temperature; 21 [ °C ].
[Edited on 30-7-2024 by semiconductive]
[Edited on 30-7-2024 by semiconductive]
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bnull
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Okay, that's very odd: it just, sort of, gave up melting? It did melt a few days ago, right? Mine melts a few seconds after heating the test tube with
a spirit lamp and gives off a strong smell of ammonia if it stays long enough over the flame. On cooling, it solidifies into a white fibrous mass. Are
you sure you didn't mistake sodium or potassium nitrate for urea? The last time I bought NaNO3 as fertilizer, it was in prills; the seller
could have sold me urea and I'd never know.
The color you saw may have come from the orange silicone. There was nothing else in the tube to do that. The color change on cooling, well, I'll give
you an instance: phenolphthalein dissolves in molten urea and the color goes from faint yellow (prill-sized amount) or red (a penknife tip full) when
molten, to colorless when solidifying, and then pink or purple as it cools (Clark's paper again and personal experience from ten minutes ago). The
dyestuff dissolves in the hot oil and partitions with urea.
A suggestion: try a small flame with only urea in the test tube. If it still refuses to melt, it is not urea. You may have accidentally converted it
to biuret. Biuret becomes violet in the presence of copper ions.
Quod scripsi, scripsi.
B. N. Ull
P.S.: Did you know that we have a Library?
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semiconductive
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| Quote: | | Okay, that's very odd: it just, sort of, gave up melting? It did melt a few days ago, right? |
It melted, under nearly the same conditions -- using the same urea, same soldering iron and same test tube, in this post:
https://www.sciencemadness.org/whisper/viewthread.php?tid=15...
But, there are two things different.
There was copper oxide in the successful melt, and I ran a little electricity through urea while melting.
But,when I added more urea to the melt, it became increasingly solid.
That's why I ended up adding an ester, eg: to try to keep it melted while electroplating.
It seems reasonable that copper oxide was breaking through the surface layer of whatever is on the urea and made a eutectic kind of melt. When I
added more urea, it dilluted the copper ions.
| Quote: | | Are you sure you didn't mistake sodium or potassium nitrate for urea? The last time I bought NaNO3 as fertilizer, it was in prills; the seller could
have sold me urea and I'd never know. |
I tried the experiment again. It's consistent.
I didn't misread a label and pull chemicals from the wrong bag. Whether the product is really urea ...
The bag is clearly labeled "the SEEP plant, UREA 46-0-0 granular fertilizer."
I have to wait until I get reagent grade before I can rule out the seller giving me fake product.
But, It did have a slight ammonia smell from it when melting with copper oxide.
The odor went away after getting above the boiling point of water for a while.
It's only pure urea prills under non-polar liquids that are refusing to melt.
Placing urea in a test tube, exposed to air, and heating tube with same soldering iron at 100% from the start; result:
Urea nearest the soldering iron began melting within seconds. Liquid wicks into remaining urea and causes fusion of the surfaces.
Mild ammonia smell is present.
But: I could only smell the urea and oils, no ammonia, when it was under kerosene or silicone oil for a while.
The thermal contact area is limited. I have to add more urea prills to get enough liquid. But it melts fine when there's enough prills to get the
liquid level up to where the iron is. In fact, I need to turn down the heat; the liquid is bubbling and smells strongly of ammonia, now.
[Edited on 31-7-2024 by semiconductive]
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bnull
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| Quote: | | But it melts fine when there's enough prills to get the liquid level up to where the iron is. |
Hold on. Is the soldering iron touching the tube at the same spot as that day?
I tried melting urea with a soldering iron just now, having paraffin wax as non-polar companion (for lack of a better word). The wax melted, then I
shook the test tube a little to dislodge the air bubbles, and moved the soldering iron to the bottom of the tube. Urea became a little stubborn to
melt, with a small puddle of liquid on the bottom and still solid urea on top. I turned off the iron and used the spirit lamp. That was when the
prills melted exactly as before.
Effect of different heat capacities perhaps? cp for urea is above 60 J/mol*K and for paraffin (average, of course) is close to 2 J/mol*K.
Quod scripsi, scripsi.
B. N. Ull
P.S.: Did you know that we have a Library?
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semiconductive
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Quote: Originally posted by bnull  | | Quote: | | But it melts fine when there's enough prills to get the liquid level up to where the iron is. |
Hold on. Is the soldering iron touching the tube at the same spot as that day?
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Yes.
You can see it in the picture, eg: the seam line of the soldering iron's barrel.
The soldering iron is aimed downward at 75 to 80 degree angle behind the tube.
This causes the hot iron barrel to make solid contact with the test tube right where the glass changes from cylindrical to spherical.
The most intense heat always comes in at the point where the test tube changes shape.
There's generally about a 15 degree Celsius difference from the outside-back glass to the front of the tube. I am now putting the thermometer closer
to the front, so it reads cold side. I have been trying to set my experiments up to be as repeatable as possible (for me, anyway.)
The power is computer controlled and changed very slowly over a period of hours.
For example, the last experiment is still running while I write this, many hours after my last post, and the power has only increased by about two
watts.
The temperature of melted urea is about 130 degrees C. Which agrees fairly closely with the literature I found. Although it stays melted even at
lower temperatures for a while. The urea also is slow bubbling ammonia gas out, at the point of contact with the soldering iron. There is liquid
urea all the way to the bottom of the test tube, but then it goes solid as the front of the test tube (farthest away from the iron) is approached.
| Quote: |
I tried melting urea with a soldering iron just now, having paraffin wax as non-polar companion (for lack of a better word). The wax melted, then I
shook the test tube a little to dislodge the air bubbles, and moved the soldering iron to the bottom of the tube. Urea became a little stubborn to
melt, with a small puddle of liquid on the bottom and still solid urea on top. I turned off the iron and used the spirit lamp. That was when the
prills melted exactly as before. |
OK. Different heat sources have different thermal transfer rates.
| Quote: |
Effect of different heat capacities perhaps? cp for urea is above 60 J/mol*K and for paraffin (average, of course) is close to 2 J/mol*K.
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I'm not sure.
I suspect it would have to be more than just the capacities.
The long time periods of my experiments allow thermal equilibrium to be approached closely. You'll notice in the picture that the urea is melted
everywhere except the front round bottom where the most surface area exists for the tube. There are no visible changes going on, as the heat input is
very slowly changing.
The location of the heat input is why I always make sure to add enough chemical to go above the rounded bottom. Theres an artificially cool spot
there.
But, I don't know:
Is urea much more heat conductive than silicone oil and kerosene, both?
For, the silicone oil was at 155 degrees in the coldest part of the tube. That's 25 degrees overheat in the coldest part of the tube.
In order for urea to not melt, at all, when in contact with 155 degree liquid at least at the top ( for hours ) ... the Urea would have to be
conducting the heat away. I could insulate the bottom of the test tube, I suppose, to test for that. I've got fumed silica, pearlite, and plaster;
so I could make an insulation cup. But it seems strange that none of the urea melted, not even the stuff on top.
Ohh! fully melted, finally!
[Edited on 1-8-2024 by semiconductive]
I need to improve the heating program a bit. I'd like to make detailed comments, but can't since there's more than one source of possible error. I
told the program to maintain power at 62.25% permanently before heading to bed.
There was some urea condensing as solid, cotton candy like, on the test tube walls. I decided to place the melt under silicone to see if it stopped
bubbling or triggered re-solidification; it didn't re-solidify for at least 4 hours; but Ammonia gas output reduced rapidly as temperature began to
rise slowly even though power input was programmed not to change, anymore. Then some brown flecks formed in the urea. The temperature dropped (not
sure when/why, need to modify program), and found tube as follows in the morning.
I've told the program to increase power slowly this morning, rather than maintain it; we'll see if the urea under silicone remelts or not.
[Edited on 1-8-2024 by semiconductive]
The answer is a definite "no.". It absorbed the silicone oil slowly, and then started blackening where the iron touched the tube. There was some
kind of chemical reaction. ugh.
Maybe it's forming biuret, or triuret ; although the temperature is not high enough according to what I see online.
I'll try it again with a new program that compensates for line voltage changes, to guarantee my heat quantity is known and doesn't vary;
Right after I do a simple test dropping a urea prill into pre-heated silicone at 131 [ °C ]. I'm going to guess it melts, since it was exposed to
air before hitting something hot; and hopefully I'll get to see see how long it can stay at 131 before turning brown or solid white.
[Edited on 1-8-2024 by semiconductive]
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semiconductive
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Posts: 311
Registered: 12-2-2017
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Ok, I've put two CC's (measured) of silicone oil in the test tube.
Nothing else is in there except the temperature probe.
I figured out about how much energy it took to get the oil to 131 [ °C ], 72% or 20.05 [ W ] of power last night.
I programmed the KASA plug to report the voltage and current being supplied to the soldering iron so I could graph it and calibrate my temperature
set-point.
Note:
This is a graph demonstrating the scientific concept of systematic error; even though I've tried to compensate for voltage changes on the power line.
Apparently, I did it a little wrong. I've got to fix this before I can get really reliable data for science experiments.
The spot where the temperature goes 'up' is in the hours between midnight and 8AM, local time. This is likely where all my neighbors have turned off
their lights and gone to bed. But it might also be a time where the climate control in the lab (basement), changes a bit. I'll have to set up a
second thermal monitoring ohm-meters in order to be able to compare what's inside the testube vs. the air around the test tube.
But, anyhow:
The temperature of the soldering iron bath rose 3 degrees during this dark of night, and I see increased power output to the soldering iron (on
average) during this time.
So, power line regulation issues are a likely cause why the temperature rose on my last experiment, instead of being evidence for a chemical
reaction.
I'll let this program run for another day or so, and get a long-term plot of the error and make sure it repeats as it hypothetically should, every
night.
Then I can try to reprogram my heater code to *properly* compensate for line voltage variations.
Well, I'm winning. Finding a source of error is half the battle to getting rid of it.
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semiconductive
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Posts: 311
Registered: 12-2-2017
Location: Scappoose Oregon, USA.
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There is definite climate control change on top of line voltage fluctuations. After several days experimenting, I've found that the the KASA smart
plug's current sensing is too coarse (around 100mW steps) to be accurate. The voltage sensing is OK, though, and matches what my multimeter says my
power line is doing.
I've written a power regulation script that is closed loop, based on the line voltage and a manually measured resistance. ( Will post a link, later.
)
Regarding the Owon temperature probes:
There are definite temperature calibration and drift problems; at least 1 [°C] scale drift/error, and 6 degree offset error.
I'll be attempting to solve those, here:
Suggestions, welcome.
Owon BT41T+ multimeter calibration
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