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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: thorium on January 31, 2008, 06:12:45 PM

Title: Can someone explain this to me
Post by: thorium on January 31, 2008, 06:12:45 PM

Is there an adapter for bucner funnel Ground Joint?

is there an apparatus specially designed to measure Melting Point? Other than filling small cappilary pipettes with the substance then dipping it into H2SO4 with a thermometer and then watching it to melt? I mean is there an easier way? A machine designed to measure the M.P?

And what about claisen adapters? what size are the ground joints? Say I have 14/23 ground joint addition funnel and I want to use my 29/32 reflux condenser at the same time is there a Claisen adapter adapted to this?

Title: Re: Can someone explain this to me
Post by: Arkcon on January 31, 2008, 06:28:09 PM

Quote from: thorium on January 31, 2008, 06:12:45 PM

is there an apparatus specially designed to measure Melting Point? Other than filling small cappilary pipettes with the substance then dipping it into H2SO4 with a thermometer and then watching it to melt? I mean is there an easier way? A machine designed to measure the M.P?

I felt the same way in college and asked the professor.  And he said no.  And in my experience in industry, he was right.  That is exactly how raw materials are identified.

In my day it was a thiele tube of cottonseed oil, you're using H2SO4, at work it was a synthetic high quality oil. In school we relied on a gentle bunsen burner flame, at work it's a thermostatically controlled electric heater for the oil bath. 

But in the end, it's a capillary of a tiny amount of solid, attached to a thermometer, a black background, and your eyes, that tells you the m.p.

Title: Re: Can someone explain this to me
Post by: ARGOS++ on January 31, 2008, 06:44:21 PM

Dear Thorium;

What you may think about (Only as one Example):   "FP62 Melting Point Apparatus (http://us.mt.com/mt/filters/products-applications_analytical-instruments_thermal-values_fp62-meltingpoint-apparatus/FP_measuring_cell_fp62_0x00024948000281c70003f2d1.jsp)”


Good Luck!
                    ARGOS⁺⁺

Title: Re: Can someone explain this to me
Post by: Arkcon on January 31, 2008, 07:01:01 PM

Quote from: ARGOS++ on January 31, 2008, 06:44:21 PM

Dear Thorium;

What you may think about (Only as one Example):   "FP62 Melting Point Apparatus (http://us.mt.com/mt/filters/products-applications_analytical-instruments_thermal-values_fp62-meltingpoint-apparatus/FP_measuring_cell_fp62_0x00024948000281c70003f2d1.jsp)”


Good Luck!
                    ARGOS⁺⁺

Ahhh...very nice.  That's what I was hoping for back in my college days.  It's about time they automated the process.  I don't want to seem lazy, but like the blurb says, it's nice to take the human bias out of the analysis.  Still, I'll bet that, at least once in a while, the instrument fails with some sample or another.

Title: Re: Can someone explain this to me
Post by: enahs on January 31, 2008, 07:50:01 PM

Is this good for a mixture where there might be a relatively large range?

Title: Re: Can someone explain this to me
Post by: ARGOS++ on January 31, 2008, 08:37:15 PM

Dear Enahs;

Could this be a part of a possible answer for you?:   "FP62 Document 1 (http://www.my-mt.com/reconditioned/Documents/FP62e.pdf)”

And also:  "ftp://ftp.dep.state.fl.us/pub/labs/lds/sops/3744.pdf”

Good Luck!
                    ARGOS⁺⁺

Title: Re: Can someone explain this to me
Post by: thorium on February 01, 2008, 07:39:34 AM

And about the vacuum pumps. As I do see there're some pumps which reduces the pressure up to 0.0001 Torrs! This is really unbelieveable! Is 1Torr=1mmHg (SO THIS MEANS 0.0001mmHg) or am I REALLY mistaken? Can't the glassware implode under this pressure?

Title: Re: Can someone explain this to me
Post by: Arkcon on February 01, 2008, 07:55:52 AM

Quote from: thorium on February 01, 2008, 07:39:34 AM

And about the vacuum pumps. As I do see there're some pumps which reduces the pressure up to 0.0001 Torrs! This is really unbelieveable! Is 1Torr=1mmHg (SO THIS MEANS 0.0001mmHg) or am I REALLY mistaken? Can't the glassware implode under this pressure?

The glassware has a pressure rating written on it, I assume the manufacturer disavows liability for exceeding it. 

As for the pumps, I don't know how accurate the gauge is. If it is a small benchtop oil valve pump then maybe not.  A washing machine sized pump attached to a freeze drier, on the other hand, might work to that low.

Given that I don't know how accurate the dial gauge on any lab pump is, you might wonder how I know if I'm over-evacuating glassware.  It could potentially be a problem, so diligence is in order, generally keeping all vacuum or pressurized glassware behind a shield, of some sort.

Title: Re: Can someone explain this to me
Post by: thorium on February 01, 2008, 08:16:14 AM

But there must be an adjustement valve on the machine. If not it wont work for all the glasswares! Is this right?

Title: Re: Can someone explain this to me
Post by: Borek on February 01, 2008, 09:31:13 AM

This is a little bit funny. Assuming external pressure of 760 mmHg difference in forces acting on glassware that has 5 mmHg inside or 0.0001 mmHg inside is in the range of 0.6%. If the glass is strong enough to be used at 5 mmHg it should be able to survive any level of vacuum. 50 mmHg vs 0.0001 mmHg is 7% difference in forces - still not that much, I suppose security margins are much higher.

Title: Re: Can someone explain this to me
Post by: Arkcon on February 01, 2008, 10:36:34 AM

Quote from: thorium on February 01, 2008, 08:16:14 AM

But there must be an adjustement valve on the machine. If not it wont work for all the glasswares! Is this right?

Hmmm... adjustment valve on a vacuum pump, to set to a specific pressure, no that's not now the bench-top oil valve or diaphragm evacuation pumps work.  Pretty much what I'd used was, you evacuated to the pressure you wanted, then blocked the vac. source with a valve.  I've used screw-type pressure regulators, but not vacuum regulators, they may exist for some applications.

Time to clear this all up, Thorioum.  What do you have, and what do you need to do?

Title: Re: Can someone explain this to me
Post by: ARGOS++ on February 01, 2008, 11:22:37 AM

Dear Thorium;

For Glassware: Forces in the direction of “Compressing” is very rarely a problem, more common are its problems for tractive power!, because the corresponding “e-Modules” differ by several factors.
As with a simple water-jet pump you can reach a vacuum of 8 till 11 Torrs (for distillation), so the additional difference is nearly marginal, as Mr. Borek pointed out. With well designed Oil pumps you can easily get low pressurs like 0.1 till 0.01 Torrs.
“Over-evacuating” is ‘not possible’ for usual laboratory glassware.

For PES or “Photon-Electron-Coincidence-Spectroscopy” such powerful pumps (LPP’s) are required to make measurement possible.
But for such low pressure a Lot of special designs are required and you have to overcome several hindrances.
You may know, that by lowering the pressure the “Freie Weglänge” (German, - in English maybe: “free length of path”) increases dramatically for every molecule you have to catch with this LPP’s!
That’s why the diameters of the metallic (Why?) tubings goes over 15 – 30 Centimeters!
Another problem is to get the whole apparatus “clean enough”! – A small forgotten fingerprint can be a disaster!


Good Luck!
                    ARGOS⁺⁺

Title: Re: Can someone explain this to me
Post by: thorium on February 01, 2008, 11:40:49 AM

I need a vacuum pump which can go below pressures 4-5 mmHg. I've found a website called Welch but I cannot understand the terms like

Ultimate Pressure (which is defined by Torr and I've found a vacuum which suits my needs => 2 Torr)

and

Maximum Vacuum (which is "Hg And at the same pump its => 29.85)

They're Reverse-Balanced (meaning that as the ultimate pressure drops Maximum Vacuum rises ???)

 If someone could explain these to me I'd be very happy :) . Which one (maximum pressure or maximum vacuum) should I look for (for the pressures that I want)?

Title: Re: Can someone explain this to me
Post by: ARGOS++ on February 01, 2008, 03:59:09 PM

Dear Thorium;

NO!,  ─  That’s not an Antagonism!

Both values mean the limit of the pump on the “identical side” of pressure/vacuum.
But both scales start not at the same “origin”:

•   ─  “Ultimate/Absolute Pressure” is the absolute Pressure the pump can not go below in any case, but it is possible that you never will reach this low level.
•    ─  “Maximum Vacuum” is in real a Δ Pressure (= Vacuum), that means how far below you can pump “below” the ambient Pressure the pump has to work in (against).

You see if you would have to start in an environmental pressure, let say of 1.5 atm the “Maximum Vacuum” set you a higher “End”-Limit as the “Ultimate/Absolute Pressure” will do it.
But if you start at 0.5 atm. you can not pump to a lower value then the “Ultimate/Absolute Pressure”, even if you have not reached the “Maximum Vacuum”.

The Diagram on the left in the Attachment may help for understanding.

Take care there are other “parameters” which should influence your decision like:
Must your pump be solvent resistant (in a Lab.), then an Oil-Pump may be not the adequate devise, and so on.

I hope to have been of help to you.

Good Luck!
                    ARGOS⁺⁺

Title: Re: Can someone explain this to me
Post by: thorium on February 01, 2008, 07:42:03 PM

You're unexplainably kind and helpful Argos++
Argos++ I've read what you wrote several times but there's something that I can't understand If the pump has got a Max Vacuum value that is 29,85 does it mean that it will go just 29,85 "Hg down the Atmospherical pressure or is this JUST an upper limit?   
 
I want to learn something does the pump that I've found has the requirements that I desire?  

And a note: The very deep (0.0001 mmHg Absolute Pressure) vacuum oil pumps haven't got a maximum vacuum value why?

Thank you very much!

Title: Re: Can someone explain this to me
Post by: ARGOS++ on February 02, 2008, 10:22:53 AM

Dear Thorium;

(Thank you for so much, - too much flowers.)

In case you set the Atmospheric pressure = “ambient” Pressure (as you did;  –  quite often the usual application.)   –   then you are correct/right!
Or even more simplified:  The “Maximum Vacuum” is simply the maximal difference in pressure between inlet and outlet the pump can reach.   (I hope that’s more “visual” for you.)
For that you may also read how vacuum is measured:   "Vacuum Gauge (http://en.wikipedia.org/wiki/Vacuum_gauge)”

To your Note: For “High Vacuum” or “Ultrahigh Vacuum” pumps (in real: wrong naming!) “Maximum Vacuum” doesn’t makes any sense, because they never should work on/in an ambient pressure as high as Atmospheric pressure, that means you should always use a “Pre”-pump of mechanical nature in combination. Such LPP pumps are working very near their physical Limits!

Another Hint: You should never use an Oil or “Ultra/High Vacuum” pump without a “Cryotrap” with liquid Nitrogen. That’s especially for chem. Labs (for Distillation, Sublimation, Reactions, etc.) where you have to face from very small till countable product decomposition. You may think about the Solubility of such Gases in Oil, etc. and the consequences for the “Ultimate/Absolute Pressure”
For this you may read on:  "Cryotrap (http://en.wikipedia.org/wiki/Cryopump)”

To your Selection:  You spent too less information, so “nobody” can make the decision for you.
But some Hints anyway:

• –   I would select a pump with a little lower “Ultimate/Absolute Pressure”, because you have to think about that you will only asymptotic come nearer to the “End”-Pressure of your 4 Torrs, and that will take time. (Also a higher “CFM” may be here of help.) Maybe something like 0.5 till 0.1 Torrs is more adequate, also because the pump will not become younger with time and you may not prefer to call every two months for service.
• –   Also to think about if your applications not in real requires a certain range of pressure like 100 till 4 Torrs, because that will be an additional “imposition” in case of an Oil Pump(for example).
• –   If your ambient pressure is the Atmospheric pressure so “Maximum Vacuum” will be very rarely a problem for “only” 4 Torrs.

How knows, at the end your decision will be a compromise in one or the other direction, not only its prize.
For Vacuum pumps you may also inspect:   "Vacuum Pumps (http://en.wikipedia.org/wiki/Vacuum_pump)”

And for Vacuum Techniques/Engineering:   "Vacuum Engineering (http://en.wikipedia.org/wiki/Vacuum_engineering)”


Good Luck!
                    ARGOS⁺⁺

Title: Re: Can someone explain this to me
Post by: thorium on February 02, 2008, 07:38:14 PM

Thank you veeery much.

So Maximum Vacuum is just an upper limit and it doesn't make any sense for a pump that's going just 2 Torrs?
Is this right? (I want to be sure because this vacuum costs over $4000 which is a really great money!)

And what's CFM?

Thank you again again and again!

Title: Re: Can someone explain this to me
Post by: ARGOS++ on February 03, 2008, 06:53:41 PM

Dear Thorium;

If I assume that in most cases with “Vacuum” you are meaning “Pump” instead, then you are half (maybe more!) wrong!  ─   and half right!

Because > 4000$ are a lot of ….:
Please! ─ Re-read/study my both last posts (#13 & #15) at least once again, with much more care!!

I DON’T sale Pumps!,   ─  And I don’t like to be responsible for your decision under such conditions!!
Not advice!, I gave you hints (including Reasons!), maybe worth to think about!
It is impossible to give Advice if I know NOT anything about application, environment, and staff.

About CFM: Many Thanks to Mr. Dr. Eugene Dakin (Eugenedakin) for the following Link:

"Vacuum Pump Sizing (CFM) (http://www.liquidcontrol.com/etoolbox/vacuum.html)”

But take very much care that the graph therein is only an Example, and differs quite a lot from Pump to Pump (mostly not only by Type) and from Manufacturer to Manufacturer!!

Sorry! that your last post is telling such worse!

Good Luck!
                    ARGOS⁺⁺

Title: Re: Can someone explain this to me
Post by: thorium on February 04, 2008, 07:50:20 AM

Ok ARGOS++ don't get angry :)

You're right! I'm not taking advises "just" reccomendations from you! Because reccomending someone something is a heavy burden... I just want to learn this:

So Maximum Vacuum is just an upper limit and it doesn't make any sense for a pump that's going just 2 Torrs?

I've read what you've wrote several times but I can't figure out anything other than this!

Thanx!

Title: Re: Can someone explain this to me
Post by: my_kemical_romance on February 05, 2008, 08:32:00 PM

okay..

Um im having trouble with charges...

I dont understand what they mean and why they are there...

CAn anyone help me...please?

Title: Re: Can someone explain this to me
Post by: ARGOS++ on February 06, 2008, 03:33:40 PM

Dear Thorium;

I never had, have, or will ever have in mind to get angry!,   ─  Not in such situations.

But as I told: I can not feel responsible in any kind for decisions somebody else is doing in situations I don’t know anything about expect 3 values of an imaginary Vacuum Pump!
That’s for everybody else also impossible.

NO!,   ─  Your last statement in its whole is wrong!,   ─  But it is an “upper” Limit!
I feel sad that till yet I was not able to empower you to do an adequate decision for your situation!

Let’s have an additional try with an Example:
(Maybe it’s wise to draw  “A Picture” with a pen and name it correctly.)

• a.)   Let it be a nice springday with an atmospheric pressure of 762 Torrs ( = 30.0 “Mercury) and with a Vacuum Pump that has left a little bit its best Conditions (“Ultimate/Absolute Pressure” = 1.0 Torr; “Maximum Vacuum” = 29.89 “Mercury;  CFM = ~5.0 L/min) with an actual “Maximum Vacuum” of only 28.92 “Mercury.
  There maybe different reasons for that:
  • ─  The Vacuum Pump got aged.
  • ─  The manufacturer had a Lack in the production of your Vacuum Pump.
  • ─  Or the Oil-Vacuum Pump colected solvents or other gases by an earlier Application,   somebody has not use a “Cryotrap”, but that you can get “corrected” by a Service who will exchange the Oil!
  • ─  And so on!
• b.)   Draw near the right border of the paper a “flask” symbolising your Application you like to evacuate and name the pressure therein with p_A and draw next left your Vacuum Pump and connect its inlet with your “flask”, and name the pressure on the outlet of the Vacuum Pump with p_O.
• c.)   For the first calculation let assume this Vacuum Pump has to work at the spring day so we can set p_O = 30.00 “Mercury. What will be the lowest pressure p_A you can get in your “flask”?
  It’s p_A = p_O  - “Maximum Vacuum” =  30.00 – 28.92  = 1.08 “Mercury, but 1.08 “Mercury corresponds to ~26 Torrs, and that’s never below 4 Torrs!
• d.)   But with the Data from the Manufacturer you will get 30.00 – 29.89  = 0.11 “Mercury = ~ 0.5 Torrs.
  So it’s easy possible with the “new” Vacuum Pump to meet its “Ultimate/Absolute Pressure”!
• e.)   What’s with your “aged” Vacuum Pump, have you to draw it away?
  Usually not, because the “Ultimate/Absolute Pressure” of the Vacuum Pump must not (but can) underlay Degradation in the same Magnitude, so you have at least two possibilities left:
  • 1.)   You may call for Service and hope for repair. At least for an Oil-Vacuum Pump you know there is a good chance. 
  • 2.)   For this Solution you have to connect (“draw”) another flask with the outlet of your Vacuum Pump and label it “Buffer”, but keep for its pressure therein still p_O.  Now draw a “cheap”  “Pre”-Vacuum Pump with a Maximum Vacuum of only ~10 – 15 “Mercury, but with a much higher CFM as your Vacuum Pump, and connect its inlet with your Buffer and label the pressure at its outlet with p_Atm. We can now start our calculation, because now   p_Atm  = 30 “Mercury:
    What’s the pressure in the Buffer?   p_O  = 30 – (>10)  = < 20 “Mercury.
    And whit’s now your p_A?    p_A =  p_O - Maximum Vacuum =  < 20 - 28.92 = < 0.0 “Mercury. In these cases the Ultimate/Absolute Pressure will be the limiting factor who still may rest at 2 – 3 Torrs.  But I can not forecast how long it will take to get there. 

Quintessence: 
For every Vacuum Pump who has to “deal” with the Atmospheric Pressure at its outlet “Maximum Vacuum” is an important limiting factor,  but such pumps are usually designed by the manufacturer (Let’s hope!) that in good conditions the Vacuum Pump will easily reach its “Ultimate/Absolute Pressure”.

WOW!,   ─   Finally it has got a very long Explanation, but I hope it has clarified a Lot, and empowers you to made a better decision for your Situation.

Good Luck!
                    ARGOS⁺⁺

P.S.:  I could be possible that I’m a few days “Out of Business”, because of 40.4°C Fever.  Sorry!.
.