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Author: Subject: How to set up a controlled drip.
beastmaster
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[*] posted on 4-8-2008 at 16:37
How to set up a controlled drip.


Often an reaction calls for a slow addition of one of the reactants to the flask as a drip. How does one set up a drip, and what equipment would be used so the rate of how many drips per min,(or whatever) could be controlled over a few hours? I have tryed out several different ideas but I can,t get a steady drip. Any help would be appreciated. I know there has to be a basic method thats is used. Thank you .
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Klute
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[*] posted on 4-8-2008 at 16:45


.... a addition funnel?

For very precise additions or larger scales, volumetric pumps can be used.




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DJF90
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[*] posted on 4-8-2008 at 17:35


Using an addition funnel, you turn the tap (from the closed position) slowly until liquid comes through as drips. Depending on how far you open the tap alters the drip rate.
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[*] posted on 4-8-2008 at 17:45


Addition funnel - does the tap arrangement or aperature vary substantially from a separating funnel?
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[*] posted on 5-8-2008 at 02:54


<< Addition funnel - does the tap arrangement or aperature vary substantially from a separating funnel? >>>


there is a separate pressure equalization tube that connects the upper chamber to the bottom chamber




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[*] posted on 5-8-2008 at 08:47


For some reactions even a pressure equalization funnel is simply too fast.

They do make stopcocks with needle valves, but eventually based upon the fluid level the drip rate will vary, and can even cut off.

Peristaltic pumps are the way to go. You can even get small ones for less than $200 not too much more than an addition funnel or two... depending on what your using you may have to get PTFE lines and some sacrificial flexible tubing for the pump.




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[*] posted on 5-8-2008 at 10:57


Constant addition funnels?
http://kimble-kontes.com/html/pg-299350.html
I've seen several on eBay--prices are always less than $200, but many needed replacement stoppers or probes.
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[*] posted on 5-8-2008 at 11:11


I personally have never seen a constant addition funnel like the ones Kontes offers, on ebay or anywhere else.

For the money you can get a decent peristaltic setup for the price of just one of those, with the added bonuse of greater precision... you know that every time the motor turns x amount of solution is going to be dispensed.

Plus you don't have to worry about breaking or cleaning the darn things.




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[*] posted on 5-8-2008 at 14:10


They're usually listed as something else, or as an unknown piece of glassware.
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Klute
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[*] posted on 5-8-2008 at 15:17


You can reduce the flow of an addition funnel stopcock by placing a small fine glass rod in the addition funnel, that partially obstructs the bottom of the funnel. Or you can get stopcocks with smaller inner hole, or simply reduce it yourself with a knife. When carefully done, I have been able to keep a rather constant flow of 1 drop 20-30sec, need to check now and then as it does tend to slow down or stop as the level goes down (different pressure).

I consider volumetric pumps to be quite a luxury for small scale reactions.. i'm sure it can be very usefull when doing molar or above scale, but a addition funnel and some attention is IMHO the best option in the lab :)

Also, a syringe and it's needle through a septum or the addition funnel's stopcock can be usefull for slow additions, leaving the plunger out and letting the fluid fall by gravity affords a slow, constant drip rate.




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[*] posted on 6-8-2008 at 06:36


Quote:
Originally posted by Klute
Also, a syringe and it's needle through a septum or the addition funnel's stopcock can be usefull for slow additions, leaving the plunger out and letting the fluid fall by gravity affords a slow, constant drip rate.


Just change the gauge of the needle you're using to change the flow rate. Of course, you're going to run into the same problem of the rate slowing as the addition proceeds.

You could always consider a syringe pump--you can get some for decent prices if you know where to look (eBay and surplus stores are a good start).

[Edited on 8/6/08 by bfesser]
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