beastmaster
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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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.... a addition funnel?
For very precise additions or larger scales, volumetric pumps can be used.
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DJF90
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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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starman
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Addition funnel - does the tap arrangement or aperature vary substantially from a separating funnel?
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Maya
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<< 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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evil_lurker
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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.
Not all chemicals are bad. Without chemicals such as hydrogen and oxygen, for example, there would be no way to make water, a vital ingredient in
beer.
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bfesser
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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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evil_lurker
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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.
Not all chemicals are bad. Without chemicals such as hydrogen and oxygen, for example, there would be no way to make water, a vital ingredient in
beer.
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bfesser
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They're usually listed as something else, or as an unknown piece of glassware.
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Klute
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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.
\"You can battle with a demon, you can embrace a demon; what the hell can you do with a fucking spiritual computer?\"
-Alice Parr
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bfesser
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| 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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