Author Topic: Slow, Controlled Solution Addition to a Rxn Flask  (Read 682 times)

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Rhodium

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Slow, Controlled Solution Addition to a Rxn Flask
« on: August 16, 2003, 11:06:00 PM »
A Simple and Inexpensive Device for Slow, Controlled Addition of a Solution to a Reaction Mixture
Peter Osvath
J. Chem. Educ. 72, 658 (1995)

A number of reactions require the slow and controlled addition of a solution containing one reagent to another. Attempting to control the flow rate over a number of hours using a conventional constant pressure addition funnel is a frustrating exercise; commercially available constant volume addition funnels are expensive and must be adjusted by trial and error each time a reaction is carried out. The use of an (expensive) peristaltic pump or syringe pump overcomes these problems but can introduce other complications. We have recently had occasion to carry out the synthesis of thioether macrocycles and cages requiring the slow and controlled addition of DMF solutions of (offensively odoriferous) thiols or (air-sensitive) thiolates to a reactant solution under nitrogen1, Although the use of a syringe pump was called for, there are obvious difficulties associated with purging the solution and assembling such an apparatus under nitrogen, and we report a simple and inexpensive solution.


Male Luer Lock tip

A Male Luer Lock tip (recovered from a broken syringe) was sweated onto the flattened tip of a pressure-equalizing addition funnel and a syringe needle was attached. Judicious selection of needle length, bore size, and reactant volume can be used to control the addition time simply and reproducibly. With a 250-mL funnel, the flow rate changes by <25% from the beginning to the end of the addition. (In fact, a reduction in the rate of addition may even be advantageous as the reaction proceeds, the reagent in the receiving flask is consumed, its concentration drops, and the rate of reaction will decrease). A piece of fine Teflon tubing of appropriate length attached to the needle can be used to reduce the flow rate even further, but this is only necessary for very slow rates of addition. For example, the time of addition of 200 mL, of an ethanolic solution could be varied from approximately 5 minutes (150mm/17 gauge) to approximately 5 h (200mm/22 gauge), and once the addition time for a particular needle length/bore is determined, the tap on the addition funnel is turned fully on, so no adjustment is necessary. When needles with a particularly fine bore are used, a small plug of glass wool should be inserted in the constriction above the tap, to filter the solution and prevent blockage of the needle. An inert atmosphere is readily maintained throughout the system. The elimination of atmospheric contamination, containment of odors and controlled slow addition have led to improved yields and less complaints from fellow inhabitants of the laboratory!

Literature Cited

1a Osvath, P.; Sargeson, A. M.; Skelton, B. W.; White, A. H. J. Chem. Soc., Chem. Commun. 1036 (1991)
1b Osvath. P.; Sargeson, A. J. Chem. Soc., Chem. Commun. 40 (1993)

lugh

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Glass Equivalent
« Reply #1 on: August 17, 2003, 03:14:00 AM »
A glass equivalent which is more corrosion resistant can bee made by following dpHarma's advice in

Post 84352 (missing)

(LaBTop: "Re: Performic MODS", General Discourse)
 :)
dpHarma
Member   posted 09-17-1999 12:22 AM          
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Got some glass tubing and a torch?
heat a short length in the middle until soft. remove from flame , grasp an end in each hand and pull firmly apart.
snap the thin section in half, adjust the length at the wide end w/ a file, scoring a mark and snapping off.
adjust legth of thin end by snapping off. if temp was right when pulled out, a capillary of about .01mm will be formed. this will auto regulate the flow from a sep or addition funnel just like an IV system with the little half-full cup just under the bag.
if the flow is too slow snap off a bit more. it will be slightly wider as you go up.
if too fast, try again.



Rhodium

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Pro & Con
« Reply #2 on: August 17, 2003, 02:39:00 PM »
That is a good complement to what I posted, but the advantage of the luer lock needle setup is that for every needle bore/length tested, you will have a reproducible result every time you use it - with exactly the same solution, of course - for example, low-viscosity solutions (acetone, ether, THF) will naturally add faster than aqueous ones. Therefore, it is imperative that you calibrate the addition speed for each solvent you intend to use.

CainTwister

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I hate to do it...
« Reply #3 on: August 22, 2003, 04:54:00 PM »
...but I'm giving up one of my trade secrets here  ::)

Invert a 2-liter bottle, melt a hole in the center of the bottle base that allows for a long-neck funnel exhaust to pass into the interior, melt two holes in each bottle "legs," pass a zip-tie through each leg's two holes and close each tie, staple each tie loop to a 2" X 2" X 24" board about 5" from each end, raise your assembledge like a tee pee with four columns and adjust the ties as needed, screw column bases into a ~30" square piece of plywood so that the bottle now hangs upside down in the center of your pyramid and the tops of each column can be screwed into a 2" X 2" piece of plywood to also support its integrity up there, fill with appropriate liquid through funnel hole, place bowl under the hanging bottle, loosen cap until your drip rate is where you want it, and replace bowl with titrant  :)