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How to sample a shaking tube

Dear Reader,

I enjoy solvent extraction, it is enjoyable and allows me to probe the very fabric of matter with a simple experiment which measures a system at equilibrium. While there are some things which solvent extraction can not do such as prove the existence (or non existence) of God there are lots of things it can do.

Now I am aware of a man who has attempted to use quantum mechanics to prove that Christianity is right this is a true story and not a joke, but we will leave that matter for another day and get back to solvent extraction.

Now lets consider the things which solvent extraction can do, we can develop new industrial processes using shaking tube experiments. In these experiments we can measure key details about the proposed process. We can work out the conditions which would be required to extract, purify and back extract metals from a liquid.

We can use it to measure binding constants in either phase, we do that by making measurements of a system with different concentrations of the binding agent.

We can also use solvent extraction to determine activity coefficents, for this we need a well understood system and some patience.

We can separate metals from each other in a small scale for an anayltical purpose, here we can clean up a sample before either using ICPMS, a radioactivity measurement or some other determination.

We can test the purity of some organic molecules using a solvent extraction experiment.

We can measure the Delta H and the Delta S of an extraction reaction using solvent extraction at different temperatures.

Sounds like great fun and excitement, but to do it we have to be able to cleanly take samples of the two layers. The top layer is easy as all you need to do is stick the tip of a pipette in it and suck some out. The lower layer is a bit harder.

Like many things there is more than one way to do it, one method is to remove the whole of the upper layer and then take the sample. This is not easy to do, it is hard to avoid leaving a little of the upper layer in the vial. So in fact we will always have to use some care and skill to sample the lower layer.

Now to help spread the love, I have taken some photos with my student (Cen Peng) to show how to do it. Start by taking the lid off the shaking vial with great care to avoid shaking it all up again into an emulsion. Now in this photo you can see me holding the opened vial with my right hand as a 200 microlitre pipette tip is about to go into the liquid. At this stage press your thumb down to the first stop of the pipette to push out much of the air.

get ready to stick it in.png

Next lower the pipette into the lower phase, make sure you have the point of the tip in the lower layer. Try not to press it against the bottom of the vial as you can seal it up. Now press a little harder on the button with your thumb to squeeze out a couple of air bubbles. Do this to remove any droplets of the upper phase from the tip of the pipette.

Squeeze out an air bubble.png

Now slowly relax your thumb and allow the pipette button to rise slowly, this will suck up some liquid into the tip. I have done this with blue liquid to make sure that you can see it more clearly. My advice at the end is to take your thumb off the button and count to five.

I have sampled the lower blue layer.png

Now with care raise the pipette out of the lower liquid.

Carefully raise the pipettee.png

Now raise it further

Final step now relax.png

Now pull it all the way out of the vial, I like to wipe the outside of the tip with a bit of clean paper tissue to remove any liquid from the upper layer. Now I tend to then pipette it out into a preweighed plastic vial. I then weigh the vial after the pipetteing to check the volume of liquid that I have dispensed. You need to know the density of the lower layer to do this, unless you also weigh out the starting aqueous layer as well.

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