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The Separating Funnel and the Partition Coefficient
Distillation separates by boiling point, and pays the latent heat to do it. This rung separates by something cheaper: by what a substance PREFERS.
Shake a mixture with two liquids that will not mix, let them settle, and every dissolved substance distributes itself between the two layers in a fixed ratio — its partition coefficient. Nernst wrote the law down in 1891. It sounds modest, and it is the single idea underneath every rung that follows in this batch: a chromatography column is nothing but this same partition repeated a few thousand times in a row, with one of the two phases held still and the other flowing past.
Build it first with your hands and a funnel, where you can watch the colour move from one layer to the other, and every later rung stops being magic.
Principiante
3 hours
Istruzioni
1
1
Find two liquids that refuse to mix
Find two liquids that refuse to mix
Pour water into a jar, add an equal depth of cooking oil, shake, and set it down. Within a minute you have two clean layers with a sharp line between them.
They separate because water molecules attract each other far more strongly than they attract oil, so mixing costs energy the system will not pay. That line is the interface, and everything in this blueprint happens across it. Note which layer sits on top: here the oil, because it is less dense. You will need to know which tap-end to drain, and getting it wrong is how a first attempt is lost.
Strumenti necessari:
Barattolo di vetro2
2
Charge the funnel — and never fill it more than two-thirds
Charge the funnel — and never fill it more than two-thirds
Close the tap. Pour in your water layer, then your solvent layer, filling no more than two-thirds. Stopper it.
The empty third is not caution, it is what lets you shake. A full funnel cannot mix and will spit past the stopper the moment you invert it. Check the tap is actually shut before anything goes in — this is the mistake everyone makes once, and the floor tells you immediately.
Materiali per questo passaggio:
Etanolo per laboratorio, 95 %50 mlStrumenti necessari:
Imbuto separatore
Becher in borosilicato3
3
Shake, and VENT
Shake, and VENT
Hold the stopper down with one hand and the tap with the other. Invert, shake gently once, then — still inverted, tap pointing away from you and everyone else — OPEN THE TAP to vent. You will often hear it hiss.
Volatile solvents build pressure the instant they are shaken, and a funnel that is not vented fires its stopper across the room. Vent after every few shakes, never point the tap at a face, and shake gently: a violent shake makes an emulsion, a milky layer that refuses to settle and can cost you the whole run.
Strumenti necessari:
Imbuto separatore
Occhiali a mascherina
Guanti in nitrile4
4
Let it settle, then drain the bottom layer only
Let it settle, then drain the bottom layer only
Stand the funnel in a ring, remove the stopper — it will not drain with the stopper in — and let the interface sharpen. Open the tap and run the LOWER layer out into a beaker, closing it the instant the interface reaches the tap.
Then pour the upper layer out of the TOP, not through the tap. Draining it past the residue in the stem is how the two layers get remixed at the last moment, and it is the commonest way a clean separation is thrown away.
Strumenti necessari:
Imbuto separatoreBecher in borosilicato5
5
Watch a dye choose a side
Watch a dye choose a side
Now make it visible. Add a drop of a water-soluble food colour to a water/solvent pair and shake: the colour stays in the water. Repeat with a plant dye extracted in alcohol and it will move the other way.
Nothing has reacted. The molecule has simply found the phase it is more comfortable in, and the ratio it settles at is a property of that molecule, not of how hard you shook. That reproducible ratio is what makes separation a measurement rather than a knack.
Materiali per questo passaggio:
Colorante alimentare1 bottigliaStrumenti necessari:
Imbuto separatore
Pipetta graduata (di Mohr)6
6
Extract three times with a third of the solvent
Extract three times with a third of the solvent
Split your solvent into three equal portions and extract three times in a row, combining the three extracts, rather than extracting once with all of it.
This is the whole trick, and the notebook proves why. Every extraction leaves a fraction behind; doing it three times leaves a fraction of a fraction of a fraction. The difference is not small, and it is the reason a chromatography column — which is thousands of tiny extractions in series — separates things a single shake never could.
Materiali per questo passaggio:
Etanolo per laboratorio, 95 %60 mlStrumenti necessari:
Imbuto separatoreBecher in borosilicatoPipetta graduata (di Mohr)7
7
How much is left behind
How much is left behind
Caricamento del notebook Jupyter…
Strumenti necessari:
Bilancia digitale8
8
Break an emulsion when you make one
Break an emulsion when you make one
Sooner or later a shake gives you a milky middle layer that will not clear. Do not shake it more.
Three things work, in order of how little they cost you: wait, because many emulsions settle in twenty minutes; add a spoonful of salt to the water layer, which raises its ionic strength and squeezes the organic phase out; or swirl gently rather than shaking on the next attempt. An emulsion is not a mistake in your technique so much as a sign the two phases are closer in character than you assumed — which is itself information about your solvent choice.
Strumenti necessari:
Imbuto separatoreMateriali
2- Segnaposto
- 1 bottigliaSegnaposto
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