LAB COFFEE TIME AND SWOP-SHOP

This place is for describing and discussing experiments by members with a focus on the practical side of things. Better methods make better science- and after 50 years I still learn new things about techniques and also the surprises that revisiting old methods reveal about science- be it physics, chemistry, or engineering. To kick it off, here's a little 'methods 101' on distillation of heavy water to remove impurities. This thread might be useful to those who never paid attention to science lessons in school, but now wish they had. Others might think some of it 'too elementary', but we have to start somewhere.

As you can see I have ended up with around three hundred grams of electrolyte and metal salts/hydroxide contaminated heavy water. It looks rather like horse-piss, but at around $1 a gram D2O is too precious to throw away.

The plan is to distill off the water leaving the impurities behind. The equipment is simple - here's a general view. The hotplate boils the D20 in the conical flask, the water-cooled condenser in the middle turns the D2O steam back into liquid, and the condensate is collected in the globe shaped flask in the left. There's a little downward-curving piece of silicon tubing on the swan-neck connector on the left btw - that is for pressure relief, and should be sitting in a beaker to catch any condensate that finds it's way out by that route.

The condenser water jacket is fed from a reservoir of cold water -flow is provided by a little aquarium pump submerged in the tank. These are very cheap - and this one has probably got 100+ hours of service so they are reliable too - which is important. If the cooling stops your precious distillate will just escape as steam.

Whisky and Gin distillers pay great attention to still-head temperatures, which they like to know to fractions of a degree. Since I am not worried about volatiles in my D2O a simpler method of control works. Although the hotplate is thermostatic, it is actually slaved to an external PID thermostat, so the thermostat on the hotplate is set to 'max' all the time, since the PID switches it on and off. Temperature sensing for the PID is via the thermocouple in the still head. The heat feedback loop is longer than I like, but I have used this crude method often enough to know it works well enough for this job. Finally, the conical flask does not sit directly on the hotplate, but on a PTFE mesh spacer. Direct contact causes -too often - cracking of the glass- even borosilicate 'oven glass'.

I'll follow up with the results -when available.

Well, Curbina they say that few experiments survive contact with reality. It's working nicely now, boiling away as you see, but I had to wrap a little insulation around the still-head and up the thermostat settings to get a decent rate of carry-over.

Well, that worked like a charm. And on the basis of this I have been gifted quite a bit more dirty D2O to re-distill by another experimenter.

The last little bit...

The distillate.

Quote from Curbina

Alan Smith , when you insulated the still-head, it was because it was cooling too fast and some of the condensed liquid kept falling back to the boiling beaker?

Yes- that was why - just to help the steam 'go round the corner' into the condenser.

It was 12.5C in my lab yesterday. That probably explains it, we are having a cool spring season here.

Here's a bit of 'pretty chemistry, making copper hydroxide from copper sulphate and sodium hydroxide. This is a simple 'double decomposition' where the sulphate ion and the hydroxide ion .change partners with the end product being insoluble copper hydroxide and soluble sodium sulphate.

So we start with 125 grams of copper sulphate heptahydrate crystals dissolved in distilled water. This is an ambient temperature experiment btw- do it hot and you get a different reaction.

Above is the sodium hydroxide - which should be added as a solution in distilled water.

Here just a small amount of sodium hydroxide has been added. A few paler blue clumps of copper hydroxide gel are forming.

And this is after more copper hydroxide has formed- the paler blue colour now begins to predominate.

And here's the end product sitting in a warm spot on top of the kiln to dry. Not exciting science, but interesting to watch.

Back to copper hydroxide. The gel has settled a little, but still needs washing to remove the residual (soluble) sodium sulphate. How can you tell? Well, apart from logic, I left some of the gel on a plate to dry in a warm place in a warm place overnight. In the picture below you can see the white sulphate crystals, squeezed out' of the drying dark blue hydroxide gel looking like a jacket of hairy fungus.

And here in the microscope...low power view, blue hydroxide, white sodium sulphate.

Because the wet copper hydroxide is a gel it can be separated from the bulk water using muslin- this is where inorganic chemistry gets most like cheese-making.

Very little hydroxide actually escapes through the muslin.

And this is the filtrate, ready for another wash