Sat 30th July (Big Badda Boom!)

It’s crazy, I keep on meaning to get the planes going, it just never quite happens.  Thinking back it probably is a contributing factor that I never get more than 3 hrs continuous sleep (changing battery packs).  Hmm maybe I should have invested in one of those large battery packs, or just jury rigged one myself.  So on the evening on the Saturday I decided it was time to get rid of the potassium.  The stuff is a damn liability in a hot car.  Sure it’s under oil, in an airtight bottle, in an airtight metal can, but even so, its just a liability to have in a car that is your ‘home’.  Turns out I had about 12 grams.  Initially I used my ‘dunking’ apparatus for 4 grams, to show that it would shatter glass easily.  So inside the dunking kit I had a glass.  This is of course the smart way to do it if you don’t want broken glass everywhere.  The kit was a bit war torn by travel and needed some patching together.  I also had to find a ‘safe’ place to do this.  Safe in this case means sufficiently isolated that you are not going to disturb anyone (easy enough in the high sierra) and far enough from vegetation as to not cause any fires.  Eventually found a ravine that would do the job, but damn was it exciting getting my little NYer down those dirt trails for all terrain vehicles.

Turns out the apparatus stuck a little, but the end result was still more than acceptable!

You will notice in this reaction the immerser is fired out.  Turns out on the immerser there was still unmelted and unreacted potassium.  This for me was a fascinating discovery in that it shows the rate that the heat is generated on the surface greatly exceeds the metals capacity to conduct heat!!

Also led to one of those ‘thats why we wear facemaks’ type moments:

I then basically emptied the rest of the potassium out (about 7-8g in total) and wrapped in in aluminium foil and attached it to the emmerser.  Knowing how these things can scale in trecherous fashions, I moved the cameras and myself further away this time.

The 8 gram reaction was again very satisfactory, and I was glad to get rid of that potassium.

The wind regrettably was very high, and only later did I get a ‘sock’ on the mike, meaning most the audio I got here was mostly wind noise 😦  .  On the grand scale of things that was a small loss.  With these sorts of en-devours you can be so frequently screwed by the smallest absent minded moment (take for instance the previous day, when just not setting the focus on the camera resulted in a nights wasted effort).

By the time it was all over I was exhausted, the more so after I had nursed the car out form the dirt trails to nowhere.  So I rolled the car back to where it was kinda overlooking Ridgecrest and watched the over the Sierra Nevada Mountains.  It was beautiful with the brillant yellow sun peering through the clouds, torn by the wind, shining through into the valley in the foreground filling it with a warm glowing orange to the misty mountains beyond.  It was while I sat there in the silence of the high plains musing on how well the potassium thing had gone, that I suddenly realized that I hadn’t really talked to anyone for days, well, least ways anyone who actually knew my name.  Did think about heading down into Ridgecrest, simply to talk to someone on skype, or phone (mine doesn’t work out here), but I was exhausted, and the sunset was pretty, and by the time it had sunk below the mountains, fatigue had caught up with me.

Potassium explodes without Hydrogens help

Thus far almost all the stuff I’ve done on the alkali metals has been on ~250 mg quantities.

Self preservation says, “Start Small!”, and with good reason, the reactions of alkali metals with water are known to scale in treacherous fashions.

In the first instance I went for 1.6 g of potassium and 3.5g of sodium.

Note here there is not the slightest hint of green gas with the potassium!

Now the potassiums the one I’m going to focus on for the moment, simply as when this is played back at about 1/100ths speed it becomes clear that the hydrogen burning plays essentially no part in the violence of this reaction.

Fascinatingly the explosion appears to go in two stages here!

So potassium, molecular weight ~40, so 1.6 g is 0.04moles. 2moles of potassium gives a mole of H₂ , so this reaction will produce about half a liter of hydrogen.  Just for scale the jug is about 3 liters.  It’s also fascinating to see the ‘springiness’ around this generated gas.

Why is Potassium gas Green?

So my interest was really spiked here when I found that when reacting with water, potassium gives off a green gas!

While the green of potassium gas has been known about for over a century, being found in Encyclopedia Britianica articles as far back as 1911, the origin of that green color is proving a little more elusive.

If you know what you are doing it’s relatively easy to observe.  250 mg of potassium is ‘the right’ scale to work with.  Much smaller and it’s all over before you get a chance to do anything, much bigger and you run into problems with the metal exploding, or the hazards of hydrogen build-up.  All you need is a heavy glass vessel of about the dimensions of a wine glass.  Insert a burning acetone taper through a small hole in the top of the vessel and to burn out all the oxygen, and then drop in your potassium.  The green color is easily visible.

I’m tempted to propose it’s due to the solvated electron, which is a hot research topic at the moment as it’s part of the principal mechanism by which radiation damage happens to DNA.  It’s known to be stable and blue in liquid ammonia, but survives only picosecond in water.

A bead of Sodium Potassium alloy in anhydrous liquid ammonia. The blue colouration is due to the presence of solvated electrons

It’s an interesting suggestion, but I think the observation that you get the green color when there is no water around pretty much kills this idea.

So how do you get a green gas.  Well the only other metallic gaseous vapor I’ve seen was mercury which I once boiled to test the calibration on a thermocouple.  That has no color at all that I can remember.

Well as you will recall virtually all the gases are colorless, e.g. all the Noble gases (helium, argon etc) and all the first period gases (nitrogen, oxygen, and mostly fluorine).  However the heavier halogens have colored vapors that get more heavily colored as you go down the group.  Chlorine, light green, bromine, brown etc.

Chlorine (left) and bromine (right).

The reason these gases have these colors is the same reason the sky is blue, Rayleigh scattering.  That’s related to the polarizability of the molecule.  This is a very different mechanism from the electronic transitions that give the classical flame colors of the alkali metals!

It’s well known that all the alkali metals have stable bound states as diatomic molecules, similar in electronic configuration to hydrogen.  IF K₂ had a similar polarizability to Cl₂, it may well be green like chlorine!  Regrettably finding the polarizability K₂ is not as easy as it sounds.  A significant difference between H₂ and K₂ is the bond energy.  H₂ has a bond energy of about 400kJ/mol, while K₂ has a bond energy of ~50kJ/mol.  For reference, the hydrogen bond, the thing that holds water together as a liquid, has a bond energy of ~20kJ/mol.  If you heat any bound state and eventually the species will gain enough energy to separate and become individual species.  With water this happens at about 100 ^oC (boiling, ~400 K).  For hydrogen it’s about 3000 K.  You can actually do the real calculations, it’s just I’m too lazy at the moment, and so I’m just going to do a linear extrapolation between these two.  That gives K₂ breaking up at about 2-300 ^oC.

Well that would fit nicely with green gas being evolved at lower temperature, but as the temperature rises, the diatomic species break up, and the relevant polarizability of the molecule, and hence the Rayleigh scattering, and hence the color is lost.

Great, so if this is a working hypothesis, then the diatomic metals should match their corresponding halogen right?  The bummer is sodium.  Sodium gives off a blue vapor when it boils.  Fluorine is almost colorless.  ARSE!

The game is not over yet!

I’ve decided I need to see sodium vapor for myself, but how to do it with only the junk I have to hand!  What I really need is a nice small sealed silica tube that can take temperatures over a thousand degrees C.  Hmmmm, thinking, thinking…..