Does old meths burn less hot?

Having failed miserably to anneal the end of a clock arbor, using the spirit lamp that came with my chemistry set some 50 years ago, and then trying with a vintage Valtock meths blowtorch and failing to get a good flame - both of which should have dealt with the job easily - I am wondering, does meths lose its potency with time? This bottle is several years old and has lost its purple colour.

As it happens, I used some 30 year old meths in a Mamod this week. It was slow to catch light, but raised steam ok. It was in a sealed plastic bottle, and I assumed some higher fractions had migrated, though the blue colour was fine.

Seems feasible that it might due to evaporation of some of the alcohol content, although I've not seen any losing the purple colour before.

Are you sure it's pure meths, though, as it puts me in mind of a chap that once asked me if petrol could go bad, as his Mini was running really roughly after topping up with a can full of "petrol" that his grandad had had in his shed since the war. He'd been told the blue colour was how it used to be back then, but - as you've probably guessed - it was actually paraffin. Good old Esso Blue! Probably worked better than forty odd year old petrol though.

Rob

No, but alcohol absorbs water enthusiastically even through a sealed cap, and because it takes a lot of heat to turn water into steam the flame will be distinctly cooler. Wet coal and wood don't perform well either!

Dave

Edited By SillyOldDuffer on 12/03/2021 18:49:44

Meths is mainly methanol which is hygroscopic. Over time it will absorb water from the atmosphre unless in a hemetically sealed container. Water conten will affet the flamability and probably the flame temperature.

Robert G8RPI.

SOD , I interested to know how water is absorbed through a sealed cap -seriously.

I believe Methylated spirit is mostly Ethanol with a bit of Methanol added to make it poisonous and a foul tasting addition to discourage consumption with a colouring to warn you. But Ethanol is also highly hydroscopic.

Rod

Many thanks,

Highly likely that it is down to water being absorbed then, given that it is a roughly 1/4 full 2 litre bottle that has been sitting in a shed for some years. I presume the dye has lost its colour due to exposure to sunlight.

Now then, how do I find a bottle of meths in these days of essential retail only?

Just checked Wilkinsons (10 minute walk away) website and they claim to stock it.

Homebase stores are open and they stock it. So do B&Q.

Well the ones I’ve been in anyway.

Also Screw Fix

Edited By MichaelR on 12/03/2021 19:15:39

It can be bought mail order.

Meths will originally be no better than about 95% ethanol, the balance made up with mainly water.

I expect it is less a matter of absorbing water, but more likely evaporation of some of the ethanol from the mixture due to a lower boiling point compared to water - leaving the mixture now with more water, in proportion to the remaining alcohol. All to do with ‘constant boiling mixtures’. Same result - lower calorific value.

We could test this! Leave the lid off and see if the level goes down or up

Rob

The level will go down.😀

Sorry to disagree but Meths is mostly Ethanol with some (poisonous) Methanol, pyridine for colour and something to make it taste bitter.

MC

Sorry AGAIN - I responded before reading subsequent comments.

MC

Same way moisture gets into hygroscopic brake fluid in a sealed system. Airborne humidity is sucked in mostly around non-perfect seals and even through not-perfectly impermeable rubber brake hoses or plastic meths bottles. Little by little over a long period it keeps accumulating . Here in the wet tropics with relative humidity 80 and 90 percent year round, brake systems corrode internally if not bled and flushed with new fluid annually.

Automotive brake systems always have a vent to atmosphere. Use silicone brake fluid - more expensive, but non-hygroscopic.

As I see it, the vapour above an ethanol/water mixture will always contain more ethanol than water, until greater than ~96.5% ethanol. That is the azeotropic point (constant boiling mixture) where the vapour pressures are equal (which is why distillation cannot produce better than about 95% alcohol however many times you might re-distill it!

This is not to say that meths will not absorb moisture if the humidity is very high, but it sure means that in most situations the ethanol will evaporate faster than the water - simply because there is a 20 degree difference in their respective boiling points.

Rest assured that the bottle of meths will empty, on standing, rather than it eventually leaking by it becoming over-filled.

As Hopper says, but with more detail.

Molecules are tiny compared with physical objects like joints and surfaces. Water has one of the smallest molecules found - about 3 picometers, or 10 billionths of an inch. In liquid form water molecules stick together into a larger body that can be sealed fairly easily, but water vapour is more difficult to contain.

A meths bottle is uusallly sealed with a screw-cap bearing on a waxed or aluminiumised cardboard disc. On the picometer scale even plastic threads that squish together are leaky, and the disc is essential. Cardboard is also full of holes, so the discs are sealed with wax, or aluminium. Aluminium is more effective than wax because its molecules form a tighter mesh, which is why aluminiumised plastic is used to package flavoursome foodstuffs and Helium Balloons.

Seals aren't really solid, they're more like a tight filter, ideally so tight nothing gets past but in practice small quantities often do. And the seal is even less perfect after the first opening damages it slightly.

Ethanol and Methanol molecules are more likely to be held successfully by the seal because their molecules are bigger, but air and water vapour can still penetrate. Once past the seal Meths has a strong affinity for water, so any that gets inside will be absorbed. This creates a slight vacuum encouraging more wet air to enter the bottle. Then day and night-time cooling create an effective suction pump. Air pressure inside rises whenever the bottle warms up forces some air out. When the bottle cools, the pressure inside drops and sucks in new wet air, which is promptly dried by the alcohol. Although the amount of water absorbed daily is tiny, it accumulates over time making it likely an old bottle of meths will collect enough water to reduce it's heat value.

Hopper mentioned hydraulics, but vapour escaping past seals is a common phenomenon. It was a major contributor to the Scott South Pole tragedy. The Scott expedition cached canned paraffin and food for use on the way back as they proceeded in stages towards the pole . They found most cans half empty on the return trip and not having enough fuel to melt snow for water weakened the men. Despite extreme cold paraffin vapour had escaped through the screw-cap and gradually emptied the can. Having to eating snow in the Antarctic didn't end well.

The blue colour of Meths is a dye. It's easily de-colourised by sunlight or a drop of household bleach.

Dave