As a part of a GC/FID project i've been looking into methods to controllably produce hydrogen at a predictable rate, and with commonly available
materials.
Electrolysis of water was the obvious choice, as the rate of production would be proportional to the current density.
Unfortunately that was a bit feeble regarding the Rate of production, also too electricity-hungry per ml of H2 produced.
detaH suggested Al & NaOH so i gave it a try.
2g of NaOH was dissolved in 100ml of distilled water to make a 0.5 [M] solution.
Theoretically this works out to produce roughly 17ml of gas per 1ml of solution reacted with elemental Al. One advantage with this reaction is that
the only gas produced is Hydrogen,
A 100ml RBF was filled with 4g of Al chippings (a large excess).
To this, a vac adapter with a length of rubber tube was attached.
The open end of the tube was led into the bottom of an inverted water-filled 25ml measuring cylinder and the bottom of the cylinder was kept under the
level of water in a bath.
1ml of the NaOH solution was injected onto the Al though the rubber tube with a hypodermic syringe and the gas evolution rate measured at 30 second
intervals over a period of 8 minutes.
Amazingly the rate of production was fairly linear until it reached 11.5ml, at which point the production effectively stopped.
The shortfall could well be something simple, like not allowing enough time etc.
The reaction is fairly exothermic, so it will vary massively when scaled up, however the dilution of the NaOH could also mitigate that.
Has anyone any experience/suggestions regarding H2 production at a controllable rate, without using words like 'platinum electrode' or
'more power' ?
[Edited on 27-11-2017 by aga]Bert - 27-11-2017 at 14:16
[Edited on 27-11-2017 by Bert]Chemetix - 27-11-2017 at 14:22
Depends on availability but Ferrosilicon and NaOH was a classic H2 generator....Lot's of sodium silicate as a byproduct, if that is useful to you.PirateDocBrown - 27-11-2017 at 15:57
I just use zinc and HCl. Or electrolyze water.NEMO-Chemistry - 27-11-2017 at 17:53
Would need to be very pure and very dry, the FID is super sensitive. They sell a machine that is supposed to make hydrogen from air, many pro labs are
apparently using them for GC.
There is a GC forum, i dont remember all the details but this topic was discussed alot on there. The coils for a GC cost a fair bit. HP do software
for that is supposed to be the best GC software, however elmer perkins i think has a free version.
The best source of air for one is a dental compressor thats oil less, and an electronic bubble counter to set it up correctly. If using Hydrogen for
the carrier gas, its highly recommended to get a detector for the oven section.....Reboot - 27-11-2017 at 18:24
Let's say the goal is steady, consistent H2 production with minimal interference. I would probably try:
1. Sheets or plates of Al (since they will vary less in their available surface area as they are consumed), amalgamated with mercury or stannous
chloride or alloyed with gallium.
2. Placed in a large excess of near-boiling water (refluxing or allowed to boil off if moisture in your hydrogen gas isn't a problem.) That way you
can keep the temperature essentially constant, keeping the reaction rate from ramping up as heat is produced.
That should give a very steady rate of H2 evolution (at least until the Al was largely consumed and you started to get substantial losses of surface
area.)
Melgar - 28-11-2017 at 06:55
The reaction of aluminum with an aqueous solution of alkali hydroxide really is the best. Aluminum is amphoteric, and will react with either three
units of an acid or one unit of a base to generate the same amount of hydrogen. All the common strong acids either form volatile gases (ie HCl) or
are reduced by aluminum (ie H2SO4) or form a ridiculously insoluble product that protects the metal from further reaction (H3PO4). With alkali
hydroxide solution, the only gas you need to worry about is water vapor.
One of the most convenient sources of reasonably pure aluminum is from large electrical cables, or electric fence wire. In this form, lengths of wire
can be placed into a narrow container with a few inches of alkaline solution at the bottom, and thus maintain a controlled output over a controlled
period of time. If your source is aluminum foil or aluminum serving trays, these can be twisted into rod shapes, to act in the same manner.
[Edited on 11/28/17 by Melgar]sclarenonz - 28-11-2017 at 07:20
melgar you're right, but the big problem for me was to try to control the reaction, the aluminum ended and I had to add more aluminum, and I have to
put more sodium hydroxide solution that occupied me enough I chose the electrolysis of lead and rays because of the fact that I can spend hours
unoccupied, the beams of the motorcycles here in Brazil gave me a much more lasting result than the stainless steel. I made several generators to give
me a satisfactory amount of hydrogen:
[Edited on 28-11-2017 by sclarenonz]Melgar - 28-11-2017 at 07:30
By "rays", I'm guessing you mean "spokes"? IIRC, those are usually made out of stainless steel. The choice of electrolyte is a lot more important
than the choice of electrode material though. Dilute sulfuric acid tends to be a popular choice because it's more electrically efficient. With
sodium bicarbonate, a lot of the hydrogen is generated from reducing sodium ions to sodium metal, then the sodium metal reacts with water. This
reaction produces a lot of waste heat.
You can control the aluminum reaction rate by changing the alkalinity of the solution it's dissolving in, or by using thicker lengths of aluminum with
less surface area.sclarenonz - 28-11-2017 at 07:58
Thank you Melgar You're a genius, I was in doubt about bicarbonate and now you gave me the answer, I'm going to use baking soda in the electrolyser,
it's going to be interesting for me because I have a co2 generator made of limestone and muriatic acid and when it's bubbling in sodium hydroxide I
have bicarbonate again, but what do I do when I run out of NaOH solution, even if I have a thick aluminum I'll always be opening it to put more naoh,
what's the solution?aga - 28-11-2017 at 08:46
Nice one Melgar.
i guess that the shape/surface area of the Al piece will also have an effect on production rate, so i'll experiment with that.Melgar - 28-11-2017 at 09:42
I guess the best solution will depend on whether you need to generate H2 in large quantities quickly, or at a low, controlled rate for a very long
time. I never really liked electrolysis reactions, because they're messy and hard to properly seal, so inevitably one of the wires will get its
copper exposed and there will be dirty transition metal salts in the liquid. Harder to make airtight too. With aluminum, you can add hydroxide
solution with an addition funnel, but since hydrogen isn't toxic, it's easier to just open the container and add the appropriate reagent when needed.
If your reaction can't have air in it, then just use an excess of aluminum and add hydroxide solution with an addition funnel. A small amount of
aluminum can generate a LOT of hydrogen.
If generating hydrogen is the goal, then it's important to have a lot of H+ ions in solution that can be reduced directly to elemental hydrogen.
Thus, acids are usually used. I guess sulfuric acid is usually used because it's cheap, but dilute phosphoric acid would probably be a better choice
for an amateur. It's less dangerous, and is less likely to dissolve something that you don't want it to dissolve. Phosphate salts are almost never
very soluble in water, so you can use a much wider range of electrode materials without having your electrolyte solution turn a grimy, opaque brown
immediately.
No matter what method you use, there will be something consumed, whether it's water or metal. And with electrolysis, you also have to figure out what
to do with the oxygen. If you use aluminum, then the oxygen just goes to oxidizing aluminum, with the hydrogen being released.aga - 28-11-2017 at 14:10
I tried electrolysis and the amount of power required kinds a rules it out.
Having Oxygen as a side product isn't a good thing either from the BOOM point of view.
Al & NaOH works almost perfectly, just need to refine the cell to make it useful.
In any event i think it will need a mass-flow regulator to get the rate constant, which a GC will need.
Incidentally, the carrier gas and flame flow rates will be different, seeing as the FID needs the flame.
It came up in the discussion with deltaH that the exothermic nature of the Al/NaOH reaction could help heat the FID chamber, reducing the condensation
problem.
The 'column' will be (at least to start with) a length of copper pipe stuffed with silica gel beads.Twospoons - 28-11-2017 at 16:07
I used Al + NaOH for a much more mundane purpose: filling balloons. I found three things: it was prone to thermal runaway, foaming, and there was
always a fine mist of NaOH along with the hydrogen.
I have also built a 4 cell electrolyser, from stacked stainless steel plate. Running 20A at about 10V it would sustain a flame on the end of a 25mm
tube.
[Edited on 29-11-2017 by Twospoons]aga - 28-11-2017 at 16:16
What reaction conditions did you use ?
Al powder, chips, solid chunks ?
What NaOH did you use ? Solution or dry NaOH on the metal with water dripped on ?
Science doesn't happen by vagueness.
Details and data or it's like a fart in the breeze,
I'm being hard-ass on this because i actually want to Make a process, not just jibber-jabber about bollocks in the internet to make me feel better
about myself.Melgar - 28-11-2017 at 18:18
This setup works for producing a pretty constant H2 stream:
I'd take the three conductors out of a length of aluminum 240VAC cable, and use them at once. One rod/wire isn't really enough.
@aga: You might just be better off regulating the pressure via a needle valve or pressure regulator, then just have a large container that the H2 can
build up some pressure in to even the flow out.
Mist can be stopped with a wadded up piece of rag or a cotton ball. Then some desiccant system for vapor, which you'd need regardless.
[Edited on 11/29/17 by Melgar]zed - 28-11-2017 at 18:42
I like hydrogen. Awesome commercial generators may be acquired on e-bay, used, for about a grand. The better ones produce 500 cc H2/min. Plenty.
That being said, they are used, and a grand is a lot of money.
Why not use aluminium powder instead of foil? There will be far more surface area for your reaction to occur.
Because then all the hydrogen will be generated during a space of about five seconds, in the form of a foam that will erupt all over your workbench.sclarenonz - 29-11-2017 at 07:24
I will put my context on the reaction of aluminum with sodium hydroxide
- I live in a wild country where instrustrialized raw materials cost a lot of money.
so I always try to recycle, to work to the extreme to get everything from scratch.
In this question, only the generators can be made with plastic bottles of soft drinks and pasting the hoses that I found on the old irons of old cars
with homemade glue: formaldehyde (distillation of wood) with urea (urine), I tried to put in the plastic aluminum, it melted completely, so I put the
bottle of beer, it warmed so much that the bottom broke and the hose melted, so I put less sodium hydroxide, the reaction slowed, but I had to open
the bottle, the most viable was bottles of 5 liters, I really liked the result, but my question is:
it is possible to make a reaction of aluminum and hydroxide and plastic bottlesaga - 29-11-2017 at 07:48
... You might just be better off regulating the pressure via a needle valve or pressure regulator, then just have a large container that the H2 can
build up some pressure in to even the flow out.
Definitely some kind of pressure regulator, or rather mass-flow regulator will be needed.
It'd be nice to get the production rate close to the required rate to avoif having to build up a big reserve of explosive gas !woelen - 29-11-2017 at 08:14
What rate do you require? With electrolysis you produce appr. 0.2 ml per second with a current of 1 A. Voltage required for electrolysis of a
NaOH-solution (using copper wire for both anode and cathode, e.g. cheap thick loudspeaker wire) is about 4.5 volts, the copper does not dissolve, also
at the anode it remains good and oxygen is produced. Power requirements are only 5W for 0.2 ml per second.
With H2SO4, NaCl and other ionized substances the copper at the anode dissolves, hence the specific choice of NaOH (or KOH).aga - 29-11-2017 at 09:36
It's only about 3ml/minute required, but that depends 100% on being able to make a column with a tiny internal diameter of 0.32mm
Chances are that it will be much bigger, so a significantly higher rate might be needed.
I found a table of recommended carrier gas flow rates, which is where that figure was calculated from :-
Gas cm/sec
H 45
He 34
N 12
Hydrogen will also be needed in much higher quantities to produce the flame for the FID.Melgar - 29-11-2017 at 09:58
Definitely some kind of pressure regulator, or rather mass-flow regulator will be needed.
It'd be nice to get the production rate close to the required rate to avoif having to build up a big reserve of explosive gas !
You can always use a pressure-release valve, but it's my experience that hydrogen is very good at escaping vessels, and will rarely build up
significant pressure in a homemade apparatus. It's also the lightest gas that exists, and dissipates very quickly.
Quote:
it is possible to make a reaction of aluminum and hydroxide and plastic bottles
Yes, but you have to be patient. First, allow the NaOH to dissolve in the water, and allow the water to cool to room temperature. Next, find
aluminum with a low surface area. I'm sure you can scavenge aluminum rods, lengths of tubing. If you can find any thick electrical cables that
nobody's collected for scrap yet, those are probably aluminum as well. If you're in a rural area, do farmers use electric fences for their animals?
If so, the ones for enclosing large land areas (rather than just a small pasture) are usually pretty thick aluminum, and you can usually find short
lengths of wire along the fence, from when animals escaped and broke the fence and the farmer had to fix it.
If you can't find anything with a low enough surface area, you might have to melt it down and recast it into ingots, but that's pretty easy. Also,
note that the reaction will gradually generate heat at a higher and higher rate for the first ten minutes or so. This is because dissolving the
passivating oxide layer is not very exothermic, but once all the oxide layer is dissolved away, the oxidization of the underlying aluminum metal is
VERY exothermic!
Quote:
With H2SO4, NaCl and other ionized substances the copper at the anode dissolves, hence the specific choice of NaOH (or KOH).
NaOH is very wasteful because of the competing metal reduction reaction, that also generates hydrogen but at a much lower efficiency. Dilute
phosphoric acid would be the best choice for amateurs, I think, because like NaOH, it won't really attack most electrode materials.
[Edited on 11/29/17 by Melgar]VSEPR_VOID - 29-11-2017 at 11:33
What reaction conditions did you use ?
Al powder, chips, solid chunks ?
What NaOH did you use ? Solution or dry NaOH on the metal with water dripped on ?
Science doesn't happen by vagueness.
Details and data or it's like a fart in the breeze,
.
1: I wasn't doing science, I was making floaty balloons for my kids
2: NaOH solution (somewhere around 100g to a litre of water - it wasn't for science), Al chunks - usually long pieces of scrap extrusion.
3: Surely any observations about the reaction are useful in designing a method to overcome potential problems?
Personally I think you have been too quick to abandon electrolysis, given the ease of control, and low cost of consumables.
Quote:
Hydrogen will also be needed in much higher quantities to produce the flame for the FID
and this is where electrolysis wins. Its used to produce H2 fast enough for welding torches.aga - 29-11-2017 at 14:48
If you do the maths, the energy required to make enough hydrogen for those torches is enormous.
Water really likes to be water, so it takes a LOT of energy to break those H-O bonds.
Others have also said it would be cheaper/easier to buy a second-hand GC, and it would.
What would be the point though ?
Personally i would like to know how one works from the ground up and make one.GrayGhost- - 29-11-2017 at 15:41
I build a homemade Kipp apparatus for chlorine. here link of model
I produced hydrogen gas by slowly dripping NaOH solution on aluminum foil balls in water and the vessel in an ice bath. The gas produced was passed
through a drying tube with calcium chloride. You could fill up balloons but if you need to hydrogenate something I found it more effective to fill a
jimmy rigged beach ball (that was completely evacuated with a vacuum aspirator) and use that to fill balloons. Did not collect the initial gas
produced and waited until it lit a flame consistently to collect.
Indeed you gotta make sure you keep temperature under control or things can runaway very fast, which actually happened the first time lol. Also I
think it's best to go with an excess of aluminum because when I went with molar equivalents I ended up having leftover NaOH, so the foil may of been
impure or some other factor.
I noticed the hydrogen produced had this faint smokeyness to it. Not exactly sure what that byproduct was because it should've been colorless and
undetectable. If anyone has a guess let me know because the following hydrogenation did work.
Have also attempted to produce via electrolysis but ended up with severe corrosion of the copper cable leading to the carbon electrodes, and very slow
rate of hydrogen production.
Calcium hydride apparently isn't that easy to source (cheaply) these days for the amateur. Even the chinese want an arm and a leg, mostly on
shipping.
