Can an O₂ absorber really be considered a ‘machine’? Well, I’d say it is a device which is engineered and constructed to provide a desired, useful function based on scientific principles. On that basis I would definitely call it a machine of sorts. And furthermore, it’s a machine with no moving parts! Exactly the sort of thing we excel at around here.
Oxygen is a good thing. But what it does, that is… oxidize stuff, isn’t good for things we want to protect and save. It makes them age. Free atmospheric oxygen corrodes metals, makes oils go rancid, bleaches and fades pigments and dyes, and most annoyingly, spoils food and vitamins.
The answer is O₂ absorbers. Oxygen absorbers are highly recommended as a means of helping preserve dry goods such as fine art, firearms and most especially, food which has been stored away for use in a future emergency. Oxygen absorbers are placed inside sealed containers and are intended to do exactly what it says on the tin… absorb atmospheric oxygen to prevent it from interacting with the preserved materials. Do NOT confuse oxygen absorbers with desiccants for humidity control. Those are entirely different.
Oxygen Absorbers
Modern oxygen absorbers will come in sizes that range from 20 cc to 2000 cc. The size of the oxygen absorber refers to the amount of free oxygen that it can take out of an environment.
Therefore, a 20 cc packet can only remove 20 cc of oxygen, while a 400 cc packet can take away 400 cc of oxygen.
Recommended oxygen absorber usage based on container size is as follows:
- 1 pint – 50 cc
- 1 quart – 100 cc
- 1 gallon – 400 cc
- 5 gallon bucket – 2000 cc
Begin Nerdy Engineering Talk
One gallon equals 3.78 liters. Chemistry buffs will recognize that one gallon of a gas consists of 3.78/22.4 moles of substance at standard temperature and pressure. If 21% of that gas is oxygen then it means:
(3.78/22.4)*(21/100) = 0.0355 moles of oxygen O₂ are present. A mole is just a number, a count of molecules.
Also note that by volume:
21% * 3.78 L = 0.7938 L
(nearly 800 cc)
O₂ absorbers work by a chemical reaction that captures free oxygen. Rusting iron is just the ticket. This reaction also releases some heat energy and is commonly employed in the design of chemical hand warmers. A 400 cc oxygen absorber is considered to be sufficient to collect all the free O₂ from a gallon jar if it’s filled with product. This is roughly ½ of the O₂ in an empty jar as we have just calculated. What we are assuming is that a jar filled with product will still have up to ½ of its volume taken up by air; and 400 cc of that is oxygen. If your jar is not filled or you think more than half the volume contains air, use more absorbing capacity. There is no danger in adding too many packets.
This means we need to scale our stoichiometry to absorb at least:
0.0355/2= 0.0178 moles of O₂.
We will be forming Fe₂O₃ rust, so each mole of Fe₂ anions will capture 1.5 moles of O₂.
Accordingly, that requires:
0.0178/1.5 = 0.0119 moles Fe₂
Now, consulting the periodic table of elements we find:
The Atomic Mass of iron (Fe) = 55.845 g/mol
Therefore, we determine that we need :
2 * 55.845 *0.0119 = 1.33 g of iron to complete the absorption.
Let’s Build It
We will use steel wool instead of iron powder in our design because it is more easily sourced. Remember, more surface area is better so use a fine wool Do not use wool with soap or anticorrosion coatings.
We want:
Grade – 0000#
Grade Name – Super Fine
Typical Use – Sanding furniture
Ordinary (non-stainless) steel wool is made from a low-carbon steel and should be in excess of 98% iron by mass. To obtain the desired iron content we need:
1.33 / 0.98 = 1.35 g. steel wool.
NOTE: There is nothing wrong with oversizing our design by using “too much” iron, other than the extra expense of course. But when you think about it, wouldn’t it be foolish to risk $100 or more in food (not to mention your very life when you are depending on that food in a survival situation!) just to save a penny? Don’t do it!! Feel free to use up to 5x the requirement and be assured of safety. If you check, you’ll find that most commercially produced O₂ absorbers are made exactly this way. I think 2-3 grams of steel wool is a fine approach here.
Optional Step:
Add sufficient bentonite clay (kitty litter) or activated charcoal and work it into the steel wool. These porous materials will hold the water necessary for catalyzing the oxidizing reaction.
A pound of activated charcoal has the same surface area as six football fields. That’s a lot of crannies for storing water so you don’t need much.
A little known fact about adding charcoal is that it will cause a small amount of carbon monoxide (CO) to form inside your container. This, in combination with the reduced oxygen atmosphere will help kill any insects that might be hiding in your storage container. Not to worry though, it isn’t enough to harm anyone and will quickly dissipate when the container is opened.
Wrap this material together in a piece of paper towel or coffee filter forming a pouch. Tape or staple it closed and secure it to the underside of the jar lid. Double sided tape might be the perfect thing here.
These units may be made up in bulk to be used as needed. Unlike commercial oxygen absorbers, there are no special storage conditions such as vacuum packing necessary. They require activation before the rust reaction will start in earnest. Simply keep them dry and you should be good, although, a couple of silica gel packets couldn’t hurt here to be sure.
How to Activate and Use Your Oxygen Absorber
NOTE: Sea water is a good rust promoter. The salinity (saltiness) of the ocean is about 35 parts per thousand. This means that in every liter of water, there are 35 grams of salt.
Make a salt water solution of 3-5 g. per 100 ml. (About ½ teaspoon per ¼ cup of water.)
Before sealing the jar, moisten the packet with about an ⅛ teaspoon of this brine solution. A simple spritz with a spray bottle to dampen the paper should be sufficient.
With salt and water present, the oxygen reacts with the iron in the steel wool to form iron oxide (Fe₂O₃) and release heat.
Wait to be sure the packet is getting warm and then proceed. Remember that we mentioned CO formation? This heat is what will cause that to happen. Apply the jar lid and ring. The absorber will draw a vacuum for sealing. The lid should ping to prove it.
There is one caution you should be aware of. This is an IMPORTANT one.
When using oxygen absorbers to store foods, it is vital that the food be dry! The moisture content must be below 10%. The reason is that botulism likes to grow in moist, low oxygen environments. Be careful.
You may read on the internet that you should not use oxygen absorbers and silica gel packets together. This is because oxygen absorbers require moisture to activate them as we have seen. Silica gel packets will absorb moisture and your oxygen absorbers may not work.
I believe this shouldn’t be a problem if you’re sure to separate them from your O₂ absorber so it has a chance to work before the moisture is drawn away. Experiment a little for yourself to see what works.