It sounds like magic, but it's just electrochemistry. All you need is a pencil, some titanium wire, and a battery to start purifying water. The voltage between the graphite and the titanium converts H2O and a little table salt (NaCl) into NaClO, sodium hypochlorite, also known as bleach. Run the reaction for a few minutes, and it will start to smell like a pool, and the bleach will kill off the bacteria in the water. For all chlorine-based water purifying products, it's good to wait at least 30 minutes for the chlorine to do its work. If you wait longer, the chlorine decomposes back into NaCl and oxygen.
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Step 1: Review Other Water Purifying Systems
I've shown several choices of water purifying systems here. The purifying pen by MSR uses the exact same electrochemical technique as the one I described. I am told it is used by the military for emergency water purifying. It's very effective and purifies a lot of water. Since it's not on the market any more, you might want to build one using the instructions I've developed.
Step 2: Scared of Chlorine?
If you're worried about the health effects of drinking chlorine bleached water, then don't drink tap water. Every town and every city purifies its water the same way, by this electrochemical process. They have fancy industrial sized reactors for this Chloro-alkali process, but it's fundamentally the same. That said, if you make chlorine using the process I describe, it is not approved by the NSF or the FDA or anything, so you would be drinking it at your own risk. I'll mention some hazards later.
Step 3: How Does It Work?
This graphic shows the reactions that take place in the electrochemical cell. We want the HClO, (or HOCl, same thing), called "sodium hypochlorite", or "bleach". Or maybe NaClO. Doesn't matter. You put the positive terminal of the battery on the graphite and the negative terminal on the titanium. Some other metals may work, but if you just pick a random wire, it will spew out yellow crap in the water when you apply the voltage, and you will get no chlorine bleach at all.
The yellow stuff might be oxidized iron or ferric chloride, which is a strong acid. Even though you made it from harmless ingredients, it's a harmful chemical, so don't touch it. Same goes for breathing it in. You can gently waft your hand above your reaction to smell it. Don't put your face right in it. Wear safety goggles and nitrile gloves.
So go on Amazon and buy some titanium wire. It's not expensive, and with steel wire or whatever you have lying around it's not going to work. The titanium is not used up in the reaction; a short piece will last you forever. For the electrode geometry, I first wind the titanium around the pencil to make a spring-like shape. Then I remove the titanium and carefully split the pencil down the middle, exposing the graphite. Then I can put the titanium wire back on the pencil without letting it touch the graphite. It's just touching the wooden part. The graphite and the wire are dry above the beaker, so I can connect the battery up there. I hold the contacts by pressing the wire and graphite to the terminals of the battery. Alligator clips and stuff would work too. Don't get the contacts or the battery wet.
You can see in the graph at the left that current doesn't flow unless you have at least some salt (NaCl) and at least ~4.5 volts, which is 3 AA batteries in series. You can do this by taping them back to back. Alternatively, you can buy a 9 volt battery and your life will be a little easier. Be careful not to get the battery terminals wet and not to shock yourself with the battery. Also don't mix up the terminals. As shown in the picture, put the positive terminal of the battery on the graphite, and the negative on the titanium. If you reverse it, sometimes the titanium changes color from silver to orange to purple. It's super cool, but not helpful.
For a preliminary test, just dump a spoonful of salt into a cup and do the reaction. It should proceed very rapidly and smell like a pool in no time. When it smells like a pool, I think that's around 5 ppm (parts per million) Chlorine, which is 5 times as much as is used in drinking water. When it smells like this, stop the reaction. You don't want it to go too far, because it can get very smelly. Like, to the point where just a whiff makes your eyes water, your throat burn for hours, and severe nausea. I've been there, and I absolutely do not recommend it.
Okay, now you've done your preliminary test, the next instructions will make sense.
Step 4: How Long to Run Your Reaction
Depending on the voltage you have available and the salt concentration you put in, the reaction will take a different amount of time to produce the same amount of chlorine bleach. This graph shows how long to leave it running to get to 1 ppm bleach, which should be enough for drinking water purifying.
You might say, oh, let's go fast, using high salt concentration and high voltage! But that's bad for two reasons. First, very high salt concentration will mean it's too salty to drink. More on that later. High voltage means you're wasting some energy, both due to overpotential and loss reactions, such as the formation of hydrogen and oxygen gas. That's right, this reaction produces explosive gas in the presence of a battery (read: ignition source). The amount of gas is extremely small, though, so as long as your container is open to air and the reaction is not left unattended for like an hour, it should be okay. I never had a problem with it. But you do it at your own risk.
For a voltage source, if you don't have a battery, a standard AC/DC converter usually works. Sometimes they are called wall warts, cell phone chargers, or power supplies for appliances, and it looks like a black box with two cords coming out. One cord goes to the wall, and the other sometimes has a DC voltage. You can read the box to see if it says it is converting to DC, which is what you want. Anything above 4.5 V will work. To connect the wire to your electrodes (the graphite and the titanium), you will have to cut and strip the DC wire and make contacts. Alligator clips make this convenient, but it is very easy to accidentally connect the positive and negative terminals, which generates a lot of current and can blow out your device or cause fire or injury. You can use electrical tape around exposed connections to avoid short circuits.
So you have this graph for how long to run the reaction, and you have some voltage source. But you need to know how much salt to put in. This image from Wikipedia shows the amount of salt in water. It's extremely hard to find any resources for how much salt the body can take. It probably depends on the other foods you eat. One solution to the problem is to produce the bleach in a small container, such as a shot glass, with 1 ppt salt (which means for 1 gram of water you want 0.001 grams of salt). Run the reaction for as long as the graph says for 1 liter. Then dump the contents into the liter bottle, and the salt concentration will be almost zero while the bleach concentration will be appropriate for 1 liter.
Then of course you need to mix it up to make sure the bleach gets around in your liter bottle. Leave it for at least 30 minutes for the bleach to kill the bacteria. Then if you leave the 1 liter (or quart) container open for some time, especially in the sun, the bleach will gradually decompose back into NaCl and oxygen, which will improve the taste.
Here are simplified instructions for a 9V battery, using the 1ppt dilution trick.
1. Wrap 12" of titanium wire around a pencil to form a spring-like shape
2. Take the titanium off the pencil, carefully keeping the spring-like shape
3. Split the pencil down the middle. If it is a cheap pencil, the graphite will break, and you will have to start over. Experiment with several brands of pencil until you get one that works. Offset cutters or diagonal cutting pliers are fairly good tools for this job.
4. Carefully put the titanium spring back on the pencil, so that it touches only the wood, not the graphite. Unwind a little titanium up near the back of the pencil for making electrical contact to the battery.
5. Prepare the solution. Get a half cup measure (120mL) and put 0.1 gram of salt (0.02 teaspoons, just a pinch) in and fill it with water. Mix it in. That's about 1ppt.
6. On the chart, you see at 9V and 1 ppt salt it should take less than 10 seconds. With the bottom of the pencil (with the titanium) in the salt water, put the positive terminal of the battery on the graphite and the negative terminal on the titanium. You may have to press hard to achieve good electrical contact. You may see a small spark. If you're hands are wet, you may feel a shock and get burned. Maybe you want to switch to alligator clips for your connections. You can tell it is running because tiny bubbles form on the electrodes. If the bubbles do not form, check your connections and maybe add a little more salt. Run the reaction for at least 10 seconds, maybe make it 20. You should start to smell the chlorine like a pool if you wait longer, like 1 minute.
7. Dump the contents of your little half-cup into a 1 liter or 1 quart bottle full of water that might have germs in it. Mixing it up dilutes the salt, so now it has only 0.1ppt salt, which is definitely fresh enough to drink. Wait 30 minutes for all the bacteria to die. Leave the container open in the sun for a while to improve the taste.
8. You can reuse the same battery and the same electrodes again and again until you stop seeing the bubbles form during the reaction. This indicates the battery is dead and needs replacing. One 9V battery should purify 698 liters of water! Obviously if you let it run for twice as long, or put in twice as much salt, you will produce more bleach concentration and you'll get half as many liters out of the same battery.
Specifically, the calculation is:
(.500 Ah*3600C/Ah)/(1.29C/L(coulombs per liter purified)*2(safety factor)) = 698 Liters
Tadaa, you have purified water! That said, this product is not certified by NSF or FDA or anyone else, including me, for use in any purpose whatever. Why am I so cautious? Have a look at some of the hazards of the chloro-alkali process on the next page.
Step 5: Hazards
As you can see, mixing the bleach with acid, ammonia, or organic materials creates very harmful products. This is significant, considering that the application is the wilderness. Suppose some animal (or human) has urinated near the drinking water. Well that will contribute some ammonia, and that will react with the bleach to make chloramines, which are harmful to the lungs. Or maybe there's some acid from a nearby fruit or acid rain; then you might have chlorine gas on your hands, which was used as a chemical weapon in the world wars. Or if you leave the reaction running too long, you may have more fully oxidized chlorine compounds, some of which are explosive, and which are in general very high pH, which can burn your skin like an acid burn.
There are some ways to catalyze the decomposition of these products, such as with ruthenium oxide, but that is kind of a stretch for the household experiment or survivalist kit. In fact, this tutorial is a little bit advanced, and if you've made it this far, you probably learned something about electrochemistry. As if you haven't had enough already, I put a short list of some links and papers you might read more about the Chloro-alkali process.