Hofmann Apparatus - Split Water 2H2+O2

Hofmann Apparatus - Split Water into Hydrogen and Oxygen

My fascination with the electrolysis of water into hydrogen and oxygen gas started at a young age. I can recall taking a plastic cup some salt, thick aluminum wire, a test tube and a 9-volt battery to my elementary school; probably 4th grade. I hooked it all up and generated some hydrogen gas.

It was so cool to magically create gas out of water. Even more exciting was the POP! that you got from lighting the test tube of hydrogen!

The basic overall reaction is: 2 H2O (liquid) → 2 H2 (gas) + O2 (gas)

Breaking water (H2O) down gives you 2 parts Hydrogen gas and 1 Part Oxygen gas.

This 3-tube apparatus for electrolyzing water was invented by August Wilhelm von Hofmann (1818–1892).

A pure water reaction can go slow. In order to speed it up an electrolyte is needed. This can be as simple as table salt (there is a risk of generating chlorine gas if there voltage and power are too great). Strong acids such as sulfuric acid (H2SO4), and strong bases such as potassium hydroxide (KOH), and sodium hydroxide (NaOH) are frequently used as electrolytes due to their strong conducting abilities. (see Wikipedia)

An in depth explanation of the reaction with dilute sulfuric acid be found here.

This Instructable will show how to construct your own Hofmann Apparatus.

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18 inches of 1" dia Clear PVC Pipe

3 ft 1/2" dia Clear PVC Pipe

3 - 1/2" Clear PVC T-Fittings

2 - 1/2" to 3/8" PVC Reducer Bushing - Spigot x FIPT

3/8" dia x 3" Threaded PVC Nipple

18" of 1/2" white PVC pipe (optional)

2" square of bicycle inner tube rubber (optional)

12 inches of 5/16 In dia graphite rod

Red and Green 1/2" PVC caps (PVC furniture caps)

2 Aquarium valves (stopcocks)

Short length of aquarium tubing

Hot Glue

Epoxy Glue

Silicone Sealant

Toothpicks

Masking Tape

Teflon Tape (pipe tape)

Heat Shrink Tubing or optionally electrical tape

Red and Black Battery Clamps (with boots)

Useful length of Red and Black silicone insulated wire

12 VDC power supply, 500mA minimum output

0.9 Liters (1 Qt. = 32 fl oz.) Automotive Battery Acid (Sulfuric Acid)

2 Liters Distilled Water

Test Tube and lighter/matches - For the POP!

There are lots of choices for the type of electrodes you can use. Electrodes can be consumed during the electrical reaction of splitting water. The least reactive commonly used electrode material is platinum. You can buy platinum electrodes on amazon and ebay, but I didn't want to spend that much money. You can use almost any metal but I kept hearing about graphite electrodes. Apparently graphite is pretty non-reactive, inexpensive and holds up well. So I opted for Graphite!

You can get your graphite from a pencil (In the pencil 'lead' graphite is mixed with a binder and is not that great for electrolysis). You can disassemble Zinc/Carbon batteries to get the graphite rod out of the center; I'm not into disassembling batteries that have their own nasty chemicals inside.. Or you can purchase, for a reasonable price, graphite rods intended for use as stir sticks for casting/melting precious metals.

I purchased a 5/16" diameter rod 12" long - I cut it in half to create my two electrodes!

I wanted my electrodes to be removable. So I installed the graphite rods in 3/8" diameter PVC pipe nipple.

To do this I cut the plastic pipe nipple in half.

Centered the rod in the nipple with toothpicks on both ends.

Applied hot glue to the non-threaded end to hold the graphite rod in place while I removed the toothpicks from the other.

Applied epoxy glue into gap between the rod and nipple on the threaded end. I masked the rod and used a syringe to apply the epoxy.

Removed the toothpicks and touched up holes left behind with more hot glue.

Applied silicone sealant over the epoxy.

The electrodes are installed in the 3/8" threaded opening in the reducer in the bottom of the vertical columns.

Apply teflon tape to insure a liquid tight fit.

Cut the Clear PVC pipe to length:

1 - 18" long 1" diameter for large diameter central tube.

2 - 13" long 1/2" diameter outer gas columns.

2 - 3-1/2" long 1/2" diameter horizontal connectors.

Clean up the cut edges. Scraping with the edge of a utility knife can remove the melted residue from the cut.

I did not find any economical clear fittings to connect the 1" pipe to the center 1/2" T-fitting. I checked and the inside diameter of the 1" pipe was almost large enough to slip over the outside of the T-fitting. The 1 inch PVC was just a little too small.

I originally thought of grinding out the inside of the 1" pipe with a Dremel tool. However, the thought came to me to heat* up the tube and stretch it over the T-fitting similar to what I had done previously. It worked perfectly.

Take care to heat the clear PVC slowly. Rotating the pipe continuously. As you heat the pipe don't be alarmed if the clear PVC turns cloudy white. I was worried that my all-clear-PVC apparatus would be ruined by an opaque section. No fear, patience paid off and as the clear PVC cooled off the cloudiness went away -- Hooray!

You also need to take special care to make sure you hold the central tube in perfect alignment with the others as it cools. I test fit the vertical gas column tubes in place so I would have references.

*WARNING - PVC can give off noxious/toxic fumes. It may be wise to heat PVC outside.

Test fit the PVC pieces and then glue them together. Be careful of external glue dripping down all over.

I found it was nicer to have excess glue outside the fitting rather than inside, so I applied the glue to the external surface of the tube or reducer, rather than the inside of the fitting.

Make sure to practice the alignment and the twist so you will end up with the right alignment when you mate the pieces.

I used aquarium valves for my stopcocks. I thought it was a nice touch to use colored caps on the gas columns. Red for Hydrogen. Green for Oxygen.

I tried to find out exactly what thread was on the aquarium valves, but was unsuccessful. It is probably just as well. I drilled a hole just smaller than the thread diameter and was able to forcefully twist the valve into place, essentially cold molding its own threads. I added a bit of hot glue around the valve opening inside the cap to insure no leaks.

I glued the caps onto the gas columns.

Be sure to practise fit so that after you have given the required twist (while slipping the cap onto the glue on PVC) that you end up with the stopcock lever oriented where you want it.

I found some very nice very flexible silicone insulated wire.

I soldered the wires onto the clamps.

I had many plans for power supplies; a ATX computer power supply (planned on using the +12V output, a small open frame DC supply, and finally after trying the previous settled on a direct plug-in wall 12V DC power supply.

The Hofmann apparatus constructed as described, and with the electrolyte described, only draws about 500mA of direct current. The power supply needs to output direct current (DC) of at least 500 mA (0.5 Amp).

Cut the connector off of the power supply cable if it has one (sorry about that). Strip back the insulation. Using a multimeter, Identify the positive and negative conductions. Connect the red (+) and black (-) clamp wires to the stripped ends of the power supply. It would be nice to dress up the splices with heat shrink tubing --. Remember to slip the heat shrink on before making your electrical connections (How many times have I forgotten - only to have to cut the splice and start over!)

The Hofmann Apparatus works so much better with an electrolyte instead of pain water. As mentioned earlier strong acids such as sulfuric acid (H2SO4), and strong bases such as potassium hydroxide (KOH), and sodium hydroxide (NaOH) make the best electrolytes.

However, what makes a good electrolyte also is poisonous and caustic and can cause burns, blindness and even death! So these are nothing to mess around with.

CAUTION - Please wear protective gear; including safety glasses, gloves (not latex), face shield, and have plenty of water available if needed for spills / accidental exposure.

WARNING - Always mix electrolyte by adding the chemical into a larger amount of water. NEVER add water to acid or bases. Violent reactions can take place.

I chose sulfuric acid since it does not react with the electrodes nor is consumed in the reaction. My easy source for sulfuric acid is car-battery acid. Many car batteries are shipped dry and acid needs to be added. It is easy to buy battery acid at automotive supply stores.

Battery acid is already diluted. However, I chose, as another instructable author did, to dilute it further. The amount of acid I purchased was 0.9 liter. I mixed it with 2 liters of distilled water. Add the Acid to the water in small amounts. This became my sulfuric acid based electrolyte.

NOTE: Do not leave the acid solution in the Hofmann Apparatus for long periods of time (>24hrs?). I left solution inside for a week and it appeared that the acid was breaking down some component of the graphite rods (perhaps some type of binder); small black flakes were appearing in solution and a few drops of solution were leaking off the external ends of the graphite rods. So demonstrate, then remove the electrolyte.

Fill the Hofmann Apparatus with the electrolyte solution. In order to get the solution up into the gas columns you will need to open the stopcocks.

I suggest connecting a length of aquarium tubing to the stopcock to direct the flow of electrolyte that invariably will come out of the stopcock as you bleed the air out the column. Do one column, then the other. You may have to add electrolyte in order have an electrolyte level high enough to force all the air out.

Remember that as gas is formed the liquid level in the center tube will raise. The center column is not long enough (have enough volume) to contain the maximum gas that can be generated. The center tube will overflow!

One way to avoid this is to use a plunger to force the electrolyte up into the gas columns without filling the center tube up to the level of the stopcocks. The plunger is described on the next step.

Connect the electrical clamps to the graphite electrodes and plug in the power supply.

The gas starts to form as shown below:

It takes about 12 minutes to generate a full column of hydrogen gas (approx 54 ml).

OPTIONAL PLUNGER: The electrolyte going into gas collection columns will be displaced by the hydrogen and oxygen generated and find its way back into the center tube. If enough electrolyte was poured into the center tube to get the air out of the columns there will be too much electrolyte in the center column at the start.

To avoid this only enough electrolyte is added to fill the gas collection columns. The electrolyte is forced up into the gas collection columns by use of a plunger.

The plunger is constructed of a 1/2" diameter PVC pipe that has a 26.5mm diameter (ID of pipe is 26mm) piece of rubber bicycle tube glued to it. I used a Cricut Maker to cut the perfect circle for the rubber disk. The disk was glued to the end of the 1/2" dia white PVC pipe.

Silicone sealant was applied between the edge of the PVC pipe and the edge of the disk of the rubber bicycle tube. A hole was punched in the rubber disk before being glued in place so that when the plunger was pulled to be removed air could be admitted so the plunger was not vacuum-locked in place.

Pour electrolyte in the center tube to the level about halfway up the gas collection columns. Insert the plunger and bring it down to the electrolyte level. Cover the end of the PVC pipe with a finger to make the plunger air tight and force the plunger down to force the electrolyte up into the gas collection columns (stopcock is open ;-). After bleeding the air, remove your finger from the end of the PVC pipe of the plunger and remove the plunger from the central pipe.

If you don't use the 'plunger method' to keep the electrolyte level low, keep watch on the electrolyte level as it climbs up the center column. Stop the reaction before it overflows or remove electrolyte as the reaction progresses.

Fill a test tube with hydrogen by holding it over the stopcock and opening the stopcock to let the hydrogen flow into the upside down test tube. (Remember Hydrogen gas is lighter than air) Stop the flow before electrolyte flows out. Cover the test tube opening with your thumb until you are ready to hold a lighted flame to the opening - POP!

Enjoy!

P.S. Note that water vapor covers the inside of the test tube after the pop. The hydrogen burned with oxygen from the atmosphere to create water.

Note: For some reason I am getting lots of Hydrogen and very little oxygen gas. I have swapped the electrical polarity (thinking it might be something wrong with one of the electrodes) and still have very little oxygen generated. Any thoughts would be appreciated.

WARNING: Mixing two parts hydrogen with one part oxygen is a gas referred to as Brown's gas. It is used for torches, internal combustion engines, etc. Do not mix the hydrogen and oxygen in this way unless you have a properly protected explosion proof setup, as Brown's gas is explosive.