Introduction: Make Lye From Salt and Gelatin

Granny's gone and so is her box of ashes for making lye. Here's a modern technique to make high quality lye (Sodium Hydroxide) at home for any number of reasons, including soap making.

We'll be using an electrochemical process called electrolysis in a two chamber apparatus. In one chamber we will put a saturated saltwater solution and a positive electrode. In the other we will put pure water and the negative electrode. The two chambers are separated by a salt bridge composed of gelatin and table salt. While Jello brand gelatin can be used, we will be using Knox unflavored gelatin. The salt bridge allows sodium ions to pass between the two chambers.

NOTE: This is dangerous. Chlorine gas is extremely toxic and hydrogen gas is highly flammable. This should only be done outdoors in a well ventilated area. Avoid breathing the fumes which escape from the chamber.

When current is applied the sodium and chlorine ions are attracted to the negative and positive poles respectively. In the positive chamber the free chlorine combines into chlorine gas which escapes while the sodium ions travel across the salt bridge to the negative electrode. Meanshile in the negative chamber the pure water separates into hydrogen gas and an a hydroxide molecule. The hydrogen gas escapes and the sodium ions from the salt bridge combine with the hydroxide molecules to form Sodium Hydroxide in solution.

This process can be used to produce NaOH solutions of up to 40% or so concentration. This can be further reduced or dried to produce higher concentrations. I believe the "raw" syrup will be sufficient for soap making.

Traditionally the lye syrup made from ashes was boiled down until sufficiently concentration. This was tested using the egg test. If an egg was placed in the solution it should sink up to an area about the size of quarter (between 1/2 and 3/4 of the egg below the surface).

If you want to get all green about it use a solar oven to heat the water for making the saturation solution and use a small solar panel or solar battery charger for the electrolysis.

Step 1: Bill of Materials

Many of these items may seem familiar if you've tried "Make a Microbial Fuel Cell - Part II" as it is the same assembly.

2 1x1x1 PVC Schedule 40 Tee
3 1" PVC Schedule 40 Connector
2 1" PVC Schedule 40 Plug
2 1x1/2 Slip vs FPT threaded adapter
1 /21" PVC Schedule 40 Threaded Cap (Optional)
Clear plastic wrap
Rubber bands
Distilled Water
Plain salt ( not iodized )
4.5-12V Power Supply with leads
Salt Bridge Medium (Pick one)
  • Knox Unflavored Gelatin/Jello brand instant gelatin
  • Hide Glue (powdered not liquid)
  • Agar
Electrodes -
Very nice stainless steel electrodes can be constructed using the reinforcing rods from automotive windshield washers
Alternatively pieces of carbon fiber rod, such as those available from model airplane and hobby shops can be

Step 2: Create the Salt Bridge

The heart, so to speak, of this electrolysis device lies in the salt bridge or membrane that spaerates the cells. Agar is most commonly used but isn't necessarily readily available. However since Agar is a gelatin I thought conventional unflavored gelatin might make a substitute.

This is my least favorite component in this design, although I like its novelty. I would really like to have something at the ends to provide mechanical stability.

To create the salt bridge we will use Knox Unflavored gelatin in place of the more common agar. I have some reservations about the stability of this innovation. My next round will focus on the use of hide glue (dry not liquid) as the binding agent. This will not substantially alter the bridge process.

Start some water to boil, you don't need and actually won't use more than a couple of tablespoons.

Take one of the coupler sections. Place the plastic wrap firmly over one end and secure with a rubber band to form a tight seal. Place the coupler open end up in a bowl.

Take one packet of Knox unflavored gelatin and pour it into the coupling. Fill the rest of the way with salt. Pour this mixture into a dry measuring cup and pour back into the coupling. You'll notice that settling left a little more room. Top off with salt, pour back into the measuring cup and add an extra tablespoon. Mix the dry ingredients thoroughly and pour back into the upright coupler.

When the water is at a strong boil pour a bit into the measuring cup and use this to gently fill the salt bridge. Pour carefully so you don't flush the tube but be generous. The mixture will settle and allow you to "top off" the water, this will disclose air bubbles flushed out and help judge when the bridge is full. Carefully picking it up and examining the bottom can also help make sure that the entire column is saturated.

Allow to stand for a few minutes and pour off any tiny bit of standing water on the top. Then place the bowl in the refrigerator for 3-4 hours.

Step 3: Assemble the Electrodes

Since I'm using the stainless steel electrodes I got from changing my wiper blades. I simply take a dry sponge and using my box cutter I cut off a strip just wide enough to fit into the threaded opening and cut this into approximately thirds. I will use 2 and discard the other.

Using a pair of cutters I clip the notches off the end and cut the remainder approximately in half. The length really isn't significant as long as it reaches will into the chamber and protrudes enough from the top to attach the power source.

Wet the sponge with a little distilled water, push the electrodes through the sponge and its on to the next step.

Step 4: Make the Brine Solution

A saturated solution is one in which the solvent (water) has achieved equilibrium between the dissolved and undissolved solvent. What does that mean? The water has absorbed all of the chemical which it can hold. The hotter the water the more chemical it can hold. So what we're going to do is heat up the water (I suggest 100 degrees but substantially above room temperature will do boiling is definitely not necessary ). and once the water has heated up add some salt and stir until fully dissolved.

Following the steps outlined in Making a Saline or Hydroxide Solution prepare 500 ml (about two cups) of a saturated saline solution using distilled water rather than tap water.

Pour about 500-700 mL of water into a pan, basically you want enough to fill your bottle with a reasonable margin of error (I used about 1 1/2 bottles) and turn up the heat. Once the water has heated up to at least 100 degrees add some salt and stir until fully dissolved.

If you use a kitchen scale measure you can measure out about 100g as a starting point otherwise just pour some in and stir. Repeat this until the salt no longer dissolves and wet salt crystals appear on the bottom of the pan (see picture). There is no particular danger in adding too much but no need to overdo it.

Remove the pan from the heat and allow to cool to room temperature. Some folks filter the solution, I was able to carefully pour off 500 mL of solution into a measuring cup without getting any surplus salt.

Step 5: Assembly and Brine Electrolysis

Now it's time to assemble the unit.

First use the plugs to plug one end of the couples. Make sure the plugs are seated fully, these are the "feet" and the device should be stable.

Next insert the couplers into one end of the "T" connectors as shown. Insert the electrode assembly into the other end to form two chambers.

Now get the salt bridge out of the fridge and insert it to connec the two "T"s to form an H as shown.

Now take a little of the distilled water and verify its pH level. This will be used for comparison.

Now fill one chamber with the saturated salt solution and attach the red lead to the electrode. This is where the salt breaks down to sodium and chlorine gas.

Now fill the other chamber with distilled water and attach the black (negative) lead to the electrode. This is the chamber where the NaOH solution will be produced.

Now connect the red and black leads to the positive and negative terminals respectively. Hydrolysis begins as soon as current begins to flow through the circuit.

From a distance of 4-6 inches a faint swimming pool like smell can be detected.


After some period of time ( depends on the amount of current flowing through the circuit ) the electrolysis is complete. This is indicated by the lack of a chlorine smell ( although there may be a better way ).

Once electrolysis is complete we can validate the results by testing the solution.

Step 6: Testing the Results

I used two separate tests to validate the production of sodium hydroxide, pH analysis and power output from an al-air fuel cell.

The main photo shows the power output from a crude al-air battery. The battery was made by laying a sheet of aluminum in a shallow pan, placing a paper towel ( porous non-conductive layer) and a carbon brush electrode (A Fluval carbon aquarium filter) on top of that.

Carefully pour the remaining NaOH solution over the paper towel making sure it is thoroughly saturated. Attach the negative pole of you voltmeter to the foil and attach the positive pole to the carbon brush.

The device immediately  ramped up to the expected voltage range of 1.4VDC ( actually 1.3 and change ). An al-air battery using saltwater produces a max theoretical voltage of .750 mVDC, the higher voltage validates the presence of sodium hydroxide in the solution.

I also tested the pH of the solution. Unfortunately I had only a swimming pool type kit which measures a limited range. As you can see the distilled water tested as pH neutral as indicated by the yellow color. After electrolysis the pH balance has shifted signficantly to the other end of the scale ( indicated by the deep reddish purple color ). This is significantly darker than the max color indicating the solution is in the expected 12-14 range.

That's it.This design should scale up to using a couple of 1/2 gallon plastic jugs, the salt bridge should be okay and doesn't need to be scaled up. You might want to really, really, really consider agar or animal hyde glue.