How to Make Hypochlorous Acid With Electrolysis

Making hypochlorous acid through electrolysis is straightforward in principle: you pass an electric current through salt water, and the chloride ions released from dissolved salt get oxidized at the anode to form chlorine species, which in the right pH range exist predominantly as hypochlorous acid (HOCl). The practice, though, involves more nuance than that one-sentence version suggests. Getting a stable, useful concentration of HOCl rather than a murky mix of unwanted byproducts depends on electrode material, salt concentration, current density, and above all, pH control.

What Happens Inside the Cell

When you dissolve ordinary table salt (sodium chloride) in water and apply a voltage across two electrodes, two main reactions happen simultaneously. At the anode (the positive electrode), chloride ions give up electrons and become dissolved chlorine gas. At the cathode (the negative electrode), water molecules pick up electrons and produce hydrogen gas and hydroxide ions. The dissolved chlorine at the anode almost immediately reacts with surrounding water to form hypochlorous acid and hydrochloric acid. That dissolved-chlorine-to-HOCl conversion is fast and happens on its own as long as the solution stays in the right pH window.

A competing reaction also occurs at the anode: the oxidation of water to produce oxygen gas. This oxygen evolution reaction consumes some of your applied current without producing any useful chlorine. Research on filter-press electrolyzers using mixed-metal-oxide anodes has modeled this side reaction alongside the chloride oxidation, and it is always present to some degree.¹ In practical terms, the oxygen evolution reaction means not all of your electricity goes toward making HOCl. Choosing the right anode material and operating conditions helps tip the balance in your favor.

Why pH Is the Single Most Important Variable

Hypochlorous acid exists in a pH-dependent equilibrium with two other chlorine species. Below about pH 3, molecular chlorine gas dominates and tends to bubble out of solution. Above roughly pH 7.5, the hypochlorite ion (OCl⁻) takes over. The sweet spot for HOCl is between those two extremes, roughly pH 3 to 7. Raman spectroscopy work on aqueous free chlorine confirmed that the equilibrium between HOCl and OCl⁻ has a pKa of 7.5, meaning that at pH 7.5 you get an even split between the two forms.² Drop the pH to around 5, and nearly all of the free chlorine is in the HOCl form. This matters because HOCl is a far more potent disinfectant than the hypochlorite ion.

During electrolysis, pH naturally drifts. The hydroxide ions produced at the cathode push the solution toward alkaline conditions, especially in an undivided cell where cathode and anode products can mix freely. If you let the pH climb above 7 or 8, you end up with sodium hypochlorite (essentially dilute bleach) rather than HOCl. Research on a zero-gap electrolysis cell demonstrated that continuously adjusting pH to between 4.5 and 5.5 in the anode compartment kept the product firmly in the HOCl zone, achieving active chlorine concentrations up to 3.6 percent.³ For a home setup, that level of concentration is well beyond what most people need. Typical consumer-grade HOCl generators produce solutions in the range of 50 to 200 parts per million (ppm), which is effective for surface disinfection and skin care.

If you are building or buying a simple single-chamber cell, adding a small amount of white vinegar (acetic acid) to the salt solution before electrolysis can help keep pH in the acidic range. Some commercial units use a membrane to separate the anode and cathode compartments, which prevents the alkaline cathode solution from neutralizing the acidic anode solution. Membrane-divided cells generally produce a more consistent product but add complexity and cost.

Electrode Materials and Why They Matter

The anode is the workhorse of the system and also the component most likely to fail. Cheap options like graphite or plain stainless steel corrode quickly in the chlorine-rich environment near the anode, contaminating your solution with dissolved metals or carbon particles. The industry standard for chlorine-generating electrolysis is a dimensionally stable anode (DSA), typically a titanium substrate coated with mixed metal oxides containing iridium, ruthenium, or tin. These coatings catalyze the chloride-to-chlorine reaction efficiently while resisting corrosion.

Durability testing on a titanium anode coated with titanium-ruthenium-iridium oxides found that it maintained performance for an estimated service life of at least 26 years during HOCl electrosynthesis in dilute chloride solutions.⁴ That kind of lifespan is relevant for continuous industrial use, but it underscores an important point for hobbyists too: investing in a proper coated-titanium anode upfront saves you from constant electrode replacement and product contamination.

For the cathode, the requirements are less demanding. Stainless steel (316 grade, which resists chloride corrosion better than 304) is commonly used in research setups and works well for DIY builds.¹ Platinum-coated titanium is another option if budget allows. The cathode side produces hydrogen gas and hydroxide ions, so it faces a less aggressively oxidizing environment than the anode.

Salt Concentration, Water Quality, and Current

Most home HOCl generators call for a dilute salt solution, somewhere around 1 to 3 grams of sodium chloride per liter of water, though some commercial units specify a teaspoon of salt per quart or similar easy-to-measure amounts. Higher salt concentrations produce higher chlorine output for a given current, but they also increase the risk of byproduct formation and can leave a salty residue if the solution is used for spraying surfaces.

Water quality deserves attention. Tap water often contains calcium, magnesium, and other minerals that can deposit on electrodes over time, reducing efficiency. It may also contain trace amounts of ammonia or organic nitrogen compounds from the municipal treatment process. As discussed in the byproducts section below, ammonia reacts rapidly with HOCl to form chloramines, which changes the chemistry of the product. Distilled or deionized water gives you a cleaner starting point.

Current density, the amount of current flowing per unit area of the electrode, affects both the production rate and the ratio of useful product to waste reactions. Too much current pushes more oxygen evolution at the anode and accelerates unwanted oxidation of the HOCl you just made. Too little current makes the process painfully slow. For small-scale setups, a low-voltage DC power supply (typically 5 to 12 volts) with currents in the range of a few hundred milliamps to a couple of amps is typical. Some builders use USB-powered circuits for portability, though the output concentrations will be modest.

Unwanted Byproducts You Should Know About

Electrolysis of chloride solutions does not produce only HOCl. One of the most significant side products is chlorate, a regulated contaminant in drinking water. Research using platinum-on-titanium electrodes found that reactions involving active chlorine species were the dominant pathway for chlorate formation, with about 13 percent of chloride being directly oxidized to chlorate.⁵ Hydroxyl radicals generated during electrolysis also contributed to both chlorate and perchlorate production in that study.

The mechanism behind chlorate formation is worth understanding in plain terms. HOCl that lingers near the anode surface can itself be further oxidized, stepping up to chlorate. Work on electrochemical treatment of wastewater showed that chlorate formed directly from the oxidation of hypochlorous acid at the anode.⁶ Interestingly, the same study found that when ammonia was present in solution, it reacted so quickly with HOCl to form chloramines that chlorate production was essentially blocked until all the ammonia was consumed. That is a double-edged observation for the home user: a little ammonia contamination in your source water might suppress chlorate, but it also means your product contains chloramines instead of free HOCl, which defeats the purpose.

Perchlorate is an even more concerning byproduct, though it forms mainly on specific electrode types. Boron-doped diamond (BDD) anodes, which are excellent at generating hydroxyl radicals, are the worst offenders for perchlorate production.⁶ Standard mixed-metal-oxide anodes produce far less perchlorate, which is one more reason to use them over exotic electrode materials for HOCl generation.

Practical steps to minimize byproducts include keeping the current density moderate, using fresh solution rather than recirculating and re-electrolyzing the same batch repeatedly, and running the cell just long enough to reach the target concentration rather than continuing to push current through an already-saturated solution. Shorter run times mean less time for HOCl to sit near the anode and get over-oxidized.

How to Test What You Have Made

You have a clear, slightly tangy-smelling liquid coming out of your electrolysis cell. How do you know it is actually HOCl at a useful concentration? The standard field test for free available chlorine (FAC) is the DPD (N,N-diethyl-p-phenylenediamine) colorimetric method. DPD test strips and liquid reagent kits are cheap and widely available from pool-supply stores. You add a reagent to a sample, and the intensity of the pink color tells you the FAC concentration.

DPD tests are good enough for most home purposes, but they have a known limitation. Both DPD and the more precise amperometric titration method are subject to interference from monochloramine and organic chloramines, which can make the measured FAC reading significantly higher than the true free chlorine residual.⁷ If your source water contained any ammonia or organic nitrogen, some of your chlorine may have been converted to chloramines, and the DPD strip would lump those in with the free HOCl reading. Performing a rapid DPD titration rather than waiting for slow color development helps reduce that interference.

For users who want greater accuracy, the FACTS method (free available chlorine test with syringaldazine) offers better specificity. Comparative testing across 16 laboratories found that FACTS was statistically equivalent to both DPD and amperometric titration for measuring free available chlorine while being less susceptible to chloramine interference.⁸ FACTS reagents are less commonly found in consumer stores but can be ordered from laboratory suppliers.

Beyond chlorine concentration, checking pH with a simple pH meter or test strip tells you whether you are in the HOCl-dominant zone. If your pH is above 7, the majority of your active chlorine is hypochlorite, not HOCl, regardless of what the DPD strip says about total free chlorine. Both tests together give you a much clearer picture of what you have actually made.

DIY Setups Versus Commercial Generators

The simplest possible DIY HOCl generator consists of a glass jar, two electrodes, a pinch of salt, and a low-voltage DC power supply. Search online and you will find people building them from repurposed phone chargers and titanium mesh off eBay. These work in the sense that they do produce some HOCl, but they offer zero control over concentration, pH, or run time, and the electrode quality is often dubious.

A step up from the jar approach is a purpose-built single-chamber flow cell where salt water is pumped past the electrodes and collected in a reservoir. Adding a small dosing pump for acid (to keep pH in the target range) and a timer on the power supply gets you much closer to a repeatable product. The electrode spacing, flow rate, and current all affect the outcome, so some trial-and-error calibration is unavoidable.

Commercial HOCl generators marketed to consumers and small businesses range from handheld spray-bottle-style units (which electrolyze a small amount of salt water right in the bottle) to countertop machines that produce a liter or more per batch. These units embed the electrode, power supply, and sometimes a membrane into a compact housing. They are convenient but opaque: you typically cannot adjust the current or see the electrode condition. Evaluation of a low-cost electrolysis-based disinfectant generator designed for resource-limited settings found it to be cost-competitive with commercial chlorinator systems in terms of both the upfront hardware cost and the per-liter cost of the disinfectant produced.⁹ That study highlights an important point: you do not need expensive equipment to produce an effective product.

The trade-off between DIY and commercial is essentially control versus convenience. A DIY setup lets you choose electrode materials, adjust operating parameters, and troubleshoot problems, but demands more knowledge and testing. A commercial unit gives you a push-button product at the cost of being locked into whatever design decisions the manufacturer made.

Safety Precautions

Electrolysis of salt water generates chlorine gas, hydrogen gas, and potentially other irritating fumes. At the dilute concentrations used for HOCl production, the amount of gas evolved is small, but it is not zero. Always run your cell in a well-ventilated area. If you can smell a strong “pool chemical” odor, your ventilation is inadequate or your pH is too low (pushing the equilibrium toward dissolved chlorine gas, which off-gasses more readily).

Hydrogen gas produced at the cathode is flammable and lighter than air. In a small enclosed space, it can accumulate near the ceiling and pose an ignition risk. Open a window or run a fan, and never operate the cell near open flames or sparking equipment.

The HOCl solution itself is a strong oxidizer. At the concentrations most home generators produce (under 200 ppm), it is gentle enough to be used as a skin-safe wound irrigant and is classified as non-hazardous at those levels. At higher concentrations, it can bleach fabrics, corrode metals, and irritate mucous membranes. Store it in opaque containers away from direct sunlight and heat, and label it clearly so nobody mistakes it for plain water.

One safety point that catches people off guard is the incompatibility of HOCl with many common chemicals. Never mix your HOCl solution with vinegar, ammonia-based cleaners, hydrogen peroxide, or rubbing alcohol. The reactions can produce chlorine gas, chloramine gas, or other toxic fumes. Treat it the way you would treat bleach in terms of what you do and do not combine it with.

Shelf Life and Storage

HOCl is not a molecule that likes to sit around. It degrades through several pathways: reaction with organic matter, photodecomposition from UV light, and a slow disproportionation reaction where HOCl molecules react with each other to form chlorate and chloride. The practical shelf life of a freshly made solution depends heavily on its concentration, pH, temperature, and how it is stored.

At room temperature in a clear container, a 200 ppm HOCl solution can lose half its strength within a week or two. Storing it in opaque (ideally HDPE plastic) bottles in a cool, dark place extends this to several weeks. Some commercial products claim stability of 30 days or more, often achieved by tightly controlling the pH at production and using UV-resistant packaging. Refrigeration slows degradation further.

For best results, make only what you plan to use within a few days. Testing with a DPD strip before each use gives you confidence that the concentration has not dropped below a useful threshold. If you are using HOCl for disinfection, the commonly cited minimum effective concentration for surface sanitizing is around 50 ppm with adequate contact time, so you have some margin even as the solution degrades.

One subtle storage concern comes back to the byproduct issue: as HOCl breaks down, some of it converts to chlorate. A solution that was initially low in chlorate can accumulate meaningful levels during storage, especially if stored warm. If your intended application involves food contact surfaces or anything where regulatory chlorate limits apply, using fresh batches is the safest approach.

Common Mistakes That Undermine Results

The most frequent error is ignoring pH entirely. Many online tutorials describe the process as “add salt, add water, turn on power, done.” If you never check pH, you have no idea whether you made HOCl, sodium hypochlorite, or mostly chlorine gas that bubbled away. A cheap pH meter or even litmus paper makes the difference between a useful product and a mystery solution.

Using iodized table salt or sea salt introduces impurities. Iodine can interfere with some chlorine test methods, and the anti-caking agents in table salt (often sodium aluminosilicate or calcium silicate) leave residue. Non-iodized salt or food-grade sodium chloride is a better starting material. Kosher salt works well because it is typically free of additives.

Running the cell too long or at too high a current does not just waste electricity. It over-produces chlorine species that have nowhere to go except toward further oxidation into chlorate, or into off-gassing as chlorine vapor. Short, controlled runs with concentration testing afterward produce a cleaner result than letting the cell run until the solution “smells right.”

Finally, neglecting electrode maintenance leads to gradual performance loss. Scale buildup from hard water, corrosion products from cheap electrode materials, and biofilm from reused water all reduce efficiency. Periodic cleaning with a dilute acid rinse (a brief soak in white vinegar works for light scale) keeps the electrodes performing consistently. If you notice darkening, pitting, or flaking on an anode, it is time to replace it before dissolved metals contaminate your output.