Homegrown Oxidizers Part Two
WSM

The Anode

A key to modern chlorate production is the MMO anode. These were hard to come by many years ago, but now are so common that surplus material can be had at reasonable cost through careful searching.

Part of the success of these MMO anodes is due to their large surface area (on a microscopic scale). The typical process of manufacture involves multiple coating and baking cycles that leave the microscopic surface looking like the cracked mud of a dry pond. These “cracked mud” layers provide much more surface area than a smooth surface would; so much so that an expanded metal mesh coated with MMO has about the same surface area as a solid sheet. This is important when calculating the size of anode required for a given power supply and cell size.


A photomicrograph showing the “cracked mud” surface appearance of the acid-cleaned surplus MMO on titanium mesh (200x magnification).

MMO on CP titanium mesh anode material. Surplus stock is shown on the left and new stock on the right.


MMO anode material; Top on solid CP titanium wire, Middle on niobium with a copper core (the bulb on the end is epoxy resin, which didn’t hold up long in an active cell) and Bottom on a thin wall CP titanium tube.

MMO anode material; Top Two: on CP titanium ribbon, roughly 1/4” wide by 0.025” thick, and Bottom: a CP titanium mesh anode, spot welded to a filled CP titanium tube for the electrical lead.

The MMO coating is more electrically conductive than the titanium substrate under it, so there’s no need to remove the coating for making electrical connections.

The surplus MMO anode mesh material is usually from industrial sources and may often be covered with contaminants. One sample had iron oxides on it (a brown smut on the surface), which was removed by soaking in hydrochloric acid (pool acid) for 15 to 60 minutes and then rinsed with water to remove the residual acid and air dried, leaving the surface clean and undamaged. If this condition exists on any MMO material found, avoid rubbing or abrading the surface with anything. Let the acid do the work; the MMO coating is not harmed by it.

If there are scratches on the MMO surface, the exposed titanium is unharmed by exposure to the cell liquor (electrolytes). Titanium, being what is known as a valve metal, is prone to passivate or “self-heal” with an insulating oxide layer when scratched. Aluminum shows similar characteristics, but is unsuitable for exposure to the conditions in the cell.

The Cathode

Cathodes (the negative electrodes) can be made from many different materials, due to an effect called cathodic protection, as long as the current flows. The better materials to use are known as valve metals due to their physical and electric properties, but of these we’ve chosen CP titanium for its outstanding performance in our tests. The acronym CP stands for commercially pure and it works better than alloyed titanium for our purpose (and is unlikely to add unwanted metal ions to our electrolyte).


CP titanium sheet metal stock for cathodes.

Stainless steel is used by some electrochemists for a cathode, but we prefer to avoid contamination by nickel, chromium and a host of other alloying metals, especially when the electrodes are exposed to the electrolyte with the power off.

If electrodes are left for long periods in the electrolyte without power they break down, not unlike a battery discharging, where irreversible damage can occur. Even with titanium electrodes, if the system powers down, remove, rinse with water and store the electrodes in a safe, dry place for the best care and longevity.

Single or Double Cathodes, and Why?

A pair of single electrodes with filled tubular titanium leads, offset for proper spacing of the PVDF compression fittings in the lid of the cell. PVDF nuts were added because the cell lid is thin plastic.

Most beginners to electrochemistry use a single anode with a single cathode. The bulk of the work is done by the anode, and if coupled with a single cathode, the anode is used at half its full potential usefulness (one side instead of both). By surrounding the anode plate with cathodes, like a sandwich, the full potential of the anode can be utilized.


An experiment where cathode plates surround the anode, and with close spacing for efficiency. PVDF (Kynar) compression fittings sealed the titanium tubing and prevented any “salt creep”, so there’s no damage to the electrical connections. The tubular leads were filled with lead-free solder and ran cool enough that no heat damage affected the plastic fittings.

Are double cathodes always preferred? No, but more work can be accomplished in a smaller space this way and it’s a consideration.

Most of the electrodes shown so far are somewhat rectangular in shape, and later research showed that longer and narrower electrodes which reach deeper into the cell tend to enhance the effect we call “hydrogen lift” which aids the mixing of ionic species in the cell plus prevent thermal layering, and thereby making a more efficient and effective system.

Hydrogen lift is a convection-like current caused by the hydrogen bubbles rising from the cathodes as the cell runs. This fluid flow draws dissolved material from the lower parts of the cell and carries it upwards to the surface of the liquor, where it flows outward and cycles downward away from the electrodes. As it nears the bottom of the cell, it’s drawn toward the electrodes to continue the cycle. This physical circulation improves the efficiency of the cell and helps to promote (along with the pH) the “bulk reaction” spoken of in technical literature, where the chlorate ions are created throughout the cell rather than just at the anode by “brute force” of the electrical current.

When we surround the anode with cathodes, the electrical stresses are more uniform on the anode, and almost the entire surface is working to produce the desired products.

Sizing the Electrodes

When we know the surface area of our anode, we can calculate the current demand of it. To calculate how much current our anode will use, we base it on the average demand of 0.3 Amps per square centimeter of surface area. Since one square inch is equal to 6.45 square centimeters we can figure our required current with this formula:

A = In²(6.45)(0.3)

Where A = maximum Amperage demand
and In² = surface area in square inches

That is, if our anode measures 1.5 by 6 inches (9 square inches or 9 In²) and we’re using one cathode about the same size as the anode, plugging this figure into the formula above shows us the answer 17.415, or an expected current demand of 17.4 Amps, maximum.

If we sandwich the anode between two cathodes, we double the current drawn and the formula is:

A = In²(2)(6.45)(0.3) = 34.83 or nearly 35 Amps

If we follow the best electrical practices, we load our power supply no more than 80% of its capacity, so we’ll need a power supply capable of supplying 44 Amps or more (or 22 Amps if a single cathode is used).

Going the other way, if we have a power supply with a maximum output of 60 Amps, we want our anode to demand no more than 80% of that or 0.80 x 60 Amps = 48 Amps. If we want to determine the area (in square inches) of an anode that will demand 48 Amps we use the formula:

___48A___
In² = (6.45)(0.3) for a single cathode, or 24.8 In²

____48A____
In² = (6.45)(0.3)(2) for double cathodes, or 12.4 In²

We then decide what width (in inches) our anode will be and dividing the square inches by that number will give us our anode length (again, in inches).

Electrode Spacing

This topic has been the subject of much debate among amateurs for many years. Some believe the anode and cathode should be as close as possible without touching each other, and others believe they should be as far apart as possible within the limits of the cell container. Most amateurs seem to compromise at somewhere in between these extremes.

We’ve observed several of these techniques and see that they all work; the electrolyte is conductive and the current flows. The difference is in the efficiency. The further the electrodes are from each other, the higher the internal cell resistance seems to be.

There is always some resistance which is normally exhibited as heat, or “wasted energy”; but in this case, heat actually improves the performance of the desired reactions so it’s not all bad. Higher heat is really a good thing here, but not too hot.

Industry uses very close electrodes, but they control the temperature actively with water cooling, flowing through hollow electrodes. Almost all amateurs depend on passive cooling, where control of the cell temperature is by limiting the electrode size and the current supplied. If the temperature goes too high, the real concern is boiling the electrolyte and warping the electrodes till they touch each other, possibly shorting the power supply, which can destroy it. We never want to allow the electrodes to touch each other when power is applied. The effect on the power supply can be catastrophic.

Temperatures should also be within the limitations of the cell’s structural materials.
Our best performing tests using double cathodes show that a spacing of about 1/8” (or 3mm) seems very effective. The cell ran at 55°C (131°F) and performed quite well. Spacers made of compatible materials, with insulating properties, can be used between the electrodes to stabilize them and keep them from shorting out electrically.

Meters


Digital panel meters to display the voltage and current output from the power supply or as it enters the cell.

To monitor the power used by our system, we use meters. Either analog or digital meters can be used. The voltmeter is wired parallel to the power supply output leads (minding the polarity, i.e., positive to positive and negative to negative) and the ammeter is wired in series with the negative lead through a device called a shunt. The shunt passes all of the current to the cell, except a tiny limited portion which the ammeter measures to display the actual current flowing though the leads.

Temperature is another value worth measuring and displaying. If the thermal sensor is exposed to the cell liquor, it won’t last long, so it needs to be protected. Some sensors can be purchased that are Teflon coated and will hold up perfectly in the cell environment.

Building a PVC Cell Tank (RC)

The most common and least expensive material to fabricate our reaction chamber of is polyvinyl chloride, or PVC. It’s available in pipes and tubes, sheets and a host of other shapes and sizes. It also comes in different colors (including clear) for a price. The simplest tank is made from pipe and fittings, though in larger sizes the fittings can be very expensive.

We’ve found that closing the bottom of the tank with PVC sheet material is much more cost effective, and when properly done, a permanent fix to the problem. Typically we look for sheet stock that is about the same thickness as the pipe wall. The pieces are cut and prepared to have the closest fit (without gaps or as close as possible; minor gaps can be filled) and then glued or cemented together. Companies that make PVC glues make them generally for three industries: pipe and plumbing, industrial and chemical. Our best results have been with the industrial or chemical grade glue.


Thick walled plastic pipes glued to plastic plates for a reaction chamber. PVC and CPVC are about the same to bond and glue. The CPVC handles greater temperature extremes but costs many times more than PVC.

Study and practice have shown us that smaller pieces are much easier to glue together than larger pieces. Large or thick pieces need to have several coatings of glue in a short space of time to soften both pieces before bonding them together. We recommend a heavy bodied cement with medium set for the best results. Weigh or clamp the pieces being bonded till they fully set. The bond is usually permanent in 24 hours at 70°F (21°C) or above. We’ve used additional cement to fill gaps and reinforce joints by applying with a disposable plastic dropper and drying for another 24 hours between every application. Care and patience makes for a neat job and a good bond.


A technique we’ve used to seal the cell lid with pliable soft tubing to help alleviate “salt creep” at the lid’s edge.

For the lid of the cell we use more sheet PVC. It’s useful to make the circular lid slightly larger in diameter than the outside diameter of the tank so it’s easier to grip. To create a soft seal on the cell lid (as shown in the photo), we first cut a thin ring of the same tube the tank is made from. Then cut the ring so the ends can overlap. Determine the amount of the ring to remove so when the ends touch, the ring will fit inside the tank with just enough of a gap for the seal to squeeze in and fill. Cut to remove that measured portion of the ring, leaving the ring in the shape of the letter “C”.

Next we mold the prepared support ring by heating it gently with a heat gun till it softens and becomes pliable at 275°F (135°C). Form it into a ring with the ends touching and one edge as flat as possible (the one that will be glued to the lid). Be careful not to scorch the ring by keeping the heat gun moving in circular movements and avoiding getting the heat gun too close while doing this. This technique takes practice and patience (it may take between 5-10 minutes to do this procedure and it’s best not to rush), but good results are worth the effort.

The formed ring can either be air cooled, or rags wet with cold water may be used to cool the PVC material till rigid again. Then center the support ring on the lid and glue it in place. The gasket is made from silicone tubing by cutting enough to make a ring that closely fits the OD (outside diameter) of the support ring on the lid. The ends of the tube are joined together by taking a short length of the same tubing, splitting it lengthwise, rolling it and inserting it in the ends of the seal to form a hollow O-ring. This O-ring seal can be bound to the cell lid with silicone caulk, which is left alone to fully cure. The caulk remains pliable in use and can be removed later to replace the seal if necessary. It’s a good idea to slightly chamfer the top inside edge of the tank so the seal will go in easier when the lid is closed on the cell.

The Bucket Cell

For simplicity and availability it’s hard to beat common 5 gallon buckets. When the bucket wears out, replace it for a few dollars and keep going. It’s prudent to place the polyethylene bucket in a secondary container, in case the bucket fails. A throw away plastic pan like those used to mix concrete for cement patching or post setting makes an excellent low-cost secondary containment. It should be large enough to hold the entire contents of the bucket.

The bucket lid needs to be modified by adding a bucket cell adapter (or BCA) to hold the electrodes and other fittings. Usually the BCA is fabricated from PVC (polyvinyl chloride) plate or sheet. Various holes are drilled and tapped in the BCA for standard pipe fittings (NPT, National Pipe Taper) and either plugged or filled with the appropriate fittings. To attach the BCA to the bucket lid, compatible machine screws can be used. It’s useful to put a gasket between the lid and the BCA to prevent salt creep getting through.


A bucket cell custom fabricated for a fellow enthusiast using 3/8” thick clear PVC plate for the BCA (bucket cell adaptor) with several ports drilled and tapped with common pipe threads and plugged.

The BCA (shown above) has the PVC plate mounted below the lid with a Viton rubber gasket (which is compatible with the electrolyte) between the lid and the BCA, in hopes of sealing them. The bolts and washers are stainless steel, and they’re mounted to threaded holes halfway through the PVC plate (so the metal isn’t exposed to the cell liquor). The BCA is made from clear PVC only because that’s what we had on hand (the typical opaque grey or white PVC works just as well and costs less), and we’re hoping it helps In observing what’s going on inside the cell, though it’ll likely be clouded with condensation from the warm cell liquor.

To cut the bucket lid we simply used a pocket knife and followed the edge of the ridge molded in the plastic, till the knife cut through. The BCA is sized to be a tight fit inside the larger ring in the bucket lid. If your bucket lid is flat, a square BCA would work also.

Generally, one should put in as many tapped holes of the appropriate sizes as possible. There are always more things to add, measure or monitor as our experience grows.

Tubular Electrode Leads, a Boon to Amateurs



Most sellers of ready-to-use electrodes sell them with flat straps for the electrical connections. These are convenient if the cell top is open, but sealing them into the lid is a major chore, and often leaks appear when the cell is in use. The solution to this problem first appeared in the APC forum blogs by the individual known as “Swede”, in the blog site called, “You'll put your eye out...”. In there, Swede shows the techniques of how he built electrodes with tubular leads, with complete descriptions and a lot of photos.

Tubular leads have the distinct advantage of being able to be securely sealed in compression fittings, which will eliminate the problem of “salt creep”. We recommend using Kynar (PVDF) compression fittings, which are as compatible in our system as is Teflon, but at about one fifth of the expense. The fittings need modifying to function this way but simply drilling through them (without the seals and nut in place) with a drill close to the size of the titanium tube works. Then replace the seals and nut.

Tubular leads can be connected to the electrodes by several methods, but the most effective means we’ve used is by resistance welding (also called spot welding). The most afford-able equipment we’ve found to do this was purchased from Harbor Freight Tools Company when they had a sale (about $160). The leads can also be attached by riveting or screws, if the fasteners are also CP titanium (but they’re hard to find and may need to be fabricated).

Electrical connections to the electrodes are made by either tapping the top end of the titanium tube to accept machine screws (to hold the ring terminal terminated leads from the power supply) or by inserting brass or stainless steel “all-thread” (threaded rod) into tapped holes in the metal filler (solder in the titanium tube leads) and attaching the power by ring terminals on these leads (held in place between washers and a machine nut) as seen in the photos above.

Electrolyte Preparation

To prepare the electrolyte we need to dissolve the salt crystals and nuggets in water. Due to impurities and additives in tap water, it’s best to use either distilled water or water purified by reverse osmosis (RO) to make our electrolyte. Filtered rain water can also be used. It’s a fact that moving water dissolves the salt seven times faster than still water. Whether moving the water by stirring or by pump, the effect is the same. Heating also works, but take care not to dissolve too much salt in the water; about 350g/liter maximum is ideal at room temperature (a bit less is okay). The following solubility charts will give you the optimal amount of salt in solution for a given temperature.

Compound--------------------------- 0°C 100°C

KCl or potassium chloride---------- 238 567

NaCl or sodium chloride------------ 357 391

Solubilities of salts in grams per liter


We should use starting materials as pure as we can find. Even if our salt is impure, it’s better to use pure water to make our electrolyte to limit the variables in our final product. Impure salt can be dissolved, filtered and recrystallized to purify it.

We still use the potassium chloride (KCl) water softener salt for our salt source but there exists agricultural sources as well (muriate of potash), which definitely need purification before use in our electrochemical cells due to impurities naturally occurring or added to it. Again, any impure salt can be dissolved, filtered and recrystallized to purify it.

Bibliography, Where to Learn More…

- APC (Amateur Pyrotechnics and Chemistry Forum), http://www.amateurpyro.com/ especially the Chemistry section and Swede’s blogs

- Science madness, http://www.sciencemadness.org/ especially the Technochemistry section

- CHLORATES AND PERCHLORATES: THEIR PRODUCTION, by James Finckbone
American Pyrotechnist Fireworks News Volume 7, Number 6 June, 1974

- Industrial Electrochemistry by C.L.Mantell, 1931

- Electrochemistry, Theoretical Principles and Practical Applications, by G. Milazzo 1963

- The Encyclopedia of Electrochemistry by C.A. Hampel 1964

- Industrial Electrochemistry by D. Pletcher 1982

- Electrochemistry in Industry by Brett 1994

- Electrode Reactions in the Chlorate Process (Doctoral Thesis) by A. Cornell 2003

- Influence of the Electrolyte on the Electrode Reactions in the Chlorate Process (Doctoral Thesis) by L. Nylen 2008

A Few Sources of Supply:

- U.S. Plastics Corp. - http://www.usplastic.com/
1390 Neubrecht Rd.
Lima, OH 45801 USA
(800) 809-4217

- Ozone Solutions, Inc. - http://www.ozonesolutions.com/products/Ozone-Compatible-Fittings
451 Black Forrest Rd.
Hull, IA 51239 USA
(712) 439-6880

- McMaster-Carr - http://www.mcmaster.com/
600 N County Line Rd.
Elmhurst, IL 60126
(630) 833-0300
or check for local listing