VERSION III of "Lead Dioxide for Dummies!"

Fellows, this is the LAST update of this document at this blog, but the document itself will see continued editing and work as the research progresses, so feel free to download it in a few weeks (months? perhaps) when it is complete. Here, the document ends rather abruptly at the pre-plating stage. It will eventually include an actual plating procedure, but that will be a new blog entry, full of photos and hopefully quite interesting. I will maintain the document indefinitely in its latest form (it is formatted for Word, an older version, '03 I believe) at the following URL...

Lead Dioxide Plating for Dummies

Updated: As of 24 Mar 2009, the document is complete and I don't anticipate any further work on it. Both the document AND this blog discusses the addition of Bismuth salts to the plating bath... Don't do this! Bismuth salts wrecked my first plating attempt. Bi has potential for other anodes, but not as an addition to a Lead Dioxide plating bath.

Changes from the previous version are too numerous to mention. It is dry, dry stuff... read it if you have a lousy wet martini on hand. If the weather breaks, I hope to do some plating very soon.

Preface: a compilation of research and an attempt to merge several patents, all relating to the plating of lead dioxide, with the ultimate goal being an anode capable of perchlorate production. A good PbO₂ anode has been a goal for many years for a lot of garage chemists.

Deposition of Lead Dioxide (PbO2) on various substrates, with refinements of Nickel and Bismuth.

With thanks to Dann2, Tentacles, Xenoid, and Rosco Bodine for inputs.

After MUCH reading and study of a large number of patents, the primary means of information for the hobbyist, I have come to a few conclusions which I hope to gather and possibly clarify here. Many of these may be misguided, or flat-out wrong, but I believe the bulk is correct, and will hopefully result in an excellent quality PbO2 plating over a variety of substrates. Possibly hundreds of home lab chemists have attempted lead dioxide anodes, and I believe the number of successful anodes may be counted on two hands.

This patent list is definitely not all-inclusive, but it does contain those that seem both reasonable and consistent with regards to both the process and the goal; a solid PbO2 anode that performs in a perchlorate cell. For chlorate (Na or KClO3) production, I highly recommend the use of MMO (Mixed Metal Oxide) anodes… these are available, inexpensive, and very effective.

The Patents

The patents themselves, in no particular order:

The Process

Some of the more important parameters that will determine the quality (especially the adherence of the plate to the substrate) include the following:

In much of the following text, I am going to refer to Bi (Bismuth), and Bismuth salts. These are applicable only to patent 8,170. Bismuth as an additive is essentially unknown in the home laboratory group effort. The patent itself emphasizes the Bismuth addition as one that greatly improves current efficiency for colder solutions, such as for wastewater management, but there are several obvious references to strengthened and improved grain structure and adherence as well, and it is these factors that appeal to the home laboratory enthusiast.

The gains from Bismuth in the bath may not offset the negatives, but Bismuth addition may be worth exploring. If desired, it may be totally ignored.

All of these patents are explicit in their use of rather complex, circulating baths. As we shall see later, the reason for the complexity is two-fold… first, the patent holder is attempting to emphasize the speed and efficiency of his process, Secondly, and far more importantly for us, a circulatory or two-bath process is critical to keep parameters under control, particularly the evolution of nitric acid. I will go into some detail on this later.

I think an important goal of this research is to create what I will call a “one pot” process… a single, monolithic bath that, while agitated or stirred, is not circulated with an outside resource. This will greatly increase the challenge of the chemistry involved, but will be worth the effort, as few home laboratories are set up for, or willing to try, a pumped, multi-vat process.

I will approach these process parameters in order.

1) Preparation of substrate: 3,707 details critical anode preparation more so than the others. The substrate must be clean, and in the case of bare valve metals, the metal itself must be mechanically scored (sandblasting is optimum) for best adhesion of the PbO2 coating. Delays in plating after cleaning a metal like Ti will allow oxides to form, and it is desirable to minimize oxidation prior to plating; therefore, cleaning should be followed quickly by plating. Cleaning may be accomplished with a solvent such as acetone, alcohol, or perhaps a dilute acid dip or etch. An acid etch may have the further benefit with a bare metal of creating a surface much more amenable to taking on a tenacious PbO2 coating.

One possibility not explored is the vacuuming of the anode, immersed in pure water and perhaps a bit of surfactant, prior to plating. This will hopefully eliminate microscopic, trapped air pockets, especially on substrates that are exceptionally rough to begin with.

Plating flat plates, especially thin plates, produces a curling of the Ti (or other substrate) due to stresses in the coating. This is undesirable, and may be corrected by using Ti tubing or rod, rotating the anode (hopefully producing an even coating), or partitioning the plate into grid-like sections by first anodizing the plate, then removing the anodizing from those areas that you want to plate PbO2 onto. The resultant anode would have “islands” of PbO2 plating, and hopefully have reduced stresses. The rate of PbO2 deposition probably has an influence on internal stresses in the PbO2 coating as well, with a reduced rate creating fewer stresses.

Movement of the anode from the preparatory bath into the plating bath should be done at a temperature designed to avoid thermal shocking of the substrate. The more I read 3,707, the more I like its approach. Gibson recommends a preparatory soak at the same or higher temperature than the plating bath. This by definition would rule out acetone as a final prep, although it would make an excellent initial degreaser. The best final prep is probably DI water with perhaps some added surfactant, at 90 degrees C, vacuumed or otherwise. The movement from the prep bath to the plating bath should be quick, and the current applied as rapidly as possible once the anode is in place.

2) Current Densities: In general, higher current densities tend to promote rougher, less capable platings, but are helpful in the early stages to create maximum adhesion. If Bismuth is used, Bismuth is preferentially plated at lower current densities; thus, the ratio of Bi/Pb in the final anode will be higher at lower current densities. A final Bi/Pb ratio of greater than about 0.10 tends to promote cracking, and it is desirable to keep the Bi/Pb ratio below 0.10. Some very simple rules of thumb with regards to Bi…

Patent 8,170 utilizes two plating baths, the first being called preplate, and the second, simply the primary plating bath. The only real difference between the two is the addition of Bismuth; the other difference is an extra addition of HNO3. This presents some possibilities, and some problems. It may, or may not be, practical to maintain two plating solutions, stored separately. If a single bath is desired, the preplate bath may be prepared, preplating completed, and then the bath may be modified with an addition of Bismuth in HNO3. For reference, the two listed solutions:

The obvious implication is that the Bismuth may be added into the bath dissolved in the required additional nitric acid. The problem that results is that your preplate bath has become a plate bath, and will remain that way, unless the Bi can be eliminated, probably by a partial neutralization of the bath, having the insoluble Bi salts precipitate, followed by vacuum filtration to recover the insoluble Bismuth salts.

The other option is to greatly reduce the Bi salts quantity, and be satisfied with a single bath having a significantly reduced capacity for plating Bismuth. Or, ignore Bismuth entirely! It is not necessary for a PbO2 anode.

3) Current profile vs. Time for the plating process: Most of the older patents use a value of amps per square foot. A much better and easier unit for us is amps per square cm. To convert, simply divide amps per square foot by 929 to yield the correct current per square cm.

From Patent Data - The bulk of current density profiles start high, and terminate low. One patent uses the following profile:

Resultant anodes are described as “smooth, hard, ceramic-like.” Aren’t they all? Patent descriptions rarely report their failures. I wish they did… it would make our job much easier.

Patent 3,707 references current densities as follows:

The other patents are very much in line in terms of general current densities. All of the later patents have an initially high density to ensure adhesion, which is then tapered off, and all have cathodic densities 1.5X to 3.0X that of the anode itself.

Yet another patent uses the following profile for anode plating:

Again, these are close to the other patents with regards to current densities.

An older patent (6,378) used a constant current density of 0.054 A/cm2, which verifies the often-quoted value of 50 mA / cm2 for the bulk of the plating time.

Spacing of the anode and cathode in most of these baths tends to be quoted as between 1 and 3 inches, or 2.5 to 7.5 cm.

An average current density profile might look something like this. Note that our anodes are smaller than those used in industry, and the times required for a satisfactory plating may be shorter. I will express the times, then, as a percentage of the total plating time, rather than a fixed time in minutes or hours. Total time will ultimately vary between setups, and of course, the amount of lead dioxide one wants to deposit.

One final thought – as home laboratory chemists, we may have the luxury of hovering over our plating project like mother hens over their chicks. If so, an even better methodology may be a nearly continuous tapering of current for the entire plating period. Rather than three distinct steps, it can be 6, 12, or more… simply (and slowly) dial the current downwards over the plating time period. Each time you check the bath, reduce the current slightly, perhaps 5%. It would still be best to have an initially high current for best adhesion of the lead dioxide to the bare substrate, but there is no hard and fast rule regarding the plating current as it nears the end. To plate in this manner, (multiple current reductions) you obviously need a lab power supply capable of CC (Constant Current) operation.

4) Acid (HNO3) Concentration and control: Most home laboratories have concentrated (not fuming) Nitric Acid, which typically measures 68% to 70% HNO3 by weight. Thus, to create a 5 g/l solution, add (5/0.70) = 7.14 grams of concentrated HNO3.

From Patent 8,170, some specifics that relate to Bismuth. If you are not experimenting with Bismuth, you may ignore the following bullets:

As Acid concentration increases…

One patent in particular (6,378), which refers to the two step plating process (alkaline, then acidic) recommends, for the acidic portion of the process, a remarkably low “less than 1.0 g/l” concentration for the HNO3. For the other patents, HNO3 concentrations were all over the place. But in every case, I could find no significant drawbacks stated with reduced acid concentrations; on the contrary, nearly every resource mentioned serious drawbacks to starting with, or allowing, the HNO3 concentration to get too high, above approx. 30 g/l. Thus, I would recommend a starting concentration of 5-10 grams/liter.

Control of Nitric Acid buildup: Patent 3,707 explicitly states Nitric Acid concentrations higher than 20 grams per liter make it physically impossible to plate lead dioxide continuously, and recommends keeping it at < 5 g/l concentration. Considering that the process creates its own HNO3, you may as well start small, below 5 g/l, just enough nitric to acidify the solution. The next step becomes control of the acid as it builds. A single mole of lead dioxide plated on an anode produces THREE moles of HNO3. To illustrate the rapid and destructive buildup of HNO3 in a plating bath, let’s make some assumptions… Your plating bath is 1 liter, contains 331.2 grams of lead nitrate (Equivalent to a 1M Pb concentration), and you start with five (5) grams of nitric acid. Your anode receives a plating of (for demo purposes) 59.8 grams of PbO2, or 0.25 moles of Pb. The Pb concentration in the bath has dropped from 1M to 0.75M. With the production of 0.25 moles of PbO2, you have produced three times that amount of nitric acid, or 0.75 moles. With a weight of 63 grams per mole, the 0.75 moles of nitric acid masses 47.25 grams. The total HNO3 concentration has increased from 5 g/l to 52.25 g/l, which is well above the maximum recommended HNO3 concentration. The plating rapidly begins to devolve into a coarse mass that will NOT adhere well, and will allow corrosive electrolytes relatively free access to the substrate. We know what happens next. The substrate fails, the lead dioxide flakes, and yet another hopeful anode is trash. Much gnashing of teeth and wailing follows.

It is common to think that the post-plating bath, rich in nitric acid, but poor in Pb ion, can be restored to its pristine state after the plating is complete. This is quite true… any number of chemical processes can be executed to bring a used bath back to its original form, but the damage is already done, and the anode is already primed for death in the perchlorate cell due to a poor plating job. I think the demo above has shown that the nitric acid levels will climb during plating, and do so rapidly enough so that the plate job will be ruined. The HNO3 concentration MUST be controlled as it is evolved. How this can be done is a subject of much debate.

Freshly prepared baths tend to start with Pb(NO3)2 in the range of 200 to 375 g/l, which, dissolved completely, creates a Pb ion concentration of 0.6M to 1.13M.

Essentially, we must give something for the HNO3 to “chew” on that both neutralizes the acid, and replenishes the Pb ion concentration to keep it in a healthy range. Classic solutions include litharge (PbO) exposed in the bath, or some other lead salt. A likely candidate is either lead carbonate, PbCO3, or Basic Lead Carbonate, 2PbCO3•Pb(OH)2. Apparently, treatment of a lead nitrate solution with sodium bicarbonate produces 2PbCO3•Pb(OH)2, while the use of sodium carbonate, Na2CO3, precipitates PbCO3. Which of these is a superior nitric acid neutralizer and lead ion replenisher, I have no idea.

Either of these can be produced by adding an excess of sodium carbonate or bicarbonate to a solution of lead nitrate. When the brilliant white precipitate ceases to form, allow the heavy lead precipitate to settle, decant the bulk of the liquor, and collect (preferably by vacuum filtration) the white lead carbonate powder, washing thoroughly with water. Dry the (Basic) lead carbonate, and save in a good container for future use.

To make use of the Lead Carbonate (or similar lead salts, may of which become soluble only in an acidic solution), it must be exposed to the plating bath liquor; further, there must be adequate agitation or stirring so as to expose the liquor to the neutralizing lead salts. One possibility is to “hang” some sort of fine polymer mesh bag containing the lead salts in a location where exchange may take place, the acid neutralized, and the lead ion concentration replenished. Another is to simply add the lead salts freely at the start of the plating process. Being both insoluble and heavy, they will settle to the bottom if the crystal size is large enough. Unfortunately, basic lead carbonate, as precipitated from a lead nitrate solution, is fairly fine, probably less than 300 mesh. Drying of the lead carbonate mass on a flat, non-stick sheet (such as aluminum foil) and spreading it out thinly, results in a salt much easier to handle. The lead carbonate dries into thin plates which are easily broken up, and can probably be added directly to a bath with little problem.

Almost all of the patents describe their plating baths as having strong agitation or stirring. If an excess of PbO or PbCO3 is present, as the nitric evolves, it should, in theory, by acidifying the bath more strongly, allow these lead salts to dissolve, and serve a double service… nitric acid concentration remains reasonable, and Pb ions are replenished. My guess for a good starting weight might be that which would replace the lead on a molecular basis.

Note: Once again, I must emphasize, NONE of the patents use a “one pot” plating process. They all use circulation. Used electrolyte is pumped from the bath, altered with litharge or some other lead salt, and reintroduced to the active plating bath.

In summary, control of the Nitric Acid may be a critical feature of plating a strong, durable anode, and is one which, perhaps, has not received the necessary attention from home lab technicians.

5) Additives: Copper nitrate is standard, but may not be necessary if Cu cathodes are used. Bismuth has been discussed. Lead sheet, and/or PbO, can be used to control pH and evolution of HNO3, and as already mentioned, excess HNO3 is not desirable.

That leaves two additives, NaF, and Ni(NO3)2.6H2O. Patent 3,463,707 recommends 0.5 g/l NaF, and most other resources mention this value, or one very close to it. Nickel Nitrate is supposed to be an excellent grain refiner for the plated PbO2, and like the surfactant, may be one of the keys in producing a good plating. Patent 2,945,791 recommends 10 g/l Ni(NO3)2.6H2O in the plating bath. Subsequent patents, especially those that were granted to the owner of this previous patent, rarely make use of Nickel Nitrate, not because of its inefficiency, or lack of efficacy, but because of its cost! Since we are working on a small scale, we can make good use of Nickel Nitrate to refine the PbO2 grain. A finer-grained PbO2 plating will yield greater strength, better resistance to attack by harsh electrolytes, and possibly a superior adherence of the PbO2 to the substrate.

6) Surfactant use: The desired surfactant for PbO2 plating is a non-ionic polyoxyethylene; these come in various chain lengths, with the longer-chained varieties finding favor. Most of the older patents refer to the Igepal family of surfactants, which are difficult to find, or purchase in small quantities. Surfactants like these see frequent use in biological laboratories, where they are used to work with (and separate) proteins. Significant effort revealed a much more common surfactant with similar qualities, Triton X-100. From Wikipedia, the description of this surfactant:

Triton X-100: (C14H22O(C2H4O)n) is a.nonionic surfactant which has a hydrophilicpolyethylene oxide group (on average it has 9.5 ethylene oxide units) and a hydrocarbon lipophilic or hydrophobic group. The hydrocarbon group is a 4-(1,1,3,3-tetramethylbutyl)-phenyl group. It is related to the Pluronic range of detergents marketed by BASF. The pluronics are triblock copolymers of ethylene oxide and propylene oxide. The part formed from ethylene oxide is more hydrophilic than the part from propylene oxide. It is very viscous at room temperature and is thus easiest to use after being gently warmed.

There are a large number of surfactants of this type, graded upon the chain length of the polymer portion of the molecule. Another very probable surfactant is Tween 20, which is nothing more than Polysorbate 20.

Polysorbate 20 (commercially also known as Tween 20) is a polysorbate surfactant whose stability and relative non-toxicity allows it to be used as a detergent and emulsifier in a number of domestic, scientific, and pharmacological applications. It is a polyoxyethylene derivative of sorbitan monolaurate, and is distinguished from the other members in the Tween range by the length of the polyoxyethylene chain and the fatty acid ester moiety. The commercial product contains a range of chemical species.

Tween 20 is a non-toxic and easily-available surfactant that should probably do a fine job. Triton X-100 is a bit tougher to locate, and has some toxicity issues, but relative to the overall toxicity of the bath, it can be considered negligible. Patent 5,791 discusses the deterioration of surfactants under the influence of high HNO3 concentrations, thus confirming again the desirability of keeping HNO3 concentration under control. Deterioration of the surfactant eventually renders the plating bath inoperative, a serious detriment to our process, but remaining surfactant and fragments can be removed with n-amyl alcohol, 1-pentanol, CAS 71-41-0. The amyl alcohol is mixed with the bath on the order of a 1:4 ratio, and the surfactant components are selectively drawn to the 1-pentanol, which is then distilled to recover the material. Complex and not something to be taken lightly, but if the choice is this process, or otherwise scrapping a large amount of Lead Nitrate, this may be a good technique.

One early patent recommended the use of gelatin, as pure as can be found. No amounts were recommended, and the patent stated that impure gelatins caused problems. With the ease of availability of polysorbate-20 and Triton X-100, I’d skip the gelatin and go right for the real stuff.

One final suggestion is the use of Kodak Photo-Flo, which apparently has characteristics much like Tween-20 or Triton X-100, albeit in a much more dilute form.

7) Thickness of PbO2 plate: When starting with a mesh anode, consensus thought is that enough lead dioxide should be plated until the gaps in the mesh are completely full. By this point, your anode will have a waffle-like appearance, but will be solid and hopefully quite strong. Patent 8,170 recommends a thickness of 0.02” to 0.2”. Given that the patent owner is producing anodes for industry, this may be on the high side for our small anodes. I am going to guess 0.100” to 0.200” as a desirable coating thickness for our anodes, with the low-side increased to perhaps compensate for coverage that may not be as effective as that achieved in a larger plating tank. Other enthusiasts from Science Madness state that the anode should be much thicker. If it turns out that the (per)chlorate electrolyte breaches the coating and destroys the substrate, then a thicker coating may definitely be more desirable.

8) Temperature: In all patent cases, the temperature was held between 60 to 90 degrees C. There are two ways to achieve this. Choices include a glass (borosilicate) vessel on a hot plate, or an immersion heater in either a glass or plastic vessel. If your choice is a plastic vessel, the two typical choices are polyethylene, usually the HD variety, or polypropylene. Of the two, the latter is MUCH preferred, as polypropylene withstands near boiling reagents better than an HDPE vessel, but I suspect HDPE would work… simply keep the heat closer to 60 C than 80 C. Another potential plastic that shows promise for nearly all phases of this process (both anode plating, and (per)chlorate production, is PET… Polyethylene Terepthalate. PET is Coke bottle plastic – crystal clear and relatively inert. One important issue may be its softening at the higher temperatures required.

I am not sure of the physical effects of temperature on the plating characteristics. Does a higher temp create a better bond of the PbO2 to the substrate, or a weaker bond? Questions like this, for now, remain unanswered. This would be an excellent area for future experimentation. For now, I am going to target 70 degrees C for my plating baths.

If your vessel is borosilicate glass, a good hot plate can be put to use. If not, some form of immersion heat will be necessary, and a means to control it. The Cadillac of temperature controllers is a ¼ DIN or similar temperature controller made for industrial process use. Most of these use PID (Proportional, Integral, Derivative) to control the temperature, meaning they are both intelligent and accurate.

Immersion heaters: These come in a nearly infinite variety, and most are quite expensive. Be prepared for sticker shock for a true lab immersion heater! Remember also that a lead nitrate plating bath uses (and creates) nitric acid, so whatever heater you select must fulfill three requirements:

Stock fish heaters may do a fine job… simply realize that they must be disassembled, and the control electronics bypassed to allow the heater to achieve the necessary temperatures. While it is possible to modify one, it will require significant tinkering. I do think I've come up with a better answer... the industrial cartridge heater.

Cartridge heaters are heating elements encased in either stainless steel, or incoloy, a space-aged nickel alloy capable of shrugging off extremes of temperature. Inconel and its family of alloys finds extensive use in jet engines at the turbine core, where temps are inconceivable. Cartridge heaters are normally used to heat molds and platens; they are inserted into bored holes in aluminum and steel molds, and they come in a wide variety of wattages and voltages.

Cartridge heaters come in two flavors, low watt density, and high watt density. The low watt density types are usually sheathed in 304SS, not good for this process due to the inevitable sheding of Fe+ ions. The high watt density types have the incoloy sheath, and come in a huge variety of sizes, voltages, and wattages. Best of all, they are remarkably inexpensive, maybe $30 for a 500 watt heater, 120V, if ordered from a company like MSC or Grainger. If purchased off eBay, new cartridge heaters can often be found for $5 to $10. Look for wattages of approximately 100 to 500, a relatively wide (3/4”) diameter, and a short length.

Incoloy is a metal acknowledged as compatible with nitric acid processes. The ends of the sheath away from the lead wires appeared to be well-sealed, either via resistance welding, or possibly spin (friction) welding. Regardless, it is best to encase the cartridge heater in a copper or titanium tube, totally sealed against ingress from harsh electrolytes. If soldering copper tube to encase a cartridge heater, electrical solder and rosin flux may be used to good effect.

With a cartridge heater correctly ensconced in a Cu tube or similar, and attached to a functional heater controller, you are ready to go.

Initial tests indicate a wattage of about 75 to 100 per liter to be adequate.

9) The Cathode: Interestingly, the current densities at the cathode are recommended to be 2X to 3X the density at the anode. In simple terms, the surface area of the cathode needs to be approximately 1/3 the area of the anode. A rule of thumb: a typical expanded mesh electrode will have an area between 1.8 and 2.3 X the measured profile. Using 2X will probably come very close. One unanswered question remains… when measuring the area of the cathode, does one measure the entire area of the immersed portion, or just that area which is in the electron flux? My thought for now is to measure only the area exposed to the flow of electrons.

If your anode is stationary, the cathode is best constructed as a cage. Despite the warnings against free iron ions floating throughout your bath, almost all of the patents recommend stainless steel as a viable cathode material. If you do choose stainless, I recommend 316SS as the least likely to corrode. Frankly, I don’t like the idea of stainless steel as a cathode, and as you’ll see later, great and tedious lengths are pursued to eliminate Fe++ from the bath.

Other recommended cathode materials are hard graphite, Copper, Titanium, and the other valve metals. Cu and Ti are probably the best.

10) How to kill a plating Bath: Despite the usual glowing terms describing their perfect anodes, enough analysis reveals some failures. I will present them based upon the apparent emphasis placed upon these parameters by the patent authors.

Nitric Acid Concentrations too high: Allow your bath to produce nitric acid, unchecked. The result will be an anode that will fail in use. High nitric acid concentrations (above 20 g/l) simply do not produce a quality lead dioxide plating.

Excess Fe in the plating bath: Patent 3,707 delves deeper into “iron pollution” than any of the others. In particular, it discusses ways to measure Fe concentration (sulfosalycilic acid), and describes techniques to keep it below 2.0 g/l. From what I’ve read, a better value is 0.00 g/l, but the iron finds its way into the plating bath via a number of sources, and apparently source #1 is the PbO used to control nitric acid concentrations. Continuous additions of litharge end up polluting the bath with iron. I can easily see this happening, as most home laboratory chemists do not buy ACS 99.999% litharge; they buy the PbO sold at pottery stores which probably has significant iron content.

The iron tends to bind or occupy the sodium fluoride added to almost all baths as Iron (III) Fluoride, Fe2F6. This compound is soluble in an acid environment. What effect it has on plating, or whether extra additions of NaF can counter any ill effect, I do not know. But the patent agrees, loss of available NaF inhibits its beneficial effect on the plating process. How to control iron? Starting with chemicals as pure as your wallet can handle is probably a good way to start. I personally have 3 kg of PbO of totally unknown iron content. If Lead Carbonate ends up being a superior scavenger of nitric acid, and we can produce it with lead nitrate of known purity, it may be another answer.

One final thought might be an addition to the bath of a reagent that binds with the free iron ions and renders them into an insoluble solid, but the acid buildup often alters the dynamics of the situation, as it dissolves iron salts deemed “insoluble” and puts them right back in the bath, where they exercise their demonic control. The iron oxides are all insoluble in cold water, but again, with acids present? I don’t know, and I’ll leave suggestions on iron control to more experienced chemists.

Starting, stopping, and restarting the plating process: Simple answer, don’t do it. Once the juice is applied, don’t let up until the anode is at a thickness you are happy with.

Poor preparation of the substrate: Rinsing a mesh or some other substrate in water will do little to prepare it for good lead dioxide adhesion. It MUST, at a minimum, be degreased. A strong detergent will do the trick, as will a solvent like acetone or MEK. Tri-Sodium Phosphate (TSP) is a powerful detergent that will eliminate every trace of grease or oil from the substrate, and if the substrate can handle it, I’d recommend boiling it in TSP for 10 to 15 minutes, followed by multiple rinsings in distilled or deionized water. From there, continue with any additional surface preparation. An analogy: In the firearms trade, a good gun blue requires a seemingly inordinate amount of preparatory work before it is blued. Without it, the blue simply will not stick, or the finish will be poor.

The Plating process begins

Thus, our plating bath so far looks something like this, remembering that any number of Bi salts may be substituted for Bismuth Nitrate.

Other satisfactory Bismuth salts are Bismuth Hydroxide Bi(OH)3 and Bismuth subnitrate 4BiNO3(OH)2 BiO(OH). In fact, just about any Bismuth salt will work, per the patent. Metallic Bismuth is another matter, and is best left for further experimentation.

Determine the surface area of your anode, and select a cathode that has an effective surface area of one half to one third of the anode. If the anode is some sort of mesh, typically titanium or MMO over Ti mesh, the surface area usually falls within the range of 1.8 to 2.3 times the dimensions of the cut mesh. If your bath has a static anode, cathodic coverage of the anode can be critical, and subject to experimentation. At a minimum, two cathodes are required, one on each side of the flattened sheet or mesh. Most of the patents describe some sort of a stainless steel “cage” surrounding the anode. Calculating the effective surface area of a cage can be daunting. If I were to use a basket or cage, I would be tempted to form one from CP (Commercially Pure) titanium, rather than stainless steel. Cu wire of course may be used. If you use a copper cathode, you may ignore additions of copper nitrate to the bath. Other metals, however, require the copper nitrate… this prevents Pb from being plated on the cathode.

Once the surface area of the anode has been determined, calculate a current profile. Referring to section three:

Note that there is no actual time; rather, the current profile is expressed as a function of percentage. If your goal is a thick lead dioxide coat, you may consider a plating session that might last several hours at a minimum. Without experimentation and experience at this process, it is hard to determine just how long a particular plating process might last, but at a minimum, keep the plating current at the 0.126 A/cm2 value until you see 100% coverage of the bare anode material. From there, it is a matter of decreasing the current over time to create a plating that adheres firmly, then transitions to a finer-grained coating on the exterior. Patent 4,130,467 recommends the addition of litharge at the rate of 5.8 g/l per hour to control nitric acid. This has been discussed to some extent, with basic lead carbonate possibly being a superior choice to litharge (PbO) when your source of litharge may be suspect, and contain significant quantities of iron.

Further, patent 4,130,467 suggests an initial nitric acid concentration of 6ml/l when using standard 68% concentrated acid. It may be possible, at this relatively low acid dosage, to monitor the pH of the bath by any conventional means, and the addition of litharge or a form of lead carbonate (or other lead salt; NOT metallic Pb) may be used to control evolved nitric, and keep the pH close to the starting value.

Any electrochemical operation generates gasses at the electrodes, usually hydrogen and oxygen. These start as micorbubbles on the anode surface, and if ignored, will grow to visible size. Obviously, any portion of the anode covered with a bubble is not being plated, and any and all means should be done to shake the bubbles free from the anode. Surfactants may help by simply preventing the bubbles from sticking in the first place. Roscoe Bodine (from SMDB) suggests the following:

This makes sense… heating of the bath drives off the volatile organics and leaves behind the desirable salts for future use. A three-step process might consist of no surfactants; then, the use of ethanol or acetic acid (I would be inclined to try ethanol first), and finally, the use of a powerful (and heavy) surfactant like Triton X-100 or Polysorbate 20. These may technically be superior, but they do eventually break down under the influence of the nitric acid (or possibly the electrochemistry itself), and the only method I have read of to remove them is the use of n-amyl alcohol (1-propanol) in a classic “sep funnel” style extraction.

Xenoid (also from SMDB) devised an ingenious device… a vibrating system whereby a small electric motor is fitted with an eccentric weight, and attached to the boom arm which holds the anode. Attaching a variable DC supply to the vibrator motor allows the user to adjust the magnitude of the vibration. I was so impressed with the idea that I immediately added my own vibrator motor, so I now have both rotation and vibration. The rotation (I am hoping) will do three things: It will help agitate and stir the bath, it will hopefully drive bubbles off the anode, and finally, it will create a more even coating than a stationary anode. Whether any of these actually happen is subject to debate and experimentation. I do know from my decorative electroplating days, that a rotating electrode produced a beautiful and very even deposition of metal on the object of interest.