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In this articleI describe how to make a ceramic substrate lead dioxide anode following the ideas of some patents regarding inert substrate electrodes, but without the problematic, labour intensive procedure of making it conductive.

A brief history: to produce chlorates and/or perchlorates by electrolysis, one needs an inert anode, which has to resist the chlorine action. Many solutions are available. Platinum is the king of materials, but it is not cheap. Graphite is good for chlorates and one can use carbon rods instead, available as gouging rods. They are cheap, not always simple to find and unuseable for perchlorate production.

Others materials are usable, but I focused my attention on lead dioxide, as written in Wouter Visser's web page and after I exchanged some e-mails with Michael Mc Ardle.

Michael gave me a great amount of data, articles and references, and we started to make some GSLD (graphite substrate lead dioxide) anodes. You can read about it on Wouter's page and see that it's not a simple task. We quickly discovered that they were weak because it's difficult to obtain a perfect coating over a carbon rod, and the latter crumbled making the lead dioxide fall to the bottom of the electrolytic cell.

Michael then discovered a patent which used a ceramic substrate. To make ceramic conductive we had to soak it in lead nitrate, dry it, soak in a basic persulfate solution to turn the nitrate into lead dioxide. These three steps were to be repeated three or four times. I was unable to get persulfates at low cost, so I used common bleach to oxidize lead nitrate. I can assure you that it is an hard work and won't be doing it again!

In the end, when making my lead nitrate from lead tetroxide (I do not buy it because it's too expensive) I got some lead dioxide in powder form. So I thought of painting a piece of ceramic tile with a paste obtained from lead dioxide and water. After the painted tile has dried I found myself with a conductive ceramic ready to be plated.
Materials:

• white cheramic tile (terra cotta seems to work, too) or other kind of ceramic (see the following text)
• lead dioxide paste (PbO2)
• lead nitrate [Pb(NO3)2]
• sodium hydroxide [NaOH] but see the text for other solutions
• a power supply (1-5 V, 1 A)
• distilled or deionized water
• kitchen paper or toilet paper, plastic spoons
• filter paper, some cans, a funnel

Optional:

• copper nitrate
• copper carbonate or hydroxide

Painting the substrate

We start with a ceramic tile (Fig. 1) and after we have chosen the anode dimensions we cut out the substrate. Ceramic easily destroyes the saw blade, so you are advised :-). Use gloves and a dust musk when cutting the ceramic, it's very messy work (and dust is not a friend on your lungs).

With sandpaper (number 100) round off the piece until it has a smooth surface (Fig. 2). If you want you can wash it to eliminate all the dust from the pores. That allows the lead dioxide layer to be more adherent but remember to let the ceramic to dry completely before start the "pasting" job.

Now comes the hardest work. Take your lead dioxide paste and with a spoon start to "paint" the substrate. It's hard because the dry ceramic adsorbs the water and causes the paste to dry instantaneously and even if it seems well adhered to the ceramic, it is not. Help the process with a piece of wetted kitchen paper. The layer is thick enough when you cannot see the white of the ceramic and when, with a spoon, you cannot crumble away the lead dioxide. You may need some practice to obtain a good result.

Electrical contacts

What now you have is a piece of ceramic which is not very conductive. Absolutely. So if you plate it putting only one contact on one side, you probabily never get the opposite side overed with (electrodeposited) lead dioxide. Some times, not only will the side (with no contact) remains uncovered, but the paste disappears (it's simple to explain why this happens, but this is not the right moment).

I prepared three contact made from three piece of 4 mm OD gouging rods and 5 cm long. I stripped off the copper cover leaving only a centimeter of it on one side so I can solder a copper wire on it. The contacts were kept in place with some cloth pegs. See the following figures.

I flattened the carbon rods for a length of 3 cm to improve the contact with the substrate. A good idea is to paint the rods with something to protect the zones which do not have to make a contact: that avoids the plating of the carbon rods and the corrosion of the copper. You can use some polystyrene dissolved in trichloroethylene, I have "widely" exeprimented with it in my electrolytic cell and it seems a good material.

The plating process

The solution used to plate the substrate varies from patent to patent. Normally, 250 g/l of lead nitrate is enough and one can use it directily without adding copper compunds or tensioactive elements. A concentrate solution of lead nitrate is yellow with a pH around 4-6. If the pH is lower the solution turn to a clear color.

Rule of thumb: more the solution used, less the manutention. During the plating the pH fall down due to the formation of HNO3. To raise the pH I add some lead hydroxyde that I make directly from my lead nitrate. Follow the link to see the notes about it.

Since you you are using a inert substrate, it do not need to strictly control the pH. Technically one can let the pH to fall down without care but I noted that if there is too much acid, too much bubbles start to form on the anode and the cover of electrodeposited lead dioxide becomes weak. So a bit of control is always a good thing.

My lead hydroxyde is in a paste form, mixed with water. I add it little at a time and if it dissolves and disappears, it means that there is enough acid to justify the correction. Even if you measure pH around 2, lead hydroxyde may NOT react and precipitate in the bottom of the cell. Don't worry, nothing of bad happens, just stop adding it.

It's of extreme importance to avoid drastic correction of the cell: that leads to a "stratification" of the lead dioxide cover which is very weak.

I was talking about the quantity of solution. Well for a 4x15 cm substrate I use at least a liter, and, belive me, the pH needs often to be corrected (I run my cell at 1 A).

The substrate is the anode in the plating process and as a cathode one can use a piece of copper wire. In the Fig. 8 I'm using a gouging rod with his copper cover.

A good cover, but not massive, for me take 8 hours or more.

Electrolysis

Here are two pictures of my electrolytic cell: this configuration works for three weeks at 10 A and 12 V. The contact was made with a piece of silver, simply taped on the electrode.

Too bad english in this post? Help me to improve, leave a comment! Thank you.