Homegrown Oxidizers Part Seven
WSM

The Holy Grail… Making Perchlorates

We’ve primarily made potassium chlorate during our efforts so far. What about pushing forward and making, the more often used and useful, potassium perchlorate?

Anodes and Other Differences from the Chlorate Process

To make chlorates we used specific anodes which optimize the production of the ClO₃^- (chlorate) ion. Will those anodes also make perchlorates? Many have theorized they will but in practice this has not proven to be the case. It appears a factor called “oxygen overpotential” isn’t high enough in the MMO anodes to push on to making perchlorates from chlorates.

There are several materials that do have the proper characteristics to effectively create perchlorates. Two of the more “readily available” perchlorate anode materials are either platinum or lead dioxide. There are several others but currently, these two appear to be a little easier to obtain. As time and technology move on, this may change, but for now let’s explore these two.

Each has positive and negative features. Platinum is very expensive and, unfortunately, is adversely affected by high chlorides and other factors in the cells, which causes it to erode. Fortunately, since only the surface is doing the work, platinum plated electrodes work as well as solid platinum, at a greatly reduced cost. The disadvantage is a shorter working life.

Industry deals with the erosion problem by recycling the platinum from the mother liquor and re-plating the electrodes, which is considered part of the cost of doing business, but impractical for most amateurs. If the cell conditions are carefully monitored and controlled, the amount of platinum lost can be minimized to what may be deemed “acceptable losses” and the quantity of homegrown perchlorates made thereby, still considered affordable.

Another option is to use lead dioxide anodes. They are considerably less expensive than platinum but more fragile, so careful handling is called for. The best type to use is the hard crystalline, beta-form PbO₂, which appears to hold up better in the conditions of the cell than other lead dioxide forms.

These may be homemade but have proven to be quite challenging without a proper laboratory setup and strict attention to detail. Getting the right form to solidly adhere to the substrate is one major hurdle, as well as determining the proper thickness of the coating, and so on. Our friend, Swede, was successful in making a workable lead dioxide (often referred to as LD by amateurs) anode and testing it successfully, but he went to great pains to do so.

We were fortunate in finding a few LD anodes to try, which were manufactured in China.


Two perchlorate anodes; the upper is lead dioxide on titanium and platinized titanium below.

The first thing we want to determine is how to configure the design of our cell using these anodes. Since we want our system compatible with the basic design of our chlorate cells, we’ve decided to modify the straps used for the electrical leads.


A tubular titanium tube, heated, flattened and closed on one end is prepared

The titanium strap is cut with aircraft shears, and the tubular lead is spot-welded onto the remaining end of the electrode strap.


The tube end is threaded and the tube carefully heated and filled with lead-free solder, cleaned up, and finished with a machine screw and two washers for attaching the electrical power.

Both types of anodes will be modified with filled, tubular titanium leads for conveniently mounting with PVDF compression fittings in the lid of our cell. The cathodes will be similarly prepared with filled tubular leads.

Besides the anode material, other differences in operating a perchlorate cell include:

• higher operating voltage (between 5 and 7 Volts DC are typical)
• shorter reaction times per kilogram of product
• higher electrolyte concentrations typically
  

Relative Solubilities

When we made potassium chlorate, we went directly from potassium chloride to chlorate so as to avoid sodium contamination, which overpowers many colors. When manufacturing oxidizer salts there are many factors to consider besides simply avoiding an inconvenient feature of just one of them. Let’s look at the details and see which method will work better for our circumstances.

Industry uses sodium salts in the process of making potassium perchlorate. The steps include salt (sodium chloride or NaCl), sodium chlorate (NaClO₃), sodium perchlorate (NaClO₄) and then potassium perchlorate (KClO₄). Why is this?

As the conversion from sodium chloride to chlorate to perchlorate occurs, each step has higher water solubility (as opposed to the potassium salts which are less soluble as their oxygen level raises). So sodium chlorate is more soluble than sodium chloride and sodium perchlorate is more soluble than sodium chlorate.

This makes potassium perchlorate manufacture and purification from sodium perchlorate very convenient, because at 25^oC, sodium perchlorate solubility is 209.6 g/100ml water and potassium perchlorate solubility is 2.062 g/100ml water (slightly less than 1% as soluble). Because of this, potassium perchlorate quickly drops out of solution, leaving the much more soluble sodium salts in solution. This also facilitates cleaning the residual sodium contamination from the potassium perchlorate crystals by a simple wash with cold deionized water, yielding an adequately pure final product.

Can we turn Potassium Chlorate into Perchlorate?

Many sources in pyrotechnic literature flatly state that direct conversion of potassium chlorate to perchlorate is not possible. The fact that industry chooses not to do so may be behind this common belief, but is this true?

Several years ago, in an effort to avoid sodium contamination, our friend Swede attempted and succeeded in converting potassium chlorate to potassium perchlorate with a platinum anode and titanium cathode, proving it’s possible, but the question remains, is it practical?

In his monograph on perchlorates (Perchlorates, Their Properties, Manufacture and Uses; Reinhold Publishing 1960) Schumacher explains about a strictly potassium perchlorate cell that “…in practice, this method is not used because of the limited solubility of potassium perchlorate compared to sodium perchlorate.”, and further describes problems of crystal fouling of the electrodes when attempting direct conversion of potassium chlorate to potassium perchlorate, “even with high electrolyte velocities”.

These conditions help explain why industry uses sodium salts to form sodium perchlorate electrolytically and then other perchlorates by double exchange, followed by fractional crystallization. For potassium perchlorate manufacture, the difference in solubilities allows us to easily separate the final product from the cell liquor and purify it to acceptable levels without many additional steps.

Desirable Cell Conditions

Associated with the electrolytic production of sodium perchlorate are a large collection of small voltage drops. Some major contributing factors are:

• anode-cathode spacing
• concentration and types of salts in the electrolyte
• anode current density
• cell temperature
• pH of the electrolyte (there is some debate on this)
• low contact resistance of connections to the electrodes
  

An efficient sodium perchlorate cell is a balancing act between all the various conditions found in the setup. Also the energy requirements of the cell are a direct correlation with the voltage drop across the cell and the current efficiency.

Industrial producers have reported power uses between 3 and 5 kilowatt/hours per kilogram of sodium perchlorate produced, depending on the cell design and operating conditions. These numbers are based on using sodium chlorate as a starting point.

Amateurs are usually tolerant of much worse efficiency because the final product is more valuable to them than the cost of the power.

More to Follow…

Most of this part is involved with theory and preparation. The next part will show a “proof of concept” operation of a small perchlorate cell and outline scaling up the process for the amateur electrochemist.

Bibliography Additions

“Perchlorates, Their Properties, Manufacture and Uses” by Joseph C. Schumacher, published by Reinhold Publishing Corporation, 1960.

“Perchloric Acid and Perchlorates” by Alfred A. Schilt, published by G. Frederick Smith Chemical Company, 1979