Preparing perchlorates
From PyroGuide
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Industrially, perchlorates are exclusively prepared by the electrochemical method. In the past, thermal decomposition of chlorate has been used but since this process is very inefficient it has been abandoned long ago. Chemical oxidation of chlorates is currently not very economical either, but it may become an option in the future. For amateur pyros, each of these methods may be used as we need not be concerned with the economy of the process as much as commercial operations do. The electrochemical method is convenient to use if you have a chlorate cell with the right anodes already, since then making perchlorates is simply a matter of operating the cell for a bit longer. If you can get chlorates in quantity for cheap, for example as a herbicide, the thermal decomposition method is an option. The method is quick, and requires no chemicals other than the chlorate starting material. Perchlorates can also be prepared by chemical oxidation of chlorates. The required chemicals are relatively expensive, but the method is quick and simple.
Electrolytic preparation
Chlorates can be oxidised in an electrochemical cell to yield perchlorates. The preferred starting material for this method is sodium chlorate, since it is very soluble. Potassium chlorate is seldom used due to its low solubility, and ammonium chlorate should never be used since it leads to the formation of sensitive and explosive NCl3 in the cell. Sodium perchlorate is conveniently converted in high yield to a number of other perchlorates (such as potassium and ammonium perchlorate) by double decomposition (metathesis) reactions. It is assumed from here on that sodium chlorate is used as starting material.
Cell construction
Electrochemical cells for perchlorate synthesis do not differ much from chlorate cells. The most important difference lies in the anode material. Not all anode materials suitable for chlorate synthesis can also be used for perchlorate synthesis. Most cell body materials used in chlorate cells may also be used in perchlorate cells. The effect of temperature and pH deviating from the optimal values is of much less importance in perchlorate cells. Finally, the voltage at which perchlorate cells operate is somewhat higher because the potential at which the conversion reactions take place is higher. The general structure of both cell types is the same: two working electrodes, and no diaphragm.
Electrode materials
Like in chlorate cells, stainless steel is a suitable cathode material. Mild steel may also be used. While copper and brass will also work, they may cause problems with copper contamination when they erode.
Anode materials for perchlorate cells should have a high oxygen over potential. What exactly that means is not further discussed here; it suffices to say that if the oxygen over potential at a certain anode material is not high enough oxygen will be evolved instead of chlorate oxidised to perchlorate. No perchlorate will be formed, and the anode material is usually attacked comparatively quickly. This holds also for chlorate cells, but the problem is less severe there since lower potentials are involved. Anode materials suitable for perchlorate synthesis are listed below. These are also described in the chlorate synthesis section but are repeated here for convenience.
Platinum: The obvious disadvantage of platinum is its high price. However, it corrodes only at a very slow rate and therefore provides an almost ideal anode material. High efficiency can be reached with platinum and processing of the electrolyte is greatly simplified due to the absence of insoluble anode erosion products.
Lead dioxide: Lead dioxide provides an economical alternative to platinum. Efficiency of lead dioxide anode based cells is usually slightly lower than that of platinum based cells, but the difference is small. Lead dioxide anodes are not easily bought and must be made. This takes some work and effort, but the anodes are cheap, fairly resistant to corrosion even at higher temperatures.
Preparing the electrolyte
Sodium perchlorate can be made directly from sodium chloride by electrolysis in which case no special electrolyte for the chlorate to perchlorate step has to be prepared. The preparation of a chloride electrolyte is described in the text on chlorates.
A cell can also be operated purely for the chlorate to perchlorate conversion. An electrolyte has to be prepared before each batch in this case. If the cell has been operated before, it is best to 'recycle' the old electrolyte and all the impure fractions obtained during extraction and purification of the product. That way no product is wasted. When the cell is operated for the first time, a fresh electrolyte has to be prepared which can be done as follows.
Preparing fresh electrolyte
1. Prepare a saturated solution of sodium chlorate. Take about 60 grams of sodium chlorate for every 100 ml of solution and bring the solution to a boil. Then allow to cool to room temperature again. Sodium chlorate will crystallise as the solution cools. The solution is then filtered to obtain a clear saturated solution.
2. Optionally, 2 to 4 g/l of potassium dichromate, potassium chromate, sodium chromate or sodium dichromate may be added to improve efficiency. These compounds are suspected carcinogens, so if you choose to add any, know the hazards involved and act accordingly. If lead dioxide anodes are used, do not add potassium dichromate as it will only reduce efficiency. Instead, 2 to 4 g/l of sodium or potassium fluoride may be used. Although not carcinogenic, the fluorides are nasty compounds as well and should be handled properly.
Recycling old electrolyte
1. If the electrolyte is not clear but contains suspended particles, remove these by filtration.
2. Dissolve any impure material left over from purification steps.
3. Re-saturate the solution with sodium chlorate, following the same procedure as described in step 1 of 'preparing a fresh electrolyte'.
4. Like described in step 2 of 'preparing a fresh electrolyte' you may now add dichromates, chromates or fluorides if you choose to do so.
Operating the cell
Voltage and current
Perchlorate cells are operated at a higher voltage than chlorate cells and temperature and pH do not need to be controlled within strict limits for optimal efficiency. Other than that, operation is much like that of chlorate cells.
Like explained in the theory section on chlorate cells, the voltage over a cell may fluctuate if the current is kept constant. In typical chlorate cells this results in a cell voltage of 3 to 4 volts, whereas in perchlorate cells the voltage is higher, 5 to 7 volts usually. The current is kept constant at an acceptable level with respect to anode erosion. A maximum current per volume as exists in chlorate cells does not exist in perchlorate cells. The current could in theory be increased indefinitely to increase the reaction rate, were it not that anode erosion increases with increasing current density (the current per unit of anode surface area). The current is therefore usually set by the surface area of the anode. As a rule of thumb maintain a current density of 200 mA/cm2.
The influence of cell temperature is two-fold: anode erosion increases with increasing cell temperature and the cell voltage is reduced at higher temperatures. The former is obviously unwanted, while a lower cell voltage means energy is saved. In industry the temperature is of course chosen to get the best of both worlds, depending on what is more expensive: energy or anodes. We need not be concerned with economy as industry does so the temperature does not matter a lot. Try to keep it between 40 and 80 deg C. When using lead dioxide anodes, it is probably best to prevent the temperature from going very high. It can make the lead dioxide crack.
Finally, some water should be added from time to time to make up for what has evaporated. Try to maintain a constant electrolyte volume.
Running times
The required time to operate a cell depends on the current. The higher the current, the less time needed. In fact, the current is a measure of the reaction rate. Therefore, the amount of electricity that went through a cell is calculated by multiplying the current going through the cell (in amperes) by the time it has been flowing (in hours). The resulting number is measured in amperage-hours (abbreviated as Ah). To convert 100 grams of sodium to sodium perchlorate 50 Ah are required if the cell operates at 100% efficiency. In real life a cell will never reach 100% efficiency, and more electricity is needed.
Example: A 200 ml cell contains initially about 100 grams of sodium chlorate (the solubility of NaClO3 in water is about 50 g/100ml at room temperature). Per 100 grams, 50 Ah are needed. So, if a cell operates at a current of 2 amperes, it would take 50/2 = 25 hours to convert all chlorate to perchlorate in a cell operating at 100% efficiency. If the cell actually operates at 80% efficiency, 100/80 * 25 = 31.25 hours (or 31 hours, 15 minutes) are needed.
If perchlorate is prepared by continuing to run a chlorate cell after all chloride has been consumed, the total run time is of course the sum of the time required for the chloride to chlorate conversion and the chlorate to perchlorate conversion.
Processing the electrolyte
When done, raw potassium perchlorate or ammonium perchlorate may be prepared from the electrolyte as follows:
Filtering
The first step is filtration. The electrolyte usually contains suspended solid particles. These consist of erosion products of the anodes, the cell walls, and the unsubmerged parts of the cathodes. These particles may be very small and are not always easily removed with common filtering paper. Filtering through a layer of diatomeous earth (sold in shops for aquarium supplies) in a filter or on a piece of cloth sometimes solves the problem. However, even that will not always remove all solid impurities. A common impurity that is hard to remove is suspended iron hydroxide, originating from corrosion of (stainless) steel cathodes. The fluffy, voluminous form of the material often gives it a white or yellowish foggy appearance. This is next to impossible to remove unless some sodium hydroxide or pool coagulant is added first. This causes the iron hydroxide particles to coagulate, making them easy to remove by filtration. Another possibility is to add hydrochloric acid to lower the pH to between 2 and 3. This will dissolve the iron hydroxide. If sodium hydroxide is then added to raise the pH to above 7 again, the iron hydroxide is precipitated in a more dense form which is easily removed by filtration, even with common filter paper.
Chromate and dichromate removal
If chromates or dichromates were used to increase the cells efficiency they should now be removed. Adding a solution of barium chloride to the electrolyte will precipitate any chromate or dichromate as the corresponding barium compounds. Add small amounts of a 10% barium chloride solution to the electrolyte. A yellow precipitate will form. If no more yellow precipitate is formed, filter to remove the barium compounds. A white (sometimes clearly crystalline) precipitate may form instead of a yellow precipitate. This is barium chlorate or perchlorate. If this happens, do not add any more barium chloride solution and filter to remove the precipitate.
Destruction of chlorate
The next step is the destruction of residual chlorate. Even when a cell is operated for many times the required running time, some chlorate is still present in the electrolyte. Since chlorates and perchlorates behave very differently in pyrotechnic compositions chlorate contamination can be very dangerous. If the cell is not operated long enough to convert most of the chlorate to perchlorate the chlorate contamination may be very severe and it is unpractical and very inefficient to attempt to destroy it all. If this is the case I suggest this step is skipped, and the raw product is extracted. Ammonium perchlorate cannot be made this way, as it would result in the formation of the dangerously unstable explosive compound ammonium chlorate. Conversion to potassium perchlorate may however be tried. This will of course afford a heavily chlorate contaminated batch of perchlorate, probably even something that can better be considered to be a chlorate/perchlorate mixture or perchlorate contaminated chlorate. This material could be used to an extremely limited extend in pyrotechnic compositions when it is treated as a chlorate, or, a better option, it can be used as a starting material for the thermal decomposition or chemical oxidation preparations of potassium perchlorate described later.
Residual chlorate is usually destroyed by the action of reducing agents. Sodium sulfite is used for this purpose in industrial setups. Iron(II)sulfate is another option, and the chemical can be bought in some gardening supply shops as it is used to supply plants with iron. A third method destroys chlorates by the action of strong acids. The cell electrolyte is acidified with hydrochloric acid to a pH of 1 to 2, and the solution is boiled. Chlorates will decompose and yield a yellow gas, chlorine dioxide. The gas will partially dissolve in the solution, imparting a bright yellow color to it. In high concentrations chlorine dioxide is dangerously explosive and sensitive but if done using proper ventilation the small amounts evolved in this reaction are very unlikely to cause dangerous levels. The gas is however quite toxic and inhalation should be avoided at all times. Never do this step inside unless a well functioning fume hood can be used. After boiling for 15 minutes raise the pH to around 8 or 9 again using sodium hydroxide. This should afford a colourless solution. A qualitative test for chlorate should now be performed to make sure all chlorate has properly been destroyed.
If any other method is used to destroy residual chlorate the pH must always be adjusted afterwards to slightly above 7. Otherwise, traces of acid may be incorporated into the product in later steps which can make it very dangerous to use in pyrotechnic compositions.
Double decomposition
A decision will have to be made at this point whether the intended product is ammonium or potassium perchlorate. Other perchlorates can be made as well but are not discussed here as they find very little use in pyrotechnics.
If residual chlorate was not destroyed the choice is simple since ammonium perchlorate is not an option. Ammonium chlorate could be formed in the process which is, as mentioned several times already, a dangerously unstable explosive compound the formation of which should at all times be avoided (even in trace amounts). If chlorate was properly destroyed, and a qualitative test indicates so, ammonium perchlorate may be prepared.
1. If potassium perchlorate is the intended product, take 70 grams of potassium chloride for every 100 grams of sodium chlorate that was started with and dissolve this in the smallest volume of water possible. If instead ammonium perchlorate is required, take 50 grams of ammonium chloride and dissolve in the smallest volume of water possible.
2. Mix this solution of either potassium or ammonium chloride with the electrolyte. A white precipitate of the corresponding perchlorate should form.
3. Boil the solution and add small amounts of water until all the perchlorate has dissolved. Due to the low solubility of potassium perchlorate a large volume of water may be needed then. If a sufficiently large container is not available the solution may be split up in several portions that are later recombined.
4. When all has dissolved, check the pH of the solution. It should be neutral or slightly alkaline (above 7). If it is not, add some dilute sodium hydroxide solution to increase the pH to between 7 and 8. When this value is overshot, hydrochloric acid may be used to lower the pH again.
5. Allow the solution to cool slowly to room temperature. The perchlorate will crystallise during this. Cool the solution further to 0 deg C, and filter. Rinse the crystals in the filter with some ice-cold water. This raw product may be further purified as described below.
Purifying the product
The product can be purified by recrystallisation. This method is especially suitable for potassium perchlorate due to its low solubility. Little product will be lost, and the purity is greatly increased. Ammonium perchlorate suffers slightly worse losses when recrystallised, but still acceptable. The impure ammonium perchlorate should be discarded since recycling could result in NCl3 formation in the cells. The losses occurring when potassium perchlorate is recrystallised are so slight recycling is hardly worth the effort (a litre of recrystallisation solution contains only a few grams of perchlorate). Recrystallising is done as follows:
1. Place the crude product in a pan and add 100 ml of water for every 20 g of raw potassium perchlorate, or 100 ml of water for every 50 gram of raw ammonium perchlorate. Bring this to a boil.
2. After it has boiled for a few minutes, add 10 ml amounts of water to the boiling solution in 5 minute intervals until all the product has dissolved.
3. Check the pH of the boiling solution. It should be neutral or slightly alkaline. If it is acidic, add potassium hydroxide solution until it is slightly alkaline (pH=7...8) again. If ammonium perchlorate is the intended product, use ammonia instead. If this is not done, traces of acid may be included in the final product making it very dangerous to use in pyrotechnic compositions.
4. Allow the solution to cool to room temperature. The purified product will crystalise.
5. Filter and rinse the crystals well with ice cold water. The filtrate should be discarded
6. The crystals may be dried in an oven at 100 deg C.