Synthesis database: Potassium chlorate synthesis (via electrolysis)

This method of potassium chlorate synthesis is used in the industry for the mass production of the mentioned compound. The process is based on the electrolysis of an aqueous solution of sodium chloride. The product of this reaction is sodium chlorate. After that, via the process of ion exchange between potassium chloride and sodium chlorate, one can obtain potassium chlorate, and sodium chloride. This step is also used in the thermal decomposition method. As an electrolyte, one can also use solutions of other metallic chlorides (for example, barium or potassium chloride). However, using sodium chloride has its advantages - for example, the product (sodium chlorate) is much more soluble in water than barium or potassium chlorate. Because of that, after the reaction of electrolysis is finished, the product is completely dissolved, and that makes filtration of the electrolyte easier, because by filtration one can remove all impurities without removing some of the desired product as well. As I already mentioned, later one can use the obtained sodium chlorate to make potassium chlorate (or barium chlorate, which is also important in pyrotechnics) via a simple ion exchange reaction. Although this method is used in the mass production of the mentioned compounds, it can also be used to make smaller amounts of these products.

As for the theory behind the method, the process still isn't completely explained, but the theory that was set more than 80 years ago by Fritz Foerster and Erich Mueller, was accepted by the scientific community. According to that teory, on the anode the following reaction occurs:

2Cl^- -› Cl_{2 (aq)} + 2 electrons

while on the cathode, hydrogen from water is reduced:

2H₂O + 2 electrons -› H₂ + 2OH^-

The chlorine that is produced on the anode, is dissolved in water, and thereby hypochlorous acid is formed, as well as hydrogen and chloride ions:

Cl_{2 (aq)} + H₂O -› HClO + H⁺ + Cl^-

In the case when there is less chlorine dissolved than it should be, the content of hydrogen ions is reduced, and thereby, all the hydroxide ions that are formed on the cathode, can not be neutralized. This can result in the increase of the pH of the reaction solution (because of that, the pH of the solution must be checked every once in a while, and in the case of an increased pH, one must add a certain amount of hydrochloric acid to lower the pH). From the last mentioned equation, one can see that hypochlorous acid is also being produced, which in the reaction with water, gives hypochlorite ions (ClO^-) and hydrogen (H₃O⁺). The exact concentrations of the dissolved chlorine, hypochlorous acid and hypochlorite ions depend on the following conditions: pH of the solution, temperature, pressure, and other. As for the formation of the chlorate, the reactions can be shown with these two equations:

2HClO + ClO^- -› ClO₃^- + H⁺ + 2Cl^-

and

2HClO + ClO^- + 2OH^- -› ClO₃^- + 2Cl^- + H₂O

One can see that for the formation of the chlorate ions, one needs to keep the concentrations of the hypochlorous acid and the hypochlorite ions as high as possible. That is accomplished by keeping the electrolyte at a slightly acidic level (pH=6). In case of such acidity, the ratio between hypochlorous acid and hypochlorite ions is optimal (2:1, as one can see in the reaction equations above). As for the temperature, the optimal values are between 60 °C and 80 °C (when the cathodes and anodes are made from ideal materials), but since in this case graphite anodes were used (which are not ideal, but are good enough), the preferred temperate was around 40°C. Higher temperatures would accelerate the decomposition of the anodes, which is almost inevitable anyway.

The first, and perhaps the most important step, is the construction of the electrolytic cells. The images above and below give a clear overview of the cell structure.

The basis of the constructure were simple glass jars. On each of the lids of these jars, 14 holes of various sizes were made. Two of them were for gas exchange, two were intended for bolts that are connected with the cathodes, and the remaining 10 holes were intended for the graphite anodes. All the mentioned parts, that went through the lid, were insulated with rubber, so that they wouldn't be in direct contact with the lid (in order to prevent eventual short circuits). In addition, to the bottom side of the lids, an additional rubber layer was attached, with narrowed holes on the places where the parts went through the lid. That way, I made sure that all the gases generated by the electrolytic cells, exit the cells through the small glass tubes that were designed for that purpose. As for the cathodes, they were interconnected in each cell by using two bolts. These bolts also allowed me to regulate and increase the distance between the cathode plates and the graphite anodes as desired, in order to prevent contact between cathodes and anodes, that would cause a short circuit. For additional safety, I also included a plastic reinforcement on the bottom of the cells that also prevented the eventual contact between anodes and cathodes.

The selection of materials from which certain parts of the cells were made, is also very important. Thereby, maybe the most important parts are the anodes. They need to be resistant to chlorine which is generated on the surface of the anodes. The almost ideal material for the anodes is platinum, which corrodes at a very low rate (compared to the graphite, about which I will say more later). Thanks to the negligible corrosion of platinum, at the end of the reaction one gets a solution with very little impurities, and because of that, the further refinement of the electrolyte is greatly simplified. The only drawback of platinum is its very high price, which is the only reason why I didn't use this material. There are various alternatives to this material. The most well known are lead(IV) oxide and graphite. The first of the mentioned is also used in lead batteries, and in this case, it is useful because it is relatively ressistant to corrosion, even when the eleytrolysis is done at higher temperatures (which increases the yield of the electrolysis). Because of the unavailability of anodes made of this material, I used graphite which was the most simple solution at the time. Graphite is cheap and can easily be found. Unfortunately, it has a few drawbacks - it is not completely resistant to the conditions in the cells during electrolysis, so it corrodes at a relatively high rate. That creates an additional problem, it pollutes the electrolyte, and that can create even more complications later, so it is necessary to filter the electrolyte after the process is complete. In spite of the mentioned drawbacks, by keeping the conditions as close to ideal, the corrosion of the graphite anodes can be reduced to a minimum.

Because of that, graphite can still be used with relatively good results. In addition, as I already mentioned, another advantage of this material is its low price. In this experiment I used graphite electrodes that are normally used for welding, and can be found in shops that sell welding equipment. These electrodes were covered with a thin layer of copper, but it was fairly simple to peel this copper off. After that, the anodes were shortened a bit, and were placed in the holes that were made through the lids of the cells. As for the cathodes, one can use a wide range of materials, because to a certain extent, they are protected against the corrosion caused by the anodes, and that allows a much wider choice of materials. A good material is stainless steel which is cheap and can be found easily. This, among other things, was also the reason I used it in this experiment. All other parts of the electrolytic cells were home made, and with these parts, care was also taken about the materials that were used - the anode fixture was made of polypropylene (PP), the gas exit tubes were made of glass, and most of the bolts were also made from stainless steel. The rubber insulation material is also resistant to corrosion.

As for the electrolyte, in this case, I used a saturated aqueous solution of sodium chloride, which was prepared by dissolving 350 grams of NaCl in one liter of distilled water. The solution was heated slowly in order to speed up the dissolution. It is a good idea to add a bit of potassium dichromate to the electrolyte (about 6 grams per 1 liter of electrolyte), in order to reduce the corrosion of the cathodes. When the mentioned compound is added, during the process of electrolysis, around the cathodes hydrated chromium oxides are formed. These oxides protect the cathodes additionaly from chlorate and hypochlorite ions. However, because of the unavailability and the fact that this compund isn't really necessary, I did not add any of this compound. In addition, potassium dichromate is known to be carcinogenic, and that is another good and clear reason to avoid this compound.

As for the power source, I used a power supply from an old computer, which was modified in order to give an alternating current of the desired value of electric current. For the formation of one mole of chlorate ions, six moles of electrons are needed. The total charge of one mole of electrons, i.e. the Avogadro's number of electrons is equal to one farad (which is equal to 96485.3415 coulombs). Since 1 Ah = 3600 C, and by taking into account the fact that 6 moles of electrons are needed, I calculated that for the synthesis of one mole of sodium chlorate, 160.8 Ah are needed. Since the cells are in a series circuit, and by using the electric current of 3 A, I calculated that the time needed to convert one mole of NaCl to one mol of NaClO₃ equals 26.8 hours. In each electrolytic cell there was 500 mL of the saturated NaCl solution, so in total, I was dealing with about 350 grams of NaCl, i.e. about 5.9 moles. Finally, the conclusion is that the reaction should be in progress for 158.12 hours in order to convert all the NaCl into NaClO₃. As for the voltage, around 3 volts are needed for the oxidation of the chloride to the chlorate ion (and for the reduction of hydrogen on the cathode), but I used a bit higher voltage (9 V) because of the electrical resistance of the cells themselves.

During the electrolysis, care must also be taken about certain conditions - temperature, pH and the concentration of sodium chloride. As for the temperature, in this case there were no problems because the temperature values never passed the safety limit (around 40 °C).

Mostly the temperature varied between 35 °C and 36 °C. The pH and the concentration of NaCl needed a bit more attention. The pH increases gradually during the electrolysis, and its ideal value is close to 6. The concentration of NaCl decreases gradually, but the optimal concentration is about 300 g/L, because as the concentration of NaCl is lowered, the corrosion of the anodes increases, which should be avoided. These problems were solved by adding a certain amount of acidified saturated solution of NaCl every 24 hours.

The needed amount of NaCl can be easily calculated by taking into account the time that passed from the last addition of the solution, since it is known how much of the NaCl passes into NaClO₃ per hour. In addition, the solution was acidified with a certain amount of hydrochloric acid. That way, with each addition of NaCl, the pH value got closer to the optimal level. When adding the mentioned solution, increased generation od chlorine occured sometimes. This shows that the solution is acidic enough because in these conditions, chlorine generation is to be expected. This problem was solved by leading the gases that are produced in the reaction, to a separate beaker containing a sodium bicarbonate solution.

After 160 hours have passed since the start of the electrolysis, the process is finished. The electrolyte is filtered a few times with the help of a medicinal gauze, in order to filter out larger unwanted particles. After that, the electrolyte was further filtered through cotton wool placed in a bottleneck (of a larger two-liter bottle) that was cut off. Gradually, by repetitive filtration, a yellow colored clear solution was obtained. Since the filtration was progressing at a very slow rate, I took a smaller amount of the already filtered solution, and the rest of the solution was slowly filtered for a few more hours. In the filtered solution, along with sodium chlorate, there was also some sodium hypochlorite. Because of that, the solution was heated until the boiling point was reached, and was kept at that temperature for about 15 minutes. Thanks to this step, all the sodium hypochlorite converted to more sodium chlorate (which is also the basis of the hypochlorite method of chlorate synthesis). After heating for 15 minutes, I checked the pH of the solution, and added a bit of sodium hydroxide solution so that the pH would get close to 8. If one assumes that all the NaCl passed into NaClO₃, that would mean that from the starting 350 grams of sodium chloride, one could get around 627 grams of sodium chlorate, which is only possible in theory (the yield of this type of homemade cells is mostly around 50%). Although the yield of the process was surely much less than 100%, I calculated the amount of needed potassium chloride for the reaction of the ion exchange by taking into account the theoretical yield of 100%. That way, I was sure that all of the sodium chlorate passed into potassium chlorate. However, some of the potassium chloride remained unused (which is not a problem because thanks to its high solubility, it remains in the solution and doesn't cause problems when extracting potassium chlorate).

Since I used 60 mL of the filtered solution, theoretically it could have contained 37.2 grams of sodium chlorate. By taking into account this value, I calculated that the needed amount of potassium chloride, for the ion exchange reaction, was equal to 26.1 grams. After adding the potassium chloride, a small amount didn't dissolve completely, so I added a bit more water in order to dissolve it completely. After that, the solution was left to cool to the room temperature, and was then further cooled to 0 °C. The result was similar to those in other methods of KClO₃ production - the crystals of potassium chlorate formed on the bottom of the beaker with the cooled solution. After filtration and drying, the mass of potassium chlorate was 10.8 grams.