The Sodium Chlorate cell

One could talk for a very long time indeed about the design for a Sodium Chlorate cell.
The amateur usually finds it difficult to know where to start when it comes to setting up a cell. It can be very confusing when one reads all the information available on the topic. Most of the reading material is geared toward the industrial producer (or sometimes the scientist who is studying the subject) who will have a different set of constraints forced upon him when compared to the amateur. The amateur will not be too sure how much quantity of product to make, how much money, time, effort and space to allocate to the project. Very often the starting point will be the power supply that is to hand, or the Anodes that can be made or purchased easily . Sometimes it will be decided to make a certain amount of product per time and having decided this, go on to obtain Anodes and a power supply.
The amateur will not be terribly interested in making the cell as small as possible, getting maximum current or power efficiency or having the smallest possible voltage across the cell.
The amateur will (very often) be interested in Anode wear rate, whether or not the power supply to hand is suitable or how much product will be produced per week.

The amateur will be using (probably) a non pH controlled cell, this is OK. All industrial setup's use pH control.

The amateur will probably use what is to hand which may be a cheap power supply, cheap Gouging rods etc. The cell container can come in the form of a bucket with a lid. The plastic that the bucket is made from must be able to withstand the cell electrolytes and fumes. A PVC bucket is ideal. Some holes are drilled in the lid to accommodate the electrodes and a vent tube. The vent tube is useful as it allows gasses to be vented out of the building in a controlled manner. A hole about 5cm in diameter is also made in the lid for looking into the cell and also for adding water or NaCl solution as required. A piece of plastic is needed for to cover this hole and stop fumes and mist from getting out of the cell onto connections etc. A stainless steel or even mild steel bucket can be used. This can then be used as the Cathode thus doing two jobs at the same time. Remove solution from container when current is stopped or it will corrode.
The electrodes should be sealed into the lid because if fumes and mist are getting on connections you will have problems trying to maintain them. Also gases coming to the surface of the solution generate a mist of the solution which will drift out of the cell. This mist will rust everything in the vicinity of the cell and beyond!.

The design parameter that always raises it's head is the Anode current density. This parameter is important because you must keep below a certain Anode current density to avoid eroding the Anode. This is particularly true with Graphite. Anode current density is the Anode current (cell current) divided by the total surface area of the Anode which is in the electrolyte. It is usually quoted in Amps per square cm. When you have decided what the Anode surface area will be and what the current density on the Anode will be, you will know what the maximum current through your cell can be. You do not have to run this maximum, you can run less current into the cell if you so wish.

Another design parameter that gets discussed is the ratio of the size of the cell (the liquid volume) to the current going into the cell. This in not a critical parameter. It has little effect on current efficiency. When the current going into the cell is raised (if you add more Anodes say and adjust power supply) and the cell volume is kept constant in order to shorten the run time of the cell, there will come a point where the heat generated by the process will not be able to escape fast enough from the cell and the temperature of the cell will be too high. If you must run a large current into a small volume of electrolyte you can keep the cell cool by putting it into a large container of water. Industrial setup's uses cells that are small in relation to the current going into them. They use cooling coils in the cells to keep the cell cool and transport heat away. This keeps the real estate used by the cells small. The amateurs cells can be large in comparison to the current going into them. The only disadvantage in doing this is that the run times of the cells can be long. This can leave the impatient amateur feeling as if nothing is happening at all as he is forced to wait perhaps two weeks before he can take out a crop of Chlorate. The cells WILL still be making Chlorate at the same rate as a smaller cell with the same current going into it.

The amateur very often comes up with some weird and wonderful arrangement of electrodes for to stop Chlorine gas escaping from the cell. It should be noted in industry that most cells use a very simple arrangement of parallel plates in a vertical arrangement. With the amateur cell the pH is not controlled. It is impossible for Chlorine gas to escape out of a cell which has a high pH (amateur cell). There will be a smell of Chlorine at the start but once the pH rises no Chlorine of any significance will come out of the cell. Add NaOH or KOH at start if there is a problem regarding smell of Cl2.

The temperature of the cell (IMO) is of little importance in non pH controlled set up. All Chlorate is made by electricity (not by species meeting in the bulk of the electrolyte as you have in pH controlled setup's) with little or no bulk chemical reactions going on. Temperature should be kept at a level so that Anodes are not damaged/eroded. Very low temperatures may cause Na or K Chloride to come out of solution.

The power supply for to supply the cell with this current will ideally be a controlled current source. With a controlled current source you set the power supply to put a certain current into the cell and the voltage across the cell will vary as the cell resistance varies.
Most power supplies for sale or to hand are NOT controlled current sources. They are controlled voltage sources (either fixed or variable).
This means that when you connect the supply to the cell the voltage across the cell in rock steady at whatever the voltage of the supply is set at (assuming the supply is not being abused) and the current put into the cell is dictated by that (rock steady) voltage and the resistance of the cell. If the supply has a variable voltage output then the current can be varied by lowering or raising the voltage of the supply.
When the power supply is a fixed voltage output (you cannot vary the Voltage with a knob. An example is a computer power supply) then the only way you can vary the current is by manipulating the electrodes in the cell or you could add a resistor or a number of diodes in series with the line going into the cell to lower the current. With a computer supply (5 Volts output) the current going into the cell will probably be within acceptable limits. If you have a fixed 10 or 12 volt supply then the current will probably be excessive and you will need a resistor or diodes to limit current. Diodes work by dropping 0.9 Volts across themselves, they effectively lower the power supply voltage seen by the cell by 0.9 Volts. Another trick with a 12 volt supply, is to put two cells in series so that less current will flow into each cell.
12 Volt battery chargers can make an acceptable supply as they tend to have controlled current characteristics. They are a half way house between a controlled current source and a controlled voltage source.
You will need a current meter in the line going into your cell (or in the power supply itself) in order to measure current. Voltage across cell can also be noted with a meter if disired.

Do not get obsessed with the voltage across the cell. The voltage across the cell will have a value no doubt. Some folks are inclined to latch onto the voltage appearing across the cell as if it were some very important, almost magical, parameter. Some times you will hear a statement like: "I ran my cell at 3.6 volts, therefore I was making Chlorate. I increased the voltage to 7.4 volts and I am now making Perchlorate". It is NOT the Voltage across the cell that decides if you are making Chlorate or Perchlorate. If the Voltage is too low not enough current will get pumped into the cell. This will give you slow production. If you have a bad connection which causes a large Voltage drop this will lower current going into the cell too. Think in terms of Anode current density.

Cathodes

Titanium is King when it comes to Cathodes. Use the CP grades as the alloy stuff is inclined to warp. It is relatively expensive. Stainless steels are next best but be aware that if they corrode (as when you do not have the cell in operation perhaps) they will release Chromium into your cell which is not wanted when using Lead Dioxide anodes. The Anode should have a Cathode on each side (surrounded) so that the current is evenly distributed on the Anode surface. Keep Cathodes small as this will decrease reduction (unwanted) reactions that occur at the Cathode surface.

pH Controlled cells

If you intend to controll the pH of the cell in order to increase the current efficiency and to decrease Anode wear then you should think of the cell as having two distinct areas.
Different reactions take place in these areas and they should be treated differently if the maximum advantage is to be taken from controlling the pH. See 'Cell Chemistry' section for details. A two compartment cell is best. The compartment containing the bulk of the liquid should be much larger that the area containing the Anode. The compartment containing the Anode should be kept cool (below 40C or so) for Graphite, but this is not so important for other anode materials. The bulk area should be allowed to get hot (70C or more) so that the bulk reactions are encouraged. The diagram below depicts a possible two compartment cell with a crude crystalizer. It will take quite some time before Sodium Chlorate will begin to appear as crystals in the bottom of the third container. [pH controlled cell and crystallizer]

A one compartment system can be used also. It should be fairly large in relation to Anode size so that bulk reactions can take place in order to obtain maximum benefit from pH control.
See Wouters page for more information on cell design.

Some cell design examples

Example 1
The designer has decided that he wants to make 1Kg of Sodium Chlorate per week. he is not too strapped for cash.

How much Anode surface area needed:

He has no Anode(s) or no power supply yet. By looking at the run times section for Sodium Chlorate on this page the following fact is noted.
In a typical Amateur cell you will get 0.00596 grams Chlorate formed per minute per Amp.
Thatís the same as 60.0768 grams formed per week per amp (There are 10,080 minutes in a week) Therefore to make 1000 Grams per week you will need to run 1000/60.0768 = 16.64 Amps (say 17).
Looking at the max. allowable current densities on the different Anode materials will give figures similar(ish) to theses: Platinum 300mA/square cm, Graphite 30mA/square cm, Lead Dioxide 300mA/square cm, and Magnetite 30mA/square cm. He may like to run his Pt, LD, or Magnetite Anodes at a higher current density as the figures above are fairly conservative. He will not want to run his Graphite Anode at a higher current density so as to keep erosion at bay.
Having decided what Anode material he is going to use he then sets about deciding how much surface area he needs on the Anode. Assuming Graphite, he will need 17/.03 = 566 square cm Min. Thatís the Anode surface area that is immersed in the electrolyte remember. He then decides if he is going to use Gouging rods, EDM Graphite or some other Graphite. He may wish to use the Graphite at a much lower Anode current density to keep Anode erosion ever lower. If he wishes to halve current density, he will do so my simply doubling the Anode surface area. If using rod shaped electrodes be advised that as the rod wears it will have a smaller surface area, the current density will rise (assuming current into cell remains the same) and wear rate will increase. It may be advisable to start with a low current density so that as the Anode wears the current density will not go above 30mA per square cm. If using flat pieces of Graphite the Anode wear will not have much effect on the surface area of Anode.

How much NaCl:

In order to make 1Kg of Chlorate you need exactly (1000 X 0.55) grams of NaCl. The 0.55 comes from the division of the molecular weights. The electrolytic cell and the Anode will not operate under such clear cut conditions. You will need at least 100 grams per litre NaCl to be left in the cell after your run so that the conditions for Graphite Anode erosion are met. If you try to use up all the Chloride the Anode will get eaten away rapidly coming toward the end of the run.
So: We need to use at least 550 grams of salt for to be converted to Chlorate + 100 grams per litre to be left in the cell at the end of the run. He will need a container that will hold approx. 3 litres of solution. This is a minimum size. If you run 17 amps into this 3 litre container you will have problems keeping it cool. Better to just use a container that is much much bigger than this. About 25 litres would be great filled with saturated NaCl solution. This will stay cool. About 330 grams NaCl will dissolve in 1 litre of water. At 17 amps you will still get the same amount of product forming (assuming similar current efficiency which is probable). It may not suit the impatient amateur as he will not be able to draw product from this larger container until a few weeks have passed. He may add Chromate, Persulphate or NaF to increase current efficiency. He will add water to the cell to keep volume steady. He may add salt solution. If he adds salt solution he needs to factor this into the run time.

Power supply:

He will probably purchase his power supply. The exact output voltage of the power supply needed to drive 17 amps into the cell is unknown. It will be somewhere between 3 and 6 volts. He will probably purchase a DC power supply having the ability to supply at least 17 amps at a fixed voltage anywhere between 1 and 10 volts. The power supply will have a nice current meter to tell him what amps are going into his cell. It will also have (though not essential) a nice volt meter to tell him what voltage is across his cell. He will be careful that he does not become too obsessed with the voltage across the cell. The current going into the cell may vary a small amount as the cell runs. The temperature, ageing of Anodes and changing concentrations of salts will vary the resistance of the cell somewhat. He may have to adjust the voltage a bit to keep current at exactly 17 amps. He may choose not to worry about the fact that the current is not staying at exactly 17 amps.
If he is very dedicated to Chlorate making he will purchase a power supply that has the ability to put a programmed current (a power supply which is a fixed current source) into his cell. He will set the supply to put 17 amps into his cell. There will be a meter for measuring current and (for interest) a meter to measure the voltage across the cell. The current going into the cell will remain rock steady at 17 amps, the voltage across the cell may vary a bit as the resistance of the cell varies.

Cathodes:

He will choose cathodes that will be at least similar surface area to the Anode(s) and place them around the Anode(s) in a sensible fashion. It is wise to place cathodes each side of Graphite Anodes in order to keep the current distribution on the Anode surface fairly even. If he were to place the Cathodes at one end of the cell, the current density on the side of the Anodes closest to the Cathodes will likely have a greater value than the side that is not toward the Cathodes which will have consequences for erosion, especially with Graphite.

Example 2
The Amateur has a computer power supply and he is thinking of purchasing some Gouging rods from EBAY and he wants to make as much Chlorate as he possibly can. He is rather strapped for cash. He has a multimeter.

How much Anode surface area needed:

Firstly test supply with a resistor to see if it is working. We have a power supply that puts out a fixed voltage of 5 volts DC and a few other voltage outputs (12V and others). We use the 5 volt output which is the main output. The supply also has a maximum current output on this 5 volt output which is hopefully stated on the supply. The maximum current that this line can supply is the maximum current you can put into you cell without risking damage to the supply. Calculate the Anode surface area from this maximum available current. Say the maximum current is 10 amps this means that you need at least an Anode surface area in the electrolyte of 10/.03 = 334 square cm. (This comes from the fact that you want to keep the Anode current density below 30mA per square cm to keep Graphite erosion at bay). How many Gouging rods do you need to give you this surface area? The area of a rod is (ignoring the small circle at the bottom of the rod) Pi X Dia. X h (remember h is the length of rod actually in the electrolyte). Use at least this amount of rods. It would be great to use twice as many to keep Anode current density real low and therefore keep erosion low. This is especially true as the Gouging rods wear, their surface area will decrease, current density will increase and erosion will increase.

How much NaCl:

Use a container that is about 10 litres or larger in size filled with saturated NaCl solution. About 330 grams NaCl will dissolve in 1 litre of water. Use a 20 (or a 30!!) litre container if you like. Calculate the run time and keep in mind that you must stop running the cell when you have got to the stage where you have 100 grams per litre Chloride so as to keep erosion on the Graphite to a sensible limit. He may add Chromate, Persulphate or NaF to increase current efficiency>

Cathodes:

He will choose cathodes that will be at least similar surface area to the Anode(s) and place them around the Anode(s) in a sensible fashion. It is wise to place cathodes each side of Graphite Anodes in order to keep the current distribution on the Anode surface fairly even. If he were to place the Cathodes at one end of the cell, the current density on the side of the Anodes closest to the Cathodes will likely have a greater value than the side that is not toward the Cathodes which will have consequences for erosion, especially with Graphite.

Running the cell:

The supply is connected and the voltage and current is measured. The voltage across the supply is measure to see that it is not being abused, ie. too much current being drawn from it. The current will be measured to ascertain how much product you will be making per week. Measure the voltage across the cell. If the voltage is below 5 volts the cell is drawing too much current and the supply is in danger of burning out. Measure the current to see what it is. Be sure that the multimeter will take the current that may be flowing. It may be lower that the supply rated current as the supply may have heated up so much that the thermal shutdown in the supply has come into play. You can put the Anodes and Cathodes further apart or put diode(s) in series with the line going into the cell in order to raise the resistance seen by the supply (this will lower the current below the maximum that the supply can handle). Run the set-up so that the current is below the maximum (say 10% below) allowable for the supply. You will have 5 volts across the supply terminals at this point because the supply is a fixed voltage type of supply that was made to supply current at a fixed five volts. If you have used diode(s) to manipulate the current through the cell, the voltage across the cell will be less that the voltage across the supply by (0.9 X [number of diodes]) volts.
The amount of Chlorate you will be making will be:
60.0768 grams per week per amp, 9 amps will give you 540 grams per week. (assuming around 50% current efficiency. You will not be able to crop this amount in the first week. It depends how much electrolyte you have started off with. You need to run the cell for the amount of time as described elsewhere on this page. He will add water or salt solution to the cell to keep the volume constant. If he uses salt solution he will need to factor this into the run time. Be patient.


Example 3
The designer has a Lead Dioxide Anode and a huge 12 Volt battery charger. He is not to concerned with making the maximum Chlorate possible per week but he is very impatient and wants Chlorate NOW.

How much Anode surface area has the Anode:

The amount of the Anode that will actually be in the electrolyte is used to calculate the surface area of the Anode. A current density of 300mA per square cm is easily accommodated with a Lead Dioxide Anode. He may wish to run his Anode at a smaller current if he so wishes but since he is impatient for product he may decide to run the Anode at a higher current density. Assuming the area of the Anode is 70 square cm, this will allow a current of 70 X 0.300 = 21 Amps to be run though cell.

How much NaCl:

The cell volume will be kept small so that Chlorate can be extracted as soon as possible. Looking at run times described elsewhere on this page he sees that with 21 amps running into the cell will give him about 21 X 0.00596 = 0.12516 grams Chlorate formed per minute, which is 180 grams per day. He will set up his cell so that he can extract Chlorate after two days, ie. 360 grams theoretical. In order to make 360 grams Chlorate, 360 X 0.55 = 198 grams Chloride are needed. Since Lead Dioxide does not get eroded by low Chloride concentration there is no need to have a quantity of 'buffer' Chloride in the cell as far as Anode erosion is concerned. It will be impossible to convert all of the 198 grams of salt into 360 grams Chlorate as run times are (as described elsewhere) based on having a decent amount of Chloride in the cell at the end of each run. He will use 270 grams NaCl dissolved in somewhat less than one litre of water. Also if he uses too little Chloride he may get Perchlorate forming if the concentration of Chloride falls below (approx.) 10 grams per litre. He may add NaF or Persulphate but not Chromate to increase current efficiency.

Cathodes:

He will choose cathodes that will be at least similar surface area to the Anode and place them around the Anode in a sensible fashion. It is wise to place Cathodes each side of the Anode in order to keep the current distribution on the Anode surface fairly even. If he were to place the Cathodes at one end of the cell, the current density on the side of the Anode closest to the Cathodes will likely have a greater value than the side that is not toward the Cathodes which will probably have consequences for erosion. Lead Dioxide is fairly robust though.

Running the cell:

The cell is less than one litre is size and has 21 Amps going into it. This cell will heat up too much. The cell can be placed sitting in a large container of water so as to keep it cool. The power supply will be set up to run 21 amps through the cell. The open circuit voltage of the (big) battery charger will be approx. 13 volts. When connected to the cell it will probably send too much (more that 21 amps) into the cell. The current going into the cell can be decreased by adding a resistor or diodes in series with the input of the cell. The voltage across the cell will be in the region of 4 to 6.5 Volts (approx.) The voltage across the charger will be 12 volts nominal. The current going into the cell will not be a perfect smoothed DC but this is not a problem. It will be difficult to measure the exact effective current because of this. There will probably be a current meter on the charger which will give an accurate enough indication of the current. If a battery of reasonable quality is connected to the charger it will act a a huge capacitor and the set up will have a nice steady current (and Voltage) going into the cell. If no battery is available it is OK. The cell will be run for 2 days and the Chlorate extracted by evaporating some of the water away and letting it crystallize out. If he does not turn of the power after 3 days he may get Perchlorate starting to form.
After a crop or two of Chlorate he may decide to be more patient regarding the process, use a much larger container and allow Chlorate to accumulate before extraction. He will add water to the cell to keep volume steady. He will not add salt solution since he want Chlorate quickly.


Example 4
The designer has a bench power supply with a maximum output current of 3 amps. He is going to purchase some Platinum wire or a Platinum clad Anode. Cash is scarce.

How much Anode surface area should he purchase:

This is entirely up to himself. He will look at the price of the wire and weep. If he wants to run the set-up at the max. current that his supply can give (looking at typical current densities used for Pt. of 300mA per square cm) he will need 3 amps/0.300 amps = 10 square cm of Platinum in the electrolyte. He will purchase Pt wire with a sensible diameter which will carry 3 amps. If he purchases Pt. of (say) A.W.G.(B & S) = 42 (thatís S.W.G. between 45 & 46) this will have a diameter of 2.5 thousands of an inch. This measly wire will not carry 3 amps. It will melt. He will purchase wire of approx. 25 A.W.G (26-27 S.W.G) which will carry 3 amps. This wire is still thin, around 19 A.W.G. would be nice so that the Anode will be self supporting.
The surface area of a cylinder is D X Pie X L. Assuming we use 19 A.W.G. (D = 0.9119mm) we need a length of wire which is
10/(3.142 X 0.09119) = 35 cm. He will need another inch to come up out of the cell, say 38 cm total length. He will bend the wire into a shape for to use as the Anode. Some like to wind the wire onto a former (say a glass tube). Winding the wire onto a tube will have some consequences for current distribution on the Anode. The Anode will have a very small current density on the wire next to the tube, it will have a larger current density on the side of the wire away from the tube. Keep this in mind. It would be good to wind the wire in a spiral of about 2 inches Dia. that is self supporting.
The price of this Anode will not be small. The smaller the Dia. of the wire you use the less Dollars you need to spend to get a certain surface area. See the section on the Pt. Anode. You could buy half the length suggested and squeeze (run through a mandrill) or hammer the wire so that it's surface area is doubled (or more) thus saving Dollars.
If he uses a Platinum clad Anode all his current carrying problems will end as the substrate of the Anode will carry 3 amps easily. He still needs 10 cm squared of Anode area in the electrolyte.
Since the Anode is so expensive he may decide to purchase less surface area and run the Anode at a higher current density, still running 3 amps into cell. Pt. will take quite a high current density without eroding. He could also purchase light wire that will not carry 3 amps but will carry (say) 0.5 amps. He will cut this wire into 6 equal lengths and have 6 Anodes coming out of the cell connected to a bus bar (like a comb arrangement). Anode surface area in the electrolyte must still be 10 square cm to give similar erosion conditions to the single Anode. The 6 Anodes will save cash and give him more work.

How much NaCl:

Looking at the run time section elsewhere on this page it is noted that 3 amps will give you (0.00596 X 3) = 0.01788 grams Chlorate formed per minute = 180.25 grams per week (current efficiency about 50%). This will use 180.25 X 0.55 = 100 grams Chloride per week. The 0.55 comes from division of the molecular weights of NaCl and Na Chlorate. Assuming that he wants to harvest Chlorate once per week he will need 100 grams Chloride in solution + (about) 100 grams Chloride per litre left at the end of the run so that erosion of the Anode is kept at bay and to keep current efficiency at a sensible level. Platinum will erode somewhat if used in a cell that has a low Chloride concentration in it. About 330 grams NaCl will dissolve in 1 litre of water. He will dissolve approx. 145 grams NaCl in about 0.45 litres water. After a week approx. 40 grams of Chloride will be left per 100ml solution. He will stop his cell at that point so that erosion of the Anode is kept at bay. He will add NaF, Chromate or Persulphate to increase current efficiency. He may decide to top up the cell with water or salt solution. If he used salt solution he must factor this into the run time.

Cathodes:

He will choose cathodes that will be at least similar surface area to the Anode(s) and place them around the Anode(s) in a sensible fashion. It is wise to place cathodes on all sides of Anodes in order to keep the current distribution on the Anode surface fairly even. If he were to place the Cathodes at one end of the cell, the current density on the side of the Anodes closest to the Cathodes will likely have a greater value than the side that is not toward the Cathodes which will have consequences for erosion. If he has a spiral type Anode he will place a rod inside the coil and a few (3 perhaps) around the outside of the coil. If he has a flat Platinised Anode he will place a sheet each side of the Anode.

Running the cell:

He connects his bench power supply up to the cell. + to Anode, - to Cathode. He increases the voltage until He gets 3 amps to flow. He will keep an eye on the current so that it does not increase above 3 amps and abuse the power supply. The current will vary somewhat as the resistance of the cell varies due to temperature and solute concentration changes. The voltage will be in the region of 3 to 6 volts. He may wish to connect a high wattage resistor (or a bulb of the correct size) in series with the power supply and increase the voltage of his power supply so that the current flowing will not be so dependent on the whims of the cell. Doing this will not increase the voltage across the cell. If he uses a 1 ohm resistor (with at least 3 watts dissipation ability) and increases the voltage of his power supply by 3 volts the current will still be 3 amps. As the resistance of the cell varies the current will not vary quite so much, as the added resistor 'swamps' or helps to mask the effects of changing cell resistance. The supply (now consisting of a voltage supply + a resistor) has some characteristics of a controlled current source. If he is not too worried about the current varying a little bit (or he doesn'tít know/care what the heck I am talking about) then he can leave the resistor out.


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