Sciencemadness Discussion Board

Solubility of salts and their percipitation

vanBassum - 23-9-2020 at 08:30

Hello guys,

First of all, this question might be very simple but since I have no real background in chemistry I have absolutely no clue. I have tried to search the internet for the answer but I couldn't find anything remotely useful to me.

Let me sketch the situation:
I have a perchlorate cell consisting of a lead dioxide anode and two titanium cathodes. I use NaCl dissolved in water as my electrolyte. From my understanding the electrolysis will first convert the chloride to chlorate and eventually to perchlorate. Now this is something I have read much about, so I understand the basics of this. Every once in a while I add some more NaCl solution to the cell to compensate for evaporation etc. There will come a point where the solution is saturated and no more salts can be dissolved in the solution. When that happens salts will begin to precipitate. My question is, what salt will precipitate out of solution first? Is this the one with the lowest solubility or will this be a mixture of salts? I know in a recrystallization the salt with the highest change in solubility per temperature will precipitate the most, but in this case the temperature is more or less constant.

My plan right now is to boil the electrolyte dry and use acetone to isolate the perchlorate. Everything that doesn't dissolve can go back into the cell and the perchlorate can be further purified. This however requires me to boil down the electrolyte solution, it would be quicker to just keep on adding NaCl solution to the cell until crystals form that I can filter out.

Can someone help me with this?

B(a)P - 23-9-2020 at 12:36

The electrolysis of chloride to perchlorate is best done in two stages, chloride to chlorate harvest recrystallise then chlorate to perchlorate. No doubt you have seen this in your reading.
The reasons being that running a chlorate cell with low chloride concentrations can damage the anode and perchlorate cells operate at a different voltage.
When operating the chlorate cell you keep adding chloride to keep it in solution at or just under its solubility limit, but not so much that you have excess solid. Eventually chlorate will start to precipitate out. Keep going until either you have enough chlorate or your electrodes no longer run efficiently because they are entirely caked in chlorate. Then repeat for perchlorate.

Maurice VD 37 - 23-9-2020 at 18:53

It depends on the nature of the chlorate or perchlorate. If you are handling sodium chlorate and perchlorate, both are extremely soluble. And the evaporation of a mixture sodium chloride + chlorate + perchlorate will deposit sodium chloride first. However, if you are handling potassium salts, it is the contrary. Potassium perchlorate is nearly insoluble in water. 100 mL can dissolve a maximum amount of 0.8 g in 100 mL water. Potassium chlorate is extremely soluble at 100°C (57 g in 100 mL water) and barely at room temperature (7 g in 100 mL). On the contrary potassium chloride is very soluble at all temperatures (34 g at 0°C and 56 g at 100°C in 100 mL)

macckone - 23-9-2020 at 18:59

This is an interesting question.
Chlorate is more soluble than chloride.
As you consume chloride, the chlorate concentration will increase above its solubility.
You have to keep adding chloride to the cell. Some people use excess chloride in a suspended container so they can see progress.
The product will deposit on the bottom below the anode once you reach the correct concentration.
Assuming the cell is kept hot, when you cool it down you will get more chlorate crystals.
There will be some chloride contamination, so you need to recrystalize the chlorate for the best results.

Good quality lead dioxide anodes can be used for both processes but it is better to do it as two stages.
I use MMO for the first stage and Lead Dioxide for the second stage as my lead dioxide is not good quality (I am lazy).
MMO is more efficient at the first stage but gets eaten up if chloride falls too low.
Platinum is more efficent for the second stage but gets eaten up if the chloride is too high.

Maurice VD 37 - 23-9-2020 at 20:29

I repeat. All what you say is only valid for sodium salts. It is not valid for potassium salts

vanBassum - 23-9-2020 at 23:57


Yes I am talking about sodium salts, because I have read that potassium salts are too insoluble to be effectively transformed from chlorate to perchlorate. While reading, I found that most people seem to use the 2 stage process although I have also read that PbO2 anodes are capable of converting chloride directly to perchlorate. Most use the 2 stage process with platinum electrodes for reasons explained by macckone. My PbO2 anode is bought from ebay and wasn't cheap, so I figure the quality is reasonably good, or at least I hope so. :) That being said, I have been thinking to go to the 2 stage process since MMO is a lot cheaper and easier to get than the PbO2. Therefore, the 2 stage process will spare my PbO2 anode.

Previously I made chlorate with MMO and I used dichromate as additive but I also read that chromates destroy the PbO2, so I am not to keen on using that. If some contamination manages to make its way to the perchlorate cell my anode will be ruined. I also read about persulfate that should work quite well with PbO2, but I haven't done my research on persulfate in a MMO chlorate cell. For now, I run without additives.

I have a batch running currently (around 72 hours now) with PbO2 and starting from chloride. The methylene blue test shows that there isn't any perchlorate yet. The electrolyte is somewhat brownish witch makes me think that the anode is being damaged. But that might also be from the first few hours, I didn't do a cleaning run or anything. The anode itself still looks good though. I might actually stop this and try to recover as much chlorate as possible and switch to the 2 stage process.

B(a)P - 24-9-2020 at 01:25

Do you hope to get solid sodium perchlorate as an end product or will you mix potassium chloride into your sodium perchlorate solution to get potassium perchlorate? If you are after the latter here is a good demonstration, quite a nice setup that they have.

vanBassum - 24-9-2020 at 01:54

Ah yes I've seen that video before that's a very nice setup, and I am a bit jealous of his glassware. :) The only concern I have is that he doesn't seem to destroy the chlorate contamination. When electrolysis is done, I like to collect the solids from the bottom of the cell. Those are of course contaminated with chloride and chlorate. To get rid of those I dissolve the perchlorate in acetone and filter the solution. Letting the acetone evaporate (or distil to recover) you are left with perchlorate and a little chlorate. Now I dissolve that in water, add HCl and boil the solution to destroy any remaining chlorate. I use the indigo carmine test to check whether any chlorate is left. If all chlorate is destroyed the solution is neutralized by boiling off the HCl or adding some NaOH. Now there is only NaClO4 and NaCl left in solution. For my purposes that is fine but you could recrystallize or again use acetone to get rid of the last bit of chloride.

The whole process summed up:
1. MMO electrolysis of NaCl to NaClO3. (NaClO3 collected as solids on the bottom of the cell.)
2. Boil and recrystallize NaClO3. (the 'waste' is returned to the cell in step 1.)
- Boiling destroys any remaining hypochlorite.
- Recrystallization gets rid of chloride.
3. PbO2 or Platinum electrolysis of NaClO3 to NaClO4. (NaClO4 collected as solids on the bottom of the cell.)
4. Get rid of the majority of NaClO3 by separation via Acetone. ('Waste' is returned to the cell in step 3.)
5. Destroy any remaining chlorate with HCl and neutralize solution. (Use Indigo carmine as test for chlorate.)
6. Optionally, separate NaCl from NaClO4 with acetone. (the 'waste' is returned to the cell in step 1.)

This will leave you with relative pure NaClO4 that can be converted to potassium if required.

I should write this stuff down in more detail for later reference.

[Edited on 24-9-2020 by vanBassum]

yobbo II - 27-9-2020 at 13:16

If you go to this site:

then sodium chlorate
then removal of product
then How to design fractional distillation processes

You will get some reading.
It can be a difficult enough subject to discuss.
What is said above is the what you can do.

I have attached file that is at the links

Attachment: (667kB)
This file has been downloaded 121 times

Attachment: (279kB)
This file has been downloaded 129 times

vanBassum - 28-9-2020 at 11:14

That site is an absolute treasure! I stumbled across it ones before. First I will download the site for my own archives before someone decides it should be pulled down.

I haven't found the time yet to read it all, but I did "scan read" the pages and the ZIP files that they are linking to and you are providing. It seems to answer my question perfectly. Thanks for the information.

vanBassum - 6-10-2020 at 00:36

A quick update: I changed my setup to 2 different cells. The first cell is a MMO for making sodium chlorate which has been running for more than a week now. I made sure to top of the cell with fresh chloride solution every day! Just now I noticed a thick layer of crystals on the bottom of the cell, so I poured the electrolyte in a second vessel and topped it of with chloride solution and placed the electrodes back in. I quickly checked if the crystals were actually chlorate by dissolving some and adding potassium chloride solution which resulted in a precipitation of potassium chlorate. I am currently running a recrystallization to get rid of any chloride and hypochlorite. Once I've got enough chlorate I'll fire up the perchlorate cell. I want to make sure I have enough chlorate to keep topping off the perchlorate cell when necessary.

I am aiming for sodium perchlorate since that can be converted to other cations like potassium of ammonium relatively easy. Besides, since the high solubility of sodium perchlorate it's easier to destroy chlorates with HCl if everything is in solution. I am sure that there is noting left when the indigo carmine tests negative.

So far so good. :)