WSM, what sort of quantities were you using of the sodium carbonate and sodium hydroxide, say per 1kg of sodium chloride?
Per 4 gallons of sodium chloride solution (350g NaCl/liter H2O, if I remember correctly), I used about 350 ml of 1M sodium carbonate solution.
Sodium carbonate solution in water is more alkaline than expected because some sodium hydroxide is also formed as the carbonate is dissolved.
When I added the 1M Na2CO3 solution to the brine, it instantly turned milky white. After sitting for a day, the fine precipitates of calcium carbonate and magnesium hydroxide, had dropped to the bottom of the 5 gallon HDPE bucket I used for a reaction vessel.
After decanting the clear brine into another new bucket, I vacuum filtered the rest of the slurry through a fine laboratory filter in a Buckner funnel, to separate the fine solids from the residual brine.
The clear, purified brine tested alkaline, so I added HCl dropwise till the brine was neutral to slightly acid, and pure enough to run in my sodium chlorate cell.
I fully expect a similar process will work in a potassium chloride brine.
Edited by WSM, 22 October 2020 - 12:09 PM.