Solubility in practice

[Swede]

I've been preparing KCl feed stock for my chlorate cell by simply allowing water at an ambient temperature to sit over a bed of KCl crystals in a big jug, with occasional agitation, for several days. The traditional way to dissolve the maximum amount of a salt is to heat, stir in an excess of the desired salt, bring to a boil, then allow to cool, with some of the salt precipitating out. The liquid is filtered, and there's your saturated solution.

 

I thought my method was fine, certainly much easier than boiling the salt stock, but I did a little test to show I was wrong. I had purchased some Chloride quantitative strips to check my cell's chloride level, and to halt chlorate production at 10% Cl-. Just for grins, I checked my lazy man's feed stock, and it was only 125 grams/l of chloride, which equals 262 grams/l of KCl salt.

 

CRC says the solubility of KCl should be closer to 350 grams/l at about 35 degrees C. To test, I prepared two identical samples.

 

Sample A: H20 at 35 degrees was mixed with a huge excess of salt, stirred for several minutes, allowed to sit (with additional stirring) for a couple hours.

 

Sample B: H20 + excess KCl was brought to a boil, stirred, and allowed to cool to 35 degrees C. The liquid was then filtered to remove floating crystals.

 

Using the titration strips, sample A (The Lazy Method) was 284 g/l KCl, close to what I tested with my big jug of feedstock. Sample B, a whopping 368 g/l KCl. The difference was HUGE. I had been cheating myself of 23% KCl.

 

Lesson: Don't be a lazy ass. Boil the salt, then cool and filter.

[baran420]

Hi Swede,

 

 

That’s fairly interesting and I am surprised no one has commented on your post until now. To make saturated solutions for stichometric reactions I used the “lazy ass” method and simply made up stock solutions based on solubility curves at the correct temperature. All salts I dissolved up went into solution beautifully, as I would have expected. However it just occurred to me after reading your post that most of the salts that I dissolved up are hydrates (Ca(NO3)2, CaCl2, CuSO4 etc etc. So naturally my solutions were not saturated, as I did not take the water of crystallisation into account (I am not a chemistv). So thanks for your post as I have realised that to make up saturated solutions one should not abide by solubility curves alone.

 

 

Cheers, AB

[Swede]

Hello baran - the funny thing is, I still find myself being lazy with the KCl. I do a sort of modified method to keep it easy, yet get maximum chloride dissolved. What I do... rather than boil the water, I use hot tap water, and perhaps add a liter or two of boiling water to the 5 gallon pail that I use to mix. Dissolving KCl is apparently endothermic, and it cools off rather rapidly, but I'm still getting good chloride concentrations.

 

The sacks of KCl I use contain big nuggets, about the size of walnuts. As they dissolve, they diminish in size. As the pail cools, if I see fine crystals falling out and layering the much bigger nuggets, then I know I have achieved true saturation for the given temperature. If fine crystals do NOT fall out during the cooling, then the solution isn't saturated, and I need to work more on it, usually by adding boiling water and agitation.

 

For cells that are 4 liters and smaller, it's no big deal to prepare a true saturated solution by boiling, but with 25 liters, it'd require a turkey fryer and a lot of propane!

[50AE]

Very good discovery Swede ! This is very useful to me, and hopefully to others

[baran420]

Hey Sweed, My KCl is obtained in 25kg bags and is "FRU. Grade" which is not overly pure but good enough. The bag is stamped "Dead Sea Works Israel" which is apparently the bigger of the KCL suppliers. It is a nice free flowing crystalline powder. Our drillers use it to condition clays in order to stop them swelling around the drill string and making them stick. The funny thing was is that I was hunting around for some KCL and getting disillusioned by the high price of "Low salt" when I noticed a driller pouring a white crystalline substance down a drill string-on closer inspection….

 

 

You must make a heap of KClO3!!!! Do you use it as the chlorate or do you thermally convert it to the perchlorate?

 

 

Cheers, AB

[Swede]

It sounds like you've found a good source of KCl. I wish my bags were free-flowing powder rather than lumps. And the lumps themselves have occasional debris embedded in them. I have to filter (through cloth) to get rid of the debris before I use it.

 

I've made a bit of chlorate but I really want perchlorate, and am stockpiling it (the chlorate) for conversion, probably electrolytic. Hardly anyone uses potassium salts for this process, but it does simplify purification, at the cost of solubility. There's no problem potassium presents that cannot be overcome with volume and raw amperage!

 

Plus, in both cases, chloride --> chlorate, and chlorate --> perchlorate, the visible crystals that form and collect are a pretty good clue about how much longer to run the cell, as opposed to sodium, which produces zero crystals until metathesis is executed. It's fun to watch the oxidizer form.

[WSM]

Hi Folks,

Somewhere in my chemistry education I recall that moving liquids dissolve about seven times as much soluble material as still liquids. I've had success using a hot plate/stirrer, even without the heat. My first "lab" stirrer involved gluing a bar magnet to the end of the shaft of a small DC motor with hot glue and spinning it under a plate of masonite (crude but effective). Teflon coated magnetic stir bars are unbeatable in most mobile liquids, usually. Larger quantities in bigger containers can be successfully done with electric lab mixers, similar to a drill motor, using a shaft with a type of propeller on the end of a compatible material (stainless steel or teflon, for example).

I've done as Swede suggests, use heat to dissolve as much as will be contained in the solvent and drop out more than the lowering temperature will keep in solution (saturated solution). I too would be interested in locating a source of low cost, clean and fine powdered KCl. Do tell. Thanks.

[Swede]

Stirring undoubtedly helps. For my lead dioxide plating rig, I created an electric stirrer using a 1/10th HP motor, a plastic shaft, and a propeller-thing on the end, so it is not magnetic, but rather direct. By using a DC motor and a variable power supply, the stirring power can be varied. I haven't used it yet, but it is an option. It seems really effective in tests.

 

Dissolving KCl on a large scale is pretty tedious, to be honest. It's simple when you're doing a liter or two, but 25 liters is a pain. The method I've finally settled on is a compromise - in the initial phase, I use hot but not boiling water, with a large surplus of KCl. I stir modestly, then simply let it sit for a few days. If I see tiny crystals that have fallen out of solution when the water cools, I know it's as saturated as it's going to get at that temperature. If not, I add more hot water, stir more.

 

In the end, long contact with the surplus of KCl forces an equilibrium. It may not be perfect, but it's easy. Decant to use.