Sciencemadness Discussion Board - KF Solubility

KF Solubility

runlabrun - 22-12-2004 at 05:28

Hey all,

Would someone happen to know how to seperate a mix of KF and K2SO4?

What is the flouride soluble in that the corresponding sulphate is not?

Both are soluble in water (to different extents) and both are insoluble in alcohol. However K2SO4 is slightly soluble in glycerol whereas KF wouldnt be (right?) so this may be a way with heated glycerol, however it would be very messy and anoying....

Any ideas? i have looked around but cant find much....
-rlr

HF toxicity

Sergei_Eisenstein - 22-12-2004 at 23:00

Maybe it is possible to precipitate your fluoride as CaF. It is very insoluble in water.

It is also the cause HF is dangerous to work with. It easily permeates the skin and will complex with calcium-ions in the blood.

runlabrun - 23-12-2004 at 03:43

CaF2 not CaF

Thats possible but i want my end product to be KF, what i have is a mix of HF and H2SO4, adding excess KOH to this will produce my troublesome mixture...

I want to seperate the K2SO4 from the KF...

I dont want to do anything with HF that shit is not good.... and btw it also gets into the calcium in your bones so you become the human representation of the far side comic "boneless chicken farm"...

So without heating the solution (~15% HF and ~40% H2SO4 rest is water) to give off HF and have to deal with that i would rather just dump in KOH and seperate the desired KF from the unwanted K2SO4....

Any ideas people?
-rlr

Marvin - 23-12-2004 at 11:50

Use fractional crystalisation from the water. I can think of something that would extract only KF but its lethal if you slip up and spill the solution on your skin, water is as safe as its going to get. Fractional crystalisation is a useful skill to learn.

I'd also be inclined to suggest leaving the bottle open in a KOH dessicator, but the fumes would probably attack the container if its glass. Its also likley a very long process outside of a vacuum.

neutrino - 23-12-2004 at 13:02

Add MgSO4. CaF2 will precipitate out and you'll be left with a soution of K2SO4.

?????????

chloric1 - 23-12-2004 at 13:58

Quote:

Originally posted by neutrino
Add MgSO4. CaF2 will precipitate out and you'll be left with a soution of K2SO4.

Hey neutrino, unless you know something about transmutation that none of us do, how do you expect to obtain CaF2 from Magnesium ions? I think you meant MgF2.

neutrino - 23-12-2004 at 16:00

Yes... all those alkaline ions tend to blur together at times...

Magpie - 23-12-2004 at 16:06

I think neutrino may be on the right track. Addition of water soluble MgSO4 (Epsom salt) should precipitate the MgF2. This could then be isolated by filtration or centrifugation. I'm wondering if it might then be possible to recover the KF by treating it with KOH since Mg(OH)2 is more insoluble (Ksp = 1.5 x 10^-11) than MgF2 (Ksp = 6.4 x 10^-9). This might be tricky, however. I definitely agree that I would not let my pH go acid enough to generate HF if there was a chance of breathing it.

Sergei_Eisenstein - 23-12-2004 at 23:37

Sorry for the CaF/CaF2 thing...

Possibly, you could also precipitate the sulfate by addition of Ba2+ (often added as BaCl2). However, I have no data about the solubility of BaF2.

Cloner - 24-12-2004 at 04:06

BaCO3 is a better option then, indeed barium will go to sulfate ions and precipitate. Carbonate helps getting rid of your HF, and it is OTC while I've never seen BaCl2 plus you'd end up with KCl + KF. Barium fluoride is somewhat soluble. Probably not great but you can use large dilution and recrystallise.

Of course, potassium fluoride is nasty by itself but since you're obviously cleared to work with this mess, you know the drill...

Magpie - 24-12-2004 at 11:29

Cloner: Where do you get BaCO3 as an OTC chemical?

Eclectic - 24-12-2004 at 17:56

KF's solubility in water is much higher than K2SO4. As the soution is concentrated, K2SO4 should crystalize out leaving KF in solution. K2SO4 should be almost insoluble in a saturated KF solution due to the common ion (K). (Unless there is some double salt thing going on).

chloric1 - 24-12-2004 at 18:50

Quote:

Originally posted by Magpie
I think neutrino may be on the right track. Addition of water soluble MgSO4 (Epsom salt) should precipitate the MgF2. This could then be isolated by filtration or centrifugation. I'm wondering if it might then be possible to recover the KF by treating it with KOH since Mg(OH)2 is more insoluble (Ksp = 1.5 x 10^-11) than MgF2 (Ksp = 6.4 x 10^-9). This might be tricky, however.

Magpie I know exactly what your after here but the differences in solubility are neglibly slim. I know of a proven procedure of making pure dichromates by digesting strontium chromate in a bisulfate solution until white strontium sulfate forms. This only works becuase strontium chromate is insoluble in cold water but about 2 grams dissolve in boiling water. So even with the major increase the proceedure takes a half hour or more and I am sure constant stirring is necessary.

The tiny difference you nstated may mean that you might need to digest the solids for hours or even days and it would be hard to tell when you had an endpoint.

chloric1 - 24-12-2004 at 18:52

Quote:

Originally posted by neutrino
Yes... all those alkaline ions tend to blur together at times...

AHHH Dont sweat it! I was just being picky so newbies won't get misinformed. I read your posts, you know what your doing. :cool:

Magpie - 24-12-2004 at 20:34

Thank you chloric 1. I was speaking only from a theortetical standpoint. I suspected these Ksp's were too close but brought it up to see if anyone had ever tried dissolving a relatively insoluble material (MgF2) by making a less soluble material[Mg(OH)2]. To me it was just an interesting possibility.

chloric1 - 25-12-2004 at 11:04

I am excited to try the dichromate synthesis from my book. It starts by oxidizing trivalent chromium in a molten salt mixture of equal weights of KNO3, K2CO3, KOH and then dissolving the solid in hot water to have aqueous chromate. You add this strontium chloride and chill and you know the rest! Could be a way to recycle chromium after organic oxidations :cool:

I still need a few more key ingredients. But I hope to try this one soon. When I have more time I may present the proceedure in word format.

[Edited on 12/25/2004 by chloric1]

[Edited on 12/25/2004 by chloric1]

runlabrun - 30-12-2004 at 05:13

thanks all for your input...

ive been doing some looking around and i think the best way would be to use epsom for MgF2

So:
2HF + H2SO4 (the mix in the bottle) + MgSO4 --> MgF2 + 2H2SO4
That right?

The MgF2 will precipitate (solubility ~0.0002g per 100g 20oC H2O i think?) and leave a solution of dilute sulphuric acid.

The precipitate can then be treated with KOH to precipitate the barely soluable Mg(OH)2 and remain a solution of KF.

Purification of the solution could be a problem though....
The MgF2 could be re-crystalised easily enough to decent purity...
But for the second part... i can see KOH + KF being left over, another re-crystallisation from anhydrous ethanol would result in KF precipiating from the KOH in ethanol solution. Does this sound fine?

Also is there a significant equilibrium with 2KF + H2O <--> 2KOH + 2HF ???
is this the reason for KF solutions not being stored in glass? So the re-crystallised KF solid should be stored in a sealed plastic bottle...

-rlr

runlabrun - 5-1-2005 at 22:22

Ok... im starting to think this wasnt such a crash hot idea.....

The bottle was 1L of aluminium cleaner containing 9.8g/L HF and 46g/L H2SO4
In theory and 100% yields ~15g of MgF2 should precipitate....

The contents of the bottle was pink, added this to a larger volume container and added 40g MgSO4 (29.5g required in theory) the excess can be easily washed out from the insoluble precipitate.
The solution was warmed in a boiling water bath to 70oC and the solution started to evolve convection currents, the excess MgSO4 started floating around and making the mixture cloudy.

So far i have seen no precipitate however there is 1L of fluid so im tempted to boil it down but the thought of un-reacted HF coming off wouldnt be a good thing....
Would the reaction between MgSO4 and HF really work? If so the excess i have added should allow to ease my worries.

The solids recovered from the solution would be re-x in cold water to remove MgSO4.

-rlr

Marvin - 6-1-2005 at 16:43

HF is a very weak acid, MgF2 may not precipitate at low pH. Try neutralising. I assume by no precipitate that the MgSO4 dissolved completely?

You wont be able to recrystalise the MgF2, its neerly insoluable, thats why it was suggested. I have doubts about the KOH extraction working well but you can let us know if you actually get any MgF2.

runlabrun - 6-1-2005 at 17:40

all the MgSO4 dissolved, took a bit of warmth (due to competitive solubility due to the exisiting SO42- from the sulphuric acid?) but eventually happened.
Ill bring up the pH and let you know....

-rlr

Update:
The mixture of MgSO4 and aluminium cleaner was warmed in boilding water bath and stirred for 40mins, added enough NaOH to bring pH to 10 where the solution was warmed as before, cooled to room temperature where a small amount of white ppt fell from the milky solution, this was filtered and the filtrate washed several times with cold water to remove MgSO4 left unreacted. There was a very faint trace of white powder on the filter paper (the paper was white so its hard to see) which was left undissolved.
Very small amount ~1g if that.... dont know if it works really well, possible the [HF]<<<[H2SO4] the MgSO4 was simply not able to react to any great extent...
Oh well...

-rlr

[Edited on 7-1-2005 by runlabrun]