Sciencemadness Discussion Board

filtering a Zinc Chloride solution

Arthur Dent - 12-11-2010 at 11:28

Hi all,

I have stumbled upon a bit of a problem synthetizing ZnCl2. I wanted to prepare a solution of ZnCl2 so I took a bunch of Zinc Carbon batteries and carefully removed the Zinc shells. I cleaned those thorougly with acetone, then distilled water. I put the Zinc in a beaker and poured some concentrated (20 BE) HCl in it.

The reaction was quite vigorous and produced foam nearly to the top of the beaker. When it subsided, I saw some effervescence on the surface of the metal, and the reaction is contionuing as of this moment. When there will be no more effervescence, I will leave a bit of excess Zn to make sure there is no more HCl in my solution.

So my problem is this: "Concentrated aqueous solutions of zinc chloride (more than 64% weight/weight zinc chloride in water) have the interesting property of dissolving starch, silk, and cellulose. Thus, such solutions cannot be filtered through standard filter papers. " taken fro the Wiki page on ZnCl2...

What if I dissolve the solution in copious amounts of distilled water? Can I filter it properly or is there a mystery filter material that's not made of paper that I can use. My goal was to have a highly concentrated solution that I could crystallize by evaporation and/or refrigeration. I know that ZnCl2 is hygroscopic, so I won't attempt to make anything anhydrous

Any suggestions or recommendations are welcome on how to filter/purify my solution as much as possible. :D Thanks!

Robert

blogfast25 - 12-11-2010 at 11:38

Well, do you know the actual concentration of your solution? To achieve 64 w % ZnCl2 by simply dissolving the scrap zinc in strong HCl would be nigh impossible because the activity of the acid tends to drop a lot at such high ionic strengths.

I bet you can filter your current solution w/o problems. I've done very similar things with zinc from batteries and filtered uneventfully...

20 BE HCl is S.G. about 1.16, right? Quite concentrated, 30 % at least I'm guessing... good stuff!

[Edited on 12-11-2010 by blogfast25]

Arthur Dent - 12-11-2010 at 13:09

Thanks for your reply!

I am unsure as to the current concentration of the solution so far, but it's just the zinc shards and the acid, so if it's 68% water and 32% HCl, I guess that when the reaction will end, i'll have something like a 15% zinc chloride soln. in water.

The acid I use comes from the hardware store (RONA in Montreal) and it's fairly clean and free of impurities. I use it to make my Aqua Regia and it does a pretty good job at dissolving metal! :D

- Robert

aonomus - 12-11-2010 at 14:16

If you have the glassware and supplies, you could always try a fritted funnel with a pad of celite.

bbartlog - 12-11-2010 at 16:17

Quote:
if it's 68% water and 32% HCl, I guess that when the reaction will end, i'll have something like a 15% zinc chloride soln. in water.


Only if HCl is used in large excess. If you use a stoichiometric quantity of HCl, you end up with a significantly higher percentage (weight/weight) solution of ZnCl2. See:

Two moles of HCl weigh ~73g (so a 32% solution weighs 228g and contains 155g water)
One mole of ZnCl2 weighs ~136g; dissolved in those same 155g water, it forms a 47% solution by weight (136g / 291g).

However, you won't get to 64% without evaporating a significant quantity of water. Concentrated ZnCl2 solution is kind of neat... quite dense.

BromicAcid - 12-11-2010 at 19:04

You could always filter through glass wool. I've also used fiberglass insulation although I am not certain it is 100% glass.

daragh8008 - 13-11-2010 at 03:26

sounds to me like you need a membrane filter/ion track polymer filter. something like these : http://cgi.ebay.com/Selection-47-mm-Diameter-Millipore-Membr...

bbartlog - 13-11-2010 at 04:49

No he doesn't. Dilution (probably not even needed) and paper filters should work fine.

daragh8008 - 13-11-2010 at 05:03

i suppose I shouldn't of said that you need a membrane filter. Perhaps I would be better to say, you could use a membrane filter. Better?

blogfast25 - 13-11-2010 at 06:39

@ Arthur Dent:

What do you want the ZnCl2 for? Anhydrous ZnCl2 isn't that difficult to achieve either, if you're interested...

As stated also by bbart: at your likely concentrations no special filtering media are required.

[Edited on 13-11-2010 by blogfast25]

Arthur Dent - 14-11-2010 at 07:27

Good to know, I'll filter my solution with a plain coffee filter. I do have a box of "grade 40" Whatman filter paper but I keep the good stuff only for fancy or sensitive reactions, I'm a cheapskate, what can I say! LOL

The zinc chloride I'm making will be used for some home-made soldering flux, by mixing it with a bit of ammonium chloride and wax with it. Plus, I like experimenting and synthetizing my own reagents... if it's a fairly harmless chemical and the reaction is not too hazardous, i'll definitely try it.

For now the most hazardous reaction I experiment is to dissolve gold plated CPUs and computer card fingers in aqua regia to recover the gold, and even at that, I always do it in very small batches at a time, unlike some experimenters I know who process that stuff a gallon at a time! :o

Robert

blogfast25 - 14-11-2010 at 09:51

Quote: Originally posted by Arthur Dent  
The zinc chloride I'm making will be used for some home-made soldering flux, by mixing it with a bit of ammonium chloride and wax with it. Plus, I like experimenting and synthetizing my own reagents... if it's a fairly harmless chemical and the reaction is not too hazardous, i'll definitely try it.

Robert


Wouldn't it have to be anhydrous for fluxing? You can almost certainly obtain anhydrous ZnCl2 by mixing the hydrate very intimately with 2 moles of dry NH4Cl (per mole of hydrate) and subliming off the salmiac. On heating, the dissociating NH4Cl (--> HCl + NH3) protects the ZnCl2 from hydrolysis. Works for several chlorode hydrates like MnCl2, also MgCl2 I believe...

smuv - 14-11-2010 at 14:35

@ Arthur, I actually once made soldering flux in a pinch; don't bother adding the wax; just use it as a liquid, and just dip the wire into it before you solder or apply the liquid flux with a paintbrush.

If you are using this for electrical soldering, make sure you remove all traces of HCl as it can lead to corrosion of the solder contact. If you are using the flux for structural soldering, leave the HCl in as it really improves the characteristics of the flux. If your flux has HCl don't worry about adding NH4Cl.

blogfast25 - 15-11-2010 at 08:21

Talking about HCl, I forgot to mention that if you're going to crystallise ZnCl2 from watery solution you better make sure you’ve got some HCl in there up to the last minute, to avoid hydrolysis to zinc (hydr)oxy chlorides. ZnCl2 solubility in water is so high that simmering to dry is the only option but w/o acid reserve you're likely to end up with a semi-soluble goo of various zinc (hydr)oxy chlorides. So your hydrate product is likely to contain some HCl. To get rid off it, fuming with excess dry NH4Cl should give a water free ZnCl2, MP about 290 C. Small amounts of zinc (hydr)oxy chlorides should not stop the product from performing.

Arthur Dent - 15-11-2010 at 16:57

@blogfast25: Thanks for the info, I'll have some excess HCl in my solution since the zinc was still reacting a tiny bit when I removed it, and i'll let the ZnCl2 slowly dessicate in my dessicating apparatus (a large ziploc bag with a beaker full of anhydrous CaCl2 ;) )

As for your process to neutralize the HCl, I do have Ammonium Chloride, but what do you mean by "fuming it with" ? I must admit i'm not familiar with the term. My lab glassware collection is rather limited but i'm always willing to acquire new stuff if it will be useful. :D

i'll keep the salt in a small amber glass bottle with a teflon-lined cap, to avoid the emission of oxidizing Cl gas.

Robert

morrie - 16-11-2010 at 05:02

I used to run a plant where we used zinc chloride as a feed material to our process. We purchased it as a granular solid. Sometimes, the air would have got to it and it would be a solid block, which was useless to us. So we gave it to the zinc plating plant up the road. They were happy to use it.

I don't know how the zinc plating place normally got their zinc chloride, but it might be worth a chat with your local plating shop.

blogfast25 - 16-11-2010 at 05:17

'fuming with NH4Cl': mix an excess of dry powdered NH4Cl with the hydrated chloride (as indicated above) intimately (use a mortar and pestle for instance to grind the powers into each other - it must be quite a homogeneous mix). Then strongly heat the mixture in a glass or ceramic receptacle using a Bunsen burner or similar: both the salmiac and the hydrate water will evaporate (the NH4Cl sublimes off - it has no liquid state at atmospheric pressure).

Because salmiac vapours are largely dissociated into HCl and NH3, the HCl protects the zinc chloride against hydrolysis. The 'fuming' method is equivalent to drying the ZnCl2 in a stream of dry, hot HCl but easier to achieve. You should be left with a glassy mass of relatively pure ZnCl2.

As regards 'avoid[ing] the emission of oxidizing Cl gas', that's not necessary: ZnCl2 is a very stable compound and does not release any Cl2. Perhaps you meant HCl? The fuming off NH4Cl gets rid of that too...

Arthur Dent - 16-11-2010 at 18:00

That's excellent! I'll try that... but definitely an "outdoors only" kind of experiment. Until I manage to make myself a fume hood, anything that releases halide gases, NO or SO2 gas is strictly outdoors and since I'm in an urban area, anything noxious has to to be very small scale otherwise i'll have angry, choking neighbors banging at my door! ;)

I'll use an old coffee pot instead of a beaker in case the heat is too much for the glass. Don't want to scrap my last good 500 ml beaker!

Robert

blogfast25 - 17-11-2010 at 05:58

Outdoors is best but I've done this kind of thing indoors with relatively small amounts (about 20 g) of NH4Cl/MnCl2 hydrate. NH4Cl is really non-toxic. Problem is that there may be a little mechanical entrainment of the target chloride, so really good ventilation is called for and outdoors/fume hood is best.

Yep, steel is good too but it will surface oxidse a bit becuse of the HCl fumes (from the NH4Cl). Also usable is decent Pyrex kitchenware as long as you heat it gently: it can take 200 - 300 C easily if you don't 'thermally shock' it (same for cooling it: allow to cool naturally to prevent thermal shock and shattering). Decent size Pyrex testubes will do OK too... especially for a (pardon the pun) 'dry run'...

And if you have access to decent scales, weigh everyting before and after: actual weight loss should correspond more or less with fumed off NH4Cl and hydrate water: end weight = starting weight - weight of NH4Cl - weight of hydrate water.


[Edited on 17-11-2010 by blogfast25]