jgourlay
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Crysallization of two salts from 1 solution?
Gents: are there two chemicals as follows?
1. Not terribly toxic
2. Will not crystallize on each other (ie, Alum+Chrome alum are out).
3. Saturated solutions of each won't react with each other (ie, NaN03+KCl are out).
I'd like to make 2 saturated solutions, pour them together, hang a seed crystal of each in the liquor, and show that concept that minerals prefer to
crystallize on their own kind.
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blogfast25
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| Quote: | Originally posted by jgourlay
[...] and show that concept that minerals prefer to crystallize on their own kind. |
Hmmm, that may not prove so easy to do: from saturated solutions crystals will usually grow on any type of impurity introduced (i.e. your
hanging seed crystals).
Many such mixtures as you're looking for will also form eutectics.
Separating crystalline, water soluble compounds by means of crystallizing them out of a solution is usually referred to as <i>fractionated
crystallisation</i>. In that technique one salt starts to crystallise out from the liquor, with the other concentrating in the liquor. This
provides a partial separation, which can be repeated (in cascade, as it were) often enough to obtain complete separation (compound A as crystals,
compound B as a solution). Fairly simple to execute fractional crystallizations (for example involving salts with different colours) should be
available through Google searches.
This more or less illustrates the principle you are alluding to.
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chief
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This should be possible with many inorganic salts; as soon as a seed-crystal is brought in it will get it's own material out of the solution; thereby
the solubility of the other material increases, and so it has no reason to grow on the substrate too.
What might be tried: Salts of the same acid (chloride, nitrate), but with different structures; eg. I believe CaCl2 OR BaCl2 hae a different structure
than MgCl2, because of the mich higher ionic radii of Ba/Ca vs. Mg; OR: A divalent salt vs. a monovalent; because of the different stoechiometric
ratio they will have different structures; therefrom crystallizing on theyr own.
But there always are exceptions, and one material may well for whatever reasons give a good substrate for another, as long as lattice-constants are
within 4-5 % of each-other ...
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jgourlay
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Chief: so....for example CuSO4 and Alum? Same acid, very different structures...
And in the cabinet already! 
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woelen
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You could also try a mix of solutions of sugar and table salt. The one is a salt, the other is a covalent organic compound. Both dissolve well in
water. It might be an interesting experiment on its own to see how such a mix behaves.
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chief
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@jgourlay: Yes, thats the sort of idea ...
@woelen: But sugar will make the solution very viscous, so either no sugar would crystallize, or it would be a diffusion-limited process. This might
be useful to affect the crystal-shape ...
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JohnWW
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The likehood of a double salt crystallizing from the solution is quite substantial. Of course, there are the alums, M(I)2SO4.M(III)2(SO4)3.24H2O,
which are hydrous sulfates (and also selenates and chromates) of a wide range of univalent (e.g. alkali metals, NH4+, Tl+, Ag+) and trivalent (e.g.
Al+++, Ga+++, Fe+++, Cr+++, V+++, Mn+++, Co+++, In+++, Tl+++, and probably some rare-earths) cations. There are also mixed halides like carnallite,
KCl.MgCl2.6H2O, and mixed carbonates, like dolomite CaMg(CO3)2 which is distinct from both calcite/aragonite and magnesite. Less soluble double salts
crystallize out under mostly hydrothermal conditions, mostly mixed silicates like feldspar and beryl and topaz.
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12AX7
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I have a variety of things that seem to... copper chloride (in excess) and salt (forms Na2CuCl4), K2SO4 (or it might be KHSO4) and CuSO4, who knows
what else. These I know best because I know that's what's in solution and I recognize the crystal forms (like the bisulfate's octahedra) or at least
the chemistry (I don't know what Na2CuCl4 is supposed to look like, but that stuff must be it!).
Simultaneous crystallization is hard to achieve, most often you get a pasty grunge of fine crystals difficult to seperate. It's like an eutectic, the
two phases grow from solution simultaneously and in direct control of each other; thus as one precipitates, the other is supersaturated and
precipitates, etc. Come to think of it, a finely filtered solution (I often gravity filter solutions through packed glass wool, which is often very
slow, but will stop everything) is probably the best, as then there are few nucleation sites.
If you want to try, you'll want to start with crystals that really like to form in the first place. Copper sulfate and potassium sulfate are two good
candidates; they are very stable crystals, grow crystals well, look and grow differently, and won't react in solution (in any further sense). I would
guess to recommend: dissolve in distilled water (K2SO4 first, as it's already low solubility), filter through colloidial-grabbing filter media, then
leave in a warm, dry place with a dust cover for a few weeks. Check every couple of days and see what happens. You might even add a seed crystal of
each type to see if you can grow each individually (should).
Tim
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chloric1
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Tim, potassium sulfate and copper sulfate form a double salt in the form of a hexahydrate. A potential candidate for the original request would also
suffice for a demonstration of common ion effect. To a saturated potassium sulfate solution, add an excess of saturated KCl solution. The sulfate
willl deposit and can be separated from the chloride still in solution. You could start with slightly less than saturated solutions and go for a more
gradual deposition. Be sure to test your chloride filtrate with barium chloride for the presence of sulfate ions.
[Edited on 12/9/2008 by chloric1]
Fellow molecular manipulator
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12AX7
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Ah, then it must be KHSO4. Well, they're octahedral- and come to think of it, I don't recall K2SO4 being octahedral.
Tim
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chief
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Maybe it will be easier to choose the other way around: One metal and 2 acids, which are structurally incompatible.
Eg. the cloride vs. carbonate or nitrate (XO3 are both planar), or vs. sulfate (SO4 2- is a tetrahedron) .
From my experience : Na2SO4 and Na2CO3, both with similar solubility-curves over the temp-range, crystallize each separately.
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chemrox
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The crystallization of minerals is a function of pressure, temperature, activity and equilibria... all interrelated. Phase diagrams and equations of
state are used to predict their formations. Look up concepts in books on petrology, mineralogy and physical chemistry. When one finds minerals
crystallizing together its because the conditions for the mineral's precipitation/formation have been met. It's not a "birds of a feather" kind of
thing. I suggest getting clear on the concept to be demonstrated before conceiving a demo. Consider pegmatities. They are what's left over after a
magma has crystaliized as a rock of a certain type that takes specific quantities of Si, O, Na, Ca, Fe, Mn, some trace elements. The left over
material forms some wonderful minerals that depend on what exactly was left from the melt. You get muscovite mica books, tourmaline, garnets, some
really exotic stuff too... Also consider the humble geode. It starts as a bubble in a lava. Hot mineralized water works its way into it and deposits
quartz if the water is rich in silica or calcite of the water is stongly basic. Sometimes more than one episode is represented and alternating layers
of different minerals are formed from different solutions. "Solutions & Equilibria" good text, I'll have to look up the author (Harvard
geochemist).
[Edited on 9-12-2008 by chemrox]
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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quantumcorespacealchemyst
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Solutions, Minerals, and Equilibria
Book by Robert Garrels
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unionised
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Sodium sulphate and sodium carbonate definitely do form a double salt.
I think salt and sugar is the best suggestion so far.
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blogfast25
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Any credible references to that effect?
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