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Name:  William
Status: student
Grade: 9-12
Location:NC
Country: N/A
Date: 12/19/2005


Question:
Are there any chemicals that will precipitate a ionic salt out of an aqueous solution?


Replies:
There are many such chemicals. In fact most water miscible organic solvents will do so. A standard "trick" for precipitating ionic salts from aqueous solution is to add ethanol, or acetone. The solubility of most ionic salts is very much lowered by the addition of relatively small amounts of water miscible organic solvents.

Vince Calder


William,

This would depend entirely in the measurable property called "solubility product" of the particular combination of cations and anions.

For example, the solubility product of Na+ and Cl- is about 36. This means that approximately 3.6M of Cl- can remain in solution in combination with Na+. However, the solubility product of Ag+ and Cl- is about 1.8x10^-10. This means that about 6.0x10^-11M of Cl- can remain in solution in the presence of Ag+.

Thus, if you look up a table of solubility products for the specific ion you wish to precipitate, and you find a low value for that particular combination of cation and anion, then you should be able to precipitate that ion with the use of the counter-ion list.

Greg (Roberto Gregorius)


Yes, William.

Most organic solvents will not dissolve ionic salts. The solvent characteristics of a mixed 2-liquid solution tend to be somewhere in between the characteristics of the separate component liquids. So if you find an organic solvent that is fully miscible with water (meaning they dissolve in each other in all percentages), then adding that solvent to a salt-saturated water solution will usually force substantial precipitation.

Isopropanol is the usual choice. "Rubbing alcohol" on the shelf at the drugstore is 70% alcohol, 30% water, and it has a pretty substantial effect when added to saturated salt-water. Isopropanol's formula is (CH3)2CHOH. You can see it has one OH group, which will try to dissolve salts, and a bunch of CH3 stuff which has little affinity for salts or water. Compare that to water with two OH groups and no CH- stuff. The result is that pure isopropanol dissolves only a tiny amount of salt compared with water. Variable mixtures are often used when polishing pure salt crystals on wet paper or cloth, so the polishing solution gradually stops dissolving the salt surface.

Acetone might also work. Its precipitation-inducing effect will be stronger, and mixed with salt-water it's more likely to be forced by the presence of salt into two separate phases (two mixed liquids, one thinks it is like acetone and has more acetone, the other thinks it is like water and will have all the remaining dissolved salt.) And be careful; acetone is more flammable and has stronger fumes which need ventilation. I would usually start with the isopropanol.

There is also the "common-ion" effect. If you try to dissolve NaCl and NaBr in the same water, The sodium from each makes the other feel like it has a higher concentration. Together, neither dissolves to the extent it did by itself. However, this effect is generally less useable for forcing precipitation than organic solvent addition.

Jim Swenson



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