FlaMtnBkr Posted June 18, 2013 Posted June 18, 2013 (edited) It has been a long time since I had chemistry and I'm not sure we learned this. How do you determine which ions will combine when two different salts are mixed together in water? I guess I don't need to know how to do it but maybe someone can tell me what would tend to form. If I add strontium chloride and potassium nitrate together in water, would it want to combine back into strontium nitrate and potassium chloride when dry? What about strontium chloride and potassium carbonate? I guess this wouldn't be good because any remaining potassium carbonate would be really (?) hygroscopic. Basically I was curious if something could be added to strontium chloride that would result in 2 salts that aren't hygroscopic when dry. Does anyone know if potassium chloride is hygroscopic? I have found conflicting information. I have some KCl and it doesn't seem very hygroscopic, if at all. Is there a measurement that predicts how hygroscopic a chemical is, other than saying something is or is not hygroscopic? Edited June 18, 2013 by FlaMtnBkr
Seymour Posted June 19, 2013 Posted June 19, 2013 Hygroscopicity is somewhat complex. Potassium nitrate, and potassium chloride when very pure are not that hygroscopic at all. However impurities, including some chloride in the nitrate or vice versa seem to make the product much more hygroscopic than either chemical pure. Ultra pure Sodium nitrate can be used in pyro very successfully, while I'd be reluctant to say the same about normal fertilizer grade. What about strontium chloride and potassium carbonate? I guess this wouldn't be good because any remaining potassium carbonate would be really (?) hygroscopic. You get Strontium carbonate precipitating out. Any remaining Potassium carbonate and Strontium chloride and the Potassium cloride formed are to be washed out. Soluble impurities and insoluble products are the blessings of an easy ion swapping exchange. Essentially when you get several salts dissolving together in one solution, the ions dissociate as they do when they dissolve, and then they get it on in a slutty orgy. While equilibrium forms and control the ratios formed between all the possible combinations of ions, but to some degree, everything possible by combining ions in the mixture will to some extent be made. Generally there is some way to manipulate changes to the equilibrium to favor your desired product. When an insoluble product is one possibility from combining ions, as it is with the SrCO3, its easy. It falls out of solution, and as it does so the strontium and carbonate are depleted, driving the equilibrium engine to create more, at random, but they are lost to the solution. It's one way. At the other end of difficulty, you can look at the solubility of all the possible chemicals formed. If it's the most or least soluble combination of ions then it will either crystallize out first, or be concentrated in the solution after all the other spectator ions and competing salts have been crystallized out. In some cases you can crash spectators or competing ions out by adding other reagents. Essentially it's about manipulating it so that what you want is either in solution or crystalline/precipitated, and what you don't want to have is not, and then separation and purification is easy.
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