SEED RANCH potassium nitrate label explained
MaLar, and others: I'm a chemist. I took a look at the SEED RANCH potassium nitrate label, and it is way different from what it should be from a standard chemical laboratory label for potassium nitrate because it is not given in modern chemical formula terms, as what is used in a laboratory for determining a chemical reaction. Using what is given on the label, those can't be used for figuring out anything regarding using potassium nitrate for black powder. First of all, "potash" is not K2O, to be correct it is K2CO3 aka potassium carbonate. K2O is potassium oxide, which plain and simple can NOT exist in the presence of water, where a K2O molecule reacts with a water molecule, H2O, to form two KOH molecules plus AN ENORMOUS AMOUNT OF HEAT. The common name for KOH is "potassium lye" aka potassium hydroxide, as opposed to just plain lye that is sodium hydroxide. Sodium hydroxide, the kind of lye you can get at hardware stores, is what you use for cleaning drains and making home made soap (fun, if you are into that kind of thing). FWIW, potassium lye makes darned good soap, too. So, in other words, there ain't a speck of K2O in that bag, and if there were, it would pose quite a hazard if it ever got wet!
So much for the useless label that is actually not all that useless, IF YOU WERE A 1930s FARMER with a sixth grade education and wanted to compare a sack of urea fertilizer with a sack of potassium nitrate fertilizer, and wanted to know how much nitrogen per dollar, err, I mean per dime you were getting. So, that's why some fertilizer bag labels give what's inside as being equivalent to something else. For me to be a chemist, I had to take botany, which covered agriculture that covered fertilizers.
Alright, about purifying potassium nitrate, or anything else that will dissolve in water, that can sometimes be pretty complex. Besides solubility, which crystal system that may cause co-crystallization problems (cubic, hexagonal, orthorhombic, icosahedron, whatever), concentration, and temperature all play a part. Potassium nitrate is one of the easiest salts to purify because of its low solubility, especially at low temperature. Look up the solubility temperature curve for potassium nitrate and compare it to something like sodium chloride, where there are big differences in solubility from 212ºF (boiling point), 150ºF, 72ºF (room temperature), 45ºF, and then 32ºF (freezing point). This is how solutions are purified on an industrial scale. Some potassium nitrate is made from sodium nitrate by dissolving potassium chloride and sodium nitrate together in water, and then slowly cooling the solution, or letting the water evaporate. Potassium nitrate is far less soluble at low temperature than sodium nitrate, so, potassium nitrate crystals form in the solution, while sodium chloride forms and stays dissolved at the lower temperature and/or higher molar concentration. Notice that this began with sodium nitrate without any potassium nitrate being present, but ended up with potassium nitrate, all of it happened while the two salts, potassium chloride and sodium nitrate were dissolved in water, and ended with potassium nitrate crystallizing forming. This reaction is how it was discovered that some particular kinds of molecules actually break apart when they dissolve in water, and the separate parts can reform into different kinds of molecules when the water evaporates.
Back in 1964, I got a 100 pound sack of agricultural grade potassium nitrate that came from Chile, where there are huge sodium nitrate deposits in the Atacama Desert. It looked like brown sugar, and was full of sand and rust flakes. So, I had to dissolve it in water and crystallize it out. It wasn't all that difficult, but took a long time. I didn't do the entire 100 pounds at once since I didn't have a large enough corrosion proof tub.