# Distinguishing between potassium chloride and sodium chloride

I have these very large blocks of salt intended for a water softener in my house, but I'm not sure if they're potassium chloride or sodium chloride, according to what I've read they're both widely used for water softeners. I've thought about distinguishing them by weight but I don't have any sodium chloride with the same granularity as the salt blocks.

• If the blocks are pure salts, flame coloring may be possible. – TAR86 May 1 '20 at 20:27
• @TAR86 Flame coloring worked great, turned out it was sodium chloride in case anyone was wondering – 0x777C May 1 '20 at 22:45
• Wonder how pure the KCl for such a use would be. Sodium gives a very strong flame test, potassium much weaker. So for even like 5% sodium (or less..) you'd see a yellow Na flame. It would help to have a piece of cobalt blue glass to look though. – MaxW May 2 '20 at 0:49
• @MaxW According to this site they're going to be upwards of 99% purity blog.watertech.com/… – 0x777C May 2 '20 at 0:54
• Flame test is the best test for distinguishing alkali metal ion but it can also be distinguished chemically. In basic qualitative analysis, there is a method of distinguishing $\ce{Na+}$ and $\ce{K+}$. For $\ce{Na+}$, potassium pyroantimonate is used and for $\ce{K+}$ sodium cobaltinitrate/picric acid is used. See: chemistry.stackexchange.com/questions/51653/… – Nilay Ghosh May 2 '20 at 5:14

$$\ce{2 KCl + MgSO4 <=> MgCl2 + K2SO4}$$