What is analytical grade salts Ehson is talking about?
AR grade (such as Analytical Reagent Grade) is a high grade of salt purity. Not of a salt mix, but of the individual components of a mix, such as sodium chloride, potassium bromide, etc. It is a set of specifications that is determined by groups such as the Amercian Chemical Society for each individual material, and then if someone meets those specifications, they can slap an AR grade label on the bottle.
The specifications limit things like heavy metals, specific contaminants that might be in each based on what it is or how it is made, etc. For example, the AR specifications for calcium hydroxide are:
AR (ACS) Grade Calcium Hydroxide Specifications
Assay Ca(OH)2 > 95%
Chloride Less than 0.03%
Magnesium Less than 0.5%
Iron Less than 0.05%
Potassium Less than 0.05%
Sodium Less than 0.05%
Strontium Less than 0.05%
Sulfur compounds (as sulfate) Less than 0.05%
Acid Insoluble Substances Less than 0.03%
Heavy Metals Less than 30 ppm
There are many other sorts of grades that one could equally use, such as pharmaceutical grade (USP, BP, etc.), food grade (FCC), water treatment grade (WCC) etc., or frankly, no grade at all if you were convinced it was appropriately pure (by your own testing or the manufacturer's, for example).
