every technical industry has its standard practices, procedures, materials, equipment, etc. when a technician reaches for a certain material that is of a certain grade, he counts on that being what is called for and by god it better meet the specs.
i have seen assays on different grades of Iodine and KI and the reagent grade(from some manufacturers) has less mercury in it than the usp grade. usp or better is all you need to know, the better grades being reagent and acs.
Below is a list of chemical grades in order of purity, from highest to lowest.
Highest quality; often equals or exceeds the latest purity standards set by the American Chemical Society (ACS). This is the only universally accepted standard. Chemicals are of the highest purity attainable.
Purity is generally equal to ACS grade. This grade is suitable for analytical work and is more than adequate for general lab use.
A grade of sufficient purity to pass certain tests prescribed by the US Pharmacopoeia (USP); acceptable for drug use. USP grade may be used for most laboratory purposes.
A grade of sufficient purity to meet the standards of the National Formulary (NF).
An upper-level intermediate quality. Exact impurities may not be known; however, lab grade is usually pure enough for most educational laboratories.
Purified, pure, or practical grade; a lower-level intermediate quality. Although this grade does contain impurities, it is usually pure enough for use in educational laboratories.
A good-quality grade used industrially. Use caution when substituting for reagent-grade or lab-grade chemicals.
When making a solution, one must first decide what degree of chemical purity is needed. Many types of chemicals are available in several grades of purity. Most of the chemicals listed in our catalog belong to one of the following 3 categories, in order from least pure to most pure.