5. _Agar._—The agar used shall be of the best quality and shall be dried
for one-half hour at 105° C. before weighing. Much of the agar on the
market contains considerable amounts of sea salts.[221][225][228] These
may be removed by soaking in water and draining before use.
6. _Gelatin._—The gelatin used shall be of light color, shall contain
not more than a trace of arsenic, copper, sulfides, and shall be free
from preservatives, and of such a melting point that a 10 per cent.
standard nutrient gelatin shall have a melting point of 25° C. or over.
Gelatin shall be dried for one-half hour at 105° C. before weighing.
7. _Litmus._—Reagent litmus of highest purity (not litmus cubes) or
azolitmin (Kahlbaum) shall be used for all media requiring a litmus
indicator.
8. _General Chemicals._—Special effort shall be made to have all the
other ingredients used for culture media chemically pure.