You can do a blood test. If your blood turns brown, the SN is good. You can also do an aquarium test. It's ok to open the SN to test it. Just reseal the bag and store it in a dark airtight container. It will be fine. The SN won't oxidize and turn into sodium nitrate instantly when you open it . This only happens if it has had a prolonged exposure to air, not if you open it up for a minute to use some. Years ago, one of my aunts used to make homemade sausages. She bought some SN that came in a glass jar. One jar lasted her for years. She would open up the jar every few months in order to measure some out. It never went bad.
It's possible that the company you bought it from sells both lab and food grade SN. Both have undergone rigorous testing and are good. Usually
the food grade is a little lower quality than the lab grade. The Lab Grade normally undergoes more rigorous testing on it to determine all impurities. The lab that uses it is using it as a reagent and they need to know all that. Food grade means that it has undergone testing, and it is safe for people to add it to food. There are no impurities in it that would harm someone.
Pure SN has a purity of 95 -99 percent. The percentage of impurities in it varies depending on how it's manufactured, but a purity of at least 95 percent is good. You should go by the manufacturer's label on the bottle to determine the purity not by what customer service says. Customer service never knows anything. There is no reason why a reputable company would lie to you about the purity of their products and risk ruining their reputation and going out of business. SN isn't illegal.
