Chemistry - Difference between lab-grade and food-grade purity?
Solution 1:
So-called food-grade stuff is meant to be consumed by humans. It's important that every single compound of a foodstuff is food-grade or at least GRAS. In the European Union the law giver has defined the state when a compound is good for consumption as "save".
Lab-grade chemicals often contain toxic materials, so when something is called lab-grade it is not save per definition.
For example: $\ce{NaOH}$ is widely used in preparation of foodstuff as well as in the lab. Now it's a fact that lab-grade (standard) $\ce{NaOH}$ (Sigma Aldrich) contains a certain amount of heavy metals. On the other hand, food-grade $\ce{NaOH}$ will not contain any heavy metals (usually food-grade $\ce{NaOH}$ will have a purity of more than $99\%$ while standard $\ce{NaOH}$ pellets are sold with purities less than $97\%$). So using lab-grade $\ce{NaOH}$ in foodstuff is a crime (at least in the EU).
So my advice is: Do not use lab-grade materials for any food-related applications.
Solution 2:
Generally laboratory grade reagents have not been prepared with human consumption as a consideration, but with regard to their use as a chemical reagent.
For example, $95\%$ ethanol is mostly ethanol and $5\%$ water. This is fine for cleaning equipment, or using as a solvent for a TLC. If you want to use ethanol as a reagent, however, you want to use absolute ethanol (which is much more expensive). Absolute ethanol is NOT safe for human consumption though, as the water has been removed from the $95\%$ (which is the azeotrope, i.e. as far as you can get through distillation of water/ethanol alone) through a process which involves adding additives such as benzene. Benzene won't cause side reactions in the way that water would, and so absolute ethanol is much better for a lab reagent than $95\%$, but worse for consumption.
In the same way, any chemical which is prepared for use as a reagent will contain impurities which don't generally cause side-reactions, while for consumption you want the impurities to be non-toxic.
Solution 3:
Background: I'm a chemist and have worked for companies in Pharma, Research Chemical Supply, and Dietary Supplements.
Lab Grade is different than Food Grade, yes. Normally Food Grade is a LOWER quality than Lab Grade.
Lab Grade normally has exhaustive testing on it to determine all impurities. The lab is using this as a reagent and they need to know all that.
Food grade means that it won't cause human disease. So really all they need to know is if there are too many heavy metals or too many bacterial contaminants. The limits are typically higher for food grade.
For instance, when I worked at a Pharmaceutical facility we worked to get a material accepted by FDA as a food grade material and not a Pharma Grade or Reagent Grade. The main reasons were insect and metal parts. The FDA regulations are much more relaxed for food grade and you can have quite a few more of each in the product for food grade than either of the other two.