If too much reagent is taken for a particular test, the excess should not be returned to the reagent bottle.?

Question

Why is this so?

Answer 1

That is correct, because contamination may have occurred. Also, another thing to consider is some reagents lose their potency when exposed to the atmosphere or oxidation can occur as well. (usually that is why they are stored in amber glass bottles)

Answer 2

Posible contamination of the excess reagent. the contamination could lead to a false test by some one else or your self during a later test.

Answer 3

It is to prevent possible contamination of the material still in the container during the return (something on the spatula or weighing paper for example). It is considered good laboratory practice to not return materials.