Be aware that not all chemicals in our labs and storage room are fresh. What is fresh enough will depend on what is the use and also in many cases on the head space in bottles and how frequently they have been opened (how much air, and hence oxygen, has been in contact with the contents of the bottle). For many chemicals the recommended protocol of marking the date when the bottle has been opened for the first time has been followed, but in other cases it has not. Avoid headaches and inconsistencies in the results of your lab work by always checking opening dates on bottles, and information about shelf-life, to make sure that you are always using fresh reactants of the required purity. We have noticed more than once that some of the chemicals in our shelves are well beyond their recommended storage life, in some cases, by many years.
This is a frequently posed question, that has no unique or simple answer. Prof. Lars Olof Björn has written a section on this in his book Photobiology: the science of life and light which is much more detailed than this short post. The problem with this question is that its meaning can be different to different persons. I will start by separating different aspects of this question into separate, and better-defined, questions that are easier to answer: Continue reading