Several grades of fixative are available:

Practical grade which is used for general fixation:
EM grade for use in electron microscopy. It is stable for over six months, has a pH 5 – 6 and proves to be an excellent fixative for structural studies.
It should also be used in histochemical or immunological reactions.

Double distilled grade which has a very low cation concentration:
Vacuum distilled grade is purified to remove all polymerised material and has no UV absorption at 235nm. It is packed in neutral glass under nitrogen for best results with enzyme histochemistry.

Any distilled glutaraldehyde is relatively unstable and has a high risk of polymerising if it is not handled properly.
It is therefore recommended that this grade of material is only purchased for use within a 3 to 4 week period and is carefully stored at 4 °C without continued defrosting and recapping.