Right � on the day, AB ate my attempts to reply, here comes another try. Pure chlorine is a gas at standard atmospheric conditions and highly toxic. For domestic and many other users it is supplied loosely trapped in water. I would guess that a metal bottle with a good seal will have retained the gas well and a plastic one perhaps less so. The only way chlorine can go off is escape into the atmosphere (i.e. slowly evaporate) with a corresponding reduction in the potency of the stock. I would suggest using it as instructed (whatever you do don't have a good sniff of the bottle) and if you can smell the pool water then there is chlorine there - otherwise you will simply not have enough chlorine to serve the purpose and you could not possibly have done any harm. Simply put, it does not expire in the way a tin of steak pie would. Only a lab will tell you the exact condition of the stuff. Note that the mixing rate should be stated in volume of water dilution - the linear size of the pool (feet or metres) is not the correct measure.