Homebrew PCBs

hallo!
i just bhought 2 litre of ammonium persulphate, since im tired of 
the mass ferric chloride makes. my only problem is that i have to 
fill a 3 litre tank with the 2 litre i have. can i mix ammonium 
persulphate with water??? and if so- how much water can i use on 
each litre of ammonium persulphate so it will keep working well?
one more question- what is the shelf life of ammonium persulphate? i 
read that once you heat it up and cool it down it wont work again. 
is that true? and what is the optimal temperature to use ammonium 
persulphate????

thanks a lot
mumin