NO! DO *NOT* use a ratchet to release that tension!
I did the same thing and ended up screwing the tensioner up and having to replace it! Plus, you run the risk of rounding off that bolt (That's the other thing I did), not to mention that there's no good way to get a ratchet in there and still get enough leverage without a pipe...hence you will round off the bolt.
Your best bet is to get a tensioner wrench from Sears, it's only about $30-40 (A long metal piece that will either have bits you can put in, or will be fit to size of 13mm) and use that. I'd caution anyone using a ratchet when changing out the serpentine belt on a Zetec, unless you are going to change out the tensioner anyways.
Also, while you've got the belt off, spin the idler pulley (Top middle pulley, may be very shiny where the belt touches it) and see if there's any amount of grab to it. If it doesn't spin very freely, you may need to replace this too.
