That's not something I'd use to make a proper recording. If I had an iPhone, I'd probably go with something like this:
AmpliTube iRig - Walmart.com
I've heard good things about Amplitube, and that's pretty cheap if even just to knock around with it a bit. Also, if your acoustic isn't wired, then just mic to it. Gotta be better than the default mic in the phone. Nice little baroque piece, btw.