Phonological rule for realizations of Italian /s/

Question

It seems that realization of Italian /s/ is not fully systematic and there are some exceptions. Is there any phonological rule for possible realizations of Italian /s/

Answer 1

While the link in ukemi's comment gives a good description of the rules governing S-voicing, the main question here has a simple answer.

/s/ and /z/ are separate phonemes in (standard/Tuscan) Italian.

This is shown by (near-)minimal pairs such as cosa /ko.sa/ "matter" and sposa /spo.za/ "bride". Both of these come directly from single Latin words, so the rules laid down by Oostendorp don't apply.

(EDIT: Michaelyus has provided a true minimal pair: presento /pre.zɛn.to/ "I present" versus presento /pre.sɛn.to/ "I foresee". Much obliged!)

In many environments, this distinction is neutralized: in fact, everywhere except between vowels within a single mono-morphemic word. In most dialects it's even neutralized in this environment—in the north they merge into /z/, while in the south they merge into /s/. So in the (relatively) near future, Italian will likely have only a single dental sibilant phoneme, just as Latin did.