Does SR protect against persistent spells?

[SangoProduction]

Say, just for sake of argument, that there is a spell that creates a pool of acid for 1 minute / level, and it is magical acid, rather than magically enhanced acid, so it affected by spell resistance.

If a creature with Spell Resistance walks through the acid, and the spell doesn't pierce the Spell Resistance, obviously it does not take any damage.

But is that first interaction with spell resistance the only chance the pool of acid gets?

[Aracor]

Yes. From d20srd.org:
Check spell resistance only once for any particular casting of a spell or use of a spell-like ability. If spell resistance fails the first time, it fails each time the creature encounters that same casting of the spell. Likewise, if the spell resistance succeeds the first time, it always succeeds. If the creature has voluntarily lowered its spell resistance and is then subjected to a spell, the creature still has a single chance to resist that spell later, when its spell resistance is up.

[SangoProduction]

Yes. From d20srd.org:

Thank you very much, good sir.

[PrismCat21]

a spell that creates a pool of acid for 1 minute / level, and it is magical acid, rather than magically enhanced acid,

Make sure that the spell is not SR:No, that it's not a Conjuration (creation) spell, and not an Instantaneous effect.
If it is any of those, it won't work. That last two making it a real physical substance.