A PH sensor likes really low bias current, but usually no need for sub µV stability. There are a few Az OP (e.g. max4238, LTC2050), that may be suitable, but not all. The lower noise ones tend to have more bias, up to the > 100 pA range.
A precision CMOS OP (e.g. lmc6081) would be an option too: less bias, but more drift and low frequency noise.