Graphene field effect transistors (FETs) are attractive candidates for sensing applications because of their high charge carrier mobility and the ideal coupling between graphene charge carriers and surface potential. However, depositing a selective layer on the graphene for these sensing applications can degrade graphene’s electrical properties, increase hysteresis, and present a challenge for maintaining its sensitivity and stability. Here, we protect the graphene by encapsulating it with an ultrathin layer (4-8 nm) of parylene, a hydrophobic polymer, and then deposit 3-5 nm sensing layers, either aluminum oxide or tantalum pentoxide for sensing pH. We demonstrate gate capacitances approaching the quantum limit with 0.6 uF/cm² and near Nernstian pH sensitivities of 55.2 mV/pH. We also observe significant improvements in field effect mobilities of 7000 cm²V^-1s^-1 and in transconductance with limited hysteresis compared to previous work. The improvements due to encapsulation have resulted in a detection limit of 0.1 mpH at a 60 Hz electrical bandwidth. We demonstrated our pH response by monitoring the change in acidity of carbonated water in real time.