Ocean acidification is a direct consequence of the atmospheric CO2 increase and represents a threat for marine ecosystems, particularly in the Arctic. High-quality seawater pH measurements with good spatial and temporal coverage are required to apprehend the ocean acidification phenomena. We are working to develop a high-accuracy, high-resolution pH sensor that has the potential to allow global ocean acidification mapping through deployment on fleets of ARGO floats and other autonomous platforms already in existence. The instrument implements a novel hybrid approach, utilizing the two different and complementary measurement techniques (potentiometric and colorimetric) to generate temporally dense and highly accurate pH data. Here we present the concept and initial results obtained from a hybrid pH sensor. Results show that the potentiometric part of the sensor is capable to operate in real ocean pressure and temperature conditions, including near-freezing temperatures typical of Arctic environmental conditions. The colorimetric part provides a stable reference to perform periodic recalibrations and remove drift.