An electrochemical sensor of a downhole tool positionable in a wellbore penetrating a subterranean formation is provided. The downhole tool has a flowline to receive downhole fluid. The electrochemical sensor includes a bulkhead carried by the downhole tool, a membrane exposed to the downhole fluid in the flowline, and an electrochemical cell supported by the bulkhead about the membrane. The electrochemical cell includes a plurality of cell sensors, at least one cell electrode, and a sensing solution. The plurality cell sensors include an analyte sensor, a temperature sensor, and a pH sensor. The sensing solution is reactive to certain analytes in the downhole fluid to generate electrical signals measurable by the plurality of cell sensors whereby parameters of chemicals in the downhole fluid may be measured.