A chemical sensor may include an electrode array for electrically interfacing with a fluid sample. The sensor can apply an electrical potential to the sample in order to effect a current flow within the sample. The sensor can measure the resulting current through the sample and determine characteristics about the fluid sample from the current measurement. In one mode of operation of the sensor, the applied electrical potential can be controlled to cause desired electrochemical reactions, such as oxidation or reduction, to occur within the sample to determine the concentration of the oxidized or reduced sample constituent. In another mode of operation, the applied electrical potential causes a current to flow simply due to the conductivity of the sample. In various embodiments, the sensor comprises a controller and a switch for switching between various modes of operation and applying appropriate electric potentials to the sample.