A sensor for the in situ detection of a target chemical species, comprising a gas permeable membrane having a sampling side and an opposing analytical side, wherein the sampling side of the membrane is capable of receiving a sample and the membrane is permeable to target chemical species present in the sample. A weak acid or a weak base is in contact with the analytical side of the membrane, and a conductivity detector is in contact with the weak acid or weak base. In use, target chemical species present in the sample permeate through the membrane and react with the weak acid or weak base, producing ionic species and changing the conductivity.