Abstract: A corrosive liquid such as sulphuric acid is concentrated by boiling in an evaporator vessel having a thermosyphon loop connected to the bottom of the vessel. Electrodes of carbon which is essentially free of any organic binders are located in one leg of the thermosyphon loop to heat the liquid by current conducted through the liquid. The electrodes are located well below the liquid surface so that boiling occurs only above the electrode level, reducing erosion of the electrodes. The electrodes are blanketed with air which flows through the porous carbon into the liquid, thereby protecting the seals and electrode connections and also introducing air into the thermosyphon loop to promote circulation of the liquid in the loop.