On-line instruments for measurement of total organic carbon (TOC). The procedure is continuous, it lasts for 6 minutes and is based on the addition in the sample of an acid, release of inorganic carbon with degassing, addition of persulfate and UV radiation and measurement of the produced carbon dioxide with NDIR detector. With applications in Environmental sector (waste water control), Pharmaceuticals (injection water), Petrochemicals, Chemical Industry, Paper industry and in Energy Plants (quality of cooling water, water of boilers, etc). G23