THERMOSHIELD C is a liquid concentrate for use as a neutralising agent in the machine reprocessing of medical instruments following the use of an alkaline product such as THERMOSHIELD basiX or THERMOSHIELD Xtreme in the cleaning step. The acidic pH value of THERMOSHIELD C keeps both instruments and the machine free of inorganic deposits and, especially, lime residues. By using THERMOSHIELD C in neutralisation it is possible to protect especially sensitive materials and prevent entrained remains of alkali from the cleaning step.