COD vials contain several hazardous reagents. Mercuric sulfate is required by regulating authorities to compensate for chloride interference. Potassium dichromate is the active color changing species in the reaction. Sulfuric acid gets the solution into an acceptable pH range for the reaction to occur. Silver sulfate serves as a catalyst for the oxidation of some of the more complex organic compounds. All four of these reagents qualify the mixture for the hazardous list. Please email me at davids@envexp.com if you would like a MSDS sheet for complete documentation.