Examples include the Collins reagent, Fenton's reagent, and Grignard reagents. ATCC also serves to set standards for biological reagent and assay quality. Sodium azide is a useful probe reagent and a preservative. Water is a common reagent in enzymatic catalysis. Oxalic acid is an important reagent in lanthanide chemistry. Silver iodide is used as a reagent. Ammonia solution can dissolve silver oxide residues, such as that formed from Tollens' reagent. Addition of preformed Grignard reagent is often used as the initiator. One method involves its conversion to the Grignard reagent, phenylmagnesium bromide. It can act as a phase transfer catalyst reagent in selected reactions. Experiments have shown that cysteine is also reactive to the reagent. It is a useful reagent for generating organobromine compounds. It is also a useful laboratory chemical reagent and precursor. Sulfur trioxide is an essential reagent in sulfonation reactions. Tripotassium phosphate has few applications except as a basic reagent in organic synthesis.