Brass analysis. Image 5 of 7. Performing a titration to determine the percentage of copper in brass. This requires sodium thiosulphate solution. A titration is the process of determining the concentration of a chemical by slowly adding a reagent of known concentration with which it has a well understood reaction. When the reaction is completed the amount of reagent not yet added is proportional to the concentration of a chemical in the solution. From the quantity of iodine we can calculate the mass of copper in the brass. The stages in this experiment are shown in images A500/566-A500/572.