Brass analysis. Image 7 of 7. Reading the burette at the end of a titration. The volume of sodium thiosulphate, the reagent, left at the end of the titration indicates how much iodine, the chemical being tested, was present in the mixture. Using this result the proportion of copper in the brass sample can be calculated from the equation of the initial reaction: 2Cu2+ + 4I- -> 2CuI + I2. This reaction and all the other previous stages in the process of analysing brass can be found in images A500/566-A500/571.