Analytical Chem istry - DePauw University

Chapter 9 Titrimetric Methods497der acidic conditions. This interference is eliminated by adding sodiumazide, NaN 3 , reducing NO 2 – to N 2 . Other reducing agents, such as Fe 2+ ,are eliminated by pretreating the sample with KMnO 4 , and destroying theexcess permanganate with K 2 C 2 O 4 .Another important example of redox titrimetry is the determination ofwater in nonaqueous solvents. The titrant for this analysis is known as theKarl Fischer reagent and consists of a mixture of iodine, sulfur dioxide, pyridine,and methanol. Because the concentration of pyridine is sufficientlylarge, I 2 and SO 2 react with pyridine (py) to form the complexes py•I 2 andpy•SO 2 . When added to a sample containing water, I 2 is reduced to I – andSO 2 is oxidized to SO 3 .pyiI + pySO i + py+ H O→ 2pyHI i + pySO i2 2 23Methanol is included to prevent the further reaction of py•SO 3 with water.The titration’s end point is signaled when the solution changes from theproduct’s yellow color to the brown color of the Karl Fischer reagent.Or g a n i c An a l y s i sRedox titrimetry also is used for the analysis of organic analytes. Oneimportant example is the determination of the chemical oxygen demand(COD) of natural waters and wastewaters. The COD provides a measureof the quantity of oxygen necessary to completely oxidize all the organicmatter in a sample to CO 2 and H 2 O. Because no attempt is made to correctfor organic matter that can not be decomposed biologically, or forslow decomposition kinetics, the COD always overestimates a sample’strue oxygen demand. The determination of COD is particularly importantin managing industrial wastewater treatment facilities where it is used tomonitor the release of organic-rich wastes into municipal sewer systems orthe environment.A sample’s COD is determined by refluxing it in the presence of excessK 2 Cr 2 O 7 , which serves as the oxidizing agent. The solution is acidified withH 2 SO 4 using Ag 2 SO 4 to catalyze the oxidation of low molecular weightfatty acids. Mercuric sulfate, HgSO 4 , is added to complex any chloride thatis present, preventing the precipitation of the Ag + catalyst as AgCl. Underthese conditions, the efficiency for oxidizing organic matter is 95–100%.After refluxing for two hours, the solution is cooled to room temperatureand the excess Cr 2 O 7 2– is determined by back titrating using ferrous ammoniumsulfate as the titrant and ferroin as the indicator. Because it is difficultto completely remove all traces of organic matter from the reagents, ablank titration must be performed. The difference in the amount of ferrousammonium sulfate needed to titrate the sample and the blank is proportionalto the COD.Iodine has been used as an oxidizing titrant for a number of compoundsof pharmaceutical interest. Earlier we noted that the reaction of S 2 O 32–