Analytical Chem istry - DePauw University

Chapter 10 Spectroscopic Methods577with an aqueous solution containing an appropriate metal ion, a coloredmetal–dithizonate complex forms that is soluble in CHCl 3 . The selectivityof dithizone is controlled by adjusting the sample’s pH. For example, Cd 2+is extracted from solutions that are made strongly basic with NaOH, Pb 2+from solutions that are made basic with an NH 3 /NH 4 + buffer, and Hg 2+from solutions that are slightly acidic.When chlorine is added to water the portion available for disinfectionis called the chlorine residual. There are two forms of chlorine residual. Thefree chlorine residual includes Cl 2 , HOCl, and OCl – . The combined chlorineresidual, which forms from the reaction of NH 3 with HOCl, consistsof monochloramine, NH 2 Cl, dichloramine, NHCl 2 , and trichloramine,NCl 3 . Because the free chlorine residual is more efficient at disinfection,there is an interest in methods that can distinguish between the differentforms of the total chlorine residual. One such method is the leucocrystal violet method. The free residual chlorine is determined by addingleuco crystal violet to the sample, which instantaneously oxidizes to givea blue colored compound that is monitored at 592 nm. Completing theanalysis in less than five minutes prevents a possible interference from thecombined chlorine residual. The total chlorine residual (free + combined)is determined by reacting a separate sample with iodide, which reacts withboth chlorine residuals to form HOI. When the reaction is complete, leucocrystal violet is added and oxidized by HOI, giving the same blue coloredproduct. The combined chlorine residual is determined by difference.The concentration of fluoride in drinking water may be determinedindirectly by its ability to form a complex with zirconium. In the presenceof the dye SPADNS, solutions of zirconium form a red colored compound,called a lake, that absorbs at 570 nm. When fluoride is added, the formationof the stable ZrF 6 2– complex causes a portion of the lake to dissociate,decreasing the absorbance. A plot of absorbance versus the concentrationof fluoride, therefore, has a negative slope.Spectroscopic methods also are used to determine organic constituentsin water. For example, the combined concentrations of phenol, and orthoandmeta- substituted phenols are determined by using steam distillation toseparate the phenols from nonvolatile impurities. The distillate reacts with4-aminoantipyrine at pH 7.9 ± 0.1 in the presence of K 3 Fe(CN) 6 , forminga yellow colored antipyrine dye. After extracting the dye into CHCl 3 , itsabsorbance is monitored at 460 nm. A calibration curve is prepared usingonly the unsubstituted phenol, C 6 H 5 OH. Because the molar absorptivityof substituted phenols are generally less than that for phenol, the reportedconcentration represents the minimum concentration of phenoliccompounds.Molecular absorption also can be used for the analysis of environmentallysignificant airborne pollutants. In many cases the analysis is carriedout by collecting the sample in water, converting the analyte to an aqueousform that can be analyzed by methods such as those described in TableIn Chapter 9 we explored how the totalchlorine residual can be determined by aredox titration; see Representative Method9.3 for further details. The methoddescribed here allows us to divide the totalchlorine residual into its componentparts.SPADNS, which is shown below, isan abbreviation for the sodium salt of2-(4-sulfophenylazo)-1,8-dihydroxy-3,6-napthalenedisulfonic acid, which is amouthful to say.NaOO 3 SOHOHNSO 3 NaONH 24-aminoantipyreneSO 3 Na