D. Harvey - Modern Analytical Chemistry (794078), страница 100
Текст из файла (страница 100)
The oxidized DPD is then titrated back to its colorless form with ferrous ammonium sulfate, with the volume of titrant being proportional to the amount of free residual chlorine. Adding a small amount of KIreduces monochloramine, NH2Cl, forming I3–. The I3– then oxidizes a portion of1400-CH09 9/9/99 2:13 PM Page 345Chapter 9 Titrimetric Methods of Analysisthe DPD to its red-colored form. Titrating the oxidized DPD with ferrous ammonium sulfate yields the amount of NH 2 Cl in the sample. The amount ofdichloramine and trichloramine are determined in a similar fashion.The methods described earlier for determining the total, free, or combinedchlorine residual also are used in establishing the chlorine demand of a water supply.
The chlorine demand is defined as the quantity of chlorine that must beadded to a water supply to completely react with any substance that can be oxidized by chlorine while also maintaining the desired chlorine residual. It is determined by adding progressively greater amounts of chlorine to a set of samplesdrawn from the water supply and determining the total, free, or combined chlorine residual.Another important example of redox titrimetry that finds applications in bothpublic health and environmental analyses is the determination of dissolved oxygen.In natural waters the level of dissolved O2 is important for two reasons: it is themost readily available oxidant for the biological oxidation of inorganic and organicpollutants; and it is necessary for the support of aquatic life. In wastewater treatment plants, the control of dissolved O2 is essential for the aerobic oxidation ofwaste materials.
If the level of dissolved O2 falls below a critical value, aerobic bacteria are replaced by anaerobic bacteria, and the oxidation of organic waste producesundesirable gases such as CH4 and H2S.One standard method for determining the dissolved O2 content of natural waters and wastewaters is the Winkler method. A sample of water is collected in a fashion that prevents its exposure to the atmosphere (which might change the level ofdissolved O2). The sample is then treated with a solution of MnSO4, and then with asolution of NaOH and KI. Under these alkaline conditions Mn2+ is oxidized toMnO2 by the dissolved oxygen.2Mn2+(aq) + 4OH–(aq) + O2(aq) → 2MnO2(s) + 2H2O(l)After the reaction is complete, the solution is acidified with H2SO4. Under the nowacidic conditions I– is oxidized to I3– by MnO2.MnO2(s) + 3I–(aq) + 4H3O+(aq) → Mn2+(aq) + I3–(aq) + 6H2O(l)The amount of I3– formed is determined by titrating with S2O32– using starch as anindicator.
The Winkler method is subject to a variety of interferences, and severalmodifications to the original procedure have been proposed. For example, NO2– interferes because it can reduce I3– to I– under acidic conditions. This interference iseliminated by adding sodium azide, NaN3, reducing NO2– to N2. Other reducingagents, such as Fe2+, are eliminated by pretreating the sample with KMnO4, and destroying the excess permanganate with K2C2O4.Another important example of a redox titration for inorganic analytes, which isimportant in industrial labs, is the determination of water in nonaqueous solvents.The titrant for this analysis is known as the Karl Fischer reagent and consists of amixture of iodine, sulfur dioxide, pyridine, and methanol. The concentration ofpyridine is sufficiently large so that I2 and SO2 are complexed with the pyridine (py)as py I2 and py SO2.
When added to a sample containing water, I2 is reduced toI–, and SO2 is oxidized to SO3.⋅⋅py ⋅ I + py ⋅ SO + py + H O → 2py ⋅ HI + py ⋅ SOMethanol is included to prevent the further reaction of py ⋅ SO with water. Thetitration’s end point is signaled when the solution changes from the yellow color of22233the products to the brown color of the Karl Fischer reagent.3451400-CH09 9/9/99 2:13 PM Page 346346Modern Analytical ChemistryOrganic Analysis Redox titrimetric methods also are used for the analysis of organic analytes.
One important example is the determination of the chemical oxygendemand (COD) in natural waters and wastewaters. The COD provides a measure ofthe quantity of oxygen necessary to completely oxidize all the organic matter in asample to CO2 and H2O. No attempt is made to correct for organic matter that cannot be decomposed biologically or for which the decomposition kinetics are veryslow.
Thus, the COD always overestimates a sample’s true oxygen demand. The determination of COD is particularly important in managing industrial wastewatertreatment facilities where it is used to monitor the release of organic-rich wastesinto municipal sewer systems or the environment.The COD is determined by refluxing the sample in the presence of excessK2Cr2O7, which serves as the oxidizing agent. The solution is acidified with H2SO4,and Ag2SO4 is added as a catalyst to speed the oxidation of low-molecular-weightfatty acids. Mercuric sulfate, HgSO4, is added to complex any chloride that is present, thus preventing the precipitation of the Ag+ catalyst as AgCl.
Under these conditions, the efficiency for oxidizing organic matter is 95–100%. After refluxing for2h, the solution is cooled to room temperature, and the excess Cr 2O72– is determined by a back titration, using ferrous ammonium sulfate as the titrant and ferroin as the indicator. Since it is difficult to completely remove all traces of organicmatter from the reagents, a blank titration must be performed. The difference in theamount of ferrous ammonium sulfate needed to titrate the blank and the sample isproportional to the COD.Iodine has been used as an oxidizing titrant for a number of compounds ofpharmaceutical interest.
Earlier we noted that the reaction of S2O32– with I3– produces the tetrathionate ion, S4O62–. The tetrathionate ion is actually a dimer consisting of two thiosulfate ions connected through a disulfide (-S-S-) linkage. In thesame fashion, I3– can be used to titrate mercaptans of the general formula RSH,forming the dimer RSSR as a product. The amino acid cysteine also can be titratedwith I3–. The product of this titration is cystine, which is a dimer of cysteine. Triiodide also can be used for the analysis of ascorbic acid (vitamin C) by oxidizing theenediol functional group to an alpha diketoneOHOHOOOHOOHOOH+ 2H+ + 2e –OOHOand for the analysis of reducing sugars, such as glucose, by oxidizing the aldehydefunctional group to a carboxylate ion in a basic solution.HOHCOHCOHCO2–HCCHHOHCOHHCOHCH2OH+ 3OH–OHCHHCOHHCOHCH2OH+ 2H2O + 2e –1400-CH09 9/9/99 2:13 PM Page 347Chapter 9 Titrimetric Methods of AnalysisOrganic compounds containing a hydroxyl, carbonyl, or amine functionalgroup adjacent to a hydoxyl or carbonyl group can be oxidized using metaperiodate, IO4–, as an oxidizing titrant.IO4–(aq) + H2O(l) + 2e– → IO3–(aq) + 2OH–(aq)A two-electron oxidation cleaves the C—C bond between the two functionalgroups, with hydroxyl groups being oxidized to aldehydes or ketones, carbonylfunctional groups being oxidized to carboxylic acids, and amines being oxidized toan aldehyde and an amine (ammonia if the original amine was primary).
For example, treatment of serine with IO4– results in the following oxidation reactionOH NH3+H2CCHOCO2–+ 2OH–O+CH2HCCO2–+ NH4+ + H2O + 2e –The analysis is conducted by adding a known excess of IO4– to the solution containing the analyte and allowing the oxidation to take place for approximately 1 h atroom temperature. When the oxidation is complete, an excess of KI is added, whichreacts with the unreacted IO4– to form IO3– and I3–.IO4–(aq) + 3I–(aq) + H2O(l) → IO3–(aq) + I3–(aq) + 2OH–(aq)The I3– is then determined by titrating with S2O32– using starch as an indicator.Quantitative Calculations The stoichiometry of a redox reaction is given by the conservation of electrons between the oxidizing and reducing agents (see Section 2C); thusMoles of electrons lost× moles reducing agentMole reducing agentmoles of electrons gained=× moles oxidizing agentmole oxidizing agentExample 9.13 shows how this equation is applied to an analysis based on a directtitration.EXAMPLE 9.13The amount of Fe in a 0.4891-g sample of an ore was determined by a redoxtitration with K2Cr2O7.
The sample was dissolved in HCl and the iron broughtinto the +2 oxidation state using a Jones reductor. Titration to thediphenylamine sulfonic acid end point required 36.92 mL of 0.02153 MK2Cr2O7. Report the iron content of the ore as %w/w Fe2O3.SOLUTIONIn this titration the analyte is oxidized from Fe2+ to Fe3+, and the titrant isreduced from Cr2O72– to Cr3+.
Oxidation of Fe2+ requires only a single electron.Reducing Cr2O72–, in which chromium is in the +6 oxidation state, requires atotal of six electrons. Conservation of electrons for the redox reaction,therefore, requires thatMoles Fe2+ = 6 × moles Cr2O72–A conservation of mass relates the moles of Fe2+ to the moles of Fe2O3Moles Fe2+ = 2 × moles Fe2O33471400-CH09 9/9/99 2:13 PM Page 348348Modern Analytical ChemistryCombining the two conservation equations gives2 × moles Fe2O3 = 6 × moles Cr2O72–Making appropriate substitutions for the moles of Fe2O3 and Cr2O72– gives thefollowing equation2 × grams Fe2 O3= 6 × M Cr O 2 – × VCr O 2 −2 72 7FW Fe2 O3which we solve for the grams of Fe2O3.6 × M Cr O272−× VCr O2272−× FW Fe2 O3=(6)(0.02153 M)(0.03692 L)(159.69 g/mol)= 0.3808 g Fe2 O32Thus, the %w/w Fe2O3 in the sample of ore isgrams Fe2 O30.3808 g× 100 =× 100 = 77.86% w/w Fe2 O3grams sample0.4891 gAs shown in the following two examples, this approach is easily extended to situations that require an indirect analysis or a back titration.EXAMPLE 9.14A 25.00-mL sample of a liquid bleach was diluted to 1000 mL in a volumetricflask.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
