Analytical Chem istry - DePauw University

Chapter 9 Titrimetric Methods443Table 9.6 Relationship Between End Point Volumes andSources of AlkalinitySource of Alkalinity Relationship Between End Point VolumesOH – V pH 4.5 = V pH 8.32–CO 3 V pH 4.5 = 2 V pH 8.3–HCO 3 V pH 4.5 > 0; V pH 8.3 = 0OH – 2–and CO 3 V pH 4.5 < 2 V pH 8.3CO 2– –3 and HCO 3 V pH 4.5 > 2 V pH 8.3and Mn 2+ . In addition, weak acid acidity may include a contribution fromorganic acids.Acidity is determined by titrating with a standard solution of NaOHto fixed pH of 3.7 (or the bromothymol blue end point) and a fixed pH8.3 (or the phenolphthalein end point). Titrating to a pH of 3.7 provides ameasure of strong acid acidity, and titrating to a pH of 8.3 provides a measureof total acidity. Weak acid acidity is the difference between the totaland strong acid acidities. Results are expressed as the amount of CaCO 3that can be neutralized by the sample’s acidity. An alternative approach fordetermining strong acid and weak acid acidity is to obtain a potentiometrictitration curve and use a Gran plot to determine the two equivalence points.This approach has been used, for example, to determine the forms of acidityin atmospheric aerosols. 4Water in contact with either the atmosphere, or with carbonate-bearingsediments contains free CO 2 that exists in equilibrium with CO 2 (g) andaqueous H 2 CO 3 , HCO – 3 , and CO 2– 3 . The concentration of free CO 2 isdetermined by titrating with a standard solution of NaOH to the phenolphthaleinend point, or to a pH of 8.3, with results reported as mg CO 2 /L.This analysis is essentially the same as that for the determination of totalacidity, and can only be applied to water samples that do not contain strongacid acidity.As is the case with alkalinity, acidity is reportedas mg CaCO 3 /L.Free CO 2 is the same thing as CO 2 (aq).Or g a n i c An a l y s i sAcid–base titrimetry continues to have a small, but important role for theanalysis of organic compounds in pharmaceutical, biochemical, agricultural,and environmental laboratories. Perhaps the most widely employed acid–base titration is the Kjeldahl analysis for organic nitrogen. Examples ofanalytes determined by a Kjeldahl analysis include caffeine and saccharin inpharmaceutical products, proteins in foods, and the analysis of nitrogen infertilizers, sludges, and sediments. Any nitrogen present in a –3 oxidationstate is quantitatively oxidized to NH 4 + . Because some aromatic heterocycliccompounds, such as pyridine, are difficult to oxidize, a catalyst is usedto ensure a quantitative oxidation. Nitrogen in other oxidation states, suchSee Representative Method 9.1 for oneapplication of a Kjeldahl analysis.4 Ferek, R. J.; Lazrus, A. L.; Haagenson, P. L.; Winchester, J. W. Environ. Sci. Technol. 1983, 17,315–324.