Analytical Chem istry - DePauw University

Chapter 9 Titrimetric Methods441When CO 2 is absorbed, Na 2 CO 3 precipitates and settles to the bottom ofthe container, allowing access to the carbonate-free NaOH. When preparinga solution of NaOH, be sure to use water that is free from dissolvedCO 2 . Briefly boiling the water expels CO 2 , and after cooling, it may be usedto prepare carbonate-free solutions of NaOH. A solution of carbonate-freeNaOH is relatively stable f we limit its contact with the atmosphere. Standardsolutions of sodium hydroxide should not be stored in glass bottlesas NaOH reacts with glass to form silicate; instead, store such solutions inpolyethylene bottles.In o r g a n i c An a l y s i sAcid –base titrimetry is a standard method for the quantitative analysis ofmany inorganic acids and bases. A standard solution of NaOH can beused to determine the concentration of inorganic acids, such as H 3 PO 4 orH 3 AsO 4 , and inorganic bases, such as Na 2 CO 3 can be analyzed using astandard solution of HCl.An inorganic acid or base that is too weak to be analyzed by an aqueousacid–base titration can be analyzed by adjusting the solvent, or by an indirectanalysis. For example, when analyzing boric acid, H 3 BO 3 , by titratingwith NaOH, accuracy is limited by boric acid’s small acid dissociationconstant of 5.8 10 –10 . Boric acid’s K a value increases to 1.5 10 –4 in thepresence of mannitol, because it forms a complex with the borate ion. Theresult is a sharper end point and a more accurate titration. Similarly, theanalysis of ammonium salts is limited by the small acid dissociation constantof 5.7 10 –10 for NH 4 + . In this case, we can convert NH 4 + to NH 3by neutralizing with strong base. The NH 3 , for which K b is 1.58 10 –5 , isthen removed by distillation and titrated with HCl.We can analyze a neutral inorganic analyte if we can first convert it intoan acid or base. For example, we can determine the concentration of NO 3–by reducing it to NH 3 in a strongly alkaline solution using Devarda’s alloy,a mixture of 50% w/w Cu, 45% w/w Al, and 5% w/w Zn.Figure 9.16a shows a typical result for thetitration of H 3 BO 3 with NaOH.− − −3NO ( aq) + 8Al() s + 5OH ( aq) + 2HO() l →8AlO( aq) + 3NH ( aq)3 223The NH 3 is removed by distillation and titrated with HCl. Alternatively,we can titrate NO 3 – as a weak base by placing it in an acidic nonaqueoussolvent such as anhydrous acetic acid and using HClO 4 as a titrant.Acid–base titrimetry continues to be listed as a standard method for thedetermination of alkalinity, acidity, and free CO 2 in waters and wastewaters.Alkalinity is a measure of a sample’s capacity to neutralize acids. The mostimportant sources of alkalinity are OH – , HCO 3 – , and CO 3 2– , althoughother weak bases, such as phosphate, may contribute to the overall alkalinity.Total alkalinity is determined by titrating to a fixed end point pH of 4.5(or to the bromocresol green end point) using a standard solution of HClor H 2 SO 4 . Results are reported as mg CaCO 3 /L.Although a variety of strong bases andweak bases may contribute to a sample’salkalinity, a single titration cannot distinguishbetween the possible sources. Reportingthe total alkalinity as if CaCO 3 isthe only source provides a means for comparingthe acid-neutralizing capacities ofdifferent samples.