Analytical Chem istry - DePauw University

372 Analytical Chemistry 2.0Each mole of Mg 2 P 2 O 7 contains two moles of phosphorous and eachmole of Mg(PO 3 ) 2 contains only one mole of phosphorous A conservationof mass, therefore, requires that two moles of Mg(PO 3 ) 2 formin place of each mole of Mg 2 P 2 O 7 . One mole of Mg 2 P 2 O 7 weighs222.6 g. Two moles of Mg(PO 3 ) 2 weigh 364.5 g. Any replacementof Mg 2 P 2 O 7 with Mg(PO 3 ) 2 must increase the precipitate’s mass.5. What additional steps, beyond those discussed in questions 2 and 3,help improve the precipitate’s purity?Two additional steps in the procedure help in forming a precipitatethat is free of impurities: digestion and reprecipitation.6. Why is the precipitate rinsed with a solution of 5% v/v NH 3 ?This is done for the same reason that precipitation is carried out inan ammonical solution; using dilute ammonia minimizes solubilitylosses when rinsing the precipitate.8B.2 Quantitative ApplicationsAlthough no longer a commonly used technique, precipitation gravimetrystill provides a reliable means for assessing the accuracy of other methodsof analysis, or for verifying the composition of standard reference materials.In this section we review the general application of precipitation gravimetryto the analysis of inorganic and organic compounds.In o r g a n i c An a l y s i sTable 8.1 provides a summary of some precipitation gravimetric methodsfor inorganic cations and anions. Several methods for the homogeneousgeneration of precipitants are shown in Table 8.2. The majority of inorganicprecipitants show poor selectivity for the analyte. Many organic precipitants,however, are selective for one or two inorganic ions. Table 8.3 listsseveral common organic precipitants.Precipitation gravimetry continues to be listed as a standard method forthe determination of SO 4 2– in water and wastewater analysis. 8 Precipitationis carried out using BaCl 2 in an acidic solution (adjusted with HCl to a pHof 4.5–5.0) to prevent the possible precipitation of BaCO 3 or Ba 3 (PO 4 ) 2 ,and near the solution’s boiling point. The precipitate is digested at 80–90 o Cfor at least two hours. Ashless filter paper pulp is added to the precipitate toaid in filtration. After filtering, the precipitate is ignited to constant weightat 800 o C. Alternatively, the precipitate is filtered through a fine porosityfritted glass crucible (without adding filter paper pulp), and dried toconstant weight at 105 o C. This procedure is subject to a variety of errors,including occlusions of Ba(NO 3 ) 2 , BaCl 2 , and alkali sulfates.8 Method 4500-SO 42– C and Method 4500-SO42– D as published in Standard Methods for theExamination of Waters and Wastewaters, 20th Ed., American Public Health Association: Washington,D. C., 1998.