Analytical Chem istry - DePauw University

370 Analytical Chemistry 2.0Fritted-glass crucibles can not withstand high temperatures and mustbe dried in an oven at temperatures below 200 o C. The glass fiber matsused in Gooch crucibles can be heated to a maximum temperature of approximately500 o C.Co m p o s i t i o n o f t h e Fi n a l PrecipitateFor a quantitative application, the final precipitate must have a welldefinedcomposition. Precipitates containing volatile ions or substantialamounts of hydrated water, are usually dried at a temperature that completelyremoves these volatile species. For example, one standard gravimetricmethod for the determination of magnesium involves its precipitationas MgNH 4 PO 4 •6H 2 O. Unfortunately, this precipitate is difficult to dryat lower temperatures without losing an inconsistent amount of hydratedwater and ammonia. Instead, the precipitate is dried at temperatures above1000 o C where it decomposes to magnesium pyrophosphate, Mg 2 P 2 O 7 .An additional problem is encountered if the isolated solid is nonstoichiometric.For example, precipitating Mn 2+ as Mn(OH) 2 and heating frequentlyproduces a nonstoichiometric manganese oxide, MnO x , where xvaries between one and two. In this case the nonstoichiometric product isthe result of forming of a mixture of oxides with different oxidation state ofmanganese. Other nonstoichiometric compounds form as a result of latticedefects in the crystal structure. 7The best way to appreciate the theoreticaland practical details discussed in this sectionis to carefully examine a typical precipitationgravimetric method. Althougheach method is unique, the determinationof Mg 2+ in water and wastewaterby precipitating MgNH 4 PO 4 • 6H 2 Oand isolating Mg 2 P 2 O 7 provides an instructiveexample of a typical procedure.The description here is based on Method3500-Mg D in Standard Methods for theExamination of Water and Wastewater,19th Ed., American Public Health Association:Washington, D. C., 1995. Withthe publication of the 20th Edition in1998, this method is no longer listed asan approved method.7 Ward, R., ed., Non-Stoichiometric Compounds (Ad. Chem. Ser. 39), American Chemical Society:Washington, D. C., 1963.Representative Method 8.1Determination of Mg 2+ in Water and WastewaterDescription o f Me t h o dMagnesium is precipitated as MgNH 4 PO 4 • 6H 2 O using (NH 4 ) 2 HPO 4as the precipitant. The precipitate’s solubility in neutral solutions is relativelyhigh (0.0065 g/100 mL in pure water at 10 o C), but it is much lesssoluble in the presence of dilute ammonia (0.0003 g/100 mL in 0.6 MNH 3 ). Because the precipitant is not selective, a preliminary separation ofMg 2+ from potential interferents is necessary. Calcium, which is the mostsignificant interferent, is removed by precipitating it as CaC 2 O 4 . Thepresence of excess ammonium salts from the precipitant, or the additionof too much ammonia leads to the formation of Mg(NH 4 ) 4 (PO 4 ) 2 , whichforms Mg(PO 3 ) 2 after drying. The precipitate is isolated by filtering, usinga rinse solution of dilute ammonia. After filtering, the precipitate isconverted to Mg 2 P 2 O 7 and weighed.Pr o c e d u r eTransfer a sample containing no more than 60 mg of Mg 2+ into a 600-mLbeaker. Add 2–3 drops of methyl red indicator, and, if necessary, adjust