Analytical Chem istry - DePauw University

Chapter 8 Gravimetric Methods365CO(NH ) ( aq) + H O() l 2NH ( aq) + CO ( g)2 2 23 2NH ( ) HO () −OH ( ) +aq + l aq + NH ( aq )3 24Because the hydrolysis of urea is temperature-dependent—it is negligibleat room temperature—we can use temperature to control the rate of hydrolysisand the rate of precipitate formation. Precipitates of CaC 2 O 4 , forexample, have been produced by this method. After dissolving the samplecontaining Ca 2+ , the solution is made acidic with HCl before adding a solutionof 5% w/v (NH 4 ) 2 C 2 O 4 . Because the solution is acidic, a precipitateof CaC 2 O 4 does not form. The solution is heated to approximately 50 o Cand urea is added. After several minutes, a precipitate of CaC 2 O 4 begins toform, with precipitation reaching completion in about 30 min.In the second method of homogeneous precipitation, the precipitant isgenerated by a chemical reaction. For example, Pb 2+ is precipitated homogeneouslyas PbCrO 4 by using bromate, BrO 3 – , to oxidize Cr 3+ to CrO 4 2– .− 3+6BrO ( aq) + 10Cr ( aq) + 22H O()l 33Br( aq)+ 10CrO22( aq) + 44H( aq)2− +4Figure 8.5 shows the result of preparing PbCrO 4 by the direct addition ofKCrO 4 (Beaker A) and by homogenous precipitation (Beaker B). Both beakerscontain the same amount of PbCrO 4 . Because the direct addition ofKCrO 4 leads to rapid precipitation and the formation of smaller particles,the precipitate remains less settled than the precipitate prepared homogeneously.Note, as well, the difference in the color of the two precipitates.A homogeneous precipitation produces large particles of precipitatethat are relatively free from impurities. These advantages, however, are offsetby requiring more time to produce the precipitate and a tendency for theprecipitate to deposit as a thin film on the container’s walls. The latter problemis particularly severe for hydroxide precipitates generated using urea.An additional method for increasing particle size deserves mention.When a precipitate’s particles are electrically neutral they tend to coagulateThe effect of particle size on color is wellknownto geologists, who use a streaktest to help identify minerals. The colorof a bulk mineral and its color whenpowdered are often different. Rubbing amineral across an unglazed porcelain plateleaves behind a small streak of the powderedmineral. Bulk samples of hematite,Fe 2 O 3 , are black in color, but its streakis a familiar rust-red. Crocite, the mineralPbCrO 4 , is red-orange in color; its streakis orange-yellow.Beaker ABeaker BFigure 8.5 Two precipitates of PbCrO 4 . In Beaker A,combining 0.1 M Pb(NO 3 ) 2 and 0.1 M K 2 CrO 4 formsthe precipitate under conditions of high RSS. The precipitateforms rapidly and consists of very small particles.In Beaker B, heating a solution of 0.1 M Pb(NO 3 ) 2 , 0.1M Cr(NO 3 ) 3 , and 0.1 M KBrO 3 slowly oxidizes Cr 3+ toCrO 4 2– , precipitating PbCrO 4 under conditions of lowRSS. The precipitate forms slowly and consists of muchlarger particles.