Analytical Chem istry - DePauw University

338 Analytical Chemistry 2.0Extraction Efficiency1008060402001 2 3 4 5 6 7pHFigure 7.31 Plot of extraction efficiencyversus pH for the extractionof the metal ion, M 2+ , in Example7.17.SHNNNHNdithizoneCalculate the extraction efficiency if we extract 100.0 mL of a 1.0 × 10 –6M aqueous solution of M 2+ , buffered to a pH of 1.00, with 10.00 mLof an organic solvent that is 0.1 mM in the chelating agent. Repeat thecalculation at a pH of 3.00.So l u t i o nWhen the pH is 1.00 the distribution ratio is6 4)−5 2 −4 2( 25 . × 10 )( 70 . × 10 )( 50 . × 10 )(. 10×10 )D =4 2 2 6 5 2(. 10× 10 )( 010 . ) + ( 2. 5× 10 )( 50 . × 10 − ) (. 1 0×10D = 0.0438and the fraction of metal ion remaining in the aqueous phase isQ aq( )1100.0 mL=( 0. 0438)( 10. 00 mL) + 100.0 mL= 0.996)−4 2At a pH of 1.00, we extract only 0.40% of the metal into the organic phase.Changing the pH to 3.00, however, increases the extraction efficiency to97.8%. Figure 7.31 shows how the pH of the aqueous phase affects theextraction efficiency for M 2+ .One advantage of using a ligand to extract a metal ion is the high degreeof selectivity that it brings to a liquid–liquid extraction. As seen in Figure7.31, a divalent metal ion’s extraction efficiency increases from approximately0% to 100% over a range of 2 pH units. Because a ligand’s ability toform a metal–ligand complex varies substantially from metal ion to metalion, significant selectivity is possible by carefully controlling pH. Table 7.9shows the minimum pH for extracting 99% of a metal ion from an aqueoussolution using an equal volume of 4 mM dithizone in CCl 4 .Example 7.18Using Table 7.9, explain how you can separate the metal ions in an aqueousmixture of Cu 2+ , Cd 2+ and Ni 2+ by extracting with an equal volumeof dithizone in CCl 4 .So l u t i o nFrom Table 7.9, a quantitative separation of Cu 2+ from Cd 2+ and Ni 2+ ispossible if we acidify the aqueous phase to a pH of less than 1. This pHis greater than the minimum pH for extracting Cu 2+ and significantlysmaller than the minimum pH for extracting either Cd 2+ or Ni 2+ Afterthe extraction is complete, buffering the aqueous phase to 4.0 allows us toextract the Cd 2+ leaving the Ni 2+ in the aqueous phase.