Analytical Chem istry - DePauw University

Chapter 7 Collecting and Preparing Samples34722. Explain how an aqueous sample consisting of Cu 2+ , Sn 4+ , Pb 2+ , andZn 2+ can be separated into its component parts by adjusting the pH ofthe solution.23. A solute, S, has a distribution ratio between water and ether of 7.5.Calculate the extraction efficiency if you extract a 50.0-mL aqueoussample of S using 50.0 mL of ether as: (a) a single portion of 50.0 mL;(b) two portions, each of 25.0 mL; (c) four portions, each of 12.5 mL;and (d) five portions, each of 10.0 mL. Assume that the solute is notinvolved in any secondary equilibria.24. What volume of ether is needed to extract 99.9% of the solute in problem23 when using: (a) 1 extraction; (b) 2 extractions; (c) four extractions;and (d) five extractions.25. What is the minimum distribution ratio if 99% of the solute in a 50.0-mL sample is to be extracted with a single 50.0-mL portion of an organicsolvent? Repeat for the case where two 25.0-mL portions of theorganic solvent are used.26. A weak acid, HA, with a K a of 1.0 × 10 –5 has a partition coefficient, K D ,of 1.2 × 10 3 between water and an organic solvent. What restrictionon the sample’s pH is necessary to ensure that 99.9% of the weak acidin a 50.0-mL sample is extracted with a single 50.0-mL portion of theorganic solvent?27. For problem 26, how many extractions are needed if the sample’s pHcannot be decreased below 7.0?28. A weak base, B, with a K b of 1.0 × 10 –3 has a partition coefficient, K D ,of 5.0 × 10 2 between water and an organic solvent. What restrictionon the sample’s pH is necessary to ensure that 99.9% of the weak baseis in a 50.0-mL sample is extracted with two 25.0-mL portions of theorganic solvent?29. A sample contains a weak acid analyte, HA, and a weak acid interferent,HB. The acid dissociation constants and the partition coefficients forthe weak acids are as follows: K a,HA = 1.0 × 10 –3 , K a,HB = 1.0 × 10 –7 ,K D,HA = K D,HB = 5.0 × 10 2 . (a) Calculate the extraction efficiency forHA and HB when a 50.0-mL sample, buffered to a pH of 7.0, is extractedwith 50.0 mL of the organic solvent. (b) Which phase is enrichedin the analyte? (c) What are the recoveries for the analyte andthe interferent in this phase? (d) What is the separation factor? (e) Aquantitative analysis is conducted on the phase enriched in analyte.What is the expected relative error if the selectivity coefficient, K HA,HB ,is 0.500 and the initial ratio of HB/HA is 10.0?