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                                                               Chapter 7 Obtaining and Preparing Samples for Analysis  209

                             2–
                     Ni(CN) 4  is greater than that for the Ni–EDTA complex. In fact, the
                     equilibrium constant for the reaction in which EDTA displaces the masking
                     agent
                                              2–
                                                   4–
                                                          2–
                                       Ni(CN) 4 +Y t NiY + 4CN   –
                                           K f  42 .  ´10 18     - 12
                                      K =     =       30  = 25 .  ´10
                                           b 4  17 .  ´10
                                                    2–
                     is very small, indicating that Ni(CN) 4  is relatively inert in the presence of
                     EDTA.



                 7F.4  Separations Based on a Change of State
                 Since an analyte and interferent are usually in the same phase, a separation often
                 can be effected by inducing a change in one of their physical or chemical states.
                 Changes in physical state that have been exploited for the purpose of a separation
                 include liquid-to-gas and solid-to-gas phase transitions. Changes in chemical state
                 involve one or more chemical reactions.

                 Changes in Physical State  When the analyte and interferent are mis-  Vapor
                                                                                                       B
                 cible liquids, a separation based on distillation may be possible if their                      A
                 boiling points are significantly different. The progress of a distillation  D
                 is outlined in Figure 7.13, which shows a plot of temperature versus  Temperature     C
                 the vapor-phase and liquid-phase composition of a mixture consisting
                 of a low-boiling analyte and a high-boiling interferent. The initial                      Liquid
                 mixture is indicated by the point labeled A. When this solution is
                 brought to its boiling point, a vapor phase with the composition indi-
                 cated by the point labeled B is in equilibrium with the original liquid  0  Mole % interferent   100
                                                                                  100         Mole % analyte       0
                 phase. This equilibrium is indicated by the horizontal tie-line between
                 points A and B. When the vapor phase at point B condenses, a new       Figure 7.13
                 liquid phase with the same composition as the vapor phase (point C)    Boiling points versus composition diagram
                 results. The liquid phase at point C boils at a lower temperature, with an equilib-  for a near-ideal solution, showing the
                                                                                        progress of a distillation.
                 rium established with the vapor-phase composition indicated by point D. This
                 process of repeated vaporization and condensation gradually separates the ana-
                 lyte and interferent.
                     Two examples of the equipment used for distillations are shown in Figure 7.14.
                 The simple distillation apparatus shown in Figure 7.14a does not produce a very effi-
                 cient separation and is useful only for separating a volatile liquid from nonvolatile
                 liquids or for separating liquids with boiling points that differ by more than 150 °C.
                 A more efficient separation is achieved by a fractional distillation (Figure 7.14b).
                 Packing the distillation column with a high-surface-area material, such as a steel
                 sponge or glass beads, provides more opportunity for the repeated process of vapor-
                 ization and condensation necessary to effect a complete separation.
                     When the sample is a solid, a separation of the analyte and interferent by subli-
                 mation may be possible. The sample is heated at a temperature and pressure below
                 its triple point where the solid vaporizes without passing through the liquid state.
                 The vapor is then condensed to recover the purified solid. A good example of the
                 use of sublimation is in the isolation of amino acids from fossil mollusk shells and
                 deep-sea sediments. 14
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