Metal ions, alkali, separation with

In this type of separation the analyte cations compete with the eluent cation for ion-exchange sites and move down the eolumn at different rates. The ionic eluent selected depends on the cations to be separated, the type of separation column and on the detector. In many cases an aqueous solution of a strong acid such as hydrochloric, sulfuric or methanesulfonic acid is a satisfactory eluent. Sample cations commonly separated include the following alkali metal ions (Li, Na+, K", Rb, Cs ), ammonium, magnesium, alkaline earths (Ca, Sr +, Ba ), and various organic amine and alkano-lamine cations. Most other metal cations are separated with a weakly complexing eluent. 

When the macroporous resin column was used with the same acidic eluent in 100 % methanol, the chromatographic separation was improved considerably. Now the alkali-metal ions are solvated with methanol and the resin matrix is probably coated with a thin layer of methanol, which makes the ions and the resin surface more compatible with one another. 

Cryptands are cyclic (mainly) polyether molecules with usually three chains linked at nitrogen caps at each end of the molecule (Figure 4.42), much like the sarcophagines but with a different capping atom and different donors. They can, depending on host cavity size, bind metal ions (alkali or alkaline earth ions preferred) or small molecules. A wide range of molecules of this family have been prepared. They can be effective in metal ion selection from a group of ions, useful in both analysis and separation of mixtures. They also help solubilize metal ions in aprotic solvents. 

Yet another group of metal ions is separated as hydroxides (i.e., hydrous oxides) when, after the H2S is removed and an oxidant is added, the solution is made alkaline with NH3. Fe(OH)3, Mn(OH)3, Al(OH)3, Cr(OH)3, and NH4VO3 will precipitate. Adding hydrogen sulfide to the filtrate precipitates CoS, NiS, and ZnS. Carbonate addition to this filtrate will bring down the alkaline earth metal (Mg, Ca, Ba, Sr) carbonates, leaving the alkali metal ions (Li, Na, K, Cs) remaining in solution. 

The proportion of hydrochloric acid in the mobile phase was not to exceed 20%, so that complex formation did not occur and zone structure was not adversely affected. An excess of accompanying alkaline earth metal ions did not interfere with the separation but alkali metal cations did. The hthium cation fluoresced blue and lay at the same height as the magnesium cation, ammonium ions interfered with the calcium zone. 

Except for sensor applications, the intercalation of alkali metal ions in metal hexacyanoferrates was used for adsorption and separation of cesium ions from different aqueous solutions with Prussian blue [43,44] and cupric hexacyanoferrate [45,46],... 

Systems of the above type incorporating larger rings form binuclear complexes with a number of alkali and alkaline earth metal ions as well as with ions such as Ag(i) and Pb(n). For example, (212) forms a symmetrical dinuclear Na+ species in which the Na+ ions are associated with the respective azacrown and crown cavities and are separated from each other by 6.40 A in the solid (Fisher, Mellinger Weiss, 1977). 

---