Dialytic methods for sample treatment in ion chromatography

This thesis presents the results of a systematic study of the parameters affecting dialysis procedures, including active (Donnan) dialysis and electrodialysis, for the pretreatment of alkaline samples prior to ion chromatographic analysis. A preliminary study of enrichment of inorganic anions using an \Elutrap\" apparatus is also presented. Dialysis methods were investigated with a view to applying membrane-based clean-up procedures to the determination of trace levels of inorganic anions in various samples following sample treatment by hydroxide fusion. Donnan dialysis was carried out by passing sodium hydroxide solutions containing inorganic anions through a cation-exchange hollow fibre immersed in a hydrogen ion donating medium. As the sample is passed through the lumen of the fibre a dialysis reaction involving exchange of sodium ions in the sample with hydrogen ions from the surrounding medium occurs resulting in total or partial neutralization of the sample. Several acid solutions including slurries of cation-exchange resins were evaluated on the basis of neutralization efficiency and penetration of the acid anion through the membrane. Cation-exchange resins in the hydrogen form slurried with 0.1 M octanesulfonic acid gave optimal performance and when the resin was stirred occasionally the total theoretical neutralization capacity of the resin was achieved. The use of an \"Elutrap\" apparatus was shown in this study to be impractical for sample clean-up and enrichment of anions. However factors affecting the migration rate of inorganic anions under applied DC potential have been elucidated. An electrodialysis process which uses an electrical field to stimulate the migration of ions through the membrane was studied for off-line neutralization of strongly alkaline samples. This method utilizes an electrodialysis cell comprising three compartments separated from each other by cation-exchange membranes. Experimental parameters such as the magnitude of the applied current or power the type of cation-exchange membrane used and the design of the cell were studied. Under optimal conditions recoveries of solute anions (Cl - Br NO3- HPO42- and SO42- in the range 3-10 lig/m1) from 1 M NaOH solution were close to quantitative except for fluoride and nitrite. It is suggested that low recoveries for these ions were due to the formation of neutral protonated species within the membrane with subsequent loss by diffusion. A flow-through electrodialysis device in which the sample was allowed to flow during the dialysis process was studied for the on-line pretreatment of strongly alkaline solutions. Variations of the shape of the sample compartments and the size of the electrodes were studied in order to optimize the flow-through cell design and to minimize the heat generated inside the cell. Quantitative recovery for fluoride ion could be achieved using a Neosepta CMS cation-exchange membrane and the ability to successfully treat samples containing fluoride represented an improvement over the off-line cell. The on-line electrodialysis method has been successfully applied to the determination of inorganic anions in vegetation samples obtained from the vicinity of an aluminium smelter following sample preparation by hydroxide fusion. This work demonstrated that when correctly applied the method could be used on different sample types which have been traditionally difficult to analyse by ion chromatography."