Dialytic methods for sample treatment in ion chromatography

摘要

This thesis presents the results of a systematic study of the parameters affectinguddialysis procedures, including active (Donnan) dialysis and electrodialysis, for theudpretreatment of alkaline samples prior to ion chromatographic analysis. Audpreliminary study of enrichment of inorganic anions using an "Elutrap" apparatus isudalso presented. Dialysis methods were investigated with a view to applyingudmembrane-based clean-up procedures to the determination of trace levels ofudinorganic anions in various samples following sample treatment by hydroxide fusion.udDonnan dialysis was carried out by passing sodium hydroxide solutions containingudinorganic anions through a cation-exchange hollow fibre immersed in a hydrogen ionuddonating medium. As the sample is passed through the lumen of the fibre, a dialysisudreaction involving exchange of sodium ions in the sample with hydrogen ions fromudthe surrounding medium occurs, resulting in total or partial neutralization of theudsample. Several acid solutions including slurries of cation-exchange resins wereudevaluated on the basis of neutralization efficiency and penetration of the acid anionudthrough the membrane. Cation-exchange resins in the hydrogen form slurried withud0.1 M octanesulfonic acid gave optimal performance and when the resin was stirredudoccasionally the total theoretical neutralization capacity of the resin was achieved. The use of an "Elutrap" apparatus was shown in this study to be impractical forudsample clean-up and enrichment of anions. However, factors affecting the migrationudrate of inorganic anions under applied DC potential have been elucidated. Anudelectrodialysis process, which uses an electrical field to stimulate the migration ofudions through the membrane, was studied for off-line neutralization of stronglyudalkaline samples. This method utilizes an electrodialysis cell comprising threeudcompartments separated from each other by cation-exchange membranes.udExperimental 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.udUnder optimal conditions, recoveries of solute anions (Cl -, Br, NO3-, HPO42- andudSO42- in the range 3-10 lig/m1) from 1 M NaOH solution were close to quantitativeudexcept for fluoride and nitrite. It is suggested that low recoveries for these ions wereuddue to the formation of neutral, protonated species within the membrane withudsubsequent loss by diffusion.udA flow-through electrodialysis device, in which the sample was allowed to flowudduring the dialysis process, was studied for the on-line pretreatment of stronglyudalkaline solutions. Variations of the shape of the sample compartments and the sizeudof the electrodes were studied in order to optimize the flow-through cell design andudto minimize the heat generated inside the cell. Quantitative recovery for fluoride ionudcould be achieved using a Neosepta CMS cation-exchange membrane and the abilityudto successfully treat samples containing fluoride represented an improvement overudthe off-line cell.udThe on-line electrodialysis method has been successfully applied to theuddetermination of inorganic anions in vegetation samples obtained from the vicinityudof an aluminium smelter, following sample preparation by hydroxide fusion. Thisudwork demonstrated that when correctly applied, the method could be used onuddifferent sample types which have been traditionally difficult to analyse by ionudchromatography.