ONLINE ANALYSIS OF ALKALINE SAMPLES WITH A FLOW-THROUGH ELECTRODIALYSIS DEVICE COUPLED TO AN ION CHROMATOGRAPH

A flow-through device suitable for on-line neutralization of strongly alkaline samples using electrodialysis is described. This device comprises an anode compartment filled with a slurry (2:1, w/v) of 1 mM octane sulfonic acid and hydrogen-form cation-exchange resin, a 300 mu l sample chamber bounded by two cation-exchange membranes, and a cathode compartment filled with dilute sodium hydroxide. An alkaline sample (up to 1 M sodium hydroxide) passed through the device at a rate of 0.1 ml/min is neutralized and with the aid of a switching valve, the treated sample can be routed directly to a suppressed ion chromatographic system for determination of inorganic anions. The design parameters for the electrodialysis devices have been studied, including the type of membrane used, the size of the electrodes, the shape of the sample compartment, and the applied power. Optimal performance was obtained with planar stainless-steel electrodes and with 2 W of power. Recoveries of inorganic anions from treated samples were close to quantitative with the exception of nitrite, for which losses were observed. The on-line electrodialysis is shown to be suitable for the treatment of sodium hydroxide solutions, but was less successful when applied to solutions of sodium carbonate and sodium tetraborate. The system has been used for the determination of fluoride in forage vegetation after preparation of the sample by fusion with sodium hydroxide. Good agreement was obtained with results from colorimetric determination of fluoride.