Dialysis membrane to prevent cadmium ion specific electrode fouling

We present a simple method of protecting a Cd(II) ion-selective electrode from fouling using a dialysis membrane. Drift due to fouling is inevitable during continuous exposure of the electrode to the bulk solution. It presents a major problem in the case of automated titrations because recalibration is not possible. The drift is due to natural organic matter present in the sediment, and the membrane prevents its accumulation on the electrode surface. This was verified by calibrating a Cd electrode exposed to sediment for varying times, both with and without a membrane covering. There are two types of time dependent biases. A primary drift occurs without exposure to sediments and is probably caused by oxidation of the surface. A more important effect due to natural organic matter accumulation causes additional drift and deteriorates the Nernstian response of the electrode. From these results, it is possible to predict the uncertainty associated with measurements of binding constants and surface site densities for a Langmuir model. In general, the biased electrode overestimates the total metal concentration, reduces the value of the stability constants and increases estimates of maximum surface site densities.