Diffusional titration errors in coulometry: Part II. Aspects of cell design, separator porosity and cell resistance

Summary Titration errors in coulometric cells where the sample compartment is separated from the auxiliary compartment by one or several membranes placed in a connecting chamber are discussed. The transport mechanism is assumed to be wholly diffusional. The error is caused by outflow of sample or inflow of products and depends upon several factors: (i) membrane type; for glass-filters a relation between conductometric and volumetric porosities is established, e c =1.14 e v 1.77 ; only non-interacting separators are discussed; (ii) titration time, 3–15 min; (iii) shape of titration curve; (iv) cell resistance. A correlation between (ii) and (iv) has been tested for a coulometric technique where both current and working electrode potential varies (v.p.c.-method). It is shown that an optimum cell-resistance may be found. Three cell models are discussed. The error is calculated for different lengths of the connecting chamber holding two membranes, for different volumes of the auxiliary compartment and for the number of titrations performed. The magnitudes of the errors are for all cases in the range 0.4–0.005%.