Amperometric Titration Largely Overestimates Chloride Concentrations in Chloroplast Extracts

Abstract

Determination of chloride contents in aqueously isolated chloroplasts from spinach by amperometric titration indicated chloride concentrations of 60 to 100 mmol 1-1 (based on a chloroplast volume of 25 μl mg-1 chlorophyll). However, when analyzed by anion chromatography, chloride contents in chloroplasts were much lower (1-8 mmol 1-1). In leaf extracts chloride concentrations obtained with both methods were rather similar. Boiling of chloroplast extracts prior to chloride titration reduced indicated chloride contents by a factor of three or four, but did not change results obtained with anion chromatography. It is concluded that chloroplasts contain large amounts of silver complexing agents giving rise to a drastic overestima­tion of chloroplast chloride concentrations when measured by amperometric titration. Boiling and centrifugation of extracts apparently precipitates these compounds only insufficiently.