Abstract
Sensitivity of calcium-selective electrodes heretofore has been limited to calcium concentrations above 10 −8 m in the absence of competing ions. We describe the use of calcium buffers to stabilize the free calcium in the reference electrode. Electrode calibration is linear to 10 −8 m and is curvilinear to 10 −11 m in the presence of 0.1 m ionic strength. Selectivity with respect to competing cations magnesium, potassium, sodium, and hydrogen is preserved. Electrode response time is less than 2 s for small changes in calcium activity. Response range is linear over 9 log units of calcium activity. Potential-time stability is less than 10 mV/h at saturation currents. Although the silver-silver chloride terminals are photosensitive throughout the visible and near-ultraviolet regions, housing the reference and indifferent in opaque barrels avoids false photovoltaic response.
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