The Use of Concentrated NaN03in tcPo2Electrodes to Diminish O2Consumption and Dehydration Drift

Abstract

We investigated: (1) The effects of NaN03 concentration, membrane material and thickness, and electrolyte layer thickness on electrode O2 consumption, stirring effect (O) and response time; (2) The effect of NaNO3 on P(H20) and electrode drift; and (3) The effect of KCl on Ag deposition at the cathode. Electrolytes containing 0–8 m NaN03, 0.01 m KCl, and 0.1 m pH 7 phosphate were tested in a 20 μ cathode tcPo2 electrode at 42dC. Increasing NaNO3 concentration reduced O2 solubility and sensitivity by a factor: e-(C/4.6). Thus, sensitivity was a linear function of solubility. Without cellophane, sensitivity with 12 μm polypropylene was 50% that of 25 μm Teflon; 7 m NaNO3 reduced sensitivity for both membranes by about 60%. 12 μ cellophane, by increasing the thickness of the electrolyte layer, minimized the difference between the membranes and exaggerated the effects of increasing NaNO3 concentration on sensitivity. O was a linear function of sensitivity. Cellophane decreased the ratio of O to sensitivity. Response times of electrodes were almost unaffected by addition of NaNO3, but were increased as expected by use of thicker membranes, and by addition of cellophane spacers. Electrolyte vapour pressure was decreased 67% by addition of 7 m NaNO3. When electrolyte vapour pressure differed significantly from that of skin or calibration solutions, water crossed the polypropylene or Teflon membranes, changing the O2 sensitivity, and causing drift both on skin and in calibration solutions. With KCl containing electrolytes, silver deposition occurs at the cathode, the rate of deposition increasing greatly as KCl concentration approaches saturation. Thus addition of NaNCO3 to O2 electrolytes lowers vapour pressure, delays dehydration, reduces O2 sensitivity by reducing permeability of the electrolyte layer to O2, and thereby reduces the gas/skin correction factor O, without significantly delaying response time, and without increasing silver deposition.