SENSOR BODY. NNER SOLUTION NNER REFERENCE ELECTRODE EXTERNAL ELECTRODE PH SENSITIVE GLASS FIGURE 114.4 a glass electrode pH sensor This reaction involves the reduction of molecular oxygen that diffuses into the cell through the oxygen permeable membrane. Since the other components of the reaction are in abundance, the rate of the reaction is limited by the amount of oxygen available. Thus, the rate of electrons used at the cathode is directly related to the available oxygen. In other words, the cathode current is proportional to the partial pressure of oxygen in the specimen being measured The electrochemical cell is completed by the silver anode. The reaction at the anode involves forming the low-solubility salt, silver-chloride, from the anode material itself and the chloride ion contained in the electro- lyte. The cell is designed so that these materials are also in abundance so that their concentration does not affect the sensor performance. This type of sensor is an example of an amperometric electrochemical sensor. Another type of electrochemical sensor that is frequently used in biomedical laboratories is the glass electrode illustrated in Fig. 114.4. The acidity or alkalinity of a solution is characterized by its pH. This quantity pH=-logIo [H* where [H+] is the activity of the hydrogen ions in solution, a quantity that is related to the concentration of the hydrogen ions. This sensor only works in an aqueous environment. It consists of an inner chamber containing an electrolytic solution of known pH and an outer solution with an unknown pH that is to be measured. The membrane consists of a specially formulated glass that will in essence allow hydrogen ions to pass in either direction but will not pass other chemical species. If the concentration of hydrogen ions in the external solution is greater than that in the internal solution, there will be a gradient forcing hydrogen ions to diffuse through the membrane into the internal solution. This will cause the internal solution to have a greater positive charge than the external solution so that an electrical potential and, hence, an electric field will exist cross the membrane. This field will counteract the diffusion of hydrogen ions due to the concentration difference and so an equilibrium will be eventually established. The potential across the membrane at this equilibrium condition will be related to the hydrogen ion concentration difference(or more accurately the activity difference) between the inner and outer solutions. This potential is given by the Nernst equatio where E is the potential measured, R is the universal gas constant, T is the absolute temperature, n is the valence of the ion, and a, and a2 are the activities of the ions on each side of the membrane. Thus the potential measured across the glass membrane will be proportional to the pH of the solution being studied. At room temperature e 2000 by CRC Press LLC© 2000 by CRC Press LLC This reaction involves the reduction of molecular oxygen that diffuses into the cell through the oxygen￾permeable membrane. Since the other components of the reaction are in abundance, the rate of the reaction is limited by the amount of oxygen available. Thus, the rate of electrons used at the cathode is directly related to the available oxygen. In other words, the cathode current is proportional to the partial pressure of oxygen in the specimen being measured. The electrochemical cell is completed by the silver anode. The reaction at the anode involves forming the low-solubility salt, silver-chloride, from the anode material itself and the chloride ion contained in the electro￾lyte. The cell is designed so that these materials are also in abundance so that their concentration does not affect the sensor performance. This type of sensor is an example of an amperometric electrochemical sensor. Another type of electrochemical sensor that is frequently used in biomedical laboratories is the glass pH electrode illustrated in Fig. 114.4. The acidity or alkalinity of a solution is characterized by its pH. This quantity is defined as pH = – log10 [H+] where [H+] is the activity of the hydrogen ions in solution, a quantity that is related to the concentration of the hydrogen ions. This sensor only works in an aqueous environment. It consists of an inner chamber containing an electrolytic solution of known pH and an outer solution with an unknown pH that is to be measured. The membrane consists of a specially formulated glass that will in essence allow hydrogen ions to pass in either direction but will not pass other chemical species. If the concentration of hydrogen ions in the external solution is greater than that in the internal solution, there will be a gradient forcing hydrogen ions to diffuse through the membrane into the internal solution. This will cause the internal solution to have a greater positive charge than the external solution so that an electrical potential and, hence, an electric field will exist across the membrane. This field will counteract the diffusion of hydrogen ions due to the concentration difference and so an equilibrium will be eventually established. The potential across the membrane at this equilibrium condition will be related to the hydrogen ion concentration difference (or more accurately the activity difference) between the inner and outer solutions. This potential is given by the Nernst equation where E is the potential measured, R is the universal gas constant, T is the absolute temperature, n is the valence of the ion, and a1 and a2 are the activities of the ions on each side of the membrane. Thus the potential measured across the glass membrane will be proportional to the pH of the solution being studied. At room temperature FIGURE 114.4 A glass electrode pH sensor. E RT nF a a = - Ê Ë Á ˆ ¯ ˜ ln 1 2