here: the ion-selective electrode(ISE)and the gas chromatograph. They were chosen because of their general use and availability and because they illustrate the use of a primary(Ise)versus a primary plus intermediate tograph) Ion-Selective Electrode(Ise) As the name implies, ISEs are used to measure the concentration of a specific ion concentration in a solution of many ions. To accomplish this, a membrane which selectively generates a potential the concentration of the ion of interest is used. The generated potential is usually an equilibrium potential called the Nernst potential, and develops across the interface of the membrane with the solution. This potential is generated by the initial net flow of ions(charge)across the membrane in response to a concentration gradient, and from thence forth the diffusional force is balanced by the generated electric force and equilibrium established. This is very similar to the so-called built-in potential of a p-n junction diode. The ion-selective membrane acts in such a way as to ensure that the generated potential is dependent mostly on the ion of interest and negligibly on any other ions in solution. This is done by enhancing the exchange rate of the ion of interest across the membrane, so it is the fastest moving and, therefore, the species which generates and maintains the potential The most familiar ISE is the pH electrode. In this device the membrane is a sodium glass which possesses a high exchange rate for H+. The generated Nernst potential, E is given by the expression: E= Eo+(RT/F) In[H+], where E is a constant for constant temperature, R is the gas constant, and F is the Faraday constant. PH is defined as the negative of the log[H*]; therefore pH=(E-E(log e)F/RT. One ph unit change corresponds to a tenfold change in the molar concentration of H* and a 59 mv change in the Nernst potential at room temperature. Other ISEs have the same type of response, but specific to a different ion, depending on the choice of membrane. Many ISEs employ ionophores trapped inside of a polymeric membrane. An ionophore a molecule which selectively and reversibly binds with an ion and thereby creates a high exchange rate for that part The ISE consists of a glass tube with the ion-selective membrane closing that end of the tube which emersed into the test solution. The Nernst potential is measured by making electrical contact to each side of the membrane. This is done by placing a fixed concentration of conductive filling solution inside of the tube and placing a wire into the solution. The other side of the membrane is contacted by a reference electrode placed inside of the same solution under test. The reference electrode is constructed in the same manner as the ise but it has a porous membrane which creates a liquid junction between its inner filling solution and the test solution. That junction is designed to have a potential which is invariant with changes in concentration of any ion in the test solution. The reference electrode, solution under test, and the ISE form an electrochemical cell. The reference electrode potential acts like the ground reference in electric circuits, and the ISE potential is measured between the two wires emerging from the respective two electrodes. The details of the mechanisms of transduction in ISEs are beyond the scope of this chapter. The reader is referred to Bard and Faulkner [ 1980] and Janata [1989 Gas Chromatograph Molecules in gases have thermal conductivities which are dependent on their masses; therefore, a pure gas can be identified by its thermal conductivity. One way to determine the composition of a gas is to first separate it into its components and then measure the thermal conductivity of each. a gas chromatograph does exactly that. The gas flows through a long narrow column, which is packed with an adsorbant solid( for gas-soli chromatography) wherein the gases are separated according to the retentive properties of the packing material for each gas. As the individual gases exit the end of the tube one at a time, they flow over a heated wire. The amount of heat transferred to the gas depends on its thermal conductivity. The gas temperature is measured a short distance downstream and compared to a known gas flowing in a separate sensing tube. The temperature is related to the amount of heat transferred and can be used to derive the thermal conductivity according to thermodynamic theory and empirical data. This sensor required two transductions: a chemical to thermal energy transduction followed by a thermal to electrical transduction. c 2000 by CRC Press LLC© 2000 by CRC Press LLC here: the ion-selective electrode (ISE) and the gas chromatograph. They were chosen because of their general use and availability and because they illustrate the use of a primary (ISE) versus a primary plus intermediate (gas chromatograph) transduction mechanism. Ion-Selective Electrode (ISE) As the name implies, ISEs are used to measure the concentration of a specific ion concentration in a solution of many ions. To accomplish this, a membrane which selectively generates a potential which is dependent on the concentration of the ion of interest is used. The generated potential is usually an equilibrium potential, called the Nernst potential, and develops across the interface of the membrane with the solution. This potential is generated by the initial net flow of ions (charge) across the membrane in response to a concentration gradient, and from thence forth the diffusional force is balanced by the generated electric force and equilibrium is established. This is very similar to the so-called built-in potential of a p-n junction diode. The ion-selective membrane acts in such a way as to ensure that the generated potential is dependent mostly on the ion of interest and negligibly on any other ions in solution. This is done by enhancing the exchange rate of the ion of interest across the membrane, so it is the fastest moving and, therefore, the species which generates and maintains the potential. The most familiar ISE is the pH electrode. In this device the membrane is a sodium glass which possesses a high exchange rate for H+. The generated Nernst potential, E, is given by the expression: E = E0 + (RT/F) ln[H+], where E0 is a constant for constant temperature, R is the gas constant, and F is the Faraday constant. pH is defined as the negative of the log[H+]; therefore pH = (E0 – E)(log e)F/RT. One pH unit change corresponds to a tenfold change in the molar concentration of H+ and a 59 mV change in the Nernst potential at room temperature. Other ISEs have the same type of response, but specific to a different ion, depending on the choice of membrane. Many ISEs employ ionophores trapped inside of a polymeric membrane. An ionophore is a molecule which selectively and reversibly binds with an ion and thereby creates a high exchange rate for that particular ion. The ISE consists of a glass tube with the ion-selective membrane closing that end of the tube which is immersed into the test solution. The Nernst potential is measured by making electrical contact to each side of the membrane. This is done by placing a fixed concentration of conductive filling solution inside of the tube and placing a wire into the solution. The other side of the membrane is contacted by a reference electrode placed inside of the same solution under test. The reference electrode is constructed in the same manner as the ISE but it has a porous membrane which creates a liquid junction between its inner filling solution and the test solution. That junction is designed to have a potential which is invariant with changes in concentration of any ion in the test solution. The reference electrode, solution under test, and the ISE form an electrochemical cell. The reference electrode potential acts like the ground reference in electric circuits, and the ISE potential is measured between the two wires emerging from the respective two electrodes. The details of the mechanisms of transduction in ISEs are beyond the scope of this chapter. The reader is referred to Bard and Faulkner [1980] and Janata [1989]. Gas Chromatograph Molecules in gases have thermal conductivities which are dependent on their masses; therefore, a pure gas can be identified by its thermal conductivity. One way to determine the composition of a gas is to first separate it into its components and then measure the thermal conductivity of each. A gas chromatograph does exactly that. The gas flows through a long narrow column, which is packed with an adsorbant solid (for gas–solid chromatography) wherein the gases are separated according to the retentive properties of the packing material for each gas. As the individual gases exit the end of the tube one at a time, they flow over a heated wire. The amount of heat transferred to the gas depends on its thermal conductivity. The gas temperature is measured a short distance downstream and compared to a known gas flowing in a separate sensing tube. The temperature is related to the amount of heat transferred and can be used to derive the thermal conductivity according to thermodynamic theory and empirical data. This sensor required two transductions: a chemical to thermal energy transduction followed by a thermal to electrical transduction