2.1. Dissolved Oxygen Electrode and Amperometry
2.1.1. Principle of Amperometry (Polarography)
Polarogram. When an electrode of noble metal such as platinum or gold is made 0.6 to 0.8 V negative with respect to a suitable reference electrode such as Ag/AgCl or an calomel electrode in a neutral KCl solution (see Fig. 2.1), the oxygen dissolved in the liquid is reduce at the surface of the noble metal. This phenomenon can be observed from a current-voltage diagram - called a polarogram - of the electrode. As shown in Fig. 2.2a, the negative voltage applied to the noble metal electrode (called the cathode) is increased, the current increases initially but soon it becomes saturated. In this plateau region of the polarogram, the reaction of oxygen at the cathode is so fast that the rate of reaction is limited by the diffusion of oxygen to the cathode surface. When the negative bias voltage is further increased, the current output of the electrode increases rapidly due to other reactions, mainly, the reduction of water to hydrogen. If a fixed voltage in the plateau region (for example, - 0.6V) is applied to the cathode, the current output of the electrode can be linearly calibrated to the dissolved oxygen (Fig. 2.2b). It has to be noted that the current is proportional not to the actual concentration but to the activity or equivalent partial pressure of dissolved oxygen, which is often referred to as oxygen tension. A fixed voltage between -0.6 and -0.8 V is usually selected as the polarization voltage when using Ag/AgCl as the reference electrode.
DO Sensor. When the cathode, the reference electrode , and the electrolyte are separated from the measurement medium by a polymer membrane, which is permeable to the dissolved gas but not to most of the ions and other species, and when most of the mass transfer resistance is confined in the membrane, the electrode system can measure oxygen tension in various liquids. This is the basic operating principle of the membrane covered polarographic DO probe (Fig 2.3).
Signal Conditioning. To read the output from the sensor, the current from the sensor is first converted to voltage by the circuit shown in Fig. 2.4 (the first 1/2 of an operational amplifier LF412). This circuit has a gain of 10,000,000:
Therefore, 0.1 micro ampere sensor current will produce an output of - 1V at pin 1 of LF412 (note that R1 can be changed to obtain other amplifier
Fig. 2.1. Setup for polarography.
Fig. 2.2. (a) Current-to-voltage diagram at different oxygen tensions; (b) calibration obtained at a fixed polarization voltage of -0.6V.
Fig. 2.3. Membrane covered polarographic oxygen sensor.
Fig. 2.4. Circuit for current to voltage conversion and application of polarization voltage.
gains). The next stage is an inverting amplifier with gain. The output from this stage is:
where R2 is the resistance in the feedback loop which can be adjusted. The application of the polarization voltage is done by a 79L05 voltage regulator that converts its input voltage of -12V to -5V. At the output of 79L05, a voltage divider (R3) is used to convert -5V to -0.7V, which is then applied to the + input of LF412. The voltage output V2 can be read either by a voltmeter or by a computer equipped with an analog-to-digital converter.
Electrode Reactions. For polarographic electrodes, the reaction proceeds as follows:
Cathodic reaction:: O2 + 2H2O + 2e- --> H2O2 + 2OH-
H2O2 + 2e- -->2OH-
Anodic reaction:: Ag + Cl- --> AgCl + e-
Overall reaction:: 4 Ag + O2 + 2H2O + 4
Cl- --> 4 AgCl + 4 OH-
the reaction tends to produce alkalinity in the medium together with a small amount of hydrogen peroxide.
Number of Electrons Involved. Two principal pathways was proposed for the reduction of oxygen at the noble metal surface. One is a 4 electron pathway where the oxygen in the bulk diffuses to the surface of the cathode and is converted to H2O via H2O2 (path a in Fig. 2.5). The other is a 2 electron pathway where the intermediate H2O2 diffuses directly out of the cathode surface into the bulk liquid (path b in Fig. 2.5). The oxygen reduction path may change depending on the surface condition of the noble metal. This is probably the cause for time-dependent current drift of polarographic sensors. Since the hydroxyl ions are constantly being substituted for chloride ions as the reaction starts, KCl or NaCl has to be used as the electrolyte. When the electrolyte is depleted of Cl-, it has to be replenished.