The cathode for a polarographic style probe is typically platinum, the anode is commonly silver, with the electrodes immersed in a potassium chloride electrolyte solution. For convenient application of the probe, the electrode system is enclosed in a housing with the cathode and membrane positioned for exposure to the sample. Only the area of membrane in contact with the cathode need be exposed to the sample.
Membrane Orientation
In order to optimize the performance of the membraned probe, the cathode is located adjacent to the membrane. The cathode is given a convex surface so that the membrane can be drawn closely over the cathode. The platinum surface is roughened to permit the necessary access of electrolyte to the cathode surface. Key features in orientation of the membrane and cathode are that the membrane must be close to the cathode and the orientation must not change during operation of the probe.
Anode Area
As indicated under theory, silver at the surface of the anode is converted to silver oxide as the determination of oxygen proceeds. When silver is not available at the surface of the anode because it has become coated with silver oxide or other reaction products, sensitivity of the probe decreases. Therefore, in order to give the probe an extended performance life, the area of silver relative to the cathode should be as large as is practical.
Membrane Thickness and Cathode Area
Molecular oxygen reaches the cathode by diffusion through a Teflon membrane. For a given thickness of membrane, at a given temperature, the number of molecules of oxygen which pass through the membrane per unit of time is directly proportional to the number of molecules which are present per unit area of water-to-Teflon interface (sometimes referred to as partial pressure) or:
Nt = f (O2)
where:
* Nt is the number of oxygen molecules arriving at the cathode per second per sq. cm of cathode area at Temperature t. * O2 is the concentration of oxygen molecules at the water-to-Teflon interface. * Teflon offers resistance to oxygen diffusion. Thus, at a given temperature and a fixed concentration of oxygen at the water-to-Teflon interface, the number of oxygen molecules arriving at the cathode per unit of time is inversely proportional to membrane thickness,
Nt = f [O2 x (1/D)]
From these relationships, it is evident that in order to have maximum sensitivity for the probe it is necessary to make the cathode area as large as is practical and to make the Teflon membrane as thin as is practical. The practical consideration for cathode size is its relationship to overall probe size, which usually is determined by where the probe must be placed. Membrane thickness must recognize desired response rate and rugged performance. Thin membranes provide quick response in addition to sensitivity because diffusion equilibrium is reached more wuickly, but thicker membranes are tougher and will provide longer service.
Membranes typically come in 1/2 mil, 1 mil, and 2 mil thicknesses. Response upscale proceeds at approximately a first-order rate. Increased thickness of membrane decreases the rate at which oxygen molecules reach the cathode. As a result, 99% completion of the upscale response is achieved in about 15, 30, and 75 seconds for 1/2, 1 and 2 mil membranes respectively.
Downscale response is significantly different from upscale response. The response ratre is essentially second order. The nature of the response indicates that an internal reaction of the probe as well as diffusion through the membrane is involved. For a 1 mil membrane, the time for a 99% response downscale is a function of the starting exposure to oxygen. From 10 mg/L 99% response is obtained in approximately 1 minute. From 100 ¥ìg/L, 99% response is obtained in about 70 seconds, and from 10 ¥ìg/L, 99% response is obtained in about 50 seconds. These results illustrate that excellent response is obtained in applications such as measurement of oxygen in boiler feed water. The probe may be calibrated at mg/L levels of oxygen and with a 1 mil membrane will require about 1 hour to reach the ¥ìg/L operating level. A 1/2 mil membrane will respond from the mg/L to the ¥ìg/L level in approximately 30 minutes.
Probe Response and Circuit Resistance
The electrical circuit for electron flow from the anode to the cathode is completed by the readout circuitry. If readout is through a galvanometer-type microampere meter, the resistance is that which is present in the winding of the meter and any resistors in series. A meter used for this service may characteristically have an internal resistance of about 2,000 ohms. If a potentiometric readout is provided, the input to the potentiometric circuitry is the voltage drop across a selected closing resistance between the cathode and anode of the probe. In either case, resistance is present in the electrical circuit between the anode and cathode.
The response rate for the probe, when exposed to a change in dissolved oxygen concentration, is influenced by the magnitude of the closing resistance. The response rates presented previously are based on a closing resistance of 2,000 ohms. If the closing resistance is decreased to 100 ohms, the downscale response for the probe using a 1/2 mil membrane will be about 99% complete in 10 seconds. If, for the same probe, the closing resistance is increased to 25,000 ohms, the downscale response will be about 99% complete in 120 seconds.
The upscale response rates of the probe is influenced to a much smaller degree by the closing resistance.
Temperature Effects on Probe Output Current
There are two factors related to temperature which must be recognized in order to correlate the output of the dissolved oxygen probe with concentration of molecular oxygen in the sample.
1. As the temperature of water decreases, kinetic energy of water and oxygen molecules decreases and molecular attraction increases. As a result, the concentration of oxygen which must be present in the water to establish a particular concentration of oxygen at the water-to-Teflon interface increases, and 2. the resistance to oxygen diffusion through the Teflon membrane increases as temperature decreases.
Both of the temperature factors serve to decrease the rate at which oxygen molecules reach the surface of the cathode as temperature decreases. Therefor, if the readout from the Dissolved Oxygen Analyzer is to display the correct reading of oxygen concentration for all samples which have the same concentration of oxygen but are at different temperatures, compensation for the overall temperature effect must be accomplished.
Temperature compensation is accomplished by the use of a suitably designed thermistor as a temperature sensor. The resistance of the thermistor is used to achieve a precise multiplication factor by the analyzer. The resulting display of dissolved oxygen concentration is corrected to within +/- 2% of the actual concentration when the samples are in the temperature range of 0oC to 50oC. Over sample ranges of +/- 10oC from the calibration temperature, the D.O. reading is within +/- 1% of the actual concentration.
Effects of Dissolved Solids on Probe Output Current
If a salt is added to and permitted to dissolve in a water sample which contains a specific concentration of dissolved oxygen (but is not saturated), the current output from the probe will increase. As a result, the display meter of the analyzer will incorrectly indicate that the dissolved oxygen concentration in the sample has increased. The reason for the increased output by the probe is that the presence of the dissolved salt decreases the molecular attraction of water and oxygen molecules in the sample. This increases the concentrationof oxygen molecules at the water-to-Teflon interface. The concentration of oxygen in the sample has not changed.