Support website

The dissolved Oxygen electrode is a galvanic MPOD (Membrane-covered Polarographic Oxygen Detector) system. They provide a convenient approach to an essential direct measurement of molecular oxygen. The membrane isolates the electrode from the sample and the oxygen is detected as it diffuses across the membrane. The probe can be used to measure any system where oxygen is present. Calibration of the sensor is readily achieved if the oxygen concentration or solubility at a specific condition is known.

A galvanic MPOD consists internally of two metals of different nobility which serve as electrodes. The more noble metal is the cathode (Silver, Platinum or Gold), the other (Lead or Zinc) is the anode. In the presence of an electrolyte, a voltage around 800 mV is produced between the two metals.

Molecules of oxygen dissolved in the electrolyte will diffuse to the surface of the cathode and pick up electrons which, in combination with water, will produce hydroxyl ions. At essentially the same time, hydroxyl ions will give up electrons at the anode and form an oxide. The resulting transfer of electrons establishes a current flow through an internal resistance which is measured as a voltage by the oxygen meter.

The membrane oxygen electrode offers the following advantages:

• The membrane encloses the two electrodes in a captured volume of electrolyte, ensuring constant electrolyte strength and purity so that ions which might otherwise “poison” the probe are not present.
• The membrane excludes materials that do not diffuse through it. As a result, most materials in the sample that might “poison” the cathode, or might cause an erroneous output from the electrode system are excluded. Potential interferences are limited to reactive gases which diffuse through the membrane, such as chlorine.

The cathode for a galvanic style probe is typically Silver (Ag) or Platinum (Pt), the anode is commonly Lead (Pb), with the electrodes immersed in an alkaline 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.

In order to optimise 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. 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.

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 cm2 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 (D).

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 recognise desired response rate and rugged performance. Thin membranes provide quick response in addition to sensitivity because diffusion equilibrium is reached more quickly, 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 rate 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 µg/l, 99% response is obtained in about 70 seconds, and from 10 µg/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 µg/l operating level. A 1/2 mil membrane will respond from the mg/l to the µg/l level in approximately 30 minutes.

There are two factors related to temperature which must be recognised in order to correlate the output of the dissolved oxygen probe with concentration of molecular oxygen in the sample.

• 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.
• 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. Therefore, if the read-out from the dissolved oxygen meter 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.

Both of the temperature factors serve to decrease the rate at which oxygen molecules reach the surface of the cathode as temperature decreases. Therefore, if the read-out from the dissolved oxygen meter 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 analyser. 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 0°C to 50°C.Effects of dissolved solids on the probe output

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 output from the probe will increase. As a result, the meter 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 concentration of oxygen molecules at the water-to-Teflon interface. The concentration of oxygen in the sample has not changed.