Tech Note 497 Detail

Dissolved Oxygen Probe Care

Affected Products:

CI-6542 Dissolved Oxygen Sensor
PS-2108 PASPORT Dissolved Oxygen Sensor
PS-2169 PASPORT Water Quality Sensor

Problem/Symptom:
Dissolved Oxygen Sensor Preparation, Storage & Calibration

PASCO Solution:

THEORY

The dissolved oxygen sensor uses a Ag/AgCl polarographic probe, which consists of two metals of different corrosion-resistance (nobility), which serve as electrodes: silver and platinum. The more corrosion-resistant platinum is the cathode. A potential of 700 mV is applied between the two electrodes, which causes dissolved oxygen molecules that diffuse to the surface of the cathode to acquire electrons which, in combination with water, produces hydroxyl (OH-) ions, while hydroxyl ions give up electrons at the anode and form an oxide.

Cathode (Reduction): O2 +2H2O+ 4e- ⇒ 4OH-

Anode (Oxidation): 4Ag+4Cl- ⇒ 4e- + AgCl
The resulting transfer of electrons establishes a current flow through an external circuit that is proportional to the oxygen concentration, but also depends on the temperature of the solution. The temperature is measured by an integrated sensor, to compensate for this dependence.
PREPARATION

Eight hours prior to the first use of the sensor, fill the Dissolved Oxygen Sensor electrode membrane cartridge and cartridge housing with polarographic solution. Calibration every 2-3 hours is recommended.

CALIBRATION

There are three valid calibration methods. All of the methods require ~20 minutes. The first two methods measure the concentration of dissolved oxygen in air at 100% relative humidity, which should give the same reading as water that is constantly aerated (defined as 100% saturation), but it may be difficult to convince the students of this fact. The last method is the most intuitive, as it directly measures the dissolved oxygen concentration in oxygen saturated water; however, it requires constant aeration of the water.

Quick Calibration Method (Fast, Moderate Accuracy, Unintuitive)

  1. Place about 5 mL (~1 cm) of deionized water into a clean soaker bottle.
  2. Check that the dissolved oxygen probe is full of fresh polarographic solution and that the membrane is intact.
  3. Connect the probe to a sensor box that is connected to a powered interface or computer.
  4. Slip the cap and o-ring of the soaker bottle over the end of the probe.
  5. Insert the probe into the soaker bottle and screw on the lid. Adjust the height of the end of the probe to about 2 cm above the top of the water.
  6. Wait for at least 20 minutes.*
  7. Shake the soaker bottle vigorously for about 10 seconds.
  8. Shake off any large water drops from the membrane.
  9. Monitor the dissolved oxygen value until the value stabilizes (2-4 minutes).
  10. Enter the calibration value.
    1. Go to the calibration screen:
      1. DataStudio: Setup/Calibrate Sensors.
      2. Xplorer GLX: Home/sensors/F4/calibrate.
      3. SPARK: Go to Home/Show/Tools/Calibrate Sensor. Enter Calibration Type: 1 point and Measurement: Dissolved Oxygen (mg/L). Click Next.
    2. Enter the standard value from Table 1 of the PS-2108 Manual corresponding to the temperature and pressure of the atmosphere as the standard value:
      1. DataStudio/SPARK: Enter the value in Calibration Point 2/Standard Value.
      2. Xplorer GLX: Enter the value in the Pt 2 (mg/L) text box.
    3. If the water is brackish or saline, use a PASCO PASPORT Salinity Sensor - PS-2195 to find the temperature and conductivity of the water and then multiply the standard value by the correction factor in the table in Tech Note 1088.
    4. Measure a value corresponding to the standard value.
      1. DataStudio / SPARK: Click the Read from Sensor button.
      2. Xplorer GLX: Press the Read Pt 2 button (F4).

Saturated Air Method (Slow, Accurate, Moderately Intuitive)

  1. Make sure the probe is full of polarographic filling solution, that the membrane is intact.
  2. Connect the probe to a sensor that is connected to a powered interface or computer.
  3. Dampen the probe tip with a wet cloth. (There should not be any drops of water on the membrane.)
  4. Insert the dissolved oxygen sensor in the soaker bottle filled with enough distilled water to cover the bottom of the soaker bottle.
  5. Make sure that the top of the probe is above the surface of the water.
  6. Tighten the lid to seal the probe above the water level.
  7. Monitor the dissolved oxygen value until the value stabilizes.* (~ 20 minutes)
  8. Enter the calibration value.
    1. Go to the calibration screen:
      1. DataStudio: Setup/Calibrate Sensors.
      2. Xplorer GLX: Home/sensors/F4/calibrate.
      3. SPARK: Go to Home/Show/Tools/Calibrate Sensor. Enter Calibration Type: 1 point and Measurement: Dissolved Oxygen (mg/L). Click Next.
    2. Enter the standard value from Table 1 of the PS-2108 Manual corresponding to the temperature and pressure of the atmosphere as the standard value:
      1. DataStudio/SPARK: Enter the value in Calibration Point 2/Standard Value.
      2. Xplorer GLX: Enter the value in the Pt 2 (mg/L) text box.
    3. If the water is brackish or saline, use a PASCO PASPORT Salinity Sensor - PS-2195 to find the temperature and conductivity of the water and then multiply the standard value by the correction factor in the table in Tech Note 1088.
    4. Measure a value corresponding to the standard value.
      1. DataStudio / SPARK: Click the Read from Sensor button.
      2. Xplorer GLX: Press the Read Pt 2 button (F4).

Bubble Method (Intuitive, Accurate, Equipment Intensive)

  1. Make sure the probe is full of polarographic filling solution and that the membrane is intact.
  2. Connect the probe to a sensor box that is connected to a powered interface or computer.
  3. Fill a small beaker with water.
  4. Bubble air through a filter at the bottom of the tank. (The bubbles should be very small.)
  5. Use a magnetic stirrer to move the water at 0.3 - 1.0 m/s.
  6. Immerse the probe in the water.
  7. Monitor the data until the value stabilizes (~ 20 minutes).
  8. Enter the calibration value.
    1. Go to the calibration screen:
      1. DataStudio: Setup/Calibrate Sensors.
      2. Xplorer GLX: Home/sensors/F4/calibrate.
      3. SPARK: Go to Home/Show/Tools/Calibrate Sensor. Enter Calibration Type: 1 point and Measurement: Dissolved Oxygen (mg/L). Click Next.
    2. Enter the standard value from Table 1 of the PS-2108 Manual corresponding to the temperature and pressure of the atmosphere as the standard value:
      1. DataStudio/SPARK: Enter the value in Calibration Point 2/Standard Value.
      2. Xplorer GLX: Enter the value in the Pt 2 (mg/L) text box.
    3. If the water is brackish or saline, use a PASCO PASPORT Salinity Sensor - PS-2195 to find the temperature and conductivity of the water and then multiply the standard value by the correction factor in the table in Tech Note 1088.
    4. Measure a value corresponding to the standard value.
      1. DataStudio / SPARK: Click the Read from Sensor button.
      2. Xplorer GLX: Press the Read Pt 2 button (F4).

For more information on calibration, see the PS-2108 manual. For troubleshooting, see Tech Note 206.

Electrode Cleaning and Storage

If you intend to use the Dissolved Oxygen probe, the probe should have a clean membrane, the inner electrode should be clean and have been completely wetted with polarographic solution for at least eight hours, and the probe should be calibrated; therefore, when removing the sensor from long-term storage, the lab coordinator should fill the probe with polarographic solution the day before measurements.

Short-term Storage ( < 2 weeks)

  1. Add ~10 ml of deionized water to the electrode storage bottle and seal it tightly over the end of the probe.
  2. (Optional) Store the probe and soaker bottle in a sealed plastic bag.

Long-term Storage ( > 2 weeks)

  1. Hold the probe below the stainless steel band and unscrew the cartridge housing at the end of the probe.
  2. Discard the old filling solution from the cartridge housing.
  3. Rinse the silver electrode with tap water.
  4. Polish the silver electrode with paper towel to remove loose silver oxide. (If the electrode is badly corroded, a silver polish should be used.)
  5. Screw the cap back on and allow the electrode to air dry.
  6. Replace the dry soaker bottle to protect the membrane from damage.

Maintenance

The long-term storage procedure will normally be all that is required to maintain the dissolved oxygen electrode; however, if the probe is abnormally slow to respond or has a damaged membrane, then servicing the electrode is recommended.

Required

CI-6541 Electrode Maintenance Kit

Procedure

  1. Unscrew and remove the cartridge housing at the end of the probe.
  2. Discard the old filling solution from the cartridge housing.
  3. Rinse the inner electrode and polish dry with a paper towel.
  4. Use the plunger to push the membrane cartridge into and through the housing so that it comes out through the threaded end of the housing.
  5. Examine the O-ring in the housing. If it is damaged, remove it with fine-tipped tweezers and insert the replacement.
  6. Use the plunger to push the replacement membrane cartridge into the housing until it is seated at the tip of the housing.
  7. Refill the probe with polarographic filling solution.

References

It takes a considerable amount of time for the sensor to stabilize to allow

  • the probe to become polarized,
  • the relative humidity to reach 100%, and
  • allow time for the water temperature to equilibrate with the atmosphere, which is where the temperature compensation ring is located.

The polarographic solution is more than just deionized water; however, its precise formulation is not an exact science: K+ and Cl- ions are added to increase electrical conductivity. Glycerol is used to increase the viscosity of the filling solution to reduce leakage through either the membrane or o-ring. The electrolyte facilitates dissolved oxygen migration and provides an electrical path to complete the current loop. The electrolyte also removes metal oxides (a by-product of the reaction) from the electrodes in order to help keep their metal surfaces clean. Electrolyte must be periodically replenished to ensure that the electrodes remain clean. One particularly simple formulation of the filling solution is as follows: 60% glycerol, 7.45% KCl, 32.55% water.


Related Technotes:
TechNote 206 DO Probe Troubleshooting

Creation Date: 06/16/2004
Last Modified: 06/12/2012
Mod Summary: