Oxygen Measurement Techniques

DO Probe

Electrochemical Method:
Composed of two metal electrodes in contact with supporting electrolyte and separated from the test solution by a gas permeable membrane. A constant voltage is placed across the cathode and anode. Oxygen diffuses through the membrane and is reduced at the cathode by the voltage. This process produces a current flow, which is detected by the meter and is proportional to the partial pressure of oxygen.

• Saves money, time, and labor
• No preparation of reagents or titration
• Allows for continuous measurement.

Advances in Probe Technology

Polarographic: Single vs. Dual Thermistor

• Added thermistor to electrolyte (in addition to sample sensor).

This allows temperature measurement in sample [water or air] AND in electrolyte. Air is poor heat sink (think of cooking in a copper vs. aluminum pot).

• People have tendency to calibrate too quickly when doing AIR calibrations.
• Double thermistor monitors differential between air and electrolyte and does not lock in calibration until the two are equal in temperature.
• Bottom Line: provides for more accurate & consistent calibrations.

Back to Top

Luminescence

A whole new technology for environmental chemistry, but which has been in use in medical field for years

Luminescence DO Technology

Luminescence technology I: Initially Blue light from an LED inside the probe excites luminerscent material on the sensor surface. Image courtesy of HACH, Inc.

Features of Luminescence DO Probe

Luminescence technology II: As oxygen molecules encounter the sensor surface, they react with the luminescent material, causing it to return to its un-excited state. This results in emission of red light which is correlated to oxygen content. Image courtesy of HACH, Inc.

• Developed by the HACH company
• Probe uses NO:
• Anode
• Electrolyte
• Cathode
• Membrane
• All of the items listed above are potential sources of error in conventional DO technology
• Research shows luminescence requires
• Less frequent adjustments during calibration
• Less maintenance
• Dis-advantages of luminescence probe
• Requires an awkward funnel and magnetic stirrer
• The funnel is difficult to use and poses a cross-contamination risk

Back to Top

Can I use the Luminescence probe for my wastewater compliance testing?

• YES!
• Changes to ch. NR 219, Wis,. Admin. Code, which took effect June 1, 2009 included approved methodologies for Luminescence technology.

DO Probe Maintenance

• Electrolyte replenishment
• Membrane failure
• Membrane rupture
• Membrane fouling
• Cathode and anode cleaning
• Follow manufacturer recommendations for interval & procedure

Troubleshooting DO Probe Problems:
see NCL website (www.nclabs.com) for solid information regarding diagnosing DO probe problems.
Some information included at the end of this presentation.

YSI 5100 On-board tools for monitoring membrane problems

• DO µA (micro-amps) should be from 8.0 to 17.0
• %/ µA (micro-amps) should be from 5.9 to 12.6%
• Replace membrane if outside this range
.
• Good tool for preventative maintenance

YSI on-board tools for probe maintenance: Step 1- Press 'Diagnosis'

>YSI on-board tools for probe maintenance: Step 1- Press 'Diagnosis'

YSI on-board tools for probe maintenance: Step 3 - The display will now show probe values for microamps(uA) and microAmp percentage (uA %).

Back to Top

GeneralProbe Maintenance

• Examine membrane daily
• Replace membrane if air bubbles, wrinkles, or if there is buildup on or under the membrane
• If gold tip (cathode) is tarnished, clean it
• The gold should have a bright "matte" finish with fine scratches-DO NOT polish too much!
• Clean with sanding disk provided in the YSI cap
• The silver anode should have a light silver color
• Clean if anode is dark
• Clean by soaking overnight in household ammonia solution
• Rinse thoroughly with tap water, DI water and electrolyte

A probe in poor condition (left) vs. a probe in good condition (right). Note the light siolver appearance of the anode and the gold matte finish of the cathode on the right.
Source: YSI Technical Note -
How to Ensure Successful & Accurate BOD Measurements

Troubleshooting: DO probe calibration problems

• Poor calibration may give the appearance of a dilution water problem when the water may be fine
• Recommend calibrating using the air-saturated WATER calibration or Winkler titration
• If air-saturated water calibration is used, use a good quality barometer in the laboratory
• Check the barometer calibration against a reliable source at least quarterly (internet, airport, local station). Remember you must re-correct for actual altitude.

Back to Top

Troubleshooting: DO Probe malfunctions

Information obtained from www.nclabs.com

1. Allow ≥ 2 hr after membrane change for the probe to stabilize. Overnight is better.
2. Warm-up instrument. Calibrate.
3. Observe readings continuously for 2 mins. w/probe in bottle.
4. Be sure the temperature is constant.
5. Watch the readings carefully.

DO NOT just record the initial reading and come back 2 minutes later.
You need to actually see what happens over the time period.

• If readings drifts slowly DOWN, a longer warm up time is required.
• If readings JUMP AROUND, the probe is not functioning properly.
• If readings STABLE in the air calibration bottle, sensor is probably OK.
• If readings stable in the air calibration bottle but not in solution, the membrane is probably defective.

Zero Oxygen Check (Response check):

• Dissolve 0.5-1 grams of sodium sulfite (Na₂SO₃) in 300 mL of lab reagent water.
• Stir slowly--avoid “tornadoes”; slowly pour into a BOD bottle.
• Calibrate your DO probe as you normally would.
• Place the probe into the "Zero Oxygen" solution
• Observe!
• Meter should read "0" within two minutes.
• (With some older YSI systems, readings below 1.0 mg/l are considered zero.)

Be sure to rinse everything thoroughly. Sodium sulfite is a significant oxygen scavenger, so residue left behind will result in oxygen depletion from samples.

Back to Top