Oxidation-Reduction Potential Probe Protocol

Oxidation-Reduction Potential Probe Protocol

Oxidation-Reduction Potential Probe Protocol

Source:American Public Health Association (APHA) 1998. Oxidation-Reduction Potential (ORP). p. 2-75-2-78. In Standard methods for the examination of water and wastewater, 20th ed. American Public Health Association, Washington, DC, USA.

Background:

Oxidation (donating electrons) and reduction (accepting electrons) reactions, also called redox reactions, mediate the behavior of many chemicals in aquatic environments. The reactivities and mobilities of important elements in biological systems (such as Fe, S, N, and C) and several metallic elements depend strongly on redox (oxidation-reduction) conditions. Redox reactions are driven by biological catalysts (such as bacteria). Reactions involving electrons are Eh-dependent; therefore, chemical reactions in aqueous media can be characterized by Eh. Eh (just like pH) represents an intensity factor, not the capacity of the system for oxidation or reduction. Eh values for soils range from +700 to -300. Anaerobic conditions exist at Eh values of +300 and lower. Negative values represent high electron activity, resulting from permanently waterlogged soils. This is due to a greater potential for electron transfer in the absence of oxidized species such as oxygen, nitrate, and oxidized forms of Fe and Mn.

Materials:

Equipment

For measuring redox potential (Eh)

  • Reference electrode: double junction Ag+/AgCl with a glass outer body and ceramic junction
  • Consists of half-cell providing a constant electrode potential
  • Fisher Scientific Catalog No. 13-620-273
  • Oxidation-reduction indicator electrode (such as the lab’s platinum electrodes)
  • At redox equilibrium, the potential difference between the ideal indicator electrode and the reference electrode equals the redox potential of the system
  • pH/millivolt meter readable to ± 1400 mV scale
  • The lab’s current pH meter, an Orion Model 250A Portable pH/ISES meter is appropriate
  • Splitter: plugs into the pH meter, has two terminal ends marked with green and red tape, connects to indicator and reference electrodes
  • Alligator clip: attaches to the copper end of the indicator electrode (platinum electrode) and plugs into the splitter

For Indicator Electrode (platinum electrode) Calibration

  • Two glass beakers for
  • Light’s solution
  • Rinse waste
  • Ring clamp and stand (to stabilize the reference electrode in the Light’s Solution)
  • Thermometer

Reagents

For calibration of reference electrode (Ag+/AgCl electrode)

  • Redox standard solution: Light’s Solution
  • A relatively stable solution with predictable oxidation-reduction potentials
  • Can be ordered or made in the lab
  • Fisher Scientific Catalog No. 433016
  • Chemicals needed/1000mL aqueous solution at 25°C

-39.21 g ferrous ammonium sulfate (Fe(NH4)2(SO4)2*6H2O

-48.22 g ferric ammonium sulfate (Fe(NH4)(SO4)2*12H2O

-56.2 mL sulfuric acid, sp gr 1.84 (H2SO4)

  • Potentials of platinum electrode vs Ag/AgCl Electrode at 25°C in Light’s Solution
    KCl (1.00M): +439 ± 10mV

Preparation:

Reference Electrode (double junction Ag+/AgCl electrode) Preparation

The two bodies must be separately filled with the appropriate solutions. A snug-fitting O-ring seal in the cap assembly holds the bodies together. To separate them, grasp the lower section of the cap with one hand and, with the other hand, pull outward on the upper lip.

Fill the inner body as follows:

  1. Remove the soaker bottle from the probe tip. Save the bottle for future storage.
  2. Add electrolyte to the inner body until the electrolyte level is approximately ¼ inch below cap
  3. Proper electrode performance requires electrolyte flow through the junction location at the tip of the inner body. Flow should be checked by performing the following procedure:
  4. Hold the electrode upright at a 45° angle between the thumb and the forefinger of the left hand, so that the filling hole faces out and is directly opposite the base of the thumb.
  5. Insert the spout of the dispensing bottle into the fill hole.
  6. Making sure that the electrode is supported by the base of the thumb, firmly press the spout into the fill hole to make an airtight seal.
  7. While maintaining the seal, squeeze the filling bottle so that the electrode becomes pressurized. A bead of liquid should form at the liquid junction in about 30 seconds; in some cases it may be necessary to maintain pressure for several minutes.

Reassemble the two bodies, positioning the inner body fill hole 180° away from the outer body fill hole.

Fill the outer body as follows:

  1. Lower the rubber sleeve to expose the fill hole.
  2. Fill the outer body with the supplied electrolyte 1.0M KNO3, or other appropriate bridge electrolyte to a level approximately ¼ inch below the fill hole.
  3. Note: To prepare 1.0M KNO3 solution, place 10.11g of reagent-grade KNO3 into a 100 ml volumetric flask and fill to mark with distilled or deionized water.
  4. Initiate flow as follows for Ceramic junction type (which is what we have)
  5. Hold the electrode upright at a 45° angle between the thumb and the forefinger of the left hand, so that the filling hole faces out and is directly opposite the base of the thumb.
  6. Insert the spout of the dispensing bottle into the fill hole.
  7. Making sure that the electrode is supported by the base of the thumb, firmly press the spout into the fill hole to make an airtight seal.
  8. Note: Normally the spout tip will not touch the internal element. While applying pressure, care should be exercised to prevent contact with this element. If contact occurs, it will be necessary to cut off a small portion of the tip.
  9. While maintaining the seal, squeeze the filling bottle so that the electrode becomes pressurized. A bead of liquid should form at the liquid junction in about 30 seconds; in some cases it may be necessary to maintain pressure for several minutes.
  10. Mount the electrode on a suitable holder and connect to the meter.

pH/mV Meter Preparation

  • We have the Orion Model 250A Portable pH/ISES meter.
  • No calibration steps needed for mV measurements.
  • Our meter has a range of -1600.0 to +1600.0 mV with a displayed resolution of 0.1 mV.

Access the absolute millivolt mode by pressing the mode key until the mV mode indicator is displayed.

Set Up:

  • Connect the splitter to the top of the pH/mV meter (there is only one port which fits the splitter)
  • On the green-taped end of the splitter, plug in the alligator clip
  • On the red-taped end of the splitter, plug in the reference electrode (double junction Ag+/AgCl electrode)
  • Secure the alligator clip onto the copper end of the platinum electrode

Procedure:

Important Notes on Using the Reference Electrode (double junction Ag+/AgCl electrode)

  • Maintain inner body electrolyte level so that the reference element is always covered. To prevent backflow, inner body electrolyte level should always be higher than outer body electrolyte level.
  • Maintain outer body electrolyte level higher than sample solution level to prevent backflow.
  • For proper electrolyte flow during measurements, ensure that fill holes are open.
  • To prevent contamination of reference and sample solutions, always rinse outer body with distilled or deionized water when transferring from one solution (or sample) to another.

Calibration and Cleaning:

Indicator Electrode (platinum electrode) Calibration

  • Working under a hood, set up ring clamp and stand to support the reference (Ag+AgCl) electrode in a glass beaker with enough Light’s Solution to immerse the probe tip
  • Turn on pH/mV meter(make sure it is set to millivolt mode)
  • Allow meter to run for up to several minutes, until mV reading is relatively stable
  • Record reading in mV
  • Expected value:
  • Potentials of platinum electrode vs Ag/AgCl Electrode at 25°C in Light’s Solution
    KCl (1.00M): +439 ± 10 mV
  • Adjust for temperature by doing one of the following:
  • Add 1.5 mV for every ⁰C over 25 ⁰C
  • Subtract 1.5 mV for every ⁰C below 25 ⁰C
  • e.g. -200 mV reading at 28 ⁰C  corrected reading -195.5 mV

Note: During past indicator (platinum) electrode calibrations of 11 probes, we have found that over time the Eh readings steadily increase (by roughly 10 mV/hr). This may be due to the Light’s Solution oxidizing as it is exposed to air while in the open beaker.

Indicator Electrodes (platinum electrodes) Cleaning

  • Before using more intense cleaning reagents (see below), try soaking the platinum electrodes (just the platinum tips) in soapy water for 15 minutes, rinse with ultrapure water, then gently wipe dry with Kimwipes
  • Can also try: soak electrodes (just the platinum tips) in 2N HCl (from acid bath) for 30 minutes, then rinse with Milli-Q water.

Storage:

Reference Electrode (double junction Ag+/AgCl electrode) Storage

For periods of non-use, the electrode should be prepared for storage. Separate the inner and outer bodies and perform the following:

  1. Inner body storage
  2. Place supplied small rubber sleeve on inner body and position to cover fill hole.
  3. Wet cotton in supplied small cot with SP135 or saturated KCl solution and place cotton over inner body junction.
  4. Store prepared inner body in an upright position.
  5. Outer body storage
  6. Insert supplied cap plug into cap end of the outer body
  7. Place supplied large rubber sleeve on outer body and position to cover fill hole.
  8. Fill soaker bottle with 1M KNO3 solution (or other electrolyte in use) and place bottle over the outer body junction.
  9. Store prepared outer body in upright position.
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