Field Measurement of Oxidation-Reduction Potential (ORP)

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Revision History

The top row of this table shows the most recent changes to this controlled document. For

previous revision history information, archived versions of this document are maintained by the

SESD Document Control Coordinator on the SESD local area network (LAN).

History Effective Date

SESDPROC-113-R2, Field Measurement of Oxidation-Reduction

Potential (ORP), replaces SESDPROC-013-R1

General: Corrected any typographical, grammatical, and/or editorial errors.

Title Page: Changed the EIB Chief from

Danny France to the Field Services

Branch Chief John Deatrick, and the Field Quality Manager from Bobby Lewis to

Hunter Johnson.

Section 2.2: Figure 6 modified for clarity.

Section 3.3: Use of overtopping cell described consistent with current practice.

April 26, 2017

SESDPROC-113-R1, Field Measurement of Oxidation-Reduction

Potential (ORP), replaces SESDPROC-013-R0

January 29, 2013

SESDPROC-113-R0, Field Measurement of Oxidation-Reduction

Potential (ORP), Original Issue

August 7, 2009

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TABLE OF CONTENTS

1 General Information ............................................................................................................ 4

1.1 Purpose........................................................................................................................... 4

1.2 Scope/Application ......................................................................................................... 4

1.3 Documentation/Verification ......................................................................................... 4

1.4 References ...................................................................................................................... 4

1.5 General Considerations ................................................................................................ 5

1.5.1 Safety .......................................................................................................................... 5

1.5.2 Records ....................................................................................................................... 6

1.5.3 Shipping ...................................................................................................................... 6

2 Background ........................................................................................................................... 7

2.1 General ........................................................................................................................... 7

2.2 Instrumentation............................................................................................................. 8

2.3 Redox Chemistry ......................................................................................................... 14

2.4 Applications ................................................................................................................. 15

2.5 Limitations ................................................................................................................... 16

3 Methodology ........................................................................................................................ 18

3.1 Standard Solutions ...................................................................................................... 18

3.2 Verification and Calibration ...................................................................................... 19

3.3 Measurement ............................................................................................................... 20

3.4 Reporting ..................................................................................................................... 21

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1 General Information

1.1 Purpose

This document describes procedures, methods and considerations to be used and observed when

conducting field Oxidation-Reduction Potential (ORP) measurements in aqueous environmental

media, including groundwater, surface water and certain wastewater. The measurement of soil

ORP is a non-standard measurement and procedures should be developed on a project-specific

basis.

1.2 Scope/Application

This document describes procedures generic to all ORP measurement methods to be used by

Science and Ecosystem Support Division (SESD) field personnel when collecting and handling

samples in the field. On the occasion SESD personnel determine that any of the procedures

described in this section are inappropriate, inadequate or impractical and that another procedure

must be used to obtain an ORP measurement, the variant procedure will be documented in the field

logbook, along with a description of the circumstances requiring its use. Mention of trade names

or commercial products in this operating procedure does not constitute endorsement or

recommendation for use.

1.3 Documentation/Verification

This procedure was prepared by persons deemed technically competent by SESD management,

based on their knowledge, skills and abilities and has been tested in practice and reviewed in print

by a subject matter expert. The official copy of this procedure resides on the SESD local area

network (LAN). The Document Control Coordinator (DCC) is responsible for ensuring the most

recent version of the procedure is placed on the SESD LAN and for maintaining records of review

conducted prior to its issuance.

1.4 References

Faulkner, S.P., W.H. Patrick, Jr., and R.P. Gambrell. 1989. Field techniques for measuring wetland

soil parameters. Soil Sci. Soc. Am. J. 53:883-890.

Megonigal, J.P., W.H. Patrick, Jr., and S.P. Faulkner. 1993. Wetland identification in seasonally

flooded forest soils: soil morphology and redox dynamics. Soil Sci. Soc. Am. J. 57:140-149.

D.K. Nordstrom and F.D. Wilde. 2005. National Field Manual, Chapter A6, Section 6.5: Reduction

Oxidation Potential (Electrode Method). USGS.

Pankow, J.E. 1991. Aquatic chemistry concepts. Lewis Publishers, Inc. Cheleas, Michigan. USA.

Pruitt, B.A. 2001. Hydrologic and soil conditions across hydrogeomorphic settings. Dissertation.

The University of Georgia, Athens, GA. USA.

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Soil Survey Staff. 1998. Keys to soil taxonomy, 8th Edition. United States Department of

Agriculture, Natural Resources Conservation Service, Washington, DC. USA.

Standard Methods. 1992. Standard Methods for the Examination of Water and Wastewater, 18th

Edition. Prepared and published jointly by: American Public Health Association, American Water

Works Association, Water Environment Federation. American Public Health Association,

Washington, DC. USA.

Stumm, W. and J.J. Morgan. 1981. Aquatic chemistry: an introduction emphasizing chemical

equilibra in natural waters, 2nd Ed. John Wiley & Sons, New York. USA.

USEPA. 2001. Environmental Investigations Standard Operating Procedures and Quality

Assurance Manual. Region 4 Science and Ecosystem Support Division, Athens, GA.

USEPA. 2007. Safety, Health and Environmental Management Program Procedures and Policy

Manual. Science and Ecosystem Support Division, Region 4, Athens, GA.

Wikipedia entry. Reduction Potential. http://en.wikipedia.org/wiki/Reduction_potential.

Retrieved April 2, 2009.

1.5 General Considerations

1.5.1 Safety

Proper safety precautions must be observed when verifying or calibrating instruments for

measurement of Oxidation-Reduction Potential. Refer to the SESD Safety, Health and

Environmental Management Program Procedures and Policy Manual (most recent version)

and any pertinent site-specific Health and Safety Plans (HASP) for guidelines on safety

precautions. These guidelines should be used to complement the judgment of an

experienced professional.

Reagents commonly used in the preparation of ORP calibration standards are toxic and

require care when handling. When using this procedure, avoid exposure to these materials

through the use of protective clothing, eye wear and gloves. Safety precautions when

handling and preparing verification solutions should include gloves and eyewear to prevent

dermal and eye contact, and a mask to avoid inhaling dust particles when handling dry

materials. Vigorous flushing should be used if the reagents or solutions come in contact

with skin or eyes. Following is specific information on commonly used solutions. The

application of the solutions is described in detail in Section 3.1, Standard Solutions, of this

procedure.

• Quinhydrone (CAS# 106-34-3) is a skin and respiratory irritant and is poisonous if

ingested. Safety precautions when handling quinhydrone should include gloves to

prevent dermal contact and a mask to avoid inhaling dust particles when mixing dry

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material to prepare calibration standards. Vigorous flushing should be used if

concentrated material comes in contact with skin or eyes.

• Zobell’s solution is also an irritant and toxic if ingested. The same handling

precautions apply when mixing and using Zobell’s solution as when using

quinhydrone. Zobell’s reacts with acid to form harmful byproducts, including

hydrocyanide gas.

• Light’s solution contains ferro- and ferric-cyanide compounds in sulfuric acid.

The components are toxic and burns are possible from contact with this solution.

• Potassium iodide solutions have lower toxicity than most calibration solution options.

General ingestion, skin contact, and eye contact precautions apply.

Unused quinhydrone, Zobell’s, Light’s or other calibration reagents and solutions should

be returned to SESD for disposal in accordance with the SESD Safety, Health, and

Environmental Management Plan (SHEMP).

1.5.2 Records

Documentation of field activities is done in a bound logbook. All records, including a

unique, traceable identifier for the instrument, should be entered according to the

procedures outlined in the SESD Operating Procedure for Logbooks (SESDPROC-010,

most recent version) and the SESD Operating Procedure for Equipment Inventory and

Management, (SESDPROC-108, most recent version).

All field ORP measurements pertinent to the sampling event should be recorded in the field

logbook for the event as outlined in the SESD Operating Procedure for Logbooks

(SESDPROC-010, most recent version), or managed electronically with appropriate

backups as described in SESD Operating Procedure for Control of Records (SESDPROC-

002, most recent version).

1.5.3 Shipping

Shipped material shall conform to all U.S. Department of Transportation (DOT) rules of

shipment found in Title 49 of the Code of Federal Regulations (49 CFR parts 171 to 179),

and/or International Air Transportation Association (IATA) hazardous materials shipping

requirements found in the current edition of IATA’s Dangerous Goods Regulations.

All shipping documents, such as bills of lading, will be retained by the project leader and

stored in a secure place.

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2 Background

2.1 General

Oxidation is the process of liberating electrons or gaining oxygen. Examples of oxidation include

conversion of elemental iron to rust, elemental sulfur to sulfate, and elemental hydrogen to water

(Pankow 1991). Reduction is the process of gaining electrons resulting in the charge on some

atomic unit in the species to be reduced. Oxidation-reduction potential (ORP) or redox potential

(hereafter, referred to as redox) is a measure of the intensity or activity of an aqueous environment

or soil to mediate reactions of important elements in biological systems (e.g., O, N, Mn, Fe, S, and

C) and other metallic elements.

Considerable confusion arises on the use of the terms oxidation and reduction as they apply to the

media under study. The following introduction reproduced from an online ‘Wikipedia’ article on

the topic lucidly explains their relationship in ORP measurement:

Reduction potential (also known as redox potential, oxidation / reduction potential or

ORP) is the tendency of a chemical species to acquire electrons and thereby be reduced.

Each species has its own intrinsic reduction potential; the more positive the potential, the

greater the species' affinity for electrons and tendency to be reduced.

In aqueous solutions, the reduction potential is the tendency of the solution to either gain

or lose electrons when it is subject to change by introduction of a new species. A solution

with a higher (more positive) reduction potential than the new species will have a tendency

to gain electrons from the new species (i.e. to be reduced by oxidizing the new species)

and a solution with a lower (more negative) reduction potential will have a tendency to lose

electrons to the new species (i.e. to be oxidized by reducing the new species). Just as the

transfer of hydrogen ions between chemical species determines the pH of an aqueous

solution, the transfer of electrons between chemical species determines the reduction

potential of an aqueous solution. Like pH, the reduction potential represents an intensity

factor. It does not characterize the capacity of the system for oxidation or reduction, in

much the same way that pH does not characterize the buffering capacity.

In short, a numerically positive redox potential or ORP represents an environment conducive to

the oxidation of an introduced substance by reduction of the original media.

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2.2 Instrumentation

ORP measurement systems are a practical implementation of electrochemical cells, which use

metal electrodes in a solution to generate an electric current or voltage. If a platinum electrode is

immersed in water with hydrogen bubbled into the solution, the H

is oxidized as follows:

= 2H

+ 2e

In the electrochemical half-cell illustrated below in Fig.1, hydrogen gas oxidizes to hydrogen ions

and free electrons, comprising an oxidation-reduction couple. This couple reaches an equilibrium

state that maintains the reference potential of the electrode. The electric potential develops on the

wire connected to the platinum electrode, but is difficult to measure in practice in the isolated half-

cell. However, when used in a complete electrochemical cell, the cell illustrated is used as a

reference to measure other half-cells against, and is called a Standard Hydrogen Electrode (SHE).

Figure 1

If, as shown in Figure 2, a SHE is connected with a salt bridge to a second half-cell in which a

reduction reaction is taking place, the electric potential between the two cells can be measured. In

the case shown, the potential of the right cell will be +0.34 Volts in reference to the standard

hydrogen electrode on the left. This would be represented as an Oxidation Reduction Potential

(ORP) of +340mV on the hydrogen scale, or simply as Eh = +340mV.

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Figure 2

In field practice, the hydrogen electrode is difficult to reproduce. To conduct field measurements,

a reference electrode is needed that is simple to maintain and will generate a potential that can be

referenced to the standard hydrogen electrode. These requirements are met by the Saturated

Calomel Electrode (SCE) and the Silver/Silver Chloride Electrode (SSCE - the SSCE is also

commonly identified as an Ag/AgCl electrode). The SCE contains a small amount of elemental

mercury, and while useful for certain applications, would rarely be used at SESD. The SSCE or

Ag/AgCl electrode is generally used as the reference cell in SESD instrumentation.

In Figure 3 below, a SHE is connected to an Ag/AgCl electrode. In this example of an

electrochemical cell, both cells reach an equilibrium potential. At that equilibrium state, the

potential of the Ag/AgCl cell is 220mV more positive than the standard hydrogen electrode.

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Figure 3

This half-cell potential of the Ag/AgCl electrode in reference to the SHE is used to convert

measurements taken with an Ag/AgCl reference back to the hydrogen scale. While the laboratory

Ag/AgCl half-cell shown has a potential of +220mV, practical reference cells have varying

potentials based on temperature and filling solutions as shown in Table 1 below.

Table 1

Half-cell Potential of Ag/AgCl reference electrode

derived from USGS NFM, Table 6.5.2 (9/2005)

Molarity of KCl filling solution

T(°C)

3.3M*

3.5M

Sat/4M

220

217

215

214

216

214

212

209

213

210

208

204

209

207

205

199

205

203

201

194

202

199

197

189

198

195

193

184

*interpolated value

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Note: YSI sondes and Thermo electrodes typically use 4M KCl filling solutions. Eureka sondes

typically use 3.3M KCl filling solutions.

In Figure 4, below, the relationship between a hydrogen electrode, a reference electrode, and a

platinum sensing electrode in an arbitrary media is shown. In this case, the ORP of the media in

reference to the silver/silver chloride electrode is 150mV. To obtain Eh, the potential of the

reference electrode in relation to a hydrogen electrode is added to the potential of the sensing

electrode in relation to the reference electrode. In practice, the potential of the reference electrode

in relation to a hydrogen electrode is not measured, but obtained from Table 1 above.

Figure 4

In Figure 5 below, a field instrument is represented as separate electrochemical cells. The

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Ag/AgCl reference electrode uses a ceramic frit or other means to provide the essential salt bridge

to the environmental media. The platinum sensing electrode is immersed in the environmental

media and connected internally in the instrument to measure the potential (voltage) between the

two electrodes.

Figure 5

In this illustration, the ORP is measured as 340 mV. This measurement is made in reference to

the Ag/AgCl reference electrode and would be reported as such, or as E

Ag/AgCl

= 340mV.

In some cases it will be desirable to report the reading on the hydrogen scale, or Eh. To do so, the

potential of the reference electrode against the SHE, obtained from Table 1, is added to E

Ag/AgCl

For our example:

340 mv Measured ORP (E

Ag/AgCl

) of sample

+ 204 mV Eh of Ag/AgCl electrode (ORP of Ag/AgCl electrode referenced to SHE)

544 mV Eh of sample

Both the +340 mV field reading and the adjusted +544 mV Eh can properly be referred to as ORP

results. It is only through specifying the reference scale that the ambiguity can be eliminated.

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In Figure 6, below, the theoretical cells shown above have been configured as a practical field

instrument. The salt bridge is commonly provided by a ceramic frit connecting the environmental

media to the reference electrode. In multi-parameter sondes, the pH probe commonly uses the

same reference electrode as the ORP probe.

Figure 6

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Redox Chemistry

In acid-base chemistry, the pH of a system is defined as the negative logarithm of the hydrogen

ion activity (simplified in practice to the hydrogen ion concentration):

pH = -log {H

}

Similarly, Pankow (1991) described the negative logarithm of the electron activity (pe) as the

master variable for describing the equilibrium position for all redox couples in a given system:

pe / - log {e

}

It can be shown (Pankow) that pe is related to Eh by

Eh = pe*(2.303*R*T)/F

Where:

R = gas constant = 8.314 J K

-1

mol

-1

T = temperature,

F = Faraday constant = 96.485*10

C mol

-1

At 25°C (298°K) this simplifies to

= pe * 0.05916

And

pe =E

/ 0.05916

According to Faulkner et al. (1989) redox is a quantitative measure of electron availability and is

indicative of the intensity of oxidation or reduction in both chemical and biological systems. When

based on a hydrogen scale, redox (E

) is derived from the Nernst Equation (Stumm and Morgan

1981):

= E

+ 2.3 Η (R Η T)/nF Η log (ϑ

{ox}

/ϑ

{red}

)

Where:

= potential of reference, mV

R = gas constant = 81.987 cal deg

-1

mole

-1

T = temperature,

n = number of moles of electrons transferred

F = Faraday constant = 23.061 cal/mole-mv

{ox} and {red} = activity of the oxidants and reductants, respectively

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2.4 Applications

When interpreted properly, redox combined with other conventional water quality parameters is

useful in developing a more complete understanding of water chemistry. Several applications of

redox are identified below:

1. Redox could be viewed as an extension of the oxygen scale. In this model, the DO probe

spans the aerobic scale and the redox probe extends that scale to measure anaerobic

conditions. Inferences to geochemistry and chemical speciation can be made from the

oxidative state of the system. Application to metal sequestration, metal-iron, -sulfide, -

methane complexation, and the subsequent bioaccumulation potential is possible.

2. Redox can be used to identify anaerobiosis at or near the water column and sediment

interface in streams, lakes, and estuaries.

3. Redox may be useful in determination of stream jurisdiction and wetland delineation in

that it can indicate conditions of soil saturation.

4. Based on redox, a pe (or EH) vs. pH stability diagram can be developed to aid in nutrient

exchange studies including the timing, release, and partitioning of important water and

sediment quality pollutants such as nitrogen and phosphorus species. Most importantly,

redox can be used to address error associated with chamber-effect during closed chamber

measurements of the water-sediment interface. Redox probes placed inside the contact

chamber and inserted approximately ten centimeters into the underlying sediment can be

used to monitor changes in sediment redox caused by the chamber, and steps can be taken

to reduce chamber-effect.

5. Redox may be useful in establishing water and sediment quality standards applicable to

wetlands.

6. Redox is used to assess the potential of a groundwater system to support various in situ

reactions with contaminants, such as reductive dechlorination of chlorinated solvents.

7. Redox can provide a useful indicator of conditions that might compromise the performance

of Clark-type dissolved oxygen (DO) probes. In general, anaerobic conditions occur at a

redox range of +150 mV to +300 mV (pH-dependent and adjusted to hydrogen reference

electrode). When redox drops below this level, DO measurements as determined with a

Clarke-type probe are highly suspect as the semi-permeable membrane does not

discriminate between partial O

and sulfides. Consequently, the meter may be reading

sulfides.

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2.5 Limitations

In most environmental media, redox reactions will not reach equilibrium due to low concentrations

or multiple redox species. Consequently, redox measurements can generally be considered semi-

quantitative in environmental media, unless certain conditions exist.

The USGS in the Interferences and Limitations Section 6.5.3A of their National Field Manual

succinctly describe some of the issues encountered in the application of ORP measurements. This

section is reproduced here, unedited:

6.5.3.A INTERFERENCES AND LIMITATIONS

Measurements should not be carried out without an awareness of the interferences and limitations

inherent in the method.

• Organic matter and sulfide may cause contamination of the electrode surface, salt bridge,

or internal electrolyte, which can cause drift or erratic performance when reference

electrodes are used (American Public Health Association and others, 2001).

• Hydrogen sulfide can produce a coating on the platinum electrode that interferes with the

measurement if the electrode is left in sulfide-rich water for several hours (Whitfield, 1974;

Sato, 1960).

• The platinum single and combination redox electrodes may yield unstable readings in

solutions containing chromium, uranium, vanadium, or titanium ions and other ions that are

stronger reducing agents than hydrogen or platinum (Orion Research Instruction Manual,

written commun., 1991).

• Do not insert redox electrodes into iron-rich waters directly after the electrode(s) contact

ZoBell’s. An insoluble blue precipitate coats the electrode surface because of an immediate

reaction between ferro- and ferricyanide ions in ZoBell’s with ferrous and ferric ions in the

sample water, causing erratic readings.

Many elements with more than one oxidation state do not exhibit reversible behavior at the

platinum electrode surface and some systems will give mixed potentials, depending on the presence

of several different couples (Barcelona and others, 1989; Bricker, 1982, p. 59–65; Stumm and

Morgan, 1981, p. 490–495; Bricker, 1965, p. 65). Methane, bicarbonate, nitrogen gas, sulfate, and

dissolved oxygen generally are not in equilibrium with platinum electrodes (Berner, 1981).

TECHNICAL NOTE:

Misconceptions regarding the analogy between Eh (pe) and pH as master

variables and limitations on the interpretation of Eh measurements are explained

in Hostettler (1984), Lindberg and Runnells (1984), Thorstenson (1984), and

Berner (1981). To summarize:

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(1) Hydrated electrons do not exist in meaningful concentrations in most

aqueous systems—in contrast, pH represents real activities of hydrated protons.

Eh may be expressed as pe (the negative logarithm of the electron activity), but

conversion to pe offers no advantage when dealing with measured potentials.

(2) Do not assume that redox species coexist in equilibrium. Many situations

have been documented in which dissolved oxygen coexists with hydrogen sulfide,

methane, and ferrous iron.

• The practicality of Eh measurements is limited to iron in acidic mine

waters and sulfide in waters undergoing sulfate reduction.

• Other redox species are not sufficiently electroactive to establish an

equilibrium potential at the surface of the conducting electrode.

(3) A single redox potential cannot be assigned to a disequilibrium system, nor

can it be assigned to a water sample without specifying the particular redox

species to which it refers. Different redox elements (iron, manganese, sulfur,

selenium, arsenic) tend not to reach overall equilibrium in most natural water

systems; therefore, a single Eh measurement generally does not represent the

system.

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3 Methodology

3.1 Standard Solutions

Care should be taken not to contaminate standards and samples and to verify the expiration date

of all standards prior to use. All meters should be verified or calibrated according to the

manufacturer’s procedures.

Standard solutions for calibration and verification should be selected to meet project requirements.

SESD generally maintains a stock of Zobell’s solution suitable for most projects. The

characteristics and use of the common standard solutions are described below.

• Zobell’s solution contains potassium ferri- and ferro- cyanide compounds. The

solution is available as prepared solutions or premeasured reagents for mixing by the

user. Zobell’s has moderate toxicity but will react with acid to form harmful

byproducts, including hydrocyanide gas. It has a shelf life ranging from several days

to several months depending on the manufacturer. Stock and working solutions of

Zobell’s should be stored in dark bottles due to its light sensitivity.

• Quinhydrone solutions are mixed at the time of use by adding quinhydrone to pH 4 or

pH 7 buffers. At 25°C, the E

of quinhydrone pH 4 and pH 7 verification solutions are

462mV and 285mV respectively. An advantage of quinhydrone solutions is that they

offer a span of calibration points that may be appropriate for particular applications.

Quinhydrone is a lightly ‘poised’ solution in that it offers less driving force towards the

calibration point: a compromised instrument is more likely to be revealed in a

quinhydrone calibration. A quinhydrone calibration/verification solution is created by

adding 10g of quinhydrone to 1L of pH 4 or pH 7 buffer solution (ASTM D1498). The

solutions are mixed on a magnetic mixing plate for a minimum of 15 minutes to create

a saturated solution with undissolved crystals remaining. Quinhydrone solutions are

usable for 8 hrs from the time of mixing.

• Light’s solution consists of ferrous and ferric ammonium sulphate in sulphuric acid.

The solution would rarely be used at SESD due to its high acidity and associated

handling difficulty. Spent solutions with a pH<2 would be regulated as a hazardous

waste. Light’s is a highly poised solution that may allow a marginally functioning

electrode to pass calibration.

• A prepared potassium iodide solution is available which has low toxicity and a long

shelf life. The solution may stain clothing or surfaces if spilled.

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3.2 Verification and Calibration

ORP instruments may be verified or calibrated, depending on the application. The approach

chosen should be selected based on project needs and information presented in Section 2.4.,

Limitations. Standard laboratory practice in making ORP measurements is to verify the accuracy

of the instrument prior to use, and this practice should be followed when true quantitative results

are required. In a verification, the instrument in its direct-reading mode is checked against a

standard solution in a pass/no-pass test, and no corrections are applied to subsequent

measurements. In most applications, the ORP information is used semi-quantitatively and for

these applications, the instruments may be calibrated to the standard solutions. In an instrument

calibration, the instrument probe is placed in the standard solution and the difference between the

standard measurement and the known ORP value of the standard is used by the instrument to make

adjustments to the subsequent measurements.

In verification of an ORP instrument, the instrument is set to absolute mV reading mode or the

internal calibration offset is zeroed out. The instrument probe should then be placed in the standard

solution and the reading verified to fall within +/-10mV of the predicted reading for the standard.

Instruments with single-purpose electrodes are most suitable for this approach. If the instrument

fails the verification, standard solution quality should be considered and instrument maintenance

performed per the manufacturer’s procedures.

In most SESD field practice, the end data use is semi-quantitative. In this case, the instruments

can be calibrated to standard solutions appropriate for the project using the manufacturer’s

recommended procedure. One minute after the calibration, the instrument should display a stable

reading within +/-10mV of the predicted reading. An instrument failing this test should be

recalibrated to determine if the problem is inadequate equilibration time. In the event of continued

instrument failure, aging or contamination of the standard solution should be considered.

Subsequently the electrode should be serviced according to the manufacturer’s procedures.

Common service procedures include cleaning the platinum electrode with mild abrasives or acids

and refilling or replacing the reference electrode.

Prior to a mobilization, all ORP instruments will be checked for proper operation and verified or

calibrated against standard solutions. During the field mobilization, each instrument will be

calibrated or verified prior to, and verified after, each day’s use or deployment.

Even though it is not necessary to re-calibrate ORP instrument at regular intervals during the

day, it may be appropriate to occasionally perform operational checks to determine if site

conditions, such as an extreme temperature change or submersion of a filling solution port have

impacted the instrument’s performance. If an operational check is warranted, the field operator

should follow the appropriate verification/calibration steps as described above.

The predicted ORP values of standard solutions will be obtained from the manufacturer of

prepared solutions, literature, or appropriate values listed in this procedure. Care is in order, as

the predicted ORP value is specific for the type of reference electrode used by the probe (either

Ag/AgCl or calomel) and the molarity of the filling solution in the reference electrode. To use the

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solution with another electrode or filling solution, the expected ORP readings for the solution

should be converted to Eh for the probes intended for the solution as per the Reporting section of

this procedure. Then a table can be compiled for the electrode in use by subtracting the E

h,ref

for

the electrode and filling solution in use. This will be done at the Field Equipment Center (FEC)

for the solutions stocked.

Verification solutions should be managed per the manufacturer’s directions regarding storage and

handling. After instrument verification or calibration, the solution cannot be returned to the stock

solution container, although a separate container of working solution can be maintained.

Spent solutions and working solutions should be returned from the field to the SESD laboratory

for proper disposal by the SHEMP, or handled as directed by the SHEMP. Properly handled stock

solutions may be returned to the FEC for use at that facility.

3.3 Measurement

ORP measurements should be conducted in a fashion that prevents the addition or loss of any

potential oxidants or reductants. Results could be compromised by exposing the sample to air or

allowing H

S to off-gas from anoxic samples. Like dissolved oxygen measurements, ORP

measurements should be conducted in situ or by using a flow-through cell evacuated of air (see

the SESD Operating Procedure for Field Measurement of Dissolved Oxygen (SESDPROC-106,

most recent version). Good results are commonly obtained with the use of an overtopping cell

where the environmental media is pumped into the bottom of a narrow cup (generally field

fabricated from a sample container) containing the instrument sensors. The sensors are continually

flushed with fresh media as the cup is allowed to overflow. Caution should be exercised at very

low flow rates where the media in the cup could potentially re-oxygenate.

When using multi-parameter probes for ORP measurements, the general guidelines for probe

deployment described in the SESD Operating Procedure for Field Measurement of Dissolved

Oxygen (SESDPROC-106, most recent version) and the SESD Operating Procedure for In situ

Water Quality Monitoring (SESDPROC-111, most recent version) apply.

ORP probes must be operated and maintained in accordance with the manufacturer’s instructions.

Reference electrodes in multi-parameter probes may require regular filling or replacement. Single

parameter ORP electrodes may require regular filling and operation in an upright position to assure

that proper salt bridge flow is maintained. Platinum electrode surfaces are easily contaminated

and polishing or cleaning of the electrodes should be performed as recommended by the

manufacturer.

Measurements in field logbooks should be recorded to the nearest mV. The type of reference

electrode in use and its filling solution should be recorded in at least one logbook as part of the

field project records.

ORP is a temperature sensitive measurement, but ORP instruments are not temperature

compensated. Consequently, the media temperature should always be recorded at the same time

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as the ORP is recorded. Likewise, as ORP is often pH dependent, pH should also be recorded at

the time of ORP measurement.

3.4 Reporting

In the absence of a specified reference scale, ORP data has no meaning. Therefore, the reference

scale used should always be specified in reporting or discussing the ORP data. ORP measurements

converted to a hydrogen scale can be reported as “E

”. Data reported as the direct field

measurement without correction might be described as “ORP referenced to Ag/AgCl electrode” or

“E

Ag/AgCl

”. The expectations of the data user should be ascertained or the measurements should be

reported in both systems.

To apply corrections to obtain E

from the direct field measurement, the known half-cell potential

of the reference electrode is added to the recorded field ORP value:

h,sample

= ORP

sample

+ half-cell potential of reference electrode

The following table, reproduced from Section 2.2, presents the half-cell potential of a silver/silver

chloride reference electrode at various temperatures and with various molarities of KCl filling

solutions.

Table 1

Half-cell Potential of Ag/AgCl reference electrode

derived from USGS NFM, Table 6.5.2 (9/2005)

Molarity of KCl filling solution

T(°C)

3.3M*

3.5M

Sat/4M

220

217

215

214

216

214

212

209

213

210

208

204

209

207

205

199

205

203

201

194

202

199

197

189

198

195

193

184

*interpolated value

Note: YSI sondes and Thermo electrodes typically use 4M KCl filling solutions. Eureka sondes

typically use 3.3M KCl filling solutions

Example:

A multi-parameter probe with a silver/silver chloride reference electrode and 4M KCl filling

solution is used to record a stream ORP measurement of 146mV. The stream temperature is

recorded as 15°C.

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From the above table, the half-cell potential of an Ag/AgCl reference electrode filled with 4M KCl

is 209mV at 15°C. Then:

h,sample

= ORP

Ag/AgCl

sample

+ half-cell potential of Ag/AgCl reference electrode

h,sample

= 146mV + 209mV

h,sample

= 355mV

As noted in Section 3.3, Measurement, ORP measurements are sensitive to temperature, and may

be sensitive to pH. As the instruments do not compensate for these parameters, ORP data should

always be reported with the temperature and pH of the media at the time of measurement.

Final reporting values of Eh or ORP should be rounded to the nearest 10mV. The following

spreadsheet formula can perform the rounding of an interim result located in spreadsheet cell ‘A1’:

=INT(A1/10+0.5)*10

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