ASTM D 1498 - 00 Oxidation-Reduction Potential of Water

 

Designation: D 1498 – 00

Standard Practice for

Oxidation-Reduction Potential of Water1 This standard is issued under the fixed designation D 1498; the number immediately following the designation indicates the year of  original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A superscript superscr ipt epsilon epsilon (e) indicates an editorial change since the last revision or reapproval.

1. Scope Scope

= absol absolute ute temperatu temperature, re, °C + 273.15, 273.15, = numb number er of electrons electrons invol involved ved in process process reaction, and  Aoxand  A red   = activiti activities es of the reactants reactants in tthe he pr process ocess.. 3.2 For definitions definitions of other items used used in this prac practice tice,, refer to Terminology D 1129. T  n

1.1 This practi practice ce covers the appara apparatus tus and procedure for the electrometri electrom etricc meas measurem urement ent of oxida oxidation tion-red -reductio uction n pote potential ntial (ORP) in water. It does not deal with the manner in which the solutions solut ions are prepared, prepared, the theoreti theoretical cal interpreta interpretation tion of the oxidation-reduction potential, or the establishment of a standard oxidation-reduction potential for any given system. The practi pra ctice ce des descri cribed bed has bee been n design designed ed for the rou routin tinee and process measurement of oxidation-reduction potential. 1.2   This standar standard d doe doess not pur purpor portt to add addre ress ss all of the safet safetyy conc concer erns ns,, if any any, asso associ ciate ated d wi with th its use. use. It is th thee responsibility of the user of this standard to establish appro priate safety and health practices and determine the applicability of regulatory limitations prior to use.

4. Summa Summary ry of Practice 4.1 This is a practice designed designed to measure the ORP which is defined defi ned as the electr electromo omotiv tivee for force ce betwee between n a nob noble le met metal al electr electrode ode and a ref refere erence nce electr electrode ode when when im immer mersed sed in a solution. solut ion. The prac practice tice describes describes the equipment equipment available available to make the measurement, the standardization of the equipment and the procedure to measure ORP. The ORP electrodes are inert and measure the ratio of the activities of the oxidized to the reduced species present.

2. Referenced Documents 2.1   ASTM Standards: D 1129 Terminology Terminology Relating to Water2 D 1193 1193 Specification for Reagent Water Water2 D 3370 Practices Practices for Samp Sampling ling Wate Waterr from Clos Closed ed Conduits2

5. Signi Significanc ficancee and Use 5.1 ORP provides provides a useful useful measur measurement ement of the oxidizi oxidizing ng or reducing nature of a particular water sample. ORP measurements are commonly made in water and wastewater samples. 5.2 Var ario ious us appl applic icat atio ions ns incl include ude mo moni nito tori ring ng the the chlordination/dechlorination chlordination/de chlorination process of water, recgonition of  oxidants/r oxida nts/reduct eductants ants pres present ent in wastewater wastewater,, monitorin monitoring g the cycle chemistry in power plants, and controlling the processing of cyanide and chrome waste in metal plating baths.

3. Terminology 3.1  Definitions of Terms Specific to This Standard: 3.1.1   oxidation-re oxidation-reduction duction potential— the the ele electr ctromo omotiv tivee force, Em, developed between a noble metal electrode and a standard reference electrode. This oxidation-reduction potential (ORP) is related to the solution composition by:

6. Interferences 6.1 The ORP electrodes electrodes reliably measured ORP in nearly all

 RT   E m 5  E o 1 2.3 nF   log  A ox /  A  A red

where:  E m  E   o F   R

 

   

   

aqueous solutions and in general are not subject to solution interfere inter ference nce from color color,, turbidity turbidity,, colloidal colloidal matt matter er,, and suspended matter. 6.2 The ORP of an aqueo aqueous us solutio solution n is sensi sensitive tive to change in temperatu temperature re of the solu solution, tion, but temp temperat erature ure corr correcti ection on is rarely done due to its minimal effect and complex reactions. Temperatur emperaturee correctio corrections ns are usually applied only when it is de desi sire red d to re rela late te the the ORP to the the acti activi vity ty of an ion ion in the the solutions. 6. 6.3 3 Th Thee OR ORP P of an aque aqueou ouss solu soluti tion on is al almo most st al alwa ways ys sensitive to pH variations even to reactions that do not appear to involve hydrogen or hydroxyl hydroxyl ions. The ORP generally tends to increase with an increase in hydrogen ions and to decrease with an increase in hydroxyl ions during such reactions. 6.4 Reproduci Reproducible ble oxida oxidationtion-reduc reduction tion pote potential ntialss cannot cannot be

= ORP ORP,, = co cons nsta tant nt that that de depe pend ndss on the the ch choi oice ce of  reference electrodes. = Faraday Faraday const constant, ant, = gas const constant, ant,

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This practice is under the jurisdiction of ASTM Committee D19 on Water and is the direct responsibility responsibility of Subcommittee Subcommittee D19.03 D19.03 on Sampling Sampling of Water Water and Water-Formed Deposits, Surveillance of Water, and Flow Measurement of Water. Currentt edition approv Curren approved ed May 10, 2000. 2000. Publis Published hed December 2000. 2000. Origin Originally ally published publi shed as D 1498 – 57 T. Last previous edition D 1498 –93. –93. 2  Annual Book of ASTM Standards, Standards,   Vol 11.01.

Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.

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D 1498 obtained ned for chem chemical ical systems systems that are not reversible. reversible. Most obtai natural and ground waters do not contain reversible systems, or may contain systems that are shifted by the presence of air. The measurem meas urement ent of end poin pointt poten potential tial in oxida oxidation tion-red -reductio uction n titration is sometimes of this type. 6.5 If the metallic metallic portion of the ORP electrode electrode is spongelike,, mate like material rialss absor absorbed bed from solut solutions ions may not be washe washed d away, even by repeated rinsings. In such cases, the electrode may exhibit a memory effect, particularly if it is desired to detectt a rela detec relative tively ly low conc concentra entration tion of a part particul icular ar speci species es immediately after a measurement has been made in a relatively concentra conce ntrated ted solut solution. ion. A brigh brightly tly poli polished shed meta metall elec electrod trodee surface is required for accurate measurements. 6.6 The ORP res result ulting ing fro from m int intera eracti ctions ons amo among ng sev severa erall chemic che mical al sys system temss pre presen sentt in mix mixed ed sol soluti utions ons may not be assignable to any single chemical.

such that only the noble metal comes in contact with the test solution. The area of the noble metal in contact with the test solution should be approximately 1 cm   2. 7.4   Electrode Assembly—A conve convention ntional al elect electrode rode hold holder er or sup suppor portt can be emp employ loyed ed for lab labora orator tory y mea measur surem ement ents. s. Many Man y dif differ ferent ent sty styles les of ele electr ctrode ode hol holder derss are sui suitab table le for various process applications such as measurements in an open tank, tan k, pro proces cesss pip pipee lin line, e, pre pressu ssure re ves vessel sel,, or a hig high h pre pressu ssure re sample line. 8. Reage Reagents nts and Materials Materials 8.1   Purity of Reagents—Reagent grade chemicals shall be used in all tests. Unless otherwise indicated, it is intended that all reagents shall conform to the specifications of the Committee on Analytical Reagents of the American Chemical Society.  3 Other grades may be used, provided it is first ascertained that the reagent is of sufficiently high purity to permit its use without lessening the accuracy of the determination. 8.2  Purity of Water — Unless otherwise indicated, reference to water shall be understood to mean reagent water conforming to Specification D 1193, Type I or II. 8.3  Aqua Regia—Mix 1 volume of concentrated nitric acid (HNO3, sp gr 1.42) with 3 volumes of concentrated hydrochloric acid (HCl, sp gr 1.18). It is recommended that only enough

7. Appar Apparatus atus 7.1   Meter —Most — Most lab labora orator tory y pH me meter terss can be use used d for measurements of ORP by substitution of an appropriate set of  electrodes and meter scale. Readability to 1 mV is adequate. Thee ch Th choi oice ce wi will ll de depe pend nd on th thee ac accu cura racy cy de desi sire red d in th thee determination. 7.1.1 Most process pH meters can can be used for measurement of ORP by substitution of an appropriate set of electrodes and meter met er sca scale. le. The These se ins instru trumen ments ts are gen genera erally lly mu much ch mor moree rugged than those which are used for very accurate measurementss in the labo ment laborator ratory y. Usual Usually ly,, these more rugge rugged d instr instruuments men ts pro produc ducee res result ultss tha thatt are som somewh ewhat at les lesss acc accura urate te and precise than those obtained from laboratory instruments. The choice cho ice of pro proces cesss ORP analyze analyzerr is gen genera erally lly based on how closely the characteristics of the analyzer match the requirement me ntss of th thee ap appl plic icat atio ion. n. Typ ypic ical al fa fact ctor orss wh whic ich h ma may y be considered include, for example, the types of signals which the analyzer can produce to drive external devices, and the span ranges available. 7.1.2 For remote remote ORP measurements the potential potential generated generated can be trans transmitt mitted ed to an exter external nal indi indicatin cating g mete meterr. Speci Special al shielded cable is required to transmit the signal. 7.2   Reference Electrode—A calomel, silver-silver chloride, or other reference electrode of constant potential shall be used. If a sat satura urated ted cal calome omell ele electr ctrode ode is use used, d, som somee pot potass assium ium chloride crystals shall be contained in the saturated potassium chloride solution. If the reference electrode is of the flowing  junction type, a slow outward flow of the reference-ele reference-electrode ctrode solution is desired. To achieve this, the solution pressure inside the li liqui quid d jun juncti ction on sho should uld be som somewh ewhat at in exc excess ess of tha thatt outside outsi de the junct junction. ion. In nonpr nonpressu essurized rized applications applications this requirement can be met by maintaining the inside solution level higher than the outside solution level. If the reference electrode is of the nonflowin nonflowing g jun juncti ction on typ type, e, the these se out outwar ward d flow and pressuriza press urization tion consi considerat derations ions shall not apply apply.. The refe referenc rencee electrode and junction shall perform satisfactorily as required in the procedure for checking sensitivity described in 11.2. 7.3   Oxidation-Reduction Electrode—A noble metal is used in the construction of oxidation-reduction electrodes. The most common com mon met metals als emp employ loyed ed are pla plati tinum num and gol gold; d; sil silver ver is rarely used. It is important to select a metal that is not attacked by the test solution. The construction of the electrode shall be

solution be prepared for immediate requirements. 8.4   Buffer —Tabl ablee 1 lis lists ts the buf buffer fer sal salts ts Buffer Stand Standard ard Salts—T available from the National Institute of Standards and Technology nol ogy spe specifi cifical cally ly for the pre prepar parati ation on of sta standa ndard rd buf buffer fer solutions. The NIST includes numbers and drying procedures. 8.4.1   Phthalate Phthalate Refer Reference ence Buf Buffer fer Solutio Solution n   (pHs  = 4.0 4.00 0 at  TABLE 1 National Institute of Standards and Technology Technology (NIST) Materials for Reference Buffer Solutions NIST Standard Sample Designation A 186-II 186II-e -e 18618 6-I-e I-e 1 85 85-g

Buffer Salt

Drying Procedure

disodi diso dium um hy hydr drog ogen en ph phos osph phat ate e pota po tass ssiu ium m di dihy hydr drog ogen en ph phos osph phat ate e p ota otass ssiiu m hy dro drog g en en p hth htha al ate ate

2 h in ov oven en at 13 130° 0°C 2 h in ov oven en at 13 130° 0°C d ryi ryin n g no t n ec ece ss ssa ry ry

A

The buffer salts listed can be purchased from the Office of Standard Reference Materials, National Institute of Standards and Technology (NIST), Gaithersburg, MD 20899.

25°C)   —Dissolve —Dissolve 10.12 g of potas potassium sium hydrogen hydrogen phtha phthalate late (KHC   8H4O4) in water and dilute to 1 L. 8.4.2   Phosphate Refer Reference ence Buf Buffer fer Solutio Solution n   (pHs  = 6.86 at  25°C)  —Dissolve 3.39 g of potassium dihydrogen phosphate (KH PO4) and 3.53 g of anhydrous disodium hydrogen phosphate (Na2HPO4) in water and dilute to 1 L. 8.5  Chromic Acid Cleaning Solution—Dissolve about 5 g of  potassium dichromate (K2Cr   2O7) in 500 mL of concentrated sulfuric acid (H2SO   4, sp gr 1.84). 2

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 Reagent Chemicals, American Chemical Society Specificatio Specifications ns,, Ame America rican n

Chemical Society, Washington, DC. For suggestions on the testing of reagents not listed by the American Chemical Society, Society, see   Analar Standards for Laboratory Chemicals,, BDH Ltd., Poole, Dorset, U.K., and the   United States Pharmacopeia Chemicals and National Formulary, Formulary, U.S. Pharmaceutical Convention, Inc. (USPC), Rockville, MD.

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D 1498 TABLE 3 Nomin Nominal al ORP of Reference Reference Quinhydrone Quinhydrone Soluti Solutions ons

—Use any commercially available “low-suds” 8.6   Detergent —Use liquid or solid detergent. 8.7   Nitric Acid  (1  (1 + 1)—Mix equal volumes of concentrated nitric acid (HNO3, sp gr 1.42) and water. 8.8   Redox Standa Standard rd Solutio Solution; n; Ferr Ferrous-F ous-Ferric erric Refer Reference ence  4 Solution —Di —Disso ssolve lve 39. 39.21 21 g of fer ferrou rouss amm ammoni onium um sul sulfat fatee (Fe(NH4)   2-(SO4)2·6H2O), 48.22 g of ferric ammonium sulfate (FeNH4(SO4)2·12H   2O) and 56.2 mL of sulfuric acid (H 2SO4, sp gr 1.84) in water and dilute to 1 L. It is necessary to prepare the solution using reagent grade chemicals that have an assay confirming them to be within 1% of the nominal composition. The solution solution should should be sto stored red in a clo closed sed glass or pla plasti sticc container. 8.8.1 The ferr ferrousous-ferr ferric ic refe reference rence solution solution is a reas reasonabl onably y stable solution with a measurable oxidation - reduction potential. Table 2 presents the potential of the platinum electrode for various reference electrodes at 25°C in the standard ferrousferric solution. 8.9   Redox Reference Quinhydrone Solutions—Mix 1 L of  pH 4 buffer solution, (see 8.4.1), with 10 g of quinhydrone. Mix 1 L of pH 7 buffer solution, (see 8.4.2), with 10 g of  quinhydrone. Be sure that excess quinhydrone is used in each soluti sol ution on so tha thatt sol solid id cry crysta stals ls are alw always ays pre presen sent. t. The These se reference solutions are stable for only 8 h. Table 3 lists the

 junctions and fill-holes of reference electrodes to reduce evaporation during prolonged storage. 10.2   ORP Electrode Cleaning—It is desirable to clean the electr ele ctrode ode dai daily ly.. Rem Remove ove for foreig eign n ma matte tterr by a pre prelim limina inary ry treatment with a detergent or mild abrasive, such as toothpaste. If this is insufficient, use 1 + 1 nitric acid. Rinse the electrode

nominal millivolt redox readings for the quinhydrone reference solutions at temperatures of 20°C, 25°C, and 30°C. 8.10   Redox Standard Solution; Iodide/Triiodide—Dissolve 664.04 g of potassium iodide (KI), 1.751 g of resublimed I 2, 12.616 g of boric acid (H 3BO3), and 20 ml of 1 M potassium hydroxide (KOH) in water and dilut to 1 L. Mix solution. This solution is stable at least one year. Solution can be stored in a closed glass or plastic container. Table 4 provides the potential of the platinum electrode for various reference electrodes at various temperatures in the standard Iodide/Triiodide solution.

in water times. cleaning procedure is immers imm ersee several the ele electr ctrode odeAn in alternative chromi chr omicc aci acid d cle cleani aning ng mix mixtur ture e to at room temperature for several minutes, then rinse with dilute hydrochlor hydro chloric ic acid, and then thoroughly thoroughly rins rinsee with water. water. If  these steps are insuf insuffficien icient, t, imme immerse rse the ORP electrode electrode in warm (70°C) aqua regia and allow to stand for 1 min. This solution dissolves noble metal and should not be used longer than the time specified. In these cleaning operations, particular care must be exercised to protect glass-metal seals from sudden changes of temperature, which might crack them.

9. Sampl Sampling ing

11. Standarization

9.1 Collec Collectt th thee sa sampl mples es in acc accord ordanc ancee wit with h Pra Practi ctices ces D 3370.

11.1 Turn on meter according according to manufacturers instructions. instructions. Check zer Check zero o on met meter er by sho shorti rting ng the inp input ut con connec nectio tion. n. The reading should be less than 6 0.5 mV. 11.2   Checking the Response of the Electrode to Standard   Redox Solutions  (see 8.8,8.9 and 8.10)—Wash the electrodes with three changes of water or by means of a flowing stream from a wash bottle. Use one or more of the solutions from sections 8.8,8.9 and 8.10 to check the response of the electrode. Fill the sample container with fresh redox standard solution and immerse the electrodes. The reading should be within 30 mV of the value expected for the standard solution. Repeat the measurement with fresh solution. The second reading should not differ from the first by more than 10 mV.

ORP, mV Buffer solution, nominal pH Temperature,°C Reference Electrode Silver/silver chloride Calomel Hydrogen

  “Sta “Standard ndard Solu Solution tion for Redox Poten Potential tial Measu Measureme rements,” nts,”  Analytical Chemistry, Vol 44, 1972, p. 1038. 4

TABLE 2 Potent Potential ial of the Platinum Electrode Electrode for Several Reference Electrodes at 25°C in Ferrous-Ferric Reference Solution Potential EMF, mV

Hg, Hg2  Cl 2, satd KCl Ag, AgCl, 1.00 M KCl Ag, AgCl, 4.00 M KCl Ag, AgCl, satd KCl Pt, H2(p = 1), H (a = 1)

+ 430 + 439 + 475 + 476 + 675

7

20

25

30

20

25

30

268 223 470

263 218 462

258 213 454

92 47 2 95 29

86 41 285

79 34 275 27

TABLE TA BLE 4 Nominal Nominal Potential Potential in mV of Platinu Platinum m Electrode in Iodide/Triiodide Standard Solution

10. Preparation 10.1   Electrode Tr Treatment  eatment —Conditio —Condition n and main maintain tain ORP electrodes as recommended by the manufacturer. If the assembly is in intermittent use, the immersible ends of the electrode should sho uld be kep keptt in wat water er bet betwee ween n me measu asurem rement ents. s. Cov Cover er the

Reference Electrode

4

Reference Electrode

20

Temperature, °C 25

30

Silver/silver chloride Ag, AgCl, sat’d KCl Standard Hydrogen Pt, H2   (p=1), H (a=1) Calomel Hg, Hg2Cl2, sat’d KCl

220 424 176

221 420 176

222 415 175

12. Proc Procedur eduree 12.1 After the electrode/meter electrode/meter assembly has been standardstandardized as described in 11, wash the electrodes with three changes of water or by means of a flowing stream from a wash bottle. Place the sample in a clean glass beaker or sample cup and insert the electrodes. Provide adequate agitation throughout the measurement period. Read the millivolt potential of the solution tio n all allowi owing ng suf sufffici icient ent tim timee for the sys system tem to sta stabil bilize ize.. Measure successive portions of the sample until readings on 3

 

D 1498 succes cessiv sivee por portio tions ns dif differ fer by no mor moree tha than n 10 mV mV.. A two suc system that is very slow to stabilize probably will not yield a meaningful ORP ORP.. 12.2   Cont Continu inuous ous Det Determ ermina ination tion of the ORP of Flo Flowing wing Streams or Batch Systems—Process ORP analyzers with their rugged electrodes and electrode chambers provide continuous measurements which are the basis for fully automatic control. Make selection of the electrodes and electrode chamber to suit the physical and chemical characteristics of the process material.. Locat rial Locatee the subm submersi ersion-st on-style yle elect electrode rode chamber so that fresh fres h solu solution tion representati representative ve of the proc process ess stre stream am or batch continuous conti nuously ly passe passess acros acrosss the elect electrodes rodes.. Agita Agitation tion may be employed in order to make the stream or batch more nearly homogeneou homog eneous. s. The ORP value is usua usually lly displ displayed ayed continuously and can be noted at any specific time. Frequently, the pH value is continuously recorded, yielding a permanent record.

14. Repo Report rt 14.1 Report the oxidation-reduction oxidation-reduction potential potential to the nearest 1 mV, interpolating the meter scale as required. When considered appropriate, appropriate, the temp temperatu erature re at which the meas measurem urement ent was made and the pH at the time of measurement, may also be reported. 15. Prec Precisio ision n and Bias 15.1 Precision and bias of the measurement measurement depends largely largely on the condition of the electrode system and on the degree to which the chemical system being measured fits the qualifications given in Sections 3 and 6. In the absence of substances that coat or poison the electrode, the precision is 6 10 mV. NOTE  1—During the revision of this standard (D 1498), the Results Advisor for Committee D19 judged this standard is incorrectly classified as a practice, because it produces a test result, and in fac it a test method. As a test method, it is subject to the requirements of D 2777, Practice for Determi Dete rminati nation on of Pre Precis cision ion and Bia Biass of App Applica licable ble Test Met Methods hods of  Commit Com mittee tee D19 on Wate ater, r, and D 584 5847, 7, Pra Practic cticee for Writing Writing Qua Quality lity Control Specifications Specifications for Standa Standard rd Test Methods for Water Analysis. However, at the time of revision, there is not sufficient data to support these requirements. In 2000, this standard was revised as a practice for the sole purpose of preventing the withdrawal of the standard during the time period necessary for Subcommittee D19.03 to obtain the interlaboratory

13. Calculation 13.1 Repor Reportt the oxida oxidationtion-reduc reduction tion potential potential in mill millivol ivolts ts directly from the meter. The electrode system employed must also be reported. 13.2 If it is desired to repo report rt the oxida oxidation tion-red -reductio uction n poten poten-tial referred to the hydrogen electrode, calculate as follows:  E h 5  E obs 1  E ref 

data necessary for the development of an acceptable precision and bias statement, and an appropriate quality control section.

where:  E h   = oxidation-reduction oxidation-reduction potential referred to the hydrogen scale, mV, oxidation-reduction ion potential potential of the the noble noble  E obs   = observed oxidation-reduct metal-reference electrode employed, mV, and  E ref    = oxidation-reduc oxidation-reduction tion potential of the the reference reference electrode as related to the hydrogen electrode, mV.

16. Keyw Keywords ords 16.1 elec electrode trodes; s; ferr ferrous-f ous-ferri erricc refe reference rence solut solutions; ions; iodid iodide/  e/  triiodide trii odide;; noble meta metall elec electrode trodes; s; ORP; oxid oxidation ation-redu -reductio ction n potential; quinhydrone; redox; redox standards; reference electrodes; reference solutions

APPENDIX (Nonmandatory Information) X1. OXIDA OXIDATION TION REDUCTION POTENTIAL

X1.1   Meani Meaning ng of the Term Term ORP —The ORP measurement establ est ablish ishes es the rat ratio io of oxi oxidan dants ts and red reduct uctant antss pre prevai vailin ling g within a solution of water or waste water. The measurement is nonspecific in contrast, for example, to the pH measurement. The ORP electrode pair senses the prevailing net potential of a solution. By this measurement, the ability to oxidize or reduce species in the solution may be determined.

measurements are useful for process control in both instances if the pH is constant and controlled. X1.2 X1 .2.2 .2 In ad addi diti tion on to co cont ntro roll of pr proc oces esse sess in wh whic ich h a specific material is treated, ORP measurements can be used to control nonspecific processes if a correlation can be established between prevailing ORP and the reaction in the process. An example is the use of ORP measurements in odor control of  municipal waste by chlorination. In some cases, the extent of  odor production correlates with ORP. Also, sewage treatment plants may be protected from unwanted oxidizing or reducing agents which might harm treatment materials if the ORP of the influent is monitored.

X1.2   Use of ORP Control Control of Waste Processe Processes: s: X1.2.1 ORP measurement measurementss are used in indus industria triall proce process ss control to monitor the treatment of unwanted materials which are amenable to oxidation or reduction. Frequently, only one species in solution is to be treated, in which case the oxidationreduction profile of the process can be predicted with some accura acc uracy cy.. Two exa exampl mples es in thi thiss cat catego egory ry are fou found nd in the plating industry: Waste cyanide is oxidized to cyanate and then (iff re (i requ quir ired ed)) to ca carb rbon on di diox oxid idee an and d ni nitr trog ogen en,, an and d wa wast stee hexavalen hexav alent-chr t-chromiu omium m is reduc reduced ed to the triv trivalent alent state. ORP

X1.3 Temperature Effects Effects on ORP Measurements: Measurements: X1.3.1 The effect effect of temperature on ORP mesurements can be understood by considering the Nernst equation:  E  5  E o 1 2.3

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 RT   log  Q nF 

 

D 1498 where  E    = measured measured poten potential tial,,  o  E  = poten potential tial when all components components involved involved in the reaction are at unit activity and 25° C,  R   = gas con consta stant, nt, T    = absol absolute ute tempe temperatur rature, e,  t  °C + 273.15, F    = Faraday Faraday,, n   = numb number er of elec electrons trons invo involved lved in in the reac reaction tion,, and Q   = produ product ct of the activit activities ies of the oxidan oxidants ts divided divided by

the plot of  E   E  versus  T  thus depends not only on  T , but on  n  as well, so that different amounts of compensation are required, depending on the value of  n  n . If the process under study is well characterized and the value of  n  n  known, automatic temperature compen com pensat sation ion is pos possib sible. le. How Howeve ever, r, if the val value ue of   n   is unknown or variable (see X1.2.1), then compensation is not possible.

the product of the activities of the reductants, each activity raised to that power whose exponent is the coefficient of the substance in the applicable chemical reaction.

analyzer may also be determined by connecting a battery of  known polarity and observing the deflection of the meter. A resistive voltage divider may be connected between the battery and anal analyzer yzer,, if neces necessary sary,, to preve prevent nt the meter from being driven dri ven of off-s f-scal calee due to app applic licati ation on of an exc excess essive ively ly hig high h potential.

X1.4   Polari Polarity ty Check —T —The he po pola lari rity ty of th thee in inpu putt of th thee

Changes in  E  o with temperature produce the same changes in   E . Fur Furthe ther, r, the slo slope pe of the curve that rel relate atess   E   and   T  depend dep endss dir direct ectly ly on   T . Fi Fina nall lly y, ch chan ange gess in ac acti tivi vity ty wi with th temperature will produce changes in   E . X1.3.2 Autom Automatic atic temperature temperature compensatio compensation n is seld seldom om attempted in ORP measurements, due to the appearance of   n   in the prelogarithmic factor of the Nernst equation. The slope of 

X1.5   Increase Precision Precision of Measurement  Measurement —If —If the system is electrochemically reversible, and a precision of better than 65 mV is desired, control the temperature of the assembly to within   61° C. Use silver/silver chloride reference electrodes with a flowing junction to avoid temperature hysteresis.

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