The Coulometric Titration of Iodide with Electrolytically Generated Bromine Using an Amperometric Endpoint Citation Wooster, Warren Scriver (1947) The Coulometric Titration of Iodide with Electrolytically Generated Bromine Using an Amperometric Endpoint. Master's thesis, California Institute of Technology. doi:10.7907/0pc3-me36. https://resolver.caltech.edu/CaltechTHESIS:04012025-165751766 Abstract No abstract. Item Type: Thesis (Master's thesis) Subject Keywords: (Chemistry) Degree Grantor: California Institute of Technology Division: Chemistry and Chemical Engineering Major Option: Chemistry Thesis Availability: Public (worldwide access) Research Advisor(s): Swift, Ernest H. Thesis Committee: Unknown, Unknown Defense Date: 1 January 1947 Record Number: CaltechTHESIS:04012025-165751766 Persistent URL: https://resolver.caltech.edu/CaltechTHESIS:04012025-165751766 DOI: 10.7907/0pc3-me36 Default Usage Policy: No commercial reproduction, distribution, display or performance rights in this work are provided. ID Code: 17123 Collection: CaltechTHESIS Deposited By: Benjamin Perez Deposited On: 01 Apr 2025 23:14 Last Modified: 01 Apr 2025 23:22 Thesis Files PDF - Final Version See Usage Policy. 6MB Repository Staff Only: item control page

THE COULOMETRIC TITRATION OF IODIDE WITH ELECTROLYTICALLY GENERATED BROMINE USING AN AMPEROMETRIC ENDPOINT BY WARREN s. WOOSTER A the1is aubmitted in partial fulfillment of the requirement• for the degree of Muter of Science CALIFORNIA INSTITUTE OF TECHNOLOGY l 9 4 7 ACKNOWLEDGEMENTS The auther wiahea tc, expre• ~ hi• gratitude f~r the aaaiatanoe and kindly advice given by Dr. Erneat H. Swift who directed thi• research. He i1 alao indebted to Mr. Ray Brown and Mr. Rollie Myer• for many uaef'ul suggestion•. THE COULOMETRIC TITRATION OF IODIDE WITH ELECTROLYTICALLY GENERATED BROMINE USING AN AMPEROMETRIC ENDPOINT Introduction: A method and apparatus for the amperometric titration of thiodiglycol was developed by Seaae, Niemann and Switt6 • In this meth- od thiodiglycol waa oxidized by means of electrolytically generated bromine, and the end point determined by observing the current flow between two platinum indicator electrodes which have a small potential difference impressed between the.m. By measuring the time of bromine generation and current flow during generation, the number of equivalents of bromine generated was determined. Further study of this method was made by Myers and Swift5 who applied it to the determination of tripositive arsenic. Neither thiodiglyool, tripositive arsenic nor their oxidation product• react with the generator cathode or the indicator electrodes. Accord- ingly, no significant indicator current flows between the indicator electrodes until the titration endpoint baa been reached. By shield- ing the generator cathode and by modification of the titration procedure, it should be possible to extend this method to substance• which establish reversible electrode potentiala. An example of such a substance is iodide ion. A study has been made of the indicator current flow characteristics during the oxidation of iodide to iodine monobromide, and a method for the analyaia of small quantities of iodide has been developed. - 2 - Reagent•: Six formal hydrochloric acid solutions were prepared from "Reagent Grade" concentrated acid. The acid available commercially waa found to contain as much as ,.5 x 10-8 equivalent• of' reducing agent per milliliter. The amount of' reducing agent present was determined by electrolytic oxidation and waa removed by boiling the eix formal acid with the calculated amount of,% hydrogen peroxide. If too much peroxide were added, the exces• was destroyed by boiling for about fifteen minutes. One formal sodium bromide solutions were prepared from "Reagent Grade 11 ealt. These solution■, when teated, were found to contain no unwanted oxidizing or reducing agenta. On occasions, laboratory di1tilled water was found to contain aa much aa 10-6 equivalent• per liter of an oxidizing agent, pre1umably chlorine. Thie waa conveniently removed by bringing the water to a boil, bubbling clean, dry air thru it while it waa being heated. Stock tenth formal eolutions of potassium iodide were made up by weight from 11 Reagent Grade 11 salt which had been dried for one hour at 110 deg. o. no iodate. The potaaaium iodide uaed was found to contain The solutions were made 0.01 Fin sodium carbonate to prevent air oxidation. Dilutions of the stock solutions, 0.005 F in sodium carbonate, were used directly for titratione. Apparatu•: The apparatus used waa essentially that employed by Myers and Swift5. The titration cell contains four electrodes made from 0.002" platinum foil, two generator electrodea each 1/2 cm2 in area, an indicator anode ~.15 cm2 in area, and an indicator cathode 5.46 cm2 in area. The generator cathode was contained within a gla•• shield, terminating in a •intered gla•• disc thru which contact with the solution was made. By keeping thi• shield filled with six formal hydrochloric acid, the generator cathode waa effectively insulated from the solution. Thia protection removed the danger of reduction of oxidation product• at the generator cathode and eliminated any indicator current due to the preaence of hydrogen in aolution. The aample to be titrated waa placed in a 4o x 80 mm weighing bottle, which was then fitted to a rubber atopper thru which the electrodes and atirrer pa••ed. During a titration the solution waa stirred continually by meana of a Cenco variable speed stirrer, turning at 1800 rpm. This •peed was determined and maintained constant by meana of a atroboscopic device mounted on the atirrer. It waa found necea- sary to run the atirrer for fifteen minutes before conatant •peed waa attained. A diagram of the indicator and generator circuit• i• given in figure l. In the indicator circuit a amall potential (u•ually l}e millivolts in these experiments) is impressed across the indicator - 4 - electrode•. Any current flowing between the electrodes ie regi•- tered on a microamm.eter. In the generator circuit a choice of two rate• of bromine generation is poasible. On the high rate a cur- rent of approximately 10 milliampere• flows thru the generator electrodea; on the low rate a current of approximately 1 milliampere flows. Current flow is determined by measuring the poten- tial drop acroe1 a standard 200 ohm reaistance by means of a potentiometer. The resistance used was calibrated by means of a Wolff potentiometer and haa a value of 199.87 ohms± 0.0~%. Procedure, When not in u1e, the electrodes were stored in a 1olution two formal in hydrochloric acid and one tenth formal in 1odium bromide. Before use, the indicator electrodes were shorted together and made the generator anode, and bromine wa1 generated on their surface• for a short period of time. This treatment waa found to increase the 1tability of electrode aen1itivity. In 1ome caae1 when the electrodes had been used for antimony or arsenic determinations, it was found advisable to generate hydrogen on their aurfaces for a short period of time, and then bromine for a longer period of time ( up ·to 50 second•. ) Sensitivity of the indicator electrodes, in microamperes of indicator current per second of bromine generation, at the appropriate rate was determined each day by generation of bromine for a certain length of time and recording of the indicator current. On - 5 - the high rate, three seconds generation gave an indicator current of about ~O microamperes; on the low rate, thirty aecond'• generation gave an indicator current of about 26 microamperes. Indicator current has been obaerved to be a linear function of the bromine concentration at constant values of acid and bromide concentration. For this reason it was necessary to determine the sensitivity under the particular conditions of concentration which were to obtain during the subsequent titrations. Daily determination of 1ensitivity was neceaaary, because it was found that the senaitivity dropped somewhat from day to day (from 11.2 microam.peres per second to 10.0 microamperes per aecond in two weeks, for example.) Thi• drop in sensitivity waa not constant, and the 1ensitivity was obaerved to improve when the electrodes were not used for several daya. To make a determination, a sample of the iodide solution waa pipetted into a 40 x bO mm weighing bottle and diluted to give a , solution of 50 ml, two formal in hydrochloric acid and one tenth formal in sodium bromide. Neither the acid concentration nor the bromide concentration was found to be critical, if the sensitivity were previously determined at the concentration used. An acid con- centration of two formal and a bromide concentration of one tenth formal were used in this work. After the initial diffusion current was determined, the indicator circuit was opened and generation atarted. When the approxi- mate concentration was known, the indicator circuit was closed about ten seconds before the end point. In the titration of an - 6 - unknown aample, it waa necessary to take trial reading• of the indicator current during the courae of a titration. After the point of maxim.um current ha ■ been reached, two reading& will give a rough indication of' the location of the end point. The point at which a minimum. indicator current pas ■ ed was noted, and generation wa■ allowed to proceed until an indicator current of 25 to 40 microampere ■ wa ■ obtained. Generation waa then atopped and the time and indicator current recorded. After each titration the indicator electrode ■ were treated aa indicated above, and the generation current waa determined. On the high rate, the potential acroaa the standard resi1tance varied a few millivolt■ (from 2.0,6 to 2.04o volte, for example) during the cour ■ e of an aft1trnoon. R1tadinga were taken after each titration, and an average value uaed for the series of titration•• On the low rate, the potential acroaa the atandard reaistance had a conatant value from day to day (0.190 volta.) This wa1 v1trified at the atart and finiah of each ■ erie ■ of titration•. End point correction wa1 made by extrapolation of the t'inal indicator current to zero current, using the previoualy determined indicator electrode sensitivity. Discussion: During the course of a titration in a 1olution two formal in hydrochloric acid and one tenth formal in sodium bromide, iodide is oxidized first to iodine and then to iodine monobromide (since the iodine - iodine monobromide potential, -0.87 vis lea ■ negative than the iodine - iodine monochloride potential, -i.06 v or the - 7 - bromide - bromine potential, -1.087 4 .) A plot of indicator current againet time of generation (eee figure 2) •how• a maximum before all the iodide haa been oxidized to iodine, and a minimum at the calculated equivalence point. If an equivalent amount ot iodine monobromide is prepared as described later, and placed in the titration cell, conditions at the end point of a titration should be approximated. It was found that addition of both iodide and iodate caused the indicator current to increaae, supporting the belief that the point of minimum current and the equiTalence point are identical. The point of maxim.um current can not be uaed as an exact point of reference in the titration for two reaaona: (l) It ia not a aharp maximum; (2) Its location does not correapond to the time required to oxidize all iodide to 1edine. It can be aeen from figure 2 that wherea• the endpoint of the titration occur• at approximately 205 aeconda, the maximum indicator current occurs at about 70 aeoonda. When the titration 1• run in hydrochloric acid with no bromide preaent (see curve B, figure 2), the maximum ia shifted still further, occurring at about 60 seconds. For these reasona, it was neceesary to utilize the point of minimum indicator current in determining the end point. It can be aeen from figure 2 that in the vicinity of the endpoint, the indicator current reache• a minimum value, then increases rapidly. If a suitable time after attaining of this minimum current is allowed to elapae (usually two aeconds on the high rate, thirty aeconds on - 8 - the low rate), the plot of indicator current is linear and can be safely used for extrapolation. In an effort to determine the reaeon for diaplacement of the current maximum, measurements were made of the solution potential, indicator cathode potential and indicator anode potential againat a saturated calomel half cell. Table II. Theae result• are tabulated in Since a fixed potential difference (1,8 millivolte) ia applied aero•• the indicator eleotrodea, the relatiYe value• of E (anode) - E (aolution) and E (1olution) - E (cathode) indicate the a.mount of control exercised by that electrode reaction over the diffu1ion current aa a whole. Thue, aa the current initially riaes, the reaction• at the indicator electrodee are: Cathode: r2 + 2e- = 21- Anode: 21- - 2e- = I2 Here the greatest potential drop ia at the cathode, and diffuaion of iodine to the cathode controls the magnitude of the diffuaion current. In the region of current maximum (time= 75 aec.), the potential drop is approximately the same at each electrode; diffuaion of iodine to the indicator cathode and of iodide to the indicator a.node control the magnitude of the diffusion current. At a time 1uf- ficient for all iodide to have been oxidized to iodine (time= 104 sec.), the aupply of iodide is exhausted, and the electrode reactions become: Cathode: I2 + 2e- = 21- Anode: I2 + 4Br- - 2e- = 2IBr2- - 9 - The fact that the greatest potential drop 1• at the indicator anode indicate• that the anode reaction, and hence diffusion of iodine to the anode, control• the magnitude of the diff'uaion current. Since both anode and cathode reactions involve diffusion of iodine, the indicator current drop• off from the iu.ximum. Aa the titration proceed•, there 1• an equilibrium between two reactions at the cathode, bromine being present in equilibrium with the iodine monobromide formed in the titration. Cathode: r2 + 2e- =2r- =2BrI 2 + 4Br- - 2e- = 2IBr 2- Br2 + 2eAnode: In the vicinity of the endpoint the concentration of iodine ia very amall, and the amall indicator current can be attributed to the iodine and bromine present in equilibrium with iodine monobromide. There is a sudden ahift of potential to the indicator cathode, the electrode reactions becoming: Cathodes Br 2 + 2e- = 2Br- Anode: 2Br- - 2e- = Br 2 After the endpoint, diffusion of bromine to the indicator cathode governs the magnitude of the indicator current. In figure 2 (curve B.) it can be seen that when iodide is titrated in the abaenee of' bromide, there is little or no indicator current at the end point. This may be explained by the •mall equi- librium constant for the reaction4 2101 (aq) = 1 2 (aq) + 01 2 (aq) K =1.6 x 10-lO - 10 - ae compared with the reaction 2IBr (aq) =I 2 (aq) + Br 2 (aq) K =1.2 x 10-~/ In an attempt to confirm these conclusion,, experiment• were made using solutions of iodine in two formal hydrochloric acid. When to such a solution was added a small amount of bromide ion• the indicator current increased only slightly (from 12.8 microamperes to 13.1 mioroamperea, for example.) But if to such an iodine solu- tion an equal amount of iodide ion were added, the indicator current increa1ed greatly (from 20.0 microamperea to ~.2 microamperea, for example.) The former experiment approximates conditions in the ti- tration when all iodide has been oxidized to iodine; the latter experiment approximates condition• in the vicinity of the point of maximum indicator current. The small diffusion current which flow• at the end point wa1 found to vary with the potential impressed acros• the indicator electrode• (aee figure)•) When the potential was greater than about 160 millivolt,, the indicator current was quite large. By use of an applied potential lesa than 140 millivolts, this current was kept at a low value (from O.) microamperes with a 12 microgram sample to 20 microamperee with a 2000 microgram sample.) At larger values of the applied potential, there is the possibility of an anode reaction involving the oxidation of iodine to higher oxidation statea. The final indicator current was found to be independent of hydrogen ion and bromide concentration, but increa1ed aa the quantity - 11 - of iodide being oxidized increased. In a study of this current, iodine monochloride was prepared by oxidation of iodide with iodate in four formal hydrochloric acid, uaing the iodine monochloride endpoint7. When a sample of this, equivalent to the amount of iodine monobromide present at the end point of a hundred aecond titration on the high rate, was placed in the titration cell, an indicator current of less than 1 microampere was observed. Upon addition of 5 ml of one formal sodium bromide, the current increased immediately to about 8 microamperea. Titration of an equivalent amount of io- dide by electrolytically generated bromine resulted in a final diffuaion current of 10 microamperea. The similarity of these values supports the belief that the final diffusion current 1• cauaed in part by iodine and bromine in equilibrium with iodine monobromide. It waa found necessary to open the indicator electrode circuit during the course of a titration. In a titration on the high rate, the indicator current was too great to be determined by the microammeter employed. During low rate titrations, the indicator current did not exceed 50 microamperea and hence could be read at all times. How~ver, continual passage of indicator current cauaed a positive error of several percent in the determination. The initial indicator current found in all iodide solutions upon acidification i• believed to be due to air oxidation. date could be detected in the potassium iodide uaed. No io- The current was relatively large (5 to 10 microam.peres) if stock iodide solu- tions which had not been treated with carbonate were uaed. Fur- - 12 - thermore, if the iodide aample1 were acidified and allowed to atand, the initial indicator current increaaed with time of atanding. Sm.all initial indicator currenta (1 to} microamperea) were obtained by uae of iodide aolutiona 0.005 F to 0.01 Fin •odium carbonate; theae 1am.ple1 were not acidified until immediately before they were to be titrated. In making calculation,, one correcta for the initial indicator current by adding the time equivalent to it, to the obaerved time of bromine generation. Attempt• were made to titrate iodide in hydrochloric acid aolution1 in the abaence of bromide. 'lbia would have the advantage of giving a very amall indicator current at the end point. cultiea were encountered. Two diffi- Indicator current valuea were attained very alowly, it being neceaaary to wait for aa long aa ~0 aecond• for a steady value to be reached. result• were alwaya obtained. Further, it waa found that high In the titration shown in figure 2 (curve B), the calculated time for the end point to be reached (including the correction for the initial indicator current) waa 207.4 aeconda. 'lbe actual endpoint, using the same correction, waa 224.2 aeconds, a value approximately &/4 high. It is believed that thi• error is due to oxidation of the electrodea in the etrong hydrochloric acid aolutions uaed. Compariaon ~ ~ micromethodes Zintl and Betz8 in aa potentiometric titration with ailver nitrate, determined 126 micrograms of iodide with an accuracy of 1%, and l} microgram• with an accuracy of 4%. - 1; - Ender• and Kautmann1 report a method in which aamplea centaining l to 10 microgram■ of iodide are oxidized to iodate with bromine water. Upon addition of potaaaium iodide, iodine is liberated and is distilled off. Treatment of the iodine formed with hydroxylamine result• in the formation of nitroua acid, by means of which aulfanilic acid is diazotized. Coupling of the diazonium compound with - napthylamine leada to the formation of a red azo dye, the concentration of which is determined colorimetrically. By thi• method quantitiea of iodide from 1 to 10 micrograms are determined with an accuracy of 4%. The complexity of the method, however, exclude• it from routine laboratory use. Kanirath 2 determined am.all quantitiea of iodide by oxidizing it to iodate, adding potaasium iodide and titrating the liberated iodine with thioaulfate. Aa in the method of Ender• and Kaufmann, aix times as mu.ch iodide reacts as was originally added. However, this method requires a sample containing 1000 microgram• of iodide. Laitinen, Jennings and Parka; using an amperometric titration with silver nitrate, determined samples containing 6;5 micrograms with an accuracy from 4% to 1;%, and sample• containing 254 micrograms with an accuracy of 10% to 11.5%. In compariaon with theae methods, the coulometric titration of iodide with electrolytically generated bromine aa here described can be conveniently used for routine determination of small amount• of iodide. As Table I shows, eamp1es of iodide as small as 1; microgra.~ s can be determined with an accuracy of± 2%. In quan- - 14 - titie• from 65 micrograma to 2000 micrograms, the determination is accurate to± 0.5%. Titration of a 130 microgram sample takes approximately 200 aeconds of low rate generation; titration of a 670 microgram sample takes approximately 100 seconds of high rate generation. '!he apparatus in its present form can not be used con- veniently for the determination of samples containing from 200 to 600 micrograms, and greater than 2000 micrograms because of the ex- cessive time required (greater than 300 ■ econds.) Provi ■ ion for an intermediate rate of generation in t."'1e apparatus would make po•sible the determination of samples containing 200 to 600 microgram■ of iodide. Summary: A coulometric method for the determination of iodide in samples containing from l; to 2000 micrograms has been developed. Iodide is oxidized by electrolytically generated bromine, the endpoint being determined by an a.mperometric method. Experimenta have shown the method to be accurate to± 2% for samples from 1~ to 50 microgr8lll.S, and accurate to z 0.5% for sample• of from 65 to 2000 micrograms. A study of the current flow characteristics of the titration has been made, and an explanation is advanced for the nature of the indicator current. LITERATURE CITED 1. Enders, G., and Kaufmann, L., z. Physiol. Chem. 243, 144 (19;6) 2. Kanirath, P., z. Anal. Chem.~, 1 (1942) ;. Laitinen, H. A., Jenning,, W. P., and Parka, T. D. Ind. Eng. Chem., Anal. Ed. 18, ;55 (1946) 4. Latimer, W. M. ~ Oxidation States .2f ~ Elements, New York 19;8 p. 5;, 58 5. Myer•, R. and Swift, E. H. - to be publi1hed 6. Sease, J. w., Niemann, o., and Swift, E. H. Anal. Chem. 1.2, 197 (1947) 7. Swift, E. H., J. Am. Chem. Soc.~' 894 (19;0) 8. Zintl, E. and Betz, K., z. Anal. Chem. 1!±, ;;o (1928) TABLE I Iodide (micrograma) Found Taken Error %Error No. ef Titration• Av. dev. % 12.8 12.7(a) -0.1 o.B 5 25.5 25.8 +o., 1.2 2 ~.7 +o.4 1.0 2 64.9 65.2(b) +o., o.46 8 64.9 65.1 +o.2 o.;1 2 129.6 1;0.; +o.7 0.54 129.6 1;0.0 +o.4 o.;1 2 194.5 195., +o.8 o.;1 2 194.5 19;.7 -o.8 o.;1 2 648.4 650.o(c) +l.6 0.25 6 648.4 650.5 +2.1 o.;2 2 669.0 671.5 +2.5 o.;7 4 0.15 669.0 672.6 +;.6 0.54 ; o.o6 1296.8 1295.; -1.5 0.12 2 1296.8 1299.0 +2.2 0.17 2 1;;8.0 1,;9.8 +1.8 0.1; 2 1;~.o 1;;9.4 +1.4 0.10 2 1625.6 1622.; -;.; 0.20 2 1625.6 1625.0(d) -o.6 o.o4 4 2007.0 2008.; +1.; o.o6 2 2007.0 2012.7 +5.7 0.29 2 ~-, (a) 11.8, 12.1, 12.5, 12.8, 14.2 (b) 64.o, 64.4, 64.6, 65.;, 65.,, 65.6, 65.7, 66.4 (c) 648.4, 649.4, 650.1, 650.1, 650.4, 651.4 (d) 162;.6, 1625.0, 1625.7, 1625.7 ' 5.5 0.9, 0.9; 0.11 -- --- o.o4 TABLE II Titration of a •ample containing 129.6 micrograms of iodide Time aeconda Current mi cream.per ea E(an. )-E(aol.) millivolts E(aol.)-E(cat.) millivolt• 0 2.0 ,1 102 4o ~o.o ,6 68 75 ,9.5 52 42 104 ,5.5 69 28 14o 25.0 91 19 211 4.o 2, 112 24o 29.5 15 90 Notes Current= indicator current E(an.) = potential of indicator anode againat calomel half cell. =potential of indicator cathode againat calomel half cell. E(eol.) =potential of aolution against calomel half cell. E(cat.) nmICATOh CThCUIT F E-2 D 5·7 S-5 0 R-7 E-1 R-3 o S-J < G""•--E> 8 R-2 S-118 ~ ,--------,....--------+-----&1+ A GENbRATOR CIHCUIT FIGURE I C SYMBOLS USED IN FIGURE 1 =DPDT Radio toggle 1witche1 s-4, s-6, S-7, s-8 =SPDT Radio toggle awitche• S-1, S-2, s-,, s-5 E-1 = Burge11 battery #2,o8 45 v. E-2 =Burge•• battery #4F4 l½ v. M-1 =Weston meter, model ,01 0-1 milliamp. M-2 =We1ton meter, modle ,01 0-50 microamp. A= To electric atopclock B =To atandard reaiatance C = To generator electrode• D = To indicator electrode• =To S-1, generator circuit G =To indicator anode terminal R-1 = ,,ooo ohm wire wound potentiometer F R-2 =600 ohm, 2 watt carbon resistance =,50 ohm, 10 watt aemi-variable wire wound resistance R-4 =1500 ohm wire wound potentiometer R-5 = 15,000 ohm wire wound potentiometer R-, R-6 = 5000 ohm, 10 watt aemi-variable wire wound reaiatance R-7 =45,000, 2 watt carbon re1i1tance R-8 =2000 ohm, 10 watt R-9 =100 ohm, 10 watt aemi-variable wire wound re1i1tance R-10 ■ emi-var1able wire wound resistance =24o ohm, 2 watt carbon re1iatance s-1• - S-1 Sw1tche1 connecting two circuit• for electrode treatment I . • : J __ _J_ __ _ . ,. ' . .. .. ----i~ - - - · . --. . ,. I. - - - -- - - - ·T: '' ::_,1_·. - ---t----,---1----1-....-+-:- :...:.~ ----:··- - ---- 1- - ' · -:- - - · -·· • · ··· I ... . - ---! - · -1-- • i . --r ' • i .• i ! ! 0 ~ -.. 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