.5 i I U ‘ 1 Q -. _ .M‘-— s“, . tt‘. p... 0-9“-..- 0 . .‘ O"#~“-‘ _-f ‘- .8 ' ”a“.- l I l 137 256 THS THE THERN’ECD‘J’i‘fiAMlCS LA' u“. is. ‘I‘r‘. ‘ ‘ r ‘.-~-‘-. ~ A93; 14119.1;151‘5151. Hgbbrm Hg LI" = Q .. - ‘ -. , t1 then? ‘;;,r ‘}‘::: chrcs (,1 n}, 5. ‘ ‘ ‘. ‘ "Pi." H‘!‘ ". 7 f“.,'\ '1 ."A'll‘i‘bAh MAM: LCLLQLE 7L. ‘ 1):". xx, 11.11.11.131: 1 1.111511. 1 10%;} I ’ ‘ I’ b ‘9' )W In. y. A P‘. 21. .' , U THE THERMODYNAMICS OF THE 333 A8313, MEI-'0“): ngBrzt H8 CELL AwThesia Submitted.to the Faculty of Michigan State College In pdrtlal Fulfillment of the Requirements.£dr the Degree of Master of Science in the Department of Chemistry by Thomas w. Dakln. '\ \ September 033$“ng 1958 ~11 & 1 11"} \1 <3 \J 'I' R ‘x' L1 EFL (111 53.113 ACKNOWLEDGEMENT Appreciation.ie expressed.hereto Dr. D. T. Ewing of the chemistry department for the materials and advice he game which aided in.the carrying out of this research. 1181. 9'“! ~,) Ft, Introduction and Literature This research was undertaken to establish with more certainty the value of the normal electroce potential of the mercuroue bromide electrode, and thereby calculate the free energy and heat of formation of mercurous bromide, and recalculate the activity coefficients of hydrobromie acid from measurements of the potential of the cell: Hz, HBr(m), ngBrz, Hg made at various m concentrations by Matthews1 and Larsonz. In view of the large amount of work done on the silver bromide electrode, and the recent papers by heston3, Harned, Keaton, and bonelson4 wherein the normal electrode potential of the AgBr electrode is determined very accurately, a meas- urement of the E° of the H823r2 electrode based only on the difference between the E°s of the AgBr and the HggBrg elect- rodes should be subject to considerable accuracy. All the calculations of the E° of the ngBr2 up to the present time have been based.on the measurements of the activity coeffic- ient of HBr by some other method, usually involving the AgBr electrode in.an.HBr solution.(Livingston5, Harnedfi, Lewis and Starch7). Gerke8, Larson?, and Matthewsl, all.based their calculations of E° on the measurement of the activity coeffic- 6 ient of HBr'determined.by Herned or Livingstons. Subsequent measurements of HBr activity coefficients by Horned, Keaton, and Donelson, and those two papers by Horned6 and LivingstonD indicate a discrepancy of around one per cent, giving, for example, .803, .802, .814 respectively for the activity coeff- icient of HBr at 0.100 molal concentration. In view of this, some doubt can be thrown on.the E0 values reported by uerkes, Larsonz, and Matthewsl. Gerke and Geddes8 report the value of the E0 of the mercurous bromide electrode as .1396v at 25.0000. They furnish a value to the International Critical Tables of .1385 v at 25.00°C. natthews1 reports a.value of .1596 v at 25.0000 using concentrations of HBr of around 1.0 molal. Larson, on the basis of Harned's6 previously published activity coefficients, gave a value of .1392? v when the HBr concentration was .9864 molal and .13855 v when the HBr waa.about 0.1 molal in his cell, both at 25.UU°C. A value calculated by Larsonz from the best Value of the cell: Hg,ngBr2,HBr(.10015m),H2(Pt) reported 8 6 activity coefficient by corks and oeddes , and Harned's was an E0 of .1591 v at 25.oo°c. The above values of the E° of the mercurous bromide electrode have a divergence of a little more than a. millivblt. They indicate that the true value probably lies between..1385 and .1396 v at 25.oo°c. As compared to the rather wide divergence of the above values for the E° of the mercurous bromide electrode, let us look at the recently reported values for the E° of the silver bromide electrode. Two types of silver bromide electrodes are now in use. One of them is the so called Maclnnea type of electrode, wherein the Agar is plated onto the electrode which is made the anode in a HBr sol- utimn. This is carried out at very low current densities. The base of the electrode is a Pt wire on which a coating of Ag has been formed either by plating or by the old method of fusing a silver oxide paste at 450°C. The more recently devised fused type of silver bromide electrode, called the hasten; electrode, is made by fusing a paste of 10% Agqus and 90% Agzo for 7% minutes at 650°C. Horned, Keston, and Donelson4 checked these two types against one another and found practically no difference. The present paper also bears out that conclusion. Both types are repro- ducible to within a few hundredths of a millivolt. Keston5 reported a value for the E0 of the silver bromide electrode of .0711 v at 25.00°C, measuring the cell: Ag,ngBr, HBr(m),HB(pt), and extrapolating the results to infinite dilution. Owen and.Foering9, measuring the cell: Ag,AgBr, KBr(m), Ne502(m1),HBoz(m2), H2(Pt), extrapolated the results to infinite dilution and obtained an E° for the AgBr electrode of .07lss v at 25.0090. Jones and beeoketromlo reported a value of .0712 v at 25.oo°c, obtained from.calculations from 4* measured the other data. Herned, Keaton, and.Donelson cell.Ag,AgBr,HBr(m),H2(Pt) and extrapolated the results to get the value of .07103 v at 25.oo°c for the E° of the AgBr electrode. In the most recent paper, Horned and Donelson11 check Owen and Foeringte results very closely after extrap- olating the results on measurements of the cells: Ag,AgBr, LiBr(m),L10H(0.01),H2(Pt) and Ag,AgBr,LlBr(m),HBr(0.Ol),H2(Pt), and report a value of .07131 v at 25.oo°c for the 3° of the AgBr electrode. The above brief survey is not at all complete in.an his- torical sense, but it is indicative of the consistency of the 8° values reported by different authors using the latest 1 technique develOped. As Horned.and Donelson.1 point out, .07115 :.00015 would include all of them. It is very likely that the true value of the E0 of the silver bromide electrode (for the reaction Ag Br- AgBr e) is within the range -.07100 to .07130 v at 25.0000. There is here a deviation of only .5 millivolt. The cell measured in this paper is: A3: A83?» HBr(m)9 HSZBrze Hg The potential of this cell is independent of the concentrat- ion of the HBr, or of the type of electrolyte furnishing the bromide ions, whether HBr, kBr, or some other bromide salt, for it is a.simple bromide ion exchange cell. The above statement is true of course only if the electrodes are prepared properly. The cell measures directly the dif- ference between the normal electrode potentials of the silver bromide and the mercurous bromide electrodes. It is a cell similar to the cell measured by Randell and Younglz, derxel5, and Bronsted14, using the corresponding chloride electrodes. Because the three recent papers by Owen and Foeringg, Harned, Keaton, and Donelsone, and Horned and Donelscnll all obtain their values of E° directly by extrapolation to infinite dilution, and because they report the values for the E° of the silver bromide electrode for various temperatures (at five degree intervals between 0° and 40°C). these 3°: are each given equal weight and used as the basis for the calculation of the E° of the mercurous bromide electrode in this paper. The E°s in these three papers are averaged to- gether at each respective temperature, and the average values obtained are assumed to be the most likely values for the E0 of the silver bromide electrode at the respective temperatures involved. The free energy of formation of the bromide ion from liquid,gaseous, and aqueous bromine was determined in 191? by Lewis and 3torch7, and more recently and probably more accurately by Jones and Baeckstromlo. The latter also de- termined the free energy of formation of silver bromide, measuring the cell: Pt-Ir,KBr(m) Br2,KBr(m),AgBr,Ag. Randall and Spencer communicated to the international Critical Tables a.value for the free energy of formation of AgBr or -22,910 cals at 25°C, based on.data obtained from the paper of Lewis and Storch7. mandall and Spencer also com— municated a value for the free energy of formation of HggBrz to the International Critical Tables of-42,702 calories at 25°C, based on.the E° reported by oerke and Goddess. A large number of workers have reported values for the heat of formation.of AgBr, many of which are too old to rely on: Bertholet15(1875), Thomsen16(1886), Kleinl7(l901), and Jouniaux;8(l904). More recent work includes the value deter- mined by Krahmer19 from e.m.f. measurements of ~24,193 cals. at 25°C, the value obtained.by hebbzo from.calorimetric measurements of 23,810 calories at 25°c, and the value reported.by woitinek21 from.cell measurements of -23,815 calories at 25°C. Shibata and Taketazz, gave a value of -23,430 calories at 25°C, based on cell measurements and on the heat of formation.of aqueous HBr given by Bertholet and Thomson, which fact throws some doubt onrthe accuracy of their value. The data.for the heat of formation.of mercurous bromide is much more meager. Nernst23, Thomsenlfi, and Varetg4 be- fore 1900 gave values from calorimetric measurements. The value in the International Critical Tables of -49,210 calories at 18°C is based on their data. he- cently, Ishikawa and Uedaz5 published values for the heat and free energy of formation.of mercurous bromide, based on cell measurements. They measured the temperature costtic- ients of the cells: H2,HBr(.1012m),HgBr,Hg, and.Pb(hetero amalgam),PbBr2(satd. sol.),HgBr,Hg. Preparation of Materials and Cells Hercury;- The mercury which was used in making up the mercurous bromide electrodes was in.the first step of purification, electrolyzed. It was made the anode in the bottom of a shallow dish, covered by an electrolyte of 3% nitric acid. The cathode, a Pt foil, was almost entirely surrounded.by a glass cup to catch any metal which dropped off the cathode. A current source of 7% volts was used, and the electrolysis was usually run for about eight hours, which was found to be sufficient time to remove all of the metallic impurities. If the mercury started with con- tained.some sludge and solid contamination, it was first run several times through a.capillary funnel into a col- umn of two feet of nitric acid (3%), before it was electrol— yzed. This in itself usually gets the mercury a mirror sur- face. After the electrolysis, the mercury was distilled twice in an all glass still, under a pressure of about 5mm. A small stream of air was led in through a capillary into the boiling mercury. The heater was an electric hot plate, on which was set a capper, asbestos covered, cylinder to surround the distilling flask. A stream of air from the air jet was directed on the condenser tube and cooled.the con- densing mercury nicely. A rather convenient and simple method of closing off the top of the distilling flask, through which the capillary tube was lead was devised. It is_detachable, does not involve any ground glass joints or glass seals to make it tight. It is made by sealing a.smaller tube about five centimeters in length onto the neck of the distilling flask, extending it upward. The inside diameter of this tube should be about the same as the outside diameter of the tube which is to be drawn into the capillary. Thus the tube extending into the capillary will fit snugly inside the ether tube for a length of about five centimeters. The top is made air tight by a.rubber tube fitted as a;sleeve over the outside tube and down onto the inside tube which goes through it. it is found from experience that this extension.does not get warm during the distillation, and.that the mercury never rises more than.a.tenth of the way up the narrow space between the inner and the outer tubes. Therefore it never gets to the rubber and effectively we have an all glass connection. ' The mercury was kept under an atmosphere of nitrogen after it was distilled. Hydrobromic Acid;- The source of the hydrobromic acid.was Baker's Analyzed.hydrcbromic acid. Constant boiling HBr was prepared from this by dictillaticn in.an all pyrex still. It was distilled four times, the middle fracticn being kept each time, and was water white when needs It was kept in the dark to prevent decomposition. Dilution of this constant boiling acid, which boiled at 123.5% at 74.21 cm, made approximately the correct concentration of acid desired. Due to the nature of the cell, and the fact that analyses were made of the cell solutions, exact dilutions were not nec- essary. Mercurcus Bromide:-» This was prepared from 0.1 molal hydrobromic acid made from.the constant boiling acid.ment- icned in the above paragraph, and 0.1 melal mercurous nitrate, made by dissolving the preper quantity of Baker's Analyzed mercurous nitrate ianater (two cc of concentrated nitric acid per liter were added to the mercurous nitrate to aid.in.dissolving it). These two solutions were evacuated, while being warmed.in.hot water, to remove dissolved gases, especially oxygen. They were then placed under an atmos- phere of nitrOgen. Evacuation.mentioned above caused the solution to boil, and this was continued for about one- half hour. Equivalent quantities of the solutions were added togeth— er. The mercurous nitrate solution was placed in an amber bottle into which a.stream of nitrogen.was bubbling. A separatory funnel containing the hydrobromic acid.was.insert- ed through a.rubber stepper. The separatory funnel was equipped with a.tube allowing nitrogen to be bubbled into it. it was kept continually filled with nitrogen. The nitrogen.escaping through the mercurous nitrate now bubbled up through the separatory funnel and.the hydrobromic acid, when the stapcock was opened. The size of the nitrOgen stream was regulated such that the pressure in the bottle just per- mitted the hydrobromic acid to drip into the mercurous nitrate from the separatory funnel. Thus, almost automatically, the precipitation was carried out, and stirred vigorously by the stream of nitrogen at the same time. After the pre- cipitation was complete, the stream of nitrOgen was ceased, and the precipitate allowed to settle. It was washed.by de- cantation with evacuated.water eight times,,using 200 cc portions (200 cc of O.l.m.mercurous nitrate formed the pre— cipitate). It was then washed five times wiFh lbO cc portions of the hydr0bromic acid which was to be used in the cell, and then placed under 250 cc.of the same solution. Some mer- (n 10 cury was added.to this, and the bottle was shaken vigorously for several minutes by hand until the mercury was finely dis- persed in.the precipitate. This was allowed to stand for at least eighteen hours to equilibrate, before it was placed in the cell. All washing was done under an atmosphere of nitrogen, and the precipitate was kept in.an amber bottle. Silver Bromide Electrodes:-» These were prepared.by two different methods. Most of the electrodes were prepared by a method similar to the one described by Brown26 for silver chloride electrodes. A Pt wire was sealed through the end of ans mm soft glass~tube so that it extended about 2: cm. It was plated with silver from a pure KAg(CN)2 sol- ution. Six of the electroues were plated at once, while arranged circularly around the anode, which was surrounded by a porous cup. The total current for the six electrodes arranged in.parallel was only two milliamps. Plating was continued for six“ hours. They were then washed over night, before coating with silver bromide. Before being plated the Pt wire was cleaned in boiling nitric acid. The plating sol- ution.contained 10 grams per liter of recrystallized KAg imm< +3; WUTOL «mg: L tam ”moi. ‘ 13 The Cells Themselves:- The cell was of the design illus- trated on page 12. Connection between the two half cells was made around the ungreased middle portion of the two way stopcock. Diffusion of one solution into the other was thus restrabned. Some of the same solution which had been evacuated and placed over the precipitated mercurous bro- mice was retained under the nitrogen in an amber bottle and was used as the electrolyte in the silver bromide half of the cell. Thus the concentration of the HBr was usually exactly the same in both half cells. In setting up the cell, tubes were extended to-the bottoms of the mercurous bromide portions of the cell vessel and nitrogen was led into these tubes for about three or four minutes to wash the air out of the cell. During this time, the bottle containing the mercurous bromide mercury mixture, covered with HBr, was.shaken vigorously. Some mercury was squirted into each of the portions of the cell vessel restricted for the mercurous bromide electrodes, forming a pool at the bottom of each, and then some of the mercurous bromide suspensiontwas pipetted in until the level of the solution;was near the top of the cell. All of the time nitrogen was.bubbling up through the sue- pension keeping oxygen away. As the tubes were cautiously removed, stoppers were inserted. The other side of the cell.was filled in a similar way while nitrogen was bubbling from the Jet indicated at the bottom of the silver bromide half of the cell vessel. The silver bromide electrodes, two of them, were necessarily exPosed to the air for a brief moment while inserting them- 14 into the cell, but were allowed to remain in the stream of nitrogen bubbling up from.the bottom to insure that no oxygen was present on then before the cell was closed and sealed. The stappers were sealed with paraffin, and the cell was placed.in the constant temperature bath. The process of filling the cell was carried out in some cases in the light of only a red lamp safe for bromide papers, in other cases , in the semidarkness of the laboratory after the shades had been pulled down, The manner of preparation in this respect is indicated with each cell. When the cell was dismantled, a sample of the electrolyte was removed from each side, and.analyzed. The acid solutions were weighed.and titrated with standard 0.1 N base, NaOH, using phenolphthalein as an indicator. The potassium bromide solutions were weighed and titrated.with a standard 0.1 N silver nitrate solution, using the adsorption.in- dicator, dichloflourescein. Constant Temperature Baths:- In order to protect the cells from the daylight in the laboratory, the water in the baths was treated with some dyes which permitted only orange red light to enter the baths. The cells were almost completely submerged in.this solutions The dyes which were found to be most suitable were a.combination, about 50% each, of Acid Fuchsin and Amaranth. The Acid Fuchsin absorbs strongly in. the green region, and the Amaranth in the blue violet regions To cool the 15° bath a stream of water from the cold water tap through some water Jackets, proved to be satisfactory, if regulated from time to time. It proved to be much.harder 15 to regulate at a constant temperature over along period of time than either of the other baths, however. Beckman thermometers, calibrated against a Bureau of Standards thermometer, measured the temperature. Potentiometer:- The potentiometer used in all the measure- ments was a Queen Potentiometer. The potentiometer permitted measurement to 1/100 of a millivolt with a fair degree of accuracy. The source of current for this was two Edison cells which maintained their voltage fairly constant over a long perioc of time. The standard was a Weston Standard cell #6o06 calibrated by the Bureau of Standards and awarded certificate test # 71606 with a voltage at 25°C of 1.08618. A.type R Leeds and Northrup high sensitive galvanometer # 78411, having an internal resistance of 500 ohms was used as a current detector. The whole of the potentiometer set up was on.an equi- potential base, a copper sheet. The wires were carried with the exception of the ends, through conduit. The galvanometer rested on a lead sheet on.a concrete block floating in sand, in order to guard against Vibration. The standard cell was kept in.a copper box suspended in the 25° constant temperature bath. Humid weather always seemed to cause highly irregular results. On such days it was found necessary to suspend measurements. At first the leads from the potentiometer to the various baths were carried through conduit. This was found later to contribute to the irregularity of the measurements on humid days. Clip leads out in.the air, gave much less irregular results on these days, although on dry days, no difference was observed. 16 17 EXperimental Data The cell diagram: Ag, AgBr, MBr(m), HgZBrz, Hg applies to all.of the cells measured in this paper, with the simple substitution of HBr or KBr as is the case for MBr, and of the prOper concentration m.as recorded for each particular cell. In order to obtain a balance with the potentiometer it was necessary to have the Ag electrode connected to the negative terminal of the potentiometer. In recording the voltage of the following cells, the notation is used as follows; th-Agn indicates that the potential measurement is between the first mercurous. bromide electrode and.the;seeend.and the second silver bromide electrode. The average values are corrected to round temperatures by the following equation; E20.UU:: Et -b- .00031(t-2o.00) , where .00031 represents the approximate temperature coefficient of the call. All potential readings are in.volts. Cell.No. I (Made up in semidarkness) Molality of HBr is .1022 Time,Hrs. Temp. Hg'-Ag’ Hg"—Agt HgI—flgtu HgntaAgtt 20 24.80 .0676 .0682‘ .0679 .0679 ?1 24.80 .0679 .0579 .0679 .0679 74 24.80 .06190 .0619 .0679 .0619 88 24.80 .06792 .0678: .06784 .06782 111 24.80 .0681 .0680 .0681 .0680 Av..Value, E20 is .0679? v Analysis of solution from the cell gave .1022 M for the Hg side and .1026 M for the Ag side. 325(corr.for concentration) is .06790 v Celero. 3 Molality of HBr is .1002‘ Time,Hrs. Temp. ‘13 24.86 #58 24.8o *41 24.86 42 2b.00 47 * o1 ' 68 ' 8o ' H33*Ag' Hg"-Ag' .06GUZi .06605 changed.Ag' electrode .06760 .0676o changed Ag't electrode 006778 .00788 .0618?) 000808 .06190 .06822 005804 006792 .06802 .06791 .06825 .067o8 (Made up in red light, din) H8I_Ag" .06760 .06750 .0675? .OoBOO .0680? .0680? .06806 .06800 .06746 Hg! IgAgII .06166 .06753 .00748 .06820 .06842 .06812 .06799 .06792 .0o781 e152 " t excluded from average Aye. Value, E25 is .06804 v No correction necessary for concentration. Cell No. 4 (Cell madeup in dim red light) Molality of HBr is .1002 Time.Hrs. Temp. Hg'eAg‘ Hg"-Ag' Hg'-Ag" Hg"-Ag" 16 24.86 .06160 .06163 .06180 .0877? 38 * .06168 .06768 .06802 .06801 Bo . ' .06166 .06181 .0o798 .06808 Ave. value, E25 is .06781 t No correction for concentration Cell No. b Time, *14 18 21 24 24% 36 38 41 .480 ##6 2 (Made up in dim red light) Molality of HBr is .1002 Hrs. Temp. 24.85 15.30 15.25 24.84 24.84 24.84 24.86 35.33 34.67 24.85 HgieAg' Hg"eAg' Hg'nAg'* .06763 .06754 .06764 .06002 .08611 .06493 .06515 .06494 .0600? .00693‘ .06678* .06190 replaced.Ag| electrode .06790 .0677? .06780 .06810 .06802 .0679? .06811 .06802 .0679? .0714? .07121 .07132 .07105 .07041 .07076 .06808 .06762 .06760 Hg"€Ag" .06164 .06605 .06515 .06778 .06770 .06789 .06790 .02102 .07023 .06748 ,4 Cell started.to decline in.e. m.f. here and became rather erratic. * These values excluded from average. Avea Value, E15 13 .06497' Ave. Value, E25 is .0619? v Ave. Value, E55 is .07115 w Analysis of the solution from the cell gave .1004 M for the Hg side and .0999M for the Ag side. Eld(corrected for conc.) is .06013 v E ‘ ,ie..06813 v 25 E50 " 13 .07151 7 19 Cell No. 8 T1m6,HrBe Temp. 3 6 *21 21% :43 46 4s 74 #91 .115 :115 117 (Made up in semi darkness) Molality of HBr is .1043 HS"A8’ Hg"-Ag' He'“A8" H8"-A8" 24.84 .06824 .06802 .0683? .06818 24.84 .06811 .06791 .0681? .06796 24.86 .06792 .06184 .06795 .06788 scrapped tapping key contacts ‘ .06830 .06820 .06836 .06826 ' .06830 .06803 .06836 .06809 ‘ .06762 .06776 .06766 .06770 35.36 .07124 .07100 .07130 .0710? 35.33 .07138 .07111 .07144 .07115 24.84 .06812 .06802 .0681? .06804 25.53 .06849 .0683? .06864 .06841 24.90 .06778 .06760 .06782 .06164 24.86 .06768 .0674? .06776 .06760 24.86 .06830 .0681? .06840 .06824 #These values corrected.to 24.85 before averaging. #These readings seem to be definitely out of line in a way that would definitely indicate that the measurement was in.srror due to dampness, and are excluded in.the first average. Ave. Value, E is .06824 t 26 Aye. Value (including * values), is .0680? v Aves Value, E5. 13 007110 1 Analysis or the solution.trom the cell gays .1039M for the fig side and .1046M for the silver side. E25(corrected for cell.) is .06802 v E35(corrected for cone.) is .0708? v 20 Cell No. 9 (Made up in semi darkness) Molality of HBr is .1068 Time,Hrs. Temp. Hgi-Ag' Hg"-Ag' H8"A8" Hg"-Ag" 2% 24.89 .06800 .06802 .06809 .06811 4% 24.88 .06798 .06804 .06812 .06819 19 14.80 .06436 .06488 .06458 .06608 49 14.55 .06437 .06474 .06431 .06469 :53, 24.81 .06785 .0675? .06772. .06775 * Excluded from average (hysteresis evident) Ave. value, E25 is .06811v Ave. value, E15 is .06473v Analysis gives .1065 m, Hg side, and .1068, Ag side. Ave. value(corrected for conc.), E25 is .06803 v' Ava. value(corrected for conc.), E15 is .06465 v Cell No. 15 (Made up in semi darkness) Molality of HBr is .100 1% 24.86 .06810 .06812 .0680? .06810 6 24.90 .06813 .0681? .06811 .06815 24- '24.86 .06829 .06832 .06825 .06828 47 24.84 .0681? .06811 .06820 .06814 60 24.86 .06760 .06772 ' .06796 .06806 :55 18.21 .06406 .06554 .06464 .06685 .88 - .06471 .06588 .06481 .06571 .72 . .06506 .06686 .06498. .06546 * N.B. These values.are somewhat erratic but they are averaged together nevertheless. Ave. value, E25 is..06814 v Eve. value, 318 is .0650? v 21 Cell No. T1me,Hrs. Temp. Hg'eAg' Hg"-Ag' Hg'-Ag" Hg"-Ag" 17 24,86 .0680? .0680? .0681? .06818 23 ' 24.87 .06799 .06798 .06809 .06808 41 18.66 ' .06420 .06426 .06410 .06412 71 15.17 .06435 .06422 .06423 .06417 :74 24.84 .06731 .06732 .06746 .06743 16 (Made up in semicarkness) Molality of KBr is .2002‘ tExcluoed from average (hysteresis evident) 22 Ave. Value, E25 is .06812 v Ave. value, E15 is .06408 v 0611 No. 17 (Made up in semidarkness) 5 24.86 .06792 .0679? .06791 .06796 26 " .06796 .06812 48 15.18 .06408 .06420 .06420 .06436 61 24.86 .06802 .06805 .06801 .06805 53 34.87 .07095 .07092 .07098 .07094 69 . .07152 .07160 .07128 .07186 72 ' .07108 .07088 .07105 .07096 Ave. value, 325 is .06804 v Ave. value, E15 is .06416 v Ave. value, E55 is .07109 v There is no correction for concentration for either of the above cells. Cell Time.Hrs. Temp. Hs'ess' Hg"-Ag' H2'-Ks" Hg"-Ag" 22% 34.76 .07108 .07098 .07108 .07098 24 34.75 .07128 .07120 .07125 .07115 24% 2 4.92 .06832 .06808 .06831 .06808 27 24.92 .06811 .06793 .06806 .06792 43 25.08 .06841 .06843 .06840 .06842 48 16.97 .06524 .06521 .06525 .06521 52 17.09 .06509 .06509 .06511 .06511 54 ' .06533 .06533 .06531 .06531 65 25.07 .06834 .0683? .06831 .06833 67 25.07 .06834 .06836 .06830 .06831 76 34.79 .07138 .07131 .07135 .07129 *89 25.07 .06765 .06779 .06755 .06772 *Excluded from average Ave. value, E25 is .06826 v Ave. value, E35 is .07126 v Ave. value, E15 is .06459 v 0811 No. 26 (made up in semidarkness) Molality of HBr is .0508 5% 25.08 .06792 .06800 .06792 .06801 7% 55.41 .07120 .07115 .07125 .07122 9 35.76 .07120 .07115 .07123 .07122 22 25.07 .06825 .06807 .06834 .06815 25 ' .06826 .06801 .06835 .06811 28 15.18 .06516 .06514 .06531 .06532 35 15.50 .06525 .06521 .06544 .06540 NO. 25 Molality of HBr is .0508 (Made up in semidarkness) 23 Cell No. 26 (continued) Av. value, E25 is Av. value, E is 55 Av. value, E15 is These last two cells, using the Keston type of AgBr electrodes used in this research were of the plated.type. There is no correction for concentration for either of the above cells. Cell No. 6 . 06809 v 607102 V 006517 V (Made up in dim red light) Molality of HBr is .2014 Time,Hrs. Temp. 14 24.88 16 24.84 19 16.10 22% 16.10 *36% 16.80 ~44 24.85 Hg'-As' .0679? .06777 .06461 006466 .06447 .06746 Hgi|gAgO .06775 .06779 .06457 .06455 .06455 .06758 fused AsBr electrodes. Hg'e421' .06785 .06796 .06488 .06445 .06445 .06748 *These values are excluded for the cell started t o decline . Ave. value, E25 is .06789 v Ave. value, E15 is .06429 v Analysis of the solutions for the above .2012M, Hg side, and..2015M. As side. Ave. value, (corrected for conc) Ave. value, (corrected for conc) 18 325: .06783 v is E15: .06423 v Nos. 25 and 26, were made up All other HgII-Ag'I .06791 .06797 .06485 .06450 .06450 .06759 has obviously cell gives 24 25 Cell No. 19 (made up in semi darkness) Molality of HBr is .5120 Time,Hrs. Temp. Hh'—As' Hg"—Ag' Hg'eAs“ Hs"-Ag" 3 24.89 .06770 .06814 .06758 .06803 6 24.87 .06783 .06774 .06762 .06751 27 24.88 .06768 .06774 .06762 .06751 ¢ell from this time on steadily declined in value. Ave. value, E25 is .0677? v Analysis of solutions after dismantling the cell, gave molality of .5120 for Ag side and .5046 for the Hg side. Correction of the Ibovesl for this, gives E25(corrected) : .06716 v Cell No. 21 (made up in semi darkness) ; HBr .5082M 8% 24.90 .06676 .06675 .06675 .06675 24 24.90 .06692 .06698 .06676 .06688 Cell declined rapidly after these readings. No correction was necessary for concentration. Ave. value, E25 is .06681 Cell No. 22 (made up in semi darkness); HBr .5082! 19 25.05 .06671 .06670 .06665 .06677 27 25.14 .06800 .06695 .06675 .06681 '43 24.97 .06628 .06626 .06620 .06624 #93 24.88 .0646? .06473 .06480 .06483 4 These readings are typical of all the cells having a molality of 0.5 HBr; they show the manner in which the e.m.f. declined as the cell stood. Bo correction was necessary for cone. in this cell. 26 Some discussion should be made of the divergent results obtained in the cells 19, 21, and 22. These cells gave an initial e.m.f. which was about one millivolt lower than the average of the other cells. These cells also showed a rapid decline in value. Due to the fact that the 0 cells declined so rapidly in value, it seems unreasonable to draw any conclusion from the fact that they were initially lower than the other cells of lower concentration. It seems likely that the result obtained is due to the AgBr electrodes which were of very small area. The AsBr plated on them would be relatively more soluble in the more con- oentrated HBr, and so much would dissolve off that they would be pregressively and permanently affected. Livingston5 came to the same conclusion. one other thing was noted in regard to these cells. When the cells were dismantled, the HszBr of the ngBrz 2 electrodes appeared decidelly darker. No such effect was observed with the weaker concentrations of HBr. Apparently in the case of these cells the dye in the constant temp- erature baths was not sufficient protection from the light. For the reasons stated above, these cells are not in- cluded in the final calculations. 27 The following two cells are included to show the type of results obtained using the H2 r2 precipitated from .02! 2B HENO and .05M KBr in air according to the method of 3 Matthews and Larson. The Hg28r2 after being washed eight times with water as indicated in their theses, was washed five times with the HBr which was to be used in the cell. From the time of contact with the acid, it was kept con- tinually under N in amber bottles. Original precipitation 2 was carried out in an amber bottle, and eXposure to light was slight if at all. It was allowed to stand under the HBr which was going into the cell, along with some Hg, and shaken occasionally from time to time, for about eighteen hours (equilibration time». Cell No. 13 (Made up in semi darkness) Molalitv of HBr is .100 Time.Hrs. Temp. Hg'—Ag' H2"9Ag' Hg'-Ag" Hg"-Ag" The e.m.f. started.at a low value around .065 v and gradually rose over two days' time until the readings below begin. 48 24.80 .06739 .06785 .06740 .06786 53 24.86 .06725 .06773 .06736 .06783 147 24.90 .06664 .06722 .06672 .06735 Cell No. 14 (Same solution as above except that it had stood.5 days with the H2 Brz . Values started around .064 v and graduafly2 rose in two days'time to indicated.values.) 53 70 79 81 139 24.90 w I 0 24.87 24.86 .06744 .06792 .06660 .06725 .06666 .06740 shock for a minute .06798 .06829 .0681? .06873 .06770 .06729 .06690 .06834 .06838 .06784 .06821 .06770 .06766 .06864 006894 .06832 28 These results indicate that if sufficient time is allowed and if sufficient shaking is given the suspension of HszBrz in HBr, in order to allow it to come to equilib- rium, about the same results will be obtained as with the method used in the preparation of the rest of the cells in this research. The results show here with the Matthews' method of preparation.indicate that much more time and care would be necessary in order toobtain consistent results, for the results are very erratic in these two cells. The advantage of the author's method over Matthews' in saving of time (the cells come to equilibrium much more rapidly) warrants its 1136. Table I Summary of all e.m.f.s for the Cell As,AsBr, MBr(m), HszBr ,Hs (cgrrected averages) Cell No. M0181 conc. E15 E25 E35 1 .1022HBr .06790 v 3 .1002 " .06804 v 4 . " .06781 5 ' .06513 .06813 .07131 6 .2014HBr (.06423) .06783 8 .lO43HBr . .06802 .07109 9 .1068HBr .06465 .06803 15 .100 HBr .0650? .06814 .07109 25 .0508HBr .06459 .06826 .07126 26 .0508HBr .06517 .06809 .07102 16 .2052KBr (.06408) .06812 17 9 (.06416) .06783 .07109 Average (All cells) .06492 v .06805 v .07111 v This condensed.data shows that there is no apparent t trend of the e.m.f. of the cell with concentration, except in the case of the already discussed and.eliminated.data. obtained.with the 0.5m HBr calls. The values are quite consistent and do not vary more.than .24 millivolts in the maximum from.the average, and the average deviation.is much less. The values obtained with the Keaton fused.AgBr electrode (cells 25 and.26) agree perfectly with the others. ‘ The temperature coefficient of this cell is constant within experimental error, and is equal to: 30 dE/dT :: .000810 volts/00 between.15 and 35°C. Because the voltage of this cell is almost a straight line function.of the temperature (see graph 1), it is perfectly prOper to interolate values. The table below gives the average values of the cell at five degree intervals between.15 and 25°C. Table 11 Temperature Ecell 15.00°0 .06492 v 20.00°c .06649 25.00 .06804 80.00 .06959 36.00 .07111 The E of this cell is represented by the following equation between 15 and 85°C. E :: .06804(1s¥(t - 25.).000310) pf the Hg,1 ,Q705 v 4828332. HBJr(m),, Agar, Ag Cell 0 o O In [0 o o 9 o O o O ,4 4 b h m 31 32 The values reported for the E0 of the silver bromide 4 electrode by Harned, Keaton and Donelaon , Owen and Foeringg, Harned and Donelaon;1 are tabulated below and averaged. The average values are assumed to be the most probable values of the E0 of the Agar electrode at the respective temper- atures. Table 111 Temp. Engr,Ag Earned, Keaton Owen & Harned & Average 8 Donelaon. Fearing Donelaon of all 3 15.00°0 .07561 .07599 .07596 .07686 20.00 .07340 .07372. .07371 .07361 25.00 .07103 .07128 .0713; .07121 30.00 .06845 .06871, .06873 .06863 35.00 .06575 .06600 .06598 .06691 This E° corresponds to the reaction: AgBr 4-10! -——> AS«+ Br- 88 The 8°62 the Mercurous Bromide Electrode This cell digram.will apply to any of the cells measured directly in this paper, with the simple substitution of KBr for HBr in several cases and the various concentrations of each individual cell for c. The polarity as marked are the terminals of the potent- iometer to which the respective electrodes of the cell were connected when a balance was obtained. A89 ASBrr HBr(c), ngBrzt Hg Half cell _ reactions: Ag 4— Br (0) ——> AgBr+ 5H828r2+0 -——> H8 + Br'(<=) Total cell reactions Ag4—5Hg28r2-——> AgBr ~k Hg __ .0 0 E0811“ 13‘Ag,AgBr“’ EHg28r2,Hg (1726:2112? Ecell as positive) 0 0 th°n E8328r2,3g“' E0811 ’Eng,sgsr The last of these equations is used to calculate the E0 for the mercurous bromide electrode, using the appropriate values of the E of the cell taken from.the table II and the values of the E0 for the AgBr electrode from.tabla 111- Table IV Temper-W 5W< 4422232582424..- 15.00°c .14078 v 20.00 .14010 v 25.00 .13925 v 80.00 .13822 v 35.00 .13702 v 34 Tegperature Coefficient of the E of the ngBr Electrode 2 The average temperature coefficients between 15.00 and 20.00°C. 20.00 and 25.00°C, and so on were calculated with the formula below, and were assumed to be the temperature coefficients at 17.50, 22.50°C, and so on.respectively. They are tabulated below. (oE°/oT) :: (526 - 8:5)/5 Temperature dEo/dT 17.50°c -.000186 volts/°C 22.50 -.000170 27.50 -.000206 32.60 -.000240 These values war platted in graph III and found to lie very nearly an a.straight line. Values at round temperatures were taken from this graph for the temperature coefficient and tabulated beloi. Their accuracy is as great as the calculated values given. Table V Temperature. dEo/dT 15.00°c -.000119 volts/0C 20.00 —.000154 25.00 -.000189 80.00 -.000223 35.00 -.000258 35 Free Energy and Heat of Reaction In calculating the free energy and heat of reaction, the following formulas are used, well known equations in chem- ical thermodynamics: --AF° :: NFEOJoules= NFEO calories When N is two faradays, NF/4.186 46106. Mrs—‘2. 850(2) °) For the reaction: 2A3+H328r2 ——> 2AgBr + 283 which is the reaction in the cell Ag, AgBr, HBr(m), Hg28r2,Hg when two faradays of plus current are passed to the right. This is the cell measured directly in this paper. The temp- erature coefficient of this cell is dE/dT = .000810 volts/°c and it is constant throughout the temperature.rang e in- volved. » Table VI Temperature -AF 15.000C 2993 calories 20.00 3066 25.00 3137 30.00 3209 35.00 3279 The heat of reaction,4iH, is constant throughout this tepperature range and is: AH = 1152 calories The constancy of this heat of reaction with temperature 36 indicates that the sum of the Specific heats of the products is equal to the sum of the specific heats of the reactants. Jones andBaeckstrom10 give for the free energy of formation of silver bromide from liquid bromine and solid silver: Ag +%Br2 AAgBr ; '-AF = 22,935 calories (25°C) We have already calculated the free energy for the following reaction: ZAg -+ Hg Br ——> 2AgBr 4- 2H3. ; -AF:=3137 calories 2 2 (25 o) Thus by doubling the first reacticn and adding it to the re- verse of the second reaction, we get: 263 +-Br2 _€> HgZBrz ; -4F:=:42,733 calories (25°C) This value compares favorably with the value obtained.by Ishikawa and UedaZSOf‘42,700'calories, obtained by a differ- ent cell measurement. For the heat of formation of silver bromide from its elements at 26°C there is given by Krahmer(l920)19, webb(l92b)20, and waitinek.(1932)2l -24,193, -23,810, and-23,815 calories respectively. The probable value is.about -23,813 calories. Ag +§Br2~>AgBr ; AH: ~23,813 calories. (2:90) The value for the heat of the following reaction is given an the previous page: 2Ag + ngBrzé 21-Ig —+ 2.383! 3 AH = 1152 «142500) Thus by doubling the first reaction.and adding it to the 57 reverse of the second reactim we get: 2Hg + Bra -—> Hg28r2 :AH =1 -48,778 calories(2500) Therefore the heat of formation of mercurous bromide (ngsrz) from liquid bromine and solid silver at 26°C is an : - 48,718 calories/ mole This compares favorably with the result obtained by Ishikawa and Uedazs of 48,940 calories. The Entropy of Formaticn of Mercurous Bromide According to the relationship, .AF‘:= H -TA§ using the values for the free energy and.heat of reaction calculated in the previous section, we get: -42‘,.733 = -48,788 -'-29BA S and then as = -20.4 calories/moie/degree Abs. 58 For the reaction: 29 + Hg28r2 -——5 2Hg + 28r— which is the reaction.in.the half cell; -HBr(m),Hg2Br2,Hg when two faradays of positive electricity arepaased to the right. The values of the temperature coefficients for this cell are taken.from-the table V, and the appropriate values for the E0 are taken from the table IV. The de- crease in free energy and.the heat of reaction.is calculated at five different temperatures and tabulated.below. Table VII Temperature -AF° lS.00°C 649l.calories 20.00 64i0 25.00 6420 30.00 6323 55.00 6317 Table VIII Temperature -¥AHP 15.00°c -8071 calories 20.00 -8840 28.00 -9017 30.00 -9488 55.00 -9981 These values were plotted in.a.graph and found to lie very nearly on a.straight line, indicating a.relationship like this: a: :2 H025( l+k(t-25)) This was calculated and found to be: H3 = -9017(l + 95.4“ -25)) which will hold with an accuracy of about 0.1% between 39 15 and 25°C. Specific Heat of Mercurous Bromide It is noted in the section dealing with heat of reaction that.the heat of this reaction is constant with temperat- ure. . Ag + fingBr ————> Hg +- AgBr 2 Thus according to Kirchoffis law, the sum of the specific heats of the reactants.is equal to the sum of the specific heats of the products. Since the International Critical Tables list the following specific heat data-for Ag, Hg, and AgBr, and none for ngBr . Kirchoff's law gives 2 a.means of calculating the specific heat of mercurous bromide. The International Critical Tables list the following data: Material Specific heat at 20°C Ag 25.2 joules/gm atom/deg Hg 27.9 ' AgBr 55.3 jouleslgm mole/deg The following relationship mus} hold: Cp(A8> + Cp(%HagBr2) == Cp(H2) +Cp(AgBr) 25.2 + - —.= 27.9 +55.5 or cp(iHeZBr2> r- filamefgicmie/aee. E0 0 f the H8 281‘2 E 18 ctrod e 1 510° 0 2 5 40°C M .00028 "1‘ /°c o rature coefficient, dE / Mercurous Bromide Electrode T, of the 2° of the .130026 590024 .00022 .00018 299016 .o°c 55. C I 38281?2 e 4H0 for the reaction- 42 \; i : 5.0°c 2:54 .o°c 35. DC 43 William D. Larsoaneasured the cell: (Pt)H2,‘HBr(m), HggBrz, Hg Cell reaction: %HgZBr2+-%H2«—%»Hg 4- Br‘.+,H+ E = E0 -‘- RT/nF" logemBr_mH Ybr’Yfi.’ The average values for the voltage of the cells he measured;are taken from Table V, p 33, of his thesis. They are tabulated below. Table IX Molggity 320 '225 '235 .050 .29940 v .50172;v .30638 v .100 .25645 .26813 .27157 .500 .21245 .2155? .21585 .500 .18568 .18650 .18814 .850 .15504 1.00 .14643 .14625 .14590 1.70 .10019 2.00 .10038 .09971 .0983? 2.75 .05905 .00784 .05559 From the above data, using the E° for the ngarz electrode determined.in this paper, the activity coefficients of the HBr were calculated. A'modification.of the equation at the top of the page is used. 0 log Y z: -logem - (E - E e HBr 2RT7F whereYis the mean activity coefficient of the HBr and E0 corresponds to the reaction for the cell at the top of the page". These activity coefficients are tabulated in table X below. Table X Mohgiity Ycoc Y25° Y55° .05 .855 .846 .825 .10 .819 .813 .795 .50 .796 .784 .755 .50 .811 .797 .765 .85 . .848 1.00 .8823 .875 .846 1.70 1.056 2.00 1.098 1.080 1.056 2.75 1.810 1.774 1.692 45 Summary The cell: Ag,AgBr,MBr(m),ngBr2,Hg has been measured at three-different concentrations with HBr as MBr, and at one concentration with KBr as MBr. Two different types of silver bromide electrodes have been used. The e.m.f. of this 0611 shows no apparent trend.with type of electrolyte (KBr or HBr), concentration, or type of AgBr electrode. The cell was also measured.at three different temperatures, 15°, 25°, and 35°C, and its temperature coefficient calculated. From the above data, the free energy and heat of reaction of the reduction of mercurous bromide by silver: 2Ag,-+.Hg2Br2 ~—9- 2Ag8r -4~ 2H3 was calculated, at five degree intervals from.l5 to 35°C. Making use of the previously, by other authors, accurately determined.E° of the AgBr electrode, the E0 of the ngsrz electrode is calculated at five degree intervals from l5 to 35°C, also the free energy and heat of the following reaction: 2@ Hg28r2 ———a> 2Hg + 28r'(a=l) Also making use of the accurately determined data.obtained recently by other authora.for the free energy and heat of formation.of silver bromide from its elements, the free energy, heat and.entropy of formation of mercurous bromide from.its elements is calculated. Using Kirchoff's law and.the International Critical Table values for the specific heat of.Ag,‘hg, and AgBr, the specific heat of HggBl‘g is calculated at 20°C Activity coefficients of HBr are recalculated from the data.of w. D. Larson using the above newly determined E0 of the mercurous bromide electrode. 1. 2. 3. 4. 5. 6. 7. 8. 46 References Matthews, J. L., PhD Thesis, Michigan.8tate College (1930). Larson, W. D., M.S. Thesis, Michigan State College (1931). Keston, A. 8., J. Am. Chem. Soc., 51, 1671 (1935). Harned, H. 3., Keston, A. 3., and.Donelson, J.u., J. Am. Chem. 30c., 58, 989 (1936). Livingston, R. 8., J. Am. Chem. 800., 48, 45 (1926). Harned, H. 3., J. Am. Chem. Soc., 51, 416 (1929). Lewis, G. N., and Storch, H., J. Am. Chem. Soc., 32, 2544 (1917). Cerke, R. H., and saddes, J. R., J. Phys. Chem., 31, 886 (1927). 9.0wen, B. 8., and Foering, L., J. Am. Chem. 600., pp, 1575 10. IL. (1936). Jones, 8., and Baeckstrom, J. Am. Chem. Soc., 56, 1524 (1934). Harned,lH. 8., anthonelson, J. G., J..Am. Chem. 800., 59, 1280 (1957). Randall, M., and Young, L. E., J. Am. Chem. 300., 59, 989 (1928). Cerke, R. H., J. Am. Chem. Soc., 35, 1684 (1922). Bronsted, J. N., Z. Physik. Chem., 59, 481 (1904). Bertholet, N., Ann. Chem. Phys., 3, 160(1875). Thomson, J.,*Thernochemische Untersuchungi, 188291886, 4VOlS. Klein, 3., Z. Physik. Chem», 39, 361 (1901). Jouniamx, A., J. Chin. Phys., 1, 609 (1904). Krahmer, C.,Z. Electrochem, 23, 97 (1920). Webb, T. J., J. Phys. Cheat, 22, 827 (1925). 21. 22. 24. 25. 26. woitinek, H., 2’. Electrochem., 518,, 359 (19:52) Shib‘ta, F. L. md Taket&, To», do Seio HirOShimo UniVQ, Ser. A, g, 243 (1932). Nernst, W., 2..Physik.Chem., g, 23 (1888). Varet, H., Ann. Chem. Phys., _8_, 79 (1876). Ishikawa, F., and. Ueda, 1., Sci. Repts. Tohoku Imp. Univ., lst -36r., _2_2_, 249 (1933). Brown, A. 3., J. Am. Chem. Soc., §_§_, 646 (1934). of J . .— r, < _ ‘ Jr" ’ 3 A .. , MI_:azx{w»-~ . ’- '<-','~’)'.. 4; =-' ‘ . ‘- I'd" 4/ , ,7 < . '7. ...‘ .-_. . aware “ .f.’ _‘ 1'3; nav‘- ‘4‘ onEmS'L‘ur UN“ ‘5 \A} 1”" D 18119 118119 ."' \‘fiw' 1p": 0 ‘ M IIIIIIIIIIIIIIIIIIIIIIIIIIIIII W“NW\\\\\\\\\\\\\\\\WW\\!\|\\|\\\\\\\\\\\\\\| 3 1293 02446 8567