JOURNAL OF RESEARCH of the National Bureau of Standards-A. Physics and Chemistry Vol. 7lA, No. 3, May-June 1967

The Heat of Formation of Boron Trifluoride by Direct Combination of the Elements 1

Eugene S. Domalski and George T. Armstrong

Institute for Basic Standards, National Bureau of Standards, Washington, D.C. 20234

IJcJnuary 27, 1967)

The energy o.f combination of c~ystalline boron in gaseous Auorine was measured in a bomb calorim­ eter. The e~pe nmental data combined wIth reasonable estimates of all known errors may be expressed by the equatIOn:

B(c)+3/2F2(g)=BF3(g), I:lH;', =-271.03 ±O.Sl kcaJ mol-'.

T?is r~sult is compared with other recent work on and related to the heat of formation f b tnfluonde. 0 oron

Key Words: Bomb calorimeter, boron, boron trifluoride, Auorine, heat of formation, Teflon.

1. Introduction The heat associated with the direct combination of the elements in a bomb calorimeter was measured An accurate value for the heat of formation of boron by Wise, Margrave, Feder, and Hubbard [41], who trifluoride is of significant importance because this found the heat of formation of BF3(g) to be - 269.88 value is involved in the thermochemistry of many ± 0.24 kcal mol- I, Another study involving the di­ boron compounds. The study of the thermochemistry rect. combination of the elements by Gross, Hayman, of boron compounds was for a long time hampered by LevI, and Stuart [5] gave - 271.20 kcal mol- I for the difficulties in measuring a suitable reaction involving heat of formation of BF3(g). elemental boron. The heat of formation of boric More recent additional measurement,s by Johnson, oxide, for instance, was uncertain to several kilo­ Feder, and Hubbard [6] showed that the calorimetric calories per mole because of the difficulty of getting work of Wise et al. [4], was correct, but reanalysis of the boron sample revealed impurities not previously comple~e. combustion of the element in oxygen, or of taken into account. A recalculation of their earlier determmmg the amount of reaction, in the absence of data gave for Il.H~98 [BF 3(g)], - 271.6 ± 0.9 kcal mol-I. complete combustion. The difficulty was apparently due to the glassy and nonvolatile character of the boric The calorimetric measurements reported by Johnson oxide formed. which tended to terminate the reaction et al. [6], were made using a boron sample of greater before completion, and made the analysis ot the Dfod­ purity in both a conventional-type combustion bomb uct a complex problem. and a two-chambered combustion bomb and led to a The thermochemistry of boron was placed on a firm value for Il.H~98 [BF3(g)] of - 271.65 ± 0.22 kcal mol-I. basis by the work of Prosen, Johnson, and Pergiel Research prior to the work of Wise et al. [4], is [1,2] 2 on the decomposition and hydrolysis of oiborane. neither sufficiently detailed nor accurate enough to and of Johnson. IV1ih er. and Prosen [31 on the heat of derive a value for the heat of formation of BF3 having formation of boron trichloride froHl the dements. an uncertainty less than several kilocalories per mole, With the aid of the heats of these reactions and other and hence, has not been considered. Gmelin [7] pro­ data, they obtained reasonably consistent values for vides a review of the earlier work on this subject for: B20 3(C), H3B03(c), B2H6(g), and BCla(g). While more the interested reader. recent work has suggested changes in some of the We felt that additional confirmatory work on the values, these changes have been small. heat of formation of BF3 was needed to establish more fully the recent work of Gross et al. [5], and Johnson et al. [6]. In addition, we have found that work in our 1 This research was sponsored by the Air Force Aero Propulsion Laboratory, Research an~ Development Divi sion, Air Force ?ystems Command. Wright-Patterson Air Force Base, laboratory on the measurement of the heats of com­ OhIO, .under USAF Delivery Order' Nq. 33(615)64- 1003, and by the Air Force Office of bustion of several refractory boron compounds has SCientific Res f':~T"c h under Order No. OAR ISSA 65-8. .. t Igures In brackets i~dica t e tile literature references at the e nd of this paper. produced values for their heats of formation very 195 sensItIve to the auxiliary value used for the heat of for individual metals to 0.001 percent. A nitrogen formation of boron trifluoride. Some systematic assay was made using the Kjeldahl method and the errors in the calculated heats of formation may be carbon content was determined by oxygen combus­ avoided by measuring the heat of combustion of boron tion of the sample and measurement of the CO2 using a similar procedure in the same apparatus. The formed. This measurement gave a higher carbon variations in the heat of formation of BF3 , as reported content than was indicated by the supplier. We by other investigators, are large enough to make a preferred our carbon analysis for the assay of our significant difference in the heats of formation of sample. The analysis for oxygen in our boron sample metallic borides if calculated from their heats of com­ was performed by both neutron activation and inert­ bustion in fluorine. gas fusion methods. The oxygen analysis obtained by inert-gas fusion is preferred over the analysis by 2. Materials neutron activation because of suspected interference by isotopic species produced from irradiation of the 2.1. Boron boron itself [8]. Table 1 summarizes the analysis of the boron sample, showing the total boron content to The sample of f3-rhombohedral boron was obtained be 99.68 percent by difference. from the Eagle-Picher Company and had been pre­ An x-ray diffraction pattern of the boron sample pared by the hydrogen reduction of boron tribromide determined by the NBS Crystallography Section on a substrate of zone refined boron. The maximum yielded lattice parameters in good agreement with particle size was 150 fL. The supplier reported traces data reported earlier. The lattice parameters were of copper and silicon and a small amount of carbon a = 10.922 A and c = 23.79 A (compared to a = 10.944 A in the sample. The sample was analyzed spectro­ and c = 23.811 A [10]) and the space group found was graphically for metallic impurities and quantitatively R3m.

TABLE 1. A nalysis of the boron sample a

Metal impurities Total

AI Fe Mg Mn Sr Ca Si Cu

< 0.001 0.079 0.002 0.014 0.002 O.OlO 0.Oi2 ...... 0.120 (0.0003)" (0.0007)"

Nonmetallic impurities

N 0 C

< 0.005 0.088 e 0.11 0.203 (O.l6l)d (0 .05)"

Assumed presence of nonme tallic impurities

BN

0.009 0.128 0.506 0.643 ______-L ______-L ______

Total boron content...... 99.677

Total boron as the element...... 99.237

a Analyses present ed in table 1 were performed by the NBS Analysis and Purification Section. unless otherwise stated. "Supplier's analysis (Eagle- Pic he r Co.). e In ert-gas fu sion (Ledoux and Co.)_ d Neutron activation analysis (General Atomic). " S upplier's analysis for carbon in boron by the met hod of Kuo. Bender. and Walker [91.

196 2.2. Teflon (Polytetrafluoroethylene) The densities used for the Te flon film , T eflon powder, and boron in making buoya ncy corrections were 2.15, The T eflon film and Teflon powder ("Teflon 7") used 2.16, and 2.35 g cm- 3 [13], respectively. Weighin gs in preparing pelleted mixtures for combustion experi­ of pelle ted mixtures and intermediate stages were me nts were the same as we have described in an earlier made to 0.01 mg. publication [11]. Here again neither the T eflon

powder nor the Teflon film were modified or treated TABLE 3. Amounts of sample and losses incurred during pellet in any s pecial way prior to use_ The energy of com­ preparation (averages) bustion, ilE~03' of the Teflon (fi lm and powder) was -10,372_8 Jg- I [11].

The fluorine used in the heat measure ments assayed I. Mass of Tefl on bag ...... g... 0.30 at 99.40 percent F 2• The fluorine was analyzed by 2. Mass of boron in mixture ...... g... .16 3. Mass of T eflon in mixture ...... g... 1.88 absorbing the F2 in mercury and observing the pres­ 4. Mass of T eAon coaling ...... g. .. 0.70 5. Loss of TeAon in sealing bag ...... mg... .32 sure and composition of the residual gases [121. The 6. Loss of mixture in pe ll eting. .. .. mg. .. .30 co mposition of the residue was determined by examina­ 7. Total loss in preparation...... mg ... .62 tion in a mass spectrometer- Table 2 shows the res ults of typical analysis of a fluorin e sample. 4. Calorimetric System TABLE 2. Composition of fluorine sam.ple No major changes had been made in th e bomb calorime ter, thermome tric syste m or combus ti on

CO IlSl i l u e nt M ule percent bomb since our earli er work [lll whi ch was carried out with the same apparatus. The apparatus will be F"l " 99.40 discussed here onl y brie fl y. 0 , 0.0960 N, .2784 An isothermal-jacket, stirred-wate r calorime te r was CO, .01 75 CF, . 1962 used ; the jacket was maintained at a constant te mpera­ Ac .0083 ture near 30 °C within 0.002 0c. T e mperature changes SO,I', .000 1 Sir, .0003 in th e calorimeter were measured to 0.0001 °C with a C,F, .0023 Mueller bridge in conjuncti on with a platinum SF,; .000 1 G-2 CIF)! .0002 resistance thermometer. Reactions we re carried out C3 F/! .0005 G!F4 or cyc lic CIF/! .000 1 in a n "A" nic kel combustion bomb, designed fo r serv­ ice with flu ori ne, having a volume of approxi mately a By diffe rence. 360 ml. Two aluminum electrodes each s uspended from the bomb head by a monel rod held a tungste n 3. Preparation of Sa m ple Pellets fuse (0.002 in ch diam) whi ch contributed about 20 ] to the combustion e nergy, assuming compl e te combus­ The first step of the procedure used to prepare the tion. The quantities of boron and Teflon in th e pelle ts boron sample for combustion .in fluorin e was to mix the we re adjusted to produce a te mperature rise in the sample with T eflon powder in a bag made of T efl on calorimeter of about 3 deg (27 to 30 °C). For proce­ film . The bagged mixture was the n pelleted and dures dealing with the loading and emptying of the provided with an additional coating of Te flon (method combustion bomb, and for details of the design and B of our earlier work [11]). Attempts to burn pelle ted construction of the fluorine manifold, our earlier work mixtures of boron and Teflon powder on whi ch no outer should be consulted [14]. T e fl on coating was provided (method A, [11]) resulted in s pontaneous combustion of the pellet during the 5. Products of Combustion flu orin e-loading procedure. However, if method B was used, it was possible to carry out the calorimetric Our previous work [11 , 14] has established that experiment, and the apparent heat transfer coefficients Teflon burns in 15 to 21 atm of fluorine to carbon tetra­ calculated for the calorime ter in these heat measure­ fluoride as the only major product. Higher fluorocar­ ments were co mparable to that of a normal combustion bons were not detected in amounts greater than 0.02 experiment in which no pre mature reaction was mole percent. The product gases were analyzed in a taking place. mass spectrometer after a bsorption of the excess Muc h care is needed in keeping trac k of the c umula­ fluorine in merc ury. It is interesting to note that the tive mass of the sample as the Teflon and boron are mass spectrome tric examination of produc t gases from added because some losses are always observed and a boron-Te fl on combustion experiment s howed no their di s tribution signifi cantly affects the results of th e sign of BF3. W e s us pect that under the conditions experiment. of the reaction of flu orin e with merc ury, an interaction Table 3 gives average values for the amounts of of some kind takes place between BF3 and the mercury T eflon and boron used in preparing a pellet and the fluoride formed during the absorption of flu orine. losses detected in the process. The sample masses A typical analysis of the residual produc t gases from were adj us ted for losses in the manner previously a combustion experiment is s hown in table 4. The described [Ill amounts of minor constituents found in the product

197 gases are greater than those expected on the basis of detail [11], in which Teflon was burned in 21 atm of the amounts present as impurities in the original fluorine. The value listed in section 2.2 [or the energy fluorine. The increments observed in the minor con· of combustion, t:.Ego3 , was determined in these experi­ stituents were probably introduced during sampling ments. Ten heat measurements were performed in and analysis procedures and were probably not in· which boron-Teflon pellets were burned in 21 atm of volved in the actual bomb process. fluorine. These measurements are summarized in Boron trifluoride was identified as a combustion table 5. In each experiment the sample pellet was product by infrared spectrometry. Examination in placed in the recess of an "A" nickel plate on the the region 650 to 400 em - 1 of a sample of the bomb bottom of the bomb. The bomb was attached to the product gases containing excess fluorine revealed the fluorine manifold and filled to 21 atm with fluorine by BF3 band at 481 em - 1 and the CF4 band at 630 em - I. the usual procedure. All bomb parts (bomb base, Spectra of the evacuated cell and of BF3 alone were bomb-head assembly, electrodes, liner and nickel taken over the region mentioned above to substantiate plate) were weighed before the first experiment and the identification. The cell used was 8 em long and after each successive experiment. The bomb parts had polyethylene windows, 0.0625 in thick. were washed with water and dried before the weigh­ ings were made. The numbered entries in table 5 are as follows: TABLE 4. Composition of residual product gases from a combustion experiment (mass spectrometric examination) (la) Mass of the boron mixed with Teflon in the pellet, corrected for weight loss in preparation, for recovery of unburned boron, and for a boron blank. C umpunent M ole pe rcent (lb) Mass of Teflon mixed with sample in the pellet,

N, 0.74 corrected for weight loss. 0 , .87 (2) Pressure of fluorine introduced into the bomb CO, . 16 CF.. 98.4 prior to combustion, corrected to 30°C. IW. SO, F, 0.008 (3) Energy equivalent of the initial calorimeter for a SiI". .026 given experiment. C, F, .01 2 SF.; .008 (4) Temperature change of the calorimeter, corrected for heat of stirring and heat transfer. (5) Total energy change in the bomb process. 6. Calibration Experiments (6) Energy liberated by the tungsten fuse assuming the fuse burns according to the reaction: Twenty calibration experiments were performed in which benzoic acid (Standard Sample 39i) was burned W(c)+ 3F2(g)= WF6(g). in 30 atm of oxygen and with 1 ml of distilled water in the nickel combustion bomb. Their consistency and From the heat of formation of WF6 [15], we calculate reproducibility have been discussed in our earlier 9.44 J mg - 1 for the energy of combustion· of the fuse. paper [11]. The average energy equivalent was cal· (7) Net energy correction for the hypothetical com­ culated to be 14,803.27 ± 0.99 J deg- I. The uncer· pression and decompression of bomb gases. tainty cited is the standard deviation of the mean. The energy equivalent is that of the standard initial oxygen t:.E cras = t:.E i(

198 0.28 [17J. The heat capacItIes at constant volume,

~~~r--: Cv , used in the calculation of entries (3) and (13) were ~~~~8 5.52 [18], 12.62 [19] , and 10.04 [16] cal deg- mol - I, - ", ' ..0'" ~"'

kJg- 1 for the r eaction: B4 C(c)+8F2(g)=4BF3(g)+CF4(g) based upon heat measure ments performed in our laboratory. These latter data will be reported in more detail in a future publication. "'i:::~ "' ~ MO '" 1.000 - ...... COV\O--'o:t'O\- Note that in adjusting th e energy of combustion of Oc-.i"":-i-M,.oOM",fOO NO NN-r-_J") the sample, e ntry (10), to the e nergy of co mbustion of CO 0 CO on ~ r..: ~...; .- '<:I' C'I:= pure boron, entry (14), the energy contributed by the I I impurities is subtracted from entry (10), and at the same time the mass of sample is reduced by the mass of the impurities. In calculating the corrections for the combustion of other impurities in the boron sample, the following heat of formation values were used and are given in kcal mol - I: B20 3 , -304.20 [25 1; BN, - 60.8 r251; MgF2' -268.7 [26 1; CaFt , -290.3 [27 1; SiF4 , ~8_ ", ~N - 385.98 [28]; FeF:1, - 235 [29]; SrF2, - 290.3 [27]; Ll")1'- \0'<:1' -\O'O!t'UjO\C'J~OOM O~"":~c-iu:iONr-..:O\ MnF3 , -238 [29]; and AIF:1, -361.0 [11]. C'JO \oC'>,)_UjOO 00 U; ..... - The raw data obtained in the benzoic acid calibration i;j ~i I I" experiments were programmed for the IBM 7094 com­ puter according to procedures outlined by Shomate [30] ~~ ~ for the computer calculation of combustion bomb MMuo - 0\0° _CO\Or-OCl"jo.n\O~ 6c-i"":-.ciM"";OM..oO calorimetric data. The energy equivalent obtained NO"- COC'l ...... OOC"l? r- C'-l ...... '<:f' was adjusted to that of the standard initial oxygen i '4 ~i calorimeter as described in section 6. The combus­ I I" tion experiments were similarly programmed, however, the only valid data calculated by the computer were the corrected temperature rises, Llte, because the pro­ gram used had not been modified to accommodate the use of fluorine as the oxidant. Atomic weights were taken from the 1961 table of ...... -' ~ """;"";"";""; -' atomic weights based on 12C = 12 and adopted by the I ~ . ••• I ~"O : ~ Q.) • : --, "0 .. International Union of Pure and Applied Chemistry o [31]. The unit of energy is the joule, and one calorie z was defin ed as 4.1840 J. = "E About 500 mg of crystalline boron was transformed .~ into boric acid solution by pyrohydrolysis 3 and the w solution examined by surface emission mass spec-

3The aut hors a rc grateful to M. W. Lerner and L. J. Pinto. U.S.A.E.C. New Brunswick Laboratory, New Brullswick. New Jersey. for perfo rming this task.

199 trometry 4 for the isotopic abundance of BIO/Bll. This ligible effect. A boron-Teflon combustion residue was study resulted in the atomic weight determination of analyzed for carbon, and the results showed an am'ount our sample of 10.812 ±0.005. As a result of the good comparable to the carbon content of residues formed agreement with the atomic weight of boron in the table from burning Teflon alone. A test for weight changes based on 12C = 12 we have used the value 10.811 g of pellets on exposure to fluorine indicated a slow mol - I from this table in our calculations. weight increase, which was not fully reversed on evaCl1- ation. A pellet which has been exposed to fluorine 8. Discussion and Results and later exposed to moist air showed additional small weight gains, indicating a hygroscopicity resulting from A residue amounting to less than 1 mg which was the exposure to fluorine. These effects we re small assumed to be unburned Teflon and/or carbon, was and slow, and no corrections were applied for them. However, they have been taken into account in as­ observed in heat measurements involving Teflon alone. sessing possible errors. No correction was applied to any experiment for this residue, and we assumed that the formation of the residue took place in all experiments approximately 9. Summary of Errors in proportion to the amount of Teflon initially present. The heat of combustion per gram of Teflon would be We have tried to estimate the overall experimental constant and the error due to residue formation would uncertainty for the heat of formation of BF3(g) deter­ be eliminated when the energy due to the combustion mined as a result of this investigation. Table 6 lists of Teflon in the pelle ted mixture was subtracted from the errors considered in making the estimate. We the total energy released in the combustion. have used the loss of sample found during the pellet­ After a boron-Teflon combustion experiment, a larger ing operation as a guide in estimating the error incurred residue was found than when only Teflon was burned, in preparing a pellet (see table 3, line 6). From this which necessitated determining the amount of un­ source we estimate an error of 0.10 percent. The two burned boron and also finding a method for gathering oxygen analyses were 0.161 and 0.088 percent and the the residue from the nickel support plate. The mass two carbon analyses were 0.05 and 0.11 percent. The of the residue was obtained by weighing the plate effect that the differences of the analyses would have before and after the experiment. The average mass upon the heat data introduces an error of 0.06 percent. found from these weighings was 3 mg. The residue was taken up from the support plate by TABLE 6, Summary of errors mixing and rubbing Na2C03 into the residue with a spatula. To determine the amount of boron, the residue mixed with Na2C03 was fused and put into Ma~nillld(' of Oeseription of e rrors e rror c xpressl·d solution with dilute acid. The pH of the solution was in percenl of adjusted, mannitol added, and the liberated acid H ~ !!II. for l)!Iron titrated with base. The mass of unburned boron 1. Weighing pell et.. 0.01 found in the residues by this analysis ranged from 0.5 2. Loss dllrin~ s ample preparation .. .10 to 1.1 mg. To determine the reliability of the above 3. Analys is of impurities.. .06 4. Reaction prior \0 ignition... .03 procedure, control experiments were performed in 5. Determining tlte amuulll of LlI1hurn ed boron. . . .06 6. Dete rmining the compos ition IIf th e cumbus tion which crystalline boron was mixed with Na2 CO:l and residue. .. .06 7. Fuse e ne rgy. . .01 the mixture was analyzed for boron using the same 8. Bomb corrosion.. .0 I procedure. As a result of the control experiments a 9. Calibration expe rime nts ... .01 10. En e r~ y of c otllbu~tion of Tefhlll [1lJ. .. .03 correction factor was applied to the boron recovered I I. Bonn} CHllIiJus tilJIl experimcll1 s ... .12 from the residues. Analysis of residues mixed with 12. Atomi(' we ight of buron... .05 13. Total e rror (perc ell!)... ;) .19 Na2 C03 was made by the NBS Analysis and Purifica­ tion Section. a (l'his is e quivale nt to 0.5 1 b -al mol - I) . We attempted to assign a composition to the residue even though the mass was subject to effects difficult An error from the reaction of the sample in the bomb to estimate such as reaction of the support plate with , prior to ignition was estimated at 0.03 percent. This I fluorine, hygroscopicity of the residue, and spattering was based upon the assumption that prereaction oc­ of molten tungsten onto the plate from the ignition curring in the bomb prior to ignition was not more process. Our estimate for the boron blank, boron than 5 J hr- I as suggested by mass increments of recovery, unburned Teflon, and tungsten account for pelleted mixtures upon exposure to fluorine. We about two thirds of the mass of the combustion residue. assumed that the determination of unburned boron was The remainder could be attributed to one of the above not in error by more than 0.1 mg (0.06 percent) and that effects, but in the absence of definite information no the additional error in estimating the total composition adjustment was made for it. of the combustion residue is similarly 0.06 percent. A test made to de termine whether the presence of Since the carbon in the boron combustion residue was boron affected the residue of Teflon, indicated a neg- comparable to the carbon from the combustion of Teflon alone, no error has been attributed to the

4 NBS Analysis a nd Purificatir)n Sec tion . uncertainty in residue left by the combustion of Teflon.

200 Errors due to the weighing of the pellet, fu se e nergy, 1 1. References and bomb corrosion were estimated at 0.01 percent. Estimates of uncertainties arising from the be nzoi c III E. J . Prosen. W. H. Johnson, a nd F. Y. Pergie l• .1 . I{ es. NBS 61, acid calibration experiments, TeRon combustion ex­ 247- 250 (1958), RP2901. periments and combustions of boron-TeRon mixtures [21 E . .1. Prosen, W. H. Johnson . a nd F. Y. Pergiel . ./ . Res. N BS 62, 43- 7 (1959), RP2927. were made by multiplying the percent standard de via­ 13 1 W. H. Johnson, R. C. Miller, and E. J. Prusen,./. Re s. NBS 62, tions of the means of the experiments by the appropri­ 213- 217 (1959), RP2956. ate factors for the Student t distribution at the 95 141 S. S. Wise, J. L. Margrave. H. M. Feder, and W. N. Hubbard . percent confidence level. Finally, we s uggest t hat the ./. Phys. Chem. 65,2157- 9 (1961). 151 (a) P. Cross. C. Hayman, P. L. Levi , and M. C. Stuart, Fulmer error present in the determination of the atomic we ight Researc h Institute Report R. 146/4/23. Nov. 1960. (b) C. is 0.05 percent as a result of the experimental findings Hayman, private communication, April, 1966. given in section 7. r6 1 C. K. Johnson, H. M. Feder, a nd W. N. Hubbard, ./. Phys. The total percent error in this study was found by Chem. 70, 1- 6 (1966). 171 Cmelin's Handbuch der anorganische Chemie, System Nummer taking the square root of the sum of the squares of the 13, Achte Auflage, Verlag Chemie C . M. B. H .. (a) Hauptband. individual errors cited. Leipzig-Berlin (1962). pp. 11 3- 116: (b) Erganzungsband Weinhe im/Be rgstrasse (1954), p. 171. r81 (a) C . H. Andersen, Ge ne ral Atom ic , San Di egu, Califurnia. 10. Heat of Formation of Boron Trifluoride private cummunicati on. March. 1966. (b) Kaman N uc lear. T echni cal Not e er N- lOS), JO June 1965. On the basis of the calorimetric data given in ta bl e 5, [91 c. Kuo. C . T. Bend er. a nd .1 . M. Walker, An a l. C he m. 35, we calculate for the s tandard heat of re action (1), 1505- 9 (1963). rl01 ./ . L. Hoa rd a nd A. E. Ne wkirk, .J. Am. C hem. Soc. 82,70- 6 (1960). B( c, ,a-rhombohedral) + 3/2F ~(g) = BF3(g) (1) [111 E. S . Domals ki and C. T. Armstrong, ./. Res. NBS 71A Wh ys. and Che m.), No.2, 105-118 (1 967). [1 21 C. 1'. Armstrong and R. S . J essup, J. Res. NBS 64A (phI's. a nd and, he nce, the standard e nthalpy of formation of C hc m. ), No. 1, 49- 59 (1960). boron triRuorid e at 298 oK, - 271.03 ± 0.14 kcal mol - I. [1 3 1 A. E. Newk irk. Pre paration a nd C hemi stry of Eleme ntary Boron. The latte r un cert ainty is the sta ndard deviation of the in Borax to Boranes. Advances in C he mi stry Series, Nu . 32. mean. We estimate our overall experimental un cer­ Ame ri can C he mi cal Socie ty, Washington, D.C. , 1961. tainty to be 0.51 kcal mol- I. 1141 E. S. Domals ki and G. T. Armstrong . .1. Res. NBS 69A Wh ys. a nd C he m.) , No.2, 137-147 (1965). Our value for the e nthalpy of formation of BF3(g) [l S I O . E. Mye rs and A. P. Brady, ./. Ph ys . Che m. 64,591- 4 (1960). is in good agree me nt with the result reported b y Gross 11 6 1 W. H. Evans. NBS . IHi va te cummunication , January 1, ]96 1. e t al. [5 1, and diffe rs from the result reporte d by John­ 11 71 W. D. Good. D. W. Scott , a nd C. Waddington, .I . Ph I'S. Che m. son et al. [6 1, by a pproximately our overall uncertaint y. 60, 1080- 9 (1956). 1181 W. H. Evans, J. Hil senrath . and H. W. Wooll e y. NBS. IJri va te J ohnson et al. [61 , have used some recent work by communicati on, July 1. ]960. Gunn [32j on the solution of BF:! in conc HF(aq), Good [191 Dow C he mi cal Company. J ANA I ~ The rmoc he mi cal T a bl es. and Milnsson [331 on the co mbusti on of boron in oxygen PB 168 370 (C lea ringhou se for Fede ral Scie ntifi c a nd T ech· in the presence of excess aqueous HF. th eir own data ni cal Information, S pringfi e ld . Virgini a. 1965). 1201 W. N. Hubbard, Flu urine Bomb Calorim etry, c h. 6. Experime nt a l on ~HJ2 ~ H [BF:!(g) I, a nd other appropriate a uxiliary d ata The rmoche mi stry. Vol. II , H. A. S kinne r. editor (Int erscie ncc to derive a value fo r ~HJ~ HH[HF' 3 H ~ 0 (a q ) 1=- 76.78 Publis he rs. In c., Ncw Yurk , ]962). kcal mol - I. Insertion of our value for ~Hf2! ' H [BF: !(g) I r2 11 .J. O . Hirschfe ld e r. C. F. C urtiss. and R. B. Bird. Mol ecular into this cycle, gives - 76.58 kcal mol- I for Theory of Gases a nd I,iquids (John Wil cy & Sons. In c .. New York . N. Y .. Copyri ght ] 954. Second Printing ]964). ~HJ2 H H [HF . 3H20(aq) I. Both our work and that of [22 1 D. Whit e, H. /-Iu , and H. L. Johnston . .I. Chem. Ph ys. 24, J ohnson et al. [61, agree in s howing that th e heats of 1149-52 (1953). formation for aq ueous solutions of HF s hould be more [231 D. R. Douslin , paper No. 11 , pp. 135- 46. PVT Relations and negative than those s uggested by Wagman et al. [34j, I nt ermolecular Pot ent ial s for Met hane and Carbon T et ra· but less negative than those indicated by Cox and flu oride in Progress in Inte rnational Researc h on Thermo· dynamics and Transport Properties . .I. F. Mas i and D. H. Ts ai. Harrop [351. In this res pect they s ubs ta nti ate our edi tors (Acade mi c Press, New York . 1962). similar finding on the heat of formation of HF(aq) as r24J C . L. Brooks and C. J. G. Raw, Trans. Faraday Suc. 54,972- 4 derived from several other reactions in ou r stud y of (1958). the heat of formation of CF~ [111. Ludwig and Cooper [251 D. D. Wagman. W. H. Evans. J. Halow. V. 13. Park er. S. M. [361 re ported for the heat of reaction (2), Bail e y, and R. H. Schumm, NBS T echni cal Not e 270- 2. May 6. 1966. [261 E. Rudziti s, H. M. Feder. and W. N. Hubbard . .1 . Ph ys. C ll e m. (2) 68, 2978- 81 (1964). [271 F. D Rossini , D. D. Wal! man , W. H. Evans, S. Levin e . and I. ~H ~! ' H=- 239.7 ± 1.2 kcal mo]- I. Co mbining our Jaffe, Selected Values of Che mi cal The rmod ynamic Prope r· ti es, NBS Circ ul a r 500 (U.S. Cove rnme nt Printing O ffi ce. data on ~HJ2!' H [BF ~(g) j with the heat of reaction (2), we Was hin gton , D.C., 1952). calculate for ~HJ2 ~H[ NF Ag)], - 31.33 kcal mol - I. [281 S. S. Wise, .J. L. Margrave. H. M. Fede r, and W. N. Hubbard, Although this is in good agreement with the heat of .1 . Phys. Che m. 67,81 5- 21 (1963). formation of NF:l (g), - 31.44 kcal mol - I reported by [29] L. Bre wer, L. A. Bromley, P . W. Gill es, and N. Lofgren, The Sinke [371, the merits of the agreement are dubious The rmodyna mi c Properti es of the Halides, paper 6, Chemi s try and Metallurgy of Mi scellaneous Mat erials : Thermodynamics. because of th e large uncertainty associated with the L. L. Quill, editor (McGraw· Hill Book Company. Inc .. New heat of reacti on (2). York. 1950).

201

248-733 0 - 67 - 2 [301 c. H. Shomate. Co mputer Calculations of Combus tion Bomb [351 J. D. Cox and D. Harrop, Trans. Faraday Soc. 61 , 1328-37 Calorimetric Data, Technical Progress Report 327, NOTS TP (1965). 3288, U.S. Naval Ordnance Test Station, China Lake, Cali ­ [361 1. R. Ludwig and W. J. Cooper, J. Chem. Eng. Data 8 , 76-7 fornia, August 1963. (1963). [311 A. E. Cameron and E. Wi c he rs, J. Am. C he rn. Soc. 84, 4175- [371 G. C. Sinke, J. Phys. Chern. 71, 359-360 (1967). 97 (1962). [32] S. R. Gunn, 1. Phys. Che m. 69, IOlO-5 (1965). [33] W. D. Good and M. Miinsson, 1. Phys. C hem. 70, 97- 104 (1966). [34J D. D. Wagman, W. H. Evans. 1. Halow , V. B. Parker. S. M. Bailey and R. H. Schumm, NBS Technical Note 270-1, October 1. 1965. (Paper 7lA3-451)

202