Thermal Stability and Titrimetric Determination of Decaborane MAE 1. FAUTH and CLAWELL F. McNERNEY Research and Development Department,
A method for the quantitative determination of decaborane consists of oxidation with potassium iodate, reaction of excess iodate with potassium iodide, and titration of the liberated iodine with sodium thiosulfate. Acetic acid was found to b e the most satisfactory solvent for the decaborane. Recoveries on the order of 98% have been obtained. Differential thermal analysis curves are presented for decaborane heated in air and in helium for the temperature range 20" to 150" C.
A
as undertaken to determine the thermal stability in air and in of decaborane (RlOHLI) helium and also to study its oxidation in solution to ascertain if direct titration methods were feasible for the determination of this compound. Because considerable quantities of decaborane were being utilized, information was desired on safe methods for handling it and on its storage properties in air. Stock ( 5 ) found that decaborane may explode when heated to 100" C. in oxygen. Simons, Balis, and Liebhafsky ( 4 ) stated that in vacuum the evolution of hydrogen from decaborane becomes appreciable a t 200 " C. Previous experiments had shon n that the direct oxidation of decaborane with nitric acid or sodium peroxide was extremely hazardous and that of the various oxidations in solution, the one using potassium iodate gave the most uniform results. .4n iodometric method for monitoring atmospheres has been decribed by Hill, Lrvinskas, and Kovick ( 2 ) . They presented the reaction as: N IKVESTIGATION
BioHlr
+ 22
I1
-P
10 BII
+ 14 H I
A niethanol solution of iodine was used by Nessner (S), who employed a closed system to prevent loss of evolved hydrogen. The following reaction is postulated: BioHir
+ 20 Iz + 30 CHIOH 10 B(OCH3) + 40 H I + 2 Hz +
EXPERIMENTAL
Differential Thermal Analysis. T h e apparatus used for differential thermal analysis has been described (1). The
U. S.
Naval Propellant Plant, Indian Head, Md.
samples used were purified by sublimation of commercial materials. For this work 0.220-gram samples were used. The reference material was alumina. The apparatus was calibrated by using the transition temDeratures of ammonium nitrate. The region from 20" to 150" C. was studied. The heating rate was 3.8" C. per minute and was Eept as uniform as possible. Decaborane was observed to undergo a weight loss of about 7 to 8yo due to sublimation. The differential thermal analysis curves are shown in Figure 1. I n air, decaborane showed a slight exotherm up to 94" C., then it began a n endotherm indicating melting a t 100" C. After melting, the curve returned to a base line with respect to the reference material, alumina, and remained there from 105" to 125" C. At 125" C. it began to decompose. Decomposition continued up to lG0" C. The decomposition of decaborane in air is always accompanied by many sharp explosive sounds, presumably due to the combustion of hydrogen split off from the decaborane molecules. I n helium an endotherm due to melting was observed a t 100" C. KO evidence of decomposition was obscrved up to 150" C. On cooling, the freezing point occurred a t 88" C., indicating supercooling. After heating in helium the sample could be retrieved and additional runs made with only slight losses of material due to sublimation. Oxidation of Decaborane in Solution. The iodometric method for the assay of decaborane presented here resulted from tests made with several inorganic oxidizers including aqueous solutions of potassium permanganate, potassium dichromate, iodine, and potassium iodate, and organic solvents such as ethyl alcohol, acetone, and acetic acid t o determine if any reaction were characterized by a n exact end point. Both ethyl alcohol and acetone, when used as solvents, reacted with the decaborane. The addition of the various oxidants, potassium permanganate, POtassium dichromate, potassium iodate, and iodine, resulted in the forniation of precipitates or colors which obscured the end point. However, the use of potassium iodate with acetone produced a rapid definite end point. When acetic acid was used as solvent, results n-ere unsatisfactory except when potassium
-0ECABORANE I N AIR CURVE OF DECABORANE IN H E L I U M CURVE OF DECABORANE I N H E L I U M ----COOLING
.......HEATING
IENDOTHERMAL Figure 1. Differential thermal analysis curves
iodate was used as oxidant. I n this case, the end point nith the iodate was relatively rapid and definite. The methods previously used for assay of commercial lots of decaborane consisted of melting point determination plus determination of boron as boric acid (H3B03)by the mannitol titration. This requires oxidation of the hydride. The methods tried for oxidation n ere the Parr bomb procedure using potassium perchlorate and sodium peroxide and oxidation n ith concentrated nitric acid. Both of these methods are extremely hazardous and several explosions occurred nhile they vere in use. Also, these methods do not differentiate hydride boron from that already present as boron oxide (B&) or boric acid. I n addition, these oxidations are timeconsuming and often fail to give accurate results on the small samples of decaborane that can safely be handled in contact 11 ith these strong oxidizing agents. Some of the commercial lots contained hard crusts of nhite crystals mixed with yellow material. The better lots nere pure white. The poorer lots nere expected to contain unknor5 n amounts of hydride polymers, oxides of boron, and boric acid. A method was desired which mould differentiate hydride boron from oxide boron and LT-ould give minimum interference from polymers that might be present. The potassium iodate oxidation of decaborane n as tried first on samples purified by sublimation, then on commercial lots. The sequence of reactions is : VOL. 32, NO. I, JANUARY 1960
91
3 BioHi4
+ 22 KIO, + 24 HzO
-*
+ 22 KI
30
NIO3
+ 5 KI + 6 HC2H302
+
6 KC2H302
2 Na2S203
+ I,
+
Xa,S40s
+ 3 12
+ 2 NaI
Procedure. STANDARDIZATION OF SODIUMTHIOSULFATE. The sodium thiosulfate solution is standardized in the customary manner with t h e modification t h a t a m-eight buret is used. The normality then is expressed in milliequivalents per gram as follon s : Meq. per gram Ka2S208= grams K2Cr207 0.049035 X grams Xa&O3
TITRATION OF POTASSIUM IODATE SODIUMTHIOSULFATE. A weight buret containing 50 ml. or more of potassium iodate solution is weighed. Approximately 50 ml. are transferred to a 500-ml. iodine flask and the buret and contents are again m-eighed. Then, 25 ml. of acetic acid and 15 ml. of 15% potassium iodide solution are added. From a previously weighed weight buret standardized sodium thiosulfate solution is added dropwise to titrate the potassium iodate solution to a starchiodide end point. The blank is calculated as grams of sodium thiosulfate per gram of potassium iodate solution. PREPARATIOP; OF SAMPLE.Because of its high toxicity, the sample consisting of 30 to 40 mg. of decaborane is packed in a size 00 gelatin capsule under a hood. The capsule and contents are weighed and the contents emptied into an iodine flask with no attempt made to dislodge any particles adhering to the capsule. The flask is stoppered immediately. The empty capsule is reweighed and the difference taken as the weight of the decaborane sample. Twenty-five milliliters of glacial acetic acid are transferred to the sample flask, which is then stoppered and set aside for 5 to 10 minutes to allow the sample to dissolve. TITR.4TION OF SAMPLE. From a weight buret a convenient portion of potassium iodate solution is rapidly added to the decaborane-acid solution. Care should be taken that the potassium iodate runs directly into the decaborane solution and does not remain on the walls of flask, so that rinsing is not necessary. The mixture is swirled five or six times and allowed to stand from 5 to 30 minutes. After standing, 15 ml. of 15% potassium iodide are added to the mixture by pouring a portion in the well a t the top of the flask. The stopper is removed to allow the solution to run into the flask, the rest of the solution is then added, and the walls are rinsed with water. Sodium thiosulfate solution is added, dropwise, from a weight buret and the sample solution is swirled to disperse the titrant until the solution has bleached to a straw color. Starch is added and titrant is added dropwise. The last 3 to 5 drops are small and each is introduced by touching the buret tip to the inside of the neck of the flask. Each WITH
92
ANALYTICAL CHEMISTRY
Table 1.
Replicates of Purified Decaborane Sample Wt., BioHiI, Grama 72 0 0 0 0 0 0 0 0 0 0
0400 0609 0486 0339 0308 035874a 031781" 033841" 032058" 033568"
Av.
98 97 98 97 98 97 97 97 98 97 98 k0
Std. dev. Weighed on microbalance.
55 91 02 96 56 66 97 76 54 61 05 37
drop is washed into the solution by rinsing the inside of the flask with water. The titration is continued until the blue has disappeared and the solution is colorless. The original weight of potassiuni iodate (in terms of the sodium thiosulfate to which it is equivalent) is taken and from this is subtracted the weight of sodium thiosulfate consumed. Per cent decaborane = ( B - A ) X meq./gram of Na2S203X
meq. &OH14 veight of sample where B is the Feight of sodium thiosulfate (NazS203)corresponding t o the original veight of potassium iodate (KI03) d is the n-eight of Sa29r0, used as titrant Milliequivalent of decaborane is taken as 0.00278 gram. DISCUSSION
A series of determinations \\-as made, during which the iodate method vas modified4 and conditions n ere varied one a t a time to study ensuing effects, It was found that storage of the decaborane-acetic acid solution for more than 2 hours is accompanied by some decomposition and lower yield. The addition of potassium iodate had to be rapid because any prolonged delay, such as the slow delivery from a pipet, promotes lower results. This precaution is accomplished by the rapid addition of potassium iodate solution from a weight buret. Varying the storage time for the reaction of the decaborane-acid mixture with the potassium iodate solution apparently is not critical and for convenience is limited to from 10 to 30 minutes. Varying the dilution from approximately 125 to 400 ml. x i s not critical. Titrating with a weight buret is more convenient for handling and measuring than n i t h a volumetric buret. Errors due to hang-up of solution on buret walls or to improper reading of meniscus levels
are eliminated, especially with solutions of 0.5.Y concentration. -4 sample weight on the order of 30 to 40 nig. is most convenient; larger quantities require inconveniently large volumes of reacting solutions, T\ hile smaller amounts tend to increase any errors. Subsequent to these modifications 10 determinations mere made maintaining conditions in accordance with the above observations. The results, shon-n in Table I, averaged 98.05y0 decaborane varying from 97.61 to 98.567,. -1s a comparison, samples of two commercial lots, L-70 and L-71, rrere assayed. The melting points, measured by means of a Koefler hot stage, nere 100.24" and 97.29' C.. resprctively. The per cent decaborane (RlOHl4)from lot L-70 was 95.91,95.98,9585)95.91%, (average 95.91%), n-hile from lot L-71 the per cent decaborane n a s 93.26, 93.53, 93.01, 93.077, (average 93.22%). -4s 7yo of lot L-71 nas unaccounted for, efforts were made to identify this material. A weighed portion was treated n i t h water and the nhite insoluble residue was separated by filtration. Without oxidation of the sample, a mannitol titration of the filtrate for boric acid gave a value of .YO%, indicating the boron oxide plus horic acid in the sample as received. -1separate sample was treated LT ith n-heptane to dissolve the decaborane. Some yellow apparently amorphous material \vas undissolved and constituted 5% of the sample. The heptaneinsoluble fraction n as then treated with n ater and a n amount corresponding to 4% of the original sample n a s found to dissolve very rapidly. 1 11ater-insoluble material corresponding to 10% of the original sample was left. On treatment of a sample of lot L-71 n ith pentane, the residue from the pentane-soluble fraction consisted principally of a crystalline material nhich melted a t 97" C. An isotropic solid !\--as left TI hich m-as partially melted a t 106" C. and left solid pieces of material that slowly turned broun at 120" C. This indicates the presence of polymeric material because sublimed samples of decaborane melt sharply at 99.8" C. and leave small amounts of a n aniorphous white material nhich is unchanged on heating to 120" C. .It the present state of the investigation, evidence is not sufficient to determine n hether polymeric boron hydrides are detected by the iodate method, and a preliminary melting point determination is therefore desirable. I n the oxidation reactions of decaborane it was assumed that the final product n-as boric acid. Stock (5) stated that under some conditions a suboxide of boron is formed nhich is resistant to further oxidation. I n this laboratory, freshly sublimed samples of
decaborane were found to dissolve readily and melt without leaving a residue. Samples which have been alloIT-ed to stand for some time develop a white opaque coating nhich is resistant to solution and is left after the bulk sample of decnborane has melted. The nature of this material has not been determined. The fact that recoveries on the order of only 98% were obtained may be clue to the presence of this material. The hydrolysis of dwaborane is writttm R S f0llOll s ( b ) :
BlaH14
+ 30 H20
-+
10 HPBOj
+ 22 H?
This would indicate t h a t 44 electrons are involved per mole of decaborane. This assumption, which has also been used by other workers in decaborane chemistry, has been utilized in the present investigation. LITERATURE CITED
(1) Gallant, IT. K. A,, “Apparatus for
Differential Thermal and Therniogravimetric Analysis,” Division of Analytical Chemistry, 133rd Meeting, ACS, San Francisco, Calif., l p r i l 1958.
( 2 ) Hill, It‘. H., Levinakas, G. J., Sovick, IT. J., “Iodometric hIonitoring of Bo-
rane-Containing Atmospheres,” University of Pittsburgh Report CCC-1024TR-129 (-4ug. 12, 1955). (3) hlessner, A. E., .Isa~. CHEY.30, 547 (1958). (4) Simons, H. L., Balis, E. IT-.) Liebhafsky, H. A Ibid., 2 5 , 635 (19j3j. (5) Stock, ii., ”“Hydrides of Boron and Silicon.” uw. 80-5. Cornel1 Univrrsitv, ”, Ithaca; N: Y . , 1933.
RECEIVED for review ?\lay 4, 1959. .4ccepted September 10, 1959. Division of Analytical Chemistry, 135th Meeting, ACS, Boston, Mass., April 1959.
Photometric Indicator Titration of Weak Bases in Acetic Acid The Modified Type II Plot KENNETH A. CONNORS and TAKERU HlGUCHl School of Pharmacy, University of Wisconsin, Madison, Wis.
b A refinement of an earlier photometric titration method is based upon an equation which takes into account solvolysis of the salt formed during the titration. Quantitative recovery data and exchange constants are reported for such weak bases as acetamide, urea, dimethylpyrone, thiourea, caffeine, antipyrine, and triphenylguanidine, titrations being performed in acetic acid with Sudan 111, Nile Blue A, p-naphtholbenzein, and malachite green as indicators. Recoveries were 98 to 101% for all samples in the 20to 30-mg. range. Some special cases of mixture analysis have been developed and data are presented. The ready accessibility, through the exchange constant, of the salt formaation constant makes this quantity a convenient measure of basicity for structural work.
I
color changes associated with the titration of weak acids and bases are frequently too gradual t o permit accurate visual determination of the end point. The recent development of methods for the linear extrapolation of photometric measurements has greatly extendcsd the range of substances \\-liich can be determined by acid-base titration (3). These methods are based upon a description of the reaction as a competition between 1,he indicator and the sample for the titrant species. For example, the titration of a base NDICATOR
sample with a n acid is represented by Equation 1: BHA+I=IHA+B (1) where I is a n indicator base, B is the sample base, and HA is the acid. The indicator is present in such small concentration that the amount of acid bound as indicator salt is negligible; solvolysis effects are also neglected. The equilibrium constant, K,,, for Equation 1 is called the exchange constant. K e x = CIHACB/CICBHA = K:H4/Kp
(2)
K,BHA is the formation constant for the salt BHA; i t describes the equilibriuni B HA4= BHA. A similar constant is defined for the indicator salt, IHA. These equations are nritten n i t h the salt and acid in the undissociated form, which is the case in acetic acid; in aqueous solution they would be JT ritten in the dissociated form. The symbolism used here follows t h a t of Kolthoff and Bruckenstein ( 5 ) . The titration equilibrium in Equation 1 is described by Equation 3.
+
+
1/X = ( K e J S ) ( I B / I ~ ~1/S (3) n-here S = milliliters of standard acid required to react stoichiometrically with the base originally present, X = milliliters of standard acid added a t any point, and I B D . 4 , the experimentally determined indicator ratio, is n-iitten for the ratio CI/CIHA. (This equality is not generally true, but is valid for the
very n-eakly basic indicators employed in this work.) A plot of 1/X US. IB/14yields a straight line, from nhich values of the end roiiit and exchange constant can be determined. This graphical photonietric titration method has been termed the Type I1 titration. I t s validity has h e m established for several systems (3). THE M O D I F I E D TYPE II TITRATION
Recent applications of the Type I1 method t o the titration of weak bases in acetic acid have revealed significant and reproducible deviations from linearity when the sample size is very small. Recoveries in excess of 100% result when this curvature is observed. Similar deviations recently have led to the derivation of a more complete titration equation for the Type I11 photometric plot ( 1 ) ; this treatment, which was developed for aqueous systems, takes into account hydrolysis of the salt formed during the titration. The equation developed for the aqueous system also describes the equilibrium in acetic acid, although not so exactly. Let X’ be the total concentration of acid added to the system a t any time prior to the end point, and let S’ be the total concentration of base initially present. Then, if dissociation is neglected for the case of acetic acid titrations,
+ S’ = CB + C B H ~
X’ = CHA CBHA
(4) (5)
The concentrations X’ and 5” can be VOL. 32, NO. 1, JANUARY 1960
93
