added and back-titrated with 0.002N bichromate using starch, grade Baker C.P. 1-4006 for iodometry, as indicator. RESULTS
The analyses were calculated so that the oxygen surplus found b y titration was subtracted from the weight of the specimen. This yielded the value for the hypothetical amount of stoichiometric metal oxide and indicated the amount of metal in the specimen (Table I). In a separate investigation it was found that the standardization of the solutions, which were used for the titration, had a probable error of &2%, if a microburet was used to reduce reading errors a t the buret, and a white background promoted the recognition of the end point. The probable error
in a series of blanks stored the same time under the same conditions as the , . inoxide specimens was ~ t 7 7 ~ ’The fluence of this error on the result decreases with the amount of surplus oxygen to be determined. For the first specimen of MnO in the table the ox) gen ratio in the blank and specimen is only 0.1. Therefore, the probable error of the blank represents in this case only ilyo of the surplus oxygen; in most other cases the percentage of the probable error is higher. The MnO monocrystal was evidently inhomogeneous, as confirmed also by electrical measurements and appearance. COO and N O monocrystals appeared very homogeneous and their oxygen surplus apparently did not depend very much upon their origin.
ACKNOWLEDGMENT
The author thanks G. L. Kichols for his valuable assistance and Keystone Carbon Co. for granting permission to publish this information. LITERATURE CITED
(1) BunsenLR. W,., Treadwell, F. P., Hall, W.T., nalytical Chemistry, Quantitative Analvsis.” 9th ed., Vol. 11, p. 598, Wile;, re; York, 1958. ( 2 ) LeBlanc, M., Sachse, H., dbhandl. sachs. A k a d . W i s s . Leipzzq, - . Math.naturw. K1. 1,82, 133 (1930). (3) Slash, G. A., Newman, R., Phys. Rev.
“.
Letters 1, 59 (1958). (4) Yamaka, E., Sawamoto, K., Phys. Rev. 112, S o . 6, 1861 (1959).
RECEIVED for review September 3, 1959. Accepted December 16, 1959.
Determination of Boron in Borohydrides and Organoboron Compounds by Oxidation with Trifluoroperoxyacetic Acid R. DONALD STRAHM and M. FREDERICK HAWTHORNE Redrtone Arsenal Research Division, Rohm & Haas Co., Huntsville, Ala.
b
Boron in borohydrides and organoboron compounds is oxidized to boric acid with trifluoroperoxyacetic acid. The boric acid is converted to mannitoboric acid and titrated by the fixed p H method to pH 6.3 with standard sodium hydroxide. The method is simple and rapid and has been applied successfully to the analysis of a varied assortment of compounds.
T
need for a rapid, accurate method of determining boron in a variety of borohydrides and organoboron compounds prompted an investigation which led to the trifluoroperoxyacetic acid oxidation method presented here. Although satisfactory decompositions and boron values have usually been obtained in this laboratory by sodium peroxide fusions (11, l a ) , a simpler, less time-consuming procedure was sought for routine use. Fusions in open crucibles with sodium carbonate, which are often used with nonvolatile metal compounds (W), can seldom be employed with the types of materials considered here. Hydrogen peroxide has been used to oxidize boron to boric acid ( I d ) ; however, the oxidation is mild and the method cumbersome. Burke (3) and Conrad and Vigler (4) employed a Parr oxygen HE
530
ANALYTICAL CHEMISTRY
bomb in analyzing borines. The boric acid formed is absorbed b y sodium carbonate. A limited number of direct specific colorimetric and spectrophotometric methods have appeared in the literature for decaborane (8, 9) and perhaps a few other compounds. Boron in organoboron compounds has been oxidized to boric acid by ignition in a combustion tube, followed by titration of the boric acid produced (1). Recently Corner (6) has devised a rapid method based on combustion in Schoniger flasks. The powerful oxidizing property of trifluoroperoxyacetic acid (7) recommends its use in converting boron to boric acid. This reagent has been found to be an excellent oxidant for boron in borohydrides and organoboron compounds. The reaction proceeds rapidly in solution in a test tube without resort to bombs or fusions. The fixed p H modification of the familiar mannitol titration of boric acid without separation of borate ion appears to be the most attractive method of determining the boric acid formed and has been employed here. By this technique, interferences from moderate amounts of weak acids present are avoided (6, IO). The trifluoroperoxyacetic acid reagent is a strong acid; hence, excess reagent may be
present during the titration without causing interference. EXPERIMENTAL
Preparation of Trifluoroperoxyacetic Acid. One milliliter (0.036 mole) of 90% hydrogen peroxide is pipetted into a n Erlenmeyer flask containing 9 ml. of acetonitrile. After t h e flask has cooled in a n ice bath, 6.2 ml. (0.044 mole) of trifluoroacetic anhydride is added slowly while t h e flask is swirled. T h e flask is then removed from t h e cooling b a t h , t h e mouth is covered with a small beaker, and t h e reagent is set aside ready for use. Larger or smaller batches of peroxy acid may be prepared by varying the quantity of reactants proportionately; however, the ratio of hydrogen peroxide to trifluoroacetic anhydride specified above should be maintained. Normally a one-day supply of acid is prepared. With certain stable samples a more concentrated reagent may be desirable. This has been achieved by doubling both the quantity of hydrogen peroxide and trifluoroacetic anhydride while leaving the amount of solvent unchanged. Trifluoroacetic anhydride is obtainable from Matheson Coleman and Bell. It is desirable to purify the anhydride as i t is received, b y a simple distillation in an apparatus protected from atmospheric moisture.
Procedure.
A sample containing
2 t o 6 mg. of boron is weighed out
a n d placed in a 6- to 7-inch test t u b e having a 12/22 female standardtaper joint. Solid samples are neighed into platinum microcombustion boats, while liquids are conveniently weighed into small glass-stoppered vials, nhich are opened just as they are placed in t h e test tube. One milliliter of acetonitx ile is added to the tube, a reflux condenser is attached, and the tube is cooled in a n ice bath for a few minutes. A sdfety shield is then placed in front of the apparatus. Two milliliters of trifluoropero.;yacetic acid reagent is added cautioudy from a pipet through the top of the condenser. After any reaction has subsided, the test tube is removed from the ice bath and placed in a boiling water bath for a period determined by the stability of the sample toward trifluoroperosyacetic acid. Many compounds are completely tleconiposed after 5 minutes’ heating, nhile some eltreine cases may require 45 minutes to 1 hour. K i t h the more stable compounds, 1 nil. of trifluoroperoxyacetic acid is usually added after about 30 minutes. K h e n decomposition is complete, the solution is quantitatii.ely n-ashed into a 150-nil. beaker with 50 nil. of distilled nater. The beaker is covered with a n a t c h glass and the solution is boiled gently for a few minutes. K h e n the solution has cooled, the cover is nashed with distilled water. and the pH iq adjusted 11-ith carbonattx-free, concentrated sodium h ~ d r o \ i d e until only slightly acidic. Tht. pH is finally adjusted potentiometrically to esactly 6.30 n-ith 0 . 1 s sodium hydroxide. Five grams of mannitol is added, and the boric acid is determined by again titrating to pH 6.30 with standard sodium hydrouide dispensed from a 5nil. buret graduated to 0.01 ml. The sodium hydroxide is standardized against boric acid by titration in exactly the manner described for a sample. =In analysis can be completed in as little as 30 minutes, depending on the time required for decomposition of the sample. RESULTS AND DISCUSSION
The versatile reagent trifluoroperouyacetic acid lends itself readily to the oxidation of borohydrides and organoboron compounds. Because of its greater oxidizing power, it oxidizes many materials that are unattacked by hydrogen peroxide. h number of reactive samples can react violently with tlie reagent; howevc‘r, by immersing the test tube in an ice bath and adding the reagent dropwise down the condenser, a smooth, controllable reaction
is obtained. Because trifluoroperoxyacetic acid is both a strong oxidant and a strong acid, it must be handled with precaution. The reagent decomposes slowly on standing; i t is advisable to prepare a fresh batch daily. Table I presents results on a number of compounds. &lost of these coinpounds were synthesized in the Rohm 8: Haas research laboratories and shown to be of good purity by physical properties and determination of other elements present. Results on purified materials have usually been accurate nithin 0.2 to 0.37, absolute. d standard deviation of 0.237, absolute was calculated b y pooling results from duplicate runs on 87 samples. The analyses n-ere carried out over about a 2-year period by three analysts. The samples represent a \vide variety of niaterials having a boron content ranging from 2.4 to 88.5%. A number of the samples required careful handling, including some which are sensitive to moisture in the air and others which are mildlj- pyrophoric. The absolute standard deviation is relatively independent of the boron concentration. For example, tlie standard deviation calculated from duplicate runs on five materials containing 60 to iO7, boron is 0.2857, absolute. Khile boric acid has long been deterniined by conversion to the relatively strong iiiannitoboric acid, the most satisfactory manner of carrying out the titration is still under discussion. The authors have had good success n-ith the fixed pH procedure and have titrated to pH 6.3 n ithout separating the borate from other ions present. As stated previously (6, IO), the pH a t nhich neutralization of mannitoboric acid is complete is dependent on the concentration of mannitol. The greater the amount of mannitol present, the lower the pH a t n hich neutralization is coniplete and at which the titration can be carried out satisfactorily. By using the fixed pH method a t a lower pH, less boric acid is neutralized before addition of the mannitol. Khatever pH is chosen for use in the fixed pH method, the sodium hydroxide used must always be standardized against boric acid, boric oxide, or sodium tetraborate b y titrating in exactly the same manner as the samples and to the same pH. LITERATURE CITED
(1) Arthur, P., Donahoo, IT.P., “Microdetermination of Boron in Organoboron Compounds,” Oklahoma Agricultural and Mechanical College, Rept. CCC1024-TR-221 (Jan. 18, 1957).
Table 1. Determination of Boron in Borohydrides and Organoboron Compounds Boron, yo Conlpourld Calcd. Found Trimethylamine phenylborane 7 26 7.15 Triethylamine phenylborane 5 66 5.31 Trimethylamine l-propylborane 9.50 0 41 Trimethylamine 2-butyl8 38 8.41 borane Pyridine phenylborane 6 40 6.63 5 91 5 . 8 5 Pyridine p-tolylborane Pyridine p-anisylborane 5 . 4 4 5.30 5 44 5.33 Pyridine o-anisylbornne Pyridine p-chlorophenylborane 5 32 5.27 Pyridine l-naphthylhorane 4 94 5.19 4 41 4.43 Pyridine diphenylborane Pyridine di-(p-tolyl)3 96 4.08 borane Pyridine di-(p-anisy1)3 55 3.57 borane Pl-ridine di-(p-chlorophenyl )-borane 3 45 3.36 Pyridine di-( p-bromo2 69 2.81 phenyl)-borane 7 26 7.37 Pyridine n-butylborane 5 91 6.00 Pyridine benzylborane 8 01 8.22 Pyridine 2-propylborane Pyridine mesityleneborane 5 13 5 , 0 9 n-Butyl di-(p-chloro3 52 3.40 phenyl)-boriiiate n-Butyl di-( p-tolyl)-borin3.90 4 06 ate 4 70 108 Tri-n-butyl borate B,B,B-Tri-( borazole 1-propyl)15.70 15. 74 B,B.B-Tri-(2-butyl)13,04 12.no borazole Triphenylphosphine methylene boron trihydride 3.73 3.85 [( CsHj)3P@CHpBH$31 Triphenylphosphine benzylidine boron trihydride [(CsHs)jP@CH(CsHa)2 . 9 5 2.86 BH@l 88 17 87 81 Decabo&e 46.14 46.07 Octyldecaborane 60.67 60.30 n-But yldecaborane
12) Blumenthal, H., AXAL. CHEM. 23, \
I
992 (1951).
( 3 ) Burke.
I\-. 31.. IKD.ESG. CHEJI., BKAL. ED. 13, 50 (1941). (4) Conrad, A. L., S’igler, 11. S.,.%SAL. CHEM.21,585 (1949). (5) Corner, AI., Analyst 8 4 , i l (1959). 16) Foote. F. J.. IND.ENG.CHEW,ASAL. ED. 4 , 39 (1932). ( 7 ) Hawthorne, 11, F., AXAL. CHEM. 28,540 (1956). (8) Hill, D. L., Gipson, E. I., Heacock, J. F., Zbid., 28, 133 (1956). (9) Hill, W,H., Johnston, 11. S., Ibtd., 27, 1300 (1955). (10) Martin, J. R. Hayes, J. R., Zbzd., 24, 182 (1952). (11) Pflaum, D. J., IVenzke, H H., ISD. ENG.CHEX., ANAL. ED.4, 392 (1932). (12) Synder, H. R., Kuck, J. A , , Johnson, J. R.,J. Am. Chenz. SOC.60, 105 (1938). RECEIVEDfor review Augnst 10, 1959. Accepted December 9, 1959. \ - I
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