Dete rm ination of S m a I I Stoic hio m et ric Devia ti o ns in Oxide MonocrystaIs H. B. SACHSE Keystone Carbon Co., Si. Marys, Pa, ,The properties of polycrystalline and monocrystalline solid state devices depend in a high degree upon stoichiometric deviations. The oxygen surplus in monocrystals of rnanganese(l1) oxide, cobalt(l1) oxide, and nickel(l1) oxide was determined with a modified Bunsen method, which permits the measurement of deviations down to 1 milliatom of oxygen per molecule of monoxide with an accuracy on the order of & l o % or better. The detection of still smaller stoichiometric deviations i s possible, however, with increased error.
S
MALL DEVIATIOKS from
the stoichiometric composition of oxides have an important effect on the electrical properties of oxide semiconductors. The first effect of this kind was reported by LeBlanc and Sachse ( 2 ) with nickel (11) oxide ( N O ) , which increases its conductivity by a factor lo6 between NiO and T\'iOl.os. Other oxides, such as cobalt(I1) oxide (COO), manganese(I1) oxide (MnO), and copper(1) oxide (CU~O), show similar effects. The stoichiometric surplus of oxide can be determined by the iodometric method which dates back to Bunsen. The oxide is dissolved in hydrochloric acid in the presence of potassium iodide, and its active oxygen, the surplus of oxygen over that in the valence state of the metal in the dissolved chloride, oxidizes a n equivalent amount of iodide ions to iodine. For oxide powders with a reasonable dissolution rate, this method can be used as described by Bunsen ( I ) . The spontaneous oxidation of iodide ions by atmospheric oxygen must be prohibited. This is no serious problem if dissolution times on the order of hours are involved. However, with dissolution times of days and weeks, the diffusion of the atmospheric oxygen into the reagents represents a source of error, nhich becomes increasingly intolerable with a decreasing stoichiometric surplus of ouygen. This is the case when densely sintered bodies or even single crystals have to be investigated that need up to 3 weeks for complete dissolution. A modified Bunsen method was necessary to deal with these cases. A reasonable approach to this prob-
lem was made by Sachse ( 2 ) ,who sealed the oxide to be investigated with the reagents into a test tube and added a small amount of sodium carbonate to the acidic solution immediately prior to sealing to replace the air in the small remaining gas volume by carbon dioxide. Deviations down to 1 atomic yo oxygen can be determined this way. More recently, the electrical and thermal properties of oxide crystals n-ith much smaller stoichiometric deviations have been measured (3, 4). Interpretation of these measurements makes it highly desirable to know the number of impurity centers, vacant atom sites, or surplus atoms in the crystal lattice. Further improvements were necessary to increase the sensitivity and reliability of the method for much smaller concentrations of surplus oxygen. The complete elimination of all parasitic oxj gen was necessary. This involved the removal of the oxygen dissolved in the reagent solution and adsorbed on the surface of the single crystal chips or powder and on the glass walls. Evacuation seemed practicable. Under practical conditions, a vacuum of 50 to 80 mm. of mercury was sufficient, as the reagent solutions developed partial pressures of hydrochloric acid and water, resulting in a heavy boiling of the liquid. PROCEDURE
Chips of the crystal to be investigated m-ere crushed to a relatively coarse powder and placed in a thin-walled glass tube approximately 2 mm. in inside diameter and 25 n m . in lcngth with one Table I.
Oside A B
open end. Subsequently, this glass tube was inserted into a borosilicate glass reaction tube 10 to 12 mm. in inside diameter and 15 cm. in length. After adding reagent solutions of 7 ml. of hydrochloric acid ( d = 1.2) and 3 ml. of potassium iodide (n = O . l ) , the neck of the tube was rinsed with a minimum amount of distilled water and subsequently connected to a high vacuum system with a manometer. After reaching a vacuum on the order of 50 mm. of mercury, the reagent solution w-as pumped for a period of 10 minutes. Fifteen minutes of gentle boiling at this vacuum were sufficient to remove most of the parasitic oxygen. It mas necessary that the pressure not be reduced belon the specific values t o avoid substantial losses of hydrochloric acid which might have resulted in a reduced dissolution rate. Small amounts of hydrochloric acid and water, which escaped, were condensed in a trap cooled with liquid oxygen. During this evacuation process, the tube with the specimen was completely degassed and slowly filled with reagent solution. I n addition to the efforts to remove the oxygen from the reagents, blank tests were run with the reagent solution only, but with the corresponding metal chloride added to reproduce the conditions in the analysis of the specimen. After sealing, the reaction tubes were stored in an oven a t 65' C. in the dark and opened after complete dissolution had taken place. The dissolution rate could be increased by removing the crystal powder from the specimen tube by placing the entire reaction tube upside down a t a n angle of 45'. Bfter complete dissolution. the rpaction tubes were broken open and rapidly rinsed out with oxygen-free water. .4 surplus of 0.013' thiosulfate was
Results of Metal Oxide Analyses
Sprcinien Surplus Keight, Metal Oxygen, Gram Milliatonis Microatoms
Formula
Probable Error in Gram-Atom Oxygen
?!In0
0 1172 1 0 .0002 1 61 32 1lnO 0 088 1 24 =!=Io. 0002 0.93 C coo 0 0891 = t O . 0003 1 18 4.54 D CoOa 0 2772 10,0003 3.9 23.8 D CoOu iz0,0003 21.3 0 2760 3 89 D CoOa 0 1331 10.0003 1 88 10.4 E Si0 0 129 1 73 i.0.0003 1.81 E Si0 0 1501 2 01 1.94 1 0 .0003 F SiOa 0 1361 1 83 2.26 1 0 .0005 a Specimens received from Sippon Telegraph and Telephone Public Corp., Japan; others from General Electric Research Laboratory.
VOL. 32,
NO. 4, APRIL 1960
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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.
