fluoride using the null-point method. The departure from direct proportionality between the amount of fluorosilicnte and volume of titrant became greater ~ i t hcontinued increase in the fluorosilicate concentration-i.e., as the amount of cerium(1V) in ewess decreased. a l . ~ , the departure from a stoichiometric reaction became greater as the acitlity of the solution as increased. This latter effect is shown in Figure 4, n-hile Figure 5 shons the effect of increaaing the ratio of ceriuni(1V) to ceriumiII1). W i e n the solutions :ititled to the half cells are 0.02.11 in crrium(1V). 0.01V in ceriuni(III), and 0.25.11 in wlfuric acid, the calibration curve is a straight line for addition of 0 to 30 ml. of 0.0LlI fluoride, xvhen the strength of the fluorosilicate solution is comparable n i t h that of the titrant. An accuracy of about 0.2% can be obtained in a fluoride determination after a TTillnrd-Winter distillation by :idopting thc procedure in which the
usual ratio of the concentration of cerium(1V) to cerium(II1) is doubled. Solutions from Pyrohydrolysis and I o n Exchange Separation. These separation processes give n distillate or a n eluate which, in t h e ideal case, is a solution of hydrofluoric acid. The normal procedure is to determine the fluoride acidimctrically. Because of the weakness of hydrofluoric acid, the end point can lack sharpness; but the real difficulty in these procedures is that the presence in the sample of anions other than fluoride can lead to high results-e.g., if chloride is present, unknown amounts of hydrochloric acid may be produced. The null-point potentiometric method may be used with great advantage t o determine fluoride in a neutralizrd aliquot of the distillate or eluate, because the method is specific for fluoride and. unlike other specific volumetric niethotls for fluoride, provides a definite and reproducible end point.
LITERATURE CITED
( I ) Busch, G. W., Carter, R. C., McKenna, F. E., in “Analytical Chemistry
of Manhattan Project,” C. J. Rodden, ed., Vol. 1, Chap. 5, bIcGraw-Hill, New York, 1950. (2) Elving, P. J., Horton, C. A., Willard, H. H., in “Fluorine Chemistry,” J. H. Simons, ed., Vol. 11, Chap. 3, Academic Press, New York, 1954. (3) Kubota, H., Surak, J. G., ANAL. CHEW31, 283 (1959). (4) Malmstadt, H. V., \%‘inefordner,J. D , Anal. Chim. Acta 20, 283 (1959). (5) ShEhyn, H., ANAL. CHEM.29, 1466 ( 195i). (6) Silverman, H. P., Bowen, F. J., Ibid., 31, 1960 (1959). (7) Sporek, X. F., Ibzd., 30,1030 (1958). (8) b niversity of Melbourne, Victoria, Australia, Rept. of Research and Investigation (1958 and 1959). (9) Warf, J. C., Cline, W. D., Tevebaugh, R. D., ASAL. CHEY.26,342 (1954). (10) Willard, H. H., Winter, 0. D., IND. ESG. CHEM., h A L . ED. 5 , 7 (1933). RECEIVEDfor review June 6, 1960. Accepted December 7 , 1960.
Potentiometric Determination of Chlorine Bound to Boron in Mixtures Containing ZChlorovinyl Boron Chlorides and Ethyl Boron Chlorides H. G. NADEAU, D. M. OAKS, and R. D. BUXTON Olin Mathieson Chemical Corp., New Hoven, Conn. b An accurate method i s described for the determination of chlorine bound to boron in organoboron compounds containing boron-chlorine and carbon-chlorine bonding. The method has been tested and used for compounds and mixtures of dichloro-2chlorovinylborane, chlorobis-2-chlorovinylborane, tris-2-chlorovinylboraneI dichloroethylborane, chlorodiethylborane, chloro-2-chlorovinylethylborane, and boron trichloride. The method i s highly reproducible and shows good accuracy over a wide range of concentration.
A
for chlorine bound to boron in mixtures containing compounds which possess both chlorine bound to lioron and to carbon necessitates a method which distinguishes each type of chlorine. While procedures are available for the determination of chlorine in organoboron compounds (Parr bomb-sodium peroxide fusion, T’olhard and Carius methods), the values arrived a t give total chlorine ULPSIS
content. Aqueous hydrolysis of chloro2-chlorovinylboranes, with subsequent titration of hydrochloric acid with either base or silver nitrate, fails to differentiate between chlorine bound to boron and to carbon, because these compounds break down in aqueous medium (above p H 3) to form hydrochloric acid, orthoboric acid, and acetylene. The hydrolysis of chlorine bound to carbon proceeds at a much slower rate than that for the chlorine bound to boron, and various attempts have been made to stabilize the 2chlorovinylboric acid formed. If the compound was dissolved in benzene and titrated with a solution of potassium methylate in benzene containing a small amount of methanol, only chlorine bound to boron was determined. The titration was carried out to a thymol blue end point. It was necessary, however, for the analysis to be rapid since on standing some breakdown of the 2-chlorovinylboric acid occurred. This procedure was found effective for chlorovinylboranes, but if sample mixtures contained chloro-
ethylboranes, the method was inaccurate. Under these conditions all of the chlorine bound to boron in the chloroethylboranes did not dissociate. Quantitative aqueous hydrolyses of these compounds are well known (a), and have been performed by the authors many times. Apparently, the poor recovery of chlorine obtained when dealing with mixtures containing alkyl boron chlorides by the benzene potassium methylate titration is related to the low polarity of the solvent. In the method presented, a polar solvent system has been chosen, and a means of stabilizing the chlorine-carbon bonds has been found. The compounds, or mixture of compounds containing both types of chlorine, are dissolved in a strong nitric acid-methanol solution. Hydrochloric acid formed through esterification of the chlorine bound to boron is titrated potentiometrically with standard silver nitrate. Chlorine bound to boron in the chloro-2-chlorovinylboranes and in the chloroethylboranes esterifies quantitatively; furthermore, the 2VOL. 33, NO. 3, MARCH 1961
341
chlorovinyldimethoxyborane is found to be very stable in this solvent system.
Table 1.
Determination of Chlorine Bound to Boron in Various Organoboron Compounds
% Chlorine Bound EXPERIMENTAL
Silver nitrate, 0.1N standardized against sodium chloride. Titrating medium, 1t o 2 concentrated nitric acid-methanol. Apparatus. Gelatin capsules, No. Reagents.
0.0.
Beckman p H meter Model H-2 equipped with glass electrode No. 4990-80 and silver-silver chloride electrode, Billett type. Dry box equipped to maintain nitrcgen atmosphere. Procedure. Since all of t h e compounds worked with are either very sensitive to air or pyrophoric, sampling is performed in a d r y box. Approximately 0.4 ml. of t h e sample is transferred to a previously tared gelatin capsule with a 5-ml. hypodermic syringe. T h e capsule is capped, removed carefully from the dry box in a suitable holder, and weighed. Immediately, the capsule is placed in a 250-ml. Erlenmeyer flask n-hich had been previously flushed with nitrogen and which contains 60 ml. of the nitric acid-methanol solution. The flask is stoppered and left to stand for 10 minutes until complete solution has occurred. The solution is quantitatively transferred to a 400-ml. beaker using a small amount of methanol as a wash, and the chlorine present as hydrochloric acid is determined potentiometrically with 0.1N silver nitrate. -4plot of millivolts us. milliliters determines the end point of the titration. RESULTS
Pure samples for reference work were difficult and impractical to obtain because of the extreme sensitivity to air which these compounds exhibit. Con-
Compound ClCH=CHBClz
to Boron
Analysis of Reference Sample, yo ClCH=CHBClz 92.50 (ClCH=CH)IBCl 3.60 BCh 2.00 Inert 1.90 (ClCH=CH)ZBCl 89.70 ClCH=CHBClI 7.40 Boron oxides 3.00 (ClCH=CH) sB 85.40 (ClCH=CH)zBCl 5.00 Boron oxides 8.60 1.00 Inert CzH6BClz 91.90 ClCH=CHBClz 5.40 BC13 0.50 Inert 2.10 (CzHdzBC1 95.70 Boron oxides 4.30 CzHS(ClCH=CH)BCl 84.30 CZHbBC12 6.00 (C1CH=CH)2BC1 9.70
48.40
I -
(ClCH=CH)2BCl (ClCH=CH)3B
CzHsBClz
(CzHs)zBCl C2H6(ClCH=CH)g BC1
47.84
Average,
yo
47.77
47.69 22.46
24.30 24.37
24.34
1.07
1.31
1.32
1.33 61.97
61.58
61.58
32.54
61.58 32.40 32.20
32.30
27.92 28.00
27.96
27.71
~~
sequently, the work was based on samples which in most cases contained impurities, but which could be determined by other methods of analysis: infrared, gas chromatography, and wet chemical methods. Total chloride was determined by the Volhard method on aqueous alkaline hydrolyzate. Total boron was determined by Parr bomb combustion and subsequent titration with carbonate-free sodium hydroxide in the presence of mannitol. Table I represents the degree of accuracy and precision obtainable with this method for a variety of compounds. Carbon and hydrogen rvere determined by the procedure of Schuele and NcNabb ( I ) . Infrared and gas chromatographic methods which were of original de-
velopment represent unpublished work of the authors. ACKNOWLEDGMENT
The authors thank the Olin Mathieson Chemical Corp. for permission to publish this work and Sidney Siggia for his encouragement to publish. This lvork was part of an rlir Force contract for the Wright 4 i r Development Center. LITERATURE CITED
W. J., McNabb, W. M., Tech. Research Rept. MCC-1023-TR317 University of Pennsylvania, Philadelphia. Pa. ( 2 ) Thoburn. J. M., Dissertation Abstr. 15, 3 4 (1955). RECEIVEDfor revien- July 20, 1960. ilccepted November 7, 1960. (1) Schuele,
*
Voltammetric Determination of Cobalt and Nickel in Hard Magnetic Alloys R. D. DeMARS
IBM Research Center, Yorktown Heights, N. Y.
b A
rapid, accurate, and extremely precise method for the simultaneous determination of nickel and cobalt in hard magnetic alloys using the techniques of voltammetry with continuously varying potential i s described. The current-voltage curves in solutions containing pyridine and potassium chloride were obtained with an allpurpose voltammetric instrument based
342 *
ANALYTICAL CHEMISTRY
on analog computer amplifiers. The method was applied to the analysis of mixtures of cobalt and nickel in standard and unknown solutions.
R
in this laboratory has pointed out the need for a more rapid and accurate method of analysis for thin cobalt-nickel alloys. The high cobalt concentrations encountered in ECENT WORK
this type of alloy limit the number of methods which can be used to analyze these samples. The determination of these metals either separately or in mixtures has been the object of numerous investigations. Recent polarographic (W), spectrophotometric ( I I ) , and electrodeposition (9) methods do not provide the accuracy, speed, or precision re-
