Anodic:
Acetonitrile complexes copper(1) but not copper(I1); therefore, the potential of the Cu(1)-Cu(Hg) couple is shifted negatively to the same extent that the Cu(11)-Cu(1) couple is shifted positively and the result is a split wave at sufficiently high acetonitrile concentrations. The cathodic and anodic waves obtained for copper(1) solutions in the presence of acetonitrile are due to the following reactions : Cathodic: Cu(CH3CN),+
+ e “=“ WHg)
+ x(CH3CN)
(15)
Cu(CH3CN),+
”=” Cuz+ + x(CH8CN) + e
(16)
The split wave, half anodic and half cathodic, was obtained for solutions in which the acetonitrile was added up to 8 hours after the preparation of copper(I), which is further evidence that hydrated copper(1) ion is stable in aqueous solution for prolonged periods of time. A similar split wave was observed from the solution obtained by saturation of the copper (I) solution with ethylene. In this case the +1 oxidation state of copper is stabilized by complexation of the copper(1) with the olefin. RECEIVED for review September 22, 1967. Accepted December 11, 1967. This work was supported by a grant from NASA awarded through the University of Missouri Space Sciences Research Center. Miss Altermatt gratefully acknowledges the support of an NSF Undergraduate Research Participation Fellowship.
A Micro Method for Determining Silicon in Organosilicon Compounds by a Modified Oxygen Flask Method James E. Burroughs, William G. Kator, and Alan I. Attia R . C . Ingersoll Research Center, Borg- Warner Corp., Des Plaines, Ill. 60018
REVERCHON AND LEGRAND ( I ) described a method for the microdetermination of silicon in organosilicon compounds by the use of a modified Schoniger technique. This procedure required specialized apparatus and was too time-consuming for routine analyses. Furthermore, the use of sodium peroxide as a flux was undesirable because many research-type samples often contain components which could spontaneously explode when mixed with this powerful oxidizing agent. To circumvent these disadvantages, as well as those of other methods for determining silicon (2,3), a simple, unique, and practical technique was developed. It involves the direct conversion of the silicon to a volatile, readily-hydrolyzed, fluorinated species by direct combustion of the organosilicon compound in the presence of a fluorocarbon. EXPERIMENTAL Apparatus. The combustion flask consisted of a 1-quart
polyethylene bottle (Fisher Scientific Co., Cat. No. 2-923). No deterioration of this container was observed at any time during these studies. To support the platinum sample carrier basket (A. H. Thomas Co., Cat. No. 6471-Q), a length of 24-gauge platinum wire (ca. 12 cm) was inserted in the bottom of a No. 4 rubber stopper such that the sample basket would be centered in the bottle during combustion. A Beckman Model B spectrophotometer was used at Sensitivity 2. Reagents. The 6N NaOH solution was prepared by dilution of a 50% NaOH reagent solution (Fisher Scientific Co., (1) R. Reverchon and Y. Legrand, Chim. Analytique, 47,194 (1965). (2) I. M. Kolthoff and P. J. Elving, “Treatise on Analytical Chemistry,” Vol. 11, Part 11, Interscience, New York, 1965, pp 107-206. (3) J. A. McHard, “Anal. Chem. of Polymers,” XII, Interscience, New York, 1959, Chapter XIV.
Cat. No. So-S-254) which was found to be essentially free of silicon. A 10% (w/v) ammonium molybdate solution was prepared from reagent grade ( N H ~ ) ~ M o ~ O ~ ~ . All ~HZO. reagents were prepared and stored in polyethylene bottles to prevent silicon contamination. Polytetrafluoroethylene powder (Halon TFE, Type G80, Allied Chemical Co.) was used as received because fluorine analysis (4) indicated a high degree of purity. Reagent grade NazSiOa+9Hz0was used for preparing the calibration curve, while octaphenylcyclotetrasiloxane (NBS Standard No. 1066), containing 14.1 silicon, was used in establishing the accuracy of the method. Procedure. Solid samples were transferred into the center of tared, prefolded Schoniger-type sample wrappers and liquids into tared No. 5 gelatin capsules (available from Arthur H. Thomas Co., Philadelphia, Pa.). Sample weights were obtained by difference. Sample sizes were taken such that the amount of silicon was between 0.2 and 1.5 mg. TO the sample was added an amount of PTFE powder such that a weight ratio of approximately 50:l existed between the amount of PTFE powder and the silicon present in the sample. The sample was loosely wrapped in the conventional manner (5) in the sample wrapper and mounted in the platinum basket. Ten milliliters of 6N NaOH and 2 ml of 3 z H ~ O were Z introduced into the polyethylene bottle, and the bottle was immersed into a large stainless steel beiker containing cold water. The bottle was then flushed rapidly with pure oxygen for 30 to 60 seconds. With the sample carrier supported on the platinum wire, the paper sample flag was ignited manually,
z
(4) R. N. Rogers and S . K. Yasuda, ANAL.CHEM., 31, 616 (1959). (5) C. N. Reilley, “Advances in Analytical Chemistry and Instrumentation,” Vol. IV, Interscience, New York, 1965, pp 75-1 16. VOL. 40, NO. 3, MARCH 1968
657
Table I. Statistical Data of Samples of Known Silicon Content Which Were Determined by the Proposed Method (CHdaCompound (C&)nSiOa (CsH5)~Si(OH)2 SiNHSi(CH3)3 14.16 12.98 34.80 Si (Theoretical) n 3 S
Si
50% Range (Rso) 95 Confidence
13 14.3.5z 0.48% *3.37% *0.25% =to.96%
9 12.89% 0.17% 11.32% =tO.lO% &0.96%
12 34.52% 0.85% 12.46% A0.50X =tl.00%
Table 11. Comparison of Data Between the Parr Bomb Method and the Proposed Method Silicon found. Z Parr Sample” Bomb method This method 978-33-4 978-35-4 978-36 0
12.7 12.2 7.95
12.4 12.3 8.00
All samples were experimental compounds.
and the entire stopper assembly was plunged into the polyethylene bottle and seated securely. Following the ignition and combustion, a few minutes were allowed for cooling of the platinum basket. The assembly was then removed from its water bath and the platinum basket was dislodged from its support by gently rotating the bottle. Without removing the stopper, the apparatus was suspended in a boiling water bath for approximately 30 minutes, After completion of the hydrolysis and cooling of the bottle, the following reagents were added in the order shown, followed by thorough mixing after each addition: 3 drops of a 1% ethanolic phenolphthalein solution, 20 ml of 5 % (w/v) boric acid, sufficient 6N HC1 to neutralize the excess NaOH to the phenolphthalein end point, 15 ml of 1N HCl, 10 ml of 10% (w/v) ammonium molybdate solution, and 2 ml of 70% HC104. The solution was quantitatively transferred to a 100-ml volumetric flask and was diluted to volume with deionized water. After allowing 5 minutes for color development, the absorbance was determined at 420 mp using 1.0-cm borosilicate glass cells. Water was used for the reference cell. A reagent blank was determined for each set of samples. The calibration curve was prepared by using aliquots of a standard solution of Na2Si03* 9Hn0 and adding the reagents as outlined above. The purity of the reagent was ascertained by classical dehydration procedures. RESULTS AND DISCUSSION
The mechanism of thermal decomposition by the oxygen flask method to yield the desired form of silicate was found to be a function of particle size of both the sample and the flux, temperature, properties of the flux, alkalinity of the absorber solution, and absorption time allotted for the combustion products. All of these parameters could be controlled within an optimum range to obtain quantitative results.
658
ANALYTICAL CHEMISTRY
Fluxes which were found unsuitable for producing the desired silicon end product included Na2C03,Na202,NaOH, Na2B407,ZnO, MgO, and various combinations of these reagents. Further studies, based on converting silicon to a volatile fluoride which could then be readily hydrolyzed to a reactive silicate form, resulted in the use of a fluorocarbon as the flux. A weight ratio of 50:l of PTFE powder to silicon in the sample was determined by factorial experiments to be optimum for obtaining quantitative recovery. The use of Hz02 and HClOd exerted a stabilizing effect on the final color development. It is believed that they may catalyze the hydrolysis of the silicon fluoride species to the metasilicate form, but the exact mechanism was not investigated. The results obtained by applying the outlined procedure to three different organosilicon compounds are shown in Table I, and are reported according to recommended statistical presentation (6, 7). The values are well within the acceptable limits of precision and accuracy established for microanalytical techniques and compare favorably with the data published for the more elaborate techniques of Reverchon and Legrand ( I ) . The limiting factor on the accuracy of this proposed method was related directly to the sensitivity of the semimicro balance which was employed for weighing the samples. A comparison of data obtained on three typical resedrch compounds by this method and the standard Parr Bomb technique is illustrated in Table 11. No attempts were made to treat the data in a statistical manner because the analyses for silicon by the Parr Bomb method had been performed much earlier than the initiation of these studies and the results at the time were in excellent agreement with the anticipated values. The molybdosilicate technique was selected for its simplicity, reproducibility, and its lack of interference from phosphorus and arsenic (8). The only major factors which were found to influence this spectrophotometric method were the presence of excess fluorine and the pH. The former difficulty was overcome by the use of boric acid, while investigations confirmed that the pH should be maintained between 1.1 and 1.2. Although the calibration curve is linear up to 2.0 mg of silicon per 100 ml of final solution, it is recommended that the 0.2- to 1.5-mg range be employed for optimum analytical results. RECEIVED for review November 8,1967. Accepted January 2, 1968. The authors thank the Borg-Warner Corporation for allocating the time and facilities for conducting these investigations, and for permission to publish the details of the proposed analytical method. (6) IUPAC Information Bulletin No. 26, August 1966. (7) W. J. Youden, “Statistical Methods for Chemists,” Wiley, New York, 1951. (8) K. Kodama, “Methods of Quant. Inorg. Analysis,” Interscience, New York, 1963, Chapter 55, p 415.
