Chromium Boride as an Electrochemical Generant for Titrations with Chromium(V1) K . S . Fletcher 111 Research Center, The Foxboro Company, Foxboro, Mass. 02035 IN RECENT WORK, lanthanum hexaboride was investigated as an electrode material for electrochemical applications ; this lead to a unique procedure for the electrochemical generation of lanthanum(II1) for use in titrations (1-3). This investigation extends this work to chromium boride (CrB), another member of this class of refractory metal borides. The constant current oxidation of this material lead to the products Cr(VI), Cr(III), and H 3 B 0 3 and it is the purpose of this investigation to test the utility of CrB for the electrochemical generation of Cr(V1) for use in titrations. Monnier and Zwahlen, using a chromium anode, generated Cr(V1) with reproducible current efficiency of about 95% (4). N o other successful procedure for electrochemical generation of Cr(V1) has been reported. EXPERIMENTAL Chromium boride, CrB, was obtained from Cerac Chemicals, Butler, Wis., in the form of a hot-pressed rod (1-inch long by 3/16-in~h diameter). The density of the rod was measured with a Beckman Model 930 Air Comparison Pycnometer as 4.9 g/cma, a value not in good agreement with the theoretical density, 6.04 g/cm3 (5). The procedure used for mounting the rod was the same described previously for the LaBs electrode ( I ) . Current-potential curves were obtained potentiostatically under quiet conditions using instrumentation previously described ( I ) . Constant currents were obtained with a constant current source built for this study having a continuously variable output from 1 to 100 mA and stability of =tO.Ol%. The current output was standardized by measuring the iR drop across a standard 10.003-ohm resistor (General Radio, Concord, Mass., Type 500-B) with a Leeds and Northrup Type K-3 Potentiometer. The Foxboro Co. Standards Laboratory provided certification, traceable to the U. S. Bureau of Standards, of the current measuring resistor and of the standard cell in the potentiometer. The cell was all glass and consisted of two 200-ml compartments separated by a fine porosity glass frit augmented by an agar plug containing K2S04. The auxiliary compartment contained a 1-cm2platinum flag electrode immersed in 0.1N H2SO1. The working compartment contained the CrB electrode and, in the case of the current-potential studies, the tip of the saturated calomel electrode discussed by Lingane (6). Fe(I1) solutions were prepared in oxygen-free solutions, 1.ON in H?S04, from Baker Analyzed Reagent Fe(NH4)z(so&.6 H 2 0 and were standardized against potassium dichromate solutions, prepared determinately from the Baker Primary Standard salt. Oxygen was purged from all cell solutions prior to use with prepurified nitrogen. (1) D. J. Curran and K. S. Fletcher 111, ANAL.CHEM.,40, 78 (1968). (2) Zbid., p 1809. (3) Zbid., 41, 267 (1969). (4) D. Monnier and P. Zwahlen, Helv. Chim. Acta, 39, 1865
(1956). (5) “ASTM-Hanawalt
Card-File of X-ray Diffraction Data” American Society for Testing Materials, Philadelphia, Pa. (6) J. J. Lingane, “Electroanalytical Chemistry,” 2nd ed, Interscience, New York, 1956, p 362.
Table I. Analysis of Chromium Boride Theoretical, Sample I, Sample 11, 82.8 80.0 80.8 Chromium 16.0 15.4 Boron 17.2 96.0 96.2 Total 100.0
x
x
RESULTS AND DISCUSSION Analysis of CrB. Two 0.1-gram samples of CrB were broken from the rod and, after weighing, were dissolved with boiling H C I 0 4 (12 hours were required). After complete dissolution, the two solutions were transferred to 200-ml volumetric flasks for dilution to volume. Analysis for Cr(V1) and H3BOa were performed on aliquots of these solutions. Cr(V1) was determined by titration with standard ferrous ammonium sulfate to the color change of sodium diphenylamine sulfonate (3, and after removal of Cr(V1) as BaCrO4, was determined by titration of its mannitol complex with NaOH (8). The results of these analyses are shown in Table
I. Current-Potential Behavior of CrB. The current-potential behavior of chromium boride was obtained using linear scan anodic or cathodic potential sweeps of 1.0 V/min. The initial potential in each case was -0.500 V us. SCE and the supporting electrolyte was 0.10N K2S04. The cathodic limit occurred at about - 1.2 V us. SCE and resulted in evolution of gas at the electrode. The anodic limit occurred at about 0 V us. SCE and resulted in oxidation of the electrode itself. For qualitative analysis of the solution products of the electrooxidation reaction, a constant current of 10 mA was impressed across the CrB anode-platinum cathode pair for 15 minutes. After this electrolysis, was identified by the rose color produced by its complex with turmeric (9), Cr(V1) was identified by its absorption maximum at 440 mp, and Cr(II1) was identified by its absorption maximum at 580 mp (IO). Electro-Oxidation of CrB. In view of the products o b tained from the electro-oxidation reaction, the following reactions can be written for material of ideal stoichiometry:
+ 3H20 CrB + 3H20
CrB
+
-+
+ H3BO3 + 3H+ + 6e Cr(V1) + H 3 B 0 3 + 3H+ + 9e
Cr(II1)
(1) (2)
(7) I. M. Kolthoff and E. B. Sandell, “Textbook of Quantitative
Inorganic Analysis,” 3rd ed, Macmillan, New York, 1952, pp 690-1. (8) Ibid., pp 535-5. (9) A. A. Benedetti-Pichler, “Identification of Materials,” Academic Press, New York, 1964. (10) W. H. Hartford, “Treatise on Analytical Chemistry”, Part 11, Vol. 8, I. M. Kolthoff, P. J. Elving, and E. B. Sandell, Eds., Interscience, New York, 1963, p 289. VOL. 41, NO. 2, FEBRUARY 1969
377
A quantitative investigation of this electro-oxidation reaction was performed using a constant current of 96.46 mA impressed across the CrB anode-platinum cathode pair for 7000 sec. The supporting electrolyte was 50 ml of 0.10N KzS04, 0.10N HZSO4,or 1.ON H2S04. After these electrolyses, the anolyte, containing Cr(VI), Cr(III), and H3B03, was quantitatively transferred t o a 100-ml volumetric flask and diluted t o volume with water. Cr(V1) was determined in 10-ml aliquots of this solution by titration against standard ferrous
Table 11. Current Efficiency for Electro-Oxidation of CrB i = 96.46 mA t = 7000 sec.
Supporting electrolyte
Q = 675.2 coulombs
0.1ON
0.1ON
0.621
0.619
1.ON HnSOd 0.618
0.820
0.816
0.811
0.199
0.197
0.193
0.773
0.790
...
17.1 79.9 97.0
16.9 79.6 96.5
KzSOr
mrnoles CrB oxidized to Cr(V1). Total mrnoles CrB oxidized (chromium titratio@ mrnoles CrB oxidized to Cr(III)c Total mrnoles of CrB oxidized (boron titration)d C.E.(C~VIII), %e C.E.(rr~r),%’ Net current efficiency, %g
16.6 79.5 96.1
a Based on titration of Cr(V1) in the electrolyticsolution containing Cr(III), Cr(VI), and H3B03. Based on titration of Cr(V1) after conversion of all Cr(II1) to Cr(V1) c Based on the difference between the above Cr(V1) titrations. d Based on the titration of HIBOX . . in the electrolvtic solution. rnmoles CrB oxidized to Cr(II1) e Calculated using C.E.(C,III, = X electrochemical mmoles 100%. where the electrochemical mrnoles was obtained usine Faraday’s law with Q = 675.2 coulombs and n = 6 equiv/mole. mmoles CrB oxidized to Cr(V1) f Calculated using C.E.(C?VIJ = X electrochemical mmoles 100% where the electrochemical mmoles was obtained using Faraday’s law with Q = 675.2 coulombs and n = 9 equiv/mole. Obtained by adding C.E.(C~VI, and C.E.(C,III,. r
I
-
I
ammonium sulfate t o the color change of sodium diphenylamine sulfonate. After these titrations, 1.5 g ammonium persulfate and 2 m l 0 . W AgN03 were added and the solution was boiled for two hours to convert Cr(II1) t o Cr(V1) (6). The total Cr(V1) was then determined as above, and the difference between the Cr(V1) found in this final titration and that found in the initial titration was equated t o the Cr(II1) produced in the electrolysis. H,BOs was determined o n separate 25-ml aliquots of the anolyte solution using the procedure described above. The mmoles of CrB oxidized, based on these titrations, are shown in Table 11. The current efficiency for electro-oxidation of CrB was determined by comparing the number of moles of Cr(II1) produced by Equation 1 and the number of moles of Cr(V1) produced by Equation 2 with the electrochemical values ( Q / n F ) with n taken as 6 and 9 equiv/mole, respectively. These data are also shown in Table 11. N o gas formation was noted at the CrB anode during these electrolyses and the values for the net current efficiency, obtained by adding the current efficiencies for generation of Cr(II1) and Cr(V1) (Table 11) are believed t o be equal within experimental precision. The analysis data (Table I) showed the rod to contain approximately 9 6 z chromium plus boron. Both the analysis data (Table I) and the current efficiency data showing mmoles of CrB oxidized based on the chromium titrations and the boron titrations (Table 11) show that the material is slightly rich in chromium. Thus the apparent 4% loss in the net current efficiency may rest on either the nonideal stoichiometry of the material employed and/or electroactive impurities in the rod. Titration of Fe(I1) with Electrochemically Generated Cr(V1). The empirically determined current efficiencies shown in Table I1 suggest that the electrochemical yield of Cr(V1) is sufficiently reproducible to allow its application for titrations. To test this, the titration of Fe(I1) with electrochemically generated Cr(V1) was studied. In each case, 9.97 or 1.99 ml of 0.01537N ferrous ammonium sulfate was added to the anode compartment of the cell containing 50 ml of 1.0N HZSO4 and one drop of 0.3% sodium diphenylamine sulfonate. After purging with nitrogen, the solutions were stirred and Cr(V1) was generated with a constant current of 96.46 mA to the color change of the indicator. At this time, the mmoles of Fe(I1) found, N F ~ ( IisIgiven ), by
Table 111. Results of Titrations of Fe(1I) with Electrogenerated Cr(V1) i = 96.46 mA
TITRATIONS OF 0.1533 MMOLE FE(II) Time to color change, sec 580.6 581 .O 574.1 578.0 579.1
mmoles Fe(II), found 0.1538 0.1539 0.1521 0.1531
Re1 error, $0.3 $0.4 -0.7
0.1534
Average mmoles Fe(I1) found Relative standard deviation =
=
-0.1
+o. 1
where 3 is the equivalents per mole of reaction of Cr(VI), C.E.(C,VI)is the empirical current efficiency for generation of Cr(V1) (determined as 79.5% for 1.ON HzS04),i = 96.46 mA, I = time to color change in seconds, n = 9 equiv/mole, and F = 96,487 coul/equiv. The data and results for these two sets of titrations are shown in Table 111. Overall precision and accuracy was a few parts per thousand.
0.1533.
i o ,4z.
CONCLUSIONS
TITRATION OF 0.0306 MMOLE FE(II) 114.6 113.2 116.7 115.9 112.0
0.0304 0.03oO 0.0309 0.0307 0.0297
Average mmoles Fe(I1) found = 0.0303. Relative standard deviation = i1 ,4%.
378
ANALYTICAL CHEMISTRY
-0.6 -2.0 +1.0 +0.3 -3.0
The current efficiency for generation of Cr(V1) from a CrB anode is not ideal and is somewhat smaller than the value reported for the chromium anode ( 4 ) . Previous work with lanthanum hexaboride has shown that the current efficiency for generation of lanthanum(II1) is constant for various conditions of supporting electrolyte, current density, and stirring rate, but is affected by the stoichiometry and the density of the sample used (2). The current efficiency for generation
of Cr(V1) from CrB would likely vary with the stoichiometry of the sample used and therefore should be determined independently. Further, since the effect of experimental conditions such as current density, supporting electrolyte composition, and stirring rate o n this empirical current efficiency is not known, this calibration procedure should be performed under conditions similar t o those t o be used in the subsequent titration. I n addition t o the material studied in this report, chromium borides with the stoichiometries Cr2B, CrsBa,
Cr3B4, and CrB, have been reported ( I I ) . Evaluation of these materials as coulometric generants for Cr(V1) would be of interest. RECEIVED for review September 23, 1968. Accepted November 5,1968. B. Aronsson, T. Lundstriim, and s. Rundqvist, ,,Borides, Silicides, and Phosphides,” Methuen and Co., Ltd., London, 1965, p 12.
Determination of Silicon in Siloxane Polymers and Silicone-ContainingSamples Employing Alkali Fusion Decomposition Methods James H. Wetters and Robert C. Smith Dow Corning Corporation, Midland. Mich. 48640
THEUTILITY of direct sodium hydroxide fusions to decompose polymeric, solid siloxanes was recently described by Greive and Sporek ( I ) . Our laboratory has employed their techniques and new fusion decomposition procedures with both fluid and solid siloxane samples. Use of potassium hydroxide in place of sodium hydroxide in direct fusions was found t o give better recovery of silicon from siloxane polymers. The usefulness of the fusion methods was greatly extended by employing an alcoholic alkali pretreatment step in which siloxane bonds were converted to silanolates. This procedure reduced the loss of volatile siloxane materials and rearrangement products. Subsequent fusion of the sample after alcohol evaporation rernoved the alkyl or aryl groups from silicon, producing silicates suitable for spectrophotometric analysis using the reduced heteropoly blue complex method. EXPERIMENTAL Apparatus and Reagents. The Technicon AutoAnalyzer system for analysis of soluble silica in water ( 2 ) was modified as shown in Figure 1. All manual spectrophotometric analyses were performed using the Cary Model 14 spectrophotometer and 1- or 5-cm cells. Most decompositions were made in 75-ml nickel crucibles with covers (Fisher Scientific Co., 8-020). Saturated sodium butylate was prepared by dissolving 4.5 grams of sodium metal in 100 ml of reagent butyl alcohol using a Teflon-lined, stainless steel be-tker. After cooling, the solution was filtered through a 60-mesh stainless steel screen and stored in a polyethylene bottle. Direct Alkali Fusion Decomposition Procedure. Generally about 5 grams of sodium hydroxide or potassium hydroxide was placed in a nickel crucible, depending upon the nature and size of the sample. About 3 mg t o 20 grams of sample was then added. With siloxane polymers, 20 mg was optimilm for spectrophotometric analysis. The lid was placed on the crucible and heat applied gently at first using a Meker type burner to melt the alkali. The alkali was held in the molten state for two t o three minutes using a hot flame (1) W. H. Greive and K. F. Sporek, Winter Meeting, ACS, Phoenix,
Ariz., January 17-21, 1966, paper No. 43. (2) Technicon AutoAnalyzer Methodology, Silica, 111 F (water analyses), Technicon Corporation.
t o completely decompose the sample. After cooling the crucible to room temperature, the fusion mass was dissolved in 50 ml of water in a Teflon-lined, stainless steel beaker. Heat was gently applied t o speed the dissolution. The crucible was removed with tongs and rinsed with water. With biological samples, the solution was filtered while basic with No. 42 filter paper and a polyethylene funnel for removal of metal hydroxides. The fusionate was neutralized with hydrochloric acid to a p H near 1.5, then transferred to a 500or 1000-ml volumetric flask and diluted to volume. About 3 to 10 pg/ml of silicon in solution was.desired for analysis. Alcoholic Alkali Evaporation and Fusion Procedure. About 5 grams of alkali was added to the nickel crucible as above. With sodium hydroxide, 15 ml of isopropyl alcohol was added. When using potassium hydroxide, 1 to 10 ml of saturated sodium butylate solution was introduced. A sample of 3 mg to 20 grams was taken, and the mixture allowed t o set at room temperature for 30 minutes. The alcohol was slowly evaporated with the crucible o n a hot plate a t low heat setting. Last traces of alcohol were removed with high heat setting. Fusion of the alkali, sample dissolution, and acidification were then carried out as described above. Analysis Procedures. Gravimetric and spectrophotometric methods were employed to evaluate recovery of silicon as reported in Table I. The 8-hydroxyquinoline (oxine) precipitation method was used essentially as described by McHard et al. (3). Sample aliquots were selected to give 0.05 to 0.3 gram of precipitate. Spectrophotometric analyses a t 815 mp of the reduced heteropoly blue complex were performed manually ( 4 ) and automatically ( 2 ) using a Technicon AutoAnalyzer assembly diagramed in Figure 1. Silicon concentrations of 0 to 15 pg/ml were rapidly analyzed using the latter system. RESULTS AND DISCUSSION
Typical silicon recovery data are given in Table I. Several linear and cyclic polydimethylsiloxanes and various other silicone materials were decomposed, and the resulting soluble (3) J. A. McHard, P. C. Servais, and H. A. Clark, ANAL.CHEM., 20, 325 (1948). (4) H. J. Horner, “Treatise on Analytical Chemistry,” Part 11, Vol. 12, Interscience, New York, N. Y., 1965, pp 287-8. VOL. 41, NO. 2, FEBRUARY 1969
379
