one of the rock samples. A rather large excess of iodide from the standard solution was added with respect to the iodine content of the rock in each case, so that any fluctuation due t o the inhomogeneity of iodine in the rock would be insignificant. The sample used as a standard was biotite granite from Westerly, R. I., with a n iodine content of 0.2 p.p.m. as determined by this method. Results of this control experiment are shown in Table 111. It was concluded that a 100% recovery of iodine was obtained from the fusion process, and no major source of interference was present in the rock samples analyzed. The blue color of the starch-iodine complex develops rapidly and is stable for at least a period of 2 hours (Figure 2). I n some cases, the maximum color intensity had not been reached in the suggested 15-minute time interval ( I ) , so the solutions were alloffed to stand for a period of 30 minutes in the dark before absorbance measurements were taken.
At very low p H values the reaction of cadmium iodide with the iodate is no longer quantitative, and a t higher p H values the reaction is somewhat slower than desired. For this reason the oxidation and liberation of iodine were carried out at a p H of 2.8, and then the p H was adjusted to 4.2 for the final colorimetry. -411 of the reagents used in the color development were tested for possible iodine contamination by varying each of the amounts used in several blank solutions. When the reagents were used in the proportion described previously, the absorbance of the blank solution was 0.005, indicating that about 0.15 pg. of iodine was introduced as a contamination from the reagents. S o studies have been made so far on the problem of introducing contamination during the powdering of silicate rock samples. lf7e are now in the process of developing a neutron activation analysis method by the use of 14.7 m.e.v. neutrons produced by the T(d,n)He4
reaction in the Cniversity of drkansas 400 KV Cockcroft-ITalton positive ion accelerator. Perhaps a comparison of the results obtained by the two methods will clarify the problem. ACKNOWLEDGMENT
The author is grateful to 1'. IC. Kuroda for invaluable advice and encouragement during the course of the n-ork. LITERATURE CITED
(1) Fellenberg, Th. von, Biochim. 2. 187,
111927).
boles; G. G., Anders, E., J . Geophys. Res. 65,4181 (1960). (3) Lambert, J. L., ANAL. CHEJI. 23, 1247 (1951). ( 4 j Sugawara, K., Koyama, T., Terada, K., Bull. C'henz. Sac. Jnonn 28. 494 (1955). (2)
RECEIVEDfor review J u n e 11, 1962. Accepted October 10, 1962. Research supported by the National Science Foundation grant G-17161.
Evaluation of Analytical Methods for Deca borane 1. J. KUHNS, R. S. BRAMANI1 and I . E. GRAHAM2 Callery Chemical Co ., Callery, Pa.
b Nine methods for the determination of decaborane were evaluated by analyzing commercial samples ranging from 90 to 97% pure. Gas chromatog ra phic, infra red, ultraviolet, iodine titration, and iodometric procedures gave comparable results a t the 95% confidence level. Decaborane was determined by these five methods without significant influence b y the impurities present. An alkaline titration method gave high results because of the acidic impurities, and elemental boron values calculated as decaborane were high because of boron A P-naphthoquinoline impurities. (benzo [ f ] quinoline) colorimetric method gave comparatively low results, for an undetermined reason. Mole per cent purity was determined by a freezing point depression method, and the average molecular weight of the impurities was calculated to b e approximately 150. The iodine titration method is recommended as a standard when high purity decaborane is unavailable for calibration purposes.
T
study was undertaken to learn which of the available methods would be most applicable for determining decaborane obtained by a diborane HIS
1700
a
ANALYTICAL CHEMISTRY
pyrolysis process. The decaborane product was a solid with all the process components more volatile than the decaborane removed, and results comparable to those reported here would be expected for decaborane obtained in a similar manner. Seven commercial samples ranging from 90 to 97y0 pure were analyzed by nine methods: gas chromatographic, infrared, ultraviolet, iodine titration, iodate titration, alkaline titration, colorimetric (p-naphthoquinoline), boron neutron absorption, a.nd freezing point. PROCEDURES
Decaborane of research purity (99.8 mole %) was used in calibrating all the methods used. The decaborane was purified by subliming it twice, and the freezing point method was used to verify that not enough impurities were present to influence the results of this study significantly Iodometric Analysis. T h e iodometric method, described b y F a u t h and McNerney, is based on t h e oxidation of decaborane with potassium iodate, followed b y a n iodometric titration (4). Although i t is based on the reduction of 44 equivalents of oxygen per mole of decaborane, in actual practice the value is somewhat below this. I n this laboratory, it was 3y0lower than the theoretical; therefore, the reagents
were standardized n-ith research uuritv decaborane. Ultraviolet Analysis. This method is based on the absorotion maximum a t 272 mp for decaborine in cyclohexane solution (9). Cyclohexane was also used as a reference solvent, and t h r instrument iyas calibrated with decaborane of research purity. A molar absorptivity of 3000 was obtained with a Beckman D K 2 spectrophotometer. This is very useful method, but the samples analyzed must be free of other materials which absorb a t 272 mp. I n one case, a pentane solution of decaborane siphoned through Tygon tubing extracted enough plasticizer to intrrfere with the analysis. G a s Chromatographic Analysis. Analyies were performed on a chromatograph with a 3-nieter, 3/s-inch column packed with 60- to 80-me~h Celite Apieaon impregnated n i t h 20 weight 70 L. It had a coIumn efficiency of 1200 theoretical plates; the retention time for decaborane relative to n-decane was 2.65 and t o naphthalene, 0.730. The helium flow rate n-as 340 nil. per niinutr and the column and detector temperature was 150' C. Cyclohexane was used a3 a solvent for the decaborane. Although the$e were the conditions for I
"
1 Present address, Arniour Research Foundation, Chicago, 111. 2 Present address, Koppers, Inc., Monroville, Pa.
Table I.
Comparison of Analytical Methods for Determination of Decaborane
Weight per cent decaborane Gas Sample chromaSO. Infrared tographic I 92.9 95.2 93.6 93.4 91.1 92.0 90.9 92.0 :3 95.2 94.0 94.2 95.0 4 95.6 94.9 96.0 94.3 5 95.8 95.4 95.0 93.2 G 97.1 91.2 96. .4 89.8 7 93.7 98.5 94.8 94.7 Std. dev. 5 0 .54 51.36 Elemental boron determined by neutron S o t included in statistical analysis.
the data obtained, a variety of instruments and columns have been used with temperatures ranging from 90" to 220°C. Squalnne, -1piezon L or AI, silicone grease, and Fluorolube are suitable partitioning liquids. Unsaturated, and hydroxylic materials are not Ictorv. For decaborane samples of the type discussed here, the recomm e n t l d conditions zrc: a 0.5-meter coluniii ( I &-inchdianief er) packed n i t h 100- to 120-mesh M i t e impregnated with 20 weight % of squalane. The operating ttmperaturc should be about 140" C. and the helium flow rate 50 cc. per minutc.. Saphthalene is recomnieiidrd a. :hi1 i n t m i s l standard, and this nictliotl is recomiii~ndedmhm impurities :we present R hich interfere with tlic other methodi. Infrared Analysis. Infrared analyses mere made with a Perkin-F>lmer Model 21 spectrophotometer using spectrograde iw-octane (2-methylheptane) as a rrference solvent. Cells with sodium chloride windows and a p a t h length of 0.2 mm. were used. T h e in.trunient mas calibrated with decaborane of research purity a t 9.92 microns, m-here the boron hydride polymer does not intrrfere, and this calibration TWS used t o calculate the decabornne conceiitrations of the samples. 0-Naphthoquinoline Colorimetric Method ( I ) . This method is based on t h e reaction of decaborane with pnaphthoquinoline t o form a colored compound which has a maximum absorbance at 490 mp. It is approximately the method of Hill and Johnston ( 7 ) hut differs in t h a t 90-minute color development time a t room temperature is changed to a &minute development time at 100" C., and p-naphthoquinoline replaces quinoline. Iodine Titration Method. This method, described by Messner (8), involves the reaction of decaborane with excess iodine in methanol. The excess iodine is titrated with sodium thiosulfate. I t is based on the reduction
UltraIodine Alkaline 8-Xaphtho- Neutron" violet titration Iodometric titration quinoline absorption 93.5 94.1 91.9 97.4 91.2 97.9 94.4 95.7 92.0 96.0 92.0 97.3 91.6 91.8 90.0 94.2 86.8 96.1 92.7 92.4 89.4 93.4 86.1 96.1 95.5 93.9 93.0 97.0 88.8 98.5 96.1 94.2 94.5 95.9 90.0 98.1 94.7 93.4 93.7 98.0 88.7 97.9 95.9 93.6 94.9 97.2 89.0 97.5 94.7 93.6 92.6 97.4 85.4 97.0 95.5 93.9 93.9 96.0 85.4 98.4 94.4 95.9 94.6 97.8 92.4 98.2 95.4 93.5 94.3 96.2 93.1 98.0 93.6 90.4 96.0 94.2 88.9 95.5 92.0 90.6 95.4 94.4 86.7 95.5 1 0 . 69 f0.80 10.66 50.80 A0.75 10.44 absorption and calculated as decaborane.
Freezing b point, mole yo 96.4 96.6 95.3 95.6 96.5 96.5 96.3 96.5 95.5 95.1 96.1 95.8 96.6 96.2 f0.20
Table II.
Analysis of Variance Data for Decaborane Determined by Eight Methods Source of Degrees of Sum of
variance freedom Among methods 7 Among samples 6 Experimental error 42 Duplication error 56 Total 111 a Significant a t 1% point.
-
squares 593.36 120.32 189.55 36.35 939.58
of 40 rather than 44 equivalents of iodine per mole of decaborane. Messner suggests t h a t 2 moles of hydrogen are evolved in the reaction. Alkaline Titration Method. Decaborane was titrated as a monobasic acid using aqueous sodium hydroxide, according t o t h e method reported by Guter and Schaeffer (6). When calibrated n i t h research purity decaborane, the results were consistently about 2% lower than theoretical; therefore, the reagents were standardized JT ith research purity decaborane. Freezing Point Method. The procedure was described by Furukawa et al. ( 5 ) . Neutron Absorption Method. Elemental boron was determined b y t h e method described by DeFord and Braman (3). Acetone was used as a solvent for the analysis. RESULTS AND DISCUSSION
The gas chromatographic, infrared, ultraviolet, iodine titration, and iodate titration methods gave results which were not significantly different at the 9570 confidence level, and i t was concluded that these five methods determined decaborane without serious interference from the impurities present. Since they are based on several different
Variance 84.76 20.05 4.51 0.65
F 18.8"
6. 94a
properties of decaborane. it is unlikely that they would all be affected in the same way. Values obtained by the alkaline titration method were significantly higher than results b y the other methods, and this indicated that acidic compounds must be present as impurities. The colorimetric method gave results significantly lower than those by other methods, but the reason was not apparent (Tables I and 11). Elemental boron was determined by the neutron absorption method; boron values calculated as decaborane were significantly high. Although the limitations of this procedure for the determination of decaborane are apparent, the results are included in the statistical analysis, because the presence of boron impurities is also apparent when the results are compared with the other data. An average of about 2 weight % carbon was found as an impurity. No boric acid or boric oxide was evident from x-ray diffraction analysis of the samples. The freezing point method for the determination of decaborane yields a value for mole percent purity; thus in the absence of knowledge of the impurities present, the results cannot be compared to those obtained b y the other VOL. 34, NO. 13, DECEMBER 1962
1701
Determination of Average Molar Absorptivity for Self-Absorption of Fluorescent Radiation in Fluorescein Solution K. K.
ROHATGI and G. S. SINGHAL
Departmenf o f Physical Chemistry, Jadavpur Universify, Calcutta 32, lnclia
b The average molar absorptivity for reabsorption of fluorescent radiation has been determined. It i s inversely related to the square root of the product of the concentration and the path length of the solution 1 / Z / b c . The magnitude of the effect of the secondary emission process has been demonstrated b y using a completely quenched solution. Similar results are obtained with solution in alcohol in the presence of KOH. These values may be used for the necessary correction for absorption re-emission processes in the measurement of fluorescence intensity under appropriate conditions.
1702
ANALYTICAL CHEMISTRY
