A Spectrophotometric Determination of Iodine in Silicate Rocks W. H. CROUCH, Jr.! Deparfmenf o f Chemistry, University o f Arkansas, Fayefteville, Ark.
b A spectrophotometric method for the determination of trace qjantities of iodine in silicate rocks has been developed. The iodine is removed from the rock b y alkaliqe fusiori, precipitatnd as silver icdide, and converted to iodate b y bromine. Iodine i s l i b e i a t d from the iodate solution in CJ qwoi?fity six times that cf the original sample b y the additiorl of excess iodide ion, and i s determined spectrophotometrically as the starchiodine chromogen at a woLrelength of 580 mp. The iodine colitent of a few silicate rock samples analyzed b y this method varied belween 0.Od a r d 0.20 p.p.m.
Ileceased Future co~res~icindence concerning this paper should be addressed to Professor P. K Kurodn, Ilepartment of Chemistrv. Universit:) of .irkansns, Fayetteville, Ark.
1698
ANALYTICAL CHEMISTRY
020t 000 0
IO
20
30
3
TIME (minutes)
CONCENTRATISN OF IODINE (pig per 2 5 m l l
Figure 1. Effect of ternpzrature on color intensity I. 11. Ill.
Figur2 2. intensity I. 11. 111. IV.
Effect of
time
on color
8.0 pg. of iodine in 10 m'. 2.7 p 3 . of iodine in 10 ml. 1.4 pg. of iodine i n 10 ml. Blank
Absorbance o t 15' to 20" C. Absorbonce at 25' C. A b s o r b a x e o t 30" C.
I. Accuracy of the Spectrophotometric Method for the Determination of Iodine in Solution
Table
I I1diiic7
I utlirics
:itltlld.
foulid.
!a. (J 0s 0 14 0.I , ? 0 1; 1) I S
0 ,2 0 0 ,2 3 0.:30
Pup.
0 . !I1 (J, 1 I
0.I4 0 !h 0 . I!) 0 . I!r
0.22 0.31
J:;rror, I
+ 2.;. 0
-21 .-I -1i.7 .i 1, I
+
-C
;.
,
ii
-4.1
:3 :3 ,
V O L 3 4 NO 13 DECEMBER 1 9 6 2
1699
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,
(2)
1247 (1951). Sugawara, K., Koyama, T., Terada, K., Bull. C'henz. Sac. Jnonn 28. 494
(4j
(1955).
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 impregnated n i t h 20 weight 70 Apieaon 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.
