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Anal. Chem. 1982, 5 4 ,
While it was initially expected that the CN emission would also be observed for COz,this was not the case; only NO lines were observed. An oxygen atom is removed from the COz molecule and forms the intermediate species NO, which is excited and emits its strongest line at 305 nm. Just as all hydrocarbons show only CN emission, oxygen-containing molecules (e.g., 02, COz) exhibit primarily the NO p emission. Table I shows data obtained for COz. In conclusion,the MTES technique can be extended to very low detection limits and affords a relatively simple, inexpensive method of determining low-level impurities in various plant gases. Its ultimate capabilities are as yet unexplored, but
828-830
further extension of this technique provides a method for detection of not only carbon and oxygen impurities in plant gases but metal impurities as well. Metal ions in a gaseous state are detected by reactions with Nz* (3). LITERATURE CITED (1) (2) (3) (4)
U.S. Patent 4 150951, 1979. Capelle, G. A.; Sutton, D. G. Appl. Phys. Lett. 1977, 30, 407. Phillips, L. F. Can. J . Chern. 1963, 47, 2060. Sutton, D. G.; Westberg, K. R.; Melzer, J. E. Anal. Chem. 1979, 57, 1399.
RECEMDfor review September 18,1981. Accepted December 24, 1981.
Quickly Dissolving Amylose Indicator in Cadmium Iodide-Linear Starch Colorimetric Reagent Jack L. Lambert," Gary T. Fina, and Earline F. Dikeman Department of Chemistry, Kansas State Universlty, Manhattan, Kansas 66506
The starch described here is pure potato amylose and is a light, dry powder that disperses and dissolves instantly in cool water. Kept in a stoppered "salt-shaker" type dispenser such as those used for meat tenderizers, the solid amylose can be kept indefinitely and added only as needed. As the concentration of starch is not critical to titrimetric end point determination, it is necessary only to ensure the minimum required to produce the blue complex with polyiodide anionic species. Potato amylose was selected for use in the cadmium iodide-linear starch colorimetric reagent because of its greater chain length and higher content in the raw starch granules compared to other common sources. This versatile reagent was first reported in 1949 (1)and has been studied (2)or used in analytical methods for the determination of selenium (3), fluoride (4,5),chloride (6),ammonia (7),bromide (8),residual iodine in rocks (lo), peroxide in edible fats (ll), chlorine (9), total iodine and iodate in seawater (12), sulfate (13), miscellaneous anions after exchange for iodate with mercury(I1) iodate (14), and iodine from a quaternary ammonium strong base resin-triiodide disinfectant (15-1 7)as well as to monitor iodine disinfectant in stored water in space vehicles in the NASA Skylab project (18). Cadmium iodide provides iodide ion in controlled excess by release from the moderately stable iodocadmium species present in solution 2CdIz
$
[CdI]+
+ [CdIJ-
F!
Cd2+
+ [CdI,I2-
Retrogradion (precipitation due to intermolecular hydrogen bonding) of starch is retarded in cadmium iodide solution, probably due to complexation of cadmium ion by the hydroxyl groups on the starch molecule. Toxicity of the cadmium retards the growth of microorganisms. The response of the reagent to oxidizing agents is nearly independent of pH between 7.0 and 0.5. At pH 7.5, a precipitate of cadmium hydroxide appears, and a t pH values lower than 0.5, dissolved oxygen slowly oxidizes the iodide. The absorbance produced at 614 nm is temperature dependent, so for precise measurements the temperature should be controlled. The reagent is essentially nonselective toward strong oxidants. The pH of the reagent prepared with pure potato amylose is approximately 6.0, which is within the 5.9-6.3 pH range recommended for prevention of hydrolytic 0003-2700/82/0354-0828$0 1.25/0
breakdown of starch. The absorptivity, e, of the reagent is 1.7 X lo4per equivalent of oxidizing agent (2). The response curve is rectilinear but exhibits a small positive intercept on the concentration axis of the polyiodide species (or the oxidant that produced the polyiodide), as noted in several studies (7, 10, 11, 19-21). Hanes (22)postulated, and Rundle et al. (23-25) later confirmed, the helical structure of the starch-polyiodide complex and presented proof that it is a compound rather than an adsorption phenomenon or a simple solution of iodine in starch. Originally, it was assumed that triiodide anion, 13-, produced the chromogen, as both iodine and iodide ion were known to be necessary to the production of color. Gilbert and Marriott (26) identified the polyiodide anion in the starch helix as 312.21-, or -I.: Lambert and Zitomer (7)proposed a model wherein Is- anions enter the starch helix sequentially and absorbance is produced only after the second 13-enters to produce Is2-. This model would explain the small positive intercept of the response curve on the concentration axis. Watanabe and co-workers (27) later concluded that the polyiodide species is 13-,(&I
