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and the sensitivity is about 21 Hz/ppb. The detector can monitor as low as ppb Of monoxide* It is that lower concentrations can be detected with longer sampling times. The relative standard deviation of the detector is about 4% and 8% in the parts-per-million and parts-per-billion ranges, respectively. The detector is rugged, inexpensive, and sensitive* Various linear ranges can be ‘Overed by varying the sample volumes. This will further enhance the usefulness of this detector.
LITERATURE CITED kevaggl, D. A.; Feldstein, M. Am. Ind. Hyg. Assoc. J . 1984, 25,64. Methods of Air Sampllng and Analysls”, 2nd ed.; American Public Health Association, Inc.: Washington DC, 1977; p 348-356. Lvsvl. I.: Zarembo. J. E.: Hanlev. A. Anal. Chem. 1959. 37. 902. Ortman. G. C. Anal. Chem. 1986, 38, 644. Morgenstein, A. s.; Ash, R. M.; Lynch, J. R. Am. Ind. HYg. ASSOC. J . 1970, 37,360. McCullough, J. D.; Crane, R. A.; Beckman, A. 0.Anal. Chem. 1947, 79, 999. Beckman, A. 0.; McCullough, J. D.; Crane, R. A. Anal. Chem. 1948, 20,674. Muller, R. A. Anal. Chem. 1954, 26,39A.
(9) Robblns, R. C.; Borg, K. M.; Robinson, E. J . Air Pollut. Control Assoc. 1988, 18, 106.
(10)Seiler, W.; Junge, C, J , &ophys, Res, 1970, 75, 2217, (1 1) Palanos, P. N. “Selective Detection of Ambient Carbon Monoxlde with a Mercury Replacement Analyzer”, paper No. 71-1128, AIAA Joint Conf. on Sensing the Environment, Palo Alto, CA, Nov 6-10, 1971. (12) Palanos, p, N, Anal. Instrum, 1972, l o , 117, (13) Scheide, E. P.; Warnar, R. B. J. Am. Ind. Hyg. Assoc. J . 1978, 3 9 , 745. (14) Hlavay, J,; Guilbault, G, G. Ana/. Chem, 1977, 49, 1980, (15) Ho, M. H.; Guilbault, G. G.; Rietz, B. Anal. Chem. 1980, 52, 1489. (16) Tomita, Y.; Ho, M. H.; Guilbault, G. G. Anal. Chem. 1979, 57, 1475. (17) Edmonds. T. E.: West. T. S.Anal. Chlm. Acta 1980. 777. 147. (18) Mercer, T. T. Anal. Chem. 1979, 57, 1026. (19) Brlstow, Q., J . Geochem. Explor. 1972, 1 , 55. (20) Schelde, E. P.; Taylor, J. K. Environ. Scl. Techno/. 1974, 8 , 1097. (21) Scheide, E. P.; Taylor, J. K. Am. Ind. Hyg. Assoc. J . 1975, 3 6 , 897. (22) Kamarkar, K. H.; Gullbault, G. G. Anal. Chlm. Acta 1974, 7 7 , 419.
RECEIVED for review March 27, 1981, Resubmitted M~~ 14, 1982. Accepted May 14,1982. The authors acknowledge the financial support of the Army Research Office, in the form of Grant No. DAAG-77-G-0266, in carrying out this research project.
Effect of Temperature on the Starch-Iodine Spectrophotometric Calibration Line Gary L. Hatch Ametek, Inc., Plymouth Products Division, 502 Indiana A venue, Sheboygan, Wisconsin 5308 I
The threshold usually exhlblted by the starch-lodlne spectrophotometrlc callbration line Is largely dependent on temperature. The molecular welght of the starch Is also an Important factor. The effect of temperature on the color Intenslty of the starch-Iodine complex is demonstrated and thermal decolorlratlon Is proposed to occur prlmarlly as a result of thermal deformation of the reactlve starch hellces. A temperature-dependent hysteresis effect for the starch-Iodine color formation-decolorlrallon reactlon Is reported.
Pieters and Hanssen (1) used a star c.. of questionable homogeny and later, along with Zitomer (5, 7), offers a colorforming mechanism which requires more than one triiodide ion to produce color. These authors (5, 7 ) propose that the threshold concentration represents the number of triiodide ions that first enter the starch helix. Drey (14) and Smith (8) refute this explanation and Drey proposed that the threshold is a result of loss of iodine by starch-induced disproportionation to hypoiodous acid according to the following sequence of reactions:
I, Numerous investigators have studied and utilized the blue starch-iodine chromogen in colorimetric iodimetry for various quantitative analyses. The basic analytical procedure relies on the obedience of the calibration line (plot of absorbance vs. concentration of iodine or species that produces iodine) to Beer’s law. Many of these studies (1-9) have revealed a nonobedience to Beer’s law in that the calibration line exhibits a positive intercept or “threshold” on the concentration axis. The results of other studies, however, have been somewhat contradictory or have shown unusual inconsistencies. Some investigators (1, 3 , 6 , 10) have revealed that the threshold can be completely overcome by lowering temperature. Smith (8), however, studied a starch preparation that produced a threshold which could not be overcome by lowering temperature, and still others (11-13) present results that show no threshold or very little temperature dependence at all. Several attempts to explain the threshold phenomenon have been made (5, 7, 14). These explanations are based on the substantiated facts that the triiodide ion (If) must be present to form color (13) and that the starch-iodine complex exists in a helical configuration within which the triiodide ions (or polyiodide ions) reside (15-18). Lambert (13) claims that
+ H,O + H++ I- + HOI Keq= 3 X 1 2 + I13starch + Is- + starch.13-(blue)
(1)
Drey does not offer any quantitative data to support this explanation but does mention that the threshold cannot be overcome by increasing the iodide ion or starch concentration or by altering pH. However, a calculation of the equilibrium amount of iodine lost due to reactions 1-3 for the usual starch-iodine reaction conditions (high 1-/12ratio, 60:l or greater, and low pH, 4.0 or less) would show that this amount is infinitesimally small compared to the normally observed M as 12). threshold concentrations (approximately Smith (8) studied two commercial starch preparations, Superlose and Superlose HAA-11-HV, a hydroxyethyl derivative of the former (both are no longer commercially available) and found that the hydroxyethyl derivative produced a nonlinear calibration line that gradually curves toward the origin. This nonlinear response is more pronounced with increased temperature. The Superlose preparation, which is described as a purified linear amylose potato starch fraction, gave linear response at
0003-2700/82/0354-2002$01.25/00 1982 Amerlcan Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 12, OCTOBER 1982
the concentrations studied a t 0 "C, 25 "C, and 35 "C. However, as the temperature was increased, the slope of the calibration lines decreased, but the observed threshold remained essentially constant. That is, the calibration lines for the different temperatures intersect a t the same point on the concentration axis. Smith predicts that the calibration line for Superlose would alicio gradually curve to the origin if the response had been studied at lower concentrations or higher temperatures. Smith provides a more plausible explanation for the observed threshold and temperature dependence by proposing that there is a temperature-dependent change in the structure and (or) number of starch helices available for reaction with triiodide ion. The molecular weight (or molecular weight distribution) of the starch has been shown to be a critical factor in the ability of starch to complex iodine (15-17). The work of Thompson and Hamori (17)indicates that a more nonlinear response and larger thresholds are obtained with lower molecular weight starch fractions. In more recent related work (19,20)a titrametric analysis of the effect of temperature on the accuracy of the starchiodine end point is prenented. From these results, Hatch and Yang (20) propose a mechanism for the observed thermal decolorization of the blue starch-iodine complex. This mechanism is based on triiodide ion being the primary color-forming unit and is in basic agreement with Smith (8). The decolorization is proporied to be a result of thermally induced deterioration or unravding of the starch helices from around the triiodide ions. The results presented1 here provide additional information on the thermal decoloiriization phenomenon and also reveal a heretofore unreported temperature-dependent hysteresis effect for the color formation-decolorization of the blue starch-iodine complex. How these thermal effects and other starch properties are related to the threshold phenomenon is discussed.
EXPERIMENTAL SECTION Apparatus and Reagents. All spectrophotometric measurements were made with a Hach Model DR-EL spectrophotometer (Hach Chemioal Co., Loveland, CO) using optically matched 2.5-cm cells. A,, (610 nm) for the starch preparation used (0.25% starch in 1% acetic acid, Fisher Scientific Co., Chicago, IL) was determined in the usual way by plotting absorbance vs. wavelength (& 2.5 nm) and the molar absorptivity, t, at 20 "C was determined to be 2.5 X lo4. All chemicals used were reagent grade or the best commercially available. The stock starch solution was newly obtained (18month guaranteed stability) and was maintained at 22 "C throughout the analyses. Stock Iodine Solution. A stock solution of 0.1 N iodine was prepared by adding 5-10 mL of distilled water to a 100-mL volumetric flask which calntained 1.28 g of resublimed iodine and 1.68 g of potassium iodide. After a 24-h period was allowed for complete dissolution, the concentrated mixture was diluted to 100 mL with distilled water. Standard Iodine Solution. A working standard iodine solution was prepared by diluting 10 mL of the stock solution to 100 mL with distilled water. The working standard was standardized daily against 0.005 64 N phenylarsine oxide (Ricca Chemical Co., Arlington, TX). Preparation of Samples for Constructing Calibration Lines. Each sample was prepared in a 250-mL glass-stoppered flask by adding an appropriate aliquot (i~0.005mL) of standard iodine solution to 200 niL of distilled water which had been adjusted to 5-6 "C. The error (
