1998
Anal. Chem. 1982, 54, 1998-2002
Detection of Carbon Monoxide in Ambient Air with a Piezoelectric Crystal Mat
H. Ho'
and George G. Guilbault *
Department of Chemlstry, University of New Orleans, New Orleans, Louisiana 70 148
Eugene P. Scheide Environmetrics, Inc., 1567 North Warson Road, St. Louis, Missouri 63 132
Carbon monoxlde is first reacted wlth mercuric oxide at 210 OC to produce mercury vapor. The mercury vapor liberated Is then adsorbed on the gold electrode of the pleroeiectric crystal, thereby Increasing the mass on the crystal and decreasing Its frequency of vibration. The change of frequency Is proportional to the amount of mercury present and is indlcative of the carbon monoxlde content of the alr. Reverslbllity can be achieved by thermal desorption. By use of a reference stream, the background due to the thermal decampositton of mercuric oxlde is compensated. The effect of moisture and interferences from other reduclng gases can be eilmlnated by use of a precolumn. The detector Is slmpie, inexpensive, and portable. Concentratlons In the part-permiillon and part-per-billion ranges can be detected by varylng the sample size.
change and added mass. This relationship enables a piezoelectric crystal to be used as a sensitive sorption detector. High selectivity of the detector can be achieved by coating the crystal with a substance which selectively adsorbs the pollutant to be detected. A number of analytical applications have been based on this principle (13-17). The ability of gold to adsorb and amalgamate mercury is well-known (18). Gold-coated piezoelectric crystals have already proven useful as an analytical tool for continuous mercury analysis (13,19-21). There is currently a need for an inexpensive and portable instrument to detect carbon monoxide in the air as well as to read out the total exposure in the working place. In this study we demonstrate a sensitive and selective, but simple and inexpensive detector for carbon monoxide, based on the conversion of mercuric oxide by carbon monoxide to mercury vapor; the mercury produced is detected with a gold-coated piezoelectric crystal.
Carbon monoxide is an odorless, colorless gas and one of the most dangerous industrial hazards. I t is also one of the most widespread poisons related to human life and activity. The toxic effects of carbon monoxide are well-known: i t combines with blood hemoglobin to produce carboxyhemoglobin, thereby reducing the effectiveness of this oxygen carrier in the bloodstream. The common methods of determination of carbon monoxide in ambient air are colorimetric (1) and infrared or nondispersive infrared (2) methods, use of oxidation catalysts like Hopcalite (3),gas chromatography by reduction to methane ( 4 ) ,and detector tubes (5). A mercury replacement method has also been reported and is widely used in several ways to provide a sensitive means of detecting carbon monoxide. McCullough et al. (6) showed that the reaction between mercuric oxide and carbon monoxide is thermodynamically favorable and can be used for determination of carbon monoxide in air. The gaseous sample is passed through a reaction tube containing hot granular red mercuric oxide and elemental mercury liberated is measured with selenium sulfide (7). On the basis of this reaction, Taller-Cooper, Inc., in the early 1950s marketed a Supersensitive CO Analyzer, that used flameless atomic absorption for measuring the mercury vapor produced (8). This instrument led to further work on mercury replacement detectors by Robbins et al. ( 9 ) , Seiler and Junge (IO),and Palanos (11, 12). In recent years, coated piezoelectric crystal detectors have become of increasing interest for detection of traces of toxic atmosphere pollutants. They have been shown to be not only highly sensitive and selective detectors but also simple, inexpensive, and portable devices which are even small enough to be carried in a worker's pocket (13). The principle of the detector is based on the linear relationship between frequency
EXPERIMENTAL SECTION Apparatus. The instrumentation consisted of a gold-coated piezoelectric crystal, the detector cell, an oscillator, a power supply, a frequency counter, and a recorder. The experimental setup is shown schematically in Figure 1. The crystals used were 9.0 MHz, AT-cut quartz plates, resonating in the thickness shear mode. Each crystal consisted of a vacuum-deposited gold electrode located on each side of the quartz plate. The crystals were obtained from International Crystal Manufacturing Co., Oklahoma City, OK, and were mounted in a HCG/U holder. An oscillator (International Crystal Manufacturing Co., Model OT-13), which drives the crystal at its fundamental resonant frequency, is powered by a regulated power supply. The applied voltage was kept constant at 9 V dc. The frequency output from the oscillator was measured by a frequency counter (Heath-Schlumberger,Model SM-4100) which was modified by a digital-to-analog converter so that the frequency could be recorded. The design of the detector cell is a modification of the one described previously (22). This design, which is shown in Figure 2, provides excellent contact between the coated crystal and gas stream and hence gives high sensitivity. The cell was made from a Pyrex 24 f 40 ground joint and the crystal is connected to the oscillator by two tungsten-nickel (W-Ni) leads protruding through a layer of glass which sealed a male joint. One end of these leads was mounted to a socket accommodating the wire lead of the crystal's holder. The other end, from the male joint, was then connected to the oscillator. This enabled not only the rapid and easy removal of crystals for thermal desorption or recoating but also provided a sealed detector cell (by means of the glass encapsulation) as well as optimum electrical contact. The piezoelectric detector and cell are available commercially from Universal Sensors (New Orleans, LA). The heating block was made from the injection port heater of the gas chromatograph and the temperature was controlled by a variable power regulator (Hewlett-Packard). The temperature was kept constant at 210 OC. The reaction chamber was made of stainless steel. AU the tubing lines were constructed of stainless steel and Teflon for carbon monoxide and mercury, respectively.
Present address: Chemistry Department,University of Alabama, Birmingham, AL.
0003-2700/82/0354-1998$01.25/0Q 1982 Amerlcan Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 12, OCTOBER 1982
1999
is energized for a certain period of time, the mercury vapor liberated is introduced into the detector cell. Since carbon monoxide is quantitatively eliminated by silver oxide, the background due to the thermal decomposition of mercuric oxide is obtained. After the solenoid valve is deenergized, the mercury on the crystal is desorbed thermally at 170 "C. The silver oxide cell is then bypassed and the solenoid valve is energized again for the same period of time; the increase in mercury vapor concentration is proportional to the carbon monoxide concentration in the sample. The crystal is desorbed after each measurement. Flgure 1. Experimental setup: (1) drying tube; (2) Ag,O tube; (3) flow controller; (4) preheated coll; (5) HgO tube; (6) solenoid valve (D) deenergized, (E) energized; (7) gold-coated plezoeiectrlc crystal; (8) oscillator; (9) frequency counter; (10) power supply; (11) digital-toanalog converter; (12) recorder; (13) flowmeter. OUTLET
TiSAMPLE INLET +
t OUTLET
II
t t SAMPLE INLET
Flgure 2. Details of deiector cell design.
Nitrogen or purified air was used as carrier gas. The flow rate was monitored by a calibrated rotameter and adjusted by a flow controller. Generation of Carbon Monoxide Standards. The standard concentrations of carbon monoxide in this study were prepared by the permeation method. A tubular permeation membrane was suspended inside a stainless steel cylinder. Carbon monoxide is contained in the cylinder and surrounds the membrane. Carbon monoxide permeated though the membrane and then mixed with the controlled flow of dilution gas inside the tube to generate precise standard concentrations. Various concentrations were obtained by adjusting the flow rate of diluent gas, the temperature of the permeation tube, and the pressure of carbon monoxide around the membrane. A certified carbon monoxide permeation source, series 57 H, was purchased from Kin-Tek Laboratories, Inc., Texas City, TX, and a Kin-Tek Precision Calibration System, Model 570, was used. Carbon monoxide was from a lecture bottle obtained from Matheson Co., LaPorte, TX. A lecture bottle control valve was used to connect the gas source to the permeation system and control the flow rate of carbon monoxide through the chamber surrounding the permeation tube at about 1 mL/min. Nitrogen was used as the dilution gas. Since carbon monoxide is highly dangerous, the waste from this gas was burned into carbon dioxide under a hood. The system is capable of generating standard concentratioins at parts-per-million levels. At partsper-billion levels a sectondary dilution is necessary. Reagents. Mercuric oxide, red powder, was obtained from J. T. Baker Chemical Co., Phillipsburg, NJ. The pellet was made from mercuric oxide powder with a hand-operated pellet press. Silver oxide was purchased from Alfa Division, Danvers, MA. Formaldehyde was generated by using a permeation tube filled with paraformaldehyde. Ethylene, sulfur dioxide, and nitrogen dioxide were used from lecture bottles and were obtained from Matheson Co., Inc., LaPorte, TX. Procedure. The reaction chamber is filled with 0.5 g of mercuric oxide pellet anid the temperature of the heating block is automatically controlled at 210 "C. The solenoid valve is deenergized and the sample is passed through silver oxide and the preheated coiled tube (210 "C) before entering the reaction chamber. Any mercury 'vapor produced is trapped by a solution of KMn04 and H2S04lbefore being vented. The flow rate is precisely measured and controlled by use of the soap bubbler method and flow controlller, respectively. When the solenoid valve
RESULTS AND DISCUSSION The principle of the detector is based on the chemical reaction between carbon monoxide and mercuric oxide at a temperature of 210 "C to produce elemental mercury. The resulting mercury vapor is determined by a gold-coated piezoelectric crystal. Mercuric Oxidexarbon Monoxide Reaction. The reaction of carbon monoxide with red mercuric oxide was reported by McCullough et al. (6) as follows:
Although this reaction is thermodynamically favorable even at 25 "C, the reaction rate is too slow to permit a quantitative analysis. In their publication, McCullough et al. (6) showed that at temperatures of 125 OC, 146 OC, and 200 OC the percentage completeness of reaction was 4%, 62 %, and loo%, respectively. In determination of the optimum temperature of this reaction, two factors, which affect the sensitivity of the detector, should be examined. First, as the temperature is increased the completeness of reaction is increased and therefore better sensitivity results. We found that at temperatures lower than 210 "C, the reaction is incomplete, particularly in the part-per-billion range. Second, the background generated by the thermal decomposition of mercuric oxide must be considered. When nitrogen or purified (carbon monoxide free) air is passed over mercuric oxide a t 210 "C, there is a small background of mercury formed. This is to be expected, since the same observations were reported by other authors (9, IO). When the temperature is increased, the background is increased; this reduces the sensitivity of the detector. Therefore the optimum temperature is a balance of these two effects, and this was found to be 210 "C (9, IO). In this study, the reaction chamber was kept at a constant temperature of 210 OC and the sample, which contains carbon monoxide, was also preheated to this temperature before introduction into the hot mercuric oxide. The background can be eliminated by using a reference stream. Carbon monoxide is known to be oxidized quantitatively by silver oxide at room temperature (9). The sample is passed through a column which is packed with granular silver oxide to provide a reference stream. Since there is no carbon monoxide in the reference stream, any response is due to the thermal decomposition background and the detector can be zeroed. Once the zero is established, silver oxide is bypassed and the sample is introduced directly to the reaction chamber. This stream, because of carbon monoxide present, has a higher mercury concentration and gives a higher response. The carbon monoxide concentrations can be obtained from the differences in these two signals. If the temperature of the reaction chamber and the flow rate of carrier gas are held constant, and the amount of mercuric oxide is relatively large (0.5 g), the background is quite stable. Detection of Mercury Vapor Produced with GoldCoated Piezoelectric Crystal. The ability of gold to amalgamate mercury is well-known. Mercury vapor liberated from the reaction chamber is adsorbed on the gold electrode of the crystal, thereby increasing the mass on the crystal and decreasing the frequency of vibration. The change of frequency is proportional to the amount of mercury present.
2000
ANALYTICAL CHEMISTRY, VOL. 54, NO. 12, OCTOBER 1982
Table I. The Effect of Surface Contamination on the Collection Efficiency of the Gold Electrode Crystala freq
response, types of crystal
Hz
fresh “homemade” gold-coated crystal commercially available gold electrode crystal without heat cleaning commercially available gold electrode crystal with heat cleaning pretreatment
121 98
BB
120
g
‘Conditions:
flow rate, 30 mL/min; sampling time, 8 s; concentration of carbon monoxide, 10 ppm.
60-
40-
3
8
E
20~
0
I
Table 11. Lifetime of Gold Electrode Crystal‘ no. of adsorptionfreq desorption cycles response, Hz 1
10 50 70 90
120 123 119 116 115
a Conditions: flow rate, 30 mL/min; sampling time, 8 s; concentration of carbon monoxide, 1 0 ppm.
Gold is one if the metals used to make the electrode on commercially available crystals. Since the collection efficiency of mercury on a gold electrode is a function of the surface area and composition, the percentage collection efficiency was investigated. Three different crystals were prepared. The first one had fresh gold coated on a nickel-plated crystal by vacuum deposition. A gold film was evaporated onto the electrode of this crystal using our conventional vacuum system. The second and third crystals used were commercially available gold electrode crystals. However, the third crystal was placed in an oven at 170 “C and hot nitrogen was passed over for 1h to flush off any contaminants. The fiist and third crystals gave the same collection efficiency, while the second one demonstrated a lower efficiency as shown in Table I. These results demonstrate that the gold electrode should be fresh for the best collection efficiency; a commercially available gold electrode crystal can be used after heat cleaning. The collection efficiency also depends on the configuration of the detector cell. Figure 2 shows the efficient and sensitive design for the detector cell. The mercury formed flows directly and perpendicularly onto the center of the electrodes on both sides of the crystal and hence gives the better sensitivity. After the measurement, mercury is desorbed from the gold surface of the electrode by thermal desorption. The crystal cell is heated up to 170 “C and hot purified air or nitrogen is passed through for several minutes. Mercury is desorbed and the gold electrode is thus regenerated. As a result, the frequency returns to the original base line and the next measurement can be made. The desorption temperature should not be over 170 “C because of the low melting soldered electrical connections. This technical problem can be solved by using hard, high-temperature-stable soldered connectors. Then the temperature can be raised for faster desorption. At 170 “C, 10 min are required for desorption even using hot purified air. However, at 300 “C, less than 1 min is required. For the best precision, the crystal should be desorbed after each measurement. Table I1 shows the lifetime of the crystal. After 70 adsorption-desorption cycles, the crystal gave almost the same response. We observed that if the crystal is desorbed immediately after each measurement the lifetime of the crystal is much longer. This may be due to the migration of mercury into the inner layers of the gold film during storage; it is difficult to desorb completely this mercury. With proper care
the lifetime of the crystal could be over 90 adsorption-desorption cycles. Effect of Flow Rate. Figure 3 shows the effect of flow rate of sample on the sensitivity of the detector to 17.5 ppm of carbon monoxide using a 4-s sampling time. The flow rate was varied from 30 to 200 mL/min. As the flow rate was increased from 30 to 90 mL/min the frequency response was the same. However, at flow rates higher than 100 mL/min the sensitivity begins to decrease. Robbins et al. (9) and Seiler and Junge (IO)showed that the flow rate does not affect the reaction between mercuric oxide and carbon monoxide up to 500 mL/min. The decreasing sensitivity at flow rates above 100 mL/min may be due to the incomplete adsorption of mercury formed on the gold electrode. At a flow rate of 200 mL/min the sensitivity is about 15% lower than at 90 mL/ min. Interferences. In order to evaluate the selectivity of detection of carbon monoxide, it is necessary to study the detector response to other gases in the atmosphere as well as possible interferences caused by these components. Thus the two steps of the detector must be evaluated: first, the interferences due to the reaction with mercuric oxide by other reducing gases; second, the interferences due to the gold-coated piezoelectric crystal itself. However, since gold is a highly specific adsorber for mercury, interferences were only found to be due to reducing gases such as hydrogen, methane, formaldehyde, and olefins, which might reduce mercuric oxide to produce mercury vapor and therefore cause errors. The concentration of hydrogen in the atmosphere is about 0.5 ppm and hydrogen is not adsorbed by silver oxide even at higher concentrations. Hydrogen is thus passed through silver oxide and can be eliminated together with the thermal decomposition background. Of course, changes in hydrogen concentration will affect the reference base; this problem can be avoided by checking the base line before measuring. However, significant changes in hydrogen concentration occur slowly or not at all in the normal atmosphere. Methane does not react with mercuric oxide at this temperture (10, 12). Formaldehyde and ethylene are quantitatively by mercuric oxide. These interferences, along with other aromatic and unsaturated hydrocarbons, could be removed effectively by using activated charcoal (6) or molecular sieve 5A (10). Inorganic gases are not interferences except for sulfur dioxide. However, sulfur dioxide can be easily removed by a molecular sieve precolumn. The effect of moisture was also studied. Water vapor probably affects the thermal decomposition of mercuric oxide rather than the direct reaction. Also water vapor is known to deactivate silver oxide (9). Therefore, water vapor should be removed from the sample. Magnesium perchlorate, anhydrous calcium sulfate, calcium chloride, and phosphorus
ANALYTICAL CHEMISTRY, VOL. 54, NO. 12, OCTOBER 1982
2001
i-.//
L
6
SAMPLINB TIME,MIN.
SAMPLING TIME ,SEC.
Flgure 4. Plots of frequency response vs. sampling time at ppm levels of CO and flow rate of 30 niL/min: TB, thermal background; A, 50 ppm TB; 6, 30 ppm TB; C, 10 ppm TB; D, 5 ppm TB; E, 1 ppm TB.
+ +
+
+
+
Flgure 6. Plots of frequency response vs. sampling time at the ppb levels of CO and flow rate of 200 mL/min: TB, thermal background; A, 150 ppb TB; B, 75 ppb TB; C, 38 ppb TB; D,5 ppb TB.
+
+
+
+
I
o 0 ! i IOL - 20 - A
30
CONCENTRATION OF CO
40
50
, PPM
Flgure 5. Plots of frequency response vs. concentration at the ppm levels of CO and flow rate of 30 mL/mln: A, 2 s sampling time; B, 4 s sampling tlme; C, 10 s sampllng time.
pentoxide were used to remove water. Calcium chloride and phosphorus pentoxide were found to be the best for efficient removal of water vapor without affecting the concentration of carbon monoxide. Response and Calibration. Plots of the frequency response vs. the sampling itime in the parts-per-million ranges are shown in Figure 4. Since the crystal responds to mass rather than concentration of mercury formed, the larger volume of sample, which gives the larger amount of mercury formed, will increase the frequency response. In Figure 4 the flow rate was kept constant a t 30 mL/min and the sampling time was changed from 1 to 16 s. As the sampling time increases the volume of sample increases and more mercury is formed; therefore the frequency response increases. However, a t 150 Hz saturation begins to occur and the response is no longer linear with the sampling time. For calibration the flow rate and sampling time should be controlled. From 1 ppm to 50 ppm a sampling time of 2 s is adequate. If the sampling time is longer than 2 s, saturation begins to occur at 50 ppm and the linearity between frequency response and concentration no longer exists. However, if the sampling time is less than 2 s the sensitivity (AF/ppm) decreases. For example, the responses to 30 ppm of carbon monoxide were 43 Hz and 89 Hz for 1 s and 2 s of iaampling time, respectively. In Figure 5 the flow rate was kept constant a t 30 mL/min and plots of frequency response vs. concentration of carbon
CONCENTRATION OF CO, PPB
Flgure 7. Plots of frequency response vs. concentration at the ppb levels of CO and flow rate of 200 mL/min: A, 2 rnin sampling time; B, 3 min sampling time; C, 5 min sampllng time.
monoxide a t different sampling periods are shown. As the sampling time is increased the sensitivity will become better while the linear range is narrower. Curve A shows the response for a 2-s sampling time; the linear range is from 1 ppm to 50 ppm and the sensitivity is about 3 Hz/ppm. In curve B, the sampling time was 4 s; the linear range is from 1 ppm to 25 ppm, and the sensitivity is about 5.7 Hz/ppm. In curve C, with 10 s of sampling time the linear range is from 1 ppm to 10 ppm and the sensitivity is 14 Hz/ppm. These results show that the range of linearity and the sensitivity are functions of the sampling time. The detector is capable of monitoring carbon monoxide in the parts-per-million and parts-per-billion levels by varying the samples sizes. At parts-per-billion levels higher flow rate and longer sampling times are required, since a larger sample volume is needed. Typical responses at the parts-per-billion levels are shown in Figure 6. It should be noted that the flow rate of 200 mL/min was used in order to reduce the sampling time. At a flow rate 200 mL/min the sensitivity of the crystal is about 15% lower than that a t 30 mL/min. However the volume of sample is increased, and good sensitivity can be obtained. Figures 6 and 7 show that from 5 to 150 ppb, a sampling time of 2 min is adequate. The sensitivity in this range is about 0.86 Hz/ppb. If the sampling time is increased to 3 min the linear range is limited to 80 ppb, while the sensitivity is increased to 1.3 Hz/ppb. For 5 min of sampling time, the departure from linearity begins to occur at 40 ppb
2002
Anal. Chem. 1082, 5 4 , 2002-2005
and the sensitivity is about 21 Hz/ppb. The detector can monitor as low as ppb Of monoxide* It is that lower concentrationscan 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. A m . 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. A m . 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
