Anesth Analg 2003;97:151-155
© 2003 International Anesthesia Research Society
TECHNOLOGY, COMPUTING, AND SIMULATION
The Color Change in CO2 Absorbents On Drying: An In Vitro Study Using Moisture Analysis
Erich Knolle, MD*,
Wolfgang Linert, PhD
, and
Hermann Gilly, PhD*,
*Department of Anesthesiology and General Intensive Care (B), University of Vienna;
Institute of Applied Synthetic Chemistry, Technical University of Vienna; and
L. Boltzmann Institute for Anesthesiology and Intensive Care, Vienna, Austria
Address correspondence and reprint requests to Erich Knolle, MD, Department of Anesthesiology and General Intensive Care (B), University of Vienna, Waehringer Guertel 18–20, A-1090 Vienna, Austria. Address e-mail to erich.knolle{at}univie.ac.at
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Abstract
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Alkali hydroxide-free absorbents change color markedly when they dry, whereas absorbents containing alkali hydroxides do not. We investigated whether this observation can be explained by the weaker hygroscopic properties of pure calcium hydroxide compared with alkali hydroxides. Samples of the alkali hydroxide-free absorbents Amsorb® or Superia® and samples of these two absorbents with 1% or 3% NaOH or 3% KOH added were dried in a moisture analyzer at 105°C to determine their moisture content and to assess the color of the samples during drying (each group, n = 5). Additionally, we repeated the experiments with pulverized samples of Baralyme® and Spherasorb®, which contain approximately 4% KOH and 1% NaOH, respectively. Amsorb® and Superia® changed color long before they were dry. After the addition of 1% NaOH, and as with the Spherasorb® samples, the drying time required for a color change was longer, and the intensity of the resulting violet was less. This effect was even stronger when 3% NaOH was added. The samples with added KOH and the Baralyme® did not change color at all on drying. We conclude that the differences in color change on drying in absorbents with varying NaOH or KOH content cannot be explained by larger water retention because of the hygroscopic properties of the alkali hydroxides.
IMPLICATIONS: In an in vitro study, the moisture content and color change on drying were determined in samples of Amsorb® or Superia® and in the same absorbents with added NaOH or KOH. With increasing concentrations of alkali hydroxide, a delay in the color change upon drying was observed. However, the moisture content did not change.
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Introduction
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Amsorb® (Armstrong, Coleraine, Northern Ireland) and Superia® (Molecular Products, Essex, United Kingdom) are recently developed absorbents that contain calcium hydroxide (Ca(OH)2) but no alkali hydroxides (1). In a previous investigation (2), we showed that in Amsorb®, a color change from colorless to violet takes place not only when the CO2 absorbing capacity is exhausted, but also when the (fresh) absorbent is dehydrated by a drying gas flow. The color change caused by drying was explained by the fact that ethyl violet cannot act as an acid-base indicator in a dehydrated environment. However, we were not able to explain why the color change was prevented almost completely by the addition of 1% NaOH. We have hypothesized that because of its strong hygroscopic property, NaOH retains some water when dried by a gas flow. If so, complete drying by heat should evaporate all the water and subsequently cause a color change from colorless to violet even in samples containing alkali hydroxides. Furthermore, in these samples, the weight loss after heat drying should be larger than in samples without alkali hydroxides, indicating a larger initial moisture content.
In the present study, we tested this hypothesis by measuring the weight reduction during heat drying using a precision moisture analyzer and then comparing the weight reduction and the color change in samples of Amsorb® or Superia® with those changes in corresponding samples to which NaOH or KOH were added. We additionally investigated a possible color change resulting from drying in samples of Baralyme® (Allied Healthcare Products, St Louis, MO) and Spherasorb® (Intersurgical, Wokingham, United Kingdom). These absorbents contain KOH or NaOH, respectively, in concentrations comparable to those added to the Superia® and Amsorb® samples.
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Methods
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For the moisture analysis, granules of Amsorb® and Superia® were ground into a fine powder. We prepared samples of pure Amsorb® (A), Amsorb® + 1% NaOH (B), Amsorb® + 3% NaOH (C), pure Superia® (D), Superia® + 1% NaOH (E), Superia® + 3% NaOH (F), and Superia® + 3% KOH (G). The NaOH and the KOH (analytical grade) were obtained from Merck, Darmstadt, Germany. Additionally, we prepared pulverized samples of Baralyme® (H), which contains a comparable percentage of KOH as the G samples, and Spherasorb® (I), which contains a comparable percentage of NaOH as the B and E samples. The weight of the samples (n = 5 in each series) was 3 ± 0.015 g. The samples were spread on a sample dish (internal diameter, 9 cm) of an electronic moisture analyzer (MA 30; Sartorius, Goettingen, Germany; resolution, 1 mg) and dried to a constant weight at 105°C for 6 min. We assumed that at this temperature, the hydroxides in the samples did not oxidize, and no degradation of any compound would occur but that all the water would be evaporated. The weight loss during drying was recorded digitally every 6 s to calculate the corresponding humidity values, with the underlying assumption being that the weight loss after complete drying reflects the initial and therefore total moisture content of the samples.
During drying, the samples were checked for a change in color every minute to determine the time to color change (TColor). Additionally, the intensity of the violet in each sample was assessed using a 4-point scale: 0 = white, the color of the fresh sample, 1 = weak violet, visible in some parts of the sample, 2 = weak violet, visible in the entire sample, and 3 = strong violet, the color at complete exhaustion of CO2 absorbing capacity. The color assessments were performed by a blinded observer who did not know the compositions of the samples.
The findings within each of three groups were compared using the Kruskal-Wallis test: the untreated absorbents (experimental Groups A, D, H, and I), the Amsorb® groups (A, B, and C), and the Superia® groups (D, E, and F). The findings in Groups F and G (Superia® with 3% NaOH or with 3% KOH, respectively) were compared using the Mann-Whitney test. For statistical calculations, the standard software package SPSS 7.5.2G (SPSS Inc, Chicago, IL) was used. P values <0.05 were considered to indicate statistical significance.
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Results
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In all samples, the weight decrease had already ended within the first 3 min of drying. The moisture content of the untreated samples varied significantly: (pure) Superia® > Baralyme® > (pure) Amsorb® > Spherasorb® (Table 1). The moisture content hardly changed in either Amsorb® or Superia® after alkali hydroxides were added.
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Table 1. The Time to Color Change (TColor), Water Content at TColor, and the Intensity of the Violet when Samples of Amsorb® and Superia® Containing Different Percentages of Added Alkali Hydroxides (NaOH or KOH) Were Drieda
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The shortest time to color change (TColor) and the most intense violet was seen in pure Amsorb® and pure Superia®. At TColor, the moisture content of both absorbents was still large (Table 1). With the addition of NaOH and with an increase in the percentage added (1% and 3%), TColor increased, and simultaneously the absorbents were significantly drier at TColor. The intensity of the violet on drying decreased significantly in both the Amsorb® and the Superia® samples after alkali hydroxides were added (Table 1). No color change was noted during the entire 6 min of drying either in the Superia® samples with added 3% KOH or in the Baralyme®. TColor and the intensity of the violet were comparable in the samples of Spherasorb®, Amsorb® with 1% NaOH, and Superia® with 1% NaOH.
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Discussion
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The present investigation has shown that the observation described in our previous study (2)—that Amsorb® changed color when drying—also applies to Superia®, another absorbent containing no alkali hydroxides. Furthermore, this study has shown that when absorbents are dried by heat, a color change like that elicited by drying with oxygen can be observed. Two findings were unexpected. First, the initial moisture content of the absorbents correlated neither with the time to nor the intensity of color change. Second, pure Amsorb® and Superia® changed color long before they were dry (Fig. 1). As more NaOH was added, the color change became less extensive and was observed at a later stage, i.e., when the moisture content was far less. When 3% KOH was added, no color change was noticed during the six-minute drying period.
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Figure 1. Water content of (A) Amsorb® and (B) Superia® samples with different content of NaOH or KOH during heat drying (105°C) in a moisture analyzer. The color change in pure Amsorb® or Superia® does not require complete dryness, but with increasing percentages of added NaOH, more drying is required to provoke a color change. With KOH, no color change occurs.
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In the first experiment of our previous study (2), a layer of Amsorb® covered different alkali hydroxide-containing absorbents that were dried by a gas flow directed from above. In this experiment, Amsorb® changed color at a very small loss of water content (1.3% ± 0.1% of the total weight of the absorbent material). We explained this finding by the particular setup thinking that the Amsorb® probably dried first. The results of the present study allow us to suggest an additional explanation, namely, that the rapid color change in Amsorb® is caused by its property of changing color long before it is completely dry. Moreover, the same property seems to explain the color change in Amsorb® in the second experiment of the previous study in which the gas flow inlet was at the bottom of the setup. In this setting, the Amsorb® changed color long before the samples were completely dry (at a water content of 8.8% ± 2.0%), although the water content of the Amsorb® layer alone could not have been less because it dried last.
The effect of NaOH, namely, of diminishing the color change, seems to depend on the amount of this alkali hydroxide added to Ca(OH)2 because the color change was weaker in the samples with 3% NaOH than in those with 1% NaOH. The extent of the color change furthermore seems to depend on which alkali hydroxide is added because a color change (though weak) was noted with 3% NaOH, whereas no color change was observed when 3% KOH was added. Moreover, a similar observation was also made with respect to Baralyme®, which contains 4.6% KOH (3). Similarly, the degree of color change in the Superia® and Amsorb® samples containing 1% NaOH (value of 2 on our grading scale) was the same as that in Spherasorb®, which contains 1.5% NaOH (as per the manufacturer’s data sheet).
In contradiction to our hypothesis, the color change did not correlate with the initial moisture content. Although the initial moisture content of pure Amsorb® is significantly different from that of pure Superia®, the time course and the intensity of color change during drying was the same in the two absorbents. Adding NaOH or KOH to Amsorb® or Superia® hardly changed the moisture content of either absorbent (no statistical difference), but both the time course and the intensity of the color change differed significantly.
The main results of the study that we wish to emphasize are that the color change in pure Amsorb® or Superia® did not require complete dryness, but the more NaOH added, the longer the drying required to provoke a color change, and with 3% KOH, there was no color change at all.
We assume that the color change in Amsorb® and Superia® on drying is caused by the color change in the pH indicator ethyl violet, which is added to many absorbents to indicate exhaustion of CO2 absorption (4). The color of the pure dry indicator is violet, and it shifts to the colorless form only when water is present and the pH value exceeds 10.3 (Fig. 2). In fact, the pH value reflects ionization of water, and so pH values cannot be determined in a dry medium. At first sight, this would explain why in Amsorb® and Superia®, ethyl violet changes color both on acidification and on drying. However, this does not explain why both absorbents changed color long before they were completely dry. At the same time, it remains to be explained why the addition of alkali hydroxides to absorbents delayed the occurrence of the color change and why the color change occurred only at a much smaller water content.
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Figure 2. Color change of ethyl violet to the violet form (right-hand side) because of protonation and dehydration.
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We would therefore like to propose an explanation for the color change during drying that takes into account the water solubility of the hydroxides. NaOH and KOH are the most water soluble of the hydroxides (at 20°C, 109 g/100 mL and 112 g/100 mL, respectively). In contrast, Ca(OH)2 is far less water soluble (at 20°C, 0.173 g/100 mL) (5). We surmise that during desiccation of an alkali hydroxide-free absorbent, the freely dissociated OH- ions from the Ca(OH)2 are quickly removed to form the inactive solid state species of the absorbent Ca(OH)2. That is why the pH in the absorbent subsequently decreases less than 10.3, yielding the observed color change to violet. This process is delayed or even hindered when the much more highly soluble alkali hydroxides are added. With increasing solubility of these hydroxides (KOH > NaOH) and at increasing concentrations, the alkalinity (concentration of OH- ions) of the absorbent increases, and increasing periods of drying are required to remove a sufficient amount of freely dissociated OH- ions for the color to change.
In clinical anesthesia, alkali hydroxide-free absorbents provide an increased margin of safety than absorbents containing alkali hydroxides because these produce minimal to zero Compound A or CO from anesthetic breakdown (1,3,6–9). Therefore, one might question the clinical relevance of investigating their property of indicating drying by change of color. However, in a recent study (10) in which we directed isoflurane through dried alkali hydroxide-free absorbents, we observed an unexpected loss of isoflurane, although CO formation was minimal. Regardless of whether the isoflurane loss proves to be caused by degradation or by adsorption, such loss should be prevented, because it may cause diminished delivery of the anesthetic that may go unnoticed. Moreover, the drying of an absorbent is always the result of an unnoticed waste of fresh gas, which should be avoided. In addition, the absorptive capacity of an absorbent decreases dramatically when dried completely (11), and, finally, a dry absorbent does not contribute to humidification of the inspired air (12). All these points considered that alkali hydroxide-free absorbents, too, should be prevented from drying out in clinical practice, even if this is not connected to a relevant production of CO or Compound A. Thus, the property of alkali hydroxide-free absorbents of changing color when drying provides an additional advantage in clinical use.
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Acknowledgments
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The authors thank Jane Neuda for editorial review. The thermogravimetric analyzer MA30 was kindly supplied by Sartorius GmbH Vienna.
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References
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- Andrews J. Inhaled anesthetic delivery systems. In: Miller R, ed. Anesthesia. 4th ed. New York: Churchill Livingston, 1994: 207–8.
- Physical properties. In: Dean JA, ed. Lange’s handbook of chemistry. 14th ed. New York: McGraw-Hill, 1992: 519–23.
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Accepted for publication February 25, 2003.
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Abstract
Introduction
