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2392 IEEE SENSORS JOURNAL, VOL. 12, NO. 7, JULY 2012 Calorimetric Sensor for H 2 O 2 /H 2 O Mist Streams Alexander J. Porkovich, Matthew D. Arnold, Galina Kouzmina, Brian Hingley, and Michael B. Cortie Abstract— Mist streams comprised of H 2 O 2 /H 2 O droplets are a recent innovation for disinfecting medical equipment, but the availability of a sensor that could monitor the concentration of H 2 O 2 applied during the treatment would be desirable. Here we describe a means to obtain a rapid estimation of H 2 O 2 concentration in this environment. The proposed sensor is based on a platinum resistance thermometer coated with a layer of MnO 2 catalyst. It may be calibrated to operate either during the mist delivery step of a disinfection cycle, or during the evacuation (drying) phase. Cooling of the sensor surface due to evaporation of H 2 O and effervescence of decomposing H 2 O 2 operates against heat generated by the decomposition reaction to produce a well- defined minimum in the temperature. The time and temperature at which this minimum occurs are well correlated, with the H 2 O 2 content of the solution used to produce the mist droplets. Index Terms— Biomedical engineering, chemical engineering, chemical sensors, sensors. I. I NTRODUCTION C ONCENTRATED solutions of H 2 O 2 in water have diverse applications in industry, including sterilization and disinfection [1, 2], bleaching [3], rocket propulsion [4], and the production of epoxides and organic peroxides [5]. These uses are due to the strong oxidizing properties of H 2 O 2 . Unfortunately, H 2 O 2 is a relatively unstable compound, and solutions of it slowly decompose to H 2 O [6, 7]. The decom- position process takes place over weeks and months, and is exacerbated by high storage temperatures or the presence of chemical impurities. Therefore, there are some applications, such as in sterilization, where it would be advisable to verify the actual H 2 O 2 concentration applied. Here we are interested in determining the H 2 O 2 concentration in a mist stream at ambient temperature. The H 2 O 2 is present in small droplets of 1μm-10 μm diameter at 30-40 wt.% concentration suspended in an air stream. These mist streams are used in a new generation of medical sterilizing cabinets. H 2 O 2 can be determined in other contexts by sensor technologies based on chemiluminescent, electrochemical or calorimetric principles [8-14]. However, the existing sensor Manuscript received July 10, 2011; accepted February 28, 2012. Date of publication March 9, 2012; date of current version May 24, 2012. This work was supported in part by Nanosonics Ltd., Australia. The associate editor coordinating the review of this paper and approving it for publication was Prof. Istvan Barsony. A. J. Porkovich, M. D. Arnold, and M. B. Cortie are with the Institute for Nanoscale Technology, University ofTechnology Sydney, Sydney 2007, Aus- tralia (e-mail: [email protected]; matthew.arnold-1@ uts.edu.au; [email protected]). G. Kouzmina and B. Hingley were with the Institute for Nanoscale Technology, University of Technology Sydney, Sydney 2007, Australia. They are now with Nanosonics Ltd., Alexandria 2015, Australia (e-mail: [email protected]; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSEN.2012.2190504 technologies are not appropriate for the H 2 O 2 -containing mist streams required in the new generation of medical sterilizing devices. Chemiluminescent sensors are problematic due to their nature: they require reagents to react with the H 2 O 2 mist and to create a color change, meaning the sensor will be disposable. Some sensors may be made to be reusable but will require chemical “refreshing”. Quantitative chemical analysis of H 2 O 2 is available by a variety of techniques but this requires condensation and collection of the mist so it can be spectrally analyzed, which is overly complex for this application. Electrochemical sensors are based on the flow of an electrical current during the reaction of H 2 O 2 with electro-active electrode materials [9, 10]. Most electrochemical sensors for H 2 O 2 have been based on enzymes, which are problematic in this case as they only operate in a specific pH range and at relatively low concentrations of peroxide [10]. Some recent work has exploited metal oxide catalysts for electrochemical analysis, but the main problem is still that the sensor becomes saturated at lower concentrations than are required here [9]. Calorimetric principles have also recently been developed to characterize hot (270 °C), concentrated (up to 10 v/v %) H 2 O 2 vapor flows [14]. The principle is that H 2 O 2 is decomposed on their surface by a catalyst which causes an increase of temperature recorded by the sensor (either thermometer or a thermocouple). In the present case, however, the analyte is a two-phase flow containing a mixture of gases (air, H 2 O vapor and H 2 O 2 vapor), and liquid (H 2 O/H 2 O 2 droplets) at about ambient temperature. Consideration of the H 2 O/H 2 O 2 phase diagram shows that, at room temperature, the H 2 O 2 vapor pressure in the gas phase in equilibrium with a 35 wt% H 2 O 2 solution would be about 20-30 Pa or only 0.02-0.03 vol.% [7]. This is considerably less than the 1-10 vol.% range used in the types of gas-phase calorimetric sensors for H 2 O 2 that have been previously reported, e.g. by Schöning et al. [14]. Therefore, the low vapor pressure of H 2 O 2 in the present process stream would produce a T that is too small to detect by standard calorimetric means. On the other hand, aqueous solutions of H 2 O 2 of 1 wt% and upwards contain a far higher activity of the substance and will readily generate a calorimetric response [11]. Since some condensation of the mist droplets on a sensor surface could be expected to occur in the present instance, and since the far greater proportion of H 2 O 2 would partition to the liquid than to the gas phase, we concluded that a calorimetric sensing technology could be a reasonable basis for monitoring the concentration of liquid-phase H 2 O 2 in mist streams. The key issue in this case is that it is the H 2 O 2 in the liquid phase that is measured, and not that in the vapor phase. Unfortunately, the heat capacity and latent heat of evaporation of the liquid H 2 O component of the mist is of comparable or 1530–437X/$31.00 © 2012 IEEE

Calorimetric Sensor for ${\rm H}_{2}{\rm O}_{2}/{\rm H}_{2}{\rm O}$ Mist Streams

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Page 1: Calorimetric Sensor for ${\rm H}_{2}{\rm O}_{2}/{\rm H}_{2}{\rm O}$ Mist Streams

2392 IEEE SENSORS JOURNAL, VOL. 12, NO. 7, JULY 2012

Calorimetric Sensor for H2O2/H2O Mist StreamsAlexander J. Porkovich, Matthew D. Arnold, Galina Kouzmina, Brian Hingley, and Michael B. Cortie

Abstract— Mist streams comprised of H2O2/H2O droplets area recent innovation for disinfecting medical equipment, but theavailability of a sensor that could monitor the concentration ofH2O2 applied during the treatment would be desirable. Herewe describe a means to obtain a rapid estimation of H2O2concentration in this environment. The proposed sensor is basedon a platinum resistance thermometer coated with a layer ofMnO2 catalyst. It may be calibrated to operate either during themist delivery step of a disinfection cycle, or during the evacuation(drying) phase. Cooling of the sensor surface due to evaporationof H2O and effervescence of decomposing H2O2 operates againstheat generated by the decomposition reaction to produce a well-defined minimum in the temperature. The time and temperatureat which this minimum occurs are well correlated, with the H2O2content of the solution used to produce the mist droplets.

Index Terms— Biomedical engineering, chemical engineering,chemical sensors, sensors.

I. INTRODUCTION

CONCENTRATED solutions of H2O2 in water havediverse applications in industry, including sterilization

and disinfection [1, 2], bleaching [3], rocket propulsion [4],and the production of epoxides and organic peroxides [5].These uses are due to the strong oxidizing properties of H2O2.Unfortunately, H2O2 is a relatively unstable compound, andsolutions of it slowly decompose to H2O [6, 7]. The decom-position process takes place over weeks and months, and isexacerbated by high storage temperatures or the presence ofchemical impurities. Therefore, there are some applications,such as in sterilization, where it would be advisable to verifythe actual H2O2 concentration applied. Here we are interestedin determining the H2O2 concentration in a mist stream atambient temperature. The H2O2 is present in small droplets of1μm-10 μm diameter at 30-40 wt.% concentration suspendedin an air stream. These mist streams are used in a newgeneration of medical sterilizing cabinets.

H2O2 can be determined in other contexts by sensortechnologies based on chemiluminescent, electrochemical orcalorimetric principles [8-14]. However, the existing sensor

Manuscript received July 10, 2011; accepted February 28, 2012. Date ofpublication March 9, 2012; date of current version May 24, 2012. This workwas supported in part by Nanosonics Ltd., Australia. The associate editorcoordinating the review of this paper and approving it for publication wasProf. Istvan Barsony.

A. J. Porkovich, M. D. Arnold, and M. B. Cortie are with the Institute forNanoscale Technology, University of Technology Sydney, Sydney 2007, Aus-tralia (e-mail: [email protected]; [email protected]; [email protected]).

G. Kouzmina and B. Hingley were with the Institute for NanoscaleTechnology, University of Technology Sydney, Sydney 2007, Australia.They are now with Nanosonics Ltd., Alexandria 2015, Australia (e-mail:[email protected]; [email protected]).

Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSEN.2012.2190504

technologies are not appropriate for the H2O2-containing miststreams required in the new generation of medical sterilizingdevices. Chemiluminescent sensors are problematic due totheir nature: they require reagents to react with the H2O2mist and to create a color change, meaning the sensor willbe disposable. Some sensors may be made to be reusablebut will require chemical “refreshing”. Quantitative chemicalanalysis of H2O2 is available by a variety of techniques butthis requires condensation and collection of the mist so itcan be spectrally analyzed, which is overly complex for thisapplication. Electrochemical sensors are based on the flowof an electrical current during the reaction of H2O2 withelectro-active electrode materials [9, 10]. Most electrochemicalsensors for H2O2 have been based on enzymes, which areproblematic in this case as they only operate in a specificpH range and at relatively low concentrations of peroxide[10]. Some recent work has exploited metal oxide catalystsfor electrochemical analysis, but the main problem is still thatthe sensor becomes saturated at lower concentrations than arerequired here [9]. Calorimetric principles have also recentlybeen developed to characterize hot (270 °C), concentrated (upto 10 v/v %) H2O2 vapor flows [14]. The principle is thatH2O2 is decomposed on their surface by a catalyst whichcauses an increase of temperature recorded by the sensor(either thermometer or a thermocouple).

In the present case, however, the analyte is a two-phaseflow containing a mixture of gases (air, H2O vapor and H2O2vapor), and liquid (H2O/H2O2 droplets) at about ambienttemperature. Consideration of the H2O/H2O2 phase diagramshows that, at room temperature, the H2O2 vapor pressure inthe gas phase in equilibrium with a 35 wt% H2O2 solutionwould be about 20-30 Pa or only 0.02-0.03 vol.% [7]. This isconsiderably less than the 1-10 vol.% range used in the typesof gas-phase calorimetric sensors for H2O2 that have beenpreviously reported, e.g. by Schöning et al. [14]. Therefore,the low vapor pressure of H2O2 in the present process streamwould produce a �T that is too small to detect by standardcalorimetric means. On the other hand, aqueous solutions ofH2O2 of 1 wt% and upwards contain a far higher activity ofthe substance and will readily generate a calorimetric response[11]. Since some condensation of the mist droplets on a sensorsurface could be expected to occur in the present instance,and since the far greater proportion of H2O2 would partitionto the liquid than to the gas phase, we concluded that acalorimetric sensing technology could be a reasonable basisfor monitoring the concentration of liquid-phase H2O2 in miststreams. The key issue in this case is that it is the H2O2 in theliquid phase that is measured, and not that in the vapor phase.Unfortunately, the heat capacity and latent heat of evaporationof the liquid H2O component of the mist is of comparable or

1530–437X/$31.00 © 2012 IEEE

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PORKOVICH et al.: CALORIMETRIC SENSOR FOR H2O2/H2O MIST STREAMS 2393

Fig. 1. Schematic diagram of the apparatus to test sensor efficacy in streamsof H2O2/H2O mist.

greater magnitude to the energy released by the decompositionof the H2O2 component. Therefore, the calorimetric signalis of a relatively complex nature. Nevertheless, as we willshow, it is possible to extract a usable estimate of H2O2concentration.

II. EXPERIMENTAL

A. Test Rig Description

The test rig used for sensor investigation, Fig. 1, comprisesthree main modules: (1) the mist generating module whichconsists of a reservoir for H2O2 solution and a submergedpiezoelectric transducer used to generate the mist by ultrasonicnebulization, (2) the mist disinfection chamber in which thesensors are located and, finally, (3) a mist exhaust modulefor decomposition of unreacted H2O2 into water and oxygen,prior to venting into the environment.

To provide consistency in mist production, the level of H2O2solution in the reservoir was controlled with a liquid levelsensor interfaced to a pump fed by a bottle of H2O2 solution toensure that the volume of liquid above the transducer remainedrelatively constant in the reservoir. The level was controlledto that required to focus all the ultrasonic energy at the liquidsurface for maximum efficiency of mist generation. Also, thepiezoceramic transducer was driven by pulse width modulationto control the rate of mist generation.

The mist generated by the transducer was injected intoa stream of laboratory air of controlled temperature andhumidity which was blown into the disinfection chamber bya fan. The combined stream of air and mist droplets wasblended over a baffle and then flowed relatively evenly throughdisinfection chamber. As it moved through the chamber, themist passed over and interacted with the sensor which wasorientated so that the catalyst coating faced the mist directly.

B. Basis of H2O2 Sensor

The sensor itself is a Pt resistance thermometer coated witha catalyst for the decomposition of H2O2. The decompositionreaction is exothermic and, all else being equal, would beexpected to raise the temperature, and hence resistance, ofthe Pt thermometer. The change in temperature was measuredusing a high-precision digital multimeter in two-wire config-uration, which was connected to a computer to convert andlog the data as temperature against time. An uncoated sensorwas also included in the system as a reference. This measuredthe temperature within the mist stream in the absence ofany catalytic effects. Calibration of the multimeters with areference resistor indicated that a precision of about 0.02 °Ccould be theoretically achieved, while testing of the RTDsthemselves indicated an overall accuracy of measurement of0.1 °C, provided that RTDs of the same type were used forboth the sensing and reference measurements.

C. Catalyst

A variety of substances, including Cu, Cu-Ni, PbO2, Ag,MnO2 and other manganese oxides, and mesoporous platinum[11, 12] will catalyze the decomposition of H2O2 via

H2O2(aq) → H2O(1) + 1/2O2(g) (1)

with �G25 °C = −134.1 kJ/mol H2O2 and �H25 °C =−191.2 kJ/mol when in a standard state aqueous solution [15].

Here we made use of MnO2 powder as a catalyst, usedin chemically pure form as supplied by May and Baker Ltd.A slurry of powdered MnO2 was prepared by mixing withethanol and dip-coating the thermometer into this. The blackMnO2 adhered to form a continuous layer on the thermometer.The X-ray diffraction pattern of the powder, Fig. 2, showedthat it was comprised of β-MnO2 (‘pyrolusite’) which hasthe rutile crystal structure. However, the broad nature of the(200), (210) and (310) peaks indicates that the material is notcompletely crystallized.

D. Effect of Vapor Phase H2O2 on Sensor Temperature

As mentioned above, it was predicted that the low vaporpressure of H2O2 in the present mist streams would notcause any perceptible deviation in the temperature of thecoated sensor relative to the uncoated one in purely gaseousenvironments. This was verified by suspending the coatedand uncoated sensors above 35% H2O2 solution in a sealedbeaker and monitoring their temperature difference over time.The environment surrounding the sensors would have con-tained air saturated with H2O2. Beaker temperatures between

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2394 IEEE SENSORS JOURNAL, VOL. 12, NO. 7, JULY 2012

Fig. 2. X-ray diffraction pattern of MnO2 catalyst powder used. Also shownare the lines for the JC-PDF card 00-024-0735 for β-MnO2 (pyrolusite).

ambient and 40 °C were investigated but no significantdeviation in temperature between the two sensors observed.Therefore, any calorimetric signal observed in the actual miststream of the test rig would necessarily have to be related tothe H2O2 content of the liquid phase of the mist rather thanto that in the gas phase.

E. Mist Characteristics

The mist in the apparatus was generated by an ultrasonicpiezoceramic transducer (Fuji Ceramic Corporation) operatedat a frequency of 2.4 MHz and was transported into the disin-fection chamber by the air flow. The air flow was maintained atthe constant level of 7.5 L/min, flowing through a cross-sectionof 1.15 × 10−2 m2. The rate of mist generation (g/min) wasfound to be a function of transducer voltage (30 V), averageoperating power (“duty cycle”), H2O2 concentration and liquidtemperature. The mist density (mass/volume) is a function ofmist generation and flow velocity. The mist generation processbecomes less efficient at higher H2O2 concentrations becauseof the increase in density and surface tension of the solutionand therefore this factor needed to be determined.

Solutions with H2O2 concentrations between 0% and 40%were tested at various transducer duty cycles, the solutionshaving been made by diluting a standardized stock solutionwith highly purified MilliQ water. The concentration of thediluted solutions was verified with a classical iodometrictitration method [16]. For the operating conditions mentionedabove and a duty cycle of 20%, the mist flux was found todecrease from 0.7 to 0.4 g.m−2s−1 as H2O2 concentration wasincreased from 0 to 40%.

The size distribution of the droplets in the present study wasevaluated using a laser diffraction technique with a Malvernanalyzer MastersizerS. Measurements were conducted by theemployment of an analogous mist generation system with anidentical Fuji ultrasonic transducer. A 35% of H2O2 solutionwas agitated at the same frequency of 2.4 MHz at ambienttemperature. Droplet size was measured in a mist stream atthe volumetric flow rate of 10 L/min.

As can be seen in Figure 3, the ultrasonic transducerproduces a distribution of droplet sizes, which range in size

Fig. 3. Typical distribution of droplet sizes produced by ultrasonic atomizerfrom 35% H2O2 fluid. These measurements were not obtained under test rigconditions and are therefore indicative only. Droplet size might also vary withH2O2 concentration.

from 0.5 μm to 10 μm. However, most particles are between3 and 5 μm, with an average size of 3.7 μm. It was calculatedthat 65% of particles were this size or smaller. (The transducervoltage used in the sizing experiments was 26 V whereas weused 30 V in the sensing experiments. In our opinion this smalldifference should not have had a major effect on droplet size.)

III. RESULTS

A series of prototypical temperature profiles on the coatedand uncoated sensors in a 20% H2O2 mist stream is shownin Fig. 4. Each cycle consists of two parts, a two minute‘mist delivery step’ (during which the mist droplets are flowedthrough the disinfection chamber), and a ‘mist evacuation’ step(during which air continues to flow through the chamber andany condensed liquids evaporate). The start and end of the mistdelivery pulse are also indicated. Three successive mist cyclesare shown. The temperatures of the sensors have been offset tobe relative to the temperature at the end of each mist pulse. Useof this datum helped suppress variability due to small changesin rig and laboratory temperature prior to the onset of anyparticular sterilization cycle. This is because the measurementsshown have always been taken after several prior sterilizationcycles, so the various parts of the apparatus (including thesensors) are not necessarily in thermal equilibrium with theambient environment. In particular, the uncoated sensor willhave been cooled by the evaporation of H2O during the prior‘mist evacuation’ step, so its starting temperature will be lowerthan ambient. In contrast, the sensor coated with catalyst startsthe measurements closer to ambient due to the heating on itssurface that occurred in the prior cycle.

Cycles with a ‘mist delivery’ step from thirty seconds tofour minutes duration were also tested. The thirty secondsmist pulse was too short to permit thermal equilibrium to bereached on the sensor surface whereas the rather long fourminute ‘mist delivery’ step simply slowed the throughput ofthe measurement process. Cycles with two and three minute‘mist delivery’ pulses were found to be convenient for accen-tuating the thermal effects.

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PORKOVICH et al.: CALORIMETRIC SENSOR FOR H2O2/H2O MIST STREAMS 2395

Fig. 4. Prototypical measurement cycles for a mist pulse of 2-min durationproduced using 20% H2O2. Differential temperature profiles on (a) bare and(b) coated sensors are shown, with the reference temperature for each sensorbeing its temperature at the end of the mist pulse, t5, a condition that wedesignate as the “wet sensor.” The mist pulse, t1−t5, is schematically indicatedby the dashed black line.

There is a delay of about 15 seconds after the transducer isturned on at t = t1 before the sensors respond at t = t2, and asimilar delay when the mist pulse is turned off at t = t5. This iscaused by the need for the mist droplets to be transported intoor out of the disinfection chamber. Furthermore, the mist pulseevidently reached the coated sensor slightly sooner than theuncoated one. However, once the mist reached the uncoatedsensor, its temperature rose rapidly to that of the mist streamat t = t3, after which it was relatively stable until the mistwas turned off at t = t5. The temperature of the reservoir isalways lower than the ambient air due to evaporative cooling,but the droplets generated from it are rapidly heated in the airstream. The net effect for the particular configuration that weused is that the temperature of the mist lay about half waybetween the ambient air and the reservoir. However, operationof the ultrasonic transducer generates heat too, which slightlyincreases reservoir temperature (about 0.2 °C during a twominute mist delivery pulse), causing the temperature of thetrailing edge of the mist pulse to be slightly elevated relative tothat of the leading edge. Once the transducer is turned off and

the mist evacuation step initiated there is an overall decline intemperature of the uncoated sensor due to evaporative coolingof its surface in the now unsaturated air stream. The uncoatedsensor is dry at about t7, after which it is slowly raised backtowards room temperature by the air stream in the rig. Theprocess of warming is however interrupted by the impositionof the subsequent mist delivery step, in this case at t = 600 s.

On the coated sensor the first effect of the mist pulse isalways to cause a small spike in temperature at t2. We believethis to be the result of the H2O2 content of the mist dropletson the leading edge of the pulse having been concentratedupwards by evaporation of their H2O into the surroundingdrier air of the ambient state that lies immediately in frontof the mist pulse. Hence these concentrated droplets generatesignificant heat on the catalyst surface when they first strikethe sensor. However, once the main body of the mist pulsereaches the sensor, the vapor phase is fully saturated withH2O and little further upwards concentration of H2O2 canoccur in the droplets by this means. At this stage the coatedsensor is actually forced down in temperature by the incomingflux of liquid. The cooling effect saturates at about t = t4.Comparison of the absolute temperatures of the coated anduncoated sensors at this point indicated that the temperatureon the coated sensor was within 0.1 °C of that of the uncoatedone, ie. the heat capacity of the H2O in the incoming flux ofmist droplets dominates the energy balance. Surprisingly, afterthe mist is turned off (t5) the coated sensor cools further, toa minimum temperature at t = t6, a characteristic and well-defined inflection point. This cooling is clearly caused, at leastin part, by evaporative cooling, a point that will be discussedlater. The coated sensor then starts to heat up rapidly, untilat t = t8 the factor causing the heating is largely exhausted.After t8 the coated sensor then maintains about the same rateof temperature rise as the uncoated sensor as both warm inthe stream of ambient air, the effect being interrupted by theonset of the next mist cycle.

The temperatures on each sensor during the mist deliverystep between t4 and t5 correspond to a state in which theconstant condensation of mist droplets has brought each sensorinto a state of relative thermal equilibrium with the mist streamand, as mentioned, these temperatures were found to be auseful datum to which the subsequent temperature transientson the sensors could be referred.

From the point-of-view of developing the present sensor,the most important thermal effects are those that correlatebest with H2O2 content. Various parameters implicit in Fig. 4were analyzed for their suitability as a means of estimatingH2O2 content. Although it was originally anticipated that asignal based on the temperature difference between the twosensors (for example, temperature difference between coatedand uncoated sensors at t5) would be suitable, this was foundto be not the case. Apparently, the various temperature changesassociated with the phenomena described are of similar mag-nitude to the variations in temperature within the mist streamand across the physical structure of the rig itself, and thedifference signal is hence swamped by (a) experimental noiseand (b) energy flows due to phase change and heat capacityof the H2O component.

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2396 IEEE SENSORS JOURNAL, VOL. 12, NO. 7, JULY 2012

Fig. 5. (a) Temperature change of a coated sensor between start and endof a 3-min mist delivery pulse, compared for different H2O2 concentrations.The ultrasonic transducer was turned on at 60 s and turned off at 240 s.(b) Temperature change as a function of H2O2 content of the liquid in thereservoir. The solid line (r2 = 0.94) was obtained by regression on the data upto 20% H2O2. (c) Reverse regression to give estimate of %H2O2 as a functionof temperature change. The dashed lines correspond to the 95% confidenceinterval for any individual predicted value of %H2O2.

The surprising cooling of the coated sensor was certainlysensitive to %H2O2 even although the energy absorbed byevaporation of H2O and H2O2 from the sensor is clearlymuch larger than the energy generated by the exothermic

catalytic decomposition of H2O2, at least during the earlierparts of the measurement cycle (t < t6). (The interplay ofthese phenomena will be considered further in the Discussionsection.) The cooling effect on the coated sensor overwhelmsthe exothermic reaction until t = t6. At that time the exother-mic nature of the decomposition reaction becomes dominantand the remaining H2O2 is decomposed, accompanied by arapid rise in temperature. The H2O2 is evidently almost totallyconsumed by t = t8.

The extent of the cooling effect depends on the H2O2concentration, a situation shown in Fig. 5(a) for mist deliverysteps of three minutes duration. If the extent of cooling duringthe mist delivery step (t1 < t < t5) is taken, then thereis a good correlation with H2O2 concentration in the lower(0-20 wt.%) range but no predictive capability at all for%H2O2 > 20%, Fig. 5(b). The difference between endingand starting temperatures of the sensor in the mist pulsecan be used to estimate the H2O2 concentration with a 95%confidence prediction interval that is about ± 4 wt.%, Fig. 5(c).Unfortunately, the temperature at which the sensor started themist delivery step was influenced by conditions prior to it (inparticular the duration of the previous mist evacuation step)so this method of obtaining H2O2 concentration was judgedto be fundamentally problematic.

Cooling of the coated sensor during the mist evacuationstep was, however, usable over a greater range of H2O2concentration. This parameter was obtained from the changein temperature between t5 and t6, Fig. 6(a). It was also notedthat the characteristic times associated with the inflection att6 on the trace of temperature vs. time for the coated sensor(ie. t6 − t5) were well correlated with the H2O2 content too,Fig. 6(b). In contrast, t7 − t5, the equivalent parameter for theuncoated sensor, was very poorly correlated to H2O2 content.The error-prone nature of determinations of t7 is evidently dueto the prolonged nature of the inflection phenomenon on theuncoated sensor, which makes determination of the time to theinflection point on it problematic.

An estimate of H2O2 concentration could therefore beobtained from either the difference-in-temperature or the time-to-inflection by performing a polynomial regression of H2O2%on the relevant thermal parameter. Furthermore, since theparameters difference-in-temperature (Fig. 6(a)) and time-to-inflection (Fig. 6(b)) are relatively independent of one another,an improved estimate for H2O2 content could then be obtainedby averaging the H2O2 concentration predicted from thedifference-in-temperature signal shown in Fig. 6(a) and thatobtained from the time-to-inflection signal shown in Fig. 6(b).The result is shown in Fig. 6(c). The 95% confidence intervalof an individual estimate obtained by this means is ±5 wt.%H2O2 but of course that of the mean of several measurementswould be much tighter (±1 wt.% H2O2, dotted lines on figure).

IV. DISCUSSION

The anomalous cooling of the coated sensor is evidentlydue in some way to the effect of the effervescence associatedwith the decomposition reaction, Fig. 7. In the absence ofsuch cooling, the temperature on the coated sensor should have

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PORKOVICH et al.: CALORIMETRIC SENSOR FOR H2O2/H2O MIST STREAMS 2397

Fig. 6. Estimation of H2O2 concentration in the liquid phase obtained byanalyzing thermal transients during the mist evacuation step. (a) Difference intemperature between end of mist pulse (t5) and inflection point (t7), for coatedsensor. (b) Time to inflection point from end of mist pulse (t7 − t5), for coatedsensor. (c) Empirical fits shown in (a) and (b) can be combined (see text) toyield a robust estimate (sensor H2O2%) of the actual H2O2 concentration.The dashed lines are the 95% confidence interval for the predicted value ofany individual value of %H2O2 using the sensor %H2O2 as an independentvariable, while the symbols show the actual %H2O2 values. The 95% CI forthe mean of several estimates of actual %H2O2 is shown as a pair of dottedlines.

been greater than that on the uncoated sensor. An enhancementof evaporative cooling due to effervescence was suspectedto be the cause, especially after the mist pulse had ended.

Fig. 7. Effervescence of H2O2-containing droplets on the surface of thecoated sensor causes intense stirring of the liquid layer and the developmentof a froth. The effect shown here was obtained by pipetting a considerableexcess of H2O2 solution onto the sensor.

(During the mist pulse itself the gas phase would be closeto saturated with H2O by the time it reaches the sensors andso further evaporative processes should be negligible.) In fact,the energy required to completely evaporate the H2O portionof, for example, a 30% H2O2 liquid phase, is of the sameorder as that generated by the complete decomposition of theH2O2 portion. Therefore, if evaporation proceeds faster thandecomposition, cooling will occur at first, followed later byheating. We suggest that the inflection point at t6 correspondsto this cross-over.

There is potentially one additional explanation for theendothermic situation that prevails at t < t6. We note thatthe decomposition reaction proceeds under non-equilibriumconditions, and in particular any kinetic energy shed bythe decomposing liquid into the process stream is ‘lost’ tothe sensor. The bubbles of O2 produced must, of necessity,be associated with significant mechanical energy due to theeffects of surface tension. When bubbles burst, there is arelease of this mechanical energy in the form of a highenergy jet of liquid, which can transport some energy awayfrom the froth. In a closed calorimeter, such kinetic energy isconverted back to thermal energy which is captured in a heatmeasurement. Here, however, this flux of energy is lost to thesensor and represents a source of cooling. We surmise that acombination of ordinary convective heat transfer (acceleratedby the perturbation of the boundary layer by effervescence)and the mechanical energy transported outwards by burstingbubbles is responsible for the surprising cooling of the coatedsensor.

V. CONCLUSION

The concentration of H2O2 in a stream of mist passingthrough a sterilizing chamber can be estimated by analysisof the transient thermal effects taking place on a platinumresistance thermometer that has been coated with a catalyst.Although the decomposition itself is exothermic, the thermal

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2398 IEEE SENSORS JOURNAL, VOL. 12, NO. 7, JULY 2012

signal is initially dominated by the evaporation and/ormechanically-induced heat loss that takes place in theeffervescing froth on sensor surfaces coated with catalyst.The H2O2 concentration in the range 0-20 wt% can beobtained from the difference in temperature on the coatedsensor between beginning and end of the mist delivery step.However, this parameter has no predictive value for higherconcentrations of H2O2. On the other hand, the time taken toreach the minimum temperature during the evacuation step,or the minimum sensor temperature during the evacuationstep relative to the starting temperature, can both be usedto estimate H2O2 concentration between 0 and about 40%H2O2. Taken in combination, these parameters can providea useful indication of the concentration of H2O2 in the mistdroplets, with the 95% confidence interval for the mean ofseveral determinations corresponding to ±1 H2O2% of thecorrect value, while the 95% confidence interval for a singleprediction spans ±5 H2O2% of the correct value.

REFERENCES

[1] N. A. Klapes and D. Vesley, “Vapor-phase hydrogen peroxide as asurface decontaminant and sterilant,” Appl. Environ. Microbiol., vol. 56,pp. 503–506, Feb. 1990.

[2] B. M. Andersen, M. Rasch, K. Hochlin, F. H. Jensen, P. Wismar, andJ. K. Fredriksen, “Decontamination of rooms, medical equipment andambulances using an aerosol of hydrogen peroxide disinfectant,” J. Hosp.Infect., vol. 62, no. 2, pp. 149–155, Feb. 2006.

[3] R. Hage and A. Lienke, “Applications of transition-metal catalysts totextile and wood-pulp bleaching,” Angew. Chem. Int. Edition, vol. 45,pp. 206–222, Dec. 2006.

[4] S. S. Pietrobon, “High density liquid rocket boosters for the spaceshuttle,” J. British Interplanet. Soc., vol. 52, pp. 163–168, May–Jun.1999.

[5] G. Grigoropoulou, J. H. Clark, and J. A. Elings, “Recent developmentson the epoxidation of alkenes using hydrogen peroxide as an oxidant,”Green Chem., vol. 5, no. 1, pp. 1–7, 2003.

[6] G. Goor, J. Glenneberg, and S. Jacobi, “Hydrogen peroxide,” in Ull-mann’s Encyclopedia of Industrial Chemistry, Electronic Release, H.Noethe, Ed. New York: Wiley, 2007.

[7] W. Eul, A. Moeller, and N. Steiner, “Hydrogen peroxide,” in Kirk-Othmer Encyclopedia of Chemical Technology, Electronic Release, M.Grayson, Ed. New York: Wiley, 2001.

[8] B. Li, Z. Zhang, and L. Zhao, “Chemiluminescent flow-through sensorfor hydrogen peroxide based on sol-gel immobilized hemoglobin ascatalyst,” Anal. Chim. Acta, vol. 445, no. 2, pp. 161–167, Oct. 2001.

[9] D. T. V. Anh, W. Olthuis, and P. Bergveld, “Electroactive gate materialsfor a hydrogen peroxide sensitive EMOSFET,” IEEE Sensors J., vol. 2,no. 1, pp. 26–33, Feb. 2002.

[10] Y. Song, L. Wang, C. Ren, G. Zhu, and Z. Li “A novel hydrogen peroxidesensor based on horseradish peroxidase immobilized in DNA films ona gold electrode,” Sensors Actuat. B: Chem., vol. 114, no. 2, pp. 1001–1006, Apr. 2006.

[11] A. J. Porkovich, M. D. Arnold, G. Kouzmina, B. Hingley, A. Dowd,and M. B. Cortie, “Calorimetric sensor for use in hydrogen peroxideaqueous solutions,” Sensor Lett., vol. 9, no. 2, pp. 695–697, Apr. 2011.

[12] N. Näther, L. M. Juárez, R. Emmerich, J. Berger, P. Friedrich, andM. J. Schöning, “Detection of hydrogen peroxide (H2O2) at exposedtemperatures for industrial processes,” Sensors, vol. 6, no. 4, pp. 308–317, Apr. 2006.

[13] S. Reisert, H. Henkel, A. Schneider, D. Schäfer, P. Friedrich, J. Berger,and M. J. Schöning, “Development of a handheld sensor system for theonline measurement of hydrogen peroxide in aseptic filling systems,”Phys. Status Solidi A, vol. 207, no. 4, pp. 913–918, Apr. 2010.

[14] P. Kirchner, B. Lia, H. Spelthahna, H. Henkelc, A. Schneiderc, P.Friedrichd, J. Kolstade, M. Keusgenf, and M. J. Schöning, “Thin-filmcalorimetric H2O2 gas sensor for the validation of germicidal effectivityin aseptic filling processes,” Sensors Actuat. B: Chem., vol. 154, no. 2,pp. 257–263, 2009.

[15] J. A. Dean, Lange’s Handbook of Chemistry, 15th ed. New York:McGraw Hill, 1999.

[16] G. Svehla, “Chapter IV. Reactions of the anions,” in Vogel’s Textbookof Macro and Semimicro Qualitative Inorganic Analysis, 5th ed. NewYork: Longman, 1979.

Alexander J. Porkovich received the B.S. (hons.) degree in nanotechnologyinnovation from the University of Technology, Sydney, Australia, in 2007,where he is currently pursuing the Ph.D. degree.

His current research interests include the development of hydrogen per-oxide sensors, specifically for the determination and discrimination of high-concentration hydrogen peroxide mists.

Matthew D. Arnold received the B.Sc. (hons.) and Ph.D. degrees in physicsfrom the University of Otago, Otago, New Zealand.

He is a Lecturer with the University of Technology, Sydney, Australia. Hiscurrent research interests include technological spheres, especially in optics,electronics, and sensors.

Galina Kouzmina received the B.S. degree in chemistry, physics, andmathematics, the M.S. degree in analytical chemistry from Kharkov NationalUniversity, Kharkiv, Ukraine, and the Ph.D. degree, which was focused on theuse of radioactive isotopes for tracing fossil fuel filtration through tectonicplates, from VNII Geosystem—National R&D Center, Moscow, Russia.

She was a Senior Scientist with Nanosonics Ltd., Sydney, Australia,from 1992 to 2009, where she was conducting research and developmentinto novel hydrogen peroxide mist technologies for bio-decontamination ofconfined spaces and medical instruments. Since 2009, she has been involvedin conducting analytical procedures with the Analytical Center, University ofNew South Wales, Sydney.

Brain Hingley received the B.E. degree in electrical engineering from theUniversity of Technology, Sydney, Australia, in 1990.

He was with the School of Chemistry, Macquarie University, MacquariePark, Australia, from 1990 to 1998, where he was involved in research onchemical instrumentation, which also included the development of microam-perometry sensors. From 1998 to 2001, he was with the Anglo-AustralianObservatory designing the control electronics for both microfiber positioningsystems and vacuum cryogenic optics. From 2001 to 2003, he was involvedin research on hydrophone sensors and arrays for military and security use.He was a Freelancer from 2004 to 2006, focusing mainly on biomedicalelectronics for research and teaching use. Since 2007, he has been involvedin the design of control and sensing electronics for medical disinfection withNanosonics Ltd., Sydney.

Michael B. Cortie received the B.Sc. degree in engineering and the Ph.D.degree in metallurgical engineering from the University of the Witwatersrand,Johannesburg, South Africa, and the M.E. degree from the University ofPretoria, Pretoria, South Africa.

He is the Director of the Institute for Nanoscale Technology, University ofTechnology, Sydney, Australia.