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Sensitivity limits of the conductivity method for measuring the concentration of atmospheric sulphur dioxide
SENSITIVITY LIMITS OF THE CONDUCTIVITY METHOD FOR MEASURING THE CONCENTRATION OF ATMOSPHERIC SULPHUR DIOXIDE
Mcrobiological
Research Establishment. Porton Down. Wiltshire. U.K.
Abstract-Sulphur dioxide was absorbed from air blown in the form of a tine jet at the surface of a small volume of equilibrium water containing hydrogen peroxide. in a stainless steel conducti\it) cell. Using a simple regenerative circuit. the conductivity could be measured to within 2 parts in IO’. Errors due to adsorption. evaporation and changes in carbon dioxide concentration were investigated. It was concluded that measurements in the ppb range are feasible, the main unsolved problem being that of interference by atmospheric ammonia.
The low-velocity impinger has been used for some time in monitoring sulphur dioxide in London air (Sash. 1961). A more sensitive and accurate version is now in operation (Brown and Derrett, 1976) but there is still need for a portable instrument which can deal with low levels of pollution. The work now reported had the aim of producing such an instrument, but included an exploration of the sensitivity limits of the conductivity method, particularly the influence of the collecting solution used. It has been the practice to add sulphuric acid in order to bring the pH down to 4 or 45 and avoid the buffering action of bicarbonate ion from atmospheric carbon dioxide. Such buffering action means that there is a non-linear relation between the conductivity of the solution and the amount of sulphur dioxide absorbed. However this is merely an inconvenience and need not prevent good measurements being taken, possibly at greatly improved sensitivity. In order to show the kind of improvement to be expected, it is necessary to anticipate a result derived in detail below. In the limiting case it is easy to show that the addition of a very small amount of acid to equilibrium water results in an increase in the hydrogen ion concentration equal to that produced by adding half of this amount to completely pure water, so that the expected conductivity increase is one-half of the full amount. This means that there is still an initial fivefold advantage in using equilibrium water (water in equilibrium with atmospheric carbon dioxide, pH cu. 5.5) over using water acidified to pH 4.5. There are two other advantages attached to the use of equilibrium water. Firstly. if the air being sampled contains only a fixed concentration of atmospheric carbon dioxide and no other acid or alkaline gas. the conductivity cannot change however much water is lost by evaporation. Secondly, it can be calculated (Bell, 1952) that for any weak acid such as carbonic, 297
the hydrogen ion concentration of its aqueous solution (in the absence of other ionizable substances) is proportional to the square root of its total concentration. Taking figures from the tables (Hodgman. 1958) on the temperature coefficient of the conductance of hydrogen ion. and the temperature coefficient of the solubility of carbon dioxide. it can be calculated that the two effects compensate each other almost exactly between 10 and 2O’C. This means that the conductivity of equilibrium water should be independent of temperature. within the practical range. which is important in the present instance where evaporative cooling could be considerable. It was therefore decided to explore the use of such water. to which had been added O.lV M.A.R. hydrogen peroxide. This was the best commercially available quality and proved to be sufficiently free of electrolytes for the purpose.
EXPERIMESTAL
PROCEDURES
ASD
RESULTS
In the original portable instrument (Kash. 1964) the cell volume was not well defined and the solution in it was isolated and therefore subject to considerable evaporative cooling. The new cell was made from a stainless steel tube 8 mm o.d. having a central solid stainless steel electrode 13 mm dia. with its end flush with the bottom of the cell, 23 mm below the rim (Fig. I). The insulation was polythene and the cell volume, measured by microsyringe. was 55 cm3. The cell was in good thermal contact with a large volume of solution, for temperature stability. The side-arm W was connected to an air-filled syringe in order that the solution level could be raised until the cell was flooded. Lowering the level then trapped an exact volume of fresh solution in the cell. Enough energy was transferred from the jet to the solution immediately below, to set up a circulation mixing the cell contents effectively. Air was pumped using a small
I
I
I
Fig. 1. Regenerative conductivity bridge: C - 0021~lF. 1 K potentiometer. Q - 470 R. R - 10 K. S - 330 K. L - I K. Z - cell. V - output voltage.
P -
Fig. I. Micro conductivir)- ccl1absorbing gns from an air jet: W - to war level control. P -- to pump. I - inlet for test air. J - Jet. S - stainless steel tube and rod electrodes. the top ends of Q hich form the cell i&f. belo% rhc jet. diaphragm pump with the air flow adjusted to 0.2 I per min using the jet as its own manometric flow meter, after calibration with a rotameter. The latter alone was not suitable because the pressure drop across it was suthcient to cause considerable unknown change in the How when it vl’as removed. as was sometimes necessary.
~&lostcircuits used for measuring ~ondllctivity are variants of a basic design comprising an oscillator. an amplifier. a rectifier and some kind of meter or recorder. Considerable elaboration is necessary if noise and drift have to he kept low. and a linear relation maintained between reading and conductivity (Brown and Derrett, 1976). A different system was used in the present work. which seemed to offer greater accuracy combined with simplicity. as long as there was no need to record. There was now no separate oscillator or detector. the celi being part of a Wien bridge connecting the output of an operational amplifier to one input (Fig. 2). With the rest of the circuit as shown. the system could go into OS& lation. and using a ten-turn potentiometer P it became possible to locate the position of incipient oscillation very closely. The dial reading on the potentiometer could then be related to the conductivity the solution in the cell. In this way the problems associated with the production and measurement of an a.c. signal of very stable amplitude were avoided. The system differs from a conventional a.c. bridge in having. in effect. a tuned high-Q circuit resulting in a much sharper end point.
The amplifier has suficient power output for osciifation to be detected aurally through a small speaker or visually by a light-emitting diode. For the purpose of laboratory tests. however. the manually operated potentiometer was replaced by one of the same value driven by a motor. The oscillation signal was taken to a separate a.c. amplifier and relay controlling this motor. connected so as to drive the potentiometer towards oscillation cut-off. Using this simple servo system it was possible to obtain almost continuous readings over an extended period during which various tests could be carried out. The component values shown in Fig. 2 are such that there was a difference of CCL10 0 units on the potentiometer dial between the oscillation-on and the oscillation-off positions. This hysteresis was useful because it enabled the relay and motor to function steadily. When the celt was replaced by a fixed resistor readings were very reproducible, e.g. using 2.2 MGZ the motor stopped automatically at a dial reading of 704.2 six times in succession. With the celi reconnected there was generally a slow drift; a plot of reading against time disclosed an error of not more than @2 units in individual readings, which may be ignored in comparison with the working reading of 170 plus up to 1000 on the potentiometer diaf. Using a series of fixed resistors in place of the cell, it was also established that the reading at osciifation cut-off was dire&y proportional to the conductjvity. as required by operational amplifier theory.
As expected. a major problem was the production of clean air for the investigation of cell characteristics. Cylinder air might have been satisfactory. but it required some humidification and it was found impossible to do this without introducing interferences. The same was true of laboratory air if passed through or over water to remove sulphur dioxide. Eventually. it was found that coarse white silica gel. equilibrated to ambient humidity. was able to remove interfering
SaGri\ it! limits
299
probably true for the primary absorption or re_jection of carbon dioside. The slowness of the conductivity changes are attributed to the slotvness of the carbon dioxide hydration and dehydration reactions (Sharma and Danckwerts. 19631. Tlteorrtictrl IW~OIN~to cdtld
Fig. 3. Readings from continuous samples: Arbitrary zeros: decrcdse for increasing conductivity. Top. control air; middle. air containing twice the normal concentration of carbon dioxide (bettveen arrows) followed bq recovery to normal air: bottom. air containing about about 2 ppb of sulphur dioxide (between arrows).
gases ivithout apparently introducing any. i&lost controls. therefore. were done using ordinary laboratory air passed through a column of such gel. The laboratory air corresponded fairly closely to open (rural) air as it was supplied through a high velocity plenum system. with inlets close to the apparatus. The result of such control sampling is shown at the top of Fig. 3. The reading remained almost steady after the first half-minute. with a slight drift during the last five minutes. The drift was theoretically equivalent to sampling an atmosphere containing 0.1 ppb of sulphur dioxide and is an indication of the lowest concentration capable of being detected by the system.
Some samples were taken from air held in a 670-l. aluminium tank. whose carbon dioxide concentration had been increased by NO ppm. or approximately doubled. Silica1 gel absorbs carbon dioxide and had to be omitted from the sampling line; the tank air was therefore cleaned up beforehand by passing it through a battery of respirator canisters. The results from one long sample are shown in Fig. 3 (centre) with first the readings from control air through silica gel. then seven minutes of tank air. and finally back to control air which Bushed out the excess carbon dioside and restored the solution (almost) to its initial state. It may be seen that the initial rates of absorption and of blow-off of carbon dioxide are about the same and that several minutes are required to establish the new equilibrium. The response of the cell to sulphur dioxide is faster than this, and the same is
clcid
Having established that the system responds as expected totvards changes in carbon dioxide concentration. we must now consider the effect of adding mineral acid. as an essential step in calibration. For the ionization of carbonic acid. \ve have ~Bcll. 1952) HB = KC where H is the hydrogen ion concentration. B that of bicarbonate ion and C that of unionized carbonic acid; the latter can be taken to be proportional to the concentration of carbon dioxide in the air above the solution. at equilibrium. If no other acid is present this reduces to HB = KC = EL where E is the concentration of hydrogen and of bicarbonate ion in equilibrium a-ater. These are equal because all of the former (neglecting the small effect of the ionization of water itself) is produced one-toone nith bicarbonate ion in the ionization process. This equation gives the square-root relation mentioned in the introduction. between the hydrogen ion concentration and the carbon dioside concentration in the air above the solution. If mineral acid is novv added to give a nominal increase in hydrogen ion concentration .A. resulting in an actual total concentration of hydrogen ion H. (H - E) of this must have come from .-I and the rest. A - (H - E). was neutralized by bicarbonate ion. The concentration of the latter is therefore reduced to E - (,-l - H + E) or (H - .-I). The dissociation equation still applies so that u-e have H(H - A) = KC = E’ as long as the carbon dioxide concentration is unchanged. This last equation gives the required final hydrogen ion concentration in terms of the initial concentration E (found experimentally to be 3.6 x IOmh. see below) and the concentration of added strong acid. in the present instance dibasic sulphuric acid from sulphur dioxide reaction with hydrogen peroxide. or. if no peroxide is present. effectively monobasic sulphurous acid. Ctrlihrutiori In order to draw a calibration curve lve needed to know the cell constant. i.e. the change in reading produced by unit change in hydrogen ion concentration. This could have been done by adding dilute sulphuric acid to equilibrium water and calculating the expected hydrogen ion concentration as just described. but it was considered more satisfactory to avoid the non-linear region in determining this important constant. This was done by taking advantage of the linear characteristics of the circuit. The 330K resistor S (Fig. 2) was temporarily replaced by one of 22K and readings were then obtainable for solutions at pH of cn. 3. when carbon dioxide effects could
-i-. NASH
Y.N
R
I
4
3
2
ppb - 1. SO2
absorbed
Fig. 4. Indirect calibration. R - potentiometer reading expected for a given amount of sulphur dioxide absorbed. Units are equal to 2.85 ng of the gas. The thin line represents the response in total absence of atmospheric carbon dioxide.
washing with alcohol. Most plastics were also found to be unsuitable. either adsorbing sulphur dioxide or giving OR active vapours. or both. The inlet tube finally adopted was of PTFE 3 mm o.d. and 1.5 mm i.d., enclosed for most of its length by a close-fitting glass tube for straightness and rigidity. The jet at the end was still made of glass but was very small and the amount of adsorption on it was not noticeable. This may be seen from Fig. 3 (bottom) giving the reading every minute for a long sample which inciuded a 3 min exposure to air containing some sulphur dioxide. The ~gjnning and end of this exposure is marked by arrows on the diagram, to demonstrate the absence of any adsorption lag. The slightly sharp recovery at the end of the exposure was consistently observed and is attributed to the carbon dioxide hydration reaction mentioned above. Note that for Fig. 3 the initial readings are arbitrary and increase in conductivity gives a decrease in reading. Erzrporatiorf
be negkcted. Simple proportion then gave the required sensitivity (for S = 330K) of 190 potentiometer units per microno~al increase in hydrogen ion concentration. As regards the amount of gas absorbed. a useful unit for low concentrations is the ppb-1.. i.e. the amount of sulphur dioxide in one litre of air at 1 ppb or proportionately. Knowing the cell volume, 55cm3, it may be calculated that this amount of gas. if fully absorbed and converted to sulphuric acid, tvould increase the concentration of hydrogen ion (initially) by 1.6 m normal. From these data the calibration curve shown in Fig. 4 was constructed. using of course the equation H(H - rl) = E’ derived above. The straight line through the origin represents the theoretical response in the absence of any atmospheric carbon dioxide. The amount of gas absorbed is read off by difference, and the aerial concentration can be found knowing the volume sampled and the collection efficiency (see below). Safffpliffg
of air containiffg
srrlphr
dioxide
When the air contained sulphur dioxide. it was found impossible to obtain consistent results with gfass infet tubing. Even at low relative humidity glass seemed to absorb the gas. and release it later, For exampIe, one inlet tube 30 cm Iong and &4 cm i.d. was washed with alcohol and dried in a stream of clean air. Control samples down it using air which had passed over silica gel were satisfactory. Successive half-minute samples were then taken from an atmosphere containing nominally 50 ppb of sulphur dioxide. The resulting potentiometer readings were converted to estimated concentrations using the procedure described above and came to 55 18. 38 and 39 ppb, showing high losses over the first minute or so of sampling. After such treatment the tube was able to contaminate clean air passed down it. for a considerable time afterwards, the effect being removed by
eflects
the sulphur dioxide concentration is very low a sample ofseveral minutes’ duration may be required and a considerable fraction of the cell contents could be lost by evaporation. Tests with &an air showed no detectable change in reading until CCL1%; of the cell volume had been lost. In order to see how evaporation would affect the reading from air containing sulphur dioxide. the experiment was repeated with water to which some sulphuric acid had been added. The r.h. was 47% and the temperature Z’C. In 8 min 12% of the cell contents had evaporated and the reading had increased by only a small amount, from an indicated 35-3.6 ppb. This relatively small change is attributed to a fortuitously good choice of cell geometry, but shows that evaporation need not be a serious problem under most conditions. If
Collection
eficief7cj
Sulphur dioxide was found to be collected with 97”/defficiency in the low-velocity impinger using cells of 1 cm3 volume or larger (Nash. 1961. 197.5).The efficiency of the smaller cell now used is probably somewhat less and has not been determined accurately. Direct calibration is difficult because of the problem of maintaining known low concentrations of sulphur dioxide in a vessel. Permeation tubes were found to be unusable because of adsorption problems in the dilution system. Some comparisons were done against the Philips coulometric recorder, sampling relatively clean outside air freshly isolated in a large chamber. For five successive samples the concentration came to 24 f. 1 ppb (assuming looO:, collection efficiency) while the reading on the recorder of the Philips instrument varied between 25 and 33 ppb, the pen deflection being rather small. On these figures, rhe collection efficiency of the impinger was between 70 and 100%. This uncertainty in no way affects the main conclusion, for which it is necessary to assume only that the efficiency is independent of concent-
Sensitivity limits
ration. Saltzman (1961) considered that thzte could be an apparent fall in collection eficiency at very low gas concentrations. but his calculations bvould not apply in the present instance where there is an irreversible reaction between gas and solution.
?kasurements made by the conductivity method are subject to interference by acidic or basic gases. the main problem in the sampling of relatively unpolluted air probably being posed by ammonia. A search rvas made for some substance capable of selectively absorbing ammonia. It was confirmed that salicylic acid was much too volatile (R. J. Sherwood. personal communication). Picric and phthalic acids were better but still gave off too much acid vapour to be usable at high sensitivity. A detailed comparison between the absorbing capacities of the latter two acids indicated that there might be a special advantage in using nonionizable strong eiectron acceptors such as tetracyano-ethylene, if they could be obtained in sufficient pure crystalline form. and the investigations are continuing.
The conductivity method is applicable to the measurement of atmospheric sulphur dioxide in the ppb range. subject to the problem of interference by
ZOI
ammonia eithsr being solved or demonstrated to fx unimportant. It would probabl) be necessary to us2 equilibrium water because of its superior performance as regards sensitivity. temperature coefficient and cvdporation
effects.
,~ci;,lowlrci~mlrt~r-I am indebted to C. J. Derrett for sug!esting the use of an operational amplifier and for providmg a prototype circuit. REFERESCES
Be11 R. P. I1952) &ids ml Baws. Chap. II. Mzthuen Monograph. London. Brown C. and Derrett C. J. (1976) .A fast response sulphur dioxide meter for laboratory srudies. Armospheric EJIcironmenr 10, 303-3O4. Hodgman C. D. (1958-59) Hum/hock of Ciwmisrr~ (Ijd Physics. 40th Edn. Chemical Rubber Pub. Co.. Clsveland. Nash T. (1961) Low-velocity gas-liquid impinger for th: continuous estimation of sulphur dioxide and other atmospheric gases. J. Sci. It~strwr. 38. 38WSI. sash %‘.(1964) A ‘personal’ measuring instrument for atmospheric sulphur dioxide. 1~. J. Air Kt7r. Polhrr. 8. 121-124. Nash T. (197% .Absorption of sulphur dioxide bv aqueous solutions. Arvtosnhrric Enuironme~~r9. 113--L Go. Saltzman 8. E. ((961) Preparation and analysis of calibrated low concentrations of 16 toxic gases. d~i,vr. Chem. 33, 1100-lI12 (does not include sulphur dioxidtt. Sharma M. M. and Danckwerts P. V. (19631 Catalysis b! Bronsted bases of the reaction between carbon dioxide and water. Trans. faraday Sot. 19. 386-395.
Mcrobiological
Research Establishment. Porton Down. Wiltshire. U.K.
Abstract-Sulphur dioxide was absorbed from air blown in the form of a tine jet at the surface of a small volume of equilibrium water containing hydrogen peroxide. in a stainless steel conducti\it) cell. Using a simple regenerative circuit. the conductivity could be measured to within 2 parts in IO’. Errors due to adsorption. evaporation and changes in carbon dioxide concentration were investigated. It was concluded that measurements in the ppb range are feasible, the main unsolved problem being that of interference by atmospheric ammonia.
The low-velocity impinger has been used for some time in monitoring sulphur dioxide in London air (Sash. 1961). A more sensitive and accurate version is now in operation (Brown and Derrett, 1976) but there is still need for a portable instrument which can deal with low levels of pollution. The work now reported had the aim of producing such an instrument, but included an exploration of the sensitivity limits of the conductivity method, particularly the influence of the collecting solution used. It has been the practice to add sulphuric acid in order to bring the pH down to 4 or 45 and avoid the buffering action of bicarbonate ion from atmospheric carbon dioxide. Such buffering action means that there is a non-linear relation between the conductivity of the solution and the amount of sulphur dioxide absorbed. However this is merely an inconvenience and need not prevent good measurements being taken, possibly at greatly improved sensitivity. In order to show the kind of improvement to be expected, it is necessary to anticipate a result derived in detail below. In the limiting case it is easy to show that the addition of a very small amount of acid to equilibrium water results in an increase in the hydrogen ion concentration equal to that produced by adding half of this amount to completely pure water, so that the expected conductivity increase is one-half of the full amount. This means that there is still an initial fivefold advantage in using equilibrium water (water in equilibrium with atmospheric carbon dioxide, pH cu. 5.5) over using water acidified to pH 4.5. There are two other advantages attached to the use of equilibrium water. Firstly. if the air being sampled contains only a fixed concentration of atmospheric carbon dioxide and no other acid or alkaline gas. the conductivity cannot change however much water is lost by evaporation. Secondly, it can be calculated (Bell, 1952) that for any weak acid such as carbonic, 297
the hydrogen ion concentration of its aqueous solution (in the absence of other ionizable substances) is proportional to the square root of its total concentration. Taking figures from the tables (Hodgman. 1958) on the temperature coefficient of the conductance of hydrogen ion. and the temperature coefficient of the solubility of carbon dioxide. it can be calculated that the two effects compensate each other almost exactly between 10 and 2O’C. This means that the conductivity of equilibrium water should be independent of temperature. within the practical range. which is important in the present instance where evaporative cooling could be considerable. It was therefore decided to explore the use of such water. to which had been added O.lV M.A.R. hydrogen peroxide. This was the best commercially available quality and proved to be sufficiently free of electrolytes for the purpose.
EXPERIMESTAL
PROCEDURES
ASD
RESULTS
In the original portable instrument (Kash. 1964) the cell volume was not well defined and the solution in it was isolated and therefore subject to considerable evaporative cooling. The new cell was made from a stainless steel tube 8 mm o.d. having a central solid stainless steel electrode 13 mm dia. with its end flush with the bottom of the cell, 23 mm below the rim (Fig. I). The insulation was polythene and the cell volume, measured by microsyringe. was 55 cm3. The cell was in good thermal contact with a large volume of solution, for temperature stability. The side-arm W was connected to an air-filled syringe in order that the solution level could be raised until the cell was flooded. Lowering the level then trapped an exact volume of fresh solution in the cell. Enough energy was transferred from the jet to the solution immediately below, to set up a circulation mixing the cell contents effectively. Air was pumped using a small
I
I
I
Fig. 1. Regenerative conductivity bridge: C - 0021~lF. 1 K potentiometer. Q - 470 R. R - 10 K. S - 330 K. L - I K. Z - cell. V - output voltage.
P -
Fig. I. Micro conductivir)- ccl1absorbing gns from an air jet: W - to war level control. P -- to pump. I - inlet for test air. J - Jet. S - stainless steel tube and rod electrodes. the top ends of Q hich form the cell i&f. belo% rhc jet. diaphragm pump with the air flow adjusted to 0.2 I per min using the jet as its own manometric flow meter, after calibration with a rotameter. The latter alone was not suitable because the pressure drop across it was suthcient to cause considerable unknown change in the How when it vl’as removed. as was sometimes necessary.
~&lostcircuits used for measuring ~ondllctivity are variants of a basic design comprising an oscillator. an amplifier. a rectifier and some kind of meter or recorder. Considerable elaboration is necessary if noise and drift have to he kept low. and a linear relation maintained between reading and conductivity (Brown and Derrett, 1976). A different system was used in the present work. which seemed to offer greater accuracy combined with simplicity. as long as there was no need to record. There was now no separate oscillator or detector. the celi being part of a Wien bridge connecting the output of an operational amplifier to one input (Fig. 2). With the rest of the circuit as shown. the system could go into OS& lation. and using a ten-turn potentiometer P it became possible to locate the position of incipient oscillation very closely. The dial reading on the potentiometer could then be related to the conductivity the solution in the cell. In this way the problems associated with the production and measurement of an a.c. signal of very stable amplitude were avoided. The system differs from a conventional a.c. bridge in having. in effect. a tuned high-Q circuit resulting in a much sharper end point.
The amplifier has suficient power output for osciifation to be detected aurally through a small speaker or visually by a light-emitting diode. For the purpose of laboratory tests. however. the manually operated potentiometer was replaced by one of the same value driven by a motor. The oscillation signal was taken to a separate a.c. amplifier and relay controlling this motor. connected so as to drive the potentiometer towards oscillation cut-off. Using this simple servo system it was possible to obtain almost continuous readings over an extended period during which various tests could be carried out. The component values shown in Fig. 2 are such that there was a difference of CCL10 0 units on the potentiometer dial between the oscillation-on and the oscillation-off positions. This hysteresis was useful because it enabled the relay and motor to function steadily. When the celt was replaced by a fixed resistor readings were very reproducible, e.g. using 2.2 MGZ the motor stopped automatically at a dial reading of 704.2 six times in succession. With the celi reconnected there was generally a slow drift; a plot of reading against time disclosed an error of not more than @2 units in individual readings, which may be ignored in comparison with the working reading of 170 plus up to 1000 on the potentiometer diaf. Using a series of fixed resistors in place of the cell, it was also established that the reading at osciifation cut-off was dire&y proportional to the conductjvity. as required by operational amplifier theory.
As expected. a major problem was the production of clean air for the investigation of cell characteristics. Cylinder air might have been satisfactory. but it required some humidification and it was found impossible to do this without introducing interferences. The same was true of laboratory air if passed through or over water to remove sulphur dioxide. Eventually. it was found that coarse white silica gel. equilibrated to ambient humidity. was able to remove interfering
SaGri\ it! limits
299
probably true for the primary absorption or re_jection of carbon dioside. The slowness of the conductivity changes are attributed to the slotvness of the carbon dioxide hydration and dehydration reactions (Sharma and Danckwerts. 19631. Tlteorrtictrl IW~OIN~to cdtld
Fig. 3. Readings from continuous samples: Arbitrary zeros: decrcdse for increasing conductivity. Top. control air; middle. air containing twice the normal concentration of carbon dioxide (bettveen arrows) followed bq recovery to normal air: bottom. air containing about about 2 ppb of sulphur dioxide (between arrows).
gases ivithout apparently introducing any. i&lost controls. therefore. were done using ordinary laboratory air passed through a column of such gel. The laboratory air corresponded fairly closely to open (rural) air as it was supplied through a high velocity plenum system. with inlets close to the apparatus. The result of such control sampling is shown at the top of Fig. 3. The reading remained almost steady after the first half-minute. with a slight drift during the last five minutes. The drift was theoretically equivalent to sampling an atmosphere containing 0.1 ppb of sulphur dioxide and is an indication of the lowest concentration capable of being detected by the system.
Some samples were taken from air held in a 670-l. aluminium tank. whose carbon dioxide concentration had been increased by NO ppm. or approximately doubled. Silica1 gel absorbs carbon dioxide and had to be omitted from the sampling line; the tank air was therefore cleaned up beforehand by passing it through a battery of respirator canisters. The results from one long sample are shown in Fig. 3 (centre) with first the readings from control air through silica gel. then seven minutes of tank air. and finally back to control air which Bushed out the excess carbon dioside and restored the solution (almost) to its initial state. It may be seen that the initial rates of absorption and of blow-off of carbon dioxide are about the same and that several minutes are required to establish the new equilibrium. The response of the cell to sulphur dioxide is faster than this, and the same is
clcid
Having established that the system responds as expected totvards changes in carbon dioxide concentration. we must now consider the effect of adding mineral acid. as an essential step in calibration. For the ionization of carbonic acid. \ve have ~Bcll. 1952) HB = KC where H is the hydrogen ion concentration. B that of bicarbonate ion and C that of unionized carbonic acid; the latter can be taken to be proportional to the concentration of carbon dioxide in the air above the solution. at equilibrium. If no other acid is present this reduces to HB = KC = EL where E is the concentration of hydrogen and of bicarbonate ion in equilibrium a-ater. These are equal because all of the former (neglecting the small effect of the ionization of water itself) is produced one-toone nith bicarbonate ion in the ionization process. This equation gives the square-root relation mentioned in the introduction. between the hydrogen ion concentration and the carbon dioside concentration in the air above the solution. If mineral acid is novv added to give a nominal increase in hydrogen ion concentration .A. resulting in an actual total concentration of hydrogen ion H. (H - E) of this must have come from .-I and the rest. A - (H - E). was neutralized by bicarbonate ion. The concentration of the latter is therefore reduced to E - (,-l - H + E) or (H - .-I). The dissociation equation still applies so that u-e have H(H - A) = KC = E’ as long as the carbon dioxide concentration is unchanged. This last equation gives the required final hydrogen ion concentration in terms of the initial concentration E (found experimentally to be 3.6 x IOmh. see below) and the concentration of added strong acid. in the present instance dibasic sulphuric acid from sulphur dioxide reaction with hydrogen peroxide. or. if no peroxide is present. effectively monobasic sulphurous acid. Ctrlihrutiori In order to draw a calibration curve lve needed to know the cell constant. i.e. the change in reading produced by unit change in hydrogen ion concentration. This could have been done by adding dilute sulphuric acid to equilibrium water and calculating the expected hydrogen ion concentration as just described. but it was considered more satisfactory to avoid the non-linear region in determining this important constant. This was done by taking advantage of the linear characteristics of the circuit. The 330K resistor S (Fig. 2) was temporarily replaced by one of 22K and readings were then obtainable for solutions at pH of cn. 3. when carbon dioxide effects could
-i-. NASH
Y.N
R
I
4
3
2
ppb - 1. SO2
absorbed
Fig. 4. Indirect calibration. R - potentiometer reading expected for a given amount of sulphur dioxide absorbed. Units are equal to 2.85 ng of the gas. The thin line represents the response in total absence of atmospheric carbon dioxide.
washing with alcohol. Most plastics were also found to be unsuitable. either adsorbing sulphur dioxide or giving OR active vapours. or both. The inlet tube finally adopted was of PTFE 3 mm o.d. and 1.5 mm i.d., enclosed for most of its length by a close-fitting glass tube for straightness and rigidity. The jet at the end was still made of glass but was very small and the amount of adsorption on it was not noticeable. This may be seen from Fig. 3 (bottom) giving the reading every minute for a long sample which inciuded a 3 min exposure to air containing some sulphur dioxide. The ~gjnning and end of this exposure is marked by arrows on the diagram, to demonstrate the absence of any adsorption lag. The slightly sharp recovery at the end of the exposure was consistently observed and is attributed to the carbon dioxide hydration reaction mentioned above. Note that for Fig. 3 the initial readings are arbitrary and increase in conductivity gives a decrease in reading. Erzrporatiorf
be negkcted. Simple proportion then gave the required sensitivity (for S = 330K) of 190 potentiometer units per microno~al increase in hydrogen ion concentration. As regards the amount of gas absorbed. a useful unit for low concentrations is the ppb-1.. i.e. the amount of sulphur dioxide in one litre of air at 1 ppb or proportionately. Knowing the cell volume, 55cm3, it may be calculated that this amount of gas. if fully absorbed and converted to sulphuric acid, tvould increase the concentration of hydrogen ion (initially) by 1.6 m normal. From these data the calibration curve shown in Fig. 4 was constructed. using of course the equation H(H - rl) = E’ derived above. The straight line through the origin represents the theoretical response in the absence of any atmospheric carbon dioxide. The amount of gas absorbed is read off by difference, and the aerial concentration can be found knowing the volume sampled and the collection efficiency (see below). Safffpliffg
of air containiffg
srrlphr
dioxide
When the air contained sulphur dioxide. it was found impossible to obtain consistent results with gfass infet tubing. Even at low relative humidity glass seemed to absorb the gas. and release it later, For exampIe, one inlet tube 30 cm Iong and &4 cm i.d. was washed with alcohol and dried in a stream of clean air. Control samples down it using air which had passed over silica gel were satisfactory. Successive half-minute samples were then taken from an atmosphere containing nominally 50 ppb of sulphur dioxide. The resulting potentiometer readings were converted to estimated concentrations using the procedure described above and came to 55 18. 38 and 39 ppb, showing high losses over the first minute or so of sampling. After such treatment the tube was able to contaminate clean air passed down it. for a considerable time afterwards, the effect being removed by
eflects
the sulphur dioxide concentration is very low a sample ofseveral minutes’ duration may be required and a considerable fraction of the cell contents could be lost by evaporation. Tests with &an air showed no detectable change in reading until CCL1%; of the cell volume had been lost. In order to see how evaporation would affect the reading from air containing sulphur dioxide. the experiment was repeated with water to which some sulphuric acid had been added. The r.h. was 47% and the temperature Z’C. In 8 min 12% of the cell contents had evaporated and the reading had increased by only a small amount, from an indicated 35-3.6 ppb. This relatively small change is attributed to a fortuitously good choice of cell geometry, but shows that evaporation need not be a serious problem under most conditions. If
Collection
eficief7cj
Sulphur dioxide was found to be collected with 97”/defficiency in the low-velocity impinger using cells of 1 cm3 volume or larger (Nash. 1961. 197.5).The efficiency of the smaller cell now used is probably somewhat less and has not been determined accurately. Direct calibration is difficult because of the problem of maintaining known low concentrations of sulphur dioxide in a vessel. Permeation tubes were found to be unusable because of adsorption problems in the dilution system. Some comparisons were done against the Philips coulometric recorder, sampling relatively clean outside air freshly isolated in a large chamber. For five successive samples the concentration came to 24 f. 1 ppb (assuming looO:, collection efficiency) while the reading on the recorder of the Philips instrument varied between 25 and 33 ppb, the pen deflection being rather small. On these figures, rhe collection efficiency of the impinger was between 70 and 100%. This uncertainty in no way affects the main conclusion, for which it is necessary to assume only that the efficiency is independent of concent-
Sensitivity limits
ration. Saltzman (1961) considered that thzte could be an apparent fall in collection eficiency at very low gas concentrations. but his calculations bvould not apply in the present instance where there is an irreversible reaction between gas and solution.
?kasurements made by the conductivity method are subject to interference by acidic or basic gases. the main problem in the sampling of relatively unpolluted air probably being posed by ammonia. A search rvas made for some substance capable of selectively absorbing ammonia. It was confirmed that salicylic acid was much too volatile (R. J. Sherwood. personal communication). Picric and phthalic acids were better but still gave off too much acid vapour to be usable at high sensitivity. A detailed comparison between the absorbing capacities of the latter two acids indicated that there might be a special advantage in using nonionizable strong eiectron acceptors such as tetracyano-ethylene, if they could be obtained in sufficient pure crystalline form. and the investigations are continuing.
The conductivity method is applicable to the measurement of atmospheric sulphur dioxide in the ppb range. subject to the problem of interference by
ZOI
ammonia eithsr being solved or demonstrated to fx unimportant. It would probabl) be necessary to us2 equilibrium water because of its superior performance as regards sensitivity. temperature coefficient and cvdporation
effects.
,~ci;,lowlrci~mlrt~r-I am indebted to C. J. Derrett for sug!esting the use of an operational amplifier and for providmg a prototype circuit. REFERESCES
Be11 R. P. I1952) &ids ml Baws. Chap. II. Mzthuen Monograph. London. Brown C. and Derrett C. J. (1976) .A fast response sulphur dioxide meter for laboratory srudies. Armospheric EJIcironmenr 10, 303-3O4. Hodgman C. D. (1958-59) Hum/hock of Ciwmisrr~ (Ijd Physics. 40th Edn. Chemical Rubber Pub. Co.. Clsveland. Nash T. (1961) Low-velocity gas-liquid impinger for th: continuous estimation of sulphur dioxide and other atmospheric gases. J. Sci. It~strwr. 38. 38WSI. sash %‘.(1964) A ‘personal’ measuring instrument for atmospheric sulphur dioxide. 1~. J. Air Kt7r. Polhrr. 8. 121-124. Nash T. (197% .Absorption of sulphur dioxide bv aqueous solutions. Arvtosnhrric Enuironme~~r9. 113--L Go. Saltzman 8. E. ((961) Preparation and analysis of calibrated low concentrations of 16 toxic gases. d~i,vr. Chem. 33, 1100-lI12 (does not include sulphur dioxidtt. Sharma M. M. and Danckwerts P. V. (19631 Catalysis b! Bronsted bases of the reaction between carbon dioxide and water. Trans. faraday Sot. 19. 386-395.