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Ultrasonics in chemical analysis
Ultrasonics in chemical analysis* R.C. Asher Instrumentation and Applied Physics Division, Building 10.5, Harwell Laboratory, Oxfordshire OXl 1 0RA, UK Ultrasonic techniques are attractive for chemical analysis for a number of reasons. In particular, they can often be used non-invasively, the energy levels are low, no radioactivity is involved,. 'moving parts" are not usually required, the response is rapid and a range of mutually compatible techniques is available. Ultrasonic transmitters and receivers are readily available commercially and, using them, sensors can be constructed for on-line analytical applications or for use in the laboratory. In addition, quite a number of complete ultrasonic analytical systems are already on the market. The principal ultrasonic parameters which can be measured are the velocity, attenuation and scattering of ultrasound and the acoustic impedance of the medium through which it is travelling. From these it is possible to determine, under favourable circumstances, chemical identity, the concentrations (or density) of solutions, mixtures and dispersions, and the particle size of dispersed phases.
Keywords: sonochemistry; chemical analysis
One does not tend to think of the use of ultrasound as a basis for techniques for chemical analysis. Nevertheless, I hope to show that, within certain limitations, it has a number of attractions. An important advantage of ultrasonic techniques is that, very often, they can be used non-invasively, i.e. the containment of the plant need not be breached. This results from the fact that containment materials, particularly metals, are usually 'transparent' to ultrasound. Other advantages of ultrasonic techniques are that: 1, they do not normally involve 'moving parts'; 2, no radioactivity is used; 3, the response is rapid; 4, the energy levels are low and (hopefully) nonhazardous; and 5, they provide a range of mutually compatible techniques (this point is somewhat philosophical). Another important feature of ultrasonic techniques, which in one respect gives them the edge over optical techniques, is that the velocity of ultrasound is relatively low compared with that of light; hence, for example, the accurate timing of ultrasonic pulses is not too difficult. The principal ultrasonic parameters which can be measured are: velocity, attenuation and scattering of ultrasound, and the acoustic impedance of the medium through which it is passing. These will be considered in turn. From these parameters one can, under favourable circumstances, determine: chemical identity, the concentration (or density) of solutions, mixtures and dispersions, and the particle size of dispersed phases. I must, however, say quite clearly that, in general, ultrasonic analytical techniques are not specific and are not suitable for trace analysis. These disadvantages, however, are shared with many other popular analytical techniques.
Generalizations
*Paper presented at the Sonochemistry Symposium, Annual Chemical Cogress, held at Warwick University, UK, 8-11 April 1986 0041-624X/87/010017-03 $03.00 © 1987 Butterworth ~" Co (Publishers) Ltd
Identification of liquids
Most of the ultrasonic techniques used for chemical analysis use compressional ultrasound (i.e. longitudinal waves). The frequency is typically in the range 0.5-10 MHz, so that, in most materials, the wavelength is somewhere between a fraction o f a millimetre and 10 mm. The power levels are exceedingly low compared with those used for ultrasonic cleaning or emulsification, etc. The ultrasound is normally generated by piezoelectric transducers of small and convenient size which are readily available commercially; these can act both as transmitters and receivers. Some form of 'coupling' such as water or grease is normally used to permit the ultrasound to be transmitted from the transducer to and from the system under study. Electronic systems are available which can energize transducers to produce either continuous waves or pulses, detect and amplify the received signals, time the echoes and carry out a wide range of data processing activities. Applications are not limited to ambient temperatures because some transducers will tolerate elevated temperatures; alternatively, ultrasonic wave guides can be used to allow the transducers to be installed in a cooler part of the plant.
Velocity At present the most fruitful ultrasonic parameter for use in chemical analysis is the velocity of ultrasound. Simple, convenient cells are available for determining this parameter. They can be installed in pipelines or tanks in production plants or used for off-line examination of samples in the laboratory. Having determined the velocity of ultrasound in the medium the data can be used in a number of ways. These ways are now considered in turn.
The velocity of ultrasound in liquids spans a wide range;
Ultrasonics 1987 Vol 25 January
17
Ultrasonics in chemical analysis: R.C. Asher 1000-1500 m s -1 is typical but there are m a n y examples outside this range. Therefore a determination of the velocity of ultrasound in a liquid can be a great help in its identification. This technique bears comparison with the more traditional techniques using melting points, boiling points, refractive index, etc. It has been used to determine the grade of petrol passing along a pipeline and, on a lighter note, it has been used to detect the difference between beer and water.
Concentration of solutions The velocity of ultrasound in a solution usually depends on the concentration of the solution. In favourable cases the velocity changes quite rapidly and moderately linearly with concentration: consequently a reasonably accurate estimate of the concentration can be made from the velocity of ultrasound. In less favourable cases the velocity-concentration curve passes through a m a x i m u m or m i n i m u m and this would result in ambiguity because two solutions may give the same figure for the velocity of ultrasound. Similarly, systems where the velocity varies only slightly with concentration are unattractive. In most cases an additional complication is that the velocity of ultrasound is also dependent on temperature and therefore a correction for this effect must be made. This correction is very often the factor which limits the accuracy with which the concentration may be determined. For most materials the velocity decreases with temperature; the notable exception is water where the temperature coefficient is positive ,-~ + 3 m s-~°C-~): many aqueous solutions show a similar effect. The use of ultrasound to determine concentrations of solutions is a moderately well established technique but is certainly not yet fully exploited. It has been used in nuclear fuel reprocessing plants to determine the concentration o f solutions of fissile material and of simple chemical feedstocks. Although I have described this application in terms of 'concentrations' the instruments can, of course, be calibrated in terms of "density" or any other units which are particularly relevant.
Mixtures of liquids The behaviour of the velocity of ultrasound in mixtures of liquids depends on the nature of the liquids. If there are two liquids, both of which are unassociated and which do not interact with each other, e.g. toluene and c a r b o n tetrachloride, then the velocity of ultrasound (at a particular temperature) is virtually linearly dependent on the composition. If, as in the case of mixtures of ethyl alcohol and carbon tetrachloride, one of the liquids (alcohol) is associated, then there is a deviation from linearity at low alcohol concentrations at which, presumably, association has not yet taken place. W h e n both of the liquids are associated more complicated behaviour is found. For example, with ethyl alcohol-water mixtures, the velocity rises to a m a x i m u m at a concentration which depends on the temperature but which is ~ 35 wt% alcohol at normal temperatures. An interesting consequence of this is that there is a particular concentration ( ~ 20 wt% alcohol) at which the velocity of ultrasound is independent o f temperature. This mixture is therefore useful in the laboratory for calibrating ultrasonic cells. Despite the attractions of measuring the composition of mixtures of liquids ultrasonically it is difficult to find practical industrial examples: the technique is used, however, in the nuclear fuel reprocessing industry for
18
Ultrasonics 1987 Vol 25 January
determining the composition of a tributylphosphatekerosene mixture used in solvent extraction plants.
Two-phase liquid systems Emulsions. A regrettably high proportion of emulsions tend to be 'opaque' to ultrasound. Fortunately, those which are sufficiently "transparent' always seem to show a simple linear relationship between ultrasonic velocity and concentration. This is in accord with a simple, naive pro rata hypothesis. This p h e n o m e n o n is the basis of an instrument for determining the composition of a waterhexanol emulsion associated with another nuclcar fuel cycle plant.
Suspensions. In contrast to emulsions, the situation with regard to the velocity of ultrasound in suspensions appears to be complex. This may be an illusion engendered by the complexity of the theory. Unfortunately the experimental data is fairly limited and, as with emulsions, there is the overriding problem that many suspensions arc rather (or very) opaque to ultrasound. The data, such as it is, is mainly for aqueous suspensions and suggests that the velocity of ultrasound changes monotonically with concentration and that the relationship does not deviate disastrously from linear: the concentration coefficient may be either positive or negative. Empirically it seems that suspensions of "dense' solids tend to show a negative concentration coefficient whereas "light" solids give a positive coefficient. Regardless of the theoretical explanations, the suspension is adequately "transparent" to ultrasound, it is often possible to determine the concentration of suspensions from the velocity of ultrasound.
Bubbly liquids. As with all two-phase systems, the theoretical treatment of the effect of bubbles on the velocity of ultrasound in liquids is complex. The experimental results are scattered and apparently not in accordance with all of the theories. It is, however, clear from experimental observations that the attenuation of ultrasound increases rapidly as more bubbles are introduced and that, therefore, most bubbly liquids are effectively "opaque' to ultrasound. It also seems that the velocity decreases as the void fraction increases and that the relationship is not very dependent on the bubble diameter. Typical experiments show that, for aqueous systems, the velocity of ultrasound drops from ~ 1500 m s -~ lbr bubble free water to ,~200 m s l when as little as a few volume per cent of air are introduced. No analytical instrument making use of the velocity of ultrasound in bubbly liquids has come to my attention.
Foams. Foams are notably 'opaque' to ultrasound. Therefore, to obtain any significant transmission, very low frequency ultrasound would have to be used since the penetration of ultrasound generally increases greatly as the frequency is decreased: this, as a consequence of its long wavelength, would imply great difficulty in measuring the velocity accurately. Therefore the possibility of characterizing foams by measuring the velocity of ultrasound seems slim.
Gases. For perfect gases, the velocity of ultrasound is given by
Ultrasonics in chemical analysis: R.C. Asher where: R is the gas constant; T the absolute temperature; 7 the ratio of the principle specific heats; and M the molecular weight. The velocity, therefore, is dependent on the molecular weight and the temperature but not (at least for perfect gases) on the pressure; this generalization also breaks down when the molecular mean free path is comparable with the ultrasonic wavelength. The range of velocities for different gases (e.g. 1300 m s-1 for hydrogen, 970 m s -1 for helium and 310 m s-1 for oxygen, all at room temperature) means that many pure gases can be identified ultrasonically. With mixtures of gases the velocity varies linearly between the velocity in one pure gas and the velocity in the other pure gas; therefore mixtures of two known gases can be analysed by determining the velocity of ultrasound in the mixture. This is especially attractive for mixtures of gases differing greatly in molecular weight. Therefore, in particular, it would appear relatively easy to analyse hydrogen-deuterium mixtures and deuterium-tritium mixtures ultrasonically. It might also be possible, in principle, to determine ortho-hydrogen/para-hydrogen ratios this way because, although these two gases have the same molecular weights, the ratios of the principal specific heat are slightly different. Gas chromatographic detectors making use of the change in the velocity of ultrasound in a carrier gas resulting from the presence of traces of elutriants are commercially available.
Attenuation Attenuation of ultrasound is difficult to measure absolutely, especially under plant conditions. The basic reason is that ultrasonic signal strength decreases not solely as a result of attenuation in the medium under study but also because of other factors such as reflection at interfaces, beam divergence, and changes in transmitter and receiver performance. These factors are not easy to keep constant. Therefore, it is most convenient for instruments to be calibrated in situ. Despite this there are successful proprietary instruments which determine when the concentration of, for example, sewage slurry reaches a predetermined level. An aspect of attenuation which does not seem to have been exploited commercially is the relationship of attenuation to particle size in two-phase systems. This might permit some deductions to be made about particle sizes. The situation becomes more promising when the 'particle' is, in fact, a bubble because, in this case, the bubble can deform in various ways in resonance with the ultrasound. As a result the absorption cross-section of a bubble increases by several orders of magnitude at its resonant frequency. Hence, from the resonant frequency, it should be possible to deduce the bubble size.
Scattering The literature shows that the scatter of ultrasound from a two-phase dispersion is dependent on the concentration of the dispersion and its particle size. Thus, the energy of the scattered ultrasound in a particular direction is dependent (up to a certain limit) on the concentration, and the angular distribution ('polar diagram') is dependent on the particle size. In principle it may be possible to devise an instrument to measure concentration and particle size in this way. However, a restriction would
probably be that the particle size which could be analysed could not be much smaller than a reasonable fraction of the wavelength of the ultrasound used; in practice this would probably impose a lower limit of a few tens of microns. Commercial instruments are available for detecting particulate matter in suspension in liquids. They are claimed to be sensitive to single particles, even if as small as a fraction of a micron, but the instrument does not have the ability to determine particle size. The scattering of ultrasound by particulate matter in suspension also means that the amplitude of an ultrasonic beam transmitted across the flowing liquid is modulated. The modulation depends in a complex way on the velocity, the mean particle size and the suspended solid concentration. Hence, if two of these parameters are known from other techniques, the third can be determined ultrasonically. An instrument based on a similar principle has been used to monitor traces of air bubbles (at the VPM level) in flowing water.
Acoustic impedance The acoustic impedance is the product of the velocity of ultrasound in a medium and its density. The acoustic impedance mismatch at an interface between two media controls the relative amounts of ultrasound reflected and transmitted across that interface. Simple on-line devices are available which determine parameters relating to this acoustic impedance mismatch between the wall of the vessel containing the medium and the medium itself. They operate, in effect, by determining the amount of ultrasound reflected back at the interface between the wall and the liquid. Therefore they have the very useful advantage of not demanding that the liquid is transparent to ultrasound. This greatly increases the range of materials which they can interrogate successfully. The technique is certainly sensitive enough to identify typical liquids and, under favourable circumstances, to measure concentrations of solutions. It appears also to offer a possibility of determining the concentration of emulsions and, perhaps, solid dispersions, which cannot be analysed ultrasonically by other techniques because they are opaque to ultrasound. Emulsions are particularly attractive in this respect because very often liquids of low density have low velocities and liquids of high density have high velocities so that the difference in acoustic impedance is attenuated. The device would almost certainly require "spot' calibration in situ.
Conclusions Determination of the velocity of ultrasound can be used to identify liquids, analyse mixtures of liquids, determine the concentrations of solutions and dispersions, and identify and analyse gases. Ultrasonic attenuation cannot be determined easily in the absolute sense but, nevertheless, instruments for determining concentrations of solutions and dispersions semi-quantitatively are available. Ultrasonic scattering has been exploited to a limited extent for detecting dispersed particulate matter and bubbles but the potential for particle size determination has not yet been fully explored. Acoustic impedance mismatch determinations may be a useful means of determining the concentration of solutions and dispersions, particularly when they are opaque to ultrasound.
Ultrasonics 1987 Vol 25 January
19
Keywords: sonochemistry; chemical analysis
One does not tend to think of the use of ultrasound as a basis for techniques for chemical analysis. Nevertheless, I hope to show that, within certain limitations, it has a number of attractions. An important advantage of ultrasonic techniques is that, very often, they can be used non-invasively, i.e. the containment of the plant need not be breached. This results from the fact that containment materials, particularly metals, are usually 'transparent' to ultrasound. Other advantages of ultrasonic techniques are that: 1, they do not normally involve 'moving parts'; 2, no radioactivity is used; 3, the response is rapid; 4, the energy levels are low and (hopefully) nonhazardous; and 5, they provide a range of mutually compatible techniques (this point is somewhat philosophical). Another important feature of ultrasonic techniques, which in one respect gives them the edge over optical techniques, is that the velocity of ultrasound is relatively low compared with that of light; hence, for example, the accurate timing of ultrasonic pulses is not too difficult. The principal ultrasonic parameters which can be measured are: velocity, attenuation and scattering of ultrasound, and the acoustic impedance of the medium through which it is passing. These will be considered in turn. From these parameters one can, under favourable circumstances, determine: chemical identity, the concentration (or density) of solutions, mixtures and dispersions, and the particle size of dispersed phases. I must, however, say quite clearly that, in general, ultrasonic analytical techniques are not specific and are not suitable for trace analysis. These disadvantages, however, are shared with many other popular analytical techniques.
Generalizations
*Paper presented at the Sonochemistry Symposium, Annual Chemical Cogress, held at Warwick University, UK, 8-11 April 1986 0041-624X/87/010017-03 $03.00 © 1987 Butterworth ~" Co (Publishers) Ltd
Identification of liquids
Most of the ultrasonic techniques used for chemical analysis use compressional ultrasound (i.e. longitudinal waves). The frequency is typically in the range 0.5-10 MHz, so that, in most materials, the wavelength is somewhere between a fraction o f a millimetre and 10 mm. The power levels are exceedingly low compared with those used for ultrasonic cleaning or emulsification, etc. The ultrasound is normally generated by piezoelectric transducers of small and convenient size which are readily available commercially; these can act both as transmitters and receivers. Some form of 'coupling' such as water or grease is normally used to permit the ultrasound to be transmitted from the transducer to and from the system under study. Electronic systems are available which can energize transducers to produce either continuous waves or pulses, detect and amplify the received signals, time the echoes and carry out a wide range of data processing activities. Applications are not limited to ambient temperatures because some transducers will tolerate elevated temperatures; alternatively, ultrasonic wave guides can be used to allow the transducers to be installed in a cooler part of the plant.
Velocity At present the most fruitful ultrasonic parameter for use in chemical analysis is the velocity of ultrasound. Simple, convenient cells are available for determining this parameter. They can be installed in pipelines or tanks in production plants or used for off-line examination of samples in the laboratory. Having determined the velocity of ultrasound in the medium the data can be used in a number of ways. These ways are now considered in turn.
The velocity of ultrasound in liquids spans a wide range;
Ultrasonics 1987 Vol 25 January
17
Ultrasonics in chemical analysis: R.C. Asher 1000-1500 m s -1 is typical but there are m a n y examples outside this range. Therefore a determination of the velocity of ultrasound in a liquid can be a great help in its identification. This technique bears comparison with the more traditional techniques using melting points, boiling points, refractive index, etc. It has been used to determine the grade of petrol passing along a pipeline and, on a lighter note, it has been used to detect the difference between beer and water.
Concentration of solutions The velocity of ultrasound in a solution usually depends on the concentration of the solution. In favourable cases the velocity changes quite rapidly and moderately linearly with concentration: consequently a reasonably accurate estimate of the concentration can be made from the velocity of ultrasound. In less favourable cases the velocity-concentration curve passes through a m a x i m u m or m i n i m u m and this would result in ambiguity because two solutions may give the same figure for the velocity of ultrasound. Similarly, systems where the velocity varies only slightly with concentration are unattractive. In most cases an additional complication is that the velocity of ultrasound is also dependent on temperature and therefore a correction for this effect must be made. This correction is very often the factor which limits the accuracy with which the concentration may be determined. For most materials the velocity decreases with temperature; the notable exception is water where the temperature coefficient is positive ,-~ + 3 m s-~°C-~): many aqueous solutions show a similar effect. The use of ultrasound to determine concentrations of solutions is a moderately well established technique but is certainly not yet fully exploited. It has been used in nuclear fuel reprocessing plants to determine the concentration o f solutions of fissile material and of simple chemical feedstocks. Although I have described this application in terms of 'concentrations' the instruments can, of course, be calibrated in terms of "density" or any other units which are particularly relevant.
Mixtures of liquids The behaviour of the velocity of ultrasound in mixtures of liquids depends on the nature of the liquids. If there are two liquids, both of which are unassociated and which do not interact with each other, e.g. toluene and c a r b o n tetrachloride, then the velocity of ultrasound (at a particular temperature) is virtually linearly dependent on the composition. If, as in the case of mixtures of ethyl alcohol and carbon tetrachloride, one of the liquids (alcohol) is associated, then there is a deviation from linearity at low alcohol concentrations at which, presumably, association has not yet taken place. W h e n both of the liquids are associated more complicated behaviour is found. For example, with ethyl alcohol-water mixtures, the velocity rises to a m a x i m u m at a concentration which depends on the temperature but which is ~ 35 wt% alcohol at normal temperatures. An interesting consequence of this is that there is a particular concentration ( ~ 20 wt% alcohol) at which the velocity of ultrasound is independent o f temperature. This mixture is therefore useful in the laboratory for calibrating ultrasonic cells. Despite the attractions of measuring the composition of mixtures of liquids ultrasonically it is difficult to find practical industrial examples: the technique is used, however, in the nuclear fuel reprocessing industry for
18
Ultrasonics 1987 Vol 25 January
determining the composition of a tributylphosphatekerosene mixture used in solvent extraction plants.
Two-phase liquid systems Emulsions. A regrettably high proportion of emulsions tend to be 'opaque' to ultrasound. Fortunately, those which are sufficiently "transparent' always seem to show a simple linear relationship between ultrasonic velocity and concentration. This is in accord with a simple, naive pro rata hypothesis. This p h e n o m e n o n is the basis of an instrument for determining the composition of a waterhexanol emulsion associated with another nuclcar fuel cycle plant.
Suspensions. In contrast to emulsions, the situation with regard to the velocity of ultrasound in suspensions appears to be complex. This may be an illusion engendered by the complexity of the theory. Unfortunately the experimental data is fairly limited and, as with emulsions, there is the overriding problem that many suspensions arc rather (or very) opaque to ultrasound. The data, such as it is, is mainly for aqueous suspensions and suggests that the velocity of ultrasound changes monotonically with concentration and that the relationship does not deviate disastrously from linear: the concentration coefficient may be either positive or negative. Empirically it seems that suspensions of "dense' solids tend to show a negative concentration coefficient whereas "light" solids give a positive coefficient. Regardless of the theoretical explanations, the suspension is adequately "transparent" to ultrasound, it is often possible to determine the concentration of suspensions from the velocity of ultrasound.
Bubbly liquids. As with all two-phase systems, the theoretical treatment of the effect of bubbles on the velocity of ultrasound in liquids is complex. The experimental results are scattered and apparently not in accordance with all of the theories. It is, however, clear from experimental observations that the attenuation of ultrasound increases rapidly as more bubbles are introduced and that, therefore, most bubbly liquids are effectively "opaque' to ultrasound. It also seems that the velocity decreases as the void fraction increases and that the relationship is not very dependent on the bubble diameter. Typical experiments show that, for aqueous systems, the velocity of ultrasound drops from ~ 1500 m s -~ lbr bubble free water to ,~200 m s l when as little as a few volume per cent of air are introduced. No analytical instrument making use of the velocity of ultrasound in bubbly liquids has come to my attention.
Foams. Foams are notably 'opaque' to ultrasound. Therefore, to obtain any significant transmission, very low frequency ultrasound would have to be used since the penetration of ultrasound generally increases greatly as the frequency is decreased: this, as a consequence of its long wavelength, would imply great difficulty in measuring the velocity accurately. Therefore the possibility of characterizing foams by measuring the velocity of ultrasound seems slim.
Gases. For perfect gases, the velocity of ultrasound is given by
Ultrasonics in chemical analysis: R.C. Asher where: R is the gas constant; T the absolute temperature; 7 the ratio of the principle specific heats; and M the molecular weight. The velocity, therefore, is dependent on the molecular weight and the temperature but not (at least for perfect gases) on the pressure; this generalization also breaks down when the molecular mean free path is comparable with the ultrasonic wavelength. The range of velocities for different gases (e.g. 1300 m s-1 for hydrogen, 970 m s -1 for helium and 310 m s-1 for oxygen, all at room temperature) means that many pure gases can be identified ultrasonically. With mixtures of gases the velocity varies linearly between the velocity in one pure gas and the velocity in the other pure gas; therefore mixtures of two known gases can be analysed by determining the velocity of ultrasound in the mixture. This is especially attractive for mixtures of gases differing greatly in molecular weight. Therefore, in particular, it would appear relatively easy to analyse hydrogen-deuterium mixtures and deuterium-tritium mixtures ultrasonically. It might also be possible, in principle, to determine ortho-hydrogen/para-hydrogen ratios this way because, although these two gases have the same molecular weights, the ratios of the principal specific heat are slightly different. Gas chromatographic detectors making use of the change in the velocity of ultrasound in a carrier gas resulting from the presence of traces of elutriants are commercially available.
Attenuation Attenuation of ultrasound is difficult to measure absolutely, especially under plant conditions. The basic reason is that ultrasonic signal strength decreases not solely as a result of attenuation in the medium under study but also because of other factors such as reflection at interfaces, beam divergence, and changes in transmitter and receiver performance. These factors are not easy to keep constant. Therefore, it is most convenient for instruments to be calibrated in situ. Despite this there are successful proprietary instruments which determine when the concentration of, for example, sewage slurry reaches a predetermined level. An aspect of attenuation which does not seem to have been exploited commercially is the relationship of attenuation to particle size in two-phase systems. This might permit some deductions to be made about particle sizes. The situation becomes more promising when the 'particle' is, in fact, a bubble because, in this case, the bubble can deform in various ways in resonance with the ultrasound. As a result the absorption cross-section of a bubble increases by several orders of magnitude at its resonant frequency. Hence, from the resonant frequency, it should be possible to deduce the bubble size.
Scattering The literature shows that the scatter of ultrasound from a two-phase dispersion is dependent on the concentration of the dispersion and its particle size. Thus, the energy of the scattered ultrasound in a particular direction is dependent (up to a certain limit) on the concentration, and the angular distribution ('polar diagram') is dependent on the particle size. In principle it may be possible to devise an instrument to measure concentration and particle size in this way. However, a restriction would
probably be that the particle size which could be analysed could not be much smaller than a reasonable fraction of the wavelength of the ultrasound used; in practice this would probably impose a lower limit of a few tens of microns. Commercial instruments are available for detecting particulate matter in suspension in liquids. They are claimed to be sensitive to single particles, even if as small as a fraction of a micron, but the instrument does not have the ability to determine particle size. The scattering of ultrasound by particulate matter in suspension also means that the amplitude of an ultrasonic beam transmitted across the flowing liquid is modulated. The modulation depends in a complex way on the velocity, the mean particle size and the suspended solid concentration. Hence, if two of these parameters are known from other techniques, the third can be determined ultrasonically. An instrument based on a similar principle has been used to monitor traces of air bubbles (at the VPM level) in flowing water.
Acoustic impedance The acoustic impedance is the product of the velocity of ultrasound in a medium and its density. The acoustic impedance mismatch at an interface between two media controls the relative amounts of ultrasound reflected and transmitted across that interface. Simple on-line devices are available which determine parameters relating to this acoustic impedance mismatch between the wall of the vessel containing the medium and the medium itself. They operate, in effect, by determining the amount of ultrasound reflected back at the interface between the wall and the liquid. Therefore they have the very useful advantage of not demanding that the liquid is transparent to ultrasound. This greatly increases the range of materials which they can interrogate successfully. The technique is certainly sensitive enough to identify typical liquids and, under favourable circumstances, to measure concentrations of solutions. It appears also to offer a possibility of determining the concentration of emulsions and, perhaps, solid dispersions, which cannot be analysed ultrasonically by other techniques because they are opaque to ultrasound. Emulsions are particularly attractive in this respect because very often liquids of low density have low velocities and liquids of high density have high velocities so that the difference in acoustic impedance is attenuated. The device would almost certainly require "spot' calibration in situ.
Conclusions Determination of the velocity of ultrasound can be used to identify liquids, analyse mixtures of liquids, determine the concentrations of solutions and dispersions, and identify and analyse gases. Ultrasonic attenuation cannot be determined easily in the absolute sense but, nevertheless, instruments for determining concentrations of solutions and dispersions semi-quantitatively are available. Ultrasonic scattering has been exploited to a limited extent for detecting dispersed particulate matter and bubbles but the potential for particle size determination has not yet been fully explored. Acoustic impedance mismatch determinations may be a useful means of determining the concentration of solutions and dispersions, particularly when they are opaque to ultrasound.
Ultrasonics 1987 Vol 25 January
19