CHAPTER- 2 (EXPERIMENTAL METHODS)

Transcription

1 CHAPTER- 2 (EXPERIMENTAL METHODS)

2 EXPERIMENTAL METHODS This chapter encloses the detailed study about experimental techniques, purification and the materials employed in the present investigation 2.1. EXPERIMENTAL TECHNIQUES & INSTRUMENTATION ULTRASONIC SOUND VELOCITY: There are mainly three experimental techniques to measure the velocity of ultrasonic sounds in pure liquids and liquid solutions. They are: (i) Continuous wave technique (ii) Pulse technique and (iii) Interferometer method. i) Continuous wave technique This technique includes optical methods based on the study 1-5 of the acoustic, grating and interferometer methods 6-8 that excite a liquid column into resonance. Variable path interferometers have been widely used and recent advances permit accuracies of a few parts per million in sound velocity measurements. A double crystal interferometer was employed to measure sound velocity at 500 MHz by Hunter and Dardy 9. Sound velocity measurements have also been carried out with other type of interferometers in which the resonance of the liquid filling a cavity was studied. Barium titanate cylinder filled with the experimental liquid, plated in such a way that one-half acts source and other half acts as receiver was used by Dobbs and Finegold 10. Fort and Moore 11 measures sound velocity values for fourteen binary liquid mixtures using single crystal interferometer at 400 KHz. The interferometer was similar to that of Hubbard and Loomis 12 but incorporated a barium titanate crystal in the place of Quartz crystal. This replacement had become necessary to 41

3 avoid the need for high voltage and consequent insulation difficulties. The uncertainty in the sound velocity measurements was estimated to be ± 0.15%. ii) Pulse technique The pulse technique has two main advantages over continuous wave methods for the measurement of absorption. First is the elimination of standing waves and the second one is the minimization of excessive local heating effects of the solution. In this method, the input transducer is activated by the signal source (a pulsed oscillator) and a train of radio frequency pulses are generated. After traveling a variable distance through the solution, the sound pulses are received by the output transducer and the attenuation is measured as pressure amplitude as a function of distance. The output signal is compared with the signal passed through a calibrated attenuation. One can also determine the sound velocity by this method by determining the delay time between the transmitted and received pulses. This technique is capable of giving accuracy of 1 part in 30,000 were developed by Green span and Tschiegg 13 Meskimin 14 Barlow and Yazgan 15, a double crystal interferometer developed by Chase 16 in which velocity (measurements) changes can be determined with an accuracy of 0.005%. The velocity measurements were made by Kiyohara and co-workers 17,18 with respect to pulses, using the pulse echo-overlap method in multiple echo - mode. iii) Interferometer method Comparing the relative merits of the various methods, interferometer is the most accurate method available for velocity measurements Hunter and Dardy 9, Dobbs and Fine gold 10, Fort and Moore 11 measured the velocity of sound for liquids and liquid mixtures by using interferometric technique with ± 0.15% uncertainty. 42

4 The author has chosen a single crystal variable path interferometer which has been employed for the present study of ultrasonic velocities supplied by Mittal enterprises (F-80X). This consists of two parts. 1. The High Frequency Generator: 1, 3, 5MHz 2. The measuring cell: 1MHz The High Frequency Generator is designed to excite the quartz crystal fixed at the bottom of the measuring cell at its resonant frequency to generate ultrasonic waves in experimental liquid filled in the measuring cell. A micrometer to observe the changes in current and two controls for the purpose of sensitivity regulation and initial adjustments of the micrometer are provided on the panel of the high frequency generator. The measuring cell: - This is specially designed double walled cell for maintaining the temperature of the liquid constant during the experiment. A fine micrometer screw has been provided at the top, which can lower or raise the reflector plate in the liquid cell through a known distance if a quartz crystal has fixed at its bottom. The sketch of the interferometer cell is shown in Fig.2.1. Experimental procedure: The high frequency generator has two knobs ADJ to adjust the position of the needle on the ammeter and the knob marked GAIN which is used to increase the sensitivity of the instrument for greater deflection. The measuring cell is made up of stainless steel (12cc capacity) tube of inner diameter 0.9cm (12cc capacity) is cemented at bottom of the cell. The ultrasonic waves move from this crystal till they reflected by a movable reflector. A fused quartz rod is used as a reflector. The reflecting surface is 1.4cm in diameter. This is coupled with micrometer screw assembly reading up to mm using Teflon couples via the steel rod. The micrometers screw activates the steel rod which is spring loaded to 43

5 take care of any back case. The micrometer assembly, along with reflector is fixed to the liquid cell with the help of a threaded cap and Teflon ring. Unscrewed the knurled cap of cell and lifted it away from double walled construction of the cell. The experimental liquid or liquid solution is poured in the middle portion of it and screwed the knurled cap. The excess liquid overflowing from the cell is wiped. Then the cell is inserted in the square base socket and clamped it with the help of a screw provided on its side. The high frequency generator is connected with cell by co-axial cable provided with the instrument. The micrometer is slowly moved in either clock wise or anti clockwise direction till the anode current on the ammeter on the high frequency generator shows a maximum or minimum. The readings of micrometer corresponding to the maximum or minimum in micrometer were recorded. About 50 readings of consecutive maximum or minimum are taken and the average of all the differences calculated (λ/2). The sound velocity is evaluated from the knowledge of λ using the formula, U = λ. f, where f is the frequency of the generator i.e., 3MHz. The values of the sound velocity are accurate to ± 0.5 ms -1. The performance of the interferometer was checked by comparing the sound velocities recorded in the present study with those of reported in the literature. The following precautions have to be taken while measuring sound velocity. i. The Generator should not be switched on without filling the experimental liquid in the cell. ii. The experimental liquid should be removed immediately and the cell should be kept clean and dry. iii. The micrometer should be kept open 25mm after use. 44

6 iv. Sudden rise or fail in temperature of circulated liquid should be avoided to prevent thermal shock to the quartz crystal. v. While cleaning the gold plating on the quartz crystal it should not be scratched vi. The Generator is allowed for 15 seconds for warming up before the observation Technical specifications: 1. High Frequency Generator: Mains voltage 220v, 50Hz, glow lamp -6.3v, 0.3amp fuse-500ma, size 41x30x13cm, weight -3.5kg approx. 2. Measuring cell: Frequency MHz, Maximum displacement of the reflector 20mm, required quantity of liquid -12cc, least count of micrometer mm, size x11.5x31cm, weight -3.75M approx. Least count =1/2x1/50x1/10mm = 0.001mm 3. Shielded cable: Length of connecting cable between the generator and the cell is 50cm approximately. Working Principle: An ultrasonic interferometer is a simple and direct device to determine the ultrasonic velocity in liquids with high degree of accuracy. The principle used in the measurement of velocity (U) is based on the accurate determination of the wavelength (λ) in the medium. Ultrasonic waves of known frequency (f) are produced by a quartz crystal fixed at the bottom of the cell. These waves are reflected by a movable metallic plate kept parallel to the quartz crystal. If the 45

7 separation between these two plates is exactly a whole multiple of the sound wavelength, standing waves are formed in the medium, these acoustic resonance gives rise to an electrical reaction on the generator driving the quartz crystal and the anode current of the generator becomes a maximum. If the distance is now increased or decreased and the variation is exactly one half wavelength (λ / 2) or multiple of it, anode current becomes maximum. From the knowledge of wavelength (λ) the velocity (U) can be obtained by the relation, U = λ. f Figure 2.1 Trouble shooting: If the deflection in the ammeter is nil or insufficient for any particular liquid, try to increase the same with the help of GAIN knob. If problem still persist the following instructions should follow. Turn trimmer A on the back side of the 46

8 generator till maximum deflection is achieved in Ammeter. Then turn trimmer B for minimum deflection. Repeat the process by turning trimmer A again for maximum followed by turning trimmer B for minimum. This can be done for a number of times till they are properly tuned. Figure 2.2. Photograph of experimental set-up Density measurements Various techniques used for density measurements are, i) Pycnometer ii) Mechanical oscillator densimeters iii) Magnetic float densimeters and iv) Specific gravity bottle. (i) Pycnometer: Measurement of density using Pycnometers still remain the simplest and least expensive procedure in terms of readily available equipment but may be the most expensive in terms of time. Filling the pycnometer reproducibly at a given temperature is a difficult task. The formation of bubbles is a frequent source of error. Irreproducible wetting of the capillary by the meniscus leads to error in measurement of the liquid level and calibrating the volume of the pycnometer as a 47

9 function of temperature introduces additional errors. Bauer and Lewin 19 have given a through treatment on the methodology, along with diagrams of various kinds of pycnometers. The recommended size for pycnometers is 10 to 30 cm 3. Use of pycnometers of size less than 10cm 3 presents difficulties in precise weighings and volume calibration whereas pycnometers of size greater than 30 cm 3 often results in the loss of balance sensitivity, greater probability of changes in volume, and the uncertainties associated with a larger glass surface. Pycnometric measurements are subject to a number of corrections, the most important ones being the corrections for buoyancy and amount of liquid or liquid mixture in the vapor phase. However careful measurements using classical method can provide very precise density values. (ii) Mechanical oscillator densimeters: Digital readout densimeters which operate on the principle of measuring the resonant frequency of an electronically excited mechanical oscillator are becoming increasingly popular. The first densimeter based on this principle was developed by Kratky and coworkers In order to define a plane of vibration and to eliminate the risk of an elliptic vibration with an ambiguous resonant frequency, Kratky et al. 23 used a hallow V-shaped glass tube as vibrator. Such a shape facilitates the filling and rinsing procedures and can also be adapted to making measurements on flowing fluids. Temperature fluctuations of the metal block holding the oscillator and variations in the mole fraction of the liquid mixture during preparation and injection can be major sources of errors but these factors can easily be controlled in the manner suggested by Goates et al. 24 and Radojkovic et al

10 Chernov and Yakovlev 26 havedescribed an amplitude method for measuring density. A flat disc probe driven by a constant frequency vibrator is immersed in the liquid sample and made to vibrate at right angles to the surface of the liquid. Density is determined from noting the change in the amplitude of vibration relative to the vibrational amplitude in a liquid of known density. Chernov and Yakovlev have applied this method to measuring densities of organic liquid mixtures. This method is capable of high precision only if the calibration curves for the probe are constructed from groups of liquids of comparable viscosity and are used for determining densities of liquids having similar viscosities. Viscosity differences greater than 3-4 cp can affect the measurements. (iii) Magnetic float densimeters: These densimeters operate on the principle of balancing the opposing effects of gravity, buoyancy and magnetic field on a float containing a permanent magnet or a soft iron core. This is done by passing current through a solenoid placed below the cell containing the liquid. Density is related to the current at which the float first lifts off from the bottom of the cell. Normally such densimeters can cover only a relatively small density range and require relatively large volumes of the sample for good precision 27. Chosen method (iv) Specific gravity bottle: The density of pure liquid and liquid mixtures is measured by employing a specific gravity bottle The volume of the specific gravity bottle is standardized by using the temperatures , , , and K. The relative density of the liquid and liquid mixtures is calculated with respect to doubled distilled water using the following relation. 49

11 Relative density (ρ) = (Wt L / Wt w )* d w where Wt L, Wt w are the weights of liquid and water respectively and d w is density of water at a particular temperature. The density values of water of different temperatures have been taken from the literature. In order to maintain the constant temperature, the specific gravity bottle with the experimental solution is immersed up to the neck in the thermostat with the thermal stability of ± 0.01K. The accuracy in the measurement of density employing the specific gravity bottle is better than ± 0.5%. In the present study the weight of the liquid and liquid mixture is measured by using a single pan electronic balance (Shimadzu AUY220, Japan) capable of measuring up to ±0.1mg Viscosity measurements Viscosities of a pure liquid and liquid mixture have been measured by using an Ostwald s glass capillary viscometer 29,30. The viscometer consists of two limbs labeled as A and B. Limb A consists of a large bulb at the bottom in which the liquid can be taken. Limb B consists of a capillary and two markings above and below of the bulb of limb B. initially the viscometer is thoroughly cleaned and dried using a hot air blow, then the solution of known concentration is poured in the limb A (nearly 10ml). The solution is sucked into the bulb B by using a rubber bulb at the top of the limb B. When the solution just crosses the upper mark, the rubber bulb is removed. 50

12 Figure 2.3 The solution now begins to fall due to the gravitational force, when the solution reached the upper mark, immediately a stop watch (1/100 sec) is switched on and the time required to pass the solution lower mark is measured. Now the solution is removed from the viscometer and it is thoroughly rinsed with the solvent acetone and dried with hot blower. In order to maintain constant temperature the viscometer is immersed in the thermostat the thermal stability of the thermostat is ±0.01K. If t and t 0 are the time of flow (of liquid and water) and ρ and ρ 0 are the densities of the liquid and water respectively, then the absolute viscosity of the liquid of liquid solution can be calculated using the relation. η = [(ρ x t) / (ρ 0 x t 0 ) ] x η 0 where η 0 is the viscosity of the pure distilled water at the experimental temperature. The viscosities of water at , , and K are collected from literature the accuracy of viscosity measurement is ± mpas. 51

13 The following precautions were taken for getting accurate and reproducible results. Care was taken while mounting the viscometer to see that the capillary is always held in vertical position. All measurements are made at a constant temperature with an accuracy of ± 0.01 K, since viscosity is highly temperature sensitive. The viscosity of water at a given temperature has been taken from literature 31. The solutions are equilibrated with thermostat for 15 minutes, so that the experimental solution attains the same temperature of the thermostat Thermostat The temperature of the solution under study is maintained constant using the electronic thermostat. This thermostat is equipped with a heater, a stirrer, a thermometer and a regulator Materials employed & Purification The mass fraction purity of the liquids (obtained from Merck) was as follows: o-xylene (0.990), m-xylene (0.990), p-xylene (0.990), mesitylene (0.990), o-cresol (0.990), m-cresol (0.990) and p-cresol (0.980) and the purity of the liquid (obtained from SRL chemicals), anisaldehyde (0.990). All the liquids used in the present study were further purified by the standard procedure

14 REFERENCES 1. Debye P and Sears P W, Proc Natl Acad Sci (U.S.) 18 (1932) Lucas R and Biquard P, Computes Rendus, 194 (1932) 2132 & Ibid, 195 (1932) Baron A, Nuoro Cimento, 5 (1957) Bergman L and Hirzal Ed, Der Ultra Schall, Stuttgart (1954) 5. Schaff s W. Molekulara Kustik, Springer, Berlin (1963) 6. Bachem C. Z Physik. 101 (1936) Hubbard J C. Phys Rev. 38 (1931) Freyer E B, Hubbard J C and Andrews D H. J Acoust Soc Amer. 51 (1929) Hunter Y L and Dardy H D. J Acoust Soc Amer. 36 (1964) Dobbs E R and Finegold L. Ibid, 32 (1960) Fort R J and Moore W R. Trans Faraday Soc. 61 (1965) Hubbard J C and Loomis, Phil Mag. 5 (1946) Green Sp M and Tschieyg C E. J Research Natl bul Standards 59 (1957) Meskimin H Y. J Accoust Soc Amer. 37 (1965) Bavlow A Y, Yazgan E and Brit. J Appl Phys 17(1966) Chase C E. Phys Fluids 1(1958) Kiyohara O, Benson G C and Grolier Y P E. Can J Chem 52 (1974) Kiyohark O, Halpim C Y and Binson G C. Ibid 55 (1977) Bauer N and Lewin S. Physical methods of Chemistry, Interscience, New York. 1(4) Chapter 2 (1971) 20. Stabinger H, Kratky O and Leopold H. Monatsh Chem. 98 (1967)

15 21. Kratky O, Leopold H and Stabinger H. Z. Angew Phys. 27 (1969) Leopold H. Elektron 19 (1979) Kratky O, Leopold H and Stabinger H. Methods in Enzymology, Academic Press, New York. 27 Part-D.(1973) Goates J R, Ott J B and Moellmer J R. J Chem Thermodyn. 9 (1977) Radojkovic N, Tasic A, Djordjevic B and Grozdanic D. J Chem Thermodyn. 8 (1976) Chernov R V and Yakovlev B V. Russ J Phys Chem. 49 (1975) Millero F J. Rev. Sci. Instrum. 3 (1967) Rama Rao G V, Viswanatha Sarma A and Rambabu C. Indian J Pure Appl Phy. 42 (2004) Kannappan V and Jaya Santhi R. Indian J Pure Appl Phy. 44 (2006) Thirumaran S and Ramesh J. Rasayan J Chem. 2 (2009) Lide D R, CRC Handbook of Chemistry and Physics, 81 st Edn., CRC Press, Inc., Perrin D D and Armarego W L F. Purification of Lab. Chem., 3 rd ed., Pregamon Press, Oxford,