EXPERIMENTAL. This chapter includes the experimental details for the measurement of

Transcription

1 EXPERIMENTAL 3.1 Introduction This chapter includes the experimental details for the measurement of hydrodynamic permeability and electro-osmotic permeability of aqueous solutions of KNO 3, Pb(NO 3 ) 2, Cd(NO 3 ) 2.4H 2 O and Al(NO 3 ) 3.9H 2 O across nylon-66, CA and Pyrex-sintered (G-3) membranes along with measurement of streaming current and conductance of membrane-permeant system. 3.2 Materials Membranes In this work, we have used nylon-66 membrane, cellulose acetate (CA) membrane and pyrex-sintered (G-3) membrane for performing the experiment. Nylon-66 membrane (Cat No P) and CA membrane (Cat No R) were purchased from Axiva Sichem Biotech, New Delhi and Pyrex-sintered (G-3) membrane was purchased from Zenith Glassware & Instruments Corporation, Kolkata Permeants Aqueous solutions of analytical grade KNO 3 (Loba Chemie Pvt. Ltd., Mumbai), Pb(NO 3 ) 2 (Thermo Fisher Scientific, Mumbai), Cd(NO 3 ) 2.4H 2 O (Qualigens Fine Chemicals, Mumbai) and Al(NO 3 ) 3.9H 2 O (SDFCL, Hyderabad) were used as permeants. Aqueous solutions of these salts in the concentration range of 10-4 to 10-3 M were prepared using doubly distilled water. 3.3 Permeability Cell The experimental set-up (permeability cell) shown in Figure 3.01, was used for measurements of hydrodynamic permeability, electro-osmotic flux and streaming current. A and A are two Pyrex glass tubes of equal length having B-19 female joints at the ends, the membrane M is fixed in the middle part. J and J are B-19 male joints inserted in female joints. B and B are two side tubes attached

2 with the cell having standard B-14 female joints at their upper ends. A glass tube C having a male B-14 joint at its lower end and a horizontal uniform graduated capillary tube F (length 10 cm and cross section area cm 2 ) at its upper end, is fitted at B. It also consists of a horizontal tube having stopcock D to adjust the liquid level in the capillary tube. At B another glass tube i.e. burette G with uniform diameter having a male B-14 joint at its lower end C is inserted. The upper part of burette is clamped with stand S for support. Figure 3.01: Experimental Set-Up for Permeability Measurement E and E, the Pt-wire electrodes were fitted in the experimental cell in such a way so as to press the membrane from both the sides and covered maximum membrane surface. Hg is liquid mercury and Cu is copper wire for making electrical contacts with the instruments. L and L indicate aqueous solutions filled in the experimental cell. 3.4 Scanning Electron Microscopic (SEM) Analysis of the Membrane The microstructures of Nylon-66 membrane and Cellulose acetate membrane were observed using scanning electron microscope (Model - QUANTA 200 F) before performing the experiment. Images of both membranes were taken at Page 67

3 different magnification range for viewing the surface morphology and pore size distribution in the membrane. 3.5 Measurement of Membrane Conductance Conductance of membrane equilibrated with different permeants was measured with the help of conductivity meter (Systronics-304, cell constant 1± 0.1). It was measured before and after electro-osmosis with the same apparatus. Measurement on conductance of membrane-equilibrated with permeants is useful to get insight into the electro-kinetic properties of the membrane porous bodies [1] which depends on the pore size and the zeta potential of the membrane. The electrical conductivity inside the membrane may be higher than the bulk conductivity [2], these effects happen due to high salt concentrations. Another conductivity meter (Elico CM 180, cell constant 1.03) was used for the measurement of specific conductance of water and all aqueous salts solutions. The conductance values of membrane-permeant system and specific conductance were found to be in increasing order due to increasing concentrations for all the solutions. 3.6 Measurement of Viscosity Coefficient of the Permeants The viscosity coefficient (η) of all the solutions was measured using Ostwald-viscometer, which consisted of a U-shaped glass tube held vertically. The liquid was drawn into the upper bulb by suction, and then allowed to flow down through the capillary into the lower bulb. Two marks (one above and one below the upper bulb) indicate a known volume. The time required for flow in between these two marks was noted. The following formula has been used for the determination of viscosity coefficient for all the permeants [3]. η = η t d t d Page 68

4 where η 1 and η 2 are the viscosity coefficients, t 1 and t 2 are time of flow and d 1 and d 2 are the densities for the permeants and conductivity water respectively. It was found that, the values of viscosity coefficient increases on increasing the concentration for each of the solutions due to increase in resistance. 3.7 Determination of Cross-Sectional Area of the Capillary Tube Cross-sectional area of the capillary tube was calculated by determining the weight (W) of Mercury (Hg) filled in a capillary of known length (l). The weight of mercury is given by W = πr l d; or πr = l where d is the density of the mercury. The cross sectional area, πr 2, of the capillary tube was found to be cm Measurement of Hydrodynamic Permeability It is a pressure-driven membrane process in which volume flow occurs due to pressure-difference across the membrane. Measurement on hydrodynamic permeabilities was done by using the technique already described by many workers [4-8]. In case of nylon-66 and cellulose acetate membranes the experimental cell was filled with the liquid under investigation and left as such for 10 to 12 hours for equilibration before use. The pyrex-sintered (G-3) membrane was cleaned with dilute HNO 3 as well as distilled water to remove traces of nitric acid [6]. Due to higher porosity of pyrex-sintered (G-3) membrane, it was left as such for 2 to 3 hours for equilibration. For maintaining the equilibrium, the cell was subsequently filled with a degassed fresh solution to confirm that the concentration of the experimental solution remained the same on both sides of the membrane. A desired hydrostatic pressure on two sides of the membrane was applied with the help of graduated burette as shown in Figure 3.01, and the rate of transport of liquid was measured by noting the time taken by liquid/air meniscus to move a Page 69

5 certain fixed distance within the horizontal exit capillary tube. The time was recorded with the help of a stop watch having least count 0.1 second. The volume flux, Jv, was calculated using the following Where, ( J ) ϕ = πr. dx dt dt = time taken dx = fixed distance in capillary tube πr 2 = cross sectional area of capillary tube 3.9 Measurement of Electro-Osmotic Permeability It is an electro-driven membrane process in which volume flow occurs due to potential difference across the membrane. Measurement on electro-osmotic permeability was done by using the technique already given by other workers [7-9]. For electro-osmotic permeability measurement across the membranes the liquid levels in the pressure head P and the capillary tube were kept at equal height to maintain pressure difference zero ( P = 0). For all the solutions varying potential-differences were applied in the range of V across the nylon-66 and cellulose acetate membranes using Pt-electrodes with the help of an electronically operated power supply (Medox-Bio, Power Supply 300 Advanced, Chennai, India, Figure 3.02). Electro-osmotic permeability measurements through pyrex-sintered (G-3) membrane for all the solutions were carried out with the help of an electronically operated power supply (Power station 300, Lab net, International, Figure 3.03). The volume flux was measured by following the rate of displacement of the solution-air meniscus in a horizontally placed graduated capillary tube with a cross sectional area of cm 2. All experiments were carried out in an air thermostat maintained at 30 ± 0.5 o C. Page 70

6 Fig. 3.02: Medox-Bio, Power Supply 300 Advanced Fig. 3.03: Labnet International, Power Station 300 The following equation was used to calculate the electro-osmotic volume flux. Where, ( J ) = πr. dx dt dt = time taken dx = fixed distance in capillary tube πr 2 = cross sectional area of capillary tube Page 71

7 3.10 Measurement of Streaming Current It is a pressure-driven membrane process and electro-kinetic phenomenon in which current flows due to pressure difference across the membrane. (I) ϕ = L T ( P) Streaming current generated during the transport of permeants by application of various pressure-differences on two sides of the membrane was measured with a Digital Picoammeter (Model PM-100, Raman Scientific Instruments, Roorkee, India) as shown in Figure 3.04, using the technique as described by other workers [5, 7, 8]. During current measurements it may be noted that the measurements were always started with the highest current range i.e A and then changed over to the lower ranges with the help of range multiplier. The reading indicated by the current range selector gave the full scale deflection. Due to less sensitivity of the instrument the streaming current at lower pressure could not be measured correctly. Fig. 3.04: Digital Picoammeter, Model PM-100, Raman Scientific Instruments Page 72

8 3.11 Reproducibility of Results and Source of Errors During experiment the results obtained may be influenced by the following factors: Incomplete wetting of the membrane The rate of flow of liquid depends upon the number of capillaries available for the transmission of the liquids. If all the capillaries are not wet poor reproducibility would be expected. Equilibrium of the membrane is likely to ensure complete removal of air contained within the pores of the membrane Blocking of the capillary channels The presences of suspended impurities in the permeating solutions are also responsible for the blocking of pores. Hence, solutions were filtered through microspores filters before use Greasy materials in the membrane Flow of permeants through membranes is greatly affected by the presence of grease in the pores as they may check the passage of permeants due to their hydrophobic nature Polarization of the electrodes During electro-osmotic measurements at higher voltages, the reproducibility of the results was likely to be affected by the evolution of gases at the electrodes. The bubbles may adhere at electrodes, thereby reducing the effect of applied electric field, resulting in a reduced flow rate. This was however, minimized by using freshly boiled conductivity water and permeants for filling the apparatus. Page 73

9 References: [1.] A. E. Yaroshchuk, T. Luxbacher, Interpretation of electrokinetic measurements with porous films: Role of electric conductance and streaming current within porous structure, Langmuir, 26 (2010), [2.] A. Szymczyk, P. Fievet, B. Aoubiza, C. Simon, J. Pagetti, An application of the space charge model to the electrolyte conductivity inside a charged micro-porous membrane, J. Membr. Sci., 161 (1999), [3.] H. A. Barnes, J. F. Hutton, K. Walters, An Introduction to Rheology, Chapter 2, Elsevier, [4.] R. P. Rastogi, K. M. Jha, Cross-phenomenological coefficients. Part 4. Electro-osmosis of acetone, Trans. Faraday Soc., 62 (1966), [5.] R. P. Rastogi, K. Singh, R. Kumar, R. Shabd, Electrokinetic studies on ionexchange membranes: III. Electroosmosis and electrophoresis, J. Membr. Sci., 2 (1977), [6.] R. P. Rastogi, K. Singh, M. L. Srivastava, Cross-phenomenological coefficients. XI. Nonlinear transport equations, J. Phys. Chem., 73 (1969), [7.] M. L. Srivastava, B. Ram, Electrokinetic studies on testosterone/aqueous electrolyte interface: Part 1. Electro-osmosis, electrophoresis, streaming potential and streaming current, J. Membr. Sci., 19 (1984), [8.] M. L. Srivastava, B. Ram, Electrokinetic studies of the testosterone aqueous d-glucose interface, Carbohydr. Res., 132 (1984), [9.] M. L. Srivastava, B. Ram, Electrokinetic phenomena at the testosterone/aqueous urea interface, J. Non-Equilib. Thermodyn., 10 (1985), Page 74