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Desalination
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A comparative study of theoretical, electrochemical and ionic transportthrough PVC based Cu3(PO4)2 and polystyrene supported Ni3(PO4)2composite ion exchange porous membranes
Mohd Arsalan, Mohammad Mujahid Ali Khan ⁎, Rafiuddin ⁎Membrane Research Laboratory, Department of Chemistry, Aligarh Muslim University, Aligarh 202 002, India
H I G H L I G H T S G R A P H I C A L A B S T R A C T
• A comparative study of the ion exchangecomposite membrane
• The membrane was found to be quitestable.
• Good practical applications• The electrochemical studies give upapplicability of the membranes.
Article history:Received 1 January 2013Received in revised form 8 March 2013Accepted 11 March 2013Available online 23 April 2013
Keywords:TMS methodComparative studiesSEM images with EDSCharge densityTGA/DTA analysis
The organic–inorganic composite ion exchange membranes, i.e. PVC based Cu3(PO4)2 and polystyrenesupported Ni3(PO4)2 were chemically prepared by sol–gel method. The physico-chemical nature was deter-mined by using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-raydiffraction (XRD) and thermo-gravimetric analysis (TGA) and differential thermal analysis (DTA) character-ization. Both the membranes were found to be crystalline in nature and no sign of visible cracks is there. Thetheoretical value of the potential was determined by Teorell–Meyer–Sievers method, electrochemical prop-erties were measured by potentiometer. The observed potential for both the membranes was little differentand positive, the potential offered by the electrolytes was in the order of KCl > NaCl > LiCl. The transportnumber, mobility ratio, distribution coefficient, charge effectiveness, and the fixed charge density of theions were calculated by potential observation. The order of surface charge density of both the membraneswas found to be LiCl b NaCl b KCl.
Nowadays, ion-exchange membranes and its related process have re-ceived much attention in both theoretical and industrial applications. Ionexchange membrane-based processes are still lacking except for some
AR Analytical reagentC1, C2 Concentrations of electrolyte solution on either side of
the membrane (mol/l)C1þ Cation concentration in membrane phase 1 (mol/l)C2þ Cation concentration in membrane phase 2 (mol/l)Ci ith ion concentration of external solution (mol/l)Ci ith ion concentration in membrane phase (mol/l)D Charge density in membrane (eq/l)F Faraday constant (C/mol)100 MPa Pressure (MPa)q Charge effectiveness of the membraneR Gas constant (J/K/mol)SCE Saturated calomel electrodeSEM Scanning electron microscopyTMS Teorell, Meyer and Sieverst+ Transport number of cationt− Transport number of anionu Mobility of cations in the membrane phase (m2/v/s)v Mobility of anions in the membrane phase (m2/v/s)Vk Valency of cationVx Valency of fixed-charge groupU U ¼ u−vð Þ= u þ vð ÞCP Copper phosphatePVC Polyvinyl chlorideNP Nickel phosphate
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reviews on specific aspects, such as modifications of ion exchange mem-branes [1–3].
The inorganic and organic membranes have lower thermal andchemical resistance as well as a shorter lifetime and are expensiveas compared to the inorganic–organic composite membranes [4–6].
The polymers are considered the main materials in preparation ofcomposite membranes due to the advantages of its good membrane-forming ability, flexibility and low cost. Thus, organic–inorganic compos-ite materials have attracted more and more interest. The compositemembranes have the basic properties of organic and inorganic materialsand put forward specific advantages for the preparation of syntheticmembranes with good thermal and chemical resistance, outstandingseparation performances, and adaptability to harsh environments [7–10].
The electrochemical studies look up the performance of the PVCbased Cu3(PO4)2 and polystyrene supported Ni3(PO4)2 compositeion exchange membranes and hence have lot of industrial applica-tions. The outstanding properties of these composite membranes ascompare to those of the previously reported composite membranesoffer important applications in chemical industry, biotechnologyproducts and waste water treatment [11–13].
The surface charge density is the main parameter that controls themembrane phenomenon and this was calculated using the membranepotential values for different electrolytes by using Teorell, Meyer, andSievers (TMS)method. Some other parameters including distribution co-efficient, transport numbers, mobility ratio, charge effectiveness etc. werealso calculated for PVC andpolystyrene based compositemembranes. The
observedmembrane potential studies are commonly used in the electro-chemical study of these composite membranes [14–19].
The structural representation of composite membranes was shownas:
2. Experimental
2.1. Materials and chemicals used
Pure crystalline polystyrene (Otto Kemi, Mumbai, India) and poly-vinyl chloride (Otto Kemi, India, AR grade) grounded and sievedthrough 200 meshes were used as a binder, 0.2 M tri-sodium phos-phate solution (E. Merck, India with purity of 99.90%), 0.2 M copperchloride (CuCl2) solution, 0.2 M nickel chloride (NiCl2) solutionfrom Otto Kemi, India and various electrolyte solutions (KCl, NaCland LiCl) were procured from (E. Merck Limited, India); of differentconcentrations were also arranged. All the used reagents were of an-alytical grade and pure deionized water was used for the preparationof solutions of the above compounds.
2.2. Instruments
Scanning electron microscopy (SEM) was used for surface mor-phology and Fourier transform infrared spectroscopy (FTIR) wasused to obtain an infrared spectrum of absorption, emission and pho-toconductivity. The X-ray diffraction (XRD) reveals the informationabout the chemical composition and crystallographic structurewhereas thermo-gravimetric analysis and differential thermal analy-sis (TGA/DTA) measure the mass change or degradation characteris-tics of material as a function of temperature and time and lastlypotentiometer was used for measuring the ionic potentials.
2.3. Synthesis of Cu3(PO4)2 and Ni3(PO4)2 materials
Copper phosphate and nickel phosphate (CP and NP) were pre-pared by sol–gel or co-precipitation method by mixing 0.2 M aqueoussolution of tri-sodium phosphate with 0.2 M copper chloride andnickel chloride solution separately, resulting to a constant stirring ofboth the solutions for 1–2 h, and also the pH of the solution mustbe maintained. The resulting precipitate was well washed almost4–5 times with deionized water to remove free electrolytes and drythe materials up to 3–5 h at 100 °C. The precipitate must be pow-dered with the help of pestle and mortar until the size of both mate-rials should be less than 200 meshes. Pure crystalline polystyrene andPVC were also grounded and sieved through 200 meshes.
2.4. Preparation of PVC based CP and polystyrene based NP compositemembranes
The synthesized precipitates that are CP and NP were mixed withPVC and polystyrene granules (less than 200 meshes) respectivelywith the help of pestle and mortar and mixing of the materials shouldbe done very carefully and uniformly. After both the mixtures were
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kept separately into two alike cast die which are having a diameter of2.45 cm. After that they should be placed in a digital furnace bymaintaining temperature till 200 °C for about 1 h to equilibrate the re-action mixture, and then transferred to a pressure device (SL-89, UK),for applying pressure of 100 MPa for the fabrication of the membranes.Tomake amechanically andmorphologically stable membrane the ratioof binders (PVC and polystyrene) with the inorganic phosphate shouldbe 1:3. If it exceeds or decreases the above mentioned ratio that is 25%and 75% the prepared membrane does not show the ideal stability andfunction. After all the well stable prepared membranes were subjectedto microscopic and electrochemical examination to check for cracksand homogeneity of the surface, resulting that they are applying forpotential observation by electrolyte separation through which furtherparameters of membrane processes can be obtained [20].
2.5. Chemical stability
By ASTMD543-95 procedure the chemical stability of themembranewas determined. By this, it is analyzing the morphological changes thatcover the alteration in color, texture, brightness, decomposition, splitsetc. By doing this the membranes were exposed to different acidic andbasic media and were evaluated after 24, 36, and 48 h.
2.6. Ionic exchange capacity
The measurement of ion-exchange capacity was done by titrationmethod. The membranes were rinsed with DM water after HNO3 treat-ment for 6 h. The H+-formmembranes were immersed in a 1.0 M NaClsolution for 1 day. The H+ ions released by the membrane throughion-exchange reaction with Na+ ions were titrated with a 0.1 M NaOHsolution, in which phenolphthalein was used as an indicator [13].
2.7. Porosity measurement
Porosity was determined as the volume of water incorporation inthe cavities per unit membrane volume from the water content data:
Porosity ¼ Ww−Wd
ALρw
where Wd is the weight of the dry membrane, A is the area of themembrane, L is the thickness of the membrane and the density ofwater is ρw.
2.8. Measurement of membrane potential
By the help of digital potentiometer (Electronics India-118) potentialsof membranes have been obtained, as it is reported by [21,22]. In ourwork I have used KCl, NaCl, and LiCl electrolyte solutions, which wereprepared from B.D.H. (A.R.) grade chemicals and DMW. It was deter-mined by reference electrodes (SCE) which kept dipped in one of the col-lared chamber of different concentrated solutions. The observationshould be at room temperature. The electrochemical setup for potentialmeasurements may be depicted as:
3. Structural characterization
3.1. Surface morphology
The surface morphology was done by scanning electron microsco-py (SEM) and the chemical compositions of composite membranewere obtained by EDX analysis [23,24]. Surface structures of compos-ite membranes were analyzed by SEM images [25]. A SEM equippedfor EDS spectroscopy is also useful to obtain information about com-position. It was carried out by Leo 4352 at an accelerating voltage of20 kV. The sample was mounted on a copper stub and sputter coatedwith gold to minimize the charging and was used to verify the micro-structure of fabricated porous membrane.
3.2. FTIR analysis
The chemical structure of organic–inorganic composites has beencharacterized by FTIR. The existence of hydrogen bonding or covalentbonding between the phases may be confirmed by FTIR spectroscopy[26]. This was done by Interspec 2020 FTIR spectrometer, Spectrolab(UK). The sample compartment was 200 mm wide, 290 mm deepand 255 mm high. The entrance and exit beam to the sample cham-ber was sealed with a coated KBr window and there was a hingedcover to seal it from the environment.
3.3. X-ray diffraction analysis
X-ray diffraction pattern of composite membranes was done byMiniflex-II X-ray diffractometer (Rigaku Corporation) with Cu Kαradiation.
3.4. The thermal stability of by TGA/DTA
By TGA/DTA (Shimadzu DTG-60H) analysis the rate of weightchanges in a material, either as a function of increasing temperature,or isothermally as a function of time and at the same time it can beused to determine the nature of material i.e. endothermic or exother-mic behaviors.
4. Results and discussion
The surface morphologies of the membranes were investigated byusing SEM with EDS characterization and are represented in Fig. 1(A),(B). It shows that the prepared membranes at 100 MPa pressure areuniformly mixed with the respective polymers and gave the informa-tion about elemental composition; therefore resulting to both themembranes being compact, porous with homogeneous nature anddo not show any visible cracks therefore it must be applicable forfurther observation [27].
Fig. 2(A) and (B) shows the spectrum of PVC based Cu3(PO4)2 andpolystyrene supported Ni3(PO4)2 composite membranes. The PVCbased Cu3(PO4)2 composite membrane there shows broad peaks at therange of 2924 to 3380 cm−1 which indicates the presence of \OHgroup in the material and also a sharp peak present at the range of1062 cm−1 indicating the C\H bonds or phosphate group in the mem-branous sample. The fundamental frequency at 566–758 cm−1 arisesdue to hindered rotations of the hydroxyl ions, whereas PVC showsstrong sharp peak at the range of 697 cm−1 indicating the presence ofC_C and the peaks present at the 3065 cm−1 indicate the presence ofwater molecule in the sample. However the polystyrene supportedNi3(PO4)2 sample indicating a strong peak at the 988–1051 cm−1
which indicates the presence of the benzene ring as well as 563–627 cm−1 range shows the presence of phosphate groups in the mate-rials. There is a strong IR band at the1091.37 which shows the benzenering and 472–619 shows the C_C presence in the binder [28].
Fig. 1. (A). (i) SEM images (surface and cross sectional) as well as (ii) EDS of PVC based Cu3(PO4)2 composite membrane. (B). (i) SEM images (surface and cross sectional) and(ii) EDS of polystyrene supported Ni3(PO4)2 composite membrane.
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In Fig. 3(A), (B) the XRD spectrum shows that there are two in-tense peaks present in PVC based Cu3(PO4)2 as well as polystyrenesupported Ni3(PO4)2 at the 2θ range of 31.7°, 44.9° and 37.2°, 44.3°respectively. The spectrum shows that the first intense peak of boththe compounds corresponds to (211) plane of the materials at differ-ent ranges [29]. The crystallite sizes of the samples were found to be~31.7 and 37.2 nm respectively and these peaks in the spectrumdemonstrated that the particles were present in the crystallineform. By the 2θ range it is clear that the PVC based Cu3(PO4)2
would show most crystallite nature than the second one due to high2θ value.
The TGA and DTA curves of the samples are shown in Fig. 4(A) thatis PVC based Cu3(PO4)2 and Fig. 4(B) polystyrene supported Ni3(PO4)2.In the PVC based Cu3(PO4)2 the weight loss was shown twice at thetemperature range 0–600 C. The first weight loss took place at84.10 °C by deducing the weight near about −0.794 mg and it shouldbe −6.087% from the starting material while the second weight losswas at the range of 436.47 °C and the weight loss was −0.677 mg
Fig. 2. (A) FTIR spectra of (i) PVC based Cu3(PO4)2 and (ii) spectra of only PVC. (B). (i) spectra of Polystyrene SUPPORTED Ni3(PO4)2 and (ii) spectra of polystyrene.
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20 30 40 50 60 700
200
400
600
800
1000
1200
Inte
nsit
y (a
.u)
2θ20 30 40 50 60 70
0
200
400
600
800
1000
1200
1400
Inte
nsit
y (a
.u)
2θ
A B
Fig. 3. (A) PVC based Cu3(PO4)2 and (B) polystyrene supported Ni3(PO4)2.
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whichwas near about−5.190%. By the reference of TGA curve the DTAcurve indicates that the particular sample shows endothermic naturewhich means that heat change must be there at more than one pointby deducing the temperature. TGA/DTA curve of polystyrene supportedNi3(PO4)2 composite material indicates that weight loss took place atthree different temperatures viz −0.809 mg (−9.953%) at 103.20 °C,−1.946 mg (−23.942%) at 436.71 °C and 0.915 mg (−11.257%) at574.37 °C. By taking the reference of TGA it is clear that the DTAcurve of the material shows the endothermic process after deductingthe temperature at only one point i.e. at 462.89 °C. Therefore it is
A
B
Fig. 4. (A) TGA/DTA PVC based Cu3(PO4)2 composite material and (B
clear that both types of material had a high hydrophilicity that couldabsorb moisture from the surrounding air [30].
The chemical stabilities of both the composite membranes weredetermined by incubating them in acidic, alkaline and strongly oxi-dant media, such as (1 M HNO3, 1 M NaCl and 1 M NaOH) solutions.Observation was completed after putting it into the above differentmedia for 12, 24, 36 and 48 h. Both the membranes have lost theirmechanical resistance which represents that the membranes were ef-fective in above media. The high thermal, chemical stability and thespecificity for cations are the exclusive features for such composite
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membranes. The low order of porosity with less thickness of suchmembrane suggests that the interstices are insignificant and diffusionwithin the membrane would happen mostly through the exchangesites [31].
It is clear that the inorganic precipitate has the capability to createpotentials due to the interphase among electrolyte solutions of differ-ent concentrations [32]. For showing the selectivity of ions the poten-tial of membranes had been regarded by using different electrolytesolutions such as KCl, NaCl, LiCl and NH4Cl. The selectivity variationof the membrane is due to the adsorption of anions which leads themembrane surface cation selective.
By means of membrane potential the measurement of ion activityis more successful in the concentration range over which the mem-brane behaves as an ideally selective one. The anion of electrolytesdid not extensively influence the potentiometric response of themembrane which represents the positive mV potential order and itfollows the Nernst equation [33]. Hence thereafter observing the po-tential for both the ion exchangeable membranes it decreases afterincreasing the concentration of electrolyte solution, which gives theresult that the above prepared membrane is perfectly cation selective(i.e. negatively charged). The plotted graph as a function of − logC2with observed potential for PVC based CF and polystyrene based NF
0
5
10
15
20
25
30
35
40
45
50
55
60
Obs
erve
d m
embr
ane
pote
ntia
l(mV
)
-logC2(mV)
KCl NaCl LiCl
3 4 51 2
1 2 3 4 50
5
10
15
20
25
30
35
40
45
50
55
60
Obs
erve
d m
embr
ane
pote
ntia
l(mV
)
-logC2(mol/l)
KCl NaCl LiCl
A
B
Fig. 5. (A) Plots of observed membrane potentials against logarithm of concentration forPVC based Cu3(PO4)2 composite membrane using various 1:1 electrolytes at 100 MPa.(B) Plots of observed membrane potentials against logarithm of concentration for polysty-rene supported Ni3(PO4)2composite membrane using various 1:1 electrolytes at 100 MPa.
composite membrane in contact with various 1:1 electrolyte solu-tions at 25 °C is given in Fig. 5(A), (B).
The charge present on the membrane leads a fundamental role inthe adsorption and transport of ions. In synthetic plus ordinary mem-branes there are some important electrochemical properties, themost important one is that they are having differences in the perme-ability of co-ions, counter ions as well as neutral molecules. In dilutesolution the number of charge is small which needed to generatethe potential and this is dependent on the porosity of the membrane[34]. If the membrane pores are broad then the more sum of chargedoes to generate good potentials on the membrane, whereas if it isnarrow, a little quantity of charge can give rise to appropriate poten-tial values. Therefore, the transport mechanism of simple electrolyteswithin the charged membrane appears to be incomplete without theestimation of the thermodynamically effective fixed charge density ofthe membrane [35].
The membranes carrying various charge densities which show,D ≤ 1. In the graph of Fig. 6(A), (B) the theoretical potential and ob-served potential were designated by dark and broken lines respectivelyand were plotted as a function of −log C2. The graph shows coincidingcurves for various electrolyte systems which gave the value of the chargedensity D within the membrane phase as indicated in Tables 1a and 1b.
The surface charge densities D of PVC based CP and polystyrenebased NP composite membranes are found to depend on the initialphase of preparation. Thus, the order of charge densities for boththe membranes was found to be in the manner of KCl > NaCl > LiCl.
0
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65
Charge Density
D = 1.0
D = 0.1
D = 0.01
D = 0.003
D = 0.0025
D = 0.0020
D= 0.0012
D= 0.0010mem
bran
e po
tent
ial (
mV
)
-logC2 (mol/l)0 1 2 3 4 5
0 1 2 3 4 55
10
15
20
25
30
35
40
45
50
55
60
65Charge Density
D = 1.0
D = 0.1
D = 0.01
D = 0.003
D = 0.0025
D = 0.0020
D= 0.0012
D= 0.0010
mem
bran
e po
tent
ial (
mV
)
-logC2 (mol/l)
A
B
Fig. 6. (A), (B) Plots of membrane potential (theoretical and observed) (mV) versus− log C2(mol/l) at different concentrations of KCl electrolyte solution for PVC basedCu3(PO4)2 as well as polystyrene supported Ni3(PO4)2composite membrane prepared atthe pressure of 100 MPa.
Table 1aObserved membrane potential in mV across PVC based Cu3(PO4)2 composite mem-brane in contact with various 1:1 electrolytes at different concentrations and pressuresat 25 ± 1 °C.
Applied pressure(MPa)
KCl NaCl LiCl
100 0.00023 0.00019 0.00010
Table 1bDerived values of membrane charge density Din eq/l for PVC based Ni3(PO4)2 compos-ite membrane by various electrolyte system using TMS equation.
Applied pressure(MPa)
KCl NaCl LiCl
100 0.00026 0.00021 0.00013
0
2
4
6
8
10m
ob
ility
ra
tio
-logC2(mol/l)
KCl NaCl LiCl
1 2 3 4 5
1 2 3 4 50123456789
10111213141516
mob
ility
rat
io
-logC2(mol/l)
KCl NaCl LiCl
A
B
Fig. 7. (A) The plot for mobility ratio of the PVC based Cu3(PO4)2 composite membranefor 1:1 electrolyte against concentration. (B) The plot for mobility ratio of the polystyrenesupported Ni3(PO4)2 composite membrane for 1:1 electrolyte against concentration.
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Due to the size factor of electrolytes charge densities are higher in thecase of KCl than in NaCl and LiCl will be less than NaCl [29,36].
According to TMS method, the membrane potential (applicable toa highly idealized system) is given by the equation at 25 °C.
where u and v are the ionic mobilities of cations and anions (m2/v/s),respectively, in the membrane phase. C1 and C2 are the concentra-tions of the membrane and D is the charge on the membrane Eq/l.The graphical method of TMS determines the fixed charge D andthe cation-to-anion mobility ratio in the membrane phase. Chargedcomposite membrane was placed at the center of the potentialmeasuring cell, which had two glass chambers for inserting the elec-trolytes of different concentrations [14–19].
The charge present on such types of membranes has key role inthe adsorption as well as transport of ions of salt or electrolyte solu-tions. Either synthetic or standard membranes, the most importantelectrochemical property is that there should be permeability differ-ences in the co-ions, counter ions as well as neutral molecules. Thepotentials of diluted electrolyte solutions are totally dependent onthe porosity of the ion exchangeable composite membranes. There-fore without the evaluation of thermodynamically effective fixedcharge density the transportation of electrolyte solution within thecharged membrane seems to be incomplete.
The surface charge density D of such organic–inorganic compositemembranes is entirely dependent on the early stage of sample prep-aration as it should be, D ≤ . The TMS Eq. (1) can also be expressedby the sum of Donnan potential, ΔΨDon among the membrane sur-faces and the external solutions, and the diffusion potential ΔΨdiff
within the membrane [14,19].
Δψm;e ¼ −ΔψDon þ Δψdiff ð2Þ
ΔψDon ¼ − RTVkF
lnγ2�C2C1þγ1�C1C2þ
!ð3Þ
where R, F and T have their standard meanings, γ1± and γ2± are themean ionic activity coefficients, and C1+ and C2+ are the cationconcentrations on the two sides of the charged membrane.
where Vk and Vx indicate the valency of cation and fixed-chargegroup on the membrane matrix, and q is the charge effectiveness ofthe membrane and is defined as follows:
q ¼ffiffiffiffiffiffiγ�K�
rð5Þ
U
Where K± is the distribution coefficient expressed as
K� ¼ Ci
Ci;C
i¼ Ci−D ð6Þ
Where Ci = ith ion concentration in the membrane phase,Ci = ithion concentration of the external solution
Δψdiff ¼ − RTω−1VkFω þ 1
� lnω þ 1ð ÞC2 þ Vx=Vkð ÞDω þ 1ð ÞC1 þ Vx=Vkð ÞD
!ð7Þ
Table 2aThe values of t+, U; ω and K±,q, Cþ evaluated using Eqs. (9), (6), (5) and (4) respec-tively, from observed membrane potentials across various electrolytes at differentconcentrations for PVC based Cu3(PO4)2 composite membrane.
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here ω = u/v is the mobility ratio of the cation to anion in the mem-brane phase.
The total membrane ΔΨm,e potential was thus obtained by simpleaddition of ΔΨDon and Δψdiff which is elaborated in Eqs. (3) and (7)respectively.
Δψm;e ¼ − RTVkF
lnγ2�C2C1þγ1�C1C2þ
!− RTω−1
VkFω þ 1
� lnω þ 1ð ÞC2 þ Vx=Vkð ÞDω þ 1ð ÞC1 þ Vx=Vkð ÞD
!ð8Þ
Δψm ¼ −RTF
tþ−t−� �
lnC2
C1ð9Þ
wheretþt−
¼ uv
ð10Þ
For the applicability of TMS theoretical equation the diffusion poten-tial and Donnan potential were separately calculated from membranepotential observation data which have been done by potentiometer, to
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
tran
spor
t num
ber(
t +)
-logC2(mol/l)
KCl NaCl LiCl
1 2 3 4 5
1 2 3 4 50.0
0.2
0.4
0.6
0.8
tran
spor
t num
ber(
t +)
-logC2(mol/l)
KCl NaCl LiCl
B
A
Fig. 8. (A) Plots showing the transport number of cation for the PVC based Cu3(PO4)2composite membrane for 1:1 electrolytes (KCl, NaCl, LiCl) vs concentration. (B) Plotsshowing the transport number of cation for the polystyrene supported Ni3(PO4)2composite membrane for 1:1 electrolytes (KCl, NaCl, LiCl) vs concentration.
0.0001 0.91 0.82 10.11 0.2000 1.0000 0.00004
get the value of transport number (t+) and mobility ratio ( ω)(Eqs. (9) and (10)). The electrolyte mobility in both of the compositemembranes was high and it follows the LiCl > NaCl > KCl order asshown in the graph. Due to higher transport number the mobilities ofelectrolyte should also be high, as shown in Fig. 7(A), (B). By free cationsof electrolytes in least as well as high concentrated solution the similartrend of mobility should be there. The transport number of cation forvarious electrolytes increases after decreasing the concentration and itfollows KCl b NaCl b LiCl, as shown in Fig. 8(A), (B). When the concen-tration of the electrolytes increases, the values of distribution coefficientdecrease, and it is clear that at any concentrations the Donnan and dif-fusion potentials can be calculated from the parameters γ1�;γ2�;C1þ;C2þω;Vx;Vk and the values of charge density D can also be calcu-lated by Eqs. (3) and (7). The values of K± and q are also derived for thesystem and shown in Tables 2a and 2b.
Table 2bThe values of t+, U; ω and K±,q, Cþ evaluated using Eqs. (9), (6), (5) and (4) respec-tively, from observed membrane potentials across various electrolytes at different con-centrations for polystyrene supported Ni3(PO4)2 composite membrane.
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5. Conclusion
It is clear from the abovementioned results, that the PVC basedCu3(PO4)2 and polystyrene supported Ni3(PO4)2 composite ion ex-change porous membranes were successfully prepared by sol–gelroute and both the composite membranes showed high stability dueto the good polymer interaction. The fixed-charge density is the essen-tial parameter for governing transport phenomena in membraneswhich totally depend on the feed composition and applied pressure be-cause of their preferential adsorption of ions and accounted for varyingthe charge density and, in turn, performance of membrane. The mem-brane potential of both the composite membranes for different 1:1electrolytes was found to follow an increasing order KCl b NaCl b LiCland surface charge density follows KCl > NaCl > LiCl order. Thereforethe membranes were found to be cation selective and show good prac-tical applications.
Acknowledgments
The authors gratefully acknowledge the Chairman, Department ofChemistry, and Aligarh Muslim University, Aligarh (India) for provid-ing the necessary research facilities such as FTIR, TGA/DT and poten-tiometric analysis. We are also thankful to the UGC for providingfinancial assistance and the Interdisciplinary Centre, Department ofApplied Physics, AMU, Aligarh for the XRD analysis along with theUSIF (AMU Aligarh) for providing SEM/EDS facility.
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