Determination Of Diffusion Coefficient
DETERMINATION OF DIFFUSION COEFFICIENT
OF ZINC (II) IONS IN POLYMERIC MEMBRANE
BY ABSORPTION SPECTROSCOPY
C. Wardak, J. Chmiel, B. Marczewska,
Department of Analytical Chemistry and Instrumental Analysis,
Faculty of Chemistry, Maria Curie-Sklodowska University,
Maria Curie-Sklodowskasq. 3, 20-031 Lublin, Poland
The diffusion coefficients of different substances in polymers are usually measured by gravimetric techniques. These coefficients can be determined by measurement of substances concentration in membranes which separate solutions. In this case the potentiometric or spectrophotometric methods or both are applied. The interferometric methods are used as well.
The value of diffusion coefficients values is important in case of using those membranes in ion-selective electrode.
In this paper it is proposed a new spectrophotometric method of the determination of zinc(II) diffusion coefficient in the polymeric membranes. In order to calculate diffusion coefficient in polymer-solvent system it is necessary to solve equation which specificate second Fick law. In case of both membrane sides diffusion (both solutions- the same composition and concentration) after simplification, this equation has a form:
(1)
where: Mt – mass of membranes absorbed substance after time t, M∞ – mass of membranes absorbed substance in moment of getting equilibrium, l – membrane thickness, D – diffusion coefficient.
We can calculate the value of substance diffusion coefficient in polymeric membrane from the graphs inclination angle in this system Mt / M∞, t1/2 (to value of Mt / M∞=0.55).
In case when polymeric membrane absorbance changes in time reach concluded:
(2)
where: At – absorbance of the membrane after time t, A∞ – absorbance of the membrane in moment of getting equilibrium.
In investigations there were used different thickness polymeric membranes (from 0.13 mm – 0.16mm).
Composition of membrane was: 33 % by weight PVC, 66,9 % TEHP [tri-(2-ethylhexyl)phosphate], 0.1 % PAN [1-(2-Pyridylazo)-2-naphthol]. These membranes were obtained by dissolving components in THF (tetrahydrofuran) and the evaporation of the solvent. The membrane contained PAN compound which gives us colored complex with zinc(II) ions. This complex has a maximum absorbance at the wave length of 558.9 nm.
The membranes were placed in special constructed cell, made possible to contact both sides of membrane with zinc(II) solutions. The cell was placed in spectrophotometer Specord M40. The absorbance was measured in the time function. The measurements were made at the constant wave length (558.9nm).
From received results and used equations (1) and (2) the diffusion coefficient Zn (II) ions was determined. The value of diffusion coefficient obtained in this way counts 3.32·10-8 cm2/s.
Related articles::