A z ZaMazama Zbmb[5

---- s^/

/ / 1

S / 1

S ' 1

Ad

A 1

f / /

/ / 1

/ / 1

/ / i

/ Area (a)

/ / /

/ / /

/ / /

// /

// /

/

1 //

1 / +

/(3)

/ /'/

Area (b) /

1 ' /

1 / /

1 S y

/ yr

Figure 1 Idealized ion exchange isotherms (see text for details).

Figure 1 Idealized ion exchange isotherms (see text for details).

where mA,B and MA B are the ion concentrations in mol dm~3 in solution and solid, respectively.

On Figure 1 the dashed line shows the case where the solid has an equal selectivity for ions A and B. The isotherm (3) describes the circumstance when A is selectively taken up, while isotherm (2) describes the circumstances when B is favoured by the exchanger.

A simple quantitative expression of the selectivity is via the selectivity factor (a) defined in eqn [6]:

where by definition:

In Figure 1 a can be calculated from area (a) divided by area (b), illustrated for a typical isotherm (1).

Not all isotherms in the literature are constructed in the formal way described above. Often they arise from solutions containing only the ingoing ion placed in contact with the exchanger, only one ion is analysed in one phase, and various units of concentration are used. These simple approaches are still valid comparisons of practical selectivities, but when isotherms are needed to generate thermodynamic data the more rigorous experimental methodology must be followed. It is also necessary to demonstrate that the exchange being studied is fully reversible to allow the laws of mass action to be applied. When inorganic exchangers are involved it may be appropriate not to dry the solid before the reverse leg of the isotherm is constructed, as heating the solid can change the number of cation sites partaking in the exchange. This is particularly so for the zeolite minerals. In cases where organic resin exchangers are examined, the resin is used preswollen (fully hydrated) to avoid discrepancies caused by the resin expanding on initial contact with the solution phase.

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Solar Panel Basics

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