Dc

F (g sā€”1 cm-2) describes the mass flux, i.e., the accumulated mass M (g) of a certain chemical per area A (cm2) available for diffusion throughout a certain time t (s):

With reference to ceramic dosimeters, M is what is being measured upon solvent extraction of the sorbent material and subsequent chemical analysis. The term Dx (cm) describes the diffusion path length, which, in the case of the ceramic membrane, equals the thickness of the membrane, i.e., 1.5 mm. The term DC (g cm-3) is the difference in aqueous chemical concentration between the sampling environment CW (g cm-3) and the inner part of the ceramic tube CI (g cm-3). The difference CW ā€” CI must be kept at its maximum during the whole monitoring period (quasi-steady-state diffusion rate) (Fig. 12.2).

This is accomplished by the solid sorbent material inside the ceramic membrane. Given a high affinity of the chemicals of concern to the sorbent (for selection see Section 12.2.2), the aqueous concentration of the chemicals in the inner part of the sampler can be assumed to approach zero. Thus, DC can be re-written as CW, the term sought for

Ceramic Interior

Sampling environment membrane of sampler

Ceramic Interior

Sampling environment membrane of sampler

Fig. 12.2. Concentration gradient across the ceramic membrane.
0 0

Post a comment