# Sephadex^{®} & Darcy’s Law

Sephadex^{®} G-10, G-25, and G-50 can be assumed to behave as rigid spheres in SEC and therefore obey Darcy’s Law. This Law describes a general relationship for flow in porous media:

U = K × ΔP × L^{-1} Equation (1)

U = linear flow rate expressed in cm/h

ΔP = pressure drop over the packed bed expressed in cm water

L = bed height expressed in cm

K = constant of proportionality depending on the properties of the bed material and the buffer Assuming a buffer with viscosity of 1 cP: U = K_{o} × ΔP × L^{-1} Equation (2)

Ko = the “specific permeability” depending on the particle size of the medium and the water regain

**Note that flow is proportional to the pressure drop over the bed and, assuming a constant pressure head, inversely proportional to the bed height. In practice this means that the pressure/flow considerations that must be made when using other SEC media do not apply to Sephadex ^{®} and that a doubling of flow rate leads to a doubling in column pressure. To a good approximation, flow rate is independent of the column diameter.**

**Flow at viscosities greater than 1 cP can be obtained by using the relationship: flow rate is inversely proportional to viscosity. High buffer viscosities can be compensated for by increasing the operating pressure to maintain a high flow rate.**

Theoretical flow (not maximum) can be calculated from equation (2) by inserting values for ΔP and L. Specific permeabilities (K) are given in Table A2.1.

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