State the van Deemter equation and briefly describe each term.

State the van Deemter equation and briefly describe each term.

• A. H = A*B*u*C; not correct
• B. H = A + B/u + C*u; A = Eddy diffusion, B/u = longitudinal diffusion, C*u = mass transfer resistance; H is the plate height.
• C. H = A - B/u - C*u
• D. H = A + D/u + E*u

Answer: (B)

The van Deemter relationship shows how column efficiency, expressed as plate height, depends on how fast the mobile phase moves. The correct form is H = A + B/u + C*u, where each term has a clear physical meaning. The A term, eddy diffusion, comes from the many different paths solute molecules can take through the packed bed, causing a velocity-independent spread. The B/u term represents longitudinal diffusion: molecules diffuse along the column, and this spreading diminishes as the linear velocity u increases because less time is available for diffusion. The C*u term accounts for mass transfer resistance between the mobile and stationary phases; as velocity rises, this transfer becomes more rate-limiting, increasing broadening. H is the plate height, a measure of peak broadening—the lower it is, the narrower the band and the more efficient the separation. At low flow, diffusion dominates and broadening is large; at high flow, mass-transfer resistance dominates and broadening also increases, giving a minimum H at an intermediate velocity. Other options either do not match the standard form or do not provide the correct interpretation of the three contributing mechanisms, so the explicit H = A + B/u + C*u with those meanings is the best description.

The van Deemter relationship shows how column efficiency, expressed as plate height, depends on how fast the mobile phase moves. The correct form is H = A + B/u + Cu, where each term has a clear physical meaning. The A term, eddy diffusion, comes from the many different paths solute molecules can take through the packed bed, causing a velocity-independent spread. The B/u term represents longitudinal diffusion: molecules diffuse along the column, and this spreading diminishes as the linear velocity u increases because less time is available for diffusion. The Cu term accounts for mass transfer resistance between the mobile and stationary phases; as velocity rises, this transfer becomes more rate-limiting, increasing broadening.

H is the plate height, a measure of peak broadening—the lower it is, the narrower the band and the more efficient the separation. At low flow, diffusion dominates and broadening is large; at high flow, mass-transfer resistance dominates and broadening also increases, giving a minimum H at an intermediate velocity.

Other options either do not match the standard form or do not provide the correct interpretation of the three contributing mechanisms, so the explicit H = A + B/u + C*u with those meanings is the best description.