I'm working to determine Kd(s) kinetically by generating association and disassociation curves. Kon (association constant, or on-rate) is in inverse seconds multiplied by inverse molar. I get that the molar comes from concentration, but where do the seconds come from?


First order association reaction:

$$\ce{A + B <=>[k_f][k_r] AB}$$

Rate of association (units: concentration per time) = $k_f\times[A]\times[B]$
Since [A] and [B] are concentrations and LHS is conc. per time the units of $k_f$ has to be such that it gives conc per time when multiplied by conc2 ([A] ×[B])
Which is conc-1 × time-1

Rate of dissociation (conc. per time) = $k_r\times[AB]$
Therefore units of $k_r$ would be time-1.

Dissociation constant is actually $\dfrac{k_r}{k_f}$; Its units therefore would be conc.

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