Calculating partition coefficients

[rt320]

I have experimental data and I'm trying to calculate partition coefficient. So I've synthesised a mineral from a seawater solution that contained Fe2+ and a range of trace metals. I'm interested in metal incorporation. 

The formula I've seen used is 

KD=M2+/Fe2+(solid) / M2+/Fe2+(solution).

Is the ratio in solution the starting ratio, from the beginning of my experiment (before any solid formed), or the final ratio, from the end of my experiment (i.e. at apparent solubility equilibrium)?

 

Can't find this simple bit of information anywhere. Thanks!