Question

Calculate the Kovats retention index for an unknown using the following retention times: 1.2 min for CH4, 11.4 min for octane, 16.0 min for the unknown, and 17.1 min for nonane

Answer 1

I= 100 [n+(N-n) [log tr(unknown)-log tr(n)]/[log tr(N)-log tr(n)]]
where n is the number carbon atoms in the smaller of the two known linear alkanes, N is the number of carbon atoms in the larger of the two known linear alkanes, tr\'(n) is the adjusted retention time of the smaller of the two known linear alkanes, tr\'(N) is the adjusted retention time of the larger of the two known linear alkanes, and tr\'(unknown) is the adjusted retention time of the unknown. The adjusted retention time is given by the equation,
tr'= tr - tm
where tr is the retention time of the solute and tm is the time required for unretained solute (CH4) to be eluted from the column. In this case, the adjusted retention time values are:
tr' (C8H18)= 11.4-1.2= 10.2 min
tr' (C9H20)= 17.1-1.2= 15.9 min
tr' (unknown)= 16-1.2- 14.8 min
So, the reaction index is,
I= 100+ [8+(9-8) [log tr(14.8)-log tr(10.2)]/[log tr(15.9)-log tr(10.2)]]= 107.59