Fishtank

Hi All, I've created a quick pen of a section of a site i'm building, the idea been as the user scrolls the nav on the left gets fixed for the height of the scroller section. I've done this bit however i'm not quite sure how to achieve the next bit. Each section on the right is related to the nav on the left when that section is in view I need to add an active class to the list item. Can anyone point me in the right direction of the best way to do this please. Thanks