Fehling's solution consists of two parts A and B. Fehling’s solution A is deep blue colour due to the bis(tartrate) complex of Cu2+. Fehling's solution B is the Rochelle salt (potassium sodium tartrate tetrahydrate) serves as a chelating agent in the solution.
Fehling solution is a complex of Cu++
Cu++=3d9
Number of unpaired e−=1
MagneticMoment=1(1+2)=3=1.73BM