We carry out plasma diagnostic analysis for a number of planetary nebulae (PNe) and H ii regions. We use N ii and O ii optical recombination lines (ORLs) with new effective recombination coefficients calculated under the intermediate coupling scheme, for a range of electron temperatures (T e) and densities (N e), and fitted against the most reliable measurements. Comparing T e derived from ORLs, collisionally excited lines (CELs), the hydrogen Balmer Jump, and/or He i if available, we find the relation T e (ORLs) < T e (He i) < T e (H i BJ) < T e (CELs), confirming the physical conditions in the bi-abundance model postulated by Liu et al., i.e. the nebula contains another cold, metal-rich and probably H-deficient component.