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 (Te) and densities (Ne), and fitted against the most reliable measurements. Comparing Te derived from ORLs, collisionally excited lines (CELs), the hydrogen Balmer Jump, and/or He i if available, we find the relation Te (ORLs) < Te (He i) < Te (H i BJ) < Te (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.