Originally Posted by clay
The "NA2" platinum starts as Pt(+4), gets reduced to Pt(+2) consuming ferrous without forming image (that's the contrast control mechanism). Then if there remains enough ferrous in that area of the print, Pt(+2) is further reduced to metalic Pt(0). Some Pt metal is deposited, so the result is a TRUE Pd/Pt print.
However, it is true that initially adding "normal" Pt(+2) to the sensitizer mix greatly negates the contrast effect of added Pt(+4). Chemically, it's not entierly clear to me why this is, but it most certainly is the case.