“…Among the techniques employed in the past is a large group of colorimetric estimations involving complexation with ferric ion (1)(2)(3)(4)(5)(6)(7)(8)(9). Other colorimetric assays involve diazotization with p -nitroaniline and nitrous acid (lo), use of Folin and Ciocalteu phenol reagent ( l l ) , complexation with cupric ion in nitrous acid (13-15), or estimation as the nitro derivative (16).…”