1) why should Cr(III) be oxidized to Cr(VI) in the first place when determining Cr(III) and Cr(VI)? 2) what reaction takes place between Cr(VI) and 1.5- diphenylcarbazide that a color change occurs? (what does the reaction look like if the medium is acidic, and what if it is basic or neutral?) 3) why do Fe, Cu, V, Mo ions and organic substances interfere with chromium analysis?