Using a first-derivative spectrophotometric method for simultaneous determination of Fe(II) and Ni(II) in mineral vitamins by interaction with 2,4-diamino-5-(2-hydroxy-5-nitrophenylazo)benzenesulfonic acid sodium salt in the presence of Tween 20. - Hashem EY; Seleim MM; El-Zohry AM (Journal of AOAC International, 2010)