AMAN SENTOSA PANGGABEAN, and BOHARI YUSUF. “DETERMINATION OF Cr(VI) BY USING CHITOSAN-1,5- DIPHENYL CARBAZIDE RESIN MODIFIED AT THE PRECONCENTRATION SYSTEM WITH COLUMN METHOD”. International Journal of Pharma and Bio Sciences, vol. 6, no. 3, Sept. 2015, pp. 101-1, https://ijpbs.net/index.php/journal/article/view/4384.