Glyphosate Quantification via Colorimetric Assay

Roundup (commercial grade, 41%, containing 360 g/L glyphosate) was diluted to prepare a 1000 mg/L glyphosate stock solution with a natural pH of 4.9–5.9. Calcium peroxide (CaO₂, 65%), ammonium molybdate (H₈MoN₂O₄, 99%), and ascorbic acid (C₆H₈O₆, 99%) was purchased from Alfa Aesar, Ward Hill, MA, USA. Calcium chloride (CaCl₂, ≥99.5%), hydrogen peroxide (H₂O₂, 30%), ammonia solution (NH₄OH, 25 wt%), ferrous sulfate (FeSO₄∙7H₂O, ≥98%), and ethyl alcohol (C₂H₅OH, 95%) were supplied from R&M Chemicals Sdn. Bhd. (Semenyih, Malaysia). Polyethylene glycol 200 [HO(C₂H₄O)nH, PEG 200], antimony potassium tartrate [K₂Sb₂(C₄H₂O₆)₂, 99%], sodium sulfite (Na₂SO₃, ≥98%, 2.0 M), sodium hydroxide (NaOH), and sulfuric acid (H₂SO₄, 95%) were procured from Sigma Aldrich (St Louis, MO, USA). Cerium (IV) sulfate tetrahydrate (CeO₈S₂∙4H₂O, ≥98%) was purchased from Acros Organics, Geel, Belgium. All the chemical reagents used in this study were of analytical reagent grade. Distilled water was used throughout the experiments to prepare the solutions and the stock solutions were freshly prepared every week. The pH of the solutions was adjusted by the addition of 0.1 M H₂SO₄ solution and 0.1 M NaOH solution.