ORIENTAL JOURNAL OF CHEMISTRY www.orientjchem.org An International Open Access, Peer Reviewed Research Journal ISSN: 0970-020 X CODEN: OJCHEG 2020, Vol. 36, No.(6): Pg. 1066-1077 This is an Open Access article licensed under a Creative Commons license: Attribution 4.0 International (CC- BY). Published by Oriental Scientific Publishing Company © 2018 Spectrophotometric Determination of Low Levels of the Orthophosphate Anion as Molybdenum Blue using Sodium Thiosulphate Reducing agent IRENE NALUMANSI 1 *, GRACE BIRUNGI 1 , BRENDA MOODLEY 2 and EMMANUEL TEBANDEKE 3 1 Department of Chemistry, Mbarara University of Science and Technology, Uganda. 2 School of Chemistry and Physics, University of Kwa Zulu-Natal, Westville Campus, South Africa. 3 Department of Chemistry, Makerere University, Uganda. *Corresponding author E-mail: inalumansi@must.ac.ug http://dx.doi.org/10.13005/ojc/360608 (Received: November 10, 2020; Accepted: December 11, 2020) ABSTRACT A simple spectrophotometric method for the determination of low levels of the orthophosphate (PO 4 3- ) in environmental water systems is reported. The method is based the formation of the phosphomolybdate from the condensation of molybdate and orthophosphate in aqueous acid medium followed by reduction with sodium thiosulphate to form phosphomolybdenum blue. The system obeys Beer’s law at 880 nm (λ max ) in the 0.005–0.06 mg P mL -1 phosphate concentration range. Molar absorptivity, Sandell’s sensitivity and correlation coefficient values for the determination were 57526 L mol -1 cm -1 , 0.2835 μg cm -2 and 0.9948 respectively. The limit of detection was 2.213 x 10 -3 mg P mL -1 . The results of PO 4 3- determination in water samples obtained using the spectrophotometric method developed in this study, compare favourably with those generated using the Murphy and Riley method which is commonly used for this analysis. Therefore, the study ably demonstrates the suitability of the present spectrophotometric method for analysis of the orthophosphate in environmental water samples. Keywords: Spectrophotometric, Phosphomolybdate, Molybdenum blue, Orthophosphate anion, Sodium thiosulphate INTRODUCTION Phosphorus (P) a vital nutrient to human, animal and plant growth occurs naturally in rocks and soils as phosphorite or hydroxyapatite, Ca 3 (PO 4 ) 2 . Ca(OH) 2 , apatite, Ca 3 (PO 4 ) 2 .CaF 2 , vivianite, Fe 3 (PO 4 ) 2 .8H 2 O and aluminium phosphate 1 . Since P serves as a nutrient in the food chain, supplementing the native P in the soil and animals’ diet where it is lacking may be a necessity. This can result in excess P moving from agricultural areas especially where animal wastes are being used as fertilisers to other environmental systems 2 . These phosphates stimulate the growth of phytoplankton and aquatic plants which provide food for many organisms including zooplanktons, fish, humans and other mammals. This may result in eutrophication of waters and its related effects of anoxic conditions 3,4,5 ; which leads to