Chem 223:Experiment 10 Spectrophotometric Determination of Total Phosphate in Water

Lab Experiment > Chem 223:Experiment 10 Spectrophotometric Determination of Total Phosphate in Water

Hanoi University of Science and Technology CHEM 223 I. Purpose of the experiment: Phosphorus, widely found in various products such as fertilizer, agricultural pesticides, dyes, processed food, and alkaline detergent, has greatly enriched our lives. However, it also contaminates environmental waters by flowing into rivers and lakes along with residential and industria ...[Show More]

Hanoi University of Science and Technology
CHEM 223
I. Purpose of the experiment:
Phosphorus, widely found in various products such as fertilizer, agricultural pesticides, dyes,
processed food, and alkaline detergent, has greatly enriched our lives. However, it also
contaminates environmental waters by flowing into rivers and lakes along with residential and
industrial waste water, or agricultural runoff. Phosphorous and nitrogen, which enhance the
growth of plankton in lakes, are used as indices of eutrophication. Consequently,
measurement of phosphorus concentration is important for water quality management.
Phosphorus occurs in 3 compounds in natural waters:
• inorganic, dissolved ortho-phosphate
• dissolved organic phosphorus compounds
• particulate phosphorus (bound in biomass or attached to particles),
which add up to the total of phosphorus content P-Total, an important parameter in
monitoring wastewater treatment plant effluents.
In this experiment, students will become familiar with the two standard approaches for the
determination of anions using spectrophotometry.
II. Introduction
There are two common spectrophotometric methods available for determining phosphate or
phosphorus concentrations:
• Molybdenum blue method
• Vanadate/molybdate method (yellow method)
Both techniques are based on the measurement of ortho-phosphate. Digestion of both
dissolved organic as well as particulate phosphorus compounds is therefore mandatory for
determining the total P content. In addition, an unfiltered sample must be acquired in order to
include all solid matters in the digestion process. Digestion is usually performed by heating
the sample with peroxodisulfate and sulfuric acid.
Inorganic phosphorus as orthophosphate, PO43-, is analyzed by reacting the phosphate with
ammonium molybdate under acid conditions to form a heteropoly acid, molybdophosphoric
acid. In the presence of vanadium, yellow vanadomolybdophosphoric acid (max=690 nm) is
formed. The intensity of the yellow color is proportional to the phosphate concentration
present in the unknown sample. The analysis will be performed on a double-beam
spectrophotometer.