Total Phosphorus Determination

Total phosphorus determination is a measure of all phosphorus found in a sample. Phosphorus exists in the form of many compounds, such as the phosphate ions (PO₄^3-), located in water, soil and sediments. Phosphates can also be in inorganic forms (orthophosphates and polyphosphates), or organic forms (organically bound phosphates). Whether these phosphorus are dissolved or particulate, this is commonly used in wastewater treatment sampling, especially for determining the health of ground waterways. At BOC Sciences, total phosphorus is measured by first digesting the sample, converting all other forms of phosphorus into orthophosphate, and then measuring the orthophosphate using the ascorbic acid method. This test is capable of measuring both dissolved and suspended orthophosphate.

Figure 1. The Basics of Phosphorus Removal. (Melendez, L. 2014)

What does a High Level of Phosphorus Means in the Environment?

Phosphorus is a naturally occurring element that is widely found in rocks, soils and organic materials, clinging to soil particles. Phosphorus is an important nutrient for aquatic organisms and in water, and is present in a particulate phase or a dissolved phase, therefore, total phosphorus is an important and major component of treatment plant effluent. Rainfall can cause varying amounts of phosphate to wash from the soil into nearby waterways, stimulating the growth of phytoplankton and aquatic plants. This may lead to an increase in fish populations and improve water quality. However, when excess phosphate enters a waterway, algae and aquatic plants can grow to the point where they take up large amounts of oxygen and affect the waterway, which can lead to eutrophication of the soil and water quality. The most common method for determining phosphorus concentrations in water bodies is developed based on colorimetric methods according to Beer's law A = ecl, in which (A) is proportional to the phosphorus concentration (c), (e) is the molar absorption coefficient and (l) is the path length of the measurement cell.

Figure 2. Phosphorus treatment in wastewater by physicochemical.

Advantages of Total Phosphorus Determination

• Relatively low measurement cost
• High digestion efficiency
• Strong sample throughput
• No specialized equipment is required
• Simple instruments and procedures
• Possibility for automation

Total Phosphorus Analysis Methods

Our total phosphorus test enables to measure all forms of phosphorus in the sample: orthophosphate, condensed phosphate and organic phosphate. We mainly utilize the ascorbic acid method to measure total orthophosphate, in which a reagent (liquid or powder) containing ascorbic acid and ammonium molybdate reacts with orthophosphate in the sample to form a blue compound. The intensity of the blue color is proportional to the amount of orthophosphate in the water.

Colorimetric Method

The principle of the colorimetric method for phosphorus determination involves the production of the phosphomolybdenum blue complexes. Once the phosphoric acid is formed, we add antimony and molybdate salts to accelerate the color formation reaction. Then, we use the vanadate method to reduce the complex, with stannous chloride (SnCl₂) or ascorbic acid being the most common reducing agents. Once a phosphomolybdenum blue complex has been formed, its absorbance can be measured. Typically, the spectrophotometer should be set to measure absorbance at 650 or 880 nm.

Digestion Method

As a nutrient, the specific form of phosphorus present in a water sample depends on the biomolecules and their breakdown products. Phosphorus can occur in protonated and deprotonated forms of orthophosphates (H₃PO₄, H₂PO₄^-, HPO₄^2-, PO₄^3-), polyphosphates such as triphosphates (H₅P₃O₁₀), organic phosphorus such as organophosphates (i.e. C-O-PO3 bond) as well as organophosphates (i.e. C-P bond) and phospholipids. In order to determine the different forms of phosphorus, a digestion step is required to convert all the required phosphorus into phosphate. Conversion of all required forms of phosphorus into phosphate prior to analysis.

• First, measurements without digestion, including orthophosphate
• Secondly, measurements after the acid digestion, and the measured species include inorganic polyphosphates in addition to orthophosphates
• Finally, measurements after an oxidazing digestion, converting all forms of phosphorus into orthophosphate including reactive phosphorus (RP) and acid-hydrolyzable phosphorus (AHP)

Figure 3. Extraction and Molybdenum Blue-based Quantification of Total Phosphate. (Ota, S.; Kawano, S. 2017)

Flow Injection Analysis

We use flow analysis to determine the phosphorus species (TP and possibly SRP). Flow injection analysis includes flow injection analysis of orthophosphate, manual digestion and flow injection analysis of total phosphorus, on-line UV/Fluorescence spectroscopy.