TP - Total Phosphorus
Phosphorus is an essential nutrient of plant, animal and human. In water, it exists primarily as orthophosphate (PO43-) or in organic compounds. The parameter total phosphorus (TP) defines the sum of all phosphorus compounds that occur in various forms. The determination is defined of the German Institute of Standardization (DIN EN ISO 6878:2004-09).
Need of monitoring
Increased phosphorus concentration leads to the eutrophication of the aquatic environment. Impacts are oxygen deficiency with deadly consequences to fish and other aquatic organisms.
This results in the need of monitoring this parameter. Phosphorus can pass via waste water discharges or drainage of agriculture areas into the waters. In addition, nowadays detergents are often used for e.g. dishwashers that contains phosphorus. Their increased usage and disposal leads to increased concentrations in the waste water. With the expansion of waste water treatment plants with phosphorus elimination the pollution that occurs from waste water discharges, sustainably reduced.
Phosphorus compounds can be determined by various colorimetric methods, which involve different advantages and disadvantages. In addition, there is a difference beteen the parameters be determined in the dissolved phase (orthophosphate PO43-) or by including the solids (total phosphate). For determination in the dissolved phase are used the molybdenum blue method or the vanadate/ molybdate method (yellow method). The molybdenum blue method is more accurate in the lower measuring ranges. Hence, these methods are used differently. Depending on the application, importance of accuracy, DIN-conformity or profitability.
Molybdenum blue method
The TP value can be determined by the molybdenum blue method, by adding a reducing agent. This can take place as follows:
Orthophosphate reacts with ammonium molybdate to a slightly yellow molybdenum phosphoric acid. By adding a reducing agent, the molybdenum blue is formed, which has a blue color:
PO43- + 12 (NH4)6Mo7O24 + 3H+ → H3P(Mo3O10)4
H3P(Mo3O10)4 + 2 SnCl2 + 8 HCL → H7P(Mo3O10)4 + 2 H2 (SnCl6)
Subsequently the TP value can detect from the molybdenum blue. Its intensity of the colour determined with a photometer at wavelengths of about 500 - 750 nm. This method is particularly suitable for the determination of low concentrations.
The QuickTOCNPO also uses the advantages of this method and measure the TP by adding the oxidant persulfate. Then the sample mixture flows into a UV reactor. There the oxidation of the sample proceeds by ultraviolet light. Molybdenum blue is formed and the result of TP detects by using a photometer.
Vanadate yellow method
Here a reaction occurs to the orthophosphate ions with ammonium molybdate and ammonium vanadate. The result is the yellow Ammoniumphosphorvanadomolybdat, which is also determined photometrically by its intensity of the colour.