Phosphate Determination by Complexometric Back Titration

This application note (download below) contains the method and procedure tips for determination of phosphates in selected soft drink and mouthwash samples by complexometric titration. A back titration is used, where an excess of bismuth nitrate is added to the sample, bismuth phosphate is then precipitated and unreacted bismuth ions are titrated against EDTA. 4-(2-Pyridylazo)resorcinol (PAR) is used as the color indicator and the color change at the endpoint of  the reaction is detected with a photometric sensor.

Reactions:

PO₄^3- + Bi(NO₃)₃  → BiPO₄ + 3 NO₃^-  (sample titration)

Bi(PAR) + EDTA → Bi-EDTA + PAR (color change)

Background

Sodium phosphate is used in mouthwash and dental gels as a buffer in an acidulated phosphate fluoride (APF) solution, an agent that helps to prevent dental caries. The recommended frequency of use for these dental products depends on the concentration of the phosphate and fluoride ions in them. Phosphates are also used as additives in carbonated soft drinks and processed foods. Even though phosphorus is an essential element for the human body, an excess intake of phosphates from food and drinks can lead to health problems, especially for people with kidney dysfunctions. Hence, it is important to know the exact concentration of phosphate ions in the mentioned products.

In a back titration an excess of titrant is added to the sample. After a sufficiently long waiting time, this excess is then titrated with a second titrant. The difference between the added amount of the first and second titrant then gives the equivalent amount of the analyte.

For more information on complexometric and back titrations see the Titration Fundamentals guide.

Figure: titration curve of a standard sample