Modulation of the pH in the Determination of Phosphate With Flow Injection and Fourier Transform Infrared Detection

Abstract

Flow injection (FI) with FTIR detection is proposed as a versatile technique for the determination of phosphate in aqueous solutions. The modulation of the pH was effected by introducing the appropriate buffer solutions into the sample line of the manifold. Spectral changes of the phosphate vibrations in the region 900–1300 cm ^{- 1} due to the dissociation or association of hydrogen ions were utilized for the determination of phosphate. Two modes were investigated, the conversion of dihydrogenphosphate (pH 5) into monohydrogenphosphate (pH 10) and into the tertiary phosphate (pH > 13). Calibration graphs of standard solutions were obtained in the concentration range 100–1000 mg l ^{- 1} (1–10 mmol l ^{- 1} ) phosphate. The standard deviation of the method was 2.0 mg l ^{- 1} . The high selectivity of the method was demonstrated by the analysis of six sugar and non-nutritive sweetened soft drink samples containing 350–600 mg l ^{- 1} phosphate. The average deviation of the results from those given by ion chromatography as an external reference method was 1.7% for non-nutritive sweetened samples and 3% for sugar containing samples. A sample frequency of 60 h ^{- 1} was obtained.