An ATR-FTIR study of different phosphonic acids in aqueous solution

An ATR-FIR study of the vibrational spectra of 1-hydroxyethane-1,1′-diphosphonic acid (HEDP), nitrilotris(methylenephosphonic acid) (NTMP) and N,N-bis(2-hydroxyethyl)aminomethylphosphonic acid (BHAMP) in aqueous solution is presented. The study was performed in the range of pH from 5 to 9, and bands assignments are given in the 2000–890 cm−1 range. However, as phosphonates display bands due to the PO stretching vibration mainly in the 900–1200 cm−1 range, the study is focused in this midinfrared region, which shows important changes as the pH changes, specially the ν(POH) at ∼925 cm−1 and ν(PO32−) at ∼970 cm−1 vibrations. IR analyses give also evidences for the zwitterionic nature of BHAMP and NTMP in solution with a strong indication that the zwitterion in both compounds remains intact throughout the pH range investigated. The successive protonation steps with the decrease of pH were evidenced in the IR spectra of the three studied phosphonates.