A Reliable Method for Quantification of Phosphonates and Their Impurities by ³¹P NMR

Abstract

A report of a phosphorus-31 nuclear magnetic resonance (³¹P-NMR) method to determine four alkylpolyphosphonates, aminotris(methylene phosphonic acid) (ATMP), 1-hydroxyethylidene diphosphonic acid (HEDP), ethylene diamine tetra (methylene phosphonic) acid (EDTMP), and diethylenetriamine penta(methylene phosphonic) acid (DTPMP), as well as impurities (orthophosphoric acid and phosphorous acid) is described. Initially, the purity of phosphonate standards was verified. Next, calibration curves using methylphosphonic acid were built and validated. The results were compared using single point calibration. Subsequently, 24 commercial samples of phosphonates were analyzed using the two NMR methods. Concentrations determined by the two techniques were statistically equivalent. Limits of detection of 1.7, 2.4, 3.4, 2.4, 0.9, and 0.8 mg mL⁻¹ were obtained for ATMP, HEDP, EDTMP, DTPMP, orthophosphoric acid, and phosphorous acid, respectively. ³¹P NMR with a single point calibration is a rapid and accurate method to analyze commercial phosphonate samples. This technique allows straightforward sample preparation and specific determination of the richness and main impurities of phosphonate samples.

Keywords:

• ³¹P NMR
• Detergents
• Internal standard
• Phosphonates
• Quantification

Acknowledgments

The authors are grateful to Depew Fine Chemical Co., Ltd. for supplying the phosphonate samples and advice during the development of this procedure.

Notes

RSD = relative standard deviation.

Levels of statistical significance: *p < 0.0001.

Phosp1: orthophosphoric acid; Phosp2: phosphorous acid.

^a weight.

^b value obtained with the calibration curve method.

^c value obtained with the single point method: [(phosphonate 1*³¹P signal area x moles mL⁻¹IS)/ IS 1*³¹P signal area] x MW_phosphonate; MW_ATMP = 299.05; MW_HEDP = 206.03; MW_EDTMPA = 436.13; MW_DTPMPA = 573.2; = 299.05; MW_Phosp1 = 98; MW_Phosp2 = 82.

Note: The mean and standard deviation (SD) of four ³¹P NMR experiments are provided.

Phosp1: orthophosphoric acid; Phosp2: phosphorous acid.

^a value obtained with the calibration curve method.

^b value obtained with the single point method: [(phosphonate 1*³¹P signal area x moles mL⁻¹IS)/ IS 1*³¹P signal area] x MW_phosphonate; MW_ATMP = 299.05; MW_HEDP = 206.03; MW_EDTMPA = 436.13; MW_DTPMPA = 573.2; = 299.05; MW_Phosp1 = 98; MW_Phosp2 = 82.