by
vmu » Sun Nov 17, 2019 9:16 am
Phosphonic acid is H-P(O)(OH)2. It is an inorganic acid.
It co-exists with extremely minor quantity of its tautomer P(OH)3 called phosphorous acid. These are the names recommended by IUPAC.
In practice, both names are used for the stable H-P(O)(OH)2.
Organic acids R-P(O)(OH)2 are called phosphonic acids.
The salts and esters of phosphonic acids are called phosphonates.
The stable organic compounds P(OR)3 are called phosphites. P(OH)3 does not form stable salts. In practice, the word "phosphite" is frequently used to name the salts of H-P(O)(OH)2.
So, you need to determine inorganic phosphonic acid H-P(O)(OH)2 or its salts in some object.
What is the object? What is the concentration range?
In several pharmaceuticals, the impurity of phosphonic acid / phosphonate is separated from the main component and other impurities (phosphoric acid / phosphate) and determined with the use of anion-exchange columns. Refractometers, conductometers, and UV detectors in indirect mode are used for detection.
Example: phosphite and phosphate in sodium alendronate according to European Pharmacopeia.
Column: Allsep Anion, 150 mm × 4.6 mm, 7 um.
Eluent: 0.2 mL HCOOH in 1000 mL H2O, 2 M NaOH to pH 3.5, F = 1.2 mL/min, 35 °C.
Sample: 2 mg/mL in water, 100 uL.
Detection: RID.
LOQ: about 0.25% (0.005 mg/mL).
When calculating and reporting the result, please don't forget to clearly state what species you are determining (or recalculating for):
- phosphonic (phosphorous) acid H3PO3;
- phosphonate (phosphite) anion HPO3^2-;
- sodium phosphite Na2HPO3;
- conditional particle PO3;
- elementary phosphorous in the form of phosphonates;
- phosphate-anion equivalent of phosphonate-anion;
- phosphoric acid equivalent of phosphonic acid;
- etc.
