Analyte: phosphate

The phosphate ion (also orthophosphate ion), PO₄^3-, is the end base of phosphoric acid, H₃PO₄, which is a triprotic acid with pKa values of 2.12, 7.21 and 12.32, and is thus existing as different ionic species depending on the pH of the solution. The phosphate ion is triply charged and carries a hydration layer comprised of 4.5 associated water molecules in aqueous solutions and this is the dominating form when the pH is above 12.3. Between pH 12.3 and 7.21, the hydrogen phosphate ion, HPO₄^2-, is the dominating species, and at even lower pH the dihydrogen phosphate ion, H₂PO₄^–, is prevailing. Some properties of the phosphate ion are summarized in the table below.

Phosphate analysis

Phosphate is frequently analysed by ion chromatography (IC) with suppressed conductivity detection along with the other common inorganic anions, and quite many different IC columns can accomplish the separation using either hydroxide eluents or carbonate-bicarbonate eluents (when hydrogen phosphate typically is the major retained ionic species). Note, however, that the retention may vary significantly between different columns depending on the nature of the ion exchange groups and their proximity. Because the dominating ionic species after the suppressor typically is the singly charged dihydrogen phosphate ion, which has a limited molar ionic conductance (33 S cm²/mol), the sensitivity for phosphate with suppressed conductivity detection is often lower than similarly retained ions.

Some HILIC columns will also retain phosphate, and this may thus be an alternative analysis technique, although the risk of precipitation could be limiting this approach. Reversed-phase (RP) and ion-exclusion chromatography (IEC, ICE) tend to not give sufficient retention for the phosphate ion.

Table of some phosphate ion properties at 25 °C

Ion	Molecular weight, M	Molar conductivity, λ₀	Diffusion coefficient, D	Ionic radius, r	Hydration shell, Δr
	(g/mol)	(S·cm²/mol)	(mm²/ms)	(pm)	(pm)
Phosphate, PO₄^3-	94.97	207	0.61	238	54
M, was retrieved from Chemical Aid Molecular Mass Calculator. Values for λ₀ & D, are from CRC Handbook of Chemistry and Physics, 75th Ed., D.R. Lide (Ed.), CRC Press Inc. (1994), Boca Raton, pp 5-90 to 5-92. Data for r & Δr, plus the number of water molecules, n, mentioned in the text, are from Y. Marcus, J. Chem. Soc. Faraday Trans., 87 (1991) 2995-2999. The pKa values mentioned in the text are from Organic Chemistry Data pKa compilation.

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Applications

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