Thermochemistry of formation of sodium polyphosphates from sodium orthophosphates

Abstract

The heats of formation of crystalline Na2H2P207, Na4PZ07, (NaP03)x, and Na5P3010 from crystalline sodium orthophosphates were measured using a differential scanning calorimeter. Heats of reaction at 298 K were calculated from the heat capacities of the phosphates which were measured from 310-620 K. Results confirm recent values for the heats of formation of NaH,PO,, Na,HPO4, Na2H2P207, Na3P309, and Na5P3010.Sodium orthophosphates undergo condensation on heating to give sodium polyphosphates. The composition of the polyphosphate depends on the Na/P ratio in the original orthophosphate or orthophosphate mixture.This paper reports the heats of formation of crystalline disodium dihydrogen pyrophosphate Na2H2P207, sodium metaphosphate (NaP03)x, tetrasodium pyrophosphate Na4P207: and sodium tripolyphosphate Na5P301 , -, (phase 11) from orthophosphates, obtained by the use of a differential scanning calorimeter. In addition, the heat capacities for several phosphates were measured in order to compare enthalpies of polyphosphate formation at 298 K with the standard heats of formation of ortho and polyphosphates recently reported by Irving et aL2* EXPERIMENTAL MATERIALSAnhydrous NaH2P04 and Na2HP04 were made from A.R. samples as described by Irving et aL2 Paper chromatography by the technique of Karl-Kroupa showed no impurities, but this method does not show phosphate impurities present in amounts of 0.5 % or less. Na4P207, Na5P3010 (phase 11), and Na3P309 were made by the methods of Bell.5 Na4P207 and Na3P,0g showed no detectable impurity when analyzed by paper chromatography. Na5P3010 was purified by four recrystallizations from aqueous ethanol by the method of Quimby.6

Electrolytic Synthesis of White Phosphorus Is Promoted in Oxide-Deficient Molten Salts

“…For the melts with the lowest anhydride population, the Tafel plots recoalesce at a −1.5 V overpotential. In line with literature reports and thermochemical reference data, we impute this high overpotential region to the reduction of orthophosphate generated at the electrode surface. ,,− Thus, we attribute this transition in the Tafel behavior to the local consumption of anhydrides near the electrode surface, a phenomenon that occurs at lower current densities for melts with lower bulk Lux acidities. We note that while a high anhydride population permits P 4 production at reduced overpotentials, any anode reaction must regenerate these anhydride linkages, potentially introducing a countervailing overpotential penalty on the anode.…”

Efficient Electrosynthesis of White Phosphorus from Molten Condensed Phosphate Salts

“…For the melts with the lowest anhydride population, the Tafel plots recoalesce at a −1.5 V overpotential. In line with literature reports and thermochemical reference data, we impute this high overpotential region to the reduction of orthophosphate (23,24,(40)(41)(42)(43)(44)(45)(46). Thus, we attribute this transition in the Tafel behavior to the local consumption of anhydrides near the electrode surface, a phenomenon that occurs at lower current densities for melts with lower bulk anhydride concentrations.…”

Electrolytic Synthesis of White Phosphorus is Promoted in Oxide-Deficient Molten Salts

“…In line with literature reports and thermochemical reference data, we impute this high overpotential region to the reduction of orthophosphate generated at the electrode surface. 21,22,[39][40][41][42][43][44][45] Thus, we attribute this transition in the Tafel behavior to the local consumption of anhydrides near the electrode surface, a phenomenon that occurs at lower current densities for melts with lower bulk Lux acidities. From another perspective, the anhydride linkages in these melts may be understood to function as preassociated Lewis acids, promoting P-O cleavage and dramatically reducing the overpotential for P4 synthesis by nearly a full volt, by direct analogy to prior reports of Lewis acid-promoted reduction of organophosphine oxides.…”