Determination of formation constants of molybdophosphates in strong acid solutions

Abstract

Sneddon, J.; Hunter, J. K.; Ottaway, J. M.; Fell, S. G. Analyst (London) 1981, 106, 288. Harnly, J. M.; Mlller-Ihll, N. J.; O'Haver, T. C. J. Aotom. Chem., in press. " Y Iare measured at only three wavelengths, the dynamic range will fall short of that which can be achieved using a bi-Gaussian wave. However, the construction of sectored wheels with more than fclur quadrants would eliminate this limitation.The equllibrlum constants for 9-, 11-, and 12-molybdophosphates were determined spectrophotometrically in strong acid solutions wlth the continuous variation method and with computer slmulatlons. For 12-moiybdophosphate, log K,, =:EXPERIMENTAL SECTION Reagents. A 0.5 M stock solution of Mo(V1) was prepared from reagent grade Na2Mo04-2Hz0 (Baker Chemical Co.). Stock solutions of HNO,, HC104, and H2S04 were prepared from the concentrated acid reagents and standardized with NaOH previously standardized with KHP. The stock phosphate solution was prepared from reagent-grade KH2P04, and the 5 M NaC10, stock solution used to control ionic strength was prepared from the reagent grade chemical (G. F. Smith Chemical Co.). This Mo(OH)S(HzO)", Mo20(0H)&O)+, M O~O ( O H )~( H~O )~~' ,

The molybdenum blue reaction for the determination of orthophosphate revisited: Opening the black box

“…When 12-MPA is finally formed, it is inevitably protonated to some extent, contrary to early opinion [36]. Its pK a values in aqueous solutions have been determined as 2.4, 4.31 and 5.46 [37]; 12-MPA should therefore exist mainly as H 3 PMoO 40 in MB reaction mixtures.…”

Microdetermination of phosphate in water by gel-phase colorimetry with molybdenum blue

“…Phosphate reacts with molybdate in a strong acidic solution to produce the yellow 12-molybdophosphate (12-MPA) species (1). In the determination of the amount of phosphate in various samples, the yellow species derived by chemical reaction is absorptiometrically employed in many laboratories.…”

Determination of orthophosphate by flow injection analysis with amperometric detection

“…Additonal methanol concentration increases after this point lead to only minimal increases in 12-MPA concentration. Figure 3 contains Job plots obtained from a second series of continuous variation experiments in which methanol concentration was held constant at 60% (v/v) and nitric acid concentration varied between 0.20 F and 3.0 F. These plots indicate that 12-MPA stability is decreased by increasing acid concentrations as has been demonstrated by others (9). The concavity at a molybdate mole ratio of 0.88 which appears at higher concentrations of nitric acid suggests the appearance of a second absorbing species whose molybdate to othophosphate ratio is smaller than that of Cyclic Voltammetry.…”

“…Continuous Variation Studies. All spectrophotometric measurements were performed with a Beckman Model 26 spectrophotometer and a matched pair of quartz cuvettes purchased from Fisher Scientific Co. 12-MPA absorbance measurements were made at 430 nm as in the studies of Kircher and Crouch (9).…”