Mechanistic investigation of molybdenum blue method for determination of phosphate

Abstract

smaller than in the absence of methanol. Curve C in Figure 3 is that calculated in the presence of 0.5M of the strong hydrogen bond donor p-bromophenol. The heteroconjugation constants of AHR~and A(HR)-r, Adenoting 3.5-dinitrobenzoate and HR, p-bromophenol are 3.6 X 101 2 and 3.9 X 103 4, respectively, as determined previously (2).The dissociation constant in AN of p-bromophenol is of the order of 10~27, that of 3,5-dinitrobenzoic acid is 10-17 (6), so that negligible proton transfer occurs in the reaction between 3.5-dinitrobenzoate and the phenol. The potentiometric titration curve in the presence of 0.5M p-bromophenol is virtually identical with that of a weak acid in water and the apparent pKdnA. is 3 units smaller than in the absence of the phenol. Use of this effect can be made in the titration of mixtures of acids in AN.In conclusion, acid-base equilibria in AN are generally much more complex than in aprotic protophylic solvents like dimethylsulfoxide. The latter solvents are much to be preferred over AN for the titration of weak acids. On the other hand, AN is one of the best solvents for the titration of weak bases, as will be shown in a subsequent paper.

Sampling, sample treatment and quality assurance issues for the determination of phosphorus species in natural waters and soils

“…Ascorbic acid acts as a 2-electron reductant [7] with the major advantages being that it is less salt sensitive and colour development is fairly independent of temperature [6]. Ascorbic acid on its own however has the major disadvantage of slow colour development [8], but the addition of antimony as a catalyst increases the rate of reduction of the complex [4].…”

Comparison of Five Antioxidant Assays for Estimating Antioxidant Capacity From Three Solanum Sp. Extracts

“…The first step is formation of 12-molybdophosphoric acid (12-MPA) (Mo VI), and the next step is a reduction of 12-MPA to phosphomolybdenum complex (Mo V) by antioxidant. The stoichiometric of Mo (VI) and 12-MPA is 6:1 [33]. In the present study which was adopted from Prieto et al [12], the amount of ammonium molybdate which was used too low, so that only a little 12-MPA formed and then react with antioxidant and gave phosphomolybdenum complex.…”

Glycochenodeoxycholic acid inhibits calcium phosphate precipitation in vitro by preventing the transformation of amorphous calcium phosphate to calcium hydroxyapatite.

“…Total calcium was measured by atomic absorption spectrophotometry (model 5 100, Perkin-Elmer Corp., Norwalk, CT) after dilution of samples with 5% trichloracetic acid/ 1% lanthanum chloride. Phosphate was determined by conductivity measurement after ion exchange HPLC (Ionpac AS4A column, Dionex, Sunnyvale, CA) or by the molybdenum blue method (9).…”