Because the diagrams reflect influence of [H+] activity on formation of various molybdate species, it would have been more appropriate to use pH scale, rather than normality of acids to plot the diagrams. However, while rigorous, such approach is quite impractical because there is no way in which pH can be measured precisely enough in the mixture of reactants within a flow cell. The advantage of the scale based on [H+] N is, that it is directly linked to concentration of acid used in preparation of reagent, in case of this work by 4x dilution (1.5.8. and 1.5.9.). Comparison of [H+] diagrams of different acids confirms that the diagrams reflect in correct way reaction equilibria: optimized conditions for sulfuric acid are located at higher acidities and higher [H+]/[Mo] ratios than those of hydrochloric or nitric acid, because sulfuric acid is not fully dissociated.
In order to limit this study to a manageable scope, the temperature (22 C) incubation time (30 seconds), concentration of potassium antimony tartarate and composition of reducing reagent (ascorbic acid + SDS), optimized in our previous work, Hatta et.al. 2019) were kept constant (1.5.8.). These limitations leave room for further investigation, such as study of the influence of antimony and longer incubation times on production of PMoB and PMo. Briefly, in presence of Sb absorbance increases dramatically within first 30 seconds of incubation time after which it gradually declines (A). The spectra of PMoB and MoB (B) reveal that at optimized conditions and 30 seconds incubation time (SPEC 1) only PMoB is produced. At longer incubation times MoB is formed, precipitates, and while overall absorbance increases (SPEC 2) the monitored absorbance at the pair of 880nm/550ref wavelengths actually falls. Therefore is no benefit in prolonging incubation time beyond 30 seconds because formation of PMoB has, in the presence of Sb already reached equilibrium HC1, S1,N1.
While the objective of this study, optimization of PMoB method, to be used for trace analysis of phosphate was achieved, many unknowns about PMoB chemistry remain hidden in the black box. While this study implies a simple situation, interaction between reaction rates of formation of PMoB and of reagent blank (MoB), the literature offers far more complex picture, where various PMoB species are formed in presence of various reductants and with and without Sb intervention. (Nagul et.al. 2015 Fig.6). Whether the differences of values of molar absorbtivities reported in literature are caused by measuring a blend of colloidal suspension (Hatta et.al, 2019) produced at suboptimal conditions, or due to formation of different PMoB compounds still has to be resolved.
Hopefully, further studies of fascinating chemistry of phospho and silica molybdates will continue, with assistance of programmable Flow Injection technique and associated instrumentation, that made this initial work possible.