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 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 2015 Fig.6). Whether the  differences of values of molar absorbtivities reported in literature are caused by measuring a blend of colloidal suspension (Hatta, 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.