Molybdic acid reaction with sodium silicate

Fig. 8-48. On-line determination of mineral acids and silicate. - Separator columns 2 IonPac AS4A concentrator columns (A) TAC-1, (B) IonPac AG5 eluent (A) 0.0017 mol/L NaHC03 + 0.0018 mol/L Na2C03, (B) 0.015 mol/L H3B03 + 0.015 mol/L NaOH flow rate (A) 2 mL/min, (B) 1 mL/min detection (A) suppressed conductivity, (B) photometry at 410 nm after reaction with sodium molybdate concentrated volume 23 mL solute concentrations 10 ppb fluoride, 10 ppb chloride, 10 ppb bromide, 10 ppb nitrate, 20 ppb orthophosphate, 20 ppb sulfate, and 23 ppb silicate (taken from [36]).

Both concentrated and somewhat diluted solutions of sodium silicates of different ratios were examined by suddenly injecting samples into rapidly stirred dilute H2S0 to convert the ionized species to the corresponding silicic acids. These were at once characterized by reaction with molybdic acid. Also colloidal species were separated from monomer and oligomers by extraction into tetrahydrofuran (THF). 

The study of the decondensation kinetics of the silicates is not easy. The NMR of Na reveals that when a solid sodium silicate is put into solution, the first step is the integral rehydration of the sodium that passes into the aqueous solvent. Afterward, there is progressive decondensation of the silicic polymers or oligomers. The decondensation reactions take place in the washing liquor, which is a dilute aqueous medium. The reactions evolve very rapidly, and the kinetics are difficult to measure by NMR. That is why other techniques, like the silico-molybdic complex, are used [16]. With this method, the quantity of silicic monomers in a solution resulting from the decondensation of the larger silicic species can be measured. The monomers formed react with the molybdic acid in controlled conditions to give a yellow complex that can be easily titrated by spectrophotometry. 

---