Rate of reduction of phosphomolybdate (PMo), of silicomolybdate (SiMo) and of molybdate (Mo) by ascorbic acid depends, besides temperature and concentration of reagents, also on the acidity of the molybdate reagent (MR).
For phosphate assay, at strong acidity (MR in 0.5 to 1.5 M sulfuric acid with addition of KSb tartarate serving as a catalyst), reduction of PMo is rapid, while SiMo is not reduced, thus allowing selective spectrophotometric measurement, since 400ppB Si response is the same as blank value. (A)
Condition for selective assay of silicate are more difficult to identify, since while lower acidity of MoR accelerates the reduction rate of Si Mo, it also promotes reduction of MR into molybdenum blue (2.2.32 fig B). Response curves obtained at lower acidity (MoR in 0,2 M sulfuric acid with no catalyst) indicate that redout for Si ( obtained at 140 to 150 sec window), can be corrected for the presence of P , by using data collected at widow located at 22 to 30 seconds (B).
While further decrease of acidity (MoR in 0,1M sulfuric acid) enhances rate of reduction of
SiMo it also promotes the reaction rate of reduction of MoR, as seen from increasing reagent blank (C).
In summary, data presented here confirm well known parameters that influence selectivity of Si and P by molybdenum blue method. What is new is demonstration of ability of microSI-LOV technique to rapidly explore and identify optimum conditions for reagent based assays.
Kinetics and Selectivity: Si & P Assay
2.2.33.A.
Note: Experiments described here were carried out in apparatus using protocol defined in 2.2.32