For the blue methylene method is recommended N,N-Dimethyl-1,4-phenylenediamine oxalate or just N,N-Dimethyl-p-phenylenediamine (without oxalate)? Does the difference affect the measurements? Does the oxalate make it more soluble or gives a longer half-life of the reactive? It is VERY important to know as there are differences in prices and depends on stock availability.
Another question is if the level of hydration of certain compounds affect the calibration curves or alters the results ? For example protocols mention using Na2SO4 (unhydrated) and NaS2.9H2O for calibration curve; but here in the lab we only have Na2SO4.10H2O and NaS2.7-9H2O.