Why is the Hach silica Method 8185 measurement at 452 nm whereas SM 4500-SiO2 C uses 410 nm?
Standard Methods versus Hach method for Silicomolybdate.
The SM 4500-SiO2 C uses oxalic, sulfuric and hydrochloric acids, whereas Hach Method 8185 uses citric and sulfamic acids. Hach uses these acids instead because of concerns with hazards. The 452 nm measurement provides better recovery with the reagent combination used in the Hach Method 8185.