Application Note

Silica Analyzer Calibration Verification: Ensuring Confidence In The Quality Of Steam Cycle Water For The 5500sc Silica Analyzer And DR3900 Spectrophotometer

Source: Hach Company
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When used together, the Hach® 5500sc Silica Analyzer [0.5-5000 μg/L SiO2] and Ultra-Low-Range (ULR) Silica Method 8282 lab analysis [3-1000 μg/L SiO2] ensure the most accurate and reliable silica monitoring results. The DR3900 spectrophotometer may be used to perform Method 8282, as well as to measure a host of other parameters. The 5500sc is an independent and self-calibrated system ready from startup with quality reagents delivered from one single source supplier. Additionally, the 5500sc may be calibrated and validated with external standards verified with the DR3900 and introduced via the “Grab Sample In” function. The “Grab Sample Out” function allows for easy online measurement verification with the DR3900. Single sourcing of reagents for each analysis eliminates variability due to reagent quality.

Both the 5500sc Silica Analyzer and DR3900 ULR Silica analysis use the sensitive heteropoly blue chemistry to measure low concentrations of silica for high purity steam cycle applications. First, any silica and phosphate ions in the sample are reacted with the molybdate ion to form phosphomolybdic and silicomolybdic acid complexes. The phosphomolybdic acid complexes are then destroyed by the addition of citric acid. Finally, the remaining silicomolybdic acid complexes are reduced with an amino naphthol sulfonic acid to form a blue colored heteropoly acid. The intensity of this blue species is proportional to the amount of silica in the original sample. Each of the reagents used for this analysis are identical for the 5500sc and DR3900.

Both the 5500sc and DR3900 implement a built-in calibration curve. However, it is critical that these calibrations be periodically verified to ensure that the curves are accurate for each specific instrument. Instrument response can drift over time due to regular wear and tear. While these issues are addressed through regular maintenance, it is recommended that verification be performed periodically to ensure that accuracy is maintained throughout the maintenance interval.