Application Note

Further Advances In Monitoring Low Level Iron In The Stream Cycle

Source: Hach Company

By Kenneth Kuruc, Denton Slovacek & Luke Johnson

In an earlier paper, it was discussed how a laser Nephelometer could be used to detect the transfer of corrosion products in a HRSG power plant at ppb levels with particles in the sub-micron size range. Also, a modified trace level iron test was documented using colorimetric methods on a lab spectrophotometer to measure total iron levels to 1ppb.

In this paper, further advances will be outlined which improve the accuracy and repeatability of the lab method, while providing further insight into the level of protection being provided to steam cycle components against FAC and other corrosion mechanisms. Focus will be in understanding the chemistry of the measurement so as to provide more specific data to plant personnel regarding the level and species of iron being observed.

The traditional colorimetric method for dissolved iron is based on the extremely sensitive ferrozine ferrous iron complex described by Stookey. Ferrozine complexes with dissolved ferrous iron to form an intensely colored purple complex. The dissolved ferrous iron concentration may be determined by measuring the absorbance of this complex colorimetrically. Modifications of this traditional method allow for the determination of both dissolved iron and particulate iron oxides at very low concentrations.