As a general rule of thumb, PTSA is best suited for cooling tower applications while fluorescein excels with boiler treatment chemicals.
Technically speaking, fluorescence is the luminescence that is caused by the absorption of light wavelength radiation. It is a unique phenomenon where particular chemicals give off visible light from emitting electromagnetic waves. Early observation of the occurrence included “kidney wood” in the 1560s, chlorophyll research in the 1830s, and the extensive investigation of wavelength change published in George Stoke’s paper “Refrangibility” published in 1852. Due to the sensitive emission profiles, spatial resolution, and specificity of fluorescence, the technology is rapidly becoming an important tool throughout industries. While there are many applications of fluorescence, the technology has gained traction only recently in the water treatment industry.
Figure 1) Wavelength emission of fluorescence
The demand to accurately dose scale and corrosion inhibitors is important for many reasons. First, historical tracers were either a detriment to the environment, difficult to test for, or just simply inaccurate. These methods for chemical introduction did not allow for online monitoring which is quite difficult to do in a cooling tower application where the system is constantly changing from water evaporation, reagent sorption, retention times and periodical bleed-off. The first fluorescent tracer technology was patented by Nalco with their discovery of PTSA (1,3,6,8-Pyrenetestrasulfonic acid, tetrasodium salt). PTSA, which is now commercially available, is a fluorescent tracer commonly blended at low concentrations with inhibitors for accurate dosing. Where historical monitoring methods are tested in parts-per-million (10-6), fluorescent tracers are precisely identifiable in parts-per-billion (10-9). At the temperatures, pHs, and other water parameters that cooling towers operate within, PTSA tracers work well relative to active chemical ingredients. Unlike molybdate or other dosing methods, fluorescent tracers do not have any known negative environmental impacts. PTSA has an excitation of 365nm and 410nm emission, so a calibrated fluorometer is required for accurate testing. Online meters can be installed for this task and can allow for continuous dosing of inhibitors. This method is an advancement compared with traditional feeding (based on water usage); however, disadvantages need to be considered. One major disadvantage of online monitoring is that the precision of dosing is dependent on the accuracy and cleanliness of online probes. PTSA is subject to hydrolysis at elevated pH, temperatures, and nitrite levels. In addition, fluorescent tracers seem to perform poorly with phosphates and can dissipate from solution. Personal experience has found PTSA to be an inaccurate method of dosing in boiler systems, where the fluorescent tracer is not representative of active inhibitor residuals. Fluorescent tracers are also dependent on background cation concentration (depicted in Table 1) which is hard to compensate for with online probe applications.
Figure 2) Molecular structure of PTSA (C16H6Na4O12S4)
Table 1) The dependence of inhibitor fluorescence intensity on inorganic cation concentration (pH 8.0, 77°F)
Fluorescein has an excitation of 494nm and emission of 520nm and has been found to work much better in high pH / high-temperature environments. Laboratory studies have found the intensity of fluorescein to increase as the pH value increased from 6.9 to 8.4 and become essentially constant with greater pH values. Like PTSA, many substances (such as quaternary amines) will cause a negative interference while laundry detergents cause positive interferences. Another advantage when compared with PTSA is that Fluorescein can be seen with the naked eye, usually a tint of green or red depending on the die when added to boiler inhibitors. With the elevated emission wavelength, fluorescein encounters a smaller cation influence proportionately compared with PTSA.
Figure 3) Molecular structure of Fluorescein (C20H10Na2O5)
While fluorescent tracer technology has greatly advanced the water treatment industry, there are still notable drawbacks. As mentioned, cation interference, pH, temperature, and turbidity all play a role in being able to accurately test fluorescent residuals. The equipment necessary for these tracers can be costly to install and maintain. However, these tracers have finally allowed for real-time chemical dosing and monitoring in cooling towers and boilers, and the future continues to look “bright”. Handheld fluorometers now offer a variety of emission wavelengths to accurately test for multiple fluorescent tracers and some even compensate for turbidity. As water treatment advances, it is worthwhile to understand the principles, advantages, and drawbacks associated with technology.
List of Resources:
 Molecular Devices. “Fluorescence Technology.” Molecular Devices, Molecular Devices, www.moleculardevices.com/technology/fluorescence. Accessed 6 Feb. 2022.
 Deak, J. (2020, July 29). Everything you need to know about Fluorescent Tracing in Cooling Water Systems. Pyxis Lab. Retrieved February 6, 2022, from https://pyxis-lab.com/2020/07/29/fluorescent-tracing-in-cooling-water-systems/
 Edinburgh Instruments Ltd. “What Are Absorption, Excitation and Emission Spectra?” Edinburgh Instruments, 25 Nov. 2021, www.edinst.com/us/blog/what-are-absorption-excitation-and-emission-spectra.
 Zhu, H., et al. “Fluorescent Intensity of Dye Solutions under Different pH Conditions.” Journal of ASTM International, vol. 2, no. 6, 2005, p. 12926. Crossref, https://doi.org/10.1520/jai12926.
 Oshchepkov, Maxim, and Konstantin Popov. “Fluorescent Markers in Water Treatment.” Desalination and Water Treatment, 2018. Crossref, https://doi.org/10.5772/intechopen.76218.
 Popov, Konstantin, et al. “Synthesis and Properties of Novel Fluorescent-Tagged Polyacrylate-Based Scale Inhibitors.” Journal of Applied Polymer Science, vol. 134, no. 26, 2017. Crossref, https://doi.org/10.1002/app.45017.