Fluorescent Tracers

     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.