Method of monitoring and controlling corrosion inhibitor dosage in aqueous systems

The concentration of an aromatic azole corrosion inhibitor in the water of an aqueous system is monitored by a fluorometric method which is based upon the measurement of the fluorescence intensity of the aromatic azole corrosion inhibitor itself, and the precise determination of concentration permits control of the dosage of such aromatic azole corrosion inhibitor.

TECHNICAL FIELD OF THE INVENTION 
The present invention is in the technical field of monitoring and 
controlling the dosage of corrosion inhibitors in aqueous systems. 
BACKGROUND OF THE INVENTION 
Corrosion of processed metals, such as steel, copper, and zinc, is a 
process whereby elemental metals, in the presence of water and oxygen, are 
converted to oxides. Although corrosion is a complicated process, it may 
be considered an electrochemical reaction involving three steps which 
occur at the anodic and cathodic sites of a metal surface, as follows: 
1. Loss of metal to the water solution in oxidized cationic form at an 
anodic site, with concomitant release of electrons ("anodic reaction"); 
2. The flow of the released electrons to a cathodic site; and 
3. Oxygen at a cathodic site uses the electrons to form hydroxyl ions 
("cathodic" reaction), which flow to an anodic site. 
These three basic steps are necessary for corrosion to proceed, and the 
slowest of the three steps determines the rate of the overall corrosion 
process. The cathodic reaction is often the slowest of the three steps 
because the diffusion rate of oxygen through water is slow. 
A corrosion control program usually depends on specific inhibitors to 
minimize the anodic or cathodic reaction, or both. Among the various types 
of corrosion inhibitors are organic compounds, which act by adsorbing or 
chemisorbing as thin layers on metal surfaces to separate the water and 
metal. These materials form and maintain a dynamic barrier between the 
water and metal phases to prevent corrosion. One series of compounds 
applied to reduce copper and copper-alloy corrosion are aromatic organic 
corrosion inhibitors. This series of organic compounds, which includes 
mercaptobenzothiazole ("MBT"), benzotriazole ("BT"), butylbenzotriazole 
("BBT"), tolytriazole ("TT"), naphthotriazole ("NTA") and related 
compounds, react with the metal surface and form protective films on 
copper and copper alloys. These compounds are active corrosion inhibition 
treatment components and are referred to generally herein as copper 
corrosion inhibitors or corrosion inhibitors, or as aromatic azoles, and 
at times as triazoles or aromatic (thio)(tri) azoles. 
Active components may be lost due to deposit, corrosion, chemical and 
microbiological degradation processes and physical losses (blowdown, 
drift, incorrect feed rates, and the like) and combinations of such 
phenomena, which are discussed in more detail below. Monitoring the loss 
of an active treatment component, particularly if such monitoring permits 
the extent of loss to be quantified and if the monitoring is continuous, 
is an indicator of treatment program performance. Moreover, if such 
monitoring is continuous and the active component loss clearly determined, 
automatic control permits the corrosion inhibitor losses to be compensated 
for an dosage to be precisely controlled. 
The conventional analytical procedure for analysis of copper corrosion 
inhibitors is a UV(ultraviolet light)-photolysis/photometric method, and 
is well known to persons of ordinary skill in the art. This method, 
however, has a number of limitations. It is not well suited for continuous 
monitoring and/or control. It provides results that are strongly dependent 
upon the operator's laboratory technique. It cannot distinguish the 
chemical structure of the aromatic azole that is present. Its observed 
response is non-linear with respect to aromatic azole dosage. It requires 
that the aromatic azole be degraded with an ultraviolet lamp in the 
presence of a color-forming reagent. The UV-photolysis/photometric method 
and its limitations are discussed in more detail below. 
It is an object of the present invention to provide a method for monitoring 
and/or controlling copper corrosion inhibitor losses and/or dosages on a 
continuous basis. It is an object of the present invention to provide a 
method for monitoring and/or controlling copper corrosion inhibitor 
concentrations that is substantially independent of an operator's 
laboratory technique. It is an object of the present invention to provide 
a method for monitoring and/or controlling copper corrosion inhibitor 
concentrations that can distinguish the chemical structure of the aromatic 
azole that is present. It is an object of the present invention to provide 
a method for monitoring and/or controlling copper corrosion inhibitor 
concentrations that provides a response that is substantially linear to 
aromatic azole concentration. It is an object of the present invention to 
provide a method for monitoring and/or controlling copper corrosion 
inhibitor concentrations that does not require UV digestion of the 
aromatic azole. These and other objects of the present invention are 
described in more detail below. 
DISCLOSURE OF THE INVENTION 
The present invention provides a method for the monitoring and/or dosage 
control of corrosion inhibitors in aqueous systems comprising measuring 
the concentration of the corrosion inhibitor within the water system by 
fluorescence monitoring. In preferred embodiment the fluorescence 
monitoring is conducted on a continuous basis. In preferred embodiment, 
the method of the present invention is employed for on-line determination 
of corrosion inhibitors in aqueous systems, particularly in industrial 
cooling water streams. In further preferred embodiment, the fluorescence 
monitoring is employed to quantify the loss of the copper corrosion 
inhibitor, and to control the dosage thereof.

PREFERRED EMBODIMENTS OF THE INVENTION 
A fluorescent compound may be characterized by its major fluorescence 
peaks, that is the excitation and emission wavelengths and fluorescence 
intensities for such major peaks. In Table 1 below there is given for 
several of the copper corrosion inhibitors a summary of such 
excitation/emission wavelengths that have been determined for use in the 
present invention. These wavelengths are listed below in nanometers 
("nm"). In addition, Table 1 sets forth the relative fluorescence of each 
compound, at corrosion inhibitor actives concentrations of 2 parts per 
million by weight ("ppm") in water within the pH ranges specified, in 
comparison to 2-naphthalenesulfonic acid ("2-NSA"), at a 0.4 ppm actives 
concentration in water, measured at 277 nm (excitation)/334 nm (emission) 
at neutral pH, which is assigned the value of 1. The relative fluorescence 
values given were determined assuming that the excitation light intensity 
and efficiency of the fluorometer optics/detector are uniform at all 
specified wavelengths. 
TABLE 1 
______________________________________ 
Analysis Conditions 
Fluorescence Wavelengths 
Relative 
Corrosion Excitation (nm)/ Fluores- 
Inhibitor 
pH Emission (nm) cence 
______________________________________ 
Naphtho- 
4 to 11 363 nm/445 nm 28 
triazole 
Naphtho- 
less than 0.5 
363 nm/445 nm 0.11 
triazole 
Tolyl- 4 to 8.5 285 nm/365 nm 0.22 
triazole 
Tolyl- less than 0.5 
280 nm/410 nm 2.34 
triazole 
Benzo- 4 to 8.5 285 nm/365 nm 0.1 
triazole 
Benzo- less than 0.5 
280 nm/390 nm 0.64 
triazole 
Butyl- 4 to 8.5 285 nm/365 nm 0.18 
benzo- 
triazole 
Butyl- less than 0.5 
280 nm/410 nm 1.52 
benzo- 
triazole 
______________________________________ 
As seen from Table 1 above, the fluorescence of the triazoles may shift as 
pH is significantly reduced. For tolyltriazole, benzotriazole and 
butylbenzotriazole, for instance, the fluorescence intensities are 
constant over a pH range of from about 4 to about 9, but increase 
dramatically as the pH is lowered below 1, and the emission wavelength is 
red-shifted at acid pH. For naphthotriazole, the fluorescence intensities 
are constant over the pH range of from about 4 to about 11. In contrast to 
tolytriazole, benzotriazole and butylbenzotriazole, however, the 
fluorescence intensities of naphthotriazole are dramatically reduced under 
acid conditions. The relative fluorencence values listed in Table 1 are a 
comparison of the aromatic azoles as 2 ppm solutions to 2-NSA as a 0.4 ppm 
solution at neutral pH because 2 ppm is a typical dosage for aromatic 
azoles in, for instance, cooling waters. Hence the relative fluorescence 
values listed in Table 1 above are comparisons between the fluorescence 
intensities of the active component aromatic azoles to the inert 
fluorescent tracer at the typical dosages of each. 
pH also affects the background fluorescence. The effect of pH reduction on 
the background fluorescence of a number of cooling tower water samples 
from representative industries is shown in Table 2 below for such water 
samples at a neutral pH ("w/o acid") and after reduction of pH to less 
than 0.5 ("w/acid") in comparison to tolytriazole ("TT"). For such 
comparison, the fluorescence of a 2 ppm (as actives) TT solution is 
assigned the percentage of 100%, and the fluorescence of both the TT 
solution and the cooling water samples are measured at 285/365 nm (ex/em) 
for the "w/o acid" measurements and at 280/410 for the "w/acid" 
measurements. In all instances the background percent fluorescence in 
comparison to TT is dramatically reduced upon acidification for all 
cooling water samples tested. Such unexpected and dramatic reductions in 
background fluorescence intensities of many of the cooling water samples, 
together with the unexpected and dramatic increases in fluorescence 
intensities of most of the aromatic azoles, at low pH conditions, provide 
a dramatic enhancement to the monitoring and dosage control capabilities 
of the method of the present invention. 
TABLE 2 
______________________________________ 
Survey of Cooling Water System Tolyltriazole 
Background Fluorescence 
Cooling Water 
Percent TT Background Fluorescence 
Sample No. w/o acid w/acid 
______________________________________ 
1 114% 14.6% 
2 82% 4.2% 
3 134% 10.9% 
4 123% 12.8% 
5 104% 10.3% 
6 158% 6.7% 
7 350% 15.2% 
______________________________________ 
Moreover, it has been found that the inert 2-NSA tracer has a relative 
fluorescence at low pH, as measured against the 2 ppm (as actives) TT 
solution at low pH that was employed for the Table 1 data above, of far 
less than 0.1. This dramatic reduction in the relative fluorescence 
intensity of the inert tracer 2-NSA versus TT at acid pH eliminates any 
significant interference to TT fluorescence measurements. As to its 
percent TT background fluorescence, for instance, 2-NSA (again as a 0.4 
ppm solution) has a relative percent TT background fluorescence of 54% w/o 
acid, which is reduced to 0% w/acid. Thus one can monitor the 
concentration of a aromatic azole such as TT, and thereby control its 
concentration, in the presence of an inert tracer such as 2-NSA without 
interference from such inert tracer. 
The determination of the fluorescence characteristics of these corrosion 
inhibitors and the dependence of the fluorescence characteristics on pH of 
the aqueous environment in which the fluorescence is measured provides 
optimizations of the fluorimetric method of monitoring and controlling 
corrosion inhibitor dosage in aqueous systems. The fluorescence intensity 
dependence on pH exhibited by some aromatic azoles within the pH ranges of 
interest, particularly the surprising and significant increase in relative 
fluorescence intensity upon pH adjustment found for some aromatic azoles, 
enhances the ability of the present invention to monitor the level of the 
corrosion inhibitors, and by virtue of such precise monitoring, to control 
the dosage thereof. The choice of emission and excitation wavelengths also 
helps in selective monitoring of specific corrosion inhibitors. For NTA, 
for example, the choice of 363 nm for excitation and 445 nm for emission, 
together with its large sensitivity, helps in distinguishing it from the 
natural background fluorescence of the water or the fluorescence of an 
inert tracer when such compound is present. For TT, if monitoring at 
typical cooling water pH ranges is required, selecting an 
excitation/emission wavelength combination of 285 nm/350 nm would render 
the signals due to TT independent of pH. 
This fluorimetric method can be described briefly as follows. A sample of 
water is taken from an aqueous system, and optionally its pH is adjusted. 
The fluorescence intensity of the sample at the desired emission 
wavelength, using the desired excitation wavelength, is measured with a 
SHADOWSCAN.TM. or other fluorescence detector. (SHADOWSCAN is a trademark 
of Nalco Chemical Company.) The measured fluorescence intensity is then 
compared to a working curve drawn up using standards in the concentration 
range of interest and this comparison provides a precise determination of 
the concentration of the corrosion inhibitor in the water sample drawn 
from the system. This method can be conducted on a continuous basis, 
providing a continuous monitoring of changes in the concentration of the 
corrosion inhibitor in the aqueous system. 
The UV-photolysis/photometric method for determining the concentration of 
aromatic azoles in an aqueous system is a spectrophotometry method which 
involves adding a spectrophotometric reagent to a water sample containing 
one or more aromatic azoles, and then irradiating the mixture with 
ultraviolet radiation for five minutes. The absorbance of the resulting 
yellow mixture is then measured at 425 nm. While this method is currently 
used in the industry, it has several significant disadvantages. The 
results of this method are extremely dependent on the time of exposure to 
UV radiation. Any deviation from the 5 minute exposure duration results in 
severe accuracy and precision problems. Other disadvantages include the 
method's poor sensitivity, its 10 minutes assay time, and the difficulties 
in using the method in continuous water streams, such as continuous 
cooling water streams. Further, the aromatic azoles are degraded during 
the irradiation, and hence an inherent drawback to this method is the 
impossibility of distinguishing one aromatic azole species from another. 
Moreover, most water samples obtained from cooling towers in the industry 
possess a natural yellow color. Therefore, distinguishing the color due to 
aromatic azole from the natural color of the water is difficult, 
especially given the low sensitivity of the UV-photolysis/photometric 
method. 
The fluorimetric method of the present invention has the following 
advantages (described in more detail below) in comparison to the 
UV-photolysis/photometric method method: 
1. The fluorescence method is simple to use, requiring no special reagents 
and little to no sample preparation. 
2. The fluorescence method has a greater sensitivity, detecting aromatic 
azoles at concentrations of just a few ppb, while the 
UV-photolysis/photometric method has a detection limit of about 250 ppb 
when equal size sample containers are used. 
3. The monitoring of aromatic azole by the fluorometric method is precise 
and accurate, irrespective of the nature of the aqueous system in which it 
is measured. 
4. The fluorescence method has a greater precision, having a relative 
standard deviation of 0.2% versus 13% for the UV-photolysis/photometric 
method. 
5. The fluorescence method has a very broad linear dynamic range which 
stretches through the useful range for applications such as cooling water. 
6. The fluorescence method provides a means for continuous analysis versus 
10 minutes for grab samples for the UV-photolysis/photometric method. 
7. The fluorescence method provides measurement readings that are stable 
and do not deteriorate with time, while the yellow color of the samples 
prepared by UV-photolysis/photometric degrade with time. 
8. The fluorescence method analysis can be performed without any 
interference from inert fluorescence tracers such as 2-NSA or vice versa. 
9. The fluorescence method can distinguish between aromatic azole species. 
10. The fluorescence method is suitable for continuous monitoring and 
dosage control. 
11. The fluorescence method is not significantly dependent upon the 
operator's laboratory technique. 
12. The fluorescence method does not require aromatic azole degradation. 
For those applications of the present invention that do not encounter any 
significant interference from an inert tracer that is being already used 
in an industrial installation, the present fluorimetric method can be 
installed in the tracer system already being used for the inert tracer. 
The method of the present invention may be used as a singular determination 
of the concentration of a given corrosion inhibitor, as intermittent 
monitoring, or as continuous monitoring, but since continuous monitoring 
is highly advantageous and difficult to achieve with standard methods, in 
preferred embodiment the present invention provides monitoring on a 
continuous or substantially continuous basis. The method of the present 
invention may be used for monitoring purposes alone, but since the precise 
monitoring provided by the present invention is most commonly desired to 
permit precise control over dosage, in preferred embodiment the present 
invention is a method for both such monitoring and such control. 
The use of inert fluorescent tracers is a widely-used known practice in 
industrial aqueous systems such as cooling waters. The substituted 
polynuclear aromatic compounds are in a class comprised of numerous 
compounds, many of which differ from one another by the number and type of 
substitutents, and by the positions of the substituents on the naphthalene 
ring, and such differences affect a given compound's fluorescence 
characteristics. As shown in the Examples below, the present fluorimetric 
method requires the selection of an emission wavelength at which the 
corrosion inhibitor's fluorescence intensity is to be measured, which 
preferably is substantially free of interference from other fluorescent 
tracers and other species present in the water being monitored. 
Undesirable interference may be encountered when some other species has 
significant fluorescence emission about the emission wavelength selected 
for monitoring the given corrosion inhibitor. In certain of the Examples 
below, the existence of potential interference was investigated and 
overcome by the emission wavelength selection, which is a powerful benefit 
of florescence analysis method. Two alternative approaches for avoiding 
interferences from other fluorescent species in the aqueous system are to 
employ an inert tracer having fluorescence characteristics that do not 
interfere with the desired fluorimetric assay or to employ measurement 
conditions (such as acid pH) in which the fluorescence of the desired 
species is enhanced relative to the background fluorescence. 
The use of such an inert fluorescent tracer in the aqueous system, and the 
monitoring of both such inert tracer and the active corrosion inhibitor by 
the fluorimetric method, is a preferred embodiment of the invention, and 
the benefits of both tracers may be realized. 
The magnitude of a fluorescent signal, or relative fluorescence, is the 
product of its fluorescent quantum yield multiplied by its light 
absorbance molar extinction coefficient and the concentration of the 
fluorescent species. The feasibility of detecting any fluorescent compound 
by a fluorimetric method is related to the fluorescence of the analyte 
(for each combination of excitation/emission wavelengths) versus the 
fluorescent background of the environment in which the fluorescent analyte 
will be measured. The aromatic azoles are inherently significantly 
fluorescent, and they are active corrosion inhibitors or corrosion 
inhibitor components. The present invention has determined a means of 
employing such inherent fluorescent characteristics to directly monitor, 
and provide precise dosage control of, such active corrosion inhibitor 
species. Moreover the present method has been found to be extremely 
precise and advantageous for determining precisely the concentrations of 
such active corrosion inhibitors by avoiding and minimizing the 
fluorescence background of aqueous systems such as cooling waters, and at 
the concentrations of the active corrosion inhibitors normally encountered 
in cooling waters down to much lower levels. The present invention is also 
advantageous because the active corrosion inhibitors are being directly 
monitored, without chemical degradation. 
As noted above, the copper corrosion inhibitors for which the fluorimetric 
method is applicable are, as a class, generally compounds containing the 
heterocyclic five-membered triazole ring structure (in unsubstituted form 
having the formula of C.sub.2 H.sub.3 N.sub.3) and an aromatic substituent 
to such ring structure, which aromatic substituents include tolyl, benzo, 
naphtho, and like aromatic substituent(s), which may be further 
substituted, for instance with alkyl substituent(s) having from about 1 to 
about 10 carbons, or halides, hydroxyls, alkenes, alkynes, sulfonates, 
carboxylates, amines, amides, and the like. By substituent is meant herein 
a radical other than hydrogen. Moreover, the substituent(s) and/or 
combinations of substituents to the aromatic substituent of a copper 
corrosion inhibitor may be any substituent within such broad definition of 
"other than hydrogen" provided that such substituent(s) and/or combination 
of substituents do not bring the compound in question outside of the 
corrosion inhibitor category. Not all of the copper corrosion inhibitors, 
however, are triazoles. Mercaptobenzothiazole (C.sub.7 H.sub.5 NS.sub.2), 
while formed in part of a heterocyclic ring and in part by an aromatic 
ring, is not a triazole, and yet has like properties as to corrosion 
inhibition and fluorescence. Hence while the terms "azoles" or at times 
"triazoles" might be used herein for the entire class of compounds, as 
done also in the field, the more apt term for this class is "aromatic 
azole(s)". 
EXAMPLE 1 
In Example 1, a fluorescence based method was used to determine 
quantitatively the concentration of tolyltriazole ("TT") by a method that 
could be used in the presence of other fluorescent species, such as an 
inert fluorescent tracer. As a preliminary matter, the fluorescence 
spectra of TT versus specific pH ranges of TT aqueous solutions were 
determined as follows. 2 ppm TT aqueous solutions were prepared in 10 mM 
buffer solutions in the pH range between 10 and 1.3. Borate buffers were 
used for solutions in the pH 8.5 to 10 range. Phosphate buffers were used 
for solutions in the pH 6 to 8.5 range. Acetate buffers were used for 
solutions in the pH 4 to 5 range. Citrate buffers were used for solutions 
in the pH 1 to 3 range. Using an excitation wavelength of 285 nm, the 
fluorescence spectra for these samples were obtained and are shown in FIG. 
1, except for samples below a pH of 4. As the pH decreased below a pH of 
4, the fluorescence intensities of the samples became stronger as the pH 
decreased, and the original calibration of the plot was no longer 
applicable. As seen in FIG. 1, an isoemission point, which is independent 
of pH, exists at about 350 nm. In the pH range of from 4 to 8.5, the peak 
position, as well as the fluorescence intensities, remain substantially 
constant. Below pH 3 (not shown in FIG. 1), a new peak centered around 412 
nm appeared, in addition to the 365 nm peak, and as the pH decreased 
further, the peak at 412 nm (due to cationic, protonated TT) increased 
further while the peak at 365 nm (due to neutral TT) decreased until the 
fluorescence of TT was represented by a single peak centered around 412 nm 
at the pH of 1.31. In this Example 1, standard solutions were prepared 
containing TT concentrations from 5 parts per billion by weight ("ppb") to 
2 ppm. 3 ml of each of these solutions was pipetted into a cuvette and 
0.075 ml of 50% H.sub.2 SO.sub.4 was added. The fluorescence intensity at 
an excitation wavelength of 280 nm and an emission wavelength of 410 nm 
was determined. The fluorescence of each of those solutions was plotted 
and a linear working curve (shown in FIG. 2) was obtained. The detection 
limit was found to be 0.005 ppm. This working curve was used to obtain the 
concentration of TT in a cooling water sample known to contain TT. The 
result obtained was 0.49 ppm of TT, compared to a concentration of 0.52 
ppm TT determined by HPLC (high pressure liquid chromatography). 
EXAMPLE 2 
To determine the extent of interferences, if any, from substances commonly 
added to cooling water streams, sixty formulations were reviewed and 
representative samples of cooling water formulations which contain other 
fluorescent substances were tested to determine the dosage required for 10 
percent interference with the fluorimetric method for monitoring aomatic 
azoles, in the acid (less than 0.5 pH) domain. For these determinations, 
each formulation was diluted with water to its typical use (dosage) 
concentration, and the fluorescence of diluted formulations were measured 
(280 nm excitation/410 nm emission wavelengths). The fluorescence spectra 
were compared to that of aqueous solutions containing 2 ppm TT, and the 
dosage of the formulation required to see a 10% interference of the TT 
spectra was determined, assuming a proportional increase in fluorescence 
intensity with dosage increase. The 2 ppm solution of TT was assigned a 
value of 100% at 280 nm/390 nm, for the determination of 10% interference 
dosage for assays of TT, which are set forth below in Table 3, together 
with the typical dosages thereof, and the types of substance in terms of 
use characteristics. 
TABLE 3 
______________________________________ 
Typical Dosage Req. 
Formulation Product for 10% 
Designation 
Substance Type 
Dosage Interference 
______________________________________ 
a Anti-microbial 
50-150 ppm 
1660 ppm 
b Corrosion Inhibitor 
50 ppm 480 ppm 
c Dispersant 40 ppm per 
&gt;1000 ppm 
ppm of iron 
d Corrosion/scale 
1000 ppm 785 ppm 
Inhibitor 
e Iron Dispersant 
10-30 ppm 1464 ppm 
f Oil Dispersant 
10-30 ppm 636 ppm 
g Antimicrobial 200 ppm 202 ppm 
i Corrosion Inhibitor 
50-100 ppm 
&gt;1000 ppm 
j Dispersant 35 ppm &gt;1000 ppm 
______________________________________ 
EXAMPLE 3 
Using aqueous solutions of BT at concentrations down to very low 
concentrations of BT actives, the fluorescence intensities were measured 
at 390 nm (excitation wavelength of 280 nm), and the intensities versus 
concentrations of BT, in ppb, were plotted to provide a working curve for 
BT, shown in FIG. 3. In comparison, FIG. 4 shows the working curves for 
both BT and TT provided by the UV-photolysis/photometric method, both of 
which are nonlinear. The detection limits for BT and TT provided by the 
UV-photolysis/photometric method working curves shown in FIG. 4 are both 
down to about 125 ppb (0.125 ppm), but are very flat for concentrations 
lower than 0.5 ppm. Moreover most industrial cooling waters have a natural 
absorbance at 425 nm. Therefore, precise analysis at low concentrations is 
extremely difficult. In comparison, the fluorimetric method working curve 
for BT (FIG. 3) is linear down to about 33 ppb, and as noted above the 
working curve for TT by the fluorimetric method is linear down to a 
concentration of about 5 ppb (as shown in FIG. 2). For a 2 ppm 
concentration of triazole, the % relative standard deviation for the 
fluorimetric method was 0.2% (with an 8 s integration time constant and 3 
replicate measurements), while the % relative standard deviation by the 
UV-photolysis/photometric method was about 13%. 
EXAMPLE 4 
Continuous monitoring/control of BT concentrations was conducted in a Pilot 
Cooling Tower using a commercial (Nalco Chemical Company) BT-containing 
water treatment formulation. FIG. 5, which is described in more detail 
below, shows the general continuous sampling technique used. A target feed 
rate of 1 ppm BT actives was chosen. Grab samples of the cooling water 
were taken from the Pilot Cooling Tower and assayed for BT using HPLC, and 
then those results were compared to the setpoint of 1 ppm BT. The results 
from this continuous monitoring/control of BT concentration are shown in 
FIG. 6. It can be seen from these results that BT levels were controlled 
at 1 ppm.+-.0.05 ppm. 
EXAMPLE 5 
The pH dependence of a 2,3-naphthotriazole ("2,3-NTA") fluorescent tracer, 
within the pH range of from 3.5 to 11.5 was determined. 2,3-NTA was found 
to have excitation maxima at 328 nm, 363 nm, and 378 nm, and emission 
maxima at 385 nm, 407 nm, and 424 nm. An isoemissive point (which provides 
a constant fluorescence over the pH range of 4 to 11) was identified at 
363 nm/445 nm. At these pH values, the relative fluorescence intensity of 
a solution containing 2 ppm of 2,3-NTA (typical usage concentration) was 
determined to be 28 times that of a solution containing 0.4 ppm of 2-NSA 
(excitation at 277 nm and emission at 334 nm). At highly acidic pH, the 
fluorescence of 2,3-NTA and 2-NSA are unexpectedly and significantly 
reduced due to reversible protonation of sulfonic acid and triazole ring, 
respectively. The reduced fluorescence of 2,3-NTA was particularly because 
the other aromatic azoles (TT, BT and BBT) showed significantly increased 
fluorescence at very acidic pH values. 
EXAMPLE 6 
An industrial cooling water system containing an open recirculating cooling 
tower was fed TT based on inert fluorescent tracer levels. The target TT 
concentration was 0.9 ppm in the recirculating system. The TT 
concentration was also monitored directly at 280 nm/410 nm using a 10% 
sulfuric acid solution to enhance TT signals and to suppress background. 
The difference between the results expected from the inert tracer readings 
and direct TT monitoring constitutes system demand. The results are shown 
in FIG. 7. It can be clearly seen that the system exhibits a large demand 
for TT. 
The water samples identified in the Examples above as cooling water were 
obtained from pilot cooling towers that simulate industrial cooling towers 
as to substances in the cooling waters (substances normally encountered in 
the feed water and substances added thereto for various purposes) and 
environmental conditions, such as temperature and the like, and 
representative industrial cooling water systems. 
A survey of potential interferences from other cooling water products shown 
in Example 2 above indicates that for most such products a 10% 
interference would require such products to be present in concentrations 
of about 10 or 100 or more times the normal dosage concentration, and 
hence no interference of any significance was determined with respect to 
TT or BT. 
In addition to the survey of potential interference from cooling water 
additives, the effect of Cu.sup.+2 ions (such as would occur from 
corrosion of copper alloys) on the fluorescence levels of the triazoles 
was investigated and determined to be substantially nonexistent. 
When a given aromatic azole can be assayed without a fluorescence intensity 
dependency in a pH range of from about a pH of 4 to about a pH of 9, such 
as tolyltriazole, the determination may be conducted in such neutral pH 
domain. The water of many aqueous systems, including cooling waters, is 
routinely within a neutral pH range (7 to 9), and hence it is possible in 
fluorimetric method assay to require no pH adjustment. This is 
particularly true for aromatic azoles with very high relative fluorescence 
(e.g., 2,3-naphthotriazole). When a given aromatic azole has a higher 
relative intensity in an acid pH domain, such as a pH of less than 0.5, 
for instance tolyltriazole, benzotriazole and butylbenzotriazole, and a 
high background fluorescence exists near neutral pH, the additional step 
of adjusting the pH may in many instances be beneficial by almost 
completely eliminating that background fluorescence interference. The 
elimination of interferences is due not only to the greater relative 
fluorescence of such aromatic azoles under acid conditions but also the 
reduction of the relative fluorescence of most other molecules, including 
2-NSA, by such acid conditions. The use of an acid domain was found to 
provide an accurate assay for TT at a concentration level of 0.52 ppm in a 
commercially-derived water sample with a large fluorescence background, of 
unknown nature, that assay by the fluorimetric method under neutral pH 
conditions was not possible. 
While the procedures described in some of the above Examples were performed 
as assays for batch samples, as noted elsewhere herein the method in 
preferred embodiment is used on a continuous basis. One preferred mode for 
an industrial intermittent or continuous method, as shown in FIG. 5, would 
be to provide a sidestream 12 to, for instance, a cooling water loop 10, 
through which small volumes of the water from the aqueous system would be 
intermittently or continuously flow. Downstream from the sidestream inlet 
14 should preferably be a filter 16 to remove solids before the assay 
point. Along the sidestream 12 there optionally may be an interconnecting 
charge point 18 for the addition of substances to the water, such as acid, 
when an acid domain assay is desired, and/or other materials, such as may 
be deemed appropriate. Downstream of such addition point 18 would be the 
assay point 20 equipped with a SHADOWSCAN.TM. fluorescence detector 
(SHADOWSCAN is a trademark of Nalco Chemical Company) with appropriate 
filters, or other fluorescence detector, set to fluoresce the aromatic 
azole and measure the fluorescence intensity at the selected wavelengths. 
Sulfuric acid is the acid of choice for acidification of water samples 
prior to assay, and the final acid concentration of such samples 
preferably is greater than 3 weight percent. 
In one embodiment, the present invention is a method for monitoring the 
concentration of a aromatic azole corrosion inhibitor in the water of an 
aqueous system, comprising fluorescing a sample of the water from the 
aqueous system, measuring the fluorescence intensity of the sample, and 
comparing such fluorescence intensity to a curve from a plot of known 
concentrations of the aromatic azole in aqueous solutions versus the 
fluorescence intensities of such solutions, wherein the fluorescence 
excitation/emission wavelengths and the pH domain the plot and the sample 
solutions are the same, whereby the concentration of the aromatic azole is 
determined by correspondence to its fluorescence intensity. In preferred 
embodiment, such method further includes the step of adjusting the pH of 
the sample prior to the fluorescing of the sample, particularly wherein 
the pH is adjusted by the addition of sulfuric acid so as to obtain a 
final acid concentration of 3 wt. percent (a pH of less than 0.5), 
although more broadly speaking adjusting the pH to a value of less than 
1.5 is also a preferred embodiment. In preferred embodiment, the aromatic 
azole is present in the water in the amount of from about 1 part per 
billion by weight to about 1000 parts per million by weight. In further 
preferred embodiment, the water has a pH within the range of from about 4 
to about 9, and the fluorescing is conducted at excitation/emission 
wavelengths of about 285/350 nm, and for such embodiment the emission 
wavelength may an isoemissive wavelength rather than a fluorescence peak. 
In other preferred embodiments the aromatic azole is tolyltriazole or 
butylbenzotriazole, the water has a pH of less than about a pH of 1.5, and 
more preferably a pH of less than about 0.5, and the fluorescing is 
conducted at excitation/emission wavelengths of about 280/410 nm; the 
aromatic azole is benzotriazole, the water has a pH of less than about 
1.5, and more preferably less than about 0.5, and the fluorescing is 
conducted at excitation/emission wavelengths of about 280/390 nm; and, the 
aromatic azole is naphthotriazole, the water has a pH within the range of 
from about 4 to about 11, and the fluorescing is conducted at 
excitation/emission wavelengths of about 363/445 nm. In another preferred 
embodiment of the invention, the fluorescing, the comparing and the 
determination of aromatic azole concentration are conducted on a 
continuous basis. In further preferred embodiment, the aqueous system is 
an industrial aqueous system, such as a steam-generating system (boiler), 
a cooling water system, a manufacturing process water system, and the 
like. 
The present invention is also a method for controlling the concentration of 
a aromatic azole corrosion inhibitor in the water of an aqueous system, 
comprising fluorescing a sample of the water from the aqueous system, 
measuring the fluorescence intensity of the sample, comparing such 
fluorescence intensity to a curve from a plot of known concentrations of 
the aromatic azole in aqueous solutions versus the fluorescence 
intensities of such solutions, wherein the fluorescence 
excitation/emission wavelengths and the pH domain, the plot and the sample 
solutions are the same, whereby the concentration of the aromatic azole is 
determined by correspondence to its fluorescence intensity, and adding to 
the aqueous system a sufficient amount of the aromatic azole to provide 
the desired total concentration of the aromatic azole in the aqueous 
system. All of the preferred embodiments set forth above for the method of 
monitoring a aromatic azole corrosion inhibitor in an aqueous system are 
also preferred embodiments to the method of controlling the concentration 
of a aromatic azole corrosion inhibitor in an aqueous system. 
The present invention is also such methods of monitoring the concentration 
of an aromatic azole and of controlling the concentration of such aromatic 
azole, further wherein the water and the sample of the water contain an 
inert fluorescent tracer, the measuring of the fluorescence intensity of 
the sample also includes the measuring of the contribution to the 
fluorescence intensity from the inert tracer, the aromatic azole 
concentration based on the contribution to the fluorescence intensity from 
the inert tracer is also determine and the system demand for the aromatic 
azole is determined from the difference between the aromatic azole 
concentration based on the contribution to the flourescence intensity from 
the inert tracer and the aromatic azole concentration based its the 
fluorescence intensity. This preferred embodiment is particularly 
demonstrated in Example 6 above. 
As the selection of a suitable excitation/emission wavelength combination, 
preferably the emission wavelength is a wavelength centered at one of the 
corrosion inhibitor's strongest peaks, when the corrosion inhibitor is in 
a pH domain in which its fluorescence spectra is independent of pH. Such a 
major peak choice is particularly preferred when there is little to no 
fluorescence interference at such wavelength from substances that may be 
present in the water sample. Another preferred selection is an emission 
wavelength that corresponds to an isoemission point of the corrosion 
inhibitor, when the corrosion inhibitor is in a pH domain in which its 
fluorescence spectra is independent of pH. Such a isoemission point choice 
is particularly preferred when there is little to no fluorescence 
interference at such isoemission point. 
When concentrations are expressed herein in parts per million ("ppm") or 
parts per billion ("ppb"), the parts are always parts by weight per 
million or billion parts by volume, wherein the weight/volume units are 
equivalent to grams/milliters. Such concentrations generally a close 
approximation to a weight/weight basis, such as parts by weight per 
million parts by weight for "ppm", but the weight/volume basis is 
nonetheless more accurate. 
Wavelengths as stated herein are always in nanometers ("nm"), and when a 
set of wavelengths are set forth separated by a slash ("/") herein, the 
first wavelength is the fluorescence excitation wavelength and the second 
is the fluorescence emission wavelength. 
INDUSTRIAL APPLICABILITY OF THE INVENTION 
The present invention is applicable to industries that require corrosion 
inhibitors for aqueous systems, such as cooling water systems, boilers, 
and other water streams, and in particular for industrial scale aqueous 
systems.