Electrolysis: processes – compositions used therein – and methods – Electrolytic analysis or testing – Tracking chemical reactions
Reexamination Certificate
2000-03-07
2002-04-23
Warden, Jill (Department: 1743)
Electrolysis: processes, compositions used therein, and methods
Electrolytic analysis or testing
Tracking chemical reactions
C205S794000, C204S434000, C073S061620, C073S061750
Reexamination Certificate
active
06375829
ABSTRACT:
TECHNICAL FIELD
This invention relates to a method and apparatus for measuring the calcium oxalate scale forming propensity of fluids and the effectiveness of calcium oxalate scale inhibitors. More specifically, this invention concerns a method of measuring the rate of calcium oxalate scale deposition on to the surface of a piezoelectric microbalance immersed in the fluid where the scale deposition is driven by an electrochemically controlled pH change in the vicinity of the microbalance.
BACKGROUND OF THE INVENTION
Calcium oxalate scale is a persistent problem in a variety of industrial processes involving water, such as pulp bleaching and sugar production. The calcium oxalate scale may remain suspended in the water or form hard deposits that accumulate on the surface of any material that contacts the water. This accumulation prevents effective heat transfer, interferes with fluid flow, facilitates corrosive processes, and harbors bacteria.
A primary detrimental effect associated with scale formation and deposition is the reduction of the capacity or bore of receptacles and conduits employed to store and convey the water. In the case of conduits used to convey scale-contaminated water, the impedance of flow resulting from scale deposition is an obvious consequence.
However, a number of equally consequential problems arise from utilization of scale-contaminated water. For example, scale deposits on the surfaces of storage vessels and conveying lines for process water may break loose and become entrained in and conveyed by the process water to damage and clog equipment through which the water is passed, e.g., tubes, valves, filters arid screens. In addition, these deposits may appear in, and detract from, the final product derived from the process, such as paper formed from an aqueous suspension of pulp.
Furthermore, when the scale-contaminated water is involved in a heat exchange process, as either the “hot” or “cold” medium, scale will be formed upon the heat exchange surfaces contacted by the water. Such scale formation forms an insulating or thermal opacifying barrier that impairs heat transfer efficiency as well as impeding flow through the system. Thus, scale formation is an expensive problem in many industrial water systems, causing delay and expense resulting from shutdowns for cleaning and removal of the deposits.
Calcium oxalate scale in biological fluids is another significant problem. In particular, kidney stones are formed of calcium oxalate, and urine analysis for calcium oxalate precipitation are used to assess the susceptibility of a patient to kidney stone formation and to monitor and screen pharmaceutical remedies.
Accordingly, there is an ongoing need for the development of new agents that prevent or inhibit the formation of calcium oxalate scales in fluids and for convenient methods of measuring the effectiveness of these inhibitors. In addition, as natural inhibitors may already be present in the solutions of interest, there is a need for effective methods of characterizing the tendency of industrial and biological solutions as such to form calcium oxalate deposits.
The effectiveness of these calcium oxalate scale inhibitors is manifested by their ability to suppress crystal growth through blocking active sites of potential centers of crystallization and preventing the agglomeration of growing crystals.
Common to the above processes is that they occur at the solid-liquid interface. Therefore the in situ measurement of the rate of crystal growth in the presence calcium oxalate scale inhibitors at the solid-liquid interface is of particular interest. Traditional measurements mostly relate to the change of the bulk properties of a test solution such as solubility, conductivity, turbidity and the like following crystal formation. There exist only a few methods for measuring crystal growth rate, and even fewer methods for conducting the measurements in situ at the solid-liquid interface.
Methods for measuring crystal growth rate at the solid-liquid interface that utilize a piezoelectric microbalance are disclosed in U.S. Pat. Nos. 5,201,215 and 6,250,140 and European Patent Application No. 676 637 A1. The principle of piezoelectric mass measurement is based upon the property of a quartz resonator to change its mechanical resonance frequency f
0
proportionally to the mass and viscoelastic properties of the deposited material. The change in frequency is expressed as follows:
Δ
f
≈
-
2
f
0
2
N
(
μ
μ
ρ
q
)
1
/
2
[
ρ
s
+
(
ρ
η
4
π
f
0
)
1
/
2
]
(
6
)
where f
0
is the unperturbed resonant frequency of the quartz crystal; N is the harmonic number; &mgr;
&mgr;
is the quartz shear stiffness, &rgr;
q
is the density of quartz; &rgr;
s
is the surface mass density of the deposit (mass/area), &rgr; is the density of the medium contacting the resonator and &eegr; is the viscosity of the medium contacting the resonator.
Where the viscoelastic properties of the system are negligible or remain constant through the measurements, the surface mass density can be measured using a simplified expression that can be used for the loading causing the resonant frequency change up to 5% (approx. 4.5 mg/cm
2
):
&rgr;
s
=−C&Dgr;f
0
where C is determined by calibration and is typically equal 1.77×10
−5
mg/(sec cm
2
Hz) for a 5 MHz quartz crystal.
However, as discussed herein, the piezoelectric microbalance described in the foregoing references is unsuitable for testing calcium oxalate solutions as it does not provide the necessary conditions for the calcium oxalate crystals to precipitate on the surface of the microbalance. Consequently, a need still exists for methods of measuring the calcium oxalate scale forming tendencies of solutions under conditions at which calcium oxalate scale forming behavior is exhibited.
SUMMARY OF THE INVENTION
We have discovered that a metal-plated quartz-crystal microbalance can be used to provide the necessary conditions for the calcium oxalate crystals to precipitate on the surface of the microbalance, in particular by controlling the solution pH proximate to the surface of the microbalance by applying an appropriate electric polarization to the metal surface (the working electrode).
However, not any material can be used for plating the quartz crystal microbalance. Thus, piezoelectric microbalances utilizing traditional gold-coated crystals cannot be used to test calcium oxalate scale inhibitors as intensive hydrogen evolution is observed at the potential that provides for the near-surface pH suitable for oxalate scale formation. This hydrogen evolution interferes with and often completely precludes deposition of calcium oxalate scale on the microbalance.
Also, the test solution should have a proper pH and concentration of calcium oxalate. The solution pH should be low enough to provide for full solubility of the constituents. However, pH's less than 2 may be too low for an electrochemical polarization to produce the pH increase at the quartz microbalance sufficient to precipitate calcium oxalate from the solution while avoiding the evolution of hydrogen bubbles. On the other hand, pH's higher than 3 may not provide for the concentration of calcium and oxalate ions in the bulk solution sufficient for a reasonable deposition rate and rapid completion of the test.
Moreover, the surface activities of the inhibitors as well as the adsorption properties of the deposition interface depend on the pH. In order to keep the screening conditions the same for various solutions an actual knowledge of the pH in the vicinity of the microbalance working electrode is required.
We have developed a method and apparatus for testing potential calcium oxalate scale inhibitors and the capacity of industrial and biological solutions to form calcium oxalate deposits that utilizes a controlled change of the pH in an oxygen-saturated acidic test solution near the deposition substrate repr
Duggirala Prasad Y.
Kouznetsov Dmitri L.
Shevchenko Sergey M.
Breininger Thomas M.
Martin Michael B.
Nalco Chemical Company
Olsen Kaj K.
Warden Jill
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