Download The Measurement of Silica

Technical
Paper
The Measurement of Silica
Author: S. P. Ellis
In-Line Instrumentation Seminar, November 17 - 19,
1993, Clearwater, FL
Introduction
With the advent of higher and higher pressure boilers and steam generators in nuclear power plants,
carryover of impurities has posed a greater risk of
corrosion to turbine blades and other downstream
piping and equipment. The turbine-generator
manufacturers have reacted to this risk by establishing ever-lowering specifications for concentrations of impurities in the feedwater. For many of the
impurities, the specifications are at or near the limit
of detection of many of the available analytical
methods used for monitoring. One such specification is for reactive silica. The term, silica, will be
used to denote dissolved, or reactive, silica
throughout this paper.
The most popular method for the measurement of
silica is the heteropoly blue (also called molybdate
or molybdenum blue) colorimetric method.
This method for silica grew up, so to speak, with the
power industry. It is used to detect silica breakthrough from strong base anion resin beds. It is also
used to quantitatively determine silica in feedwater,
as well as steam and steam condensate samples.
Determination of the silica break from a resin bed
does not require an analytical test having a low
limit of detection. Further, when low pressure boilers are in question, easily detectable quantities of
silica (10 - 20 ppb, or even higher) are not only
tolerated, but are often considered to be desirable.
Nevertheless, for higher pressure systems, silica
at concentrations above 5 ppb is considered to
be intolerable.
All owners and operators of ion exchange deionization equipment are highly interested in the accuracy and precision of methods used for measuring
silica. Failure of the equipment to produce water in
conformance with prescribed specifications is, of
course, quite costly. This is even more true for
mobile water treatment companies, since transportation costs are often very high. Thus, this research
was approached from the standpoint of trying to
define the variability in the silica measurement. In
so doing, it might be possible to work out acceptance criteria which would allow equipment to
operate, at least for a period of time, without imposing contractual penalties. In some cases this is
already being done in some form.
At the same time, the dilemma of the power producer is also recognized. The status of the warranty
for the turbine-generator is not trivial by any
means, it remains absolutely necessary to do all
that one can to ensure that conformance to the
vendor’s specifications is maintained. Therefore, the
power producer must use some measure to demonstrate the quality of the feedwater and the
makeup water to the boiler, even if there is some
question concerning precision and bias of the test.
Numerous laboratory instruments using 50 mm or
100 mm cells have been used to obtain silica measurements at or below 5 ppb. Further, in-line instrument manufacturers report obtaining values as low
as 0.5 ppb in their marketing literature. At least one
manufacturer reports values of 3 ppb or less with
both laboratory and in-line equipment using cell
pathlengths of 1-inch1.
Error in in-line silica measurements were a part of
the EPRI RP 2712 study2. The results of that study
indicated a sizeable error in both precision and bias.
This paper describes the initial attempts to determine the degree of error involved in the measurement of silica, both by laboratory and inline
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TP1055EN.doc Jun-09
instrumentation. This paper is largely introductory,
and somewhat tutorial. The results of laboratory
studies which were designed to quantify error in the
measurements at low concentrations are presented.
Results of comparisons of various methods of determining blank values, as well as some data concerning the effects of the amount of reagent, and
elapsed times prior to reading the absorbance after
addition of all reagents are also given.
The absorption spectrum for the silica-molybdenum
blue complex is shown in Figure 1. The most important feature of the spectrum is the intense, broad
absorption band centered at about 815 nm.
Because of this broad band, lower resolution spectrophotometers and even colorimeters may be used
successfully. That is, an instrument with very low slit
widths is unnecessary.
Experimental
Silica measurements were made using a Shimadzu
Model UV-1201 UV/VIS spectrophotometer, and a
Bausch & Lomb Spectronic 20 spectrophotometer
(now owned and marketed by Milton-Roy). The latter
instrument was factory-modified by replacing the
standard focusing lens with a red lens and
installing a nonstandard detector to make it more
sensitive at 815 nm, the wavelength used for measurement. Additional electronic modifications were
installed by Update Instruments to provide more stability.
Reagents used in the test include an acidic ammonium molybdate solution, an oxalic acid solution
and an amino acid solution. All of these solutions
were obtained from Hach Chemical Co. in Loveland
CO. A 1 ppm working standard solution of silica was
also obtained from Hach.
Solutions for analysis were generated by diluting
the stock standard solution with deionized water to
produce solutions whose concentrations ranged
from zero to 100 ppb silica added. Each analytical
solution was prepared 10 times and each of these
solutions was analyzed once. A standard curve was
prepared to compare the results of the tests and
help illustrate the error in the measurements.
Background
Before presenting the analytical results, it is worthwhile to revisit some of the spectroscopic considerations which go into the colorimetric analysis of
silica. These considerations include the absorption
spectrum and the relationship between concentration and response (Absorbance). Sources of errors in
measurements, the relationship of those errors to
detection limits, and the ability of the instrument to
detect small (0.5 to 2 ppb) differences in concentration are also generically discussed.
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Figure 1: Absorption Spectrum of Heteropoly Blue
Complex
The absorption of light may be used to quantify the
amount of an analyte in solution. A set of sol
utions containing known amounts of the analyte in
question are prepared and analyzed at the appropriate wavelength. The relationship between the
concentration of an analyte in solution and the
absorbance of the solution is given by the BeerLambert equation:
A = abc
where, A = absorbance
a = absorption coefficient
b = cell pathlength
c = concentration
For a given analyte, the absorption coefficient is
constant. Therefore, the slope of the curve, which is
theoretically a straight line, is dependent entirely
upon the cell pathlength. The effect of this relationship is shown in Figure 2.
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Figure 2: A vs. Conc.
Figure 3: A vs. Conc. with Error
While any standard curve will theoretically produce
results, the errors, which are associated with all
measurements, may prevent determinations at low
concentrations or discrimination between two similar concentrations. If errors in the measurement of
the analyte are considered, a curve such as that
shown in Figure 3 may be produced. It should be
noted that the error bars are sufficiently high in this
case to make adjacent points statistically indistinguishable. Indeed, sets of points even further away
from any given point may be indistinguishable from
one another. Obviously, this type of situation can
overcome any theoretically derived limit of detection or ability of the test to discriminate between
two very similar concentrations of analyte.
These values may be contrasted with absorbance
values calculated from the efficiency data of
modern detectors. A good estimate of that efficiency is that they can detect increments of about
0.1% transmittance4,5. Thus, using a value of
99.9%T, and converting that value to absorbance
(A = log (100/%T)), one obtains an absorbance of
about 0.0004 in a 10 mm cell. This value translates
to 0.001 for a 1-inch cell and 0.004 for a 100 mm
cell. It will be noted that these values are quite close
to those calculated for 1 ppb silica solutions from
actual experimental data. It can, then, be
assumed that the quality of the detector is theoretically sufficient to measure silica at the 1 ppb level of
concentration.
Actual absorbance values for the concentrations of
silica may now be considered. Although spectral
efficiency data were not located, the absorbance at
low ppb concentrations can be calculated from
available data, if conformance to the Beer-Lambert
equation is assumed. The values of expected
absorbances of 1 ppb solutions using 1-inch and
100 mm path cells from two independent sources
are presented in Table 1.
The sources of error in optical instrumental analysis
arise from three main sources; chemical, spectroscopic/optical and electronic. In practice, the latter
two categories of error are not so easily separated.
The electronics that power a lamp, for example, can
be made more stable which, in turn, makes the
lamp more stable. Historically, however, the instrumental sources of error are commonly related to
lamp instability, the sample itself, detector instability, optics and reagent impurities.
Table 1: Expected Absorbances of a 1ppb Silica Solution
Most of the time, the operator must use the instrument as it is purchased. He may not have the ability
or the time to modify and improve the instrument.
The operator must, therefore, spend his time on
activities that he can control, such as
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reagent purity, sample integrity and careful conduct
of the analytical procedure. He must also know the
limitations of the instrumentation, which he operates, in order to make appropriate judgements concerning data quality.
Results and Discussion
In an effort to quantify the error associated with the
points used to construct a standard curve for a
laboratory instrument, it was decided to first try to
incorporate as much of the error that a single
operator could make. Therefore, ten standards at
each concentration were prepared and each solution was measured once to attain some degree of
statistical significance. The mean values of these
standards, measured in both a 1-inch cylindrical
cuvette and a 100 mm rectangular cell, were
plotted versus absorbance. The results are shown
in Figure 4.
minations of single solutions at each concentration
should diminish the errors significantly.
Another approach could be to standardize at a high
concentration at which the standard deviation
would be expected to be lower. This action would
minimize the error at the upper end of the curve.
However, a good standardization would still rely on
a low error about the zero, which is not necessarily true.
In order to study the error associated with the blank
(or Zero) measurement, several methods for the
colorimetric determination of silica were investigated. These methods came from power plants,
ASTM, Standard Methods and Hach Chemical Co.
All, of course, were similar. But almost all had small
deviations from one another; particularly in the
handling of the blanks. Five methods of handling
the blank were investigated.
1. Reagents 1, 2 and 3 were added to deionized
water, in order, with 10 min. and 2 min. reaction
times allowed between reagent additions. Five
minutes was allowed between the addition of
reagent 3 and measurement.
2. Reagent 2 was added to a 1:1 solution of concentrated hydrochloric acid in deionized water.
No molybdate or amino acid reducing reagents
were added.
3. Reagents 1, 2 and 3 were added to deionized
water, but not in order. The reagents were
added in quick succession, starting with reagent
2, then 1 and then 3.
4. Reagents 1 and 2 were added to deionized
water in quick succession, and allowed to react
for 10 min. before measurement.
5. Reagents 1 and 2 were added together and
allowed to react for 2 min. Then reagent 3 was
added and the solution was allowed to react an
additional 5 min. Finally, deionized water was
added and the measurement was taken.
Figure 4: SiO2 Standard Curves
Clearly, the errors shown in this study are quite
large in comparison to those which could be
obtained by “normal” standardization techniques.
Simply using the usual approach of multiple deterPage 4
In each case, the spectrophotometer was zeroed
using deionized water, and the measurement made.
Measurements were made on ten different solutions of each blanking method, and each solution
was analyzed once. The results of these tests are
presented in Table 2.
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Table 2: Comparison of Blank Methods
Some of the silica methods which were considered
warned against waiting too long to measure the
absorbance after the color had developed. Therefore, a quick test was run to see if time delays were
a problem. A 50 ppb silica standard was prepared
and analyzed using 10 min. and 2 min. reaction
times between reagent additions. The test solution
was measured immediately after mixing with the
final reagent, and at several intervals thereafter.
The results of this test are shown in Table 3.
Table 3. Time Effects on Absorbance
Conclusions
Laboratory measurements of silica are, as all
measurements, beset by errors. If these errors are
not recognized and dealt with, results can be
misleading, at best. We unfortunately have a tendency to believe a pretty digital readout or a rock
steady meter when we should remain skeptical. Results of the testing described above lead to the following conclusions.
1. Modern instruments are theoretically capable of
determining 1 ppb silica in a cell with as short a
pathlength as 10 mm.
2. Errors in measurement raise this limit to as high
as 20 ppb in a 1-inch pathlength cell.
3. The use of a 100 mm pathlength cell lowers the
limit of detection to about 3 ppb, even though
the magnitudes of the errors associated with
the development of the standard curves are
similar to those for the standard curve using a
1-inch pathlength cell.
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4. Modifications in the way the standard curves
are developed should decrease the magnitudes
of the errors. This should also decrease the limit
of detection somewhat.
5. The errors associated with the zero, or blank,
determination are of about the same magnitude as the other standard concentrations used
in this study. Therefore, they cannot be ignored.
6. The time between the addition of the last reagent and reading the absorbance does not
appear to be important from about 5 minutes to
at least 45 minutes.
The experiments described in this paper were
designed to introduce a rather high amount of
error into the determination of silica. Operator error
is also high in manual methods compared to welloperating automatic systems. While it may be
assumed that in-line instruments have less error
associated with a measurement, error still exists.
Such error may be sufficient to disallow the measurement of 1 ppb or sub-ppb concentrations of silica.
In future work, the emphasis will be shifted to the
study of response and error associated with in-line
instruments. Of particular importance to us is the
ability of the instruments to respond to small
increments in concentration and the ability to discriminate between close concentrations.
References
1. K. R. Doerr, D. Harp and L. Liou, Low Level Silica
Verification for Analyzer Users, Ultrapure Water,
July/August, 20-25, 1991.
2. J. K. Rice, D. M. Sopocy, R. B. Dooley, Quantification of Continuous Instrument Error, 1990 International Conference on Waterborne Trace
Substances, August 1990, Baltimore, MD.
3. A. B. Carlson and C. V. Banks, Spectrophotometric Determination of Silicon, Analytical Chemistry, 24(3), 472-477, 1952.
4. R. E. Hanson and M. V. Buell, Photoelectric Technique for Spectrophotometry Between 194 and
225 Millimicrons, Analytical Chemistry, 31(5),
878-81, 1959.
5. J. Salpeter, Personal Communication, October
1993.
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