United States
Environmental Protection
Agency
Environmental Monitoring
Systems Laboratory
Las Vegas NV 89193-3478
Research and Development
EPA/600/S4-88/026 Oct. 1988
v°/EPA Project Summary
The Determination of pH by
Flow Injection Analysis and by
Fiber Optrode Analysis
Stephen H. Pia, Donna P. Waltman, and Daniel C. Hillman
Two new protocols for measuring
pH have been developed. The first
measures pH colorimetrically using
an indicator dye mixture in a flow
injection analysis (FIA) procedure.
The second measures pH using a
fiber optic chemical sensor (FOCS or
optrode) specifically developed for
pH determinations. The FOCS meth-
od measures pH by monitoring the
fluorescence of fluorescein deriv-
ative bonded to the distal end of a
fiber optic cable. The FIA method
currently has a precision and
accuracy of about ±0.2 pH units and
can measure 100 samples/hour. The
matrix may affect the precision and
accuracy but has not been fully
investigated. The FOCS method has a
precision of ±0.05 to 0.20 pH units
and an accuracy of ±0.1 to 0.6 pH
units. The bias is largely due to
inadequacy of the calibration model,
which needs further development.
About 10-60 samples can be anal-
yzed. The response time is matrix
dependent. It varied from 10 seconds
to 7 minutes in the solutions studied,
with slowest response in dilute,
poorly buffered samples.
This Project Summary was
developed by EPA's Environmental
Monitoring Systems Laboratory, Las
Vegas, NV, to announce key findings
of the research project that is fully
documented in a separate report of
the same title (see Project Report
ordering information at back).
Introduction
An important parameter determined
for acid deposition characterization and
monitoring is pH. It has been measured
during all of the EPA's Aquatic Effects
Research Program Surface Water
Surveys in both the field (on-site) and
analytical laboratories. Laboratory mea-
surements with pH electrodes were
generally precise and accurate ( + 0.05
pH units), but proved to be time
consuming and labor intensive. When
used in the field, electrodes and meters
performed marginally (± 0.5 to ±1 pH
unit) Future surveys and long-term
monitoring projects will require field pH
measurements using either in-situ or
closed-system techniques. This ne-
cessitates the development of new
methods for determining pH in the field.
In an initial step to develop a field
method, this study was undertaken to
fully characterize two indicator-based
pH methods and check their suitability for
field use. The first involves the use of an
indicator dye mixture with a pH
dependent absorbance in a FIA method.
The second method uses an optical fiber
coated with fluorescein, the fluorescence
of which at 530 nm is pH dependent.
Procedure
All chemicals were ACS analytical
reagent grade or better. Solutions were
prepared in ASTM Type II water and
were stored in high density polyethylene
bottles at 4 degrees centigrade. Real lake
water samples were field audit samples
from the Eastern Lake Survey. They were
originally taken from Big Moose Lake and
Bagley Lake and were homogenized and
stabilized for use in the survey. The pH
and other physical and chemical prop-
erties of these real samples were ex-
tensively characterized.
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In the flow injection analysis method,
a pH indicator reagent is mixed with
sample and injected into a deionized
water carrier stream. The absorbance of
the effluent is monitored at 555 nm. The
pH indicator reagent is a proprietary
mixture of pH indicator dyes whose
absorbance at 555 nm is proportional to
pH.
A fiber optic chemical sensor (FOGS
or optrode) for pH is constructed by
immobilizing fluorescein to the distal end
of a fiber optic. Fluorescein is a
fluorescent compound which emits light
at 530 nm when excited at 485 nm. The
intensity of fluorescence at 530 nm is pH
dependent. Sample pH is measured by
dipping the probe into the sample and
monitoring the fluorescence at 530 nm
while exciting at 485 nm.
The following issues were addressed
in evaluating the methods: calibration
curves, precision, accuracy, sample
analysis rate, matrix effects, and real
sample analysis. The basic method
analytical characteristics were deter-
mined using stable, well characterized
pH buffers. The effects of several matrix
variables (ionic strength and buffering
capacity) on the analytical characteristics
were studied using acetate, phosphate,
and sulfuric acid solutions. Finally, the
performance of the method was tested
with two real lake water samples.
Results and Discussion
The analytical figures of merit of the
three methods of pH determination are
given in Table 1. The FIA method
compares very well with the pH electrode
for the routine determination of pH in
natural surface waters. It has acceptable
precision and bias and has a very high
sampling rate. Improvement in both
precision and accuracy are possible by
improving the FIA hardware (more
accurate and precise solution handling,
using syringe pumps). Also the linearity
and applicable pH range may be
increased by modifying the indicator
reagent mixture and/or monitoring more
than one wavelength.
The results of the FOGS pH method
were encouraging, but the technique
needs more development than the FIA
method. The optrodes investigated were
prototypes and consequently the
analytical characteristics varied from
optrode to optrode. The bias was
dependent upon the optrode sensitivity
and varied from ±0.1 to ±0.6 pH unit.
The bias is magnified by the inadequacy
of the calibration model. The linear
model chosen for the optrode does not
explain all of the variation. A polynomial
model will be tested when the software
becomes available. Another optrode
characteristic is a limited lifetime.
Optrode sensitivity decreases from day
to day. Finally, while the optrodes are
generally faster than electrodes, the
achievable sampling rate is affected by
ionic strength and buffering capacity.
Conclusions and
Recommendations
The FIA method appears to be the
more promising of the two new
techniques at this time. With further
work, a method suitable for long-term,
unattended monitoring of pH in surface
waters could be developed. The FOGS
pH optrode wili require manufacturing
changes and further characterizations in
order for it to become analytically viable.
Potential interferences in the FIA
method should be further investigated.
For example, the bias for dilute acid
standards was larger than anticipated.
The bias appeared to be related to the
pH or acid used rather than ionic
strength. Also, interferences from
potential organic components in surface
waters should be studied. Finally, a
larger set of real samples should be
analyzed and compared to pH electrodi
data.
In the near future, the use o
chemically sensitive field effect transis
tors (Chemfet) for pH measurement wil
be studied. Preliminary indications an
that Chemfets will be an excellent tool fo
measuring pH directly or as a detector ii
a FIA method.
Table 1.
Comparison of Analytical Figures of Merit of Electrode, FIA, and
FOCS pH Methods
pH Electrode
FIA
FOCS
Rate (samples/hr) 4-20
pH Range 3-10
Bias (pH Unit) ±0.05
Precision (pH Unit) ±0.05
100
3.6-6.8
±0.01
±0.05 to ±0.20
10-60
3.6-7.5
±0.1 to ±0.6
±0.02 to±0.2
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Stephen H. Pia, Donna P. Waltman, and Daniel C. Hillman are with Lockheed
Engineering and Management Services Company, Inc., Las Vegas, NV
89109
Edward M. Heithmar is the EPA Project Officer (see below).
The complete report, entitled "The Determination of pH by Flow Injection
Analysis and by Fiber Optrode Analysis," (Order No. PB 88-235 502/AS;
Cost $74.95, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Monitoring Systems Laboratory
U.S. Environmental Protection Agency
Las Vegas, NV 89193-3478
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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Penalty for Private Use $300
EPA/600/S4-88/026
0000329 PS
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