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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Official Business
Penalty for Private Use $300

EPA/600/S4-88/026
                                        0000329   PS

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