WATER QUALITY -- New Parameter Codes for pH, Alkalinity, Specific Conductance, and Carbonate/Bicarbonate
In Reply Refer To: September 19, 1980
EGS-Mail Stop 412
Quality Water Technical Memorandum No. 80.27
Subject: WATER QUALITY -- New Parameter Codes for pH, Alkalinity,
Specific Conductance, and Carbonate/Bicarbonate
Beginning October 1, 1980, the field and laboratory values for the
subject constituents will be entered into WATSTORE under separate
parameter codes. In addition, a more careful distinction than has
been made in the past will be made of the methods used to
determine alkalinity and carbonate/bicarbonate ion concentrations
and of the assignment of parameter codes to values obtained by
those methods.
Either by request from the field or by necessity for quality
control checks, the National Water Quality Laboratories (NWQL)
often measure pH, specific conductance, and alkalinity even though
field values may be reported on the log-inventory form. Both
values need to be stored, and parameter codes are being
established for that purpose.
The measurement of alkalinity, bicarbonate, and carbonate and the
reporting of those measurements are complicated by a mixture of
problems including past field measurement methods that may have
produced less accurate data than those that could be acquired
through incremental titration and parameter codes that were not
sufficiently descriptive to allow certainty in their use in
WATSTORE. This memo is intended to specify acceptable field
measurement methods and clarify the assignment of parameter codes.
Prior to 1964, the accepted procedure for determination of
hydroxide, carbonate, and bicarbonate ions was to titrate with a
standard acid to fixed endpoints at pH 10.4, 8.3, and 4.5. The
concentrations of the ions were equated to the volume of a
standard acid added to the sample after conversion to proper
reporting units. In 1964, Barnes (WSP 1535-H, 1964) showed that
the actual stoichiometric endpoints of the bicarbonate portion of
the titration could vary between pH 5.38 and 4.32, depending on
the temperature and ionic strength of the sample. This kind of
endpoint variability can be expected with respect to the other two
ions, the chemical principles Barnes used in his arguments apply
to those endpoints as well. It is quite clear from this work that
titration to fixed endpoints yields measures of hydroxide,
carbonate, and bicarbonate that are probably in error by variable
and indeterminate amounts. The only acceptable method for
measuring these constituents is to titrate in increments of acid
small enough to accurately distinguish the inflection points of
the pH vs. acid-volume curve, which are the stoichiometric
endpoints. Titration to fixed pH endpoints is still in common use
in water-quality work; however, and many WRD field personnel
continue to use the method in the field because they have received
no directions to discontinue its use.
Since 1964, the measurement of "alkalinity", as defined by the
acid needed to drive the sample pH to 4.5, has been made
extensively. Our files now contain hundreds of thousands of
measurements made by titration to pH 4.5. The method is recognized
by EPA and USGS, as well as by other organizations devoted to
methods standardization, and the term "alkalinity" has been
attached to this measurement so frequently that the term and the
measurement are now virtually inseparable.
The different measurements and parameters that may be derived from
titration of a sample with a standard acid are illustrated in the
enclosed figure. The dots on the curve are the inflection points,
the true stoichiometric endpoints of the titration of hydroxide
(OH), carbonate (C03), and bicarbonate (HC03). In this example,
the inflection points are all at pH's greater than the previously
accepted endpoints 10.4, 8.3, and 4.5, but they could as easily be
less than those values. It can be seen that l) volume A correctly
represents OH(-), but volume B does not, 2) volume C correctly
represents C03(-2) CO3(2_), but volume D does not, 3) volume E
correctly represent HCO3(-) but F does not, 4) G correctly
represents the alkalinity due to C03 and HC03- but H does not, 5)
volume I represents the alkalinity as now accepted by common use,
namely, the acid needed to drive the sample pH to 4.5.
It is also clear that the measurements represented by volumes B,
D, F, and H are those most commonly reported and that they are not
rigorously correct.
In accordance with the foregoing analysis, after October 1, 1980,
1) data will no longer be entered from the field under parameter
codes 71830, 00445, 00440, and 00430; these codes will henceforth
be used only for data retrieval; 2) values of OH(-), C03(-2), and
HC03(-) will be entered only if they are determined by incremental
tltration to the inflection points and expressed only as mg/L of
the ion; new paramenter codes have been requested for these latter
determinations and will be transmitted to you along with the
approved field procedures as soon as they are available; 3) values
of alkalinity by titration to pH 4.5 obtained in the field and lab
will be entered under parameter codes 00431 (field) and 00410
(lab).
In addition, 4) specific conductance values determined in the
field and will be entered under parameter codes 00094 (field) and
00095 (lab) and 5) pH values determined in the field and lab will
be entered under parameter codes 00400 (field) and 00403 (lab).
R. J. Pickering
Enclosure
Distribution: A, B, S, PO, FO
Key Words: Water quality, analytical methods, data handling,
field measurements, parameter codes, pH, specific conductance,
alkalinity, carbonate, bicarbonate, hydroxide.
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In Reply Refer To: October 8, 1980
EGS-Mail Stop 412
Memorandum
To: Distribution
From: Chief, Quality of Water Branch, Reston, Virginia
Subject: Water Quality--Quality of Water Branch Technical
Memorandum No. 80.27
The figure enclosed in the subject memorandum was incomplete.
Please substitute the enclosed figure in your copy of Quality of
Water Branch Technical Memorandum No. 80.27.
R. J. Pickering
Enclosure
Distribution: A, B, S, PO, FO