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 

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


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


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


Distribution: A, B, S, PO, FO