Provisional Method for Carbonate, Dissolved; Bicarbonate, Dissolved; and Carbonate Alkalinity, Dissolved; Electrometric Tritration,
Incremental, Field
In Reply Refer To: December 11, 1981
EGS-Mail Stop 412
QUALITY OF WATER BRANCH TECHNICAL MEMORANDUM 82.05
Subject: WATER QUALITY--Provisional Method for Carbonate,
Dissolved; Bicarbonate, Dissolved; and Carbonate
Alkalinity, Dissolved; Electrometric Tritration,
Incremental, Field
Attached is a description of the incremental field tritration
for carbonate, bicarbonate, and carbonate alkalinity, which
is similar to the method described by Barnes in U.S.
Geological Survey Water-Supply Paper 1535-H (1964). The
method of Barnes was referred to in Quality of Water Branch
Technical Memorandums 80.27 and 81.04 as the only acceptable
method for determination of the carbonate species.
This method is provisional and will remain so until the
precisions of the determinations in the field have been
established. We would urge the users of this method to assist
in the determination of the field precision in the following
manner. At an appropriate sampling site when the streamflow
is stable, collect six replicate samples and determine the
carbonate, bicarbonate, and carbonate alkalinity in each by
this method. Report the original data plus any statistical
analyses you may choose to make to this office. We would also
appreciate any comments, discussions of difficulties, or
suggestions for any improvements that might be made to the
method description.
After further review and incorportation of any comments, the
method will be published in the forthcoming Field Methods
TWRI.
R. J. Pickering
Chief, Quality of Water Branch
Attachment
This memorandum supplements Quality of Water Branch Technical
Memorandums 80.27 and 81.04
Key words: water quality, analytical methods, carbonate,
bicarbonate, carbonate alkalinity
WRD Distribution: A, B, S, FO, PO
PROVISIONAL METHOD
Carbonate, Dissolved; Bicarbonate Dissolved;
Carbonate Alkalinity, Dissolved; Electrometric Titration,
Incremental, Field
Parameters and Codes: Carbonate, dissolved (mg/L as C03):
99445
Bicarbonate, dissolved (mg/L as HC03):
99440
Carbonate Alkalinity, dissolved
(mg/L as CaC03): 99430
1. Application
1.1 This procedure is applicable to surface and ground
waters, either filtered or unfiltered, although an unfiltered
sample is preferred (see Sections 3.4 and 3.5).
1.2 Accurate values for pH, and carbonate and bicarbonate
ion concentrations in an aqueous solution are essential in
studies involving carbonate chemistry and equilibrium
calculations. These parameters are especially subject to
rapid changes in water samples owing to loss of dissolved
gases. Determinations by this procedure are to be made in the
field to minimize exchange of carbon dioxide gas between the
sample and its surroundings.
2. Summary of Method
Although method I-1030-78 (Skougstad and others, 1979), a
fixed-pH endpoint (pH 4.5) titration, is commonly used to
determine alkalinity, accurate determination of alkalinity
due to carbonate species, as well as determination of the
concentrations of those species, can be made by constructing
a titration curve from measurements of pH versus volume of
strong acid titrant added to the sample in small increments.
The endpoints for titration of successive proton absorbing
species are taken as the inflection points of the titration
curve, or as the maximum rates of change of pH per volume of
titrant added (see fig. 1).
A detailed description of the theory of potentiometric
titration of carbonate and bicarbonate, sampling procedures,
and the occurrence of alkalinity, are given in Barnes (1964),
Wood (1976), and Hem (1970).
3. Interferences
3.1 Any ionized substance that reacts with a strong acid can
contribute to alkalinity if the reaction occurs at a pH above
that of the specified endpoint; examples are salts of weak
organic and inorganic acids. The weak inorganic bases
SiO(OH)3, H2B04, NH3 and Al(OH)2(H20)4+ will accept a
hydrogen ion in the titration to the first endpoint near pH
8.3. The weak inorganic bases H2P04 and Al(OH)(H20)5 as well
as most of the weak organic bases found in natural waters
will accept a hydrogen ion in the titration between the first
endpoint and the second endpoint near pH 4.5. Corrections for
these interferences may be required.
3.2 The hydroxide ion (OH-) will contribute significant
alkalinity when the initial sample pH exceeds 11.0 units. In
this case, a correction for the OH- ion should be applied to
the calculation of carbonate.
3.3 All surface waters carry particulate matter ranging in
concentrations from several lO's of milligrams per liter to
several lOO's of grams per liter. Compostion varies from
mainly organic to mainly inorganic rock fragments and is
largely controlled by the geologic terrain traversed by the
stream. Particulates can take up some strong acid by
dissolution, adsorption, or ion exchange and, thereby, cause
anomalously high measurements. For this reason, filtration
through a 0.45 micron pressure-type stainless steel barrel
filter using an inert gas such as nitrogen or argon for
pressure may be needed for some samples. Experience indicates
that small amounts of particulate matter do not interfere
appreciably. The effect of particulate matter should be
determined by comparing shapes of and analytical values
obtained from titration curves determined on filtered versus
unfiltered samples.
3.4 Ground water obtained from wells usually has much less
particulate matter than surface water but may contain
sufficient particulate matter to justify filtration. For
discussion of ground-water sample filtration see Wood (1976).
An in-line filter that permits exclusion of the atmosphere
during filtration is recommended.
3.5 Wind-borne dust can be a source of contamination of the
sample and direct sunlight can warm the sample appreciably,
causing loss of carbon dioxide. Every precaution should be
taken to minimize these effects. The field procedure is best
performed in an enclosed van-type vehicle containing all the
reagents and equipment required for on-site determination of
unstable constituents.
4. Apparatus
4.1 Beakers, 150 mL capacity, glass or disposable plastic.
4.2 Bottle, 500 mL capacity, plastic squeeze, for rinsing
with distilled or deionized water.
4.3 Buret, 25-mL capacity with 0.1 mL graduations. A lO-mL
semi-micro buret with .02 graduations may be used for samples
containing less than 200 mg/L of carbonate and bicarbonate.
4.4 Buret stand and holder.
4.5 pH electrode, combination; Orion 91-62 or equivalent and
filling solution.
4.6 pH meter, battery operated, with expanded scale or scale
length of at least 15 cm for detailed work of +0.02 pH units.
4.7 Pipets, class A volumetric, 50 mL, or other appropriate
volume.
4.8 Stirrer, portable magnetic with small teflon coated
stirring bar. If this is not available, hand stirring with
glass stirring rod or with hand-held battery-powered cocktail
stirrer with glass stirring rod will suffice.
4.9 Thermometer, 0! to 50!C, graduated in 0.1!C.
5. Reagents
5.1 pH buffer solutions, pH 4, 7 and 10 accurate to +.02
units and changes with temperature specified.
5.2 Sodium carbonate standard solution, 0.01639N: 1.00 mL =
1.00 mg HC03: Dry 1.0 g primary standard Na2C03 at 150!C to
160!C for 2 h. Cool in a desiccator. Dissolve 0.8685 g in
carbon dioxide-free water prepared by boiling for 15 minutes
and cooling without agitation; dilute to 1,000 mL in a
volumetric flask.
5.3 Sulfuric acid standard solution, 0.01639N: 1.00 mL =
1.00 mg HC03: Cautiously add 0.5 mL concentrated H2S04 (sp gr
1.84) to 950 mL water. After the solution has been thoroughly
mixed, standardize by titrating 25.00 mL Na2C03 standard
solution to pH 4.5. Adjust the concentration of the sulfuric
acid standard solution to exactly 0.01639N by dilution with
water or by addition of dilute acid as indicated by the first
titration. Confirm the exact normality by restandardization.
Keep the solution in a sealed glass bottle or volumetric
flask until used. Although the sulfuric acid standard
solution is reasonably stable if protected from ammonia
fumes, the normality should be verified at least monthly
(NOTE 1).
NOTE 1. It may be found more convenient to prepare standard
sulfuric acid that is not exactly 0.01639N but the exact
normality of which is known. Such standards may be used if
the appropriate factor is applied in the calculations.
6. Procedure
6.1 Standardize the pH meter as described in method
I-1586-78 (Skougstad and others, 1979).
6.2 Rinse electrode thoroughly (at least 3 times) with
sample water, and adjust temperature of titrant to +2!C of
the sample temperature.
6.3 Measure and record pH value of a freshly-collected
representative sample.
6.4 Pipet an appropriate volume of sample (usually not more
than 50 mL) into a clean dry 150 mL beaker and insert pH
probe and teflon stirring bar. (See section 3.4 and 3.5 for
guidelines on need for prior filtration).
6.5 If pH is greater than 8.3, add sulfuric acid standard
solution dropwise and carefully record the volume delivered
in 0.02 mL increments (0.05 increments with a 25-mL burette)
and record the pH after each addition until pH is below 8.0.
Stir gently with magnetic stirrer or other appropriate
stirring device while adding titrant and making readings.
Allow 15-20 seconds for equilibration after each acid
addition.
6.6 If initial pH is less than 8.3, skip step 6.5 and go
directly to step 6.7
6.7 Titrate rapidly to pH 5.0 and record the volume of
titrant at pH 5.0 to the nearest 0.02 mL (0.05 mL on 25-mL
buret). Allow 15-20 seconds for equilibration.
6.8 From pH 5.0 to 4.0, add acid dropwise in 0.02 mL
increments and record the pH after each addition, allowing
15-20 seconds for pH equilibration after each. The most
sensitive part of the titration curve is usually between
pH 4.8 and 4.3.
7. Calculations
7.1 The true endpoints are determined by either 7.lA or
7.lB.
7.lA Construct a titration curve plotting the volume of the
titrant (sulfuric acid standard solution) added against pH.
The endpoints are the inflection points of the curve, the
points at which the pH changes are greatest for a given
amount of titrant (see fig. 1).
7.lB Plot the rate of change of pH with change in titrant
volume
_ pH
_ mL of titrant
against the total volume of titrant. The endpoints are the
volumes of titrant delivered at which there occur maximum
rates of change of pH per volume of titrant added (see
fig. 1).
7.2 Calculate carbonate
C03 (mg/L) = 1000 x [mLa at ep near pH 8.3] x .9835
mLs
(See NOTE 2).
7.3 Calculate bicarbonate
HC03 (mg/L) = 1000 x [(mLa at ep near pH 4.5)
mLs -2(mLa at ep near pH 8.3)] x 1.0
(See NOTE 2).
where mLs = volume of sample in mL mLa = volume of titrant
(surfuric acid standard solution) added in mL ep = endpoint
determined by 7.lA or 7.lB
NOTE 2: If the sulfuric acid standard solution has normality
different from 0.01639N, compute new multiplying factors as
follows:
For C03 Factor = 0.9835 x Acid Normality
.01639N
For HC03 Factor = 1.00 x Acid Normality
.01639N
7.4 Calculate carbonate alkalinity
Carbonate Alkalinity (mg/L as CaC03) =
8. Report
Report carbonate (99445), bicarbonate (99440), and carbonate
alkalinity (99430) as follows: less than 1000 mg/L, whole
numbers: 1000 mg/L and above, three significant figures.
9. Precision
The precision of this method in the field has not been
established.
References
Barnes, Ivan. 1964. Field measurement of alkalinity and pH.
U.S. Geological Survey Water-Supply Paper 1535-H, p. 17.
Hem, John D. 1970. Study and interpretation of the chemical
characteristics of natural water. U.S. Geological Survey
Water-Supply Paper 1473, second edition, p. 363.
Skougstad, M., Fishman M., Friedman, L.C., Erdman, D.E. and
Durran, S.S. (editors) 1979. Methods for determination of
inorganic substances in water and fluvial sediments.
Techniques of Water-Resources Investigations of the United
States Geological Survey, book 5, chapter Al, p. 626.
Wood, Warren W. 1976. Guidelines for collection and field
analysis of groundwater samples for selected unstable
constituents. Techniques of WaterResources Investigations of
the United States Geological Survey, Book 1, chapter D2,
p. 24.