TITRATION ALKALINITY
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SUMMARY: Seawater is collected from discrete depths
using PVC sampling bottles attached to the CTD/rosette
system. Subsamples of approximately 50 ml are measured
gravimetrically and titrated with 0.1 N HCl using an
automated potentiometric titration system. Total
alkalinity is determined using a Gran plot corrected
for interference from bisulfate and hydrogen fluoride.
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1. Principle
Titration alkalinity is one of four parameters needed to describe
the carbonate system in seawater. Titration alkalinity is therefore
useful for assessing the magnitude and direction of ocean-atmospheric
carbon dioxide flux and the saturation state of calcium carbonate
within the interior of the ocean. Titration or total alkalinity is
usually defined as the quantity of hydrogen ions in millimoles (mmol)
required to neutralize the weak bases in 1 kilogram of seawater
(Grasshoff, 1983). A more exact definition (Dickson, 1981) of total
alkalinity is the quantity of hydrogen ion required to neutralize
bases formed from weak acids with pKs of a > 4.5. Our determination
of total alkalinity employs the potentiometric titration of a seawater
sample with hydrochloric acid. Our methods are slight modifications
of those developed for seawater analysis (Grasshoff, 1983 and references
therein). Our titration procedure employs an open cell and a computer-
controlled titration system . The computer-controlled system
incorporates an automated high-precision burette and pH meter.
2. Precautions
Alkalinity samples should be drawn directly from the Niskin bottle
using clean drawing tubes. Care should be taken to avoid contamination
from ship's equipment or the atmosphere.
3. Water Sampling
Alkalinity samples are collected using the same techniques employed
when collecting DIC samples (see Chapter 6).
3.1. Drawing the sample
3.1.1. Samples are drawn into a clean 300 ml glass reagent bottle.
3.1.2. The bottle should be flushed with at least one volume of water.
3.1.3. The tube is withdrawn so that the bottle remains brimful when
the tube is completely withdrawn.
4. Preserving the Sample
4.1. Some of the sample is removed from the reagent bottle using a 10
ml pipet. Enough water is removed so that about 1 cc of air is
contained in the bottle when the glass stopper is inserted.
4.2. 100 µl of a saturated HgCl2 solution is added to the sample.
The tapered ground glass bottle neck is dried with a kimwipe
and the bottle is sealed with a ground glass stopper covered
with light coating of ApiezonR grease. The stopper is pressed
firmly into the bottle to make a good seal and is secured with
polyethylene tape of a large rubber band.
5. Potentiometric Titration
5.1. The sample is brought to 25°C and approximately 50 ml is placed
in a tared 100 cc beaker. The weight of the sample is recorded
to the nearest milligram.
5.2. A clean stirbar is added to the beaker and the beaker is placed
on a stirplate. The antidiffusion burette tip and the
potentiometric electrode is placed in the beaker.
5.3. The temperature of the stirred sample is measured to the nearest
0.1°C. The temperature of the sample is recorded and the
automated titration is initiated.
5.4. The automated titration procedure adds a 1 to 1.5 ml aliquot of
approximately 0.1 N HCl depending upon the anticipated sample
alkalinity. After reaching a stable mv reading, the mv value is
recorded and a 15 microliter HCl aliquot is added. After the
electrode output stabilizes a second 15 µl aliquot is added.
This process is continued until the mv value reaches a
predetermined value beyond the V2 inflection point. Typically
30 to 35 aliquots are added to reach the predetermined mv value.
6. Computation of Total Alkalinity
Total alkalinity is determined from the volume of acid required
to reach the second endpoint and the acid normality. The inflection
point (V2) is determined using a Gran plot (Gran, 1952) which corrects
for the interference from both bisulfate and hydrogen fluoride. Sulfate
and fluoride concentrations are calculated from salinity and the
equilibrium constants for bisulfate and hydrogen fluoride (Khoo et al.,
1979; Dickson and Riley, 1979).
7. Preparation of HCl
The accuracy of the total alkalinity measurement is determined, in
large part, by the uncertainty in the normality of the HCl. The
approximately 0.1 N HCl solution is made by dilution of concentrated
HCl into a 0.7 M solution of high-purity NaCl. The normality of the
HCl solution is ascertained by titration of solutions made from dried
high-purity sodium carbonate and borax.
8. Precision and Accuracy
The precision of our total alkalinity determinations is approximately
+4 µequiv/kg. This gives a coefficient of variation of approximately
0.2% for typical seawater samples. Because we lack a liquid seawater
standard of known alkalinity, the accuracy of our total alkalinity
determinations is not well known. In order to help ensure the accuracy
of our alkalinity measurements we regularly collect intercalibration
samples for Dr. Keeling's laboratory at Scripps Institution of Oceanography.
To date, alkalinity values for replicate samples analyzed in both
laboratories are typically within 10 µequiv/kg.
9. Equipment and Supplies
kimwipes and applicator sticks
300 ml ground glass stoppered reagent bottles
10 ml automatic pipet
Apiezon grease
analytical balance
volumetric flasks, 100 ml beakers, 50 ml glass pipets and mercury
thermometer
Automated titration system
PC and software
Brinkmann model 655 Dosimat and 5 ml top
Orion model 940 pH meter
Orion combination electrode
stir plate
10. Reagents
distilled deionized water (DDW)
high purity sodium carbonate
high purity sodium chloride
reagent grade hydrochloric acid
11. References
Almgren, T., D. Dyrssen and S. Fonselius. 1983. Determination
of alkalinity and total carbonate. In: Methods of Seawater
Analysis, M. E. K. Grasshoff and K. Kremling, editors, Verlag-
Chemie, pp. 99-123.
Dickson, A. G. 1981. An exact definition of total alkalinity
and a procedure for the estimation of alkalinity and total
inorganic carbon from titration data. Deep-Sea Research, 28A,
609-623.
Dickson, A. G. and J. P. Riley. 1979. The estimation of acid
dissociation constants in seawater media from potentiometric
titrations with strong base. TheThe ionic product of water (Kw).
Marine Chemistry, 7, 89-99.
Gran, G. 1952. Determination of the equivalence point in
potentiometric titration. Part II. Analyst, 77, 661-671.
Khoo, K. H., R. W. Ramette, C. H. Culberson and R. G. Bates.
1977. Determination of hydrogen ion concentrations in seawater
from 5 to 40oC: standard potentials at salinities from 20 to
45 o/oo. Analytical Chemistry, 49, 29-34.
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