ALKALINITY
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SUMMARY: Seawater samples are collected at discrete
depths using CTD-rosette sampling protocols. Subsamples
for alkalinity are collected and immediately preserved
with HgCl2 for subsequent analysis in the laboratory.
The alkalinity is determined by a potentiometric titration,
and the second end point (V2) is determined with a
modified Gran Plot.
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1. Principle
The total alkalinity is defined as the number of milliequivalents
of H+ required to titrate one kilogram of seawater to the bicarbonate
equivalence point (~pH 4.5). The classical chemical formulation
(Harvey, 1966) of this definition is:
Alk(t) = [HCO31-] +2[CO32-] +[B(OH)41-] +[OH1-] -[H+]
In high precision work corrections for other proton acceptors
(e.g. [H3PO3-] and [SiO(OH)3-), should be taken into account (Edmond,
1970; Dickson, 1981; Bradshaw and Brewer, 1988). From the knowledge
of the total alkalinity, dissolved inorganic carbon, salinity, and
nutrient content of seawater the entire carbonate "balance" can be
calculated from the dissociation constants of carbonic acid, boric
acid and other interacting species.
2. Precautions
Samples should be drawn as soon as possible and preserved to halt
biological activity. Samples bottles should be completely sealed and
stored in the dark cool place.
3. Water Sampling
3.1. Drawing the sample
3.1.1. Samples are drawn into combusted 300 ml glass reagent bottles
in the same manner as dissolved oxygen.
3.1.2. The drawing tube is completely filled with sample while
manipulating the tube to dislodge bubbles. The drawing tube
is flushed and inserted to the bottom of the sample bottle.
3.1.3. The sample bottle is overflowed with at least two volumes
of sample.
3.1.4. The tube is slowly withdrawn with the sample flowing.
3.2. Preserving the sample
3.2.1. Use a pipet to remove water from the neck of the sample
bottle of sample. Enough water is removed so that
approximately 1 ml of air is contained in the bottle when
the glass stopper is inserted.
3.2.2. 100 µl of saturated HgCl2 is added to each sample. The
tapered ground glass bottle neck is dried with a Kimwipe
wrapped on an applicator stick. The bottle is sealed with
a ground glass stopper coated with a light covering of
Apiezon grease. The stopper is pressed firmly into the
bottle to make a good seal, and is secured with polyethylene
tape or a large rubber band.
4. Calibration of Electrode (NBS and SWS)
4.1. NBS
4.1.1. Soak electrode in storage solution overnight.
4.1.2. Place electrode in buffer 7 and soak for 15 minutes.
4.1.3. Pour out old buffer and replace with fresh buffer.
4.1.4. Record relative millivolt reading when electrode stabilizes.
4.1.5. Take temperature of buffer.
4.1.6. Repeat steps 2 through 5 with buffer 4.
4.2. Seawater buffer preparation
4.2.1. 0.4 M Tris and 0.4 M Bis seawater buffers are prepared
according to the method of Dickson (1992).
4.2.2. NaCl, KCl, and NaHSO4 are dried and added gravimetrically.
4.2.3. MgCl2 and CaCl2 are added volumetrically from stock
solutions that have been standardized with AgNO3.
4.2.4. HCl is added from a 0.1 M stock solution carefully prepared
from a Dilut-it.
4.2.5. Buffers are stored in tightly capped 250 ml polyethylene
Nalgene bottles.
4.3. Calibration
4.3.1. Place electrode in Tris buffer overnight.
4.3.2. Pour out old buffer and replace with fresh buffer.
4.3.3. Record relative millivolt reading when electrode stabilizes.
4.3.4. Take temperature of buffer.
4.3.5. Place electrode in Bis buffer for approximately 15 minutes.
4.3.6. Repeat steps 2 through 6.
5. Preparation and Standardization of Acid Titrant
5.1. Volumetrically prepare a solution of 0.1000 N HCl in 0.7 M
sodium chloride.
5.2. Standardize the above solution against recrystalyzed sodium
tetraborate (borax) which is stored in a dessicator over a
saturated solution of sodium chloride and sodium citrate
(Vogel, 1978).
5.3. Prepare at least two concentrations of borax primary standard.
5.3.1. Make a 0.0035 to 0.004 M solution of sodium tetraborate.
5.3.2. Weigh out approximately 25 g of the borax solution and
titrate in a similar manner to a seawater titration.
5.3.3. Calculate the endpoint using a Gran Plot.
5.4. Calculate normality of the acid titrant using the known quantity
and concentration of borax primary standard.
5.5. As a final check on the accuracy of our acid standardization,
a subsample is sent to Dr. Andrew Dickson's laboratory at
Scripps Institution of Oceanography for high precision
coulometric determination of the acid normality.
6. Titration of Samples
6.1. Turn on computer, Dosimat, printer, and pH meter. Load the
alkalinity program. Dispense approximately 20 ml of acid
through the Dosimat to purge any air bubbles and to pump fresh
acid into the lines.
6.2. Throughly rinse and dry a 50 ml beaker.
6.3. Place beaker on balance and tare. Weigh out approximately
50 grams of sample.
6.4. Rinse electrode with seawater and gently dry by touching the
bottom of the electrode with a Kimwipe. Do not wipe the
electrode dry.
6.5. Place sample on magnetic stir plate. Put stir bar into sample
taking care not to splash any sample out of the beaker. Adjust
to moderate stirring. Arrange electrode and buret tip as to
not interfere with the stir bar but to make sure electrode
junction and tip are submerged in sample.
6.6. Start titration program. The program will request a preadd
volume. This volume should be sufficient to adjust the pH of
the sample to about 4. The program will then add HCl in 0.015
ml increments and record the millivolt reading after each
increment is added. The titration will automatically terminate
when a specified millivolt reading (equivalent to pH 3.0) is
reached.
6.7. After the titration has terminated, check and record the sample
temperature.
6.8. Remove beaker from stir plate and pour out sample. Rinse with
tap water for approximately 3 minutes and then rinse with
deionized distilled water. Dry beaker for next sample.
7. Calculations
Data is fit by a modified Gran Function (f2=(Vo+V)([H+]+[HS04-]+[HF]))
using Matlab. A best fit Eo and slope are obtained by minimizing the sum
of the residuals for those points in the range pH 3-3.5. The accuracy
of the calculated slope is determined by comparing to the measured slope
for the buffers described above.
8. Quality Control
As a safeguard on the quality of our results, we maintain a set of
secondary standards which are run with each analysis. These are made
from a large surface seawater sample, which is preserved with HgCl2
and subdivided into 300 ml reagent bottles. These are sealed as
described above and stored in a cool dark place.
9. Precision and Accuracy
The precision of our titration procedure is approximately 3 µeq/kg.
An absolute alkalinity standard is not yet available, and the accuracy
of the procedure is determined primarily by the standardization for
the normality of the titrant. We intercalibrate the determination
of the acid normality with Dr. Andrew Dickson's laboratory at Scripps
Institution of Oceanography, where high-precision coulometric methods
are used to determine the acid normality.
10. Equipment and Supplies
Niskin bottles and rosette
300 ml combusted reagent bottles
Apiezon grease
Computer interfaced 665 Dosimat and Orion EA 940 Ion Analyzer
Calibrated volumetric glassware
Analytical and topload balance
Magnetic Stirrer
Thermometer
Glass Electrodes
11. Reagents
Hydrochloric acid (0.1000 N)
Sodium tetraborate (borax)
Distilled and deionized water
NaCl
HgCl2 (saturated solution)
12. References
Bradshaw, A. L. and P. G. Brewer. 1988. High precision
measurements of alkalinity and total carbon dioxide in
seawater by potentiometric titration: Presence of unknown
protolytes? Marine Chemistry, 23, 69-86.
Dickson, A. 1981. An exact definition of total alkalinity
and a procedure for the estimation of alkalinity and total
inorganic carbon dioxide from titration data. Deep-Sea
Research, 28A, 609-623.
Dickson, A. 1992. pH buffers for seawater media based on
the total hydrogen ion concentration scale. Deep-Sea Research,
submitted.
Edmond, J. M. 1970. High precision determination of titration
alkalinity and total carbon dioxide content of sea water by
potentiometric titration. Deep-Sea Research, 17, 737-750.
Harvey, H. W. 1966. The Chemistry and Fertility of Sea Waters.
Cambridge Univ. Press, 240 pp.
Vogel, A. I. 1978. Textbook of Quantitative Inorganic Analysis,
4th Ed., pp. 300-301. Longman, New York.
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