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.
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.
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.
|3.1.||Drawing the sample
|3.2.||Preserving the sample
|4.2.||Seawater buffer preparation
|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.
|5.4.||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.|
|6.1.||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.|
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.
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.
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.
Niskin bottles and rosette
300 ml combusted reagent bottles
Computer interfaced 665 Dosimat and Orion EA 940 Ion Analyzer
Calibrated volumetric glassware
Analytical and topload balance
Hydrochloric acid (0.1000 N)
Sodium tetraborate (borax)
Distilled and deionized water
HgCl2 (saturated solution)
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.