Hawaii Ocean Time-series (HOT)
in the School of Ocean and Earth Science and Technology at the University of Hawai'i
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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.
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. Water Sampling
4. Calibration of Electrode (NBS and SWS)
5. Preparation and Standardization of Acid Titrant
6. Titration of Samples
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