ALKALINITY


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.


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 The tube is slowly withdrawn with the sample flowing.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

11. Reagents

12. References