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.
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.
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.
Alkalinity samples are collected using the same techniques employed when collecting DIC samples (see Chapter 6).
|3.1.||Drawing 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.|
|5.1.||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.|
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).
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.
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.
kimwipes and applicator sticks
300 ml ground glass stoppered reagent bottles
10 ml automatic pipet
volumetric flasks, 100 ml beakers, 50 ml glass pipets and mercury thermometer
PC and software
Brinkmann model 655 Dosimat and 5 ml top
Orion model 940 pH meter
Orion combination electrode
distilled deionized water (DDW)
high purity sodium carbonate
high purity sodium chloride
reagent grade hydrochloric acid
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 ‰. Analytical Chemistry, 49, 29-34.