Alkalinity in seawater and estuarine water - some limitations in the two-point method (original) (raw)
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Improvements in a fast potentiometric seawater alkalinity determination
Ciencias Marinas, 2000
In the present paper, we introduce some methodological improvements in a potentiometric technique for determining seawater alkalinity. This fast method (about two minutes per sample) uses open flasks and an ordinary titration system without thermostatic bath. A new potassium hydrogen phthalate-borax 4.4 buffer is proposed to calibrate the electrode near the end point of the titration. The pH values of this new buffer at different pH-scales and temperatures were determined using spectrophotometry. The combined use of this easy and improved two end-point method for alkalinity determination and certified reference material enables the alkalinity to be determined with an accuracy lower than 1µmolkg–1(0.05%).
Marine Chemistry, 2004
A flow-through analyzer has been developed for the rapid, potentiometric determination of total alkalinity (TA) in seawater in order to capture large temporal and spatial variations of the CO 2 system in coastal zones, where the carbon cycle is still not well understood despite the potential large contribution of such areas to the global carbon cycle. This analyzer requires small amounts of seawater (3 ml min À 1 ), and has realized continuous measurements of TA with a response time of 4 -5 min, with long-term precision of about 0.1% (ca. 2 Amol kg À 1 ), and accuracy of a similar level over a range of TA (more than 200 Amol kg À 1 ) when appropriate standard solutions are used. This analyzer is fully automated and can achieve stable measurements (within 0.15%; 3 Amol kg À 1 ) over 1 -2 days without regular recalibration for drift, which will enable us to carry out continuous, in situ measurements of TA in coastal waters. D
GEOCHEMICAL JOURNAL, 2014
The total alkalinity (A T) of seawater is one of the parameters required to characterize carbonate systems, which are essential for studying the greenhouse effects of carbon dioxide (CO 2) on the Earth's climate. The A T of seawater was measured by potentiometric open-cell titration using HCl-without the addition of sodium chloride (NaCl)-and calculated by a nonlinear least-squares regression. In addition, the equation for calculating the titrant density was studied over a range of concentrations and temperatures. The similarity of the pH electrode response to the ideal Nernst value (k s) was measured using pH buffers for seawater and pure water. New methods for the calculation of A T using k s were proposed. Open-cell titrations for a certified reference material (CRM) were conducted using two electrodes with k s of 0.997 and 0.989. The measured values for both electrodes were in good agreement with the CRM value of 2356.78 ± 0.26 µmol/kg. A T was successfully calculated using k s. The dilution effect by the titrant, free of NaCl, on electromotive force measurement was also examined and found to be negligible for the calculation of CO 2 parameters.
A precise and rapid analytical procedure for alkalinity determination
Marine Chemistry, 1987
A potentiometric analytical method is proposed for the determination of the alkalinity of seawater. The precision is 0.1% and each determination takes 3 min. The technique is very easy to use, even on board ship since it is carried out in open flasks. A polynomial equation is also proposed which greatly simplifies the theoretical expression.
Analytica Chimica Acta, 2008
a b s t r a c t This paper examines the performance of a previously reported, closed cell, potentiometric titration technique [J.M. Hernández-Ayón, S.L. Belli, A. Zirino, Anal. Chim. Acta 394 (1999) 101] for the simultaneous determination of pH, total inorganic carbon (TCO 2 ), total alkalinity (TA), and organic alkalinity (OA) in coastal seawater samples. A novel interpretation of the titration data, as recently proposed by Hernández-Ayón et al. [J.M. Hernández-Ayón, A. Zirino, A.G. Dickson, T. Camiro-Vagas, E. Valenzuela-Espinoza, Limnol. Oceanogr.: Methods 5 (2007) 225] who applied it to waters of unusually high organic matter content, was applied here to fjord surface waters collected over the duration of a phytoplankton bloom.
Titration alkalinity of seawater
Marine Chemistry, 1993
The titration system is described that was used to measure the total alkalinity of seawater (TA) during the Joint Global Ocean Flux Study (JGOFS) sponsored by the National Oceanic and Atmospheric Administration (NOAA) in the equatorial Pacific. It consists of a piston titrator, a pH meter, and a glass thermostated cell. Since the new pH meters and titrators have RS232 interfaces the system can be easily connected to a personal computer. The computer programs used to carry out the titration and to determine TA, pHsw (pH on the seawater scale), and TCO2 from the full titration curve are described. A typical titration takes 20 min and consists of 25 points. Six separate titration cells were calibrated to be used on three systems at sea. The reliability of the electrodes was examined by titrations of 0.7 m NaCI with HC1 at a pH near 3 and using seawater buffers at a pH near 8. Although most electrodes did not have Nernstian behavior over the entire pH range, all gave precise values of TA for a given solution. The individual ceils were calibrated using standard Na2CO3 and seawater standards prepared in our laboratory and Certified Reference Material (CRM) provided by Dickson. The cells gave reliable values of TA, but the values of pHsw were low (0.02) and values of TCO2 were high (20 #mol kg -1) due to the non-Nernstian behavior of the electrodes at a pH near 8.0. If the slope determined from the buffers is used, the titrations yield reliable values of TA, TCO 2 and pHsw. Measurements on Dickson standards with the three ceils at sea indicate that the systems have a reproducibility of ±2-4 #mol kg-1 in TA. The titration values of TCO2 determined on the CRMs and the samples collected at sea were about 17 -4-6/~mol kg -1 (fall) and 20 4-6/zmol kg 1 (spring) too high. This offset in TCO2 is independent of depth and is due to the non-Nernstian behavior of the electrodes. The offset is not due to unknown protolytes.
Analytica Chimica Acta, 1999
This study reports the potential contribution of organic bases to the alkalinity of seawater samples. The concentration of organic bases in these samples was inferred from the difference between the measured alkalinity and that calculated from a knowledge of pH and concentrations of the various inorganic acid-bases species such as total carbon, total boron, and so on. Significant concentrations of such organic bases were measured in cultures of the marine microalgae Rhodomonas sp. (800 µmol kg -1 ) and Isochrysis aff. Galbana (400 µmol kg -1 ), as well as in three marine environments (northern gulf of California, México; San Quintín Bay, B.C., Mexico; and San Diego Bay). These three sites are characterized by significant biological activity and restricted mixing, and the organic bases were found at concentrations greater than 50 µmol kg -1 in each of these three locations.
Development of a reference solution for the pH of seawater
Analytical and …, 2007
A method that uses a Harned cell to perform potentiometric pH measurements has been optimized and applied to an aqueous solution of simulated seawater that contains sodium perchlorate, sodium sulfate, sodium hydrogen carbonate and boric acid and has an ionic strength I of 0.57 mol kg −1 . The standard metrological approach developed for the measurement of pH in low ionic strength aqueous solutions was maintained, but a few modifications were necessary, and measurement procedures and calculations were modified ad hoc from those adopted in conventional protocols. When determining the standard potential of the cell, E°, NaClO 4 salt was added to a 0.01 mol/kg HCl solution to attain the same ionic strength as the test solution and to investigate possible specific effects related to the high levels and the nature of the background electrolyte. An appropriate value of γ ±HCl (0.737) was then selected from the literature, based on a realistic value for I. Finally, in order to convert the acidity function at zero chloride molality into pH, a suitable value of γ Cl (0.929) was calculated. As a result, we obtained pH=8.18 (T=25°C) with an associated expanded uncertainty U=0.01 (coverage factor k=2). The aim was to establish a sound basis for the pH measurement of seawater by identifying the critical points of the experimental and theoretical procedure, and to discuss further possible developments that would be useful for achieving a reference solution.