Improvements in a fast potentiometric seawater alkalinity determination (original) (raw)
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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.
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.
Alkalinity in seawater and estuarine water - some limitations in the two-point method
Marine Chemistry, 1988
In a recent paper, van den Berg and Rogers proposed a two-point potentiometric method for estimating the alkalinity of estuarine waters. There are several problems associated with the application of such methods. Potential users should consider these problems before they finally decide whether a one-or two-point method or an alkalinity titration is the suitable choice. The purpose of this note is to discuss errors that can be expected if the procedure of van den Berg and Rogers is used. Some of these errors can be decreased by proper correction, while others are more difficult to compensate for.
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.
Determination of seawater pH from 1.5 to 8.5 using colorimetric indicators
Marine Chemistry, 1989
The apparent dissociation constants of five sulfonephthalein indicators (thymol blue, bromophenol blue, bromocresol green, bromocresol purple and phenol red) were determined in 35%o seawater at 25 ° C. Measurements of seawater pH were made using the ratio of the optical absorbance of the acidic and basic components of a particular indicator and the apparent dissociation constant of the indicator. The suite of indicators allows quantitative determination of seawater pH over the pH range 1.5-8.5. For seven seawater samples ranging from pH 1.5 to 8.0 the average difference between pH measurements made with indicators and conventional electrodes was + 0.005. Measurements of seawater pH with indicators is not limited to single indicator solutions. Simultaneous addition of phenol red and bromocresol green allows pH determinations from 3.0 to 8.2, which are suitable for All~Minity titrations. The pK.' values of the indicators were determined independently of potentiometric pH measurements. The pH indicators provide an absolute free hydrogen ion molality scale independent of the problems of electrode drift and liquid junction errors associated with potentiometric pH measurements.
Spectrophotometric Calibration of pH Electrodes in Seawater Using Purified m-Cresol Purple
Environmental Science & Technology, 2012
This work examines the use of purified meta-cresol purple (mCP) for direct spectrophotometric calibration of glass pH electrodes in seawater. The procedures used in this investigation allow for simple, inexpensive electrode calibrations over salinities of 20−40 and temperatures of 278.15−308.15 K without preparation of synthetic Tris seawater buffers. The optimal pH range is ∼7.0−8.1. Spectrophotometric calibrations enable straightforward, quantitative distinctions between Nernstian and non-Nernstian electrode behavior. For the electrodes examined in this study, both types of behavior were observed. Furthermore, calibrations performed in natural seawater allow direct determination of the influence of salinity on electrode performance. The procedures developed in this study account for salinity-induced variations in liquid junction potentials that, if not taken into account, would create pH inconsistencies of 0.028 over a 10-unit change in salinity. Spectrophotometric calibration can also be used to expeditiously determine the intercept potential (i.e., the potential corresponding to pH 0) of an electrode that has reliably demonstrated Nernstian behavior. Titrations to ascertain Nernstian behavior and salinity effects can be undertaken relatively infrequently (∼weekly to monthly). One-point determinations of intercept potential should be undertaken frequently (∼daily) to monitor for stable electrode behavior and ensure accurate potentiometric pH determinations.