Compatibility of activity coefficients estimated experimentally and by Pitzer equations for the assessment of seawater pH (original) (raw)
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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.
Marine Chemistry, 2017
Measurements of pH in seawater are important to determine the natural and anthropogenic trends in the oceans. Spectrophotometry or glass electrode potentiometry measurements of pH require calibration with help of buffers. One common buffer solution is the Tris/Tris•H + couple, which should be well characterised both experimentally and theoretically for optimum accuracy. Chemical speciation modelling in the complex seawater medium is best addressed with an ion interaction approach, with Pitzer equations being the most widely used. The published Pitzer coefficients for Tris and Tris•H + in artificial seawater are based on isopiestic measurements, which necessarily give strong weight to the third virial coefficient C for the key interaction between Tris•H + and chloride. However, in low salinity waters it is the second virial coefficient B that is of greater importance. We have used Harned cell measurements of Tris solutions at ionic strengths up to 1 mol kg-1 to reassess the relevant Pitzer parameters, and have found improved agreement with experimental measurements in artificial seawater. We suggest that additional measurements should be undertaken to address the remaining differences between model calculations and experimental data in artificial seawater. We have used the revised Pitzer parameters to reassess the acid-base constant of the indicator m-cresol purple in low salinity waters.
Marine Chemistry, 2011
An important property of aqueous solutions is pH because it affects chemical and biochemical properties such as chemical reactions, equilibrium conditions, and biological toxicity. With the increasing uptake of fossil fuel CO 2 into the oceans, a decrease in pH is important to consider at this time. Unfortunately, many different methods for assessing pH have been used by different groups. The objectives of this review were to (1) briefly examine the concept of pH as it was introduced and developed, up to the current scientific developments, assumptions, and recommendations, (2) critically assess the various approaches that different scientific groups have adopted for pH, balancing their preferences and arguments, (3) compare measuring vs. modeling pH, and (4) issue recommendations on an optimized approach or approaches for pH. The main conclusions of this review are: (1) pH definitions and conventions are highly variable, which leads to highly variable estimates of pH. For example, for seawater at S A = 35.165 g/(kg soln), t = 25°C, P = 1.0 atm, and fCO 2 = 3.33E-4 atm, model calculated pH values varied from 8.08 to 8.33 on the various pH scales; (2) An acceptable nomenclature is needed to keep pH variability unambiguous, due to alternative definitions and conventions. A nomenclature example is given in this paper. It is the (still unsolved) task of international bodies such as IUPAC or IOC to develop and promote such widely recognized conventions; (3) pH can be accurately estimated based on measurement (potentiometric, spectrophotometric) and modeling approaches. Accuracy via different definitions and conventions clearly requires consistency with respect to experimental measurements, equilibrium constants, activity coefficients, and buffer solutions that are used for specific approaches; (4) "Total" pH accuracy that includes the Bates-Guggenheim convention is ±0.01 pH units. Removing the Bates-Guggenheim convention from the accuracy calculation can lead to "conventional" accuracies of ±0.004 pH units; (5) pH extensions to high solution concentrations are capable using the Pitzer modeling approach. Modeling can, in principle, lead to pH estimates that are more accurate than measurements, which is illustrated with two Pitzer models for natural waters made up of the major components of seawater. But this principle still needs to be proven; (6) It is recommended that ocean scientists use the free concentration or activity of the proton to examine the effect of pH on processes in the oceans.
The estimation of the pK∗HA of acids in seawater using the Pitzer equations
Geochimica et Cosmochimica Acta, 1983
The equations of Pitzer have been used to calculate the stoichiometric ionization constants, pKfi,, for acids in NaCl media at 25°C. The calculated results for the ionization of HAc, HZO, B(OH)3, HzCOj, H3P04, H2PO;, HPO:-, H3As04, H2AsO; and HAsO:-are in good agreement with the measured values, providing higher order interaction terms (0 and q) are used. The pKfiA measurements of these acids in NaCl media containing Mg'+ and Ca'+ were used to determine Pitzer specific interaction parameters at I = 0.7. With these Pitzer coefficients, it was possible to make reliable estimates for the activity coefficients of anions in seawater (S = 35) that form strong interactions with Mg*+ and Ca2'. The calculated activity coefficients yield reliable estimates for the pKfA of acids in seawater.
Talanta, 2019
The comparison of pH measurements in seawater collected at different locations or occasions, is meaningful if the same measurand (i.e. the quantity intended to be measured) is determined, if adequate measurement procedures are used, including the selection of calibrators, and if the measurement uncertainty is known. Depending on the purpose of this evaluation, the measurement uncertainty should be smaller than a defined target value. The measured pH should have a sound physical-chemical meaning to allow the adequate assessment of its impacts. In the present procedure TRIS-TRIS HCl solutions, of different molality ratios, prepared in artificial seawater with reference values estimated by primary measurements, were used to obtain proper calibrators for the pH meter used for the analysis of seawater samples. This work presents the uncertainty evaluation of pH measurements in seawater, performed by potentiometry using a combination glass electrode, from the interpolation uncertainty eva...
The use of buffers to measure the pH of seawater
Marine Chemistry, 1993
The pH of seawater can be measured in the field using potentiometric and spectrophotometric methods. The use of pH standards or buffers is an important aspect of the calibration of both methods in a laboratory on a common concentration scale. The buffers can also be used to monitor the performance of pH meter and spectrophotometer during a cruise. A procedure is described for the determination of the pH of seawater, where the proton concentration is expressed as moles kg-H20 -l using seawater buffers. The buffers are prepared in synthetic seawater in the laboratory by the methods outlined by Bates and coworkers. We have prepared four buffers (Bis, Tris, Morpholine and 2-Aminopyridine) that cover a pH range from 6.8 to 8.8. The emf values of the buffers were measured with a H2, Pt/AgC1, Ag electrode system after their preparation and bottling for use at sea. The measured emf values were found to be in good agreement (4-0.05 mV) with the original measurements of Bates and coworkers from 0 to 45°C. The measured pH of these buffers are in good agreement (-4-0.001 pH units) with the values calculated from the equations of Dickson on the total pH scale based on Bates et al. Studies are underway to access the long term stability of these buffers. We have also used these buffers to calibrate systems used to make potentiometric and spectrophotometric measurements of pH on seawater relative to the H2, Pt/Ag, AgCi electrode from 5 to 45°C.
Reassessment of pH reference values with improved methodology for the evaluation of ionic strength
Analytica Chimica Acta, 2005
The conflict between pH as empirical number in routine control and the pH value regarded as conveying some information concerning the effective concentration or activity of hydrogen ions, a H , has caused much confusion. There are, however, reasons to conclude that the overwhelming amount of thermodynamic data is not sufficiently accurate-either due to ignorance of metrological concepts or due to insufficiently specified measurement processes of fundamental chemical quantities pH. The commonly used seven reference buffer solutions to which primary pH values have been conventional assigned, represent a selection out of a more extensive list, recommended by NBS (now NIST) in 1962. From then onwards conventions concerning the Debye-Hückel model of electrolyte solutions and ionic strength have been revised and the pH(S) values reassessed in conformity but only for these seven reference buffer solutions. The others have, so far remained unchanged, locking harmonisation of the conventionally assigned pH(S) values. In this work, ionic strength is calculated through complete equations derived from the acidity constants. Concentrations of the various species involved in the conventional assignment of pH and their corresponding activity coefficients are therefore, more rigorously known. The process proves particularly useful for poliprotic acids with overlapping acidity constants, where the ratio is less than 10 3. As a consequence, conventionally assigned pH values of reference buffer solutions are recalculated and corrections are introduced as appropriate.
A chemical equilibrium model for natural waters
1998
This paper reviews the present status of the Pitzer chemical equilibrium model, which can be used to characterize the one-atmosphere activity coefficients of ionic and non-ionic solutes in natural waters as a function of temperature and ionic strength. The model considers the ionic interactions of the major seasalt ions (H, Na, K, Mg, Ca, Sr, Cl, Br, OH, HCO 3 , B(OH) 4 , HSO 4 , SO 4 , CO 3 , CO 2 , B(OH) 3 , H 2 O) and is based on the 25 • C model of Weare and co-workers. The model has been extended by a number of workers so that reasonable estimates can be made of the activity coefficients of most of the major seasalt ions from 0 to 250 • C. Recently coefficients for a number of solutes that are needed to determine the dissociation constants of the acids from 0 to 50 • C (H 3 CO 3 , B(OH) 3 , H 2 O, HF, HSO − 4 , H 3 PO 4 , H 2 S, NH + 4 etc.) have been added to the model. These results have been used to examine the carbonate system in natural waters and determine the activity of inorganic anions that can complex trace metals. The activity and osmotic coefficients determined from the model are shown to be in good agreement with measured values in seawater. This model can serve as the foundation for future expansions that can examine the activity coefficient and speciation of trace metals in natural waters. At present this is only possible from 0 to 50 • C over a limited range of ionic strengths (<1.0) due to the limited stability constants for the formation of the metal complexes. The future work needed to extend the Pitzer model to trace metals is discussed.
Journal of Chromatography A, 2008
Ion chromatography (IC) presents new possibilities for assessing information about environmental samples, namely waters of various compositions, ranging from high-purity water to highly saline ones. Constant proportion between major ions present in seawater, has been assumed in the past, from which the first practical equation relating chlorinity and salinity has been developed, being later substituted by a practical salinity scale, derived from conductivity measurements relative to a standard seawater, according to internationally accepted recommended procedures. Seawaters are characterized by salinity values around 35 while derived saline solutions may present considerable changes in ionic composition, conductivity, hence on salinity. Natural and anthropogenic phenomena may introduce new issues requiring clarification for which qualitative and quantitative information from additional sources is useful, e.g. ionic composition from IC. The different ranges of concentration of major and minor species present in seawater and derived saline solutions are a challenge for the optimization of a practical methodology for composition assessment in two single IC runs, one for anions and another one for cations, which has been attained in this work. Composition of saline solutions determined by IC was critically assessed in terms of anion-cation balance and further related to conductivity and salinity measurements aiming to evaluate the quality/completeness of ion chromatographic analyses performed at preselected conditions and to search for other meaningful relations for efficient recognition/distinction between saline solutions of different types.