The pK∗ of TRISH+ in Na-K-Mg-Ca-Cl-SO4 brines—pH scales (original) (raw)
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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.
Use of the Pitzer Equations to Examine the Dissociation of TRIS in NaCl Solutions †
Journal of Chemical & Engineering Data, 2009
The TRIS buffer system is frequently used to calibrate electrodes and indicators that are used to determine the pH of natural waters. Most of the potentiometric and spectrophotometric systems used to measure the pH are calibrated with TRIS buffers. Since most natural waters contain high concentrations of NaCl, it is useful to be able to determine the pK of TRIS using ionic interaction models. In this paper, the dissociation constants of TRIS in NaCl solutions from (0 to 100)°C and (0 to 5) mol · kg -1 have been fitted to Pitzer equations. This allows one to estimate the values of the pH for TRIS buffers for natural brines over a wide range of conditions.
Spectroscopic measurements of the pH in NaCl brines
Geochimica et Cosmochimica Acta, 2009
Spectrophotometric measurements of the pH in natural waters such as seawater have been shown to yield precise results. In this paper, the sulfonephthalein indicator m-cresol purple (mCP, H 2 I) has been used to determine the pH of NaCl brines. The indicator has been calibrated in NaCl solutions from 5 to 45°C and ionic strengths from 0.03 to 5.5 m. The calibrations were made using TRIS buffers (0.03 m, TRIS/TRIS-HCl) with known dissociation constants pK TRIS in NaCl solutions [Foti C., Rigano C. and Sammartano S. (1999) Analysis of thermodynamic data for complex formation: protonation of THAM and fluoride ion at different temperatures and ionic strength. Ann. Chim. 89, 1-12]. The values of pH were determined from pH ¼ pK mCP þ logfðR À e 1 Þ=ðe 2 À Re 3 Þg where R = 578 A/ 434 A, the ratios of the indicator absorbance maximum at 578 and 434 nm, e 1 = 0.00691, e 2 = 2.222 and e 3 = 0.1331 [Clayton T. and Byrne R. H. (1993) Spectrophotometric seawater pH measurements: total hydrogen ion concentration scale calibration of m-cresol purple and at-sea results. Deep-Sea Res. 40, 2115-2129]. Measurements were also made in NaCl solutions with different levels of TRIS (0.01-0.11 m). At low levels of TRIS buffer (<0. , the values of pK mCP increased significantly. This effect can lead to erroneous values of pK mCP at low ionic strengths in estuaries and lakes. The measured values of pK mCP in NaCl as a function of ionic strength (I/m) and temperature (T/K) were fitted to the equation (r = 0.0072) pK mCP ¼ À29:095 þ 2639:2=T þ 5:0417 ln T À 0:3307I 0:5 À 186:80I 0:5 =T À 0:28346I þ 296:44I=T þ 0:12841I 1:5 À 68:23I 1:5 =T These results should be useful in determining the pH of NaCl brines in natural waters from 0 to 50°C.
Marine Chemistry, 2016
The pH on the total proton scale of the Tris-HCl buffer system (pHTris) was characterized rigorously with the electrochemical Harned cell in salinity (S) 35 synthetic seawater and S = 45-100 synthetic seawater-derived brines at 25 and 0°C, as well as at the freezing point of the synthetic solutions (-1.93°C at S = 35 to-6°C at S = 100). The electrochemical characterization of the common equimolal Tris buffer [RTris = mTris/ H Tris m = 1.0, with mTris = H Tris * Tris pK = stoichiometric equilibrium dissociation constant of Tris-H + , equivalent to equimolal pHTris. This consistency allows reliable use of other RTris variants of the Tris-HCl buffer system within the experimental conditions reported here. The results of this study will facilitate the pH measurement in saline and hypersaline systems at below-zero temperatures, such as sea ice brines.
Accreditation and Quality Assurance, 2015
Calibration of pH meters is usually performed with reference pH buffer solutions of low ionic strength, I B 0.1 mol kg-1. For seawater pH measurements (I & 0.7 mol kg-1), calibration buffers in high ionic strength matrix are required. The Harned cell, in association with the Nernst equation and a model for estimating the chloride ion activity coefficient, c Cl À ; is the basis of the primary method for pH assignment to reference pH buffers. The semi-empirical Pitzer model is, in principle, adequate to estimate c Cl À of complex solutions, namely seawater. Nevertheless, no assessment of the validity of the model for this matrix is known to the authors. This work aims at estimating the adequacy of the Pitzer model by assessing the metrological compatibility of mean activity coefficients, in this case c AE ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi c H þ c Cl À p estimated experimentally with the Harned cell, c Exp AE ; and using the Pitzer model, c Ptz AE. The measurement uncertainty considered in the compatibility test was estimated using the bottom-up approach, where components were combined by the numerical Kragten method after checking its adequacy. The compatibility of the estimated c AE was assessed for solutions with increasing complexity and an ionic strength of 0.67 mol kg-1. c Exp AE and c Ptz AE are metrologically compatible for a confidence level of 95 % where the relative standard uncertainty of their difference ranged from 1.1 % to 3.1 % in all chloride solutions to approximately 6.3 % when sodium sulfate was also present. This led to assume the validity of the Pitzer model equations to estimate c Cl À ; required to define reference pH values of buffer solutions with high ionic strength. Keywords Seawater Á pH Á Activity coefficients Á Experimental and theoretical approaches Á Uncertainty Á Compatibility & Bárbara Anes
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.
The effects of pressure on pH of Tris buffer in synthetic seawater
Marine Chemistry, 2017
Equimolar Tris (2-amino-2-hydroxymethyl-propane-1,3-diol) buffer prepared in artificial seawater media is a widely accepted pH standard for oceanographic pH measurements, though its change in pH over pressure is largely unknown. The change in volume (ΔV) of dissociation reactions can be used to estimate the effects of pressure on the dissociation constant of weak acid and bases. The ΔV of Tris in seawater media of salinity 35 (Δ Tris *) was determined between 10 and 30 °C using potentiometry. The potentiometric cell consisted of a modified high pressure tolerant Ion Sensitive Field Effect Transistor pH sensor and a Chloride-Ion Selective Electrode Highlights Pressure dependence of equimolar Tris buffer pH in synthetic seawater was quantified to 200 bar Δ Tris * was quantified between 10 and 30 °C in synthetic seawater Results are in excellent agreement with reported values measured in 0.725 mol kg-1 NaCl solution.
Limnology and Oceanography, 1986
The emf measurements for the TRIS buffer in seawater have been used to define buffer solutions that can be used to determine the pH on a free or total proton scale for estuarine waters. The pH is related to the stoichiometric dissociation constant (K*) of TRISH I-, the concentration of buffer (mTRrs) and salinity (5) by pH = pK* + (aS + bS2)mTRrs where a = -9.73 x 10e5 and b = 6.988 x 10-5. The values of pK* were fit to equations of the form pK*=AIT+B+ClogT where A, B, and C arc functions of salinity and T is the absolute temperature.
Measurement of total alkalinity in hypersaline waters: Values of< i> f< sub> H
1988
Total alkalinity of a water sample can be determined simply and rapidly by measuring the pH change induced by a single addition of acid to that sample. The technique requires a value for an empirical factor, fH, that is related to the total activity coefficient of H ÷ for the particular water sample. [H varies with ionic strength and ionic composition of the water sample. [H was determined for 30 saline lakewater samples ranging in ionic strength from 0.22 to 3.14 M. [H decreased from 0.78 at 0.22 M to 0.65 at 0.46 M and increased to 1.52 at 2.94 M. The trend offH followed the trend of the total activity coefficient of H ÷ predicted for seawater by the specific ion interaction model.
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.