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The experimental determination of water solubility (log S0) and Setschenow coefficient (km) of a compound is a time-consuming activity, which often needs large amounts of expensive substances. This work aims at establishing two “open-source” chemometric models based on a regression tree that is able to predict the two abovementioned quantities. The dataset used is the largest to appear up to now for the collection of km values, containing information on 295 molecules and it is relevant also for the collection of logS0 values (321 molecules); for each of them 32 descriptors were taken from freely available databases. Information about water solubility and Setschenow coefficients, necessary to train the models, were taken from available literature. Validation was performed on a separate test set of molecules. The precision reached in the prediction is fully satisfying, being RMSEP = 0.6086 and 0.0441 for logS0 and km, respectively.

Solution thermodynamics of three aminophosphonic acids are studied in this work. In particular, the protonation constants [ log ( K i H /mol kg−1)] of N-phosphonomethyliminodiacetic acid (NTAP), N,N-Bis(phosphonomethyl)glycine (NTAP2) and Nitrilotris(methylenephosphonic acid) (NTAP3) and the solubility of NTAP were determined by potentiometric measurements (ISE-[H+] glass electrode) in three different ionic media, namely NaCl(aq), (CH3)4NCl(aq) and (C2H5)4NI(aq) at different ionic strengths (in the range 0 log K i H on ionic strength and temperature was studied by Debye-Huckel type equation and by Specific ion Interaction Theory (SIT). At infinite dilution, the refined values of the first protonation constants are: log ( K i H /mol kg−1) = 11.4 ± 0.1, 12.4 ± 0.1 and 13.3 ± 0.1 for NTAP, NTAP2 and NTAP3, respectively and the enthalpy changes relative to the first protonation step are: −15.7 ± 0.3, −20.5 ± 0.4 and –33.5 ± 0.2 kJ mol−1 (same order). The basicity trend: NTAP3 > NTAP2 > NTAP is observed in all the ionic media. The differences in the values of the protonation constants obtained in the three ionic media were also interpreted in terms of formation of weak complex and eleven Na+/L species are obtained using the so called ΔpK method. Solubility investigations performed on NTAP allowed us to determine the Setschenow and the activity coefficients of the neutral species. The values of the total and specific solubility in pure water are: log ( S 0 T /mol kg−1) = −1.431 ± 0.003 and log ( S 0 0 /mol kg−1) = −2.293 ± 0.006, respectively. The formation constants of the three ligands with Ca2+ and Mg2+ cations were determined at T = 298.15 K in NaCl(aq) at I = 0.151 mol kg−1, finding the presence of the ML and MHL species. The stability of the Ca2+/L is slightly higher than that of the Mg2+/L ones.

In this paper, potentiometry and calorimetry were used to determine the thermodynamics of interaction between risedronate and four bivalent metal cations, namely: Mg2+, Zn2+, Pb2+, and Cu2+ in aqueous NaCl solutions at different ionic strengths and at T = 298.15 K. The data analysis allowed us to ascertain that the main species formed were the MLH2, MLH, ML and M2L; however scarcely soluble species precipitated at acidic pH values, between 4 and 7 depending on the metal cation involved, probably due to the formation of the neutral M2L(s) species. Comparison of the stability constants with other similar ligands suggests that metal complexation occurs through the phosphonate with an additional stabilizing contribution due to the pyridine moiety. The dissection of the free energy term revealed that the entropic change is the main contribution to the stability of the complexes for all equilibria, except those regarding the formation of the M2L species, and the main forces involved in these processes are entropic in nature, such as the desolvation of both the ligand and the metal cations. The sequestering ability was evaluated computing the pL0.5 parameter at different pH and ionic strength values; the highest ones were found for Cu2+ in acidic solutions and at low ionic strength, whereas pL0.5 values for both Pb2+ and Zn2+ were slightly lower. Results reported in this work may be helpful in the assessment of the use of risedronate as effective chelating agent and for understanding the mechanism of action of this molecule as a drug.

The speciation of Al3+ in aqueous solutions containing organic and inorganic ligands important from a biological (citrate (Cit3-), gluconate (Gluc-), lactate (Lac-), silicate (H2SiO42-), carbonate (CO32-), fluoride (F-)) and industrial (Gantrez®; polymethyl-vinyl-ether-co-maleic acids; GTZ S95 and GTZ AN169) point of view is reported. The stability constants of Al3+/Lz- complexes (Lz- = ligand with z- charge) were determined by potentiometry at T = 298.15 K and 0.10 ≤ I/M ≤ 1.00 in NaCl(aq) (in NaNO3(aq) only for Al3+/GTZ S95 and Al3+/Gluc- acid systems). For Al3+/Cit3-, Al3+/Lac- and Al3+/GTZ AN1694- systems, the investigations were also carried out at 283.15 ≤ T/K ≤ 318.15. The dependence of the thermodynamic parameters on ionic strength and temperature was modelled with a Debye-Huckel type equation. Different speciation schemes of Al3+/Lz- systems were obtained, including protonated, simple metal-ligand, polynuclear and hydrolytic mixed species. At I → 0 M and T = 298.15 K the stability trend for the AlL(3-z) species is: 14.28 ± 0.02, 13.99 ± 0.03, 10.16 ± 0.03, 3.16 ± 0.08, 2.84 ± 0.10 for GTZ S95, GTZ AN169, Cit3-, Gluc- and Lac-, respectively. From the investigations at different temperatures, it results that the entropic contribution is the driving force of the reactions. The sequestering ability of the ligands towards Al3+ was investigated determining the pL0.5 parameter at different experimental conditions, finding the following trend: Cit3- » Gluc- > GTZ S954- > GTZ AN1694- > Lac- for the organic ligands, and pL0.5: F- » CO32- > H2SiO42- for the inorganic ones.

This work is focused on the determination of the thermodynamic parameters of interaction between a biodegradable complexone, as MGDA (methylglycinediacetic acid), towards the major constituents of natural fluids, such as H+, Na+, K+, Ca2+ and Mg2+. The determination of their thermodynamic parameters of interaction was performed by potentiometric and calorimetric measurements in different experimental conditions, in order to determine the protonation and complex formation constants, together with the relative enthalpy changes of reaction. The protonation data were modeled as a function of ionic strength to obtain data in standard conditions using the EDH (extended Debye-Huckel type equation) and the SIT (Specific ion Interaction Theory) approaches. Ionic strength dependence parameters were obtained, enabling the calculation of the equilibrium constants at any value within the ionic strength range here considered (0 The species determined for the Ca2+ and Mg2+/MGDA (defined below as L) systems, namely the ML, MLH and M2L. For Na+, K+ and (CH3)4N+, the M2L species was not determined, but the neutral MH2L was found for Na+ and K+. The stability trend is: Mg2+ > Ca2+ >> Na+ > K+ > (CH3)4N+. In all cases, the experimental data determined show that the stability of all the complex species decreases with increasing the ionic strength. The studies conducted in this work have shown that the MGDA is an effective chelating agent, with a capacity for mobilization comparable with that of NTA. The values of the enthalpy changes are negative, indicating an exothermic reaction for the first and the second protonation step and for the formation of the CaL− species. On the contrary, the third protonation step and the formation reaction of the MgL− species is endothermic. The entropic contribution to the stability of the species was found to be predominant, except for the first protonation constant. Two “case studies” were conducted to evaluate the possible use of MGDA in real systems, the first representing a water for industrial use and second a urine with presence of precipitated calcium oxalate.

This work is focused on the development of sustainable stone conservation materials, based on BPA-free epoxy-silica hybrid resins, with consolidating and hydrophobic properties, for long-term treatments. For this purpose, a cycloaliphatic diol, with minor health and environmental associated issues with respect to classic phenolic derivatives, was selected to synthesize a BPA-free bio-based epoxy resins precursor, 2,2,4,4-tetramethyl-1,3-cyclobutanediol diglycidylether (CBDO-DGE). Fourier transform infrared (FT-IR) and Raman spectroscopies were employed to assess the synthesis and clean-up procedures. In addition, both 1H-NMR and 13C-NMR in solution were used to ascertain the structure and purity of the bio-based epoxy. The development of the epoxy thermosets by 1,8- diaminooctane (DAO), triethylenetetramine (TETA) and 5-amino-1,3,3-trimethylcyclohexanemethylamine (IPDA) curing agents was followed by attenuated total reflection infrared (ATR FT-IR) and Raman spectroscopies, whereas their suitability as organic counterpart of the hybrids was established by differential scanning calorimetry (DSC) and thermogravimetric (TGA) measurements. In addition, epoxy-silica hybrids were obtained by exploiting sol-gel technology, carrying out the epoxy hardening reactions also in the presence of (3-glycidyloxypropyl)trimethoxysilane (GPTMS) and octyltriethoxysilane (OcTES), as silica-forming additives. To investigate the properties of the resulting hybrid materials imparted by the selection of the different reactants, various blends were studied by a combination of scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM-EDS), ATR FT-IR and Raman analysis. Furthermore, the assessment of the hybrids tunability as materials for stone conservation was ascertained in terms of thermostability and hydrophobicity by TG-DTA, DSC, dynamic mechanical analysis (DMA) and contact angle measurements.

The estimation of thermodynamic parameters of N-Acetyl-L-cysteine (NAC) protonation were determined in NaCl(aq), (CH3)4NCl(aq), (C2H5)4NI(aq), employing various temperature and ionic strengths conditions, by potentiometric measurements. The interaction of NAC with some essential metal cations (e.g., Ca2+, Mg2+ and Zn2+) was investigated as well at 298.15 K in NaCl(aq) in the ionic strength range 0.1 ≤ I/mol dm−3 ≤ 1.0. The values of protonation constants at infinite dilution and at T = 298.15 K are: log K1H = 9.962 ± 0.005 (S H) and log K2H = 3.347 ± 0.008 (COO-H). In the presence of a background electrolyte, both log K1H and log K2H values followed the trend (C2H5)4NI ≥ (CH3)4NCl ≥ NaCl. The differences in the values of protonation constants among the three ionic media were interpreted in terms of variation of activity coefficients and formation of weak complexes. Accordingly, the determination of the stability of 4 species, namely: NaL−, NaHL0(aq), (CH3)4NL−, (CH3)4NHL0(aq) was assessed. In addition, as regards the interactions of Mg2+, Ca2+ and Zn2+ with NAC, the main species where the ML0(aq), ML(OH)−, and ML22−, that were found to be important in the chemical speciation of NAC in real multicomponent solutions. The whole set of the data collected may be crucial for the development of NAC-based materials for natural fluids selective decontamination from heavy metals.

Ethylenediamine-N,N'-disuccinic acid is a biodegradable alternative to EDTA, therefore its use for the sequestration of Ca(2+), Sn(2+), Cu(2+), Zn(2+) and Fe(3+) is analyzed. New data on its binding ability towards these cations were obtained with potentiometric, voltammetric and calorimetric measurements at different ionic strengths and at T = 298.15 K. Real multi-component fluids, namely fresh water, urine, sea water, saliva and blood plasma were chosen as case studies to evaluate the sequestering ability of EDDS in comparison with EDTA. Speciation diagrams were drawn in selected conditions, considering all interactions among the "natural" components of the fluid and those studied in this work, EDDS and EDTA (cL = 1 mmol dm(-3)) as sequestering agents and the cited metal cations (cM ∼ 10(-5) mol dm(-3)). The comparison of the sequestering ability of EDDS and EDTA is done using pM and pL0.5. In blood plasma the plasma mobilizing index was adopted. It was found that EDDS is a good alternative to EDTA, which tends to bind Ca(2+) and Mg(2+) more than EDDS. In particular, EDTA cannot be used as a sequestrant for Sn(2+) when cCa > cEDTA. EDDS is more efficient than EDTA at pH < 8, particularly in urine, where carbonate is absent. In sea water, the sequestering ability of EDDS towards Fe(3+) is higher than that of EDTA. In blood plasma, the PMI of EDDS towards Cu(2+) is higher than that of EDTA. Thermodynamic information, in terms of ΔH and ΔS, for the protonation and metal complex formation reactions are reported.

Protonation constants of 2-aminobenzoic acid, (anthranilic acid, vitamin L1) (1), 4-aminobenzoic acid (PABA) (2), and 2-aminoterephthalic acid (3) have been obtained from potentiometric measurements at 25 °C (298.15 K) and ionic strengths between 0 and 5 mol ⋅ dm − 3 NaCl and between 0 and 3 mol ⋅ dm − 3 (CH 3 ) 4 NCl. Values for the step-wise protonation constants log K 1 and log K 2 , extrapolated to zero ionic strength and infinite dilution, were determined as follows. For 2-aminobenzoic acid: 4.958 ± 0.003 (error estimate at the 95% confidence interval) and 2.060 ± 0.007; for 4-aminobenzoic acid: 4.868 ± 0.008 and 2.420 ± 0.004; for 2-aminoterephthalic acid: 5.089 ± 0.004 and 3.594 ± 0.006. Differences between protonation constants obtained using various concentrations of NaCl (aq) and (CH 3 ) 4 NCl (aq) for ionic strength adjustment are interpreted in terms of formation of weak complexes between the ligands and the ions of the background electrolyte. Values of log K for the formation of 1:1 complexes of the aminobenzoate anion with the tetramethylammonium cation have been estimated. The dependence of the solubility was studied as a function of NaCl (aq) and (CH 3 ) 4 NCl (aq) concentration. Similar behaviour was found for the three ligands. The total solubility increases with increasing NaCl (aq) concentration, whereas the opposite trend is observed in (CH 3 ) 4 NCl (aq) . Total and specific solubility was also determined, from which Setschenow coefficients and activity coefficients were deduced for the various species. Activity coefficient variation was examined in terms of both a Debye–Huckel type equation (DHt) and a SIT (Specific ion Interaction Theory) model.

In this paper, a study, at T = 298.15 K, on the thermodynamic properties and the complexing ability towards Zn 2 + of 2-mercaptopyridine-N-oxide (pyrithione, PT) and 2-hydroxypyridine-N-oxide (HPNO) is reported. For this purpose, different analytical techniques were used, potentiometry, voltammetry, atomic absorption and UV–visible spectroscopy. The equilibrium constants of the H + /HPNO and Zn 2 + /HPNO systems, together with the solubility of the Zn(HPNO) 2 species were determined in NaNO 3 aqueous solutions at different ionic strengths (0 I /mol·dm − 3 ≤ 3.2). For pyrithione, since reliable protonation constants have been recently published, the study was focused on its complexing ability towards Zn 2 + , and to the determination of the total and intrinsic solubility of the neutral Zn(PT) 2 species in NaCl at different ionic strengths (0 I /mol·dm − 3 ≤ 3.1). The speciation schemes of the two investigated Zn 2 + /L systems are similar, and consist of the ZnL + and ZnL 2 species. Generally, the formation constant values of the Zn 2 + /PT species are higher than those of the Zn 2 + /HPNO ones, for example, the stability of the ZnL + species is log K = 5.07 and 6.26 (at I = 0.1 mol·dm − 3 ) for HPNO and PT, respectively. At infinite dilution, the total solubility of the ZnL 2 species is: log S 0 T = − 4.291 ± 0.007 and − 1.241 ± 0.004 for the Zn(PT) 2 and Zn(HPNO) 2 , respectively. For these two species, the solubility product, K S0 , was also determined. The dependence on ionic strength of the equilibrium constants was modeled by the extended Debye–Huckel type equation (EDH) and the Specific ion Interaction Theory (SIT). Using this last model, the activity coefficients of all the species involved in the protonation and complex formation reactions were calculated. Furthermore, the formation of the mixed complex Zn(PT)(HPNO) was evidenced by means of spectrophotometric titrations, with a stability constant log β = 11.36 ± 0.06. In the case of pyrithione, the speciation study was performed also in a multicomponent solution simulating the composition of soaps.

In this paper, new data are reported on the protonation of phytate at T = 298.15 K in different ionic media and ionic strengths, namely NH4Cl(aq) (0.1 ⩽ I/mol · kg−1 ⩽ 1.9) and NaNO3(aq) (0.1 ⩽ I/mol · kg−1 ⩽ 5.4). A complete set of phytate protonation constants, with general formula HiPhy (with 1 ⩽ i ⩽ 7) was proposed. The data were modeled by a logarithmic empiric equation, as well as an Extended Debye–Huckel (EDH) and specific ion interaction theory (SIT) models. A strong similarity was found between protonation data in NH4Cl(aq), NaNO3(aq) and those previously reported in NaCl(aq), KCl(aq) and KNO3(aq), so that an unique set of thermodynamic protonation data has been proposed for phytate protonation at I ⩽ 0.15 mol · kg−1. Thermodynamic parameters obtained in these conditions can be useful to model the behavior of phytate in real aqueous systems at low ionic strengths. An empirical relationship has also been proposed for all the protonation data at 0 < I/mol · kg−1 ⩽ 0.15: TΔS (±1.5) = 11.7 − 0.72 · ΔG. Using literature data for the phytate protonation in (C2H5)4NI(aq), the weak complexes between Na+ or NH4+ and phytic acid have been calculated by the ΔpK method (i.e., by the comparison of the protonation constants determined in an interacting medium, such as NH4Cl(aq), and those obtained in a non-interacting supporting electrolytes, like tetraalkylammonium salts). In terms of weak complex formation constants, the results are similar for NaNO3(aq) and NH4Cl(aq), and twelve HiMjPhy species are reported together with their stability. For example, for the formation of the M6Phy species, it is: log β = 25.9 and 25.6 (±0.2) for M = Na+ and NH4+, respectively, at I = 0.1 mol · dm−3. This paper represents an advance in the understanding of the acid-base behavior of phytic acid in a wide number of ionic media at different ionic strengths.

This paper reports the results of a study on the interactions of tin(II) with different low molecular weight (poly)carboxylates (acetate (ac), malonate (mal), succinate (suc), adipate (adip), 1,2,3,-propanetricarboxylate (tca), 1,2,3,4-butanetetracarboxylate (btc), mellitate (mlt)), polyamines (putrescine (ptr), cadaverine (cdv), spermidine (spd), spermine (sper)), amino acids (glycinate (gly), aspartate (asp), glutamate (glu)), and S-donor ligands (thiolactate (tla), thiomalate (tma), penicilamine (pen)), at T = 298.15 K and I = 0.15 mol L− 1 in NaNO3(aq). Depending on the ligand, different MpLqHr species are formed (including the mixed ML(OH)), and the corresponding stability constants are reported. In the case of the Sn2 +-ac, Sn2 +-adip, Sn2 +-mlt and Sn2 +-asp systems, measurements at different ionic strengths (0.15 ≤ I/mol L− 1 ≤ 1.0) in NaNO3(aq) were also carried out, while experiments at different temperatures (298.15 ≤ T/K ≤ 313.15) have been performed only in the case of the Sn2 +-adip system, in order to evaluate and model the dependence of the stability of tin(II)/ligand species on these parameters. The sequestering ability of the investigated ligands toward tin(II) has been also evaluated by calculating various pL0.5 values (i.e., the total ligand concentration necessary to bind 50% of cation present in trace), a semiempirical parameter already proposed for the quantification of this ability in different conditions. Some empirical relationships are also presented, for a rough but immediate estimation of the stability of various tin(II)/ligand species as a function of the nature and number of the functional groups of different ligands.

The acid base properties of 1,2-dimethyl-3-hydroxypyridin-4-one (also known as deferiprone, def, figure 1 ), together with the solubility and the distribution ratio have been studied potentiometrically at different temperatures and ionic strengths in NaCl, KCl and in (CH3)4NCl aqueous solutions. The total solubility of deferiprone is fairly high (0.100 mol · dm−3 in pure water) and decreases with increasing salt concentration (salting out effect); this behaviour is greater in NaCl than in (CH3)4NCl aqueous solutions. From the analysis of the solubility and the distribution measurements it was possible to determine the Setschenow and the activity coefficients of the neutral species. Deferiprone shows two protonation steps, whose protonation constants are log K 1 H = 10.088 and log K 2 H = 3.656 at infinite dilution and T = 298.15 K. The ionic strength dependence of the protonation constants was interpreted both in terms of variation of the activity coefficients, using the Debye–Huckel, the SIT (Specific ion Interaction Theory) and the Pitzer approaches, or considering the formation of weak species with the ions of the supporting electrolyte (e.g. Na+, K+ and Cl−). Moreover, temperature gradients were provided for the two protonation constants. The stepwise protonation enthalpy values are negative in all cases (e.g. ΔH1 = −19.2 kJ · mol−1 and ΔH2 = −13.8 kJ · mol−1 at infinite dilution and T = 298.15 K) and become more negative increasing both temperature and ionic strength. It was observed that the proton binding process is mainly entropic in nature for the first protonation step and enthalpic for the second. The results are in good agreement with literature data.

The 1-butyl-3-methylimidazolium tetrafluoroborate, [bmim]BF 4 , is an ionic liquid when pure. The acid base properties of aqueous solutions of [bmim]BF 4 were examined by potentiometric titration. Then, protonation constants of glycine in binary mixtures of water and [bmim]BF 4 were determined, from potentiometric titration data obtained at T = 298.2 K and various ionic strengths of sodium chloride (0.25 ⩽ I ⩽ 1.00 mol · dm −3 ). The ionic strength dependence of the equilibrium constants of glycine was fitted to an extended Debye–Huckel equation to derive ionic strength dependence parameters. Comparison with literature data showed that, even at high concentrations, [bmim]BF 4 is similar to the electrolyte [(C 2 H 5 ) 4 NI], in that it does not interact significantly with other solutes. Solvatochromic parameters in the binary mixtures of water and [bmim]BF 4 have been calculated by using Redlich–Kister equation. Kamlet–Abboud–Taft (KAT) solvatochromic equation allows us to determine the values of calculated protonation constants in different media.

An extensive study of the tin(II)/phytate (Phy) system was carried out in NaNO 3(aq) , at different ionic strengths (0.10 ⩽ I /mol · L −1 ⩽ 1.00) and temperatures (278.15 ⩽ T /K ⩽ 328.15), by potentiometric and voltammetric techniques. The stability and formation enthalpy changes of six SnH q Phy species were determined. To better characterise this system, some potentiometric titrations were also carried out in mixed ionic media (NaNO 3(aq) + NaCl (aq) and NaNO 3(aq) + NaF (aq) ) at total ionic strength I = 1.00 mol · L −1 . The formation of some ternary mixed SnH q PhyCl and SnH q PhyF species (charges omitted for simplicity) was found. The formation enthalpies of the complex species were calculated, at I = 0.40 mol · L −1 in NaNO 3(aq) , by the dependence of stability constants on temperature obtained by potentiometric titrations, in the range 278.15 ⩽ T /K ⩽ 328.15. The complex formation process is endothermic, and the main contribution to tin(II) complexation by phytate is entropic in nature. For example, for the SnPhy species we have, at T = 298.15 K and I = 0.40 mol · L −1 in NaNO 3(aq) : Δ H = 57.7 ± 2.8 kJ mol · L −1 , Δ G = −99.9 ± 1.7 kJ mol · L −1 , and T Δ S = 158 ± 3 kJ mol · L −1 . The ionic strength dependence of the formation constants of the simple tin(II)/phytate species, was modelled by the Debye–Huckel and the SIT approaches. The sequestering ability of phytate towards tin(II) was evaluated by calculating the pL 0.5 values (i.e., the total ligand concentration necessary to bind 50% of cation present in trace) at different ionic strengths, ionic media, and pH. The sequestering ability increases with increasing the pH, whilst it decreases with increasing the ionic strength (the same behaviour shown by the stability constants). Moreover, taking into account the different sequestering ability of phytate towards tin(II) in the different ionic media, the trend: pL 0.5 = 5.70 (in NaNO 3(aq) + NaF (aq) ) > pL 0.5 = 5.16 (in NaNO 3(aq) + NaCl (aq) ) > pL 0.5 = 4.86 (in NaNO 3(aq) ) was observed at pH 8.1 and I = 1.00 mol · L −1 . This is due to the presence of a second ligand (Cl − or F − ) that stabilizes the complex species with the formation of ternary complex species. Some empirical relationships were also found.

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, H2I) 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 pKTRIS 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 = pK m CP + log { ( R - e 1 ) / ( e 2 - Re 3 ) } where R = 578A/434A, the ratios of the indicator absorbance maximum at 578 and 434 nm, e1 = 0.00691, e2 = 2.222 and e3 = 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 ( pK m CP = - 29.095 + 2639.2 / T + 5.0417 ln T - 0.3307 I 0.5 - 186.80 I 0.5 / T - 0.28346 I + 296.44 I / T + 0.12841 I 1.5 - 68.23 I 1.5 / T These results should be useful in determining the pH of NaCl brines in natural waters from 0 to 50 °C.