Entire Ionic Strength Range Research Articles

Influence of Humic Acid on the Transport of Two Types of Synthesized Zinc Oxide Nanoparticles in Quartz Sand

In this study, transport and retention behaviors of the two types of nZnO prepared with separate manufacturing methods were compared/analyzed according to the presence/absence of Suwannee River humic acid (SRHA) adsorbed into the sand surface and the SRHA suspended in bulk solution, and to changes in the solution ionic strength (0.1–10 mM) in sand-repacked water-saturated columns. In the absence of suspended SRHA, nZnO-1 breakthrough was observed only in SRHA-coated soil, and the breakthrough amount decreased with the increase in the ionic strength (23.8% to 17.2% at 0.1 mM to 10 mM, respectively). In contrast, nZnO-2 breakthrough was not observed over the entire ionic strength range, regardless of the SRHA sand coating. With the presence of suspended SRHA, neither nZnO-1 nor nZnO-2 showed a significant difference in the breakthrough amount regardless of sand coating or ionic strength. However, the breakthrough amount of nZnO-1 was higher than that of nZnO-2 (51.5% versus 37.7% at 10 mM with 1 mg/L SRHA). From confirming the difference in transport between the two types of nZnO, the amount of SRHA adsorbed into nZnO-1 was less than the amount adsorbed into nZnO-2 (0.29 mg/g versus 0.64 mg/g at 10 mM with 1 mg/L SRHA). This result was considered to be due to the larger nZnO-1 breakthrough amount than the nZnO-2 breakthrough amount, which was caused by the larger amount of suspended SRHA that could occupy the deposition sites in the nZnO-1 suspension. Because the rate of SRHA deposition on the sand surface was higher than on nZnO, nZnO transport improved in the presence of suspended SRHA, and changed according to the amount of suspended SRHA.

Understanding Mono- and Bivalent Ion Selectivities of Nanoporous Graphene Using Ionic and Bi-ionic Potentials.

Nanoporous graphene displays salt-dependent ion permeation. In this work, we investigate the differences in Donnan potentials arising between reservoirs, separated by a perforated graphene membrane, containing different cations. We compare the case of monovalent cations interacting with nanoporous graphene with the case of bivalent cations. This is accomplished through both measurements of membrane potential arising between two salt reservoirs at different concentrations involving a single cation (ionic potential) and between two reservoirs containing different cations at the same concentration (bi-ionic potential). In our present study, Donnan dialysis experiments involve bivalent MgCl2, CaCl2, and CuCl2 as well as monovalent KCl and NH4Cl salts. For all salts, except CuCl2, clear Donnan and diffusion potential plateaus were observed at low and high salt concentrations, respectively. Our observations show that the membrane potential scaled to the Nernst potential for bivalent cations has a lower value (≈50%) than for monovalent cations (≈72%) in the Donnan exclusion regime. This is likely due to the adsorption of these bivalent cations on monolayer graphene. For bivalent cations, the diffusion regime is reached at a lower ionic strength compared to the monovalent cations. For Mg2+ and Ca2+, the membrane potential does not seem to depend upon the type of ions in the entire ionic strength range. A similar behavior is observed for the KCl and NH4Cl membrane potential curves. For CuCl2, the membrane potential curve is shifted toward lower ionic strength compared to the other two bivalent salts and the Donnan plateau is not observed at the lowest ionic strength. Bi-ionic potential measurements give further insight into the strength of specific interactions, allowing for the estimation of the relative ionic selectivities of different cations based on comparing their bi-ionic potentials. This effect of possible ion adsorption on graphene can be removed through ion exchange with monovalent salts.

Bioenergetic Constraints on Microbial Hydrogen Utilization in Precambrian Deep Crustal Fracture Fluids

ABSTRACTPrecambrian Shield rocks host the oldest fracture fluids on Earth, with residence times up to a billion years or more. Water–rock reactions in these fracture systems over geological time have produced highly saline fluids, which can contain millimolar concentrations of H2. Mixing of these ancient Precambrian fluids with meteoric or palaeo-meteoric water can occur through tectonic fracturing, providing microbial inocula and redox couples to fuel blooms of subsurface growth. Here, we present geochemical and microbiological data from a series of borehole fluids of varying ionic strength (0.6–6.4 M) from the Thompson Mine (Manitoba) within the Canadian Precambrian Shield. Thermodynamic calculations demonstrate sufficient energy for H2-based catabolic reactions across the entire range of ionic strengths during mixing of high ionic strength fracture fluids with meteoric water, although microbial H2 consumption and cultivable H2-utilizing microbes were only detected in fluids of ≤1.9 M ionic strength. This pattern of microbial H2 utilization can be explained by the higher potential bioenergetic cost of organic osmolyte synthesis at increasing ionic strengths. We propose that further research into the bioenergetics of osmolyte regulation in halophiles is warranted to better constrain the habitability zones of hydrogenotrophic ecosystems in both terrestrial subsurface, including potential future radioactive waste disposal sites, and other planetary body crustal environments, including Mars.

Improved thermodynamic model for interaction of EDTA with trivalent actinides and lanthanide to ionic strength of 6.60 m

The dissociation constants of ethylenediaminetetraacetic acid (H4EDTA), and the stability constants of Am3+, Cm3+and Eu3+ with EDTA4− have been determined at 25°C, over a range of concentration varying from 0.1 to 6.60m NaClO4 using potentiometric titration and an extraction technique, respectively. The formation of only 1:1 complex, M(EDTA)−, where (M=Am3+, Cm3+ and Eu3+), was observed under the experimental conditions. The observed ionic strength dependencies of the dissociation constants and the stability constants have been described successfully over the entire ionic strength range using the Pitzer model. The thermodynamic stability constant: logβ1010=20.55±0.18 for Am3+, logβ1010=20.43±0.20 for Cm3+ and logβ1010=20.65±0.19 for Eu3+ were calculated by extrapolation of data to zero ionic strength in an NaClO4 medium. In addition, logβ1010 of 20.05±0.40 for Am3+ was obtained by simultaneously modeling data both in NaCl and NaClO4 media. For all stability constants, the Pitzer model gives an excellent representation of the data using interaction parameters β(0), β(1), and Cϕ determined in this work. The improved model presented in this work would enable researchers to model accurately the potential mobility of actinides (III) and light rare earth elements to ionic strength of 6.60m in low temperature environments in the presence of EDTA.

Protein adsorption to poly(ethylenimine)-modified Sepharose FF: II. Effect of ionic strength

In Part I of this work, we have studied the effect of ionic capacity (IC) on bovine serum albumin (BSA) adsorption equilibria and kinetics to poly(ethylenimine) (PEI)-grafted Sepharose FF, and found a critical IC (cIC, 600mmol/L), above which both protein capacity and uptake rate increased drastically. In this work, five PEI-Sepharose FF resins of typical ICs reported earlier were selected to explore the effect of ionic strength (IS) on the adsorption equilibria and kinetics of BSA. Commercially available DEAE (IC=160mmol/L) and Q Sepharose FF (IC=269mmol/L) resins were used for comparisons. It is found that at similar ionic capacities, protein adsorption capacities on both the PEI-Sepharose FF resins and the commercial resins decreased with increasing IS, but on the capacity sensitivity to salt concentration, the former was lower than the latter. In addition, the effective diffusivities (De) of the former were smaller than the latter in the entire IS range studied. The low IS sensitivity of adsorption capacity of the PEI-Sepharose FF resins could be interpreted by the increase of pore accessibility with increasing IS; the smaller De values in the PEI-Sepharose FF resins were considered due to the lack of surface diffusion in the PEI-Sepharose FF resins of low PEI densities. For the PEI-Sepharose FF resins of high ICs (520, 740 and 1220mmol/L), both protein capacity and De values increased first and then decreased with increasing IS. The increasing trend of protein capacity in the low IS range was considered due to the increase of accessible pores for BSA. The rise–fall trend of De was attributed to the dependencies of the “chain delivery” effect on protein capacity and binding strength, both of which are related to IS. Moreover, the IS sensitivity of the De for the resins of ICs>cIC (740 and 1220mmol/L) was much higher than those of ICs<cIC, further proving that the “chain delivery” effect in PEI layer did contribute significantly to the overall mass transfer at IC>cIC. Furthermore, the two PEI-Sepharose FF resins of ICs>cIC kept high adsorption capacities and De values up to 200–300mmol/L NaCl. Therefore, the operating IS ranges for these two PEI-Sepharose FF resins can be much broader than the traditional ion-exchange media.

Buffers for the Physiological pH Range: Studies of 4-(N-Morpholino)butanesulfonic Acid (MOBS) from 5 to 55 °C

In this study, we report the pH values of two buffer solutions without chloride ion and eight buffer solutions with NaCl with an ionic strength I=0.16 mol⋅kg−1. Electromotive force (emf) techniques have been used to get the cell potentials at 12 temperatures from 5 to 55 °C, including 37 °C. An extended form of the Bates-Guggenheim convention is used in the entire ionic strength range, 0.04 to 0.16 mol⋅kg−1. The residual liquid junction potentials (δEj) of the buffer solutions of MOBS have been estimated from previous measurements with a flowing junction cell. These values of δEj have been used for correction in order to ascertain the operational pH values of four buffer solutions of MOBS at 25 and 37 °C. These solutions are recommended as pH standards for physiological application in the pH range 7.4 to 7.7.

Influence of Extracellular Polymeric Substances (EPS) on Deposition Kinetics of Bacteria

The significance of extracellular polymer substances (EPS) on cell deposition on silica surfaces was examined by direct comparison of the deposition kinetics of untreated "intact" bacteria versus those from the same strain but with EPS removal via cation exchange resin (CER) treatment using a quartz crystal microbalance with dissipation (QCM-D). Four bacterial strains, mutant Escherichia coli BL21 (gram-negative, nonmotile), Pseudomonas sp QG6 (gram-negative, motile), Rhodococcus sp QL2 (gram-positive, nonmotile), and Bacillus subtilis (gram-positive, motile), were employed to determine the influence of EPS on cell deposition. Experiments were conducted in both monovalent (NaCl) and divalent (CaCl2) solutions under a variety of environmentally relevant ionic strength ranging from 1 to 100 mM at pH 6.0. The effectiveness of EPS removal via CER method was ensured by biochemical composition analysis of EPS solutions and further confirmed by FTIR analysis. Comparable zeta potentials were observed for untreated and CER treated bacterial cells in both NaCl and CaCl2 solutions, indicating that removal of EPS from cell surfaces via CER treatment did not affect the electrokinetic properties of the cell surfaces for all four strains. However, observed deposition efficiencies (alpha) were greater for untreated cells relative to those with CER treated cells across the entire ionic strength range examined in both NaCI and CaCl2 solutions for all four bacterial strains. These results strongly demonstrated that the removal of EPS from cell surfaces for all four strains decreased the deposition of bacteria on silica surfaces. This study clearly showed that the enhancement of cell deposition on silica surfaces due to the presence of EPS on cell surfaces was relevant to all bacterial strains examined regardless of cell types and motility.

Role of a Conserved Salt Bridge between the PAS Core and the N-Terminal Domain in the Activation of the Photoreceptor Photoactive Yellow Protein

The effect of ionic strength on the conformational equilibrium between the I 2 intermediate and the signaling state I ′ 2 of the photoreceptor PYP and on the rate of recovery to the dark state were investigated by time-resolved absorption and fluorescence spectroscopy. With increasing salt concentration up to ∼600 mM, the recovery rate k 3 decreases and the I 2 / I ′ 2 equilibrium (K) shifts in the direction of I ′ 2 . At higher ionic strength both effects reverse. Experiments with mono-(KCl, NaBr) and divalent (MgCl 2, MgSO 4) salts show that the low salt effect depends on the ionic strength and not on the cation or anion species. These observations can be described over the entire ionic strength range by considering the activity coefficients of an interdomain salt bridge. At low ionic strength the activity coefficient decreases due to counterion screening whereas at high ionic strength binding of water by the salt leads to an increase in the activity coefficient. From the initial slopes of the plots of log k 3 and log K versus the square root of the ionic strength, the product of the charges of the interacting groups was found to be −1.3 ± 0.2, suggesting a monovalent ion pair. The conserved salt bridge K110/E12 connecting the β-sheet of the PAS core and the N-terminal domain is a prime candidate for this ion pair. To test this hypothesis, the mutants K110A and E12A were prepared. In K110A the salt dependence of the I 2 / I ′ 2 equilibrium was eliminated and of the recovery rate was greatly reduced below ∼600 mM. Moreover, at low salt the recovery rate was six times slower than in wild-type. In E12A significant salt dependence remained, which is attributed to the formation of a novel salt bridge between K110 and E9. At high salt reversal occurs in both mutants suggesting that salting out stabilizes the more compact I 2 structure. However, chaotropic anions like SCN shift the I 2 / I ′ 2 equilibrium toward the partially unfolded I ′ 2 form. The salt linkage K110/E12 stabilizes the photoreceptor in the inactive state in the dark and is broken in the light-induced formation of the signaling state, allowing the N-terminal domain to detach from the β-scaffold PAS core.

CryptosporidiumOocyst Surface Macromolecules Significantly Hinder Oocyst Attachment

The role Cryptosporidium parvum oocyst surface macromolecules play in controlling oocyst adhesion (deposition) kinetics to quartz surfaces has been investigated utilizing a radial stagnation point flow system. Deposition kinetics and corresponding attachment efficiencies of viable oocysts were compared with those after treatment with a digestive enzyme (proteinase K) to cleave these surface macromolecules. Low deposition rates were observed with viable oocysts over the entire range of ionic strengths (KCl) investigated, even at ionic strengths as high as 100 mM where the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of colloidal stability predicts the absence of an electrostatic energy barrier. "Electrosteric" repulsion between the oocyst surface macromolecules and the quartz surface is surmised to cause these low deposition rates and attachment efficiencies. However, after removal of these surface macromolecules by the digestive enzyme, increased attachment efficiencies were observed over the entire range of ionic strengths. This significant increase in the deposition kinetics was seen despite the oocysts having a more negative zeta potential following the removal of the surface macromolecules. After treatment with proteinase K, the oocysts no longer experienced electrosteric repulsive forces, and their deposition kinetics followed the general behavior predicted by DLVO theory.

Activity coefficients in (hydrogen chloride + holmium chloride) (aq) from T = (278.15 to 328.15) K

Activity coefficients of HCl in (hydrogen chloride + holmium chloride) (aq) have been calculated from the observed e.m.f.s using the Nernst equation. The temperatures ranged from (278.15 to 328.15) K at 5 K intervals and at constant total ionic strengths of (0.01, 0.025, 0.05, 0.1, 0.25, 0.5, 1.0, and 1.5) mol · kg −1. Electromotive-force measurements were made on the cell without liquid junction of the type: Pt | H 2 ( g , p = 101 . 325 kPa ) | HCl ( m A ) , HoCl 3 ( m B ) | AgCl ( s ) , | Ag ( s ) The results of the activity coefficients of HCl for this mixed electrolyte mixture have been interpreted in terms of the simpler Harned’s equations and the ion-interaction model of Pitzer. Results show that the quadratic term is sufficient for the full range of Y B (the ionic strength fraction of the salt) to 0.9 at all the ionic strengths studied. The Pitzer’s mixing parameters S θ H , Ho and Ψ H,Ho,Cl (including higher order electrostatic effects) and θ H,Ho and Ψ H,Ho,Cl (excluding higher order electrostatic effects) have been determined. These values at T = 298.15 K are: S θ H , Ho = 0 . 115 , Ψ H,Ho,Cl = −.071; and θ H,Ho = −.663, Ψ H,Ho,Cl = 0.165. The parameters obtained in this study reproduce the activity coefficients of HCl in the mixtures within 0.015 over the entire range of ionic strengths and within 0.009 for I ⩾ 0.05 mol · kg −1 over the entire temperature range.

Role of Surface Proteins in the Deposition Kinetics ofCryptosporidium parvumOocysts

A radial stagnation point flow system was used to investigate the influence of Cryptosporidium parvum surface properties on oocyst deposition kinetics onto solid surfaces. To determine the role of oocyst surface proteins in adhesion, the deposition kinetics of viable oocysts were compared with the deposition kinetics of oocysts treated (inactivated) with either heat or formalin. Results showed a significantly higher deposition rate with formalin and heat-treated oocysts compared to viable oocysts under identical solution ionic strengths. Low deposition rates and corresponding attachment efficiencies were observed with viable oocysts over the entire range of solution conditions investigated, even at high ionic strengths where DLVO theory predicts the absence of an electrostatic energy barrier. An "electrosteric" repulsion between the viable Cryptosporidium oocyst and the quartz substrate, attributed to proteins on the oocyst surface, is surmised to cause this low deposition rate. Inactivation of the oocysts with either formalin or heat resulted in increased attachment efficiencies over the entire range of ionic strengths examined. It is hypothesized that formalin and heat treatments alter the structure of surface proteins and thus reduce steric repulsion. Formalin treatment was also found to impart an increased hydrophobicity to the oocyst surface and thus greater enhancement in oocyst deposition kinetics compared to heat treatment.

Adhesion Kinetics of ViableCryptosporidium parvumOocysts to Quartz Surfaces

The transport and deposition (adhesion) kinetics of viable Cryptosporidium parvum oocysts onto ultrapure quartz surfaces in a radial stagnation point flow system were investigated. Utilizing an optical microscope and an image-capturing device enabled real time observation of oocyst deposition behavior onto the quartz surface in solutions containing either monovalent (KCl) or divalent (CaCl2) salts. Results showed a significantly lower oocyst deposition rate in the presence of a monovalent salt compared to a divalent salt. With a monovalent salt, oocyst deposition rates and corresponding attachment efficiencies were relatively low, even at high KCl concentrations where Derjaguin-Landau-Verwey-Overbeek (DLVO) theory predicts the absence of an electrostatic energy barrier. On the other hand, in the presence of a divalent salt, oocyst deposition rates increased continuously as the salt concentration was increased over the entire range of ionic strengths investigated. The unusually low deposition rate in a monovalent salt solution is attributed to "electrosteric" repulsion between the Cryptosporidium oocyst and the quartz surface, most likely due to proteins on the oocyst surface that extend into the solution. It is further proposed that specific binding of calcium ions to the oocyst surface functional groups results in charge neutralization and conformational changes of surface proteins that significantly reduce electrosteric repulsion.

Activity Coefficients of (Hydrogen Chloride + Europium Chloride) (aq) Using Harned's Rule and the Pitzer Formalism

Activity coefficients of HCl in (hydrogen chloride + europium chloride) have been determined at temperatures ranging from 278.15 K to 328.15 K at 5 K intervals and at eight constant total ionic strengths of (0.025, 0.05, 0.1, 0.25, 0.5, 1.0, 1.5, and 2.0) mol kg-1. Experimental cell potentials were obtained using a cell without liquid junction of the type Pt|H2(g, p = 101.325 kPa)|HCl (mA), EuCl3 (mB)|AgCl(s)|Ag(s). The treatment of Harned has been used to describe the mean molal activity coefficient of hydrochloric acid as a function of ionic strength in acid salt mixtures. Results indicate that the quadratic form is sufficient for the full range of YB (the ionic strength fraction of the salt) to 0.9 at all the experimental ionic strengths. The ion-interaction model of Pitzer has also been used to calculate Pitzer's mixing parameters ϑH,Eu and ΨH,Cl,Eu for the system H+ + Eu3+ + Cl- + H2O. The values found, ϑH,Eu = 0.2 ± 0.03 and ΨH,Cl,Eu = 0, were similar to those for other trivalent ions studied previously. The parameters used in this study reproduce the activity coefficients of HCl in the mixtures to within 0.016 over the entire range of ionic strengths and within 0.009 for I > 0.05 mol kg-1 over the entire temperature range.

Charge Reversal Mutations in a Conserved Acidic Patch inAnabaenaFerredoxin Can Attenuate or Enhance Electron Transfer to Ferredoxin:NADP+Reductase by Altering Protein/Protein Orientation within the Intermediate Complex

A series of charge reversal mutations in a highly conserved acidic patch on the surface ofAnabaenaferredoxin (Fd), comprising residues D67, D68, and D69, have been constructed by site-directed mutagenesis. One such mutant, D68K, has a rate constant for electron transfer (et) toAnabaenaferredoxin:NADP+reductase (FNR) at low ionic strength (I= 12 mm) which is 2.5 times larger than wild type (9000 vs 3600 s−1). This mutant Fd became indistinguishable from the wild-type protein in its reactivity atI≥ 100 mm. The other mutants showed various degrees of impairment in their et reactions with FNR over the entire range of ionic strengths. The degrees of such impairment for the D67K and D69K mutants were similar to that of the double mutant D67K/D69K. The double mutant D68K/D69K had et activity intermediate between these mutants and wild type, whereas incorporation of the “super” mutation, D68K, into the double mutant, resulting in the D67K/D68K/D69K triple mutant, did not significantly alter the impairment caused by the D67K/D69K double mutation. Binding constants for complex formation (Kd) between the oxidized mutant proteins and oxidized FNR (except for that of the triple mutant which was not measurable), and the kinetically determinedKdvalues for the intermediate Fdred:FNRoxcomplex, showed no correlation with et rate constants or with the extent of charge reversal. These results indicate that hydrophobic interactions play a key role in determining complex stability. They also provide strong support for the contention that the specific protein/protein geometry within the Fdred:FNRoxintermediate complex is the major determinant of the et rate constants in this series of mutants, and that this is optimized largely by hydrophobic rather than electrostatic interactions. When electrostatic forces are dominant, as they are at low ionic strength, this can lead to nonoptimal et orientations.

Internal architecture of the core nucleosome: fluorescence energy transfer studies at methionine-84 of histone H4.

Chicken histone H4, labeled separately at Met-84 with N-[[(iodoacetyl)amino]ethyl]-5-naphthylamine-1-sulfonic acid and 5-(iodoacetamido)fluorescein, was reassociated with unlabeled histones H2A, H2B, and H3 and 146 base pairs of DNA to produce fluorescently labeled nucleosomes having physical characteristics virtually the same as those of native core particles. Four types of particles were prepared containing respectively unlabeled H4, dansylated H4, fluoresceinated H4, and a mixture of the two labeled H4 molecules. Quantitative singlet-singlet energy-transfer measurements were carried out to determine changes in the distance between the two Met-84 H4 sites within the same nucleosome following conformational transitions which we have reported earlier. In the ionic strength range 0.1-100 mM NaCl, the distance between these sites is less than 2 nm except at 1 mM. Between 100 and 600 mM monovalent salt the distance separating the donor and acceptor fluors at Met-84 H4 increases to 3.8 nm. The conformational change centered around 200 mM NaCl is cooperative. Our results and those of others indicate that there is little unfolding of the histone octamer, at least around Met-84 H4, in the entire ionic strength range studied. A mechanism involving the rotation of the globular portion of H4 is proposed to account for this transition which occurs at physiological ionic strengths.

Quasielastic light-scattering studies on dinucleosomal-sized DNA: ionic-strength dependence.

AbstractQuasielastic light‐scattering (QELS) studies were performed on a monodisperse preparation of dinucleosomal‐sized DNA having an average of 375 base pairs. The QELS data were obtained at several time intervals at each angle examined over the range 30° &lt; θ &lt; 60° for the ionic strength solvents 500, 50, 10, and 1 mM in KCl. What results from this study is that two relaxation domains persist over the entire ionic strength range where the “splitting” between these two domains increases as the ionic strength is lowered. The ordinate intercept of Dapp vs Cp/2Cs, where Cp is the polyion concentration and Cs is the added salt concentration, is shown to give the “true” hydrodynamic value for the diffusion coefficient. The “splitting phenomenon” is interpreted in terms of two models currently in vogue: small ion–polyion coupled modes and translational–rotational coupled modes for rigid rods in congested solutions. Although neither model is in perfect agreement with the experimental results, the small ion–polyion coupled mode theory appears to provide a better quantitative description of the observations.

Activity coefficients for the system HCl + CaCl2+ H2O at various temperatures. Applications of pitzer's equations

Activity coefficients of hydrochloric acid in mixed solutions with calcium chloride have been measured at total ionic strengths of 0.1, 0.25, 0.5, 1.0, 1.5 and 2.0 mol kg–1 at nine different temperatures from 278.15 to 318.15 K at intervals of 5 K, and at total ionic strengths of 2.5, 3.0, 3.5, 4.0, 4.5 and 5.0 mol kg–1 from 278.15 to 318.15 K at intervals of 10 K, using e.m.f. cells of the type Pt, H2(g, 1 atm)|HCl(mA), CaCl2(mB)|AgCl|Ag. The results have been interpreted in terms of Pitzer's equations, from which the calculations of the activity coefficients of calcium chloride in the mixtures have been made. The values of SθMN and ψMNX(including higher-order electrostatic effects) and θMN and ψMNX(excluding higher-order effects) were determined based on Pitzer's treatment. The trace activity coefficients of HCl, γtrA(when yB, the ionic strength fraction of CaCl2 in the mixture, equals 1), are also reported. Within the experimental error, Harned's rule is obeyed for hydrochloric acid at all experimental temperatures and over the entire range of ionic strengths studied. In the case of calcium chloride, Harned's rule is a good description only for I= 0.1, 0.25 and 0.5 mol kg–1, over the range of temperatures examined.