Sorption behavior of Am(III) onto granite

Sorption behavior of Am(III) onto granite was investigated. The distribution coefficient (K _d ) of Am(III) onto granite was determined in the solution of which pH was ranged from 2.9 to 11.4 and ionic strength was set at 10⁻² and 10⁻¹. TheK _d values were found to increase with increasing pH and with decreasing ionic strength. The obtained data were successfully analyzed by applying an electrical double layer model. The optimum parameter values of the double layer electrostatics and adsorption reactions were obtained, and the selective adsorption behavior of Am(III) onto the granite was discussed.     

Sorption of Am(III) on natural sediment: experiment and modeling

Release of long-lived radioactivity to the aquatic bodies from various nuclear fuel cycle related operations is of great environmental concern in view of their possible migration into biosphere. This migration is significantly influenced by various factors such as pH, complexing ions present in aquatic environment and sorption of species involving radionuclides on the sediments around the water bodies. ^241/243Am are two major radionuclides which can contribute a great deal to radioactivity for several thousand years. In the present study, ²⁴¹Am sorption on natural sediment collected from site near a nuclear installation in India, has been investigated under the varying conditions of pH (3–10) and ionic strength [I = 0.01–1 M (NaClO₄)]. The sorption of Am increased with pH of the aqueous medium [10% (pH 2) to ~100% (pH 10)], which was explained in terms of the increased negative surface charge on the sediment particles. There was marginal variation in Am(III) sorption with increased ionic strength (within error limits) of the aqueous medium suggesting inner-sphere complexation/sorption process. Sediment was characterized for its elemental composition and structural phases using Energy Dispersive X-Ray (SEM-EDX) and X-Ray Diffraction (XRD) techniques. Zeta-potential measurement at I = 0.1 M (NaClO₄) suggested that Point of Zero Charge (pH_PZC) was ~2, indicating the presence of silica as major component in the sediment. Kurabtov plot using sorption data as a function of pH at fixed I = 0.1 M (NaClO₄) indicated the presence of multiple Am(III) species present on the surface. Potentiometric titration of the suspension indicated the presence of mineral oxide like behavior and assuming a generic nature (≡XOH) for all types of surface sites, protonation–deprotonation constants and total number of sites have been obtained. The sorption data has been modeled using 2-pK Diffuse Double Layer Surface Complexation Model (DDL-SCM). ≡XOAm²⁺ has been identified as the main species responsible for the sorption profile.     

Extraction of Am(III) and Eu(III) with dialkyldi (or mono) thiophosphinic (or phosphoric) acids

Extraction of Am(III) and Ln(III) from NaClO₄ medium with di(2-ethylhexyl)dithiophosphoric acid (DEHDTP), di(2-ethylhexyl)monothiophosphoric acid (DEHMTP), di(2-ethylhexyl)monothiophosphinic acid (DEHMTPI), dihexyldithiophosphinic acid (DHXDTPI), diheptyldithiophosphinic acid (DHPDTPI), dioctyldithiophosphinic acids (DODTPI), dinonyldithiophosphinic acid (DNDTPI), di(1-methylheptyl)dithiophosphinic acid (DMHDTPI) and di(2-ethylhexyl)dithiophosphinic acid (DEHDTPI) in xylene has been investigated. The order of the extraction selectivity for Am(III) is DEHDTPI > DEHDTP > DEHMTPI > DEHMTP, DHPDTPI > DODTPI > DHXDTPI > DNDTPI, DMHDTPI > DEHDTPI > DODTPI, for extractants with 2-ethylhexyl alkyl, straight chain alkyl, branch chain alkyl, respectively. Using 0.1 mol/l NaClO₄ solution as aqueous phase, the slope values of the logD-pH and logD-logC curves are not integers, and the slope values for Am(III) are slightly higher than those for Eu(III), for all extractants. The relationship between the slope value and extraction conditions can be described as: logS = alg(C _HA/C _M ^S/4)+b. In the presence of macro Eu(ClO₄)₃, the formula, logSF _Am/Ln = B-2log(C _HL-D _Ln/(D _Ln + 1)C _Eu), can well describe the relationship between separation factor and the extraction condition. A high separation factor (SF _Am/Eu = 2500) is obtained by solvent extraction with 0.5 mol/1 DEHDTPI in toluene from 1 mol/l NaNO₃ solution.     

The influence of humic acid on the pH-dependent sorption of americium(III) onto kaolinite

This work investigates the sorption of americium [Am(III)] onto kaolinite and the influence of humic acid (HA) as a function of pH (3–11). It has been studied by batch experiments (V/m = 250:1 mL/g, C _Am(III) = 1 × 10⁻⁵ mol/L, C _HA = 50 mg/L). Results showed that the Am(III) sorption onto the kaolinite in the absence of HA was typical, showing increases with pH and a distinct adsorption edge at pH 3–5. However in the presence of HA, Am sorption to kaolinite was significantly affected. HA was shown to enhance Am sorption in the acidic pH range (pH 3–4) due to the formation of additional binding sites for Am coming from HA adsorbed onto kaolinite surface, but reduce Am sorption in the intermediate and high pH above 6 due to the formation of aqueous Am-humate complexes. The results on the ternary interaction of kaolinite–Am–HA are compared with those on the binary system of kaolinite–HA and kaolinite–Am and adsorption mechanism with pH are discussed. Effect of different molecular weight of HA, with three HA fractions separated by ultrafiltration techniques, on the Am sorption to kaolinite were also studied. The results showed that the enhancement of the sorption of Am onto kaolinite at the acidic pH conditions (pH 3–4) was higher with HA fractions of higher molecular weight. Also, the Am sorption over a pH range from 6 to 10 decreased with decreasing molecular weight of HA.     

Americium(III) Adsorption on Polyethylene from very Dilute Aqueous Solutions

The Am(III) adsorption from very dilute aqueous solutions on polyethylene was studied as a function of the pH value, the composition of dissolved salts and the presence or absence of adsorbent SiO₂. It was found that the Am(III) adsorption on polyethylene is decreased with increasing H⁺ concentration and dissolved salt concentrations and in the presence of SiO₂ as an adsorbent. The mechanisms of Am(III) adsorption on polyethylene were discussed. Based on the observations in this paper, extreme care is necessary to minimize and correct the Am(III) adsorption on the container walls in order to improve the accuracy of Am(III) distribution coefficient (K _d) between a solid and an aqueous solution.     

U(VI) mono-hydroxo humate complexation

The complexation of the uranyl ion with humic acid is investigated. The humic acid ligand concentration is described as the concentration of reactive humic acid molecules based on the number of humic acid molecules, taking protonation of functional groups into account. Excess amounts of U(VI) are used and the concentration of the humic acid complex is determined by the solubility enhancement over the solid phase. pH is varied between 7.5 to 7.9 in 0.1M NaClO₄ under normal atmosphere and room temperature. The solubility of U(VI) in absence of humic acid is determined over amorphous solid phase between pH 4.45 and 8.62. With humic acid, only a limited range of data can be used for the determination of the complexation constant because of flocculation or sorption of the humic acid upon progressive complexation. Analysis of the complex formation dependency with pH shows that the dominant uranyl species in the concerned pH range are UO₂(OH)⁺ and (UO₂)₃(OH)₅ ⁺. The complexation constant is evaluated for the humate interaction with the to UO₂(OH)⁺ ion. The stability constant is found to be logβ = 6.94±0.3 l/mol. The humate complexation constant of the uranyl mono-hydroxo species thus is significantly higher than that of the nonhydrolyzed uranyl ion (6.2 l/mol). Published data on the Cm³⁺, CmOH²⁺ and Cm(OH)₂ ⁺ humate complexation are reevaluated by the present approach. The higher stability of the hydrolysis complex is also found for Cm(III) humate complexation.     

Evaluation of radiation stability of N,N-didodecyl N′,N′-di-octyl diglycolamide: a promising reagent for actinide partitioning

The radiation stability of N,N-di-dodecyl-N′,N′-di-octyl-3-oxapentane-1,5-diamide (D³DODGA) was studied by γ-irradiation of the solvent up to a absorbed dose of 1,000 kGy. The effect of γ-irradiation on the radiolytic degradation of D³DODGA was assessed by measuring the distribution ratio of Am(III) (D _Am(III)) as well as the third phase formation in the irradiated D³DODGA-n-dodecane solution. The D _Am(III) in the irradiated solution decreased with increase of absorbed dose. The critical aqueous concentration of Nd(III) above which the third phase forms, increased with increase of absorbed dose. However, the limiting organic concentration of Nd(III) remained at ~25 mM irrespective of the absorbed dose. Recovery of Am(III) from the radiolytically degraded organic phase showed that back extraction of Am(III) was quantitative in a few contacts using dilute nitric acid. Our studies clearly indicated that radiolytic degradation of D³DODGA in n-dodecane is marginal even at the absorbed dose of 1,000 kGy, and therefore D³DODGA is a potential candidate for minor actinide partitioning.     

Sorption of 241Am by Aspergillus nigerspore and hyphae

Biosorption of ²⁴¹Am by a fungus A. niger, including the spore and hyphae, was investigated. The preliminary results showed that the adsorption of ²⁴¹Am by the microorganism was efficient. More than 96% of the total ²⁴¹Am could be removed from ²⁴¹Am solutions of 5.6-111 MBq/l (C _o) by spore and hyphaeof A. niger, with adsorbed ²⁴¹Am metal (Q) of 7.2-142.4 MBq/g biomass, and 5.2-106.5 MBq/g, respectively. The biosorption equilibrium was achieved within 1 hour and the optimum pH range was pH 1-3. No obvious effects on ²⁴¹Am adsorption by the fungus were observed at 10-45 °C, or in solutions containing Au³⁺ or Ag⁺, even 2000 times above the ²⁴¹Am concentration. The ²⁴¹Am biosorption by the fungus obeys the Freundlich adsorption equation. There was no significant difference between the adsorption behavior of A. nigerspore and hyphae. This revised version was published online in July 2006 with corrections to the Cover Date.     

Dimethylglyoximkomplexe des Kobalts(III) mit aromatischen Diaminen Über α-Dioximkomplexe der Übergangsmetalle, 37. Mitt.

Zusammenfassung Durch Oxydation von Kobalt(II)salz-Lösungen in Anwesenheit von Dimethylglyoxim und aromatischen Diaminen wurden unter doppelter Umsetzung 41 Salze der [Co(HD)₂(o-Phenylendiamin)₂]⁺, [Co(HD)₂(m-Phenylendiamin₂])⁺, [Co(HD)₂-(2-Methyl-p-phenylendiamin)₂]⁺ und [Co(HD)₂(N-Dimethyl-p-phenylendiamin)₂]⁺-Kationen erhalten. Die Koordination von zwei Diaminliganden im Kobalt(III)-bis-dimethylglyoximin-Kern bestätigt dietrans-Konfiguration der [Co(HD)₂(Diamin)₂]⁺-Komplexe. Diese Annahme wurde auch durch UR-spektroskopische Untersuchung bestätigt.

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On-dioxime complexes of transition metals, XXXVII. Cobalt(III)-dimethylglyoxime complexes with aromatic diamines

The oxidation of cobalt(II) salts in presence of dimethyl-glyoxime and aromatic diamines, has been studied. A series of 41 novel complex salts of the cations [Co(HD)₂(o-phenylendiamine)₂]⁺, [Co(HD)₂(m-phenylendiamine)₂]⁺, [Co(HD)₂(2-methyl-p-phenylendiamine)₂]⁺ and [Co(HD)₂(N-dimethyl-p-phenylendiamine)₂]⁺ has been prepared and characterized by means of double decomposition reactions.The coordination of 2 diamine molecules to the Co(III)-dimethylglyoximine-skelet confirms thetrans-configuration of the [Co(HD)₂(diamine)₂ ⁺ complexes.

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Mit 2 Abbildungen     

Quantum chemical study of the structure of bicyclo[2.2.0]hex-1(4)-ene and its dimers

The RHF, B3LYP, and PBE0/6-311G** quantum chemical methods are used to determine the point symmetry group and the equilibrium structure of bicyclo[2.2.0]hex-1(4)-ene (I, D _2h ), its two stable dimers (tricyclo[4.2.2.2^2,5]dodeca-1.5-diene (II, D _2h ) and 2,5-dimethylenetricyclo[4.2.2.0^1,6]decane (III, C ₂)), and pentacyclo [4.2.2.2^2,5]dodecane (IV, D ₂) that is a hypothetical intermediate in the dimerization reaction of the molecules of I. The relation of total energies is obtained with regard to zero-point vibrations: E(III) < E(II) ≪ E(IV) ≪ 2E(I).     

Coordination of perchlorate ion in the secondary solvation sphere of Am3+ in the tracer concentrations in mixed system of dimethyl sulfoxide/water with 1.00M (H,Na)(Cl,ClO4)

The stability constants, β₁, of monochloride complex of Am(III) have been determined in a mixed system of dimethyl sulfoxide (DMSO) and water at 1.00 mol·dm⁻³ ionic strength using solvent extraction. The values of β₁ in mixed DMSO+H₂O solutions decrease rapidly with an increase in the mole fraction of DMSO (X _s ) in mixed solvents and become negative ones, which is not in a definition of stability constant, inX _s >0.04. The variation of β₁ inX _s ≦0.02 was accounted for by the size-variation of the primary solvation sphere around Am(III), which was present as a solventshared ion-pair, and by a little effect due to an invasion (coordination) of ClO₄ ⁻ into the secondary solvation sphere of Am³⁺. On the other hand, it was concluded that the β₁ obtained by solvent extraction inX _s >0.02 was an apparent value, because of a large effect due to an invasion (coordination) of ClO₄ ⁻ into the secondary solvation sphere of Am(III).     

Chromatographic separation of americium from europium using bis-2,6-(5,6,7,8-tetrahydro-5,9,9-trimethyl-5,8-methano-1,2,4-benzotriazin-3-yl) pyridine

The separation of americium(III) from europium(III) was achieved utilizing a bis-2,6-(5,6,7,8-tetrahydro-5,9,9-trimethyl-5,8-methano-1,2,4-benzotriazin-3-yl) pyridine (CA-BTP) chromatographic resin. The extraction chromatographic materials were prepared using various concentrations of CA-BTP. This new, hydrolytically stable extractant was impregnated on an inert polymeric support at 40% loading. The uptake of Am(III) and Eu(III) by this material from 0.1 to 4.0 M aqueous HNO₃ solutions was measured. The resulting dry weight distribution ratios, D _w , indicated a strong preference for Am(III) with little affinity for Eu(III). These results are similar to recently reported solvent extraction studies indicating a maximum uptake of Am(III) in the 0.5–1.0 M HNO₃ range. The resin preparation, performance, and characterization of the Am/Eu separation are reported herein.

     

Radiolytic stability of N,N-didodecyl-N’,N’-diethylhexyl diglycolamide

The radiolytic degradation of N,N-di-dodecyl-N′,N′-di-2-ethylhexyl-3-oxapentane-1,5-diamide (D³DEHDGA) was studied at various absorbed doses of γ-radiation. The degradation was assessed by measuring the variation in the extraction behavior of Am(III) in irradiated solvent systems composed of neat D³DEHDGA or 0.1 M D³DEHDGA/n-dodecane in the presence and absence of nitric acid. The distribution ratio of americium (D _Am(III)) decreased with increase of absorbed dose. The presence of n-dodecane and nitric acid enhanced the radiolytic degradation of the solvent. The third phase formation behavior of Nd(III) decreased with increase of absorbed dose and the third phase was not observed at absorbed doses above 100 kGy. The recovery of Am(III) from the irradiated system was near quantitative in five contacts. The study revealed that the extraction and stripping behavior of the irradiated solvent was quite satisfactory for partitioning of minor actinides from real wastes.     

Experimental study of the kinetics of reaction of chlorine atoms with tetrahydrofuran and fully deuterated tetrahydrofuran

The overall rate constants for H-abstraction (k_H) from tetrahydrofuran and D-abstraction (k_D) from fully deuterated tetrahydrofuran by chlorine atoms in the temperature range of 298-547 K were determined. In both cases, very weak negative temperature dependences of the overall rate constants were observed, described by the expressions: k_H = (1.55 ± 0.13) × 10⁻¹⁰ exp(52 ± 28/T) cm³ molecule⁻¹ s⁻¹ and k_D = (1.27 ± 0.25) × 10⁻¹⁰exp(55 ± 62/T) cm³ molecule⁻¹ s⁻¹. The experimental results show that the value of the kinetic isotope effect (k_H/k_D), amounting to 1.21 ± 0.10, is temperature independent at 298-547 K.     

Extraction of Sm(III), Gd(III) and Ho(III) ions with picrolonic acid in methylisobutylketone

Summary Picrolonic acid (HPA) in methylisobutylketone (MIBK) (0.01 mol^. dm^-3) has been used for the extraction of lanthanide(III) ions such as Sm(III), Gd(III) and Ho(III) (Me) (~3^. 10^-6mol^. dm^-3) from pH 1-2 buffer solutions of 0.1 mol^. dm^-3(H⁺, Cl^-) ionic strength and quantitative extraction (>95%) was found at pH 2. Through slope analysis the composition of the organometallic adduct responsible for the extraction came out to be M(PA)₃. The conditional equilibrium constant values, log K_ex, were deduced to be 2.60±0.01, 2.09±0.01 and 1.44±0.03 for these lanthanide(III) ions, respectively. The metals in concentration up to ~2.5^. 10^-4mol^. dm^-3can be quantitatively extracted by the proposed system. Among the various anions, fluoride, oxalate and cyanide ions (~3.0^. 10^-4mol^. dm^-3) and, among the cations, Zn(II) Cu(II), Co(II) and Fe(III) reduced the lanthanide extraction. The extraction of various other metal ions at the optimized conditions of Me extraction for this series of lanthanide ions was also studied and high separation factors (10²-10³) were obtained showing the good selectivity of this extraction system.     

Influence of metal loading and humic acid functional groups on the complexation behavior of trivalent lanthanides analyzed by CE-ICP-MS

The complexation behavior of Aldrich humic acid (AHA) and a modified humic acid (AHA-PB) with blocked phenolic hydroxyl groups for trivalent lanthanides (Ln) is compared, and their influence on the mobility of Ln(III) in an aquifer is analyzed. As speciation technique, capillary electrophoresis (CE) was hyphenated with inductively coupled plasma mass spectrometry (ICP-MS). For metal loading experiments 25 mg L⁻¹ of AHA and different concentrations (c_Ln(Eu+Gd) = 100–6000 μg L⁻¹) of Eu(III) and Gd(III) in 10 mM NaClO₄ at pH 5 were applied. By CE-ICP-MS, three Ln-fractions, assumed to be uncomplexed, weakly and strongly AHA-complexed metal can be detected. For the used Ln/AHA-ratios conservative complex stability constants log β_LnAHA decrease from 6.33 (100 μg L⁻¹ Ln³⁺) to 4.31 (6000 μg L⁻¹ Ln³⁺) with growing Ln-content. In order to verify the postulated weaker and stronger humic acid binding sites for trivalent Eu and Gd, a modified AHA with blocked functional groups was used. For these experiments 500 μg L⁻¹ Eu and 25 mg L⁻¹ AHA and AHA-PB in 10 mM NaClO₄ at pH-values ranging from 3 to 10 have been applied. With AHA-PB, where 84% of the phenolic OH-groups and 40% of the COOH-groups were blocked, Eu complexation was significantly lower, especially at the strong binding sites. The log β-values decrease from 6.11 (pH 10) to 5.61 at pH 3 (AHA) and for AHA-PB from 6.01 (pH 7) to 3.94 at pH 3. As a potential consequence, particularly humic acids with a high amount of strong binding sites (e.g. phenolic OH- and COOH-groups) can be responsible for a higher metal mobility in the aquifer due to the formation of dissolved negatively charged metal-humate species.     

Can the Theoretical Fitting of the Proton‐Nuclear‐Magnetic‐Relaxation‐Dispersion (Proton NMRD) Curves of Paramagnetic Complexes Be Improved by Independent Measurement of Their Self‐Diffusion Coefficients?

The self‐diffusion (D_c) coefficients of various lanthanum(III) diamagnetic analogues of open‐chain and macrocyclic complexes of gadolinium used as MRI contrast agents were determined in dilute aqueous solutions (3–31 mM ) by pulsed‐field‐gradient (PFG) high‐resolution ¹H‐NMR spectroscopy. The self‐diffusion coefficient of H₂O (D_w) was obtained for the same samples to derive the relative diffusion constant, a parameter involved in the outersphere paramagnetic‐relaxation mechanism. The results agree with an averaged relative diffusion constant of 2.5 (±0.1)×10^?9 and of 3.3 (±0.1)×10^?9 m² s^?1 at 25 and 37°, respectively, for 'small' contrast agents (M_r 500–750 g/mol), and with the value of bulk H₂O (2.2×10^?9 and 2.9×10^?9 m² s^?1 at 25° and at 37°, respectively) for larger complexes. The use of the measured values of D_c for the theoretical fitting of proton NMRD curves of gadolinium complexes shows that the rotational correlation times (τ_R) are very close to those already reported. However, differences in the electronic relaxation time (τ_SO) at very low field and in the correlation time (τ_V) related to electronic relaxation were found.     

端基取代的长链硅烷二阶超极化率的量子化学研究

对端基取代的一维无限长反位硅烷H₂N-(SiH₂-SiH₂)_n-NO₂的二阶超极化率进行了系统的量子化学研究. 通过仔细检验和选择外场强度, 采用9个外场强度(0.0000, ±0.0008, ±0.0012, ±0.0016, ±0.0020 a.u.)计算的体系能量来确定4阶场强展开式中的5个系数, 从而得到可靠的二阶超极化率. 建议数据拟合时用二阶超极化率单元值的平均值形式γ(n)/n作为拟合对象, 同时用1/n的2阶多项式作为拟合函数, 以得到无限长链的二阶超极化率极限值. 拟合数据范围的选择应该使该数据范围得到的极限值与其临近数据范围得到的极限值的均方偏差最小. 分子构型的优化使计算的二阶超极化率增加大约20%, 在基组中增加极化函数使二阶超极化率在无限长链时的极限值减少大约15%. 相关效应的影响最大, MP2的结果比RHF的结果增加近一倍. 根据本文最高水平MP2/6-31G(d)//RHF/6-31G的计算, 端基取代的一维无限长反位硅烷H₂N-(SiH₂-SiH₂)_n-NO₂的二阶超极化率的每单元极限值为0.8364×10⁶ a.u.     

Photoinduced Degradation of the Herbicide Clomazone Model Reactions for Natural and Technical Systems

The photodegradation of the herbicide clomazone in the presence of S₂O₈^2? or of humic substances of different origin was investigated. A value of (9.4 ± 0.4) × 10⁸ m ^?1 s^?1 was measured for the bimolecular rate constant for the reaction of sulfate radicals with clomazone in flash‐photolysis experiments. Steady state photolysis of peroxydisulfate, leading to the formation of the sulfate radicals, in the presence of clomazone was shown to be an efficient photodegradation method of the herbicide. This is a relevant result regarding the in situ chemical oxidation procedures involving peroxydisulfate as the oxidant. The main reaction products are 2‐chlorobenzylalcohol and 2‐chlorobenzaldehyde. The degradation kinetics of clomazone was also studied under steady state conditions induced by photolysis of Aldrich humic acid or a vermicompost extract (VCE). The results indicate that singlet oxygen is the main species responsible for clomazone degradation. The quantum yield of O₂(a¹Δ_g) generation (λ = 400 nm) for the VCE in D₂O, Φ_Δ = (1.3 ± 0.1) × 10^?3, was determined by measuring the O₂(a¹Δ_g) phosphorescence at 1270 nm. The value of the overall quenching constant of O₂(a¹Δ_g) by clomazone was found to be (5.7 ± 0.3) × 10⁷ m ^?1 s^?1 in D₂O. The bimolecular rate constant for the reaction of clomazone with singlet oxygen was k_r = (5.4 ± 0.1) × 10⁷ m ^?1 s^?1, which means that the quenching process is mainly reactive.     

Effects of Pressure and pH on the Physical Stability of an I-Motif DNA Structure

The thermodynamic stability of a cytosine(C)-rich i-motif tract of DNA, which features pH-sensitive [C..H..C]⁺ moieties, has been studied as function of both pressure (0.1–200 MPa) and pH (3.7–6.2). Careful attention was paid to correcting citrate buffer pH for known variations that stem from changes in pressure. Once pH-corrected, (i) at pH >4.6 the i-motif becomes less stable as pressure is increased (K_D decreases), giving a small negative volume change for dissociation (Δ_DV°) of the i-motif – a conclusion opposite to that which would be drawn if the buffer pH was not corrected for the effects of pressure; (ii) the i-motif's melting temperature increases by more than 30 K between pH 6.5 and 4.5, the consequence of an enthalpy for dissociation (Δ_DH°) of 77(3) and 90(3) kJ (mol H⁺)⁻¹ at 0.1 and 200 MPa, respectively; (iii) below pH 4.6 at 0.1 MPa (pH 4.3 at 200 MPa) the melting temperature decreases as a result of double protonation of cytosine pairs, and Δ_DH° and Δ_DV° change signs; and (iv) the combination of Δ_DH° and Δ_DV° lead to the melting temperature at pH 4.3 being 3 K higher at 200 MPa than at 0.1 MPa.