Simple Inorganic Solutions Research Articles

Type I Antifreeze Proteins Enhance Ice Nucleation above Certain Concentrations

In this study, we examined the effects that antifreeze proteins have on the supercooling and ice-nucleating abilities of aqueous solutions. Very little information on such nucleation currently exists. Using an automated lag time apparatus and a new analysis, we show several dilution series of Type I antifreeze proteins. Our results indicate that, above a concentration of ∼8 mg/ml, ice nucleation is enhanced rather than hindered. We discuss this unexpected result and present a new hypothesis outlining three components of polar fish blood that we believe affect its solution properties in certain situations.

Solubility of hydroxyapatite by solid titration at pH 3–4

Objectives The solubility isotherm (S) of hydroxyapatite (HAp) is of fundamental importance to saliva chemistry, dental caries and related contexts. It has previously been shown that the locus of the S[HAp] is substantially lower than is commonly reported, and of different slope, probably due to HAps incongruent dissolution. The aim of the present study was to determine the S[HAp] over a wider pH range and to identify the precipitate formed at equilibrium in HAp solid titration. Methods The solid titration technique of Leung and Darvell (Leung VW-H, Darvell BW. Calcium-phosphate system in saliva-like media. J Chem Soc Faraday Trans 1991; 87(11):1759–64.) was used to investigate the solubility behaviour of HAp at 37.0 ± 0.1 °C in 100 mM aqueous KCl. The pH range studied overlapped that of earlier work from pH 3.6 to 5.2, for a reproducibility check and validation, and extended to pH ∼2.9. XRD and EDX were used to identify the precipitates. SEM and TEM were used to observe the morphology. Results The previous S[HAp] reported by Chen et al. (Chen Z-F, Darvell BW, Leung VW-H. Hydroxyapatite solubility in simple inorganic solutions. Arch Oral Biol 2004; 49(5):359–67.) was reconfirmed. An abrupt change of slope of S[HAp] was detected at pH ∼3.9. No other phase than HAp was found at pH 3.2, 3.6 and 4.1. In particular, brushite (dicalcium phosphate dihydrate) was not detected, even below pH 3.9, where instead calcium-deficient HAp was formed. Conclusion The solid titration method was reconfirmed as reproducible and to yield HAp from pH 2.9 to 5.2. The expected brushite did not appear, but rather a stable calcium-deficient HAp was consistently formed. The chemistry of calcium phosphates needs to be reevaluated.

Diffusion coefficients of metals and metal complexes in hydrogels used in diffusive gradients in thin films

Diffusion coefficients of metal ions and metal–ligand complexes in hydrogels were measured at ionic strengths of 0.1–100 mmol L −1 using a diffusion cell. The three different types of hydrogel were all based on polyacrylamide and have been commonly used in the technique of diffusive gradients in thin films (DGT), where accurate diffusion coefficients are essential for the assessment of the concentrations of labile metal in solution. There was little difference in the diffusion coefficients of either Cu and Cd measured in gels with thicknesses of 0.4, 0.8 and 1.6 mm, showing that any diffusion boundary layer in solution was negligibly small. The diffusion coefficients of Pb, Ni, Cu and Cd in simple inorganic solutions was independent of ionic strength (1–100 mmol L −1), except at very low ionic strengths (0.1 mmol L −1) where the value of D was 50% lower. This observation is consistent with the gel having a positive charge that creates a Donnan partitioning of cations at the gel surface. Diffusion coefficient of Pb complexes decreased with increasing size of the ligand, in the order of diglycolic acid (DGA), nitrilotriacetic acid (NTA), fulvic acid and humic acid. In freshwaters the contribution of fulvic acid species to measured DGT mass should be considered. No significant difference was found between the rate of diffusion of Cu and Cd through the mostly commonly used gel and the filter membrane at the face of the DGT device. Diffusion coefficients obtained using a gel with small pore size (restricted gel) plus filter membrane were higher than those obtained using the restricted gel alone. It may be necessary to modify the standard DGT equation for restricted gel deployments, to take into account the difference in diffusion between the gel and the filter membrane.

The effects of database parameter uncertainty on uranium(VI) equilibrium calculations

The propagation of database parameter uncertainty has been assessed for aqueous and mineral equilibrium calculations of uranium by Monte Carlo and quasi-Monte Carlo simulations in simple inorganic solution compositions. The concentration output distributions of individual chemical species varies greatly depending on the solution composition modelled. The relative uncertainty for a particular species is generally reduced in regions of solution composition for which it is predicted to be dominant, due to the asymptotic behaviour imposed by the mass balance constraint where the species concentration approaches the total element concentration. The relative uncertainties of minor species, in regions where another species comprising one or several of the same components is predicted to be dominant with a high probability, also appear to be reduced slightly. Composition regions where two or several species are equally important tend to produce elevated uncertainties for related minor species, although the uncertainties of the major species themselves tend to be reduced. The non-linear behaviour of the equilibrium systems can lead to asymmetric or bimodal output distributions; this is particularly evident close to equivalence points or solubility boundaries. Relatively conservative estimates of input uncertainty can result in considerable output uncertainty due to both the complexity of uranium solution chemistry and the system interdependencies. The results of this study suggest that for some modelling scenarios, “classical” speciation calculations based on mean value estimates of the thermodynamic values may result in predictions of a relatively low probability compared to an approach that considers the effects of uncertainty propagation.

Hydroxyapatite solubility in simple inorganic solutions

Objective: To use solid titration and laser-scattering end-point detection to determine the hydroxyapatite (HAP) solubility isotherm at 37 °C in a plain KCl solution, with CO 2 present, and with additional phosphate. Design: Pulverised HAP solid was used to perform a titration to saturation. A very sensitive detector capable of detecting the light scattered at small forward angles from a laser beam was adopted to monitor the onset of precipitation (or failure to dissolve) near the equilibrium point. Each addition of HAP solid caused a step increase in scattered light and this signal decreased quasi-exponentially with time. When a steady but elevated scattering was obtained, small portions of 1 M HCl were added to lower the pH by about 0.5–2 units, according to need, to dissolve excess solid and allow a further titration. The scatter signal and pH data, plotted against the amount of HAP added, were used to estimate the actual end-point by interpolation and thus construct each point on the solubility isotherm. Results: The solubility isotherm for HAP in plain KCl solution in the absence of CO 2 obtained now differs substantially from the results of previous solubility studies (i.e. much lower). The increase in solubility due to CO 2 reported earlier was confirmed, while the effect of excess phosphate was found to be to increase the apparent solubility of HAP, contrary to elementary mass-action expectations. Conclusions: The solid titration method is a more reliable approach than that of conventional excess solid method with respect to the determination of the HAP-carrying capacity. The solubility of HAP appears to be substantially lower than previously reported, and a reconsideration of all aspects of the system is warranted.

PH and ionic strength dependence of the ASV response of cadmium, lead and zinc in solutions which simulate natural waters

Anodic stripping voltammetry (ASV) peak currents of Pb, Zn and Cd (5 × 10 −8 M) in low ionic strength (1 m M), bicarbonate-buffered solutions were suppressed by increasing the pH (range 4–8). Increasing the ionic strength (1–500m M) of Pb solutions reduced the pH-dependent response, especially when NaClO, rather than KCl was used as the supporting electrolyte. At low ionic strengths and pH > 6, Pb solutions showed a broadened or double peak indicating an alternative oxidation mechanism. Use of NaCl as a supporting electrolyte showed that the cation controlled the current. This and other evidence, such as the time-dependent response, indicates that low currents were due to adsorption of trace metal onto the cell assembly. The inability to predict pH and ionic strength dependent changes of stripping currents measured in simple inorganic solutions illustrates the many problems associated with using ASV for speciation studies on untreated natural waters which contain extremely low concentrations of trace metals.

THE QUANTITATIVE EFFECT OF X-RAYS ON ASCORBIC ACID IN SIMPLE SOLUTION AND IN MIXTURES OF NATURALLY OCCURRING COMPOUNDS

Data on the x-ray induced reaction of ascorbic acid in simple inorganic solution, in solutions containing serum albumin, in plasma, and in muscle have been presented. The reaction occurred in the presence of serum albumin and in human plasma but was relatively small in excised rat muscle.