Jean-Yves Gal

About a synthetic saliva for in vitro studies

Numerous artificial salivas have been used during studies in odontology. These salivas have compositions, which are more or less the same as that of natural saliva. In this article, we are presenting a discussion about the various media described in the related literature. A review of nearly 60 artificial salivas was carried out to clarify the role of some of the compounds most frequently met in the proposed formulae. The study focused on the buffer effect, the role played by CO(2) gas and the presence of calcium ions, hydrogenocarbonates, hydrogenophosphates and thiocyanates. The SAGF medium, which we proposed some years ago, was used as a reference and some in vitro behavioral tests of dental biomaterials were studied in a comparative way. Using the SAGF medium allowed us to specify the mode of fluoride ions release from glass ionomer cements and the corrosion behavior of the dental amalgams.

Calcium carbonate solubility: a reappraisal of scale formation and inhibition

Considerable disparity exists in the published thermodynamic data for selected species in the Ca(2+) /CO(2)/H(2)O system near 25 degrees C and 1 atm pressure. Some authors doubt the significance of CaCO(3)(0)aq) complexes although there is experimental evidence of their occurrence. Evaluation of all the published experimental and estimated data for aqueous calcium carbonate species confirms that the consistent set of constants given by Plummer and Busenberg in 1982 is the best available, and suggests a formation constant log beta = 3.22 for CaCO(3)(0)(aq). This value was confirmed by additional experimental data and calculations using a specially developed computer program. The solubility s and solubility product K(s) are critically evaluated for each solid polymorph (amorphous CaCO(3), ikaite, vaterite, aragonite and calcite) using a hydrated ion pair model and we give coherent explanations for the calcium carbonate precipitation/dissolution process and the existence of supersaturated waters. The practical cases of scale formation and its inhibition by phosphonate-type compounds are discussed and explained with the same model, taking into account the CaCO(3)(0)(aq) species.

Mechanisms of scale formation and carbon dioxide partial pressure influence. Part I. Elaboration of an experimental method and a scaling model

Scale formation in industrial or domestic installations is still an important economic problem. The existence of a metastable domain for calcium carbonate supersaturated solutions and its breakdown are observed under conditions rarely well defined. In most cases it is the pH rise caused by the carbon dioxide loss that involves calcium carbonate precipitation. Before studying this problem, we suggest in this first part, a new model for the evolution of the calcocarbonic system that takes into account the hydrated forms of CaCO3: CaCO3 amorphous, CaCO3 x 6H2O (ikaite) and CaCO3 x H2O (monohydrate). According to this model, the precipitation of any one of these hydrated forms could be responsible for the breakdown of the metastable state. After this first step, the solids evolve into dehydrated forms. At first, the metastable domain spread of the calcium carbonate supersaturated solutions was studied by the elaboration of computer programs in which the formation of CaCO3(0)(aq) ion pairs was taken into account. These ion pairs are supposed to evolve through dehydration to form the various calcium carbonate solid form precursors. This thermodynamic study was then compared to the experimental methods of the critical pH. Here the pH rise was caused by adding sodium hydroxide under different conditions for sodium hydroxide addition speed, agitation mode and ageing of solutions. For the highest speed of sodium hydroxide addition, the CaCO3 ionic product reached the value of the amorphous calcium carbonate solubility product, and the reaction of the amorphous calcium carbonate precipitation was of the homogenous type. Decreasing the reagents addition speed caused an extension of the titration time. Then, the breakdown of the metastable state was obtained with the CaCO3 x H2O heterogeneous precipitation. This clearly illustrates the probable ageing of the precursors of the solid states that are considered in this model.

Formation constants β2 of calcium and magnesium fluorides at 25°C

Abstract Until now, scientific work on the reactions of the complexation in water of fluoride ions with calcium or magnesium ions considered only soluble order 1 complexes (CaF+ and MgF+). The precipitation of the compounds CaF2 and MgF2 did not take into account the preliminary formation in solution of these order 2 complexes (CaF20 and MgF20). We therefore studied their formation using a direct potentiometric method in the F− ion-selective electrode at 25°C. This experiment, together with a computerized thermodynamic study, allowed us to determine the values of formation constants at ionic strength equal to zero: log β CaF 2 0 =5.7 and log β CaF 2 0 =3.2 , as well as limiting solubilities: SCaF2=10−4.8 and SMgF2=10−5 mol l−1. Then, some examples were studied showing the necessity to take into account the formation constant of CaF2: the supersaturation of solutions, the validity of results from the fluoride selective electrode, the difference between released fluoride in distilled water and in artificial saliva in restorative dentistry, the fluoride concentration in hard waters and its health consequences.

Thermodynamic study in aqueous solutions of weakly soluble ionic compounds

Any investigation for a better knowledge of precipitation/dissolution problems necessitates the availability of all the beta formation constants of the uncharged soluble species (ion-pair). Several difficulties dealing with solubility measurements are briefly reviewed, especially related to phase structure variations, time-lag or supersaturation phenomena. Thanks to some thermodynamic considerations, the evolution of the uncharged soluble species with hydration and solid phase modifications can give a new explanation about the observed dispersion in literature values for some weakly soluble ionic compounds. When not given elsewhere, the evaluation of thermodynamic data of interest (formation constants, solubility product, etc.) is made possible according to given methods.

Mechanisms of scale formation and carbon dioxide partial pressure influence. Part II. Application in the study of mineral waters of reference.

In the first part, we have designed a new model of evolution for the calco-carbonic system which includes the hydrated forms of CaCO3: CaCO3 amorphous, CaCO3 x 6H2O (ikaite) and CaCO3 x H2O (monohydrate) (J. Eur. Hydr. 30 (1999) 47). According to this model, it is the precipitation of one or other of these hydrated forms which could be responsible for the breakdown of the metastable state. After this first step, the precipitates evolve to dehydrated solid forms. Through the elaboration of computer programs in which the CaCO3(0) (aq) ion pair formation was considered, this model was compared to experimental data obtained by the critical pH method applied to synthetic solutions. In the present article, the same method was applied for four French mineral waters, at 25 degrees C under study. Three samples formed a precipitation during the sodium hydroxide addition. For these three cases, this precipitation began for the CaCO3 H2O saturation. The added volume of sodium hydroxide was more than what was required for neutralizing free CO2 initially in solution. These results indicate that during a spontaneous scaling phenomenon, the pH rises at the same time by loss of the initial free CO2 and of the one produced by the hydrogen carbonate ions decomposition. Then we calculated, at various temperatures for the three studied scaling waters: CO2 partial pressures and loss of total carbon corresponding to the solubility products of CaCO3 hydrated forms. The results show that the partial pressure monitoring of the carbon dioxide is important in managing the behavior of scaling waters.

Experimental study of platinized poly-2,5-dimethoxyaniline electrodes

Abstract In the domains of chemical catalysis and analytical chemistry, the modification of metal electrodes by the deposition of multilayers was developed about twenty years ago. The purpose of our work was to compare the behavior of an electrode prepared with a substituted polyaniline with that of electrodes regularly used in analytical chemistry: platinum, platinized platinum and glassy carbon. We initially present results obtained with poly-2,5-dimethoxyaniline (PDMAn) on two classic systems: the couple Fe(CN)63–/Fe(CN)64– and the couple O2/OH–. The first example is often studied during training in electrochemistry because of the good results generally obtained. Levich’s equation is effectively well verified. On the other hand, reduction of oxygen and oxidation of OH– ions are always limited by the rate of the electron transfer reactions. It seems to us that the study of the behavior of these two systems with platinum electrodes covered with PDMAn, platinized or not, could be interesting. The oxidation of isopropanol in a weakly acid medium was also investigated. In the field of analytical chemistry, a more detailed study is mandatory before deciding on the possibility of using electrodes modified with films of substituted polyaniline. An electrode, which would not be platinized, does not seem useful; the use of electrodes prepared with platinized polymers deposited on substrates less expensive than platinum, (glassy carbon for example), may be more interesting. It seems that H2O2 formation over a wide domain of potential would be the best result for oxidations and new experiments will be investigated.

Repartition of various soluble and insoluble species in supersaturated solutions

The hydrated ion-pair model for uncharged soluble species in equilibrium with many solid polymorphs is applied to examples fiom various fields of interest. The role of such precursors and their changes on ageing can explain the often observed evolution of a hydrated precipitate to successively more dehydrated solid forms. The cases studied concern the precipitation of calcium carbonates, oxalates and phosphates.