Salem M. Hamza

Spontaneous precipitation of calcium sulphate dihydrate in the presence of some metal ions

Abstract The precipitation of calcium sulphate dihydrate from aqueous solutions has been studied at super-saturations sufficient for spontaneous precipitation but low enough to enable highly reproducible experiments to be carried out with respect to induction periods and rates of precipitation. The induction periods and rates of precipitation are both strongly dependent on degree of saturation. Over a range of supersaturation, 1.5–4.1, the precipitation reaction appears to be controlled by a surface polynucleation process. The influence of some metal ions on the precipitation process have been studied at several additive concentrations. The reduction in precipitation rate can be interpreted in terms of a Langmuir-type adsorption insotherm.

Dissolution of calcium carbonate crystals: a constant-composition kinetic study

A highly reproducible constant-composition technique has been used to study the rates of dissolution of calcium carbonate crystals over a range of undersaturation. The technique can be used even at very low under saturation. Over a range of relative undersaturation, 0.05–0.33, the dissolution reaction appears to be controlled by a surface process which, in contrast to a bulk diffusion reaction, is markedly inhibited by the presence of additives. The influence of a number of polyphosphonates on the rate of reaction has been investigated. The reduction in dissolution rates can be interpreted in terms of a Langmuir-type adsorption isotherm. The effect of the pKa values of phosphonates on their inhibition of dissolution rates of calcite crystals has been discussed.

Thiopental and Phenytoin as Novel Ionophores for Potentiometric Determination of Lead (II) Ions

Two novel polymeric membrane sensors for the analysis of Pb(II) have been developed based on two therapeutic drugs, thiopental (TP) and phenytoin (PT) as two new ionophores and potassium tetrakis(p-chlorophenyl) borate (KTpClPB) as a lipophilic additive, in plasticized PVC membranes. The sensors show a Nernstian response for Pb(II) ions over the wide concentration ranges of 1×10−2 – 7×10−6 M and 1×10−2 – 8×10−6 M for the sensors based on thiopental and phenytoin, respectively. The proposed sensors have a fast response time and can be used for more than nine weeks without any considerable divergence in potentials. The sensors exhibit comparatively good selectivity with respect to alkaline, alkaline earth and some transition and heavy metal ions. They were employed for direct determination of lead in solder alloys and in galena rocks with a good agreement with the obtained results by atomic absorption spectroscopy.

Effect of some amino acids on the rate of dissolution of calcite crystals in aqueous systems

Abstract The dissolution of calcite seed crystals has been investigated in the presence of some amino acids by constant composition method in which the undersaturation and ionic strength were maintained constant. The crystal dissolution rates are markedly inhibited by the addition of amino acids. The order of the degree of inhibition is L-arginine > glycine > phenyl alanine > alanine. Application to a kinetic Langmuir-type model suggests that adsorption of the amino acids at the active dissolution sites is the cause of the reduction in the dissolution rates.

Dissolution of magnesium fluoride crystals in the presence of some minerals salts

Abstract Dissolution of magnesium fluoride seed crystals has been studied in the absence and presence of some mineral salts under conditions in which the under-saturation was maintained constant and controlled by a specific fluoride ion electrode. Over a range of relative under-saturation, 0.3–0.75, the dissolution reaction appears to be governed by a surface polynucleation process which is markedly inhibited by the presence of additives. The action of mineral salts on the rate of dissolution of magnesium fluoride crystals can be interpreted in terms of a Langmuir-type adsorption isotherm.

Influence of some phosphates and polyphosphate on the precipitation of calcium sulfate dihydrate in sodium chloride solutions

Abstract The inhibiting effect of phosphates and polyphosphates on the precipitation rate of calcium sulfate dihydrate (gypsum) in sodium chloride solutions has been studied using constant composition technique in which the super-saturation and ionic strength were maintained constant at 25°C. The presence of phosphoric acid H3PO4, orthophosphate NaH2PO4, metaphosphate Na2HPO4,pyrophosphate PYP and hexametaphosphate HMP, even at relatively low concentration 10-6 M, markedly reduced the rates of precipitation. The action of additives can be interpreted in terms of adsorption, following the Langmuir isotherm, of additives at the active crystal growth sites. Dependence of the degree of inhibition with the change in driving force has been observed. The order of the degree of inhibition on the rate of precipitation of gypsum for various additives studied is as follows : H3PO4 > NaH2PO4 > Na2HPO4> HMP > PYP.

Influence of polyphosphonates on the precipitation of strontium sulfate (Celestite) from aqueous solutions

Abstract The precipitation of strontium sulfate from aqueous solutions in the presence of polyphosphonates at 0.7 mol l-1 sodium chloride and 25°C has been studied. Over a range of relative super-saturation (2.0 - 6.2) the process appears to follow a surface controlled precipitation mechanism which is markedly inhibited by the presence of additives. The presence of phosphonates even at relatively low concentrations (2.5 - 20 x 10-6 mol l-1) were retarding the rate of precipitation. The retardation effect of these additives has been attributed to the blocking of active sites by adsorption of additives molecules at the crystal precipitation surfaces. The action of additives can be interpreted in terms of a Langmuir-type adsorption isotherm.

The kinetics of dissolution of calcium fluoride in the presence of some metal ions

Abstract The kinetics of dissolution of calcium fluoride crystals has been studied in sodium chloride solutions at 25°C in the presence of zinc, magnesium, lead and aluminium ions, using a constant-composition method, in which the undersaturation and ionic strength were maintained constant. With the exception of zinc, the addition of metal ions, even at relatively low concentration (5X10 -5 mol dm -3 ), markedly retards the rates of dissolution of calcium fluoride crystals. Moreover, the effect was enhanced as the relative undersaturation decreased. The subsequent blocking of dissolution sites by the adsorbed ions can be interpreted in terms of a Langmuir isotherm. Direct adsorption experiments have also been made by using magnesium ions. Comparison of the kinetically derived equilibrium constants with the directly measured adsorption equilibrium constant showed increasing kinetic equilibrium constants with decreasing relative undersaturation.