Crystallization experiments, calcium

Chemically pure reagents were used. Cadmium was added as its sulfate salt in concentrations of about 50 ppm. Lanthanides were added as nitrates. For the experiments with other metal ions so-called "black acid from a Nissan-H process was used. In this acid a large number of metal ions were present. To achieve calcium sulfate precipitation two solutions, one consisting of calcium phosphate in phosphoric acid and the other of a phosphoric acid/sulfuric acid mixture, were fed simultaneously in the 1 liter MSMPR crystallizer. The power input by the turbine stirrer was 1 kW/m. The solid content was about 10%. Each experiment was conducted for at least 8 residence times to obtain a steady state. During the experiments lic iid and solid samples were taken for analysis by ICP (Inductively Coupled Plasma spectrometry, based on atomic emission) and/or INAA (Instrumental Neutron Activation Analysis). The solid samples were washed with saturated gypsum solution (3x) and with acetone (3x), and subsequently dried at 30 C. The details of the continuous crystallization experiments are given in ref. [5]. 

Figure 4. Plots of solution total calcium concentration and pH as a function of time for a typical calcite-seeded crystallization experiment in simulated natural...

To test the results of the simulations presented in Section II and to find out where the practical limits of resolution are in solid state proton NMR, we carried out a series of experiments on single crystals of calcium formate [Ca(HCOO)2] and malonic acid [CH2(COOH>2]. The former compound was chosen because it represents a fairly diluted proton spin system with correspondingly weak dipole-dipole interactions. Malonic acid with the close pair of protons in its methylene group is, on the other hand, representative of a large class of organic compounds with strong dipole-dipole interactions. 

Crystallization experiments were then carried out in supersaturated solutions of calcium carbonate. Under these conditions, rhombohedral calcite crystals were observed in the recognition sites of the calcite-imprinted polymer. Very few crystals... 

Zhang et al. used Langmuir monolayers to study the initial stage of nucleation and crystallization of calcium phosphate [160]. The monolayers used in their study consisted of dipalmitoylphosphatidylcholine, arachidic acid, and octadecylamine. The experiments show that the adsorption of calcium ions to the respective mono-layer is a prerequisite for subsequent nucleation. The authors also demonstrate that calcium phosphate forms through a multistage assembly process, in which first an amorphous calcium phosphate dihydrate layer forms, which then recrystallizes to form a crystalline hydroxyapatite layer (Fig. 12). This transformation of an amorphous phase to a crystalline phase provides direct evidence of a multistep crystallization process, which the authors claim is similar to the processes occurring in biomineralization. 

Calcium carbonate has normal pH and inverse temperature solubilities. Hence, such deposits readily form as pH and water temperature rise. Copper carbonate can form beneath deposit accumulations, producing a friable bluish-white corrosion product (Fig. 4.17). Beneath the carbonate, sparkling, ruby-red cuprous oxide crystals will often be found on copper alloys (Fig. 4.18). The cuprous oxide is friable, as these crystals are small and do not readily cling to one another or other surfaces (Fig. 4.19). If chloride concentrations are high, a white copper chloride corrosion product may be present beneath the cuprous oxide layer. However, experience shows that copper chloride accumulation is usually slight relative to other corrosion product masses in most natural waters. 

The crystal growth rates can be directly determined from the second and third moment as described above. The calculated rates for calcium oxalate here are in the range 0.75 x 10 to 4.7 x 10 m/s. Literature values for the growth rate of calcium oxalate monohydrate vary considerably 1.08 x 10 m/s (Kavanagh, 1992), 3.4 X 10 to 5.0 x 10 m/s (Garside etal., 1982) and 2.8 x lO to 1.11 X 10 m/s (Nielsen and Toft, 1984). The values obtained from the experiments are therefore within the range of the literature data. It should be borne in... 

Experiment.—On a small hand balance, having two sheets of paper counterpoised in the pans, weigh out equal quantities (about 0-5 g.) of quinone and quinol dissolve the two substances separately in warm water and mix the solutions. Magnificent green needles of quinhydrone crystallise almost at once. After some time collect them at the pump, wash with water, and dry between filter papers over calcium chloride in a non-evacuated desiccator. Boil a few of the crystals with water and smell the vapours. 

A disadvantage of the conventional precipitation method in which the supersaturation was allowed to decrease during the reactions, was that different calcium phosphate phases could form and subsequently dissolve during the course of the reactions. In the present work, the constant composition method was used to investigate the influence of sodium chloride, potassium chloride, and potassium nitrate, as background electrolyte upon the rate of crystallization of HAP in solutions supersaturated only with respect to this phase. These experiments were made in solutions containing totaj... 

In this experiment, tap water with added phosphate was used as influent. Concentration of phosphate was adjusted to an adequate range from 2 to 23 mg/jg. Calcium chloride and sodium hydroxide solution were added to maintain calcium concentration from 70 to 100 mg/jg and pH of the effluent from 9.0 to 9.5. Using this equipment, we performed experiments to obtain efficiency of phosphate removal, relationship between phosphate concentration, and crystallization rate and factors affecting phoshate removal. 

The supersaturation is too low in all experiments to be measured accurately, but it seems reasonable to assume that the effect of residence time is imposed through the kinetics. Another observation is that the D-value for cadmium uptake in anhydrite is about ten times higher than in HH or DH. An explanation for this higher D seems to be related to the crystal structures of the calcium sulfates. Only the AH structure matches with an anhydrous CdSO phase, while no hemi- or dihydrate phase of CdS04 exists. 

Preparation of Chromium. Perform the experiment in the presence of your instructor, in a fume cupboard Wear eye protection or a mask ) Roast chromium(III) oxide in an iron crucible. Triturate potassium dichromate crystals in a mortar, melt the powder in a porcelain crucible with the flame of a gas burner, pour the substance onto a glazed tile plate, and after it solidifies again triturate it into a powder. Weigh 12 g of roasted chromium(III) oxide, 12 g of the fused potassium dichromate, and 10 g of an aluminium powder, and thoroughly mix all the substances in a mortar. Spill 10 g of a calcium fluoride powder onto the bottom of a chamotte crucible, and then the prepared mixture. (If there is no chamotte crucible in your laboratory, make a box from asbestos paper in the form of a crucible and fasten it with wire.) Tamp the substance with a pestle and make a hollow in the middle using a test tube (see Fig. 120). 

In these experiments, synthetic zeolites of the faujasite-type without binding substance were used. Calcium and nickel-calcium samples in ionic form were obtained by ion exchange under conditions ensuring stability of the crystal structure (5). Platinum addition was carried out by ion exchange with Pt(NH3)6Cl4 (6). 

Dupre61 in 1901 reported experiments which indicated that the picrates of calcium, lead, and zinc, formed in situ from melted picric acid are capable of initiating the explosion of that material. Kast62 found that the dehydrated picrates are more sensitive than those which contain water of crystallization. The data tabulated... 

An interesting analogous in vitro experiment was performed by Xu el al. [92] in which the presence of a polyelectrolyte in solution resulted in a layer of amorphous calcium carbonate forming under a structured monolayer. This subsequently transformed into a thin layer of polycrystalline calcite. The crystal growth in this in vitro system occurs by phase transition of amorphous calcium carbonate into calcite and not by dissolution of amorphous phase and reprecipitation of calcite crystals. 

In vitro experiments by Iijima and her colleagues demonstrate the advantage of separating the cation and the anion in terms of collagen mineralization [97]. The setup used involved diffusion of calcium and phosphate from opposite sides of collagen membranes produced from the Achilles tendon - a tissue that normally never mineralizes. Crystal formation, density, morphology and orientation were drastically influenced by the orientation of the fibers within the... 

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