Precipitation experiments, spontaneous

Panic Disorder. Panic attacks are commonly experienced by patients with PTSD. In addition, the avoidant symptoms of PTSD resemble agoraphobia, and the hyperarousal symptoms of PTSD bear some semblance to certain panic symptoms. The key discriminating factor is that patients with panic disorder will, at some point in the course of the illness, experience spontaneous panic attacks arising in the absence of any traumatic reminder or other precipitating stressor. 

FIGURE 8.29 At low concentrations of solute, dissolving is accompanied by a decrease in free energy so it is spontaneous. At high concentrations, dissolving is accompanied by an increase in free energy so the reverse process, precipitation, is spontaneous. The concentration of a saturated solution corresponds to a state of lowest free energy at the temperature of the experiment. 

The results confirm previous suggestions based upon approximate stoichiometry determinations using conventional precipitation experiments that at sufficiently low supersaturation HAP can precipitate without the need for precursor formation both at the surface seed crystals (6)and spontaneously from solution (56). 

Frimer, A. A., J. Org. Chem., 1977, 42, 3194-3196, footnote 7 A new method of preparation involves interaction of ally lie halides in solvents with 98% hydrogen peroxide in presence of silver ion and base at ambient temperature under argon. The reactions must be run in the dark to prevent precipitation of metallic silver, which will catalyse decomposition of the hydroperoxide or excess hydrogen peroxide. In an experiment not run in the dark, the hydroperoxide from 3-chlorocyclohexene ignited spontaneously after isolation and concentration. 

In their non-agitated experiments, these authors used an experimental technique similar to that of Suess and Fiitterer (1972) for generating spontaneous precipitation. The problems with this technique were previously discussed. After... 

It is imperative for the technical chemist to keep an accurate record of all his experiments, and the student should become accustomed to recording all of the work he performs in his laboratory notebook. He should not write up the directions he followed, but should describe the procedure he actually used in sufficient detail so that he later can repeat the experiment exactly on the basis of his notes, without introducing any new conditions. It is necessary, therefore, to make an accurate record of quantities used, the temperature maintained, and the reaction time, as well as the nature and size of the apparatus. The notes should also include observations made during the course of the reaction, such as the appearance or disappearance of a color or of a precipitate, hbera-ation of gas, spontaneous increase in temperature, etc. It should also be noted how the rate of the reaction was controlled, how its completion was determined, how the purity of the product was established, etc. 

Numerous kinetic studies have been made of the spontaneous precipitation of calcium phosphates from solutions containing concentrations of lattice ions considerably in excess of the solubility values (33, 34). Although attempts, are usually made to control the mixing of reagent solutions, it is difficult to obtain reproducible results from such experiments since chance nucleation of solid phases may take place on foreign particles in the solution. Many of these difficulties can be avoided by studying the crystal growth of well-characterized seed crystals in metastable supersaturated solutions of calcium phos.phate. 

---