Precipitation optimum temperature

Note The optimum temperature for this process is 25° C. If the solution is cold it will be difficult, if not impossible to cause precipitation. If necessary, warm the j ar in a basin of warm water. Do not allow it to become hot or the toxin will be destroyed. 

The effect of temperature is complex. Solubility usually increases so that the supersaturation ratio decreases with increasing temperature. On the other hand, the size of the critical agglomerate for nucleation decreases with increasing temperature [Equation (8-5), Section 8-4]. Therefore an optimum temperature exists for precipitate formation. 

Macerate (grind) one-half cake of yeast or half an envelope of dry yeast in 50 mL of water in a beaker, add 0.35 g of disodium hydrogen phosphate, and transfer this slurry to a 500-mL round-bottomed flask. Add a solution of 51.5 g of sucrose in 150 mL of water, and shake to ensure complete mixing. Fit the flask with a one-hole rubber stopper containing a bent glass tube that dips below the surface of a saturated aqueous solution of calcium hydroxide (limewater) in a 6-in. test tube (Fig. 1). The tube in limewater will act as a seal to prevent air and unwanted enzymes from entering the flask, but will allow gas to escape. Place the assembly in a warm spot in your desk (the optimum temperature for the reaction is 35°C) for one week, at which time the evolution of carbon dioxide will have ceased. What is the precipitate in the limewater ... 

Optimum operating conditions (type and concentration of the precipitant agent, temperature, aging time, agitation, pH) for the precipitation are required in various situations. 

Unlike LiCo02 cathode material that is synthesized via a relatively simple solid state process, the LiNii.y.zMnyCoz02 compounds are usually prepared using a multistep process based on the modified mixed-hydroxide approach. The first step is predpitalion of a mixed metal hydroxide, Nii.y.zMuyCoz(OH)2, which is then reacted with a lithium salt in air or oxygen, with the optimum temperature of 800-900 C. The mixed metal hydroxide precipitation (alternatively, mixed metal carbonate) is a crucial step, requiring rigorous parameters control (Fig. 1.4). 

Titanium and Niobium. These are added to give stabilization against the harmhil precipitates, such as chromium carbides. These elements combine with carbon and nitrogen and do not allow carbon and nitrogen to precipitate as carbides and nitrides which lead to intergranular corrosion. The optimum temperature for formation of TiC (titanium carbide) is 900-950°C. 

Because of constitutional complexity, the exact chemistries of nickel-base superalloys must be controlled carehiUy in order to avoid the precipitation of deleterious topologically close-packed (TCP) phases and extraneous carbides after long-term high temperature exposure. Heat-treatment schedules and thermomechanical treatments in the case of wrought alloys also are important to provide optimum strength and performance. 

Precipitation is affected by pH, solubiUty product of the precipitant, ionic strength and temperature of the aqueous stream, and the presence of metal complexes. For each metal precipitant, there is an optimum pH where its solubiUty is lowest and hence, the highest removals may be achieved. When an aqueous stream contains various metals, the precipitation process caimot be optimized for each metal, sometimes making it difficult to achieve effluent targets for each. SolubiUty products depend on the form of the metal compound and ate lowest for metal sulfides, reflecting the relative insolubiUty of these compounds. For example, the solubiUty product for lead sulfide [1314-87-0] is on the order of compared to 10 for lead carbonate. Metal... 

The optimum precipitate is obtained by a more elaborate heal treatment the alloy is solution heat-treated (heated to dissolve the impurity), quenched (cooled fast to room temperature, usually by dropping it into oil or water) and finally tempered or aged for a controlled time and at a controlled temperature (to cause the precipitate to form). 

The study of the product distribution from the isomerization of 3,3-dimethylbutene proved useful for evaluating the strength of the acid centers in aluminas (36). Pure alumina from aluminum isopropoxide which was calcined at 700° showed optimum activity. Heating at higher temperatures decreased the number of acid sites as well as their acid strength. Aluminas obtained from potassium or sodium aluminate contained alkali in amounts of 0.08 to 0.65%, depending on the way of precipitation and on the number of washings. 

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