Colloid precipitation

The vanadium pentoxide catalyst Is prepared as follows Suspend 5 g. of pure ammonium vanadate in 50 ml. of water and add slowly 7 5 ml. of pure concentrated hydrochloric acid. Allow the reddish-brown, semi-colloidal precipitate to settle (preferably overnight), decant the supernatant solution, and wash the precipitate several times by decantation. Finally, suspend the precipitate in 76 ml. of water and allow it to stand for 3 days. This treatment renders the precipitate granular and easy to 6lter. Filter the precipitate with suction, wash it several times with cold 5 p>er cent, sodium chloride solution to remove hydrochloric acid. Dry the product at 120° for 12 hours, grind it in a mortar to a fine powder, and heat again at 120° for 12 hours. The yield of catalyst is about 3 - 5 g. 

Fouling can be characterized by mechanism and location. Membranes can foul in three places on, above or within the membranes (refer to the sidebar on the next page). The term agglomeration in the general sense, describes colloidal precipitates... 

A colorless, colloidal precipitate was formed and stirred thoroughly for about 15 minutes, whereupon it was filtered by suction. The raw product thus obtained was washed with water until It contained only about Va% water-soluble salts. After drying for 12 hours In a vacuum apparatus at 60°C and under a pressure of 12 mm Hg, the product had the form of hard pieces. The pieces were comminuted to powder in a ball mill and the powder was passed through a sieve (3,600 meshes per cm ). The small residue on the sieve was again pulverized and passed through the same sieve. The yield was 870 g, or 99% of theoretical, calculated on the assumed formula... 

In the second method diphenylcarbazide is employed as an adsorption indicator. The end-point is marked by the pink colour becoming pale violet (almost colourless) on the colloidal precipitate in dilute solution (ca 0.01 M) before the opalescence is visible. In 0.1M solutions, the colour change is observed on the precipitated particles of silver cyanoargentate. 

At a pH of 8.5 or more, all the iron is in the ferric form, usually as a colloidal precipitate. If the iron level is very low, it tends to be ignored for most general purposes, but if the concentration rises to perhaps 0.3 ppm Fe or more, it causes discoloration upon precipitation and fouling and must be removed. 

The different growth modes discussed above have been exemplified also from structural studies. Froment and Lincot [247] used structural characterization methods, such as TEM and HRTEM, to determine the formation mechanisms and habits of chemically deposited CdS, ZnS, and CdSe thin film at the atomic level. These authors formulated reaction schemes for the different deposition mechanisms and considered that these should be distinguished to (a) atom-by-atom process, providing autoregulation in normal systems (b) aggregation of colloids (precipitation) ... 

In the case of aerated protein icings, however, stabilizers are essential. Where hot water is used, gelling-type stabilizers work best (agar, gelatin, Irish moss extract). In cold foams, prepared with cold water, cellulose-type gums are used to contain water in the foams colloidal precipitates, such as are formed through the reaction of carrageenin and protein, are very helpful. 

EXPERIMENTAL PROCEDURES AUTOMATED TURBIDIMETRIC TITRATION. A method for the automated aqueous turbidimetric titration of surfactants has been published (10) in which anionic surfactants are titrated against N-cetylpyridinium chloride to form a colloidal precipitate near the equivalence point. N-cetylpyridinium halides have a disadvantage in that they have the tendency to crystallise out of solution (15), consequently the strength of the solution may alter slightly without the knowledge of the operator, also the crystals suspended in solution may cause damage to the autotitrator. In view of these drawbacks hyamine was preferred as the titrant. 

EMULSION MECHANISM. The above results show that the indicator was unnecessary because the method is essentially a turbidimetric one. However, the occurence of a sharp minimum in the titration curve at low salt concentrations, suggests that the formation of a colloidal precipitate is not the underlying mechanism for the turbidity. Consequently, efforts were made to clarify the underlying physico-chemical mechanism of the titration. 

The 02(aq) reacted with the Mn11, forming a colloidal precipitate of oxidized manganese, likely composed of a mixture of Mnm and MnIv. We can write this reaction in a simple form as,... 

Cadmium sulfate hydrate, 4 515 physical properties of, 4 509t Cadmium sulfate monohydrate, 4 515 physical properties of, 4 509t Cadmium sulfide, 4 503, 515-516, 518, 521 colloidal precipitation color, 7 343t color and bad gap, 7 335t physical properties of, 4 509t piezochromic material, 6 607 Cadmium sulfide photodetectors, 19 137 Cadmium sulfide photoconductor, fabrication and performance of, 19 155-156... 

Siloxane compounds, in vitreous silica manufacture, 22 414 Siloxane materials, 20 240 Siloxane oligomers, in silicone polymerization, 22 555-556 Siloxanols, silylation and, 22 703 Silsesquioxane hybrids, 13 549 Silsesquioxanes, 15 188, 22 589-590 SilvaGas process, 3 696, 697 Silver (Ag), 22 636-667. See also Silver compounds. See Ag entries Argentothiosulfate complexes Batch desilverizing Lead-silver alloys Palladium-silver alloy membranes analytical methods for, 22 650-651 applications of, 22 636-637, 657-662 as bactericide, 22 656, 657, 660 barium alloys with, 3 344 in bimetallic monetary system, 22 647-648 in cast dental gold alloys, 8 307t coke formation on, 5 266 colloidal precipitation color, 7 343t colloidal suspensions, 7 275 color, 7 334, 335... 

Free metal ions Inorganic ion pairs complexes Organic complexes chelates Metals species bound to high molecular weight DOM Metals in colloids Metals sorbed onto colloids Precipitates... 

The size of surfactant precipitate structures may vary widely, from colloidal precipitate, too small to be seen visually, to large crystals. This is an important property, because some deleterious effects of surfactant precipitation could be minimized or beneficial effects of precipitation could be negated if the precipitate were of colloidal dimensions. Examples of the importance of precipitate structure and size include the aforementioned oil field process and recovery of surfactant from surfactant-based separations via precipitation-fiItering. Effects of using surfactant mixtures on precipitate structures would be useful to know. 

The total concentration of complex formed during the complexation is proportional to X. During template polymerization of acrylic acid, a stable colloidal precipitate resulted in the systems under investigation, and turbidity measurements could be used, assuming that direct reading from the turbidimeter (in logarithmic scale) is proportional to the amount of polymeric product. The assumption was checked by calibration procedure. The light absorption (%) is proportional to the concentration of poly(acrylic acid)-poly(vinyl pyrrolidone) mixture.100% conversion was assumed when no increase in turbidity was detected by the recorder. In the case when copolymers were used as templates, the apparatus was calibrated for each copolymers separately. 

PQ Corp. gel, colloidal, precipitated North America, Latin America 20... 

In this chapter we describe characteristic validation procedures of the Heavy Metals Limit Test in the Japanese Pharmacopoeia (JP) [1]. Although an equivalent test is commonly listed in both the United States Pharmacopoeia and the European Pharmacopoeia, there are differences in the color reagents and conditions of sample preparation of the JP procedure. Heavy metals are defined in the JP as poisonous metallic impurities such as Pb, Bi, Cu, Cd, Sn, and Hg that form colored colloidal precipitates with sodium sulfide TS in a slightly acidic solution of pH 3 to 4. The level is expressed as the equivalent quantity of lead. 

Adsorbed, occluded, and included impurities are said to be coprecipitated. That is, the impurity is precipitated along with the desired product, even though the solubility of the impurity has not been exceeded. Coprecipitation tends to be worst in colloidal precipitates... 

The catalyst is prepared by suspending 20 g. of c.p. ammonium metavanadate in 200 cc. of water and adding slowly 30 cc. of concentrated hydrochloric acid (sp. gr. 1.19). The reddish-brown semi-colloidal precipitate is washed several times with water by decantation and finally suspended in 300 cc. of water and allowed to stand at room temperature for three days. This treatment makes the precipitate granular and easy to filter. The precipitate is collected on a filter using a pump and washed several times with water to free it from hydrochloric acid. It is then dried at 120° for twelve hours, finely powdered and again dried for twelve hours at 120°. 

Iron needs to be removed by an oxidation process and filtered off. Iron levels of 1 to 10 ppm can often be found in subsurface waters and this may rise to perhaps 25 ppm in seriously anaerobic water usually sulfides are also present. Both iron and manganese can present problems if present in cooling water, and it is necessary to confirm whether some or all of the iron found has originated from the makeup supply or if it is an indicator of corrosion within the system itself. Ferrous (Fe2+) and ferric (Fe3+) iron may be present in the cooling water depending on the pH. At a pH level of 8.5 or more all the iron will be in the ferric form, usually as a colloidal precipitate. 

Copper-cadmium granules Mix 25 g 40 to 60 mesh Cd granules with 6 N HC1 and shake well. Decant off HC1 and rinse the granules with distilled water. Add 100 mL 2% CuS04 solution to the cadmium granules. Swirl for 5 to 7 min. Decant and repeat this step with fresh CuS04 solutions until a brown colloidal precipitate of Cu appears. Wash out the precipitate with water. 

The composition of the insoluble compound (precipitate) obtained from the analyte must be known and stable. Poorly soluble substances may form colloidal suspensions (particle diameters from 10 7 to 10 4 cm). The formation of a colloidal suspension can be minimized or prevented by carrying out the precipitation from a dilute solution of the analyte, at a temperature close to the boiling point of water and with constant stirring. The relative supersaturation affects the particle size and is expressed as Q - S/S, where Q is the instantaneous concentration of the added species and S is the equilibrium solubility of the compound that precipitates. Particle size seems to be inversely proportional to relative supersaturation. The electric double layer formed during precipitation keeps the colloidal precipitate particles from coming into contact with each other, thus preventing further coagulation. 

Figure 7. Formation of gels from iron molybdate precipitates [5]. A very fine colloidal precipitate forms in all cases but undergoes different changes suggested by the arrows, according to concentration and Mo Fe proportions, as well as other conditions mentioned in the figure. In the upper part domain, the colloidal precipitate seems to dissolve to form a solution whose viscosity increases until formation of a gel. In the right-hand domain, the gel directly forms from the colloidal system but can become transparent if the reaction takes place in the temperature range 293-313K.

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