PH-induced precipitation

FA can interact with clay minerals and are known to form stable complexes with metal ions and hydrous oxides [59,61]. The operational technique for isolation of HA involves a pH-induced precipitation and it is likely that accessory minerals may be associated with the precipitation process. Complexes of HA and clay minerals are also formed, the increased ash content of HA suggesting that amorphous silica, iron hydroxides, and clay may aggregate with the HA fraction [58,60,61]. 

Fig. 1. pH-induced precipitation of a random copolymer of methacrylic acid and methacrylate (commercialized as Eudragit S 100 by Rohm Pharma GMBH, Weiterstadt, Germany) (open squares) and p-amino-phenyl-a-D-glucopjrranoside-modified copolsmier (open circles) measured as turbidity at 470 nm. Some decrease in turbidity at lower pH values is caused by flocculation and sedimentation of the poljmier precipitate. Redrawn from Ref 8. 

Most noticeable is the large change in pH induced by the precipitation of 0.196 mmol kg-1 of calcium carbonate, o... 

If a surface precipitate of metal hydroxy-polymer has formed on an adsorbent, the -pH relationship for the coated adsorbent should resemble closely that observed for particles consisting purely of the hydroxy-polymer or the hydrous oxide of the metal (15). This kind of evidence for Co(ll), La(lII), and Th(lV) precipitation on silica colloids was cited by James and Healy (15). It should be noted, however, that the increase in C toward a maximum value often occurs at pH values well below that required thermodynamically to induce bulk-solution homogeneous precipitation of a metal hydrous oxide (15, 16). If surface precipitation is in the incipient stage under these conditions, it must be a nucleation phenomenon. James and Healy (15) argue that the microscopic electric field at the surface of a charged adsorbent is sufficiently strong to lower the vicinal water activity and induce precipitation at pH values below that required for bulk-solution precipitation of a metal hydrous oxide. 

For mixed metal oxides obtained from their hydroxide or carbonate precursors after calcination, it is generally difficult to determine whether the as-prepared precursor is a single-phase or multiphase solid solution [35]. Non-aqueous solvents appear superior for achieving two dissimilar metal oxides such as MM Oz or MM 04 precipitates such reactions cannot be carried out simultaneously in aqueous solution due to the large variations in pH necessary to induce precipitations [41,42]. Table 6.1 summarizes some of the nanoparticulate semiconducting metal oxides and mixed metal oxides prepared via co-precipitation techniques. The general procedure of achieving metal loaded nanoparticles on an oxide support is shown in Fig.6.5. 

Decrease in pH. After heating at 140°C for 20 min, the pH of milk has decreased to about 5.8 due to acid production from pyrolysis of lactose, precipitation of soluble calcium phosphate as Ca3(P04)2, with the release of H+, and dephosphorylation of casein with subsequent precipitation of the liberated phosphate as Ca3(P04)2 with the release of H+. The heat-induced precipitation of Ca3(P04)2 is partially reversible on cooling so that the actual pH of milk at 140°C at the point of coagulation is much lower than the measured value and is probably below 5.0. 

In the absence of calcium, /3-lactoglobulin solubility increases as pH is increased from 6.4 to 7. Addition of calcium at any pH causes a decrease in solubility. At any ratio of pH and calcium ion concentration, the increased charge on the protein induced by pH change is balanced by added calcium ions to hold the level of denaturation constant (de Wit 1981). This suggests that calcium-induced precipitation of /3-lactoglobulin occurs by an isoelectric mechanism. However, Hillier et al. (1979) found that an increase in the calcium concentration up to 0.4 mg/ml slowed heat denaturation of /3-lactoglobulin, but additional calcium had little effect. 

The primary proteins in milk are the caseins. These may be removed from milk by acid and heat-induced precipitation at pH 4.6 followed by centrifugation. The remaining supernatant (called whey) contains primarily a-lactalbumin and /3-lactoglobulin. These two proteins may be separated by gel filtration or with a Cu(II)—IDA-agarose affinity column. 

H-Gly-Pro-Hyp-OH (3.0 g, 10.5 mmol) was dissolved in 10% Na2C03 (54 mL) and stored at 4°C. N-Fluoren-9-ylmethyl succinimidyl carbonate (4.05 g, 12.0 mmol) was dissolved in DME (45 mL) and stirred at 4°C. The aq Na2C03 soln was added slowly to the DME soln, and the reaction proceeded for 2.5 h at 4°C and 21 h at rt. The soln was filtered, and H20 (360 mL) was added to the filtrate. The aqueous layer was extracted with Et20 (300 mL), acidified to pH 2 with coned HQ, reduced to halfvolume at 80 °C under reduced pressure, and stored at 4 °C for 24 h to induce precipitation. The aqueous layer was decanted from the oily precipitate, reduced to ca. 30 mL at 81 °C under reduced pressure, and stored at 4°C for 24 h. Again, the aqueous layer was decanted from the oily precipitate. Both oily precipitates were dissolved in MeOH (20 mL), then EtOAc (250 mL) was added. A white residue was recovered by evaporation at 73 °C for 1 h under reduced pressure yield 3.23 g (60%). The identity of the... 

Additionally, H2 formed at the cathode increase the pH of the wastewater thereby inducing precipitation of Cr3+ andFe3+ as corresponding hydroxides Cr(OH)3 and Fe(OH)3 (Golder et al. 2007). 

Apart from raising the pH level, precipitation can be induced in a variety of other ways. Anionic species, for example, can be deposited on the surface of suspended carriers by decreasing the pH level. This procedure has been used for vanadium(V) and Mo(VI). Oxidation at a pH level where the ions of the lower valency are soluble and the oxidized species insoluble, can also be utilized to precipitate from a homogeneous solution. Iron(n/m) and Mn(III/IV) are cases in point. Oxidation can be... 

Albumin nanoparticles can be prepared by controlled desolvation, pH-induced coacervation, and/or chemical cross-linking with glutaraldehyde. Briefly, the pH of an aqueous solution of albumin is raised to about 9.0 and nanoparticles are precipitated by adding a miscible cosolvent such as acetone [32], A study reports attempts to optimize the desolvation method to prepare albumin nanoparticles of a more controlled particle size and narrower particle size distribution [33],... 

The increase of atmospheric CO2 may have decreased the pH of precipitation very slightly, but PcOj in soils is far more important for the acid-base status of surface waters. Variation in forest soil Pco is related to the temperature and moisture content of soils as well as the release of excess soil CO2 to the atmosphere. Warmer conditions increase the rate of microbial and root respiration in the soil, thereby increasing soil Pco above the long-term average value and producing short-term increases in runoff ANC, and vice versa. Norton et al. (2001) found that intra-seasonal variations in Pco caused by variable snowpack thickness could induce variation in ANC in runoff of 10-15 peqL Such variability is comparable to variability in ANC caused by a 15-20 p.eqL change in SO4 in runoff. Decline in soil Pco, despite increased temperature and possibly increased soil respiration could result from a lower soil moisture content and a greater efflux of soil CO2. 

The uniqueness of the ferritin structure arises from the metabolic requirement to organize and utilize dissolved iron at concentrations and pH levels that induce precipitation of potentially toxic solid phases. Not only is iron solubilized by micelle encapsulation, but homeostatic control is also maintained. The study of ferritin therefore provides an important example of biological control of solid state reactions that involve the formation and organization of nanometer-size inorganic solids in biological time and space. 

The incipient wetness catalyst was prepared with an aqueous solution of Ni(N03)2.6 H2O added slowly to the support (Silica Ketjen 77, 270 m g" ). The solution concentration was adjusted to obtain catalysts with ca. 10% nickel content. The precipitation-deposition catalyst was prepared by precipitating nickel from an aqueous solution of nickel nitrate slurried with the support. The precipitation was produced by a slow and homogeneous change in the pH induced by urea thermal decomposition. 

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