Dissolution precipitation

The physical chemist is very interested in kinetics—in the mechanisms of chemical reactions, the rates of adsorption, dissolution or evaporation, and generally, in time as a variable. As may be imagined, there is a wide spectrum of rate phenomena and in the sophistication achieved in dealing wifli them. In some cases changes in area or in amounts of phases are involved, as in rates of evaporation, condensation, dissolution, precipitation, flocculation, and adsorption and desorption. In other cases surface composition is changing as with reaction in monolayers. The field of catalysis is focused largely on the study of surface reaction mechanisms. Thus, throughout this book, the kinetic aspects of interfacial phenomena are discussed in concert with the associated thermodynamic properties. 

A big concern in swimming pools is prevention of etching and scaling (ie, precipitation of CaCO ) which can be controlled by maintenance of a proper degree of saturation with respect to calcium carbonate. The calcium carbonate dissolution-precipitation equiUbrium is represented by ... 

Chemical reactions can also affect the k and k terms and thereby influence or control coUoidal stabUity (21,121). Pertinent examples are dissolution, precipitation, hydrolysis, precipitation, and chemical complexing. The last reaction may involve either simple species, eg. 

Central pore being filled by the dissolution precipitation mechanism... 

The starting material and shock-activated powder were mixed with 5-wt% MgO and heated for various periods. At the end of each period the phase content of the samples was determined with x-ray diffraction. In this environment it is thought that the phase is formed by a dissolution-precipitation process as shown in Fig. 7.8. As indicated in Fig. 7.9, the shock-activated silicon nitride displays substantially enhanced dissolution rates that are strongly dependent on shock pressure between 22 and 27 GPa. 

There is considerable metallographic and electrochemical evidence in support of each theory and it is of interest to note that two of the most authoritative works on corrosion appear to support opposite views — Uhlig favours the selective dissolution of zinc theory, whereas Fontana and Greene favour the dissolution-precipitation theory. 

For the extraction of rubber and rubber compounds a wide variety of solvents (ethyl acetate, acetone, toluene, chloroform, carbon tetrachloride, hexane) have been used [149]. Soxtec extraction has also been used for HDPE/(Tinuvin 770, Chimassorb 944) [114] and has been compared to ultrasonic extraction, room temperature diffusion, dissolution/precipitation and reflux extraction. The relatively poor performance of the Soxtec extraction (50% after 4h in DCM) as compared with the reflux extraction (95% after 2-4 h in toluene at 60 °C) was described to the large difference in temperature between the boiling solvents. Soxtec was also used to extract oil finish from synthetic polymer yam (calibration set range of 0.18-0.33 %, standard error 0.015 %) as reference data for NIRS method development [150]. 

S, Soxhlet S , Soxtec R, reflux SF, shake-flask US, ultrasonics SFE, supercritical fluid extraction MAE, microwave-assisted extraction PFE, pressurised fluid extraction (ASE , ESE ) D/P, dissolution/precipitation. 

Wieboldt et al. [560] have described SFE-SFC-FTIR analysis of hindered phenol primary antioxidants and phosphite secondary antioxidants in PE. SFE is more selective for the lower-range low-MW polymer than Soxhlet-type extraction. This yields a chromatogram with less interference from low-MW polymer peaks in the region where the additive components elute. As a result, SFE appears to be a better choice than Soxhlet-type extraction for the selective removal of additives from flaked polymer. SFE and dissolution/precipitation methods were compared for a PVC/stabiliser system [366]. 

A limited list of solvent/nonsolvent combinations for dissolution/precipitation is given in Table 3.50. More extensive listings may be found elsewhere... 

Table 3.51 Main characteristics of dissolution/precipitation methods...

Dissolution Precipitation, solvent evaporation, redissolution RPLC, FTIR, NMR... 

Applications Dissolution/reprecipitation is claimed to be the most widespread approach to polymer/additive analysis [603], but recent round-robins cast some doubt on this statement. Dissolution appears to be practised much less than LSEs. However, in cases where exhaustive extraction is difficult, e.g. for polyolefins containing high-MW (polymeric) additives, a dissolution/precipitation method is preferred. 

In a comparative qualitative study El Mansouri et al. [632] examined dissolution/precipitation and Soxhlet extraction of phosphorous antioxidants in PP. The dissolution (refluxing toluene)/precipitation... 

Cortes et al. [634] have recently used /rSEC-GC/LC in a comparative quantitative study of dissolution and dissolution/precipitation of PC/(2,4-di-f-butylphenol, nonylphenol isomers, Tinuvin 329, Irgafos 168) and ABS/(nonylphenol isomers, Tinuvin P, BBP, Vanox 2246, Tinuvin 328/770, Topanol CA, Acrawax). For the ABS sample the dissolution approach determined a four-fold higher concentration for Vanox 2246 than by dissolution/precipitation of the sample, indicating that precipitation can yield low (incorrect) results for additives which exhibit solubility dependence. Using both sample preparations equivalent concentrations were observed for the additives of the PC sample, except for Tinuvin 329. 

The oligomer distribution is highly dependent on the method of isolation of the oligomers (TLC, Soxhlet or dissolution/precipitation). Jung and Lee [513] have determined a dimer of 2-phenylbenzoxazole in a polyester resin by means of TLC. A comparative study of the separation of styrene oligomers (up to dodecamers) by TLC and OPLC has appeared [514]. 

RPLC-PDA is frequently used for quality control, such as the determination of free Irganox 1098 in PA4.6 (at 278 nm after dissolution/precipitation), of free Irganox 1010/1076 in PP (at 278 nm after extraction with MTBE, thus avoiding dissolution of polymer waxes), of Luperco 802 in PP (at 218 nm, after extraction with HCC13), and of Tinuvin 122 in HDPE (at 225 nm as diol). The advantages of the use of HSLC over conventional LC in QC of plastics and additives have been demonstrated, e.g. for AOs in PE, mixed phthalate esters and residual terephthalic acid in PET and partially cured epoxy resins [557],... 

Little is known about the kinetics of dissolution, precipitation, and oxidation-reduction reactions in the natural environment. Consequently, simulating the kinetics of even more complicated injection- zone chemistry is very difficult. 

---