Shift factors precipitated

The common ion effect (Chapter 3) is a further important factor affecting solubilities. Addition of A or B to the above system (equation (5.28)) will shift the equilibrium to the left and reduce the solubility of AB. In practice, this situation would arise when an excess of a precipitating reagent has been added to an analyte solution. Such an excess leads to the possibility of complexation reactions occurring which will tend to increase the solubility of AB. For example, when aluminium or zinc is precipitated by hydroxyl ions, the following reactions with excess reagent can occur... 

An internal electrochemical mechanism was proposed long ago for deposition on certain metal substrates, since the rate of deposition sometimes depended on the nature of the substrate [11].) The standard potential of Reaction (5.3) is -l- 0.08 V, considerably more positive than the rednction potential of S to (-0.45 V). Free sulphide, if formed, would be in a very low concentration, since it will be removed continually by precipitation of PbS this will move the S rednction potential strongly positive according to the Nemst equation [Eq. (1.32)]. This positive shift will be even greater than normal because of the non-Nemstian behavior of the S /S couple when [S] > [S ] (at least in alkaline solntion) [12]. In opposition to this, the solubility of S in the (slightly acidic) aqneons solntions is very low, which will move the potential in the opposite direction. Add to this the very small concentration of S in acid solution [Eq. (1.15)], and it becomes clear that it is not trivial to estimate the feasibility of the formation of PbS by free snlphide. The non-Nemstian behavior of the sulphur-rich S /S couple and the lack of knowledge of the solnbility of free S in the deposition solntion are the two factors that complicate what would have been a tractable thermodynamic calcnlation. 

If there is introduced into the solution from some other source an ion that is in common with an ion of the insoluble solid, the chemical equilibrium is shifted to the left, and the solubility of that solid will be greatly decreased from what it is in pure water. This is called the 11 common-ion effect." This effect is important in gravimetric analysis, where one wishes to precipitate essentially all of the ion being analyzed for, by adding an excess of the "common-ion" precipitating reagent. There is a practical limit to the excess, however, which involves such factors as purity of precipitate and possibility of complex formation. You can calculate the solubility under a variety of conditions, as illustrated in the following problem. 

The DNA sequencing chemistry begins with a base-modification reaction, the extent of which determines the frequency of DNA cleavage in the subsequent phosphate-elimination reaction. The number of bases modified in each molecule depends on the concentration of dimethylsulphate (G and G A reactions) and hydrazine (C T reactions) as well as the temperature and duration of the reaction. For speed and convenience the Maxam-Gilbert procedure makes use of temperature shifts and dilution to control the rate and extent of these reactions. The reagents are mixed at 0°C, incubated at 20°C for the required time and the DNA precipitated with cold sodium acetate and ethanol to slow down or halt the reaction. A fixed concentration of the different reagents is usually used so the main factor determining the extent of reaction is the time of incubation at 20°C. 

One of the key factors for yield improvement is to precipitate or remove MgX2 from solution. This can be accomplished by going to less polar solvents, such as heptane, or by forming insoluble complexes, such as MgX2 dioxane. Potential explanations for this effect are removal of a Lewis acid, shift in the Schlenk equilibrium, or the change in solvent polarity that affects the nucleophilicity of the Grignard reagent. 

Of the mechanisms suggested, dilution is a likely influence. Dilution certainly occurred during water level recovery after the mines closed. The amount and influence of dilution cannot be precisely quantified, but isotope modeling suggests dilution factors of 45 to 85%. These dilution factors could saturate the water with respect to siderite, gypsum, anhydrite and cerussite, but not smithsonite or anglesite (2). The pH during oxidation is unknown, but pH 5 or below in the mine environment is not an unrealistic hypothesis, and low pH makes the calculated carbon isotope values shift toward the observed ratios. Siderite precipitation... 

For liquid dosage forms, altered bioavailability upon storage is generally manifest in precipitation of API or other formulation components. Precipitation can result from a number of factors. With small molecules, precipitation can be caused by shifts in the pH of the solution (suspension). Such shifts can be due to absorption of carbon dioxide, chemical degradation of a component that generates an acid or base or loss of a buffer component due, for example, to oxidation. Another factor with small molecules is precipitation due to an increase in the API particle size. This effect, called Ostwald ripening, is caused by the gradual dissolution of smaller... 

The second factor involves changes in the middle-latitude precipitation pattern caused by a poleward shift of the middle-latitude rainbelt, a region associated with large-scale cyclonic disturbances. In the high-C02 atmosphere, warm, moisture-rich air penetrates further north than in the normaI-C02 atmosphere. This is caused by a greater transport of moisture from lower to higher latitudes. Thus, precipitation increases significantly in the northern half of the rainbelt, whereas it decreases in the... 

---