Shelf life reaction kinetics

One most often undertakes kinetic studies to develop an understanding of the reaction mechanism. Other motives sometimes apply one can learn about the stability, or shelf life, of a material and the practicality of preparing a given substance in the laboratory or commercially. This book is directed toward individuals who wish to be able to read in their own fields of interest the scientific literature that uses these techniques for the study of chemical reactions and the deduction of their mechanisms. It is also intended to be of use to those who plan to undertake these studies on their own. 

We now turn briefly to the problem of peptide stability in the solid state [8] [88], First, we note that most - if not all - reactions discussed in the previous and subsequent sections can also occur in the solid state, although the kinetics and mechanisms of the reactions can be quite different from those observed in solution. Moisture content, the presence of excipients that act as catalysts, and surface phenomena are all factors whose roles are all-but-im-possible to predict. As a result, each formulation poses a new challenge to pharmaceutical scientists. As a rule, solution data cannot be used to predict the shelf-life of solid formulations, and extrapolating from one solid formulation to another can be misleading. 

Chemical reaction kinetics can be used to evaluate degradation data at accelerated conditions and predict the drug product assay at normal conditions for periods longer than the proposed shelf life. This is applicable to limited cases because the reaction kinetics is often complex for drug products. The following example illustrates the procedure to follow to calculate the API concentration with time for a drug product stored at normal conditions when such data are not yet available. 

In summary, the shelf life from the actual data is 41 months, that estimated from first-order kinetics is also 41 months, and the one predicted from zero-order kinetics is 39 months. All three values are similar. However, the results reveal that the actual degradation reaction of the drug product is more likely to be of first order. 

All explosives undergo thermal decomposition at temperatures far below those at which explosions take place. These reactions are important in determining the stability and shelf life of the explosive, The reactions also provide useful information on the susceptibility of explosives to heat. The kinetic data are normally determined under isothermal condi-... 

However, an understanding of the reaction kinetics in hydrolysis, oxidation, and photodegradation may increase the knowledge of the degradation mechanism and often provide elegant alternatives to prevent instability and an accurate determination of the product shelf life (the expiration date). 

For the degradation of the antibiotic clindamycin, which is held at pH 4.0 in an aqueous solution, kinetic measurements show that at 343 K the reaction is first-order with a rate constant equal to 2.49 x 10 s . Over the temperature range 320 K to 360 K, the activation energy was found to be 123.3 kJ mol k Calculate the rate constant at 295 K. Since the degradation reaction is first-order, it turns out that the time taken for 1% decomposition turns out to be close to 0.01/A r and this time is independent of the initial concentration of the antibiotic. Hence, is it possible to conclude that if the compound were to be stored at 295 K, it would remain safe for use at the end of an economically acceptable shelf-life ... 

This heterogeneity in the kinetics can make a rate constant deceptive since it will change with the extent of reaction. This is just one challenge in making extrapolations of degradation with time a rapid rate due to a reactive API form may not continue once the reactive API form is consumed. Fortunately, the matter is somewhat simplified in pharmaceutical stability testing since only a small amount of degradation determines the shelf-life. 

The zinc and manganese dioxide electrode reaction kinetics in alkaline electrolyte are faster than the same reactions in the Leclanche and zinc chloride electrolyte. These differences in reaction rate determine the differences in cell performance. The alkaline cell delivers superior capacity, higher-rate discharge capability, lower internal resistance, lower leakage, and longer shelf life. Alkaline cells have excellent high-rate... 

The kinetics of quality deterioration in dried onion flakes (NEB and thiosulfinate loss) and dried green beans (chlorophyll-u loss) were studied as a function of water activity and temperature and empirical equations and mathematical models developed that successfully predicted the shelf life of the dried products as a function of temperature and (Table 25.11 and Table 25.12) [88]. Above the mono-layer (flw, 0.32-0.43) for onion, increasing moisture contents resulted in greater reaction rates for browning and thiosulfinate loss. Very little browning was observed over a storage period of 631 d at 20 C and... 

In conjunction with the future development of better quantitive chemical markers, the most significant imminent trend in food quality control is the development and application of new sensor technologies and other instrumental methods for the in-line, on-line, and off-line quantitative determination of these chemical markers. This trend is well-illustrated in many chapters in this book (e.g.. Chapters 4-7, 9, 12, 15, 16, 18, 19, 21-23). Furthermore, accurate kinetic models are needed to be able to predict remaining shelf-life as well as to optimize product quaUty that depends on the intricate interplay of various chemical reactions under various processing conditions. 

Either hydrolysis or condensation may be controlled using acidic or basic catalysis. Essentially, when examining these two reactions, one would ideally like to be able to freeze them in time in order to obtain solutions with long shelf life. Although this is only possible by, for example, cooling down a solution/mixture, it is preferable to use a solution of monomeric-hydrolyzed silanes as a primer solution and catalysis conveniently allows this. The kinetic constant of the hydrolysis and/or condensation reaction vary as a function of the pH of the solution. If acidic catalysis is examined for hydrolysis, then the constant, plotted as log k as a function of pH, exhibits a V-shaped curve with a minimum specific value of pH. This value corresponds to the slowest kinetics of the reaction, usually around pH 6 for hydrolysis and 10 for condensation. O Figure 11.4 shows an example of such a shape. The position of the minimum of the curve will itself depend on the structure of the molecule. 

Measurements were performed by using Besthorn s Hydrazone method which includes spectrophotometric analysis of quinones produced by enzyme. Enzyme electrodes were characterized in terms of maximum reaction rate (V ax) and Michaelis-Menten constant (KJ. In addition to kinetic parameters, stability of enzyme electrodes towards environmental conditions such as pH and temperature was investigated. Usage stability and shelf-life analysis were also examined. 

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