principal components shows the number of independent properties being measured by the tests considered.
Following from this, it is necessary to establish the number of independent properties that are necessary to predict product performance in service with the goals of rendering any specification more meaningful and allowing a high degree of predictability of product behavior. On a long-term approach it might be possible to obtain new tests of a fundamental nature to replace, or certainly to supplement, existing tests. In the short term, selecting the best of the existing tests to define product quality is the most beneficial route to predictability.
On the other hand, the precision of a test method is the variability between test results obtained on the same material, using a specific test method. The precision of a test is usually unrelated to its accuracy. The results may be precise, but not necessarily accurate. In fact, the precision of an analytical method is the amount of scatter in the results obtained from multiple analyses of a homogeneous sample. To be meaningful, the precision study must be performed using the exact sample and standard preparation procedures that will be used in the final method. Precision is expressed as repeatability and reproducibility.
The intra-laboratory precision or the within-laboratory precision refers to the precision of a test method when the results are obtained by the same operator in the same laboratory using the same apparatus. In some cases, the precision is applied to data gathered by a different operator in the same laboratory using the same apparatus. Thus, intra-laboratory precision has an expanded meaning insofar as it can be applied to laboratory precision.
Repeatability or repeatability interval of a test (r) is the maximum permissible difference due to test error between two results obtained on the same material in the same laboratory.
The repeatability interval (r) is, statistically, the 95% probability level or the differences between two test results are unlikely to exceed this repeatability interval more than five times in a hundred.
The inter-laboratory precision or the between-laboratory precision is defined in terms of the variability between test results obtained on the aliquots of the same homogeneous material in different laboratories using the same test method.
The term reproducibility or reproducibility interval (R) is analogous to the term repeatability but it is the maximum permissible difference between two results obtained on the same material but now in different laboratories. Therefore, differences between two or more laboratories should not exceed the reproducibility interval more than five times in a hundred.
Acetogenesis
Acetogenesis is the third phase of anaerobic digestion in which simple molecules created through the acidogenesis phase are further digested by acetogens to produce largely acetic acid, as well as carbon dioxide and hydrogen.
The acid-producing bacteria, involved in the second step, convert the intermediates of fermenting bacteria into acetic acid (CH3COOH), hydrogen (H2) and carbon dioxide (CO2). These bacteria are facultatively anaerobic and can grow under acid conditions. To produce acetic acid, the bacteria need oxygen and carbon. For this, they use the oxygen solved in the solution or bounded-oxygen whereby the acid-producing bacteria create an anaerobic condition which is essential for the methane producing microorganisms. Moreover, these bacteria reduce the compounds with a low molecular weight into alcohols, organic acids, amino acids, carbon dioxide, hydrogen sulfide and traces of methane.
From a chemical standpoint, this process is partially endergonic (i.e., only possible with energy input), since bacteria alone are not capable of sustaining that type of reaction. An endergonic reaction is a chemical reaction in which total amount of energy is a loss – it takes more energy to initiate the reaction than the energy produced by the reaction and, thus, the total energy is a negative net result.
An acetogen is a microorganism that generates acetate (CH3COO−) as an end product of anaerobic respiration (fermentation) and can produce, in most cases, acetate as the end product) from two molecules of carbon dioxide (CO2) and four molecules of molecular hydrogen (H2). This process (acetogenesis) is different from acetate fermentation, although both occur in the absence of molecular oxygen (O2) and produce acetate.
Acetogens are found in a variety of habitats, generally those that are anaerobic (lack oxygen). Thus, acetogenesis is a process through which acetate is produced from carbon dioxide and an electron source (such as hydrogen and carbon monoxide) by anaerobic bacteria. In this reaction, carbon dioxide is reduced to carbon monoxide and formic acid (HCO2H) or directly into a formyl group, the formyl group is reduced to a methyl group and then combined with the carbon monoxide and Coenzyme A produce acetyl-CoA. Two specific enzymes participate on the carbon monoxide side of the pathway: (i) CO-dehydrogenase, which catalyzes the reduction of the carbon dioxide and (ii) acetyl CoA synthase, which combines the resulting carbon monoxide with a methyl group to give acetyl-CoA.
The key aspects of the acetogenic pathway are several reactions that include the reduction of carbon dioxide to carbon monoxide and the attachment of the carbon monoxide to a methyl group. The first process is catalyzed by specific enzymes (carbon monoxide dehydrogenase enzymes) and the coupling of the methyl group (provided by methylcobalamin) and the carbon monoxide is catalyzed by acetyl CoA synthetase.
The accumulation of hydrogen can inhibit the metabolism of the acetogenic bacteria and present knowledge suggests that hydrogen may be a limiting feedstock for methanogens. This assumption is based on the fact that addition of hydrogen-producing bacteria to the natural biogas-producing consortium increases the daily biogas production. At the end of the degradation chain, two groups of methanogenic bacteria produce methane from acetate or hydrogen and carbon dioxide. These bacteria are strict anaerobes and require a lower redox potential for growth than most other anaerobic bacteria.
See also: Acidogenesis, Anaerobic Digestion, Methanogenesis.
Acid Catalyst
A catalyst alters the rate of a reaction without changing the reaction thermodynamic parameters through another route. Normally, the one in the energy of the transition state is lower. There are two types of catalysts which can affect the reaction pathway: (i) homogeneous catalysis and (ii) heterogeneous catalysts.
Since the acidity of the catalyst support reflects the reaction pathway and plays a key role in the catalyst performance, it is possible to differentiate between homogeneous and heterogeneous catalyst. The homogeneous catalyst has lower ability of acidic sites rather than heterogeneous catalysts and it requires higher temperature in its reaction system, whereas the solid structure and the surface of a heterogeneous catalyst give its ability for higher strength and locations for acidic sites.
Many mineral acid catalysts that are active in homogeneous catalysis can be made suitable for heterogeneous catalysis by supporting the catalyst on an inorganic oxide. Strongly acidic heterogeneous catalysts are prepared by supporting Brønsted acids such as trifluoro-sulfonic acid, sulfuric acid, phosphoric acid, and Lewis acids (such as such as boron trifluoride, BF3, and antimony pentafluoride, SbF5) on
