Equilibrium constant

About: Equilibrium constant is a research topic. Over the lifetime, 11920 publications have been published within this topic receiving 297117 citations.

l-(+)-Lactate: Determination with Lactic Dehydrogenase and DPN

Abstract: Publisher Summary This chapter discusses the enzymatic determination of L-(+)-lactate with lactic dehydrogenase (LDH) and diphosphopyridine nucleotide (DPN). LDH catalyzes the oxidation of L-lactate by DPN. The equilibrium of the reaction, which lies far to the left, has a constant K c of 2.9 ×10 –12 (moles/l) (25°C). The reaction products must be removed from the mixture to obtain quantitative oxidation of L-lactate. Protons are bound by the use of an alkaline reaction medium and pyruvate is trapped as the hydrazone. The basic equation for the spectrophotometric assay of L-lactate is given. The equilibrium constant for this reaction is K c ≈ 7 × 10 2 at pH 9.5 and 25°C. Relatively high concentrations of DPN and LDH are necessary to obtain a quantitative and sufficiently fast reaction. The course of the reaction is followed spectrophotometrically by the increase in optical density because of the formation of DPNH. The LDH preparation should have a specific activity of at least 15,000 units/mg (according to Bucher) or 270 units/mg (according to Racker). Contamination by malic dehydrogenase and glycerol-l-phosphate dehydrogenase should not exceed 0.03% (relative to the LDH activity).

A thermodynamic analysis of methanation reactions of carbon oxides for the production of synthetic natural gas

Abstract: Synthetic natural gas (SNG) can be obtained via methanation of synthesis gas (syngas). Many thermodynamic reaction details involved in this process are not yet fully understood. In this paper, a comprehensive thermodynamic analysis of reactions occurring in the methanation of carbon oxides (CO and CO2) is conducted using the Gibbs free energy minimization method. The equilibrium constants of eight reactions involved in the methanation reactions were calculated at different temperatures. The effects of temperature, pressure, ratio of H-2/CO (and H-2/CO2), and the addition of other compounds (H2O, O-2, CH4, and C2H4) in the feed gas (syngas) on the conversion of CO and CO2, CH4 selectivity and yield, as well as carbon deposition, were carefully investigated. In addition, experimental data obtained on commercial Ni-based catalysts for CO methanation and three cases adopted from the literature were compared with the thermodynamic calculations. It is found that low temperature, high pressure, and a large H-2/CO (and H-2/CO2) ratio are favourable for the methanation reactions. Adding steam into the feed gas could alleviate the carbon deposition to a large extent. Trace amounts of O-2 in syngas is unfavourable for SNG generation although it can lower carbon deposition. Additional CH4 in the feed gas almost has no influence on the CO conversion and CH4 yield, but it leads to the increase of carbon formed. Introduction of a small amount of C2H4, a representative of hydrocarbons in syngas, results in low CH4 yield and serious carbon deposition although it does not affect CO conversion. CO is relatively easy to hydrogenated compared to CO2 at the same reaction conditions. The comparison of thermodynamic calculations with experimental results demonstrated that the Gibbs free energy minimization method is significantly effective for understanding the reactions occurring in methanation and helpful for the development of catalysts and processes for the production of SNG.