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Weak Acids (weak + acid)
Selected AbstractsA theoretical explanation for the retention mechanism of ion exclusion chromatographyJOURNAL OF SEPARATION SCIENCE, JSS, Issue 17 2003Bronis, aw K. G Abstract Ion Exclusion Chromatography is classically used for the separation of weak acid anions. Dilute strong acids (e.g. sulphuric or perchloric acid) or just water are used as eluents. To increase the exclusion effect, strong cation exchangers, characterized by high concentration of functional groups, are applied. The inner column volume of commercially available columns is increased by increasing their size in comparison to traditional ones (usually 300×7.8 mm ID). The description of the retention mechanism of this technique implicitly assumes that both mobile and stationary phases are typical aqueous solutions, and their dielectric constants are thus equal. This equality implies the equality of solute dissociation constants in both phases. Another implicit assumption is that the dead- and inner volumes of the column are constant, and independent of the mobile phase composition. The present paper shows that stationary and mobile phases are generally characterized by different physicochemical parameters. Thus, they cannot be considered as regular aqueous solutions. Additionally, we show that weak cation exchanger resins, which are characterized by a relatively small concentration of the functional groups, and weak acid based buffers can also be used in IEC. This would expand the possible applications of this method and enable, for example, the separation of strong acids (anions). The influence of ionic strength on the retention and dead- and inner column volumes is also discussed. Finally we also briefly describe the retention mechanism of Electrostatic Ion Chromatography. [source] pH regulation in an acidophilic green alga , a quantitative analysisNEW PHYTOLOGIST, Issue 2 2009Birgit Bethmann Summary ,,Short-term cytosolic pH regulation has three components: H+ binding by buffering groups; H+ transport out of the cytosol; and H+ transport into the vacuole. To understand the large differences plants show in their tolerance to acidic environments, these three components were quantified in the acidophilic unicellular green alga Eremosphaera viridis. ,,Intracellular pH was recorded using ion-selective microelectrodes, whereas constant doses of weak acid were applied over different time intervals. A mathematical model was developed that describes the recorded cytosolic pH changes. Recordings of cytosolic K+ and Na+ activities, and application of anion channel inhibitors, revealed which ion fluxes electrically compensate H+ transport. ,,The cytosolic buffer capacity was , = 30 mM. Acidification resulted in a substantial and constant H+ efflux that was probably driven by the plasmalemma H+ -ATPase, and a proportional pH regulation caused by H+ pumped into the vacuole. Under severe cytosolic acidification (, 1 pH) more than 50% of the ATP synthesized was used for H+ pumping. While H+ influx into the vacuole was compensated by cation release, H+ efflux out of the cell was compensated by anion efflux. ,,The data presented here give a complete and quantitative picture of the ion fluxes during acid loading in an acidophilic green plant cell. [source] Microtubule arrays in fucoid zygotes are sensitive to cytoplasmic pHPHYCOLOGICAL RESEARCH, Issue 1 2001David C. Henderson SUMMARY Regulation of microtubule (MT) arrays and embryo-genesis by cytoplasmic pH (pHc) was investigated in zygotes of the brown alga Pelvetia compressa (J. Agardh) De Toni. pHc was clamped to (set to) acidic values using a weak acid, propionic acid (PA), and to alkaline values using a weak base, methylamine (MA). Acidification of pHc from the normal value of 7.4,7.5 to about 7.0 caused disruption of microtubule arrays. The nucleating activity was delocalized from the centrosomes and dispersed over the nuclear envelope, the number of MTs decreased, and MTs failed to extend into the cell cortex. Alkalinization to about pH 8.0 also caused dispersal of nucleating activity, but distinct centrosomes remained. MTs coursed in various directions following MA treatment, giving the array a disorganized appearance. Two MT-dependent processes, rhizoid morphogenesis and cell division, were found to be perturbed by small changes in pHc. [source] Geological and Geochemical Characteristics of the Hydrothermal Clay Alteration in South KoreaRESOURCE GEOLOGY, Issue 4 2000Sang-Mo KOH Abstract: Hydrothermally altered areas forming pyrophyllite-kaolin-sericite-alunite deposits are distributed in Chonnam and Kyongsang areas, Cretaceous volcanic field of the Yuchon Group. The Chonnam alteration area is located within depression zone which is composed of volcanic and granitic rocks of late Cretaceous age. The clay deposits of this area show the genetic relationship with silicic lava domes. The Kyongsang alteration area is mainly distributed within Kyongsang Basin comprising volcanic, sedimentary and granitic rocks of Cretaceous and Tertiary age. Most of the clay deposits of this area are closely related to cauldrons. Paleozoic clay deposit occurs in the contact zone between Precambrian Hongjesa granite gneiss and Paleozoic Jangsan quartzite of Choson Supergroup. Cretaceous igneous rocks of the both alteration areas belong to high K calc-alkaline series formed in the volcanic arc of continental margin by subduction-related magmatism. Chonnam igneous rocks show more enrichment of crustal components such as K, La, Ce, Sm, Nd and Ba, higher (La/Yb)cn ratio, and higher initial 87Sr/86Sr ratio (0. 708 to 0. 712) than those of Kyongsang igneous rocks. This might be due to the difference of degree of crustal contamination during Cretaceous magmatism. The most characteristic alteration minerals of Chonnam clay deposits are alunite, kaolin, quartz, pyrophyllite and diaspore which were formed by acidic solution. Those of Kyongsang clay deposits are sericite, quartz and pyrophyllite which were formed by weak acid and neutral solution. The formation ages of the clay deposits of two alteration areas range from 70. 1 to 81. 4 Ma and 39. 7 to 79. 4 Ma, respectively. The Daehyun clay deposit in Ponghwa area of Kyongsang province shows the alteration age range from 290 to 336 Ma. This result shows the different alteration episode from the hydrothermal alteration of Cretaceous to early Tertiary in the Kyongsang and Chonnam alteration areas. These data indicate, at least, three hydrothermal activities of Tertiary (middle to late Eocene), late Cretaceous (Santonian to Maastrichtian) and Paleozoic Carboniferous Periods in South Korea. [source] Facilitating student understanding of buffering by an integration of mathematics and chemical concepts,BIOCHEMISTRY AND MOLECULAR BIOLOGY EDUCATION, Issue 2 2004Robert Curtright Abstract We describe a simple undergraduate exercise involving the titration of a weak acid by a strong base using a pH meter and a micropipette. Students then use their data and carry out graphical analyses with a spreadsheet. The analyses involve using mathematical concepts such as first-derivative and semi-log plots and provide an opportunity for collaboration between biochemistry and mathematics instructors. By focusing on titration data, rather than the titration process, and using a variety of graphical transformations, we believe that students achieve a deeper understanding of the concept of buffering. [source] Study of electromigration effects on a pH boundary during the on-line electrokinetic preconcentration by capillary electrophoresisELECTROPHORESIS, Issue 16 2010ina Vítková Abstract A contribution to the description of electrokinetic effects on the pH boundary formed by sodium borate pH 9.5 and sodium phosphate pH 2.5 electrolytes for on-line preconcentration of weak acids is presented in this article. Simulations of electrokinetic injections together with experimental studies using contactless conductivity detection verified that the preconcentration is induced mainly by dissociation changes of analytes on the pH boundary and transient ITP state. Moreover, a study of the addition of organic solvent to the injection electrolyte was performed with impressive results. Subnanomolar LODs of hydroxybenzoic acids were achieved with 80% of methanol in the injection electrolyte which represents more than 70,000-fold preconcentration in comparison with classical CZE method. [source] Estimation of the soil,water partition coefficient normalized to organic carbon for ionizable organic chemicals,ENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 10 2008Antonio Franco Abstract The sorption of organic electrolytes to soil was investigated. A dataset consisting of 164 electrolytes, composed of 93 acids, 65 bases, and six amphoters, was collected from literature and databases. The partition coefficient log KOW of the neutral molecule and the dissociation constant pKa were calculated by the software ACD/Labs®. The Henderson-Hasselbalch equation was applied to calculate dissociation. Regressions were developed to predict separately for the neutral and the ionic molecule species the distribution coefficient (Kd) normalized to organic carbon (KOC) from log KOW and pKa. The log KOC of strong acids (pKa < 4) was not correlated to these parameters. The regressions derived for weak acids and bases (undissociated at environmental pH) were similar. The highest sorption was found for strong bases (pKa > 7.5), probably due to electrical interactions. Nonetheless, their log KOC was highly correlated to log KOW. For bases, a nonlinear regression was developed, too. The new regression equations are applicable in the whole pKa range of acids, bases, and amphoters and are useful in particular for relatively strong bases and amphoters, for which no predictive methods specifically have been developed so far. [source] Solubilization of two structurally related anticancer drugs: XK-469 and PPAJOURNAL OF PHARMACEUTICAL SCIENCES, Issue 1 2006Yan He Abstract The efficiency of a solubilization technique is determined by the physical,chemical properties of the drug. This study investigates the solubilization on two structurally related anticancer drugs, XK-469 and PPA. XK-469 is much less polar than PPA with an intrinsic solubility of 0.000274 mg/mL, which is about 10000 fold less than that of PPA. Fortunately, its physical,chemical properties make it much more formulatable. An ionizable drug can be solubilized by pH adjustment with cosolvency, micellization, or complexation. Both XK-469 and PPA are weak acids with pKa values of 2.7 and 2.9, respectively. Thus, they can be solubilized by pH adjustment. At pH 4.55, neither cosolvency, micellization nor complexation has much effect on the solubility of PPA. However, these techniques can significantly increase the solubility of XK-469. In fact, the solubility of XK-469 in 20% HP,CD at pH 4.55 is 5.85 mg/mL, which is more than 20000 times greater than its intrinsic solubility. With the solubilization descriptors obtained from the experimental data for both unionized and ionized drug species at pH 1.0 and pH 4.55, the solubility of each drug at any pH and excipient concentration can be estimated. Then, a solubilization technique can be chosen for preparing a desired final drug concentration. © 2005 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 95:97,107, 2006 [source] Some cesium and potassium salts increase the water permeability of astomatous isolated plant cuticlesJOURNAL OF PLANT NUTRITION AND SOIL SCIENCE, Issue 1 2007Salem Elshatshat Abstract Salts were applied as aqueous solution to the outer surfaces of astomatous isolated cuticles, and the water was allowed to evaporate. Effects of salt residues on the surfaces of cuticles on water permeability of cuticles were measured at 25°C. A surface dose of 0.2 mol m,2 Cs2CO3 and K2CO3 increased the water permeability of pear leaf cuticles by factors of 9.9 and 3.9, respectively. Na2CO3 was barely effective, and Li2CO3 as well as (NH4)2CO3 had no influence on water permeability. Potassium applied as sulfate, nitrate, or chloride had no effect on the water permeability of pear leaf cuticles, while K+ salts of weak acids (0.2 mol m,2) were effective, as they increased water permeability by factors of 5.4 (K+ -acetate), 3.9 (K2CO3), and 2.0 (K2HPO4), respectively. Sensitivity of Idesia polycarpa leaf cuticles to treatment with K2CO3 at 0.2 mol m,2 was greatest as water permeability was increased 35-fold, while the water permeability of cuticles from other species tested (astomatous leaf cuticles from Hedera helix, Citrus aurantium, Prunus laurocerasus, Pyrus communis, and Populus canescens; fruit cuticles from Capsicum annuum and Lycopersicon esculentum) increased only by factors ranging from 1.7 to 3.9. Data are discussed in relation to swelling and ion-exchange properties of cuticles. [source] Effect of temperature on the chromatographic retention of ionizable compounds.JOURNAL OF SEPARATION SCIENCE, JSS, Issue 6-7 2008Abstract We propose a general simple equation for accurately predicting the retention factors of ionizable compounds upon simultaneous changes in mobile phase pH and column temperature at a given hydroorganic solvent composition. Only four independent experiments provide the input data: retention factors measured in two pH buffered mobile phases at extreme acidic and basic pH values (e. g., at least ± 2 pH units far from the analyte pKa) and at two column temperatures. The equations, derived from the basic thermodynamics of the acid,base equilibria, additionally require the knowledge of the solute pKa and enthalpies of acid,base dissociation of both the solute and the buffer components in the hydroorganic solvent mixture. The performance of the predictive model is corroborated with the comparison between theoretical and experimental retention factors of several weak acids and bases of important pharmacological activity, in mobile phases containing different buffer solutions prepared in 25% w/w ACN in water and at several temperatures. [source] Analysis of water solubility data on the basis of HYBOT descriptorsMOLECULAR INFORMATICS, Issue 9-10 2003Part 2. Abstract Solubility data of 787 organic liquids (electrolytes and non-electrolytes) with diverse structures has been quantitatively described by physicochemical property descriptors. Special effects like intra - and intermolecular hydrogen bonds have been shown to be very important for water solubility. It is found that an important part of the solute-solvent interaction is neglected in all correlations of logS with (only) logP, as in this case the solute H-bond donor effect is not considered. As expected intramolecular hydrogen bonds lead to reduced solubility, whereas intermolecular hydrogen bonds (both HB donors and acceptors) of solutes result in higher solubility. An exception to the latter rule are carboxylic acids which due to intermolecular HB-induced dimerization in the pure liquid phase of acids show a three times lower solubility as expected on the basis of their molecular properties. A volume-related term (molecular polarizability ,) was found to have an essential negative contribution to solubility. For the first time the solubility increasing effect of partial ionization of weak acids and bases in saturated aqueous solutions has been quantitatively considered for sets of compounds by exact calculation of the pH determined by the solutes aqueous solubility and pKa value(s). [source] Antibiotic therapy , rationale and evidence for optimal drug concentrations in prostatic and seminal fluid and in prostatic tissueANDROLOGIA, Issue 5 2003Kurt G. Naber Summary. The theoretical background of drug penetration into the prostate is outlined, emphasizing the phenomenon of ion-trapping and the role of nonionic diffusion of weak acids, bases and amphoteric drugs across biological membranes with a pH gradient. Determination of drug concentrations in human prostatic secretion are problematic because of possible urinary contamination. Studies have been carried out mainly in healthy volunteers. The results have to be interpreted with caution, if not care was taken to rule out or at least identify urinary contamination. Analysing the concentrations of various fluoroquinolones in prostatic and seminal fluid as well as in prostatic tissue, it becomes obvious that the fluoroquinolones differ not only in plasma concentrations but also in their penetration ability to these sites. In spite of intensive investigations, our knowledge is still limited concerning the mechanisms that govern the transport of antibiotic drugs into and their activity in the various prostatic compartments and how the findings can be applied clinically. Nevertheless, overall the concentrations at the site of infection of most of the fluoroquinolones should be sufficient for the treatment of chronic bacterial prostatitis and vesiculitis caused by susceptible pathogens. [source] | ||||||||||||||||