Monoprotic Acids Research Articles

Environmentally sensitive hydrogels: N‐isopropyl acrylamide/Acrylamide/ Mono‐, Di‐, Tricarboxylic acid crosslinked polymers

Environmentally sensitive hydrogels responsive to various stimuli such as temperature, pH, ionic strength of the medium and the solvent were prepared by using N‐isopropyl acrylamide (NIPAM), acrylamide (AAm) and monomers that have various number of carboxylic acid (XA) functionality using N,N′‐methylene bisacrylamide (Bis) as crosslinker. Hydrogels were prepared via free radical polymerization reaction in aqueous solution. P(NIPAAm‐co‐AAm) and p(NIPAAm‐co‐AAm)/XA hydrogels that contain monoprotic crotonic acid (CA) exhibit a lover critical solution temperature (LCST) at 28°C, whereas p(NIPAAm‐co‐AAm)/IA (IA:itaconic acid), and P(NIPAAm‐co‐AAm)/ACA (ACA:acotonic acid) hydrogels exhibit a lover critical solution temperature at 30.7°C and 34.4°C, respectively. Spectroscopic and thermal analyses were performed for the structural and thermal characterizations of the prepared hydrogel. The swelling experiments as equilibrium swelling percentages by gravimetrically were carried out in different solvents, at different solutions temperature, pH, and ionic strengths to determine their effects on swelling characteristic of hydrogels. POLYM. ENG. SCI., 55:843–851, 2015. © 2014 Society of Plastics Engineers

Dynamical Approach to Multiequilibria Problems for Mixtures of Acids and Their Conjugated Bases

Mathematical methods are described for the determination of steady-state concentrations of all species in multiequilibria systems consisting of several acids and their conjugated bases in aqueous solutions. The main example consists of a mixture of a diprotic acid H2A, a monoprotic acid HB, and their conjugate bases. The reaction equations lead to a system of autonomous ordinary differential equations for the species concentrations. The traditional equilibrium approach is briefly reviewed. Single variable polynomials are determined that are satisfied by the equilibrium proton concentrations. The remaining species are then given explicitly by rational functions of these proton concentrations, the equilibrium constants, and the initial concentrations of the other species. A quintic polynomial was derived to determine the equilibrium proton concentration for the example system. It is shown to reduce to a quartic polynomial in the absence of the second acid HB. An alternative dynamical approach to the equilibrium problem is described that involves the formulation of the kinetic rate equations for each species, which together constitute a nonlinear system of ordinary differential equations. The equilibrium concentrations are determined by evolving the initial concentrations via this dynamical system to their steady state. This dynamical approach is particularly attractive because it can easily be extended to determine equilibrium concentrations for arbitrarily large multiequilibria systems. With the equations provided here and some knowledge of computing software, the fast and accurate computation of equilibrium concentrations becomes feasible for the education of upper-division undergraduate and graduate students as well as for the study of research problems. This dynamical method also serves to introduce students to nonlinear dynamical systems, which are essential for the study of dynamic problems in chemistry, for example, oscillatory reactions.

Mutual diffusion coefficients of L-glutamic acid and monosodium L-glutamate in aqueous solutions at T = 298.15 K

Mutual diffusion coefficient values for binary aqueous solutions of both L-glutamic acid (H2Glu) and sodium L-glutamate (NaHGlu) were measured with the Taylor dispersion technique, at T=298.15K, and concentrations ranging from (0.001 to 0.100)mol·dm−3. The results were discussed on the basis of the Onsager–Fuoss and the Nernst theoretical equations, by considering the H2Glu as a weak acid (monoprotic acid, with K2=5.62·10−5). The smaller values found for the acid with respect to those of the salt, confirm this association hypothesis. From the diffusion coefficient values at infinitesimal concentration, limiting ionic conductivities as well as the hydrodynamic radius of the hydrogen glutamate ion (HGlu−) were derived and analyzed in terms of the chain methylene groups. The effect of different phenomena, such as association or complexation, were also taken into consideration and discussed. Values for the dissociation degree for H2Glu were also estimated.

The difference between the aggregates of short-tailed and long-tailed cationic calix[4]arene in water as detected using fluorescein dyes

Water-soluble cationic calixarenes readily form aggregates in water. The properties of these aggregates, however, markedly vary along with alterations in the molecular structure. In a set of our recent studies, such aggregates were examined by various methods, and first of all using acid–base indicator dyes. The latter acted in fact as monoprotic acids exhibiting one color transition, and their pKa shifts against the values in water were relatively close for different calixarenes. In the present paper, we report the behavior of a much more complicated dye, fluorescein, and two relative compounds, in aqueous solutions of two cationic calix[4]arenes. The last named possess four choline groups on the upper rim and the sole difference is the length of the alkoxy groups at the lower rim: O(CH2)2CH3 and O(CH2)7CH3. In water, both calixarenes form aggregates of similar size and with similar zeta-potential. However, their influence on the acid–base and tautomeric equilibria of fluorescein clearly demonstrate that the aggregates of the long-tailed calixarene resemble the common micelles of colloidal surfactants to a higher degree than those of tetrapropyloxy compound.

Analysis of variation matrix array by bilinear least squares–residual bilinearization (BLLS–RBL) for resolving and quantifying of foodstuff dyes in a candy sample

Bilinear least square (BLLS) method is one of the most suitable algorithms for second-order calibration. Original BLLS method is not applicable to the second order pH-spectral data when an analyte has more than one spectroscopically active species. Bilinear least square-residual bilinearization (BLLS–RBL) was developed to achieve the second order advantage for analysis of complex mixtures. Although the modified method is useful, the pure profiles cannot be obtained and only the linear combination will be obtained. Moreover, for prediction of analyte in an unknown sample, the original algorithm of RBL may diverge; instead of converging to the desired analyte concentrations. Therefore, Gauss Newton-RLB algorithm should be used, which is not as simple as original protocol. Also, the analyte concentration can be predicted on the basis of each of the equilibrating species of the component of interest that are not exactly the same.The aim of the present work is to tackle the non-uniqueness problem in the second order calibration of monoprotic acid mixtures and divergence of RBL. Each pH-absorbance matrix was pretreated by subtraction of the first spectrum from other spectra in the data set to produce full rank array that is called variation matrix. Then variation matrices were analyzed uniquely by original BLLS–RBL that is more parsimonious than its modified counterpart. The proposed method was performed on the simulated as well as the analysis of real data. Sunset yellow and Carmosine as monoprotic acids were determined in candy sample in the presence of unknown interference by this method.

3-D Surface Visualization of pH Titration “Topos”: Equivalence Point Cliffs, Dilution Ramps, and Buffer Plateaus

3-D topographic surfaces (“topos”) can be generated to visualize how pH behaves during titration and dilution procedures. The surfaces are constructed by plotting computed pH values above a composition grid with volume of base added in one direction and overall system dilution on the other. What emerge are surface features that correspond to behavior in aqueous solutions. Equivalence point breaks become cliffs that pinch out with dilution. Buffer effects become plateaus. Dilution alone generates 45° ramps. Limitations of the Henderson–Hasselbalch equation can be seen by noting the conditions over which a plateau remains relatively flat. Because dissociation is driven by dilution, the surfaces demonstrate when the solution of a weak acid becomes indistinguishable from that of a strong acid. Surfaces are presented for hydrochloric acid, HCl (a strong acid); acetic acid, CH3COOH (a weak monoprotic acid); oxalic acid, HOOCCOOH (a weak diprotic acid) and l-histidine dihydrochloride, C6H9N3O2·2HCl (a weak triprotic acid). Supplementary materials include suggested use of topos in lecture, as worksheets and in support of laboratory activities for first-year college courses and third-year analytical chemistry courses. Also provided is pH TOPOS, the macro-enabled spreadsheets that quickly generate surfaces for any mono-, di-, or triprotic acid desired. Only a change of acid dissociation constants, Ka values, is required.

Retention prediction and separation optimization of ionizable analytes in reversed-phase liquid chromatography by organic modifier gradients in different eluent pHs

The influence of eluent pH on retention of ionizable analytes during organic modifier gradients in reversed-phase high performance liquid chromatography is studied. Two approaches are examined for the retention prediction of solutes under organic modifier gradient conditions at any constant mobile phase pH: In the first approach an analytical solution of the fundamental equation of gradient elution in linear organic modifier gradients for monoprotic acids/bases under certain assumptions is proposed. The second approach is based on an empirical model arising from the evaluation of the gradient retention data. Both approaches were successfully applied to describe the retention behavior of 16 OPA derivatives of amino acids obtained in 19 simple mono-linear organic modifier gradient runs performed between two given acetonitrile contents with different gradient duration and at different eluent pHs, in a particular pH range where amino acids behave as weak monoprotic acids. Further, this study provides a reliable way for optimizing gradient separations of ionogenic compounds with respect to mobile phase pH.

Sulfur-Containing Chitin and Chitosan Derivatives as Trace Metal Adsorbents: A Review

Chitosan has low uptake for most heavy metals as well as high solubility in monoprotic acids. To address these problems, sulfur-containing functional groups have been introduced to enhance the sorption of heavy metals via formation of strong coordination bonds with heavy metals and also to improve its stability. The approaches to sulfur enrichment to the chitosan structure include grafting of sulfur-containing molecules to C2 amine or C6 hydroxyl groups. Crosslinking and grafting agents such as glutaraldehyde and epichlorohydrin have also been used in chitosan modification. In this review, special emphasis is placed on the synthesis pathways of sulfur-functionalization of chitin and chitosan and their metal uptakes. Generally, the uptake of soft acid metals such as Hg(II) and Cd(II) has been shown to increase with increasing sulfur content of modified chitosan. However, other factors such as the availability of thiol and amine groups as well as surface area of the modified chitosan also influence metal sorption.

Solubility–pH profiles of some acidic, basic and amphoteric drugs

The solubility vs. pH profiles of five ionizable drugs of different nature (a monoprotic acid, a monoprotic base, a diprotic base and two amphoteric compounds showing a zwitterionic species each one) have been determined through two different methodologies: the classical shake-flask (S-F) and the potentiometric Cheqsol methods using in both instances the appropriate Henderson–Hasselbalch (H–H) or derived relationships. The results obtained independently from both approaches are consistent. A critical revision about the influence of the electrolyte used as buffering agent in the S-F method on the obtained solubility values is also performed. Thus, some deviations of the experimental points with respect the H–H profiles can be attributed to specific interactions between the buffering electrolyte and the drug due to the hydrotrophic character of citric and lactic acids. In other cases, the observed deviations are independent of the buffers used since they are caused by the formation of new species such as drug aggregates (cefadroxil) or the precipitation of a salt from a cationic species of the analyzed compound (quetiapine).

Theoretical and experimental investigations on migration behavior of weak monoprotic acids in continuous two‐dimensional ion exchange/reversed phase CEC

A continuous 2D ion exchange/RP CEC system was constructed in this report and retention characteristics of weak monoprotic acids in the column were investigated theoretically. The transport equations were deduced for predicting migration behavior of weak monoportic acids based on mixing model combining ion exchange, RP, and electrophoretic separation mechanism. The influences of separation voltage, length of capillary, pH value, and ionic strength of buffer and concentration of organic modifier in mobile phase on the separation were well described by the equations. It was verified that the migration rate of solute was linearly related to voltage in the case of lower voltage, decreased with the increase of the first-dimensional length, and [H+]. The retention time of solute increased with enhancing ionic strength and concentration of organic modifier. These theoretical results were all demonstrated by the relevant experiments. The relevant application was performed for the aqueous extract of Rhizoma gastrodiae, showing the obvious potential in method development for the analysis of complex samples.

Electrolytic Nature of Aqueous Sulfuric Acid. 2. Acidity

In part 1 of this study, I reported that the Debye-Hückel limiting law and the smaller-ion shell (SiS) model of strong electrolyte solutions fit nicely with the experimental mean ionic activity coefficient (γ(±)) of aqueous sulfuric acid as a function of concentration and of temperature when the acid is assumed to be a strong 1-3 electrolyte. Here, I report that the SiS-derived activity coefficient of H(+), γ(H(+)), of the 1-3 acid is comparable to that of aqueous HCl. This agrees with titration curves showing, as well-known, that sulfuric acid in water is parallel in strength to aqueous HCl. The calculated pH is in good accord with the Hammett acidity function, H(0), of aqueous sulfuric acid at low concentration, and differences between the two functions at high concentration are discussed and explained. This pH-H(0) relation is consistent with the literature showing that the H(0) of sulfuric acid (in the 1-9 M range) is similar to those of HCl and the other strong mineral monoprotic acids. The titration of aqueous sulfuric acid with NaOH does not agree with the known second dissociation constant of 0.010 23; rather, the constant is found to be ~0.32 and the acid behaves upon neutralization as a strong diprotic acid practically dissociating in one step. A plausible reaction pathway is offered to explain how the acid may transform, upon base neutralization, from a dissociated H(4)SO(5) (as 3H(+) and HSO(5)(3-)) to a dissociated H(2)SO(4) even though the equilibrium constant of the reaction H(+) + HSO(5)(3-) ↔ SO(4)(2-) + H(2)O, at 25 °C, is 10(-37) (part 1).

Diseño de hojas de cálculo como herramientas para el aprendizaje del equilibrio ácido-base: uso de la barra de desplazamiento

This article presents a simple and conceptual application of the spreadsheet and the scroll bar to find the composition of a system in chemical equilibrium, for various situations related to monoprotic acid equilibrium. This method allows to simulate different situations such as a) weak acid dilution, b) common ion effect, c) buffer solutions, and d) the composition of a very dilute acid or an acid of very low acid-dissociation constant.

On the Existence of Hydrogen Salts of Monoprotic Acids

The notion that acid salts exist only for diprotic and polyprotic acids is found in many high school and university textbooks, although the “only” condition is not always stated explicitly. A fairly simple experiment shows that there is a pronounced exothermic effect when pure acetic acid is added to potassium acetate. Experiments with similar reactions between other pure monoprotic acids and their corresponding salts were also carried out. In all cases, a pronounced exothermic effect could be detected and an explanation is offered in terms of formation of acid salts. All of the above salts contain strong hydrogen bonds in their structure (as may be unequivocally demonstrated by means of IR spectroscopy) and the existence of such intermolecular attractions is one of the factors leading to the exothermic effect.

Effect of temperature and solvent composition on acid dissociation equilibria, I: Sequenced [formula omitted] determination of compounds commonly used as buffers in high performance liquid chromatography coupled to mass spectroscopy detection

A new automated and rapid potentiometric method for determining the effect of organic-solvent composition on pKa has been developed. It is based on the measurements of pH values of buffer solutions of variable solvent compositions using a combined glass electrode. Additions of small volumes of one precisely thermostated solution into another, both containing exactly the same analytical concentrations of the buffer components, can produce continuous changes in the solvent composition. Two sequences of potential measurements, one of increasing and the other of decreasing solvent content, are sufficient to obtain the pKa values of the acidic compound within the complete solvent-composition range in about 2h. The experimental design, procedures, and calculations needed to convert the measured pH into the thermodynamic pKa values are thoroughly discussed. This rapid and automated method allows the systematic study of the effect of solvent compositions and temperatures on the pKa. It has been applied to study the dissociation constants of two monoprotic acids: formic acid and triethylamine:HCl in acetonitrile/water mixtures within the range from 0 to 90% (v/v) at temperatures between 20°C and 60°C. These volatile compounds are frequently used to control the pH of the mobile phase in HPLC, especially in methods coupled to mass-spectrometry detection. The obtained pKa values are in excellent agreement with those previously reported. The results were fitted to empirical functions between pKa and temperature and composition. These equations, which can be used to estimate the pKa of these substances at any composition and temperature, would be highly useful in practical work during chromatographic method development.

A theoretical study of the molecular structures and vibrational spectra of the N2O⋯(HF)2

Abstract Theoretical calculations using both the MP2 and B3LYP levels of calculation with a 6-311++G(3df,3pd) basis set have been performed to determine stable structures and molecular properties for the H-bonded complexes involving nitrous oxide (N2O) and two HF molecules. Five complex have been characterized as minima since no imaginary frequency was found. Three complex are predicted to be relatively more stable with binding energies varying from 14 kJ mol−1 to 23 kJ mol−1 after BSSE and ZPE corrections. Our calculations have revealed that the second complexation with HF preferably occurs with the first complexed HF molecule, i.e., forming the X⋯H F⋯H F skeleton with X = O or N instead the F H⋯N N O⋯H F one. As expected, the H F chemical bonds are increased after complexation due to intermolecular charge transfer from “n” isolated pair of the X atom (X = N, O or F) to the σ∗ anti-bonding orbital of HF. For the strongly bounded complex, the doubly complexed HF molecule acts as a bridge between the two end molecules while transferring electrons from N2O to HF. Both possess the same amount of residual charge but with opposite signs. The H F stretching frequency of the monoprotic acid is shifted downward after complexation whereas its IR intensity is much enhanced. This increase has been adequately interpreted in terms of equilibrium hydrogen charge and charge-flux associated to the H F stretching using the CCFOM model for infrared intensities. This procedure has also allowed to analyze the new vibrational modes arising upon H-bond formation, especially those associated with the out-of-plane and in-plane HF bending modes, which are pure rotations in the HF isolated molecule.

Gel strength and swelling of acrylamide‐protic acid superabsorbent copolymers

AbstractThe viscoelastic and swelling properties of polyacrylamide‐based superabsorbent copolymers were investigated as a function of the ionic comonomer structure. Superabsorbent copolymers were synthesized by free‐radical crosslinking copolymerization of acrylamide and one of the monoprotic acids (acrylic acid and crotonic acid) or the diprotic acids (maleic acid and itaconic acid) as the investigated ionic comonomer. The reaction composition of all components, i.e. monomer, comonomer, initiator, co‐initiator, and crosslinker, was fixed to be the same for the synthesis of all four superabsorbent copolymer systems. Viscoelastic measurements were performed in all systems where the particles were closely packed. The network structures of all systems were evaluated via viscoelastic and swelling measurements. The results indicated that superabsorbent polymers (SAPs) with high water absorbency were accompanied by low gel strength and the calculated high value of molecular weight between crosslinks ($\bar {M}{}_c$) and low value of effective crosslinking density (νe). Diprotic acid‐containing SAPs showed higher water absorbency over monoprotic acid‐containing and non‐ionic ones. The differences in $\bar {M}{}_c$ and νe values of each system were explained with respect to the differences in the monomer reactivity ratio and hydrophilicity of the comonomers. Copyright © 2010 John Wiley & Sons, Ltd.

Production of monoprotic, diprotic, and triprotic organic acids by using electrodialysis with bipolar membranes: Effect of cell configurations

To investigate the production of monoprotic, diprotic, and triprotic organic acids by using electrodialysis with bipolar membranes, acetic acid, oxalic acid, and citric acid were chosen as the target products. The effects of cell configuration and acid species on process performances were evaluated by considering such factors as the stack voltage drop, acid yield, current efficiency, and energy consumption. The results indicated that BP-C was the most acceptable cell configuration when salt concentration was 0.13 mol dm −3 and current density was 10 mA cm −2. The whole electrodialysis with bipolar membranes process has reached a highest current efficiency of 75%. The energy consumptions of acetic acid, oxalic acid, and citric acid were 3.8–15.7, 7.3–21.1, and 8.1–22.7 kWh kg −1 with the experimental conditions, respectively. The produced acid concentration at a given time follows the order of acetic acid > oxalic acid > citric acid.

Determination of dissociation parameters of weak acids in different media according to the isohydric method

The isohydricity (pH constancy) principle is referred to the pair of solutions: weak acid (HL, C0mol/L) and strong acid (HB, Cmol/L) when mixed e.g., according to titrimetric mode. Such a case takes place if the relation C0=C+C2×10pK1 is valid, where pK1=−logK1, K1 – dissociation constant for a weak monoprotic acid HL. This principle, outlined and formulated in earlier paper (Michałowski et al., Talanta 82 (2010) 1965), is the basis for a sensitive method of pK1 determination, confirmed for a series of weak acids in presence of basal electrolytes or in water+organic solvent (dimethyl sulphoxide, methanol, isopropanol) media. The results of titrations were elaborated according to principles of regression analysis, with use of least squares method. A new criterion for precision of the results obtained according to this method is formulated. The pK1 values obtained are comparable with ones found in literature.

Modeling the electric transport of HCl and H3PO4mixture through anion-exchange membranes

The electric transport of the mixture of hydrochloric and phosphoric acids through strong base (Neosepta ACM) and weak base (Selemion AAV) anion-exchange membranes was investigated. The instantaneous efficiency of HCl removal from the cathode solution, <TEX>$CE_{Cl}$</TEX>, with and without <TEX>$H_3PO_4$</TEX> was determined. It was found that <TEX>$CE_{Cl}$</TEX> was 0.8-0.9 if the number of moles of elementary charge passed through the system, <TEX>$n_F$</TEX>, did not exceed ca. 80% of the initial number of HCl moles in the cathode solution, <TEX>$n_{Cl,ca,0}$</TEX>. The retention efficiency of <TEX>$H_3PO_4$</TEX> in that range was close to one. The transport of acid mixtures was satisfactorily described by a model based on the extended Nernst-Planck and Donnan equations for <TEX>$n_F$</TEX> not exceeding <TEX>$n_{Cl,ca,0}$</TEX>. Among the tested model parameters, most important were: concentration of fixed charges, the porosity-tortuosity coefficient, and the partition coefficient of an undissociated form of <TEX>$H_3PO_4$</TEX>. For the both membranes, the obtained optimal values of fixed charge concentration, <TEX>$\bar{c}_m$</TEX>, were up to 40% lower than the literature values of <TEX>$\bar{c}_m$</TEX> obtained from the equilibrium measurements. Regarding the <TEX>$H_3PO_4$</TEX> equilibria, it was sufficient to consider <TEX>$H_3PO_4$</TEX> as a monoprotic acid.

Anodic aqueous electrophoretic deposition of titanium dioxide using carboxylic acids as dispersing agents

The dispersion of anatase phase TiO 2 powder in aqueous suspensions was investigated by zeta-potential and agglomerate size analysis. The iso-electric point (IEP) of anatase was determined to be at pH 2.8 using monoprotic acids for pH adjustment. In comparison, it was found that the use of carboxylic acids, citric and oxalic, caused a decrease in zeta-potential through the adsorption of negatively charged groups to the particle surfaces. The use of these reagents was shown to enable effective anodic electrophoretic deposition (EPD) of TiO 2 onto graphite substrates at low pH levels with a decreased level of bubble damage in comparison with anodic EPD from basic suspensions. The results obtained demonstrate that the IEP of TiO 2 varies with the type of reagent used for pH adjustment. The low pH level of the IEP and the ability to decrease the zeta-potential through the use of carboxylic acids suggest that the anodic EPD of anatase is more readily facilitated than cathodic EPD.