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Leading Agriculture and Horticulture Solution and technology provider in the South Gujarat region. Acetic acid, a weak acid, donates a proton (hydrogen ion, highlighted in green) to water in an equilibrium reaction to give the acetate ion and the hydronium ion.
An acid dissociation constant, Ka, (also known as acidity constant, or acid-ionization constant) is a quantitative measure of the strength of an acid in solution.
Due to the many orders of magnitude spanned by Ka values, a logarithmic measure of the acid dissociation constant is more commonly used in practice. The acid dissociation constant for an acid is a direct consequence of the underlying thermodynamics of the dissociation reaction; the pKa value is directly proportional to the standard Gibbs energy change for the reaction. The quantitative behaviour of acids and bases in solution can be understood only if their pKa values are known.
The equilibrium constant for this dissociation reaction is known as a dissociation constant.
The acid loses a proton, leaving a conjugate base; the proton is transferred to the base, creating a conjugate acid. In solution chemistry, it is common to use H+ as an abbreviation for the solvated hydrogen ion, regardless of the solvent. The hydroxide ion OH?, a well known base, is here acting as the conjugate base of the acid water. A broader definition of acid dissociation includes hydrolysis, in which protons are produced by the splitting of water molecules. Although Ka appears to have the dimension of concentration it must in fact be dimensionless or it would not be possible to take its logarithm. This is a form of the Henderson–Hasselbalch equation, from which the following conclusions can be drawn.
The buffer region extends over the approximate range pKa ± 2, though buffering is weak outside the range pKa ± 1. In water, measurable pKa values range from about ?2 for a strong acid to about 12 for a very weak acid (or strong base). A buffer solution of a desired pH can be prepared as a mixture of a weak acid and its conjugate base. When the difference between successive pK values is less than about four there is overlap between the pH range of existence of the species in equilibrium.
For substances in solution the isoelectric point (pI) is defined as the pH at which the sum, weighted by charge value, of concentrations of positively charged species is equal to the weighted sum of concentrations of negatively charged species. The self-ionization constant of water, Kw, is thus just a special case of an acid dissociation constant. An Amino acid is also amphoteric with the added complication that the neutral molecule is subject to an internal acid-base equilibrium in which the basic amino group attracts and binds the proton from the acidic carboxyl group, forming a zwitter ion . Thus the zwitter ion, NH3+CHRCO2-, is amphoteric because it may either be protonated or deprotonated.
Historically, the equilibrium constant Kb for a base has been defined as the association constant for protonation of the base, B, to form the conjugate acid, HB+. DMSO is widely used as an alternative to water because it has a lower dielectric constant than water, and is less polar and so dissolves non-polar, hydrophobic substances more easily.
In aprotic solvents, oligomers, such as the well-known acetic acid dimer, may be formed by hydrogen bonding. These facts are obscured by the omission of the solvent from the expression that is normally used to define pKa, but pKa values obtained in a given mixed solvent can be compared to each other, giving relative acid strengths. As of 2008, a universal, solvent-independent, scale for acid dissociation constants has not been developed, since there is no known way to compare the standard states of two different solvents.
Ka is the dissociation constant of a substituted compound, Ka0 is the dissociation constant when the substituent is hydrogen, ? is a property of the unsubstituted compound and ? has a particular value for each substituent.
Alcohols do not normally behave as acids in water, but the presence of a double bond adjacent to the OH group can substantially decrease the pKa by the mechanism of keto-enol tautomerism. The standard enthalpy change can be determined by calorimetry or by using the van 't Hoff equation, though the calorimetric method is preferable.
Biological fluids are generally heterogeneous media that contain suspended or fragmented cells, proteins, or even crystalline particles.
The determination of the concentration of each chemical form of the metal is clearly not feasible, due to the very complicated speciation. If it is so complicated to evaluate the metal speciation in healthy individuals, more and more difficult will be the determination in the cases of overloaded metal-induced pathologies and human metal intoxications. It is well recognized that various human iron intoxications and overloaded metal-induced pathologies have been treated efficiently by administration of a chelating agent. The semi-empirical quantity widely used to quantify the complexing ability of a chelating agent towards a metal (M) ion is the pM value.
Because of the complexity of biological systems, effects of antidotal chelators are often better described quantitatively from results of animal experiments or clinical treatments than by theoretical calculations.
The use of software enabling the calculation of multiple chemical equilibria was initially put forward as a powerful tool for tackling these issues. In this regard, it is important to highlight the importance of the development of easy, fast and handily methods to directly assess the free metal, when it is possible, or at least the bioavailable fraction. Through SPE techniques, it has been demonstrated that not only the pre-concentration and separation of a target analyte is possible, but also to have information about speciation of several metal ions. According to this strategy, by using a selective metal sorbent, it is possible to assess the free iron(III) concentration in very complex matrixes, such as the blood plasma or urine. Following this novel idea, deferoxamine (DFO), a strong selective iron(III) chelator, also used in chelation therapy, is immobilized on the MCM-41 mesoporous silica (DFO-MS)16 to develop a device capable of detecting iron(III), and evaluating its free concentration. Urine was selected as the testing matrix, indeed the urine samples of iron overload patients have a high concentration of iron(III), complexed with the ligand employed in the chelation therapy. For these reasons, we decided to demonstrate the feasibility of total and free iron determination directly in that biological medium, so we defined the Fe(III) sorption mechanism on DFO-MS in 0.1 M KNO3 and also in urine. An Orion420 pH-meter, with a combined glass electrode, was used to determine the pH of all the solutions. The DFO-mesoporous silica (MS) MCM-41 type self assembled monolayer (DFO-MS) was prepared according to the previously described and optimized pathways,16 as the scheme of Fig. The DFO was used in the one pot reaction under two forms: mesylate salt, as received or free amine. The experimental sorption profiles represent the ratio of sorbed to total metal ion f reported vs. A constant amount of DFO-MS (50 mg) was immersed in a 0.1 M KNO3 solution with a fixed Fe(III) concentration (10–5 M) in the presence of EDTA. Urine samples of individuals, with no discernible pathology, were selected as test solutions. After determining K* in urine media, specific experiments were dedicated to asses the total and the free Fe(III) according to a strategy, previously successfully employed in a large variety of natural samples and beverages with commercial resins.11,13,15 To this purpose Fe(III) and a strong iron chelator (deferiprone) were added to urine samples to simulate a condition of an iron overload patient under chelation therapy.
The complexing properties of the functionalized MS (DFO-MS) toward Fe(III) have to be verified. In the case of the DFO-MS, it was assumed, in a rough estimate, that the protonation constants of the DFO, covalently bound on the MS, were the same of the ligand in solution.
The sorption profiles of Fe(III), in the presence of EDTA as a competitive ligand, were reported in Fig.
MEDUSA program19 is applied to calculate ?M at the ionic strength of interest and in function of the pH. In this chapter eqn (1) is used to characterize the sorption reaction of iron(III) on DFO-SAMMS, as explained above, being ?ex the unknown parameter.
As previously discussed, urine is an interesting matrix for the analysis of the free iron because the urine of iron overload patients usually exhibits a high concentration of iron(III), complexed with the ligand employed in the chelation therapy.
Information about the speciation of iron at this stage is important for many reasons and, generally speaking, the possibility to assess the free iron in such media is of overwhelming importance, as already pointed out.
For the reasons, it was decided to investigate the Fe(III) sorption on the functionalized silica directly on this media. As it can be seen, the iron concentration in urine is not reported, because it is usually too low to be detected. Our urine samples were collected from individuals, with no discernible pathology, with iron concentration below the LOD of the ICP-OES; for this reason, we have spiked the real samples with iron(III) to simulate the condition of an iron overload patient. To evaluate the partition coefficient K* between Fe(III) and DFO-MS, in the range of pH and ionic strength of urine media, competitive ligands are needed. To understand the effect of diverse urine, samples obtained from different volunteers, spiked with the same Fe(III) concentration and equal additions of the deferiprone, were employed. The desorption profiles are described by the formation of the only single complex in the solid phase and the values of the log?ex, previously obtained in synthetic solutions and under different conditions are here confirmed in a real and complexed media, as human urine.
The original tool to perform the free metal evaluation was now ready for use; the next item was a simulation of the in-field test.
The “free metal sensor” was tested on four SPU samples (A, B, C and D), treated as reported in the Experimental section.
To have an estimate of the ionic strength, the main ions were detected, and the pH was measured.
Table 5 Ionic composition of four different urine samples used in the experiments shown in Fig. As reported in the Experimental section, each sample was divided into different subsamples and put to equilibrate with different amounts of DFO-MS.
In Table 6 for each sample, the pH, the nominal deferiprone concentration (cL) and the amount of metal experimentally determined by direct quantification (ctot,dir) are reported (second, third and fourth columns).


Self-assembly of a synthetic phospholipid membrane by adding copper ions to an emulsion of an oil and a detergent, forming vesicles and tubules (credit: Itay Budin and Neal K. Chemists have taken an important step in making artificial life forms from scratch, creating self-assembling cell membranes, the structural envelopes that contain and support the reactions required for life and make a living organism from non-living molecules. Molecules that make up cell membranes have heads that mix easily with water and tails that repel it. By assembling an essential component of earthly life with no biological precursors, they hope to illuminate life’s origins.
January 27, 2012 by saberjim …and p**s off the ignorant creationists who will only proclaim that it is a miracle from god. The Kurzweil Accelerating Intelligence newsletter concisely covers relevant major science and technology breakthroughs (daily or weekly) via e-mail. Utkarsh deals in innovative and result oriented products for Horticulture and agriculture like plants, seeds, natural and chemical fertilizers and pesticides, fungicides etc.
It is the equilibrium constant for a chemical reaction known as dissociation in the context of acid-base reactions.
For aqueous solutions of an acid HA, the base is water; the conjugate base is A? and the conjugate acid is the hydronium ion. Furthermore, in all but the most concentrated solutions it can be assumed that the concentration of water, [H2O], is constant, approximately 55 mol·dm?3.
The illusion is the result of omitting the constant term [H2O] from the defining expression. Hydrochloric acid is said to be "fully dissociated" in aqueous solution because the amount of undissociated acid is imperceptible. In practice the mixture can be created by dissolving the acid in water, and adding the requisite amount of strong acid or base.
The constant for dissociation of the first proton may be denoted as Ka1 and the constants for dissociation of successive protons as Ka2, etc. The species distribution diagram shows that the concentrations of the two ions are maximum at pH 5.5 and 10.
Since the proton carries a positive charge extra work is needed to remove it; that is the cause of the trend noted above. In the case that there is one species of each type, the isoelectric point can be obtained directly from the pK values. When more than two charged species are in equilibrium with each other a full speciation calculation may be needed. At that temperature both hydrogen and hydroxide ions have a concentration of 10?7 mol dm?3. Thus, for exothermic reactions, (the standard enthalpy change, ?H, is negative) K decreases with temperature, but for endothermic reactions (?H is positive) K increases with temperature. The reason for this is that when the solvent is in its standard state its activity is defined as one. A simple example is provided by the effect of replacing the hydrogen atoms in acetic acid by the more electronegative chlorine atom. It turns out, these influences are more subtle than that of a dielectric medium mentioned above. When both the standard enthalpy change and acid dissociation constant have been determined, the standard entropy change is easily calculated from the equation above. Increased mobilization of the iron(III) in experiments on animals or humans, most often evaluated from urinary output, is usually used as an assessment tool for chelation therapy.
Many trace metal ions are present mostly bound to macromolecules (as proteins, but also nucleic acids, sugars, etc.) in biofluids. Fractionation is practically the most general procedure to assess at least classes of species in biological samples. Soon after the first computer programs were developed, they were in fact actively applied by coordination chemists, not only for calculations on laboratory solutions but also for simulating naturally occurring mixtures of metal ions and ligands. Such experimental evaluations could also be employed to verify computer modeling of chemical speciation. The sorbed metal fraction decreases, increasing the strength of the natural ligand and reducing the amount of the resin.
This information could be paramount to establish the efficiency of a chelator, and to estimate the Non-Transferrin-Bound Iron (NTBI) if the sensor is used directly to test biological fluids of iron overload patients.
The amount of free iron in such samples is an indication about the effectiveness of the drug, depending on the nature of the ligand and on the integrity of the expelled ligand; so the possibility to assess the free iron is of overwhelming importance, either to establish to the NTBI, or to give information about drug half life. Iron standard solution for ICP of 1000 mg L?1 (Fluka) was used to obtain the proper Fe(III) concentration in the solution phase. 0.4 g of MS (MCM-41 or MSU-H type), previously dried at 130 °C were added to the mixture reaction, left stirring overnight under nitrogen, at thermostatted temperature.
In this second case, the mesylate salt was dissolved in methanol and an equivalent amount of NaOH was added to the solution; after 20 minutes, the solvent was removed and the residue was washed three times with acetonitrile and dried before use.
In particular, two different competition experiments were performed, following the idea that the equilibria of interest are those established at pH around neutrality.
A known amount of Fe(III) (the mmol of Fe(III) were always in defect with respect to the active sites of the materials) was sorbed at pH around 3 on DFO-MS (about 30 mg) in 0.1 M KNO3 media.
The fresh urine sample was filtered to eliminate all the solid material and methanol was added, in order to prevent bacteria growing. For this purpose, different subsamples of a unique urine previously enriched with iron(III) were left to equilibrate with DFO-MS.
It is fundamental to accurately define the value of the partition coefficient K* to assess the free metal concentration.
Of course, this is not proved, but it can be used in a first approximation, and if the exchange constants of Fe(III) on the solid phase, not significantly differ from those in solution, the approximation can be accepted. 2 Sorption profile as a function of pH in solution without ligand and with EDTA as a ligand for Fe(III). 2 with the white circles (experiment with lower EDTA concentration), the value of the exchange constant obtained was log?ex = 40(1). In addition, to avoid the formation of microorganism, methanol was added in order to reach a 10% concentration in volume. If the iron is free in solution, without any other iron chelator, a quantitative sorption of the metal into the solid phase is expected.
6 the sorbed ion is reported, for each urine sample, with symbols and the fitting with lines. 6 Competition of deferiprone on DFO-MS in different iron enriched urine samples, (reported with different symbols) at pH 7. With these data, it is possible to calculate the theoretical values of ?Mt, reported in the fifth column. Craig Venter recently announced the creation of the “first synthetic living cell,” only its genome was artificial.
In water, they form a double layer with heads out and tails in, a barrier that sequesters the contents of the cell. From commercially available precursors, the scientists needed just one preparatory step to create each starting lipid chain. Utkarsh also provides field-to-field services to the farmers, corporate and institutions with the help of well trained technical team. Acids with a pKa value of less than about ?2 are said to be strong acids; a strong acid is almost completely dissociated in aqueous solution, to the extent that the concentration of the undissociated acid becomes undetectable. The underlying structural factors that influence the magnitude of the acid dissociation constant include Pauling's rules for acidity constants, inductive effects, mesomeric effects, and hydrogen bonding. These calculations find application in many different areas of chemistry, biology, medicine, and geology. The Bronsted–Lowry definition applies to other solvents, such as dimethyl sulfoxide: the solvent S acts as a base, accepting a proton and forming the conjugate acid SH+. Activities of the products of dissociation are placed in the numerator, activities of the reactants are placed in the denominator. Nevertheless it is not unusual, particularly in texts relating to biochemical equilibria, to see a value quoted with a dimension as, for example, "Ka = 300 M". This is known as solvent leveling since all such acids are brought to the same level of being strong acids, regardless of their pKa values. When the pKa and analytical concentration of the acid are known, the extent of dissociation and pH of a solution of a monoprotic acid can be easily calculated using an ICE table. Phosphoric acid values (above) illustrate this rule, as do the values for vanadic acid, H3VO4. Acetonitrile is less basic than DMSO, and, so, in general, acids are weaker and bases are stronger in this solvent. This process, known as homoconjugation, has the effect of enhancing the acidity of acids, lowering their effective pKa values, by stabilizing the conjugate base. This is an example of a linear free energy relationship as log Ka is proportional to the standard fee energy change. The diketone 2,4-pentanedione (acetylacetone) is also a weak acid because of the keto-enol equilibrium. Fumaric acid is (E)-1,4-but-2-enedioic acid, a trans isomer, whereas maleic acid is the corresponding cis isomer, i.e. In the following table, the entropy terms are calculated from the experimental values of pKa and ?H.
Alternatively, the efficiency of a drug is estimated by calculating the complexing ability of a chelating agent towards Fe(III).


However, it is difficult to apply any of the classical structure elucidation techniques, because of the small quantities, usually microgram, of the analyte.2–4 Identification is possible only for stable species and when standards are available for comparison. Chelating agents can affect metal toxicity by mobilizing the toxic metal into (mainly) urine or through the intestine. The DFO-MS was finally filtered off, washed several times with acetonitrile and dried under vacuum (see scheme in Fig.
After equilibration a small amount of solution was collected in a new disposable testing tube.
In all the original samples the concentration of Fe(III) was always below the LOD; consequently standard solution of Fe(III) was always added to the samples.
Then different competitive ligands were added to each subsample and the relative desorption curves were obtained. Four tubes with the same volume (V) of SPU samples were put in contact with different amounts of DFO-MS (w) and left to equilibrate overnight. Instead of determining an operational value of K*, valid only in that strictly particular set of experimental conditions, we followed the strategy already reported for chelating resins.11,15,17 Once defined the sorption reactions and the exchange constants, it is possible to calculate the value of K* under any condition.
It is clear the effect of competition, since the sorption profiles with EDTA (see triangles and empty circles) were moved to the neutral pH values with respect to the profile in the absence of ligand, reported with the empty squares. Then iron(III) was desorbed, step by step, increasing the concentration of EDTA from 0 to a quantity sufficient to permit the complete desorption of Fe(III).
4, the sorption of iron(III) at pH 7 on DFO-MS in the presence of increasing amount of three different ligands (DFO, deferiprone, and oxalate) in solution is shown. It is evident that, as the strength and the concentration of the added ligand increase, the metal exhibits higher difficulty to enter the solid phase. Instead of that, as first point of the experiments, a sorption of around 70% was always registered.
It is important to highlight that the log?ex was not affected by the changing of the ligand employed for competition, by the pH and the origin of the urine samples. The chemists created similar molecules with a novel reaction that joins two chains of lipids.
For example, many compounds used for medication are weak acids or bases, and a knowledge of the pKa values, together with the water–octanol partition coefficient, can be used for estimating the extent to which the compound enters the blood stream. When an exception to the rule is found it indicates that a major change in structure is occurring. Some pKa values at 25oC for acetonitrile (ACN)[21][22][23] and dimethyl sulfoxide (DMSO)[24] are shown in the following tables. Homoconjugation enhances the proton-donating power of toluenesulfonic acid in acetonitrile solution by a factor of nearly 800.[28] In aqueous solutions, homoconjugation does not occur, because water forms stronger hydrogen bonds to the conjugate base than does the acid.
To obtain the pKa value for use with aqueous solutions it has to be extrapolated to zero co-solvent concentration from values obtained from various co-solvent mixtures.
Hammett originally[33] formulated the relationship with data from benzoic acid with different substiuents in the ortho- and para- positions: some numerical values are in Hammett equation. In aromatic compounds, such as phenol, which have an OH substituent, conjugation with the aromatic ring as a whole greatly increases the stability of the deprotonated form. Neutral methylamine molecules are hydrogen-bonded to water molecules mailnly through one acceptor, N-HOH, interaction and only occasionally just one more donor bond, NH-OH2. However, a number of variables and problems have to be considered in the choice of the opportune chelating therapy. From this parameter and knowing the total metal content (ctot), the free metal concentration [M] can be obtained.
15, 17 and 18) referred to chelating resins as the solid phase, enables us to establish the value of the partition coefficient K* of the metal ion between the solution and the solid phase.
Then small portions of NaOH were added and, after each addition, the pH at equilibrium was registered. The Fe(III)-DFO-MS obtained was put to equilibrate in a 0.1 M KNO3 solution with PIPES buffer (10–2 M) at pH about 7. To be sure to obtain a reliable partition coefficient K*, not dependent on the media and on the variability of different samples, other experiments were performed considering iron(III) enriched urine of other individuals. The pH was measured, small amounts of solution were collected in new disposable testing tubes and analyzed for the iron content by ICP-OES. Because our purpose is to apply the material as a sensor in biological fluids, the characterization should also be extended at neutral pH; so the study of the sorption profile in the presence of a competitive ligand is indispensable to avoid the Fe(III) hydrolysis and to move the sorption profile curve towards basic values. The values of ?L and ?M are known and depend on the ligand involved and on its concentration. This value is justified by the presence of other ligands in urine, like proteins or inorganic anions (such as phosphate ion), responsible for iron(III) complexation and able to compete with the DFO anchored to the silica.
Consequently the log?ex value could be considered “universal” for urine samples, so we can calculate K* at any pH of any urine samples.
Fully artificial life will require the union of both an information-carrying genome and a three-dimensional structure to house it.
Nature uses complex enzymes that are themselves embedded in membranes to accomplish this, making it hard to understand how the very first membranes came to be. Acid dissociation constants are also essential in aquatic chemistry and chemical oceanography, where the acidity of water plays a fundamental role. In the case of VO2+ (aq), the vanadium is octahedral, 6-coordinate, whereas vanadic acid is tetrahedral, 4-coordinate. Hence, methylamines are stabilized to about the same extent by hydration, regardless of the number of methyl groups. In theory, pFe has to be calculated taking into account all the complexation equilibria involving the metal and the possible ligands.
In chronic metal-induced disease, where life-long chelation becomes fundamental, toxicity or side effects of the administered chelator must be taken into account. Nevertheless, such an approach suffers from the lack of the necessary data to derive a reliable model.1 Thus in silico studies (performed on computer or via computer simulation) can only be complementary to the empirical approach (“in vitro” and “in vivo”), but they cannot be considered as a substitute to experimental measurements.
The solution was checked for the iron content and the concentration was always not significantly different from blank values. The total iron was checked directly in solution before equilibration with the sorbent, since the concentration was largely higher than the LOQ of the ICP-OES.
It was also experimentally verified that the methanol added as the stabiliser in the urine media, did not interfere with Fe3+ binding since the same sorbed fraction was obtained both in the absence and presence of methanol (Fe(III) sorption 75% without methanol and 72% with methanol). In living organisms, acid-base homeostasis and enzyme kinetics are dependent on the pKa values of the many acids and bases present in the cell and in the body. In stark contrast, corresponding methylammonium cations always utilize all the available protons for donor NH-OH2 bonding.
Nevertheless, complexation reactions in complex systems such as serum and urine may hardly be accurately modelled by computer software.
The experimental data were fitted with eqn (1): in this case the variable K* was the unknown parameter. From the fitting of the experimental data by eqn (1), the value of K* can be determined (note that we are using eqn (1) still in MODE 1). In chemistry, a knowledge of pKa values is necessary for the preparation of buffer solutions and is also a prerequisite for a quantitative understanding of the interaction between acids or bases and metal ions to form complexes. The reason for this large difference is that when one proton is removed from the cis- isomer (maleic acid) a strong intramolecular hydrogen bond is formed with the nearby remaining carboxyl group.
The experimental determination of the bioavailable fraction of iron(III) in biological fluids would therefore be of the utmost relevance in the clinical practice.
After each addition the system was left to equilibrate; then a small amount of solution was collected in a new disposable testing tube, and analyzed by ICP-OES for iron content.
In the hypothesis that only the complex was formed, from K* it was possible to calculate the ?ex by the eqn (2). If a sorption model is supposed, and consequently a sorption reaction with precise stoichiometry, it is possible to obtain the exchange constant (?ex) using eqn (2). This favors the formation of the maleate H+, and it opposes the removal of the second proton from that species. The efficiency of the therapy could be more easily estimated as well as the course of overload pathologies. The value of log?ex = 40.1(2) was found, in very pretty good agreement with the results obtained by the previous sorption profiles. In this context, the aim of the present work was the development of a sensor to assess the free iron directly in biological fluids (urine) of patients under treatment with chelating agents.
In the proposed device (DFO-MS), the strong iron chelator deferoxamine (DFO) is immobilized on the MCM-41 mesoporous silica. The characterization of the iron(III) sorption on DFO-MS was undertaken, firstly in 0.1 M KNO3, then directly in urine samples, in order to identify the sorption mechanism. The stoichiometry of the reaction in the solid phase was found to be: with an exchange constant (average value) of log?ex = 40(1). The application of DFO-MS to assess pFe in SPU (Simulating Pathology Urine) samples was also considered. The results obtained were very promising for a future validation and subsequent application of the sensor in samples of patients undergoing chelation therapy.



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