Acidic analyte dissociation

A sample contains a weak acid analyte, HA, and a weak acid interferent, HB. The acid dissociation constants and partition coefficients for the weak acids are as follows Ra.HA = 1.0 X 10 Ra HB = 1.0 X f0 , RpjHA D,HB 500. (a) Calculate the extraction efficiency for HA and HB when 50.0 mF of sampk buffered to a pH of 7.0, is extracted with 50.0 mF of the organic solvent, (b) Which phase is enriched in the analyte (c) What are the recoveries for the analyte and interferent in this phase (d) What is the separation factor (e) A quantitative analysis is conducted on the contents of the phase enriched in analyte. What is the expected relative erroi if the selectivity coefficient, Rha.hb> is 0.500 and the initial ratio ofHB/HA was lO.O ... 

Kelleher, N.L., Zubarev, R.A., Bush, K., Eurie, B., Eurie, B.C., McLafferty, E.W. and Walsh, C.T. (1999) Localization of labile posttranslational modifications by electron capture dissociation the case of gamma-carboxyglutamic acid. Analytical Chemistry, 71, 4250-4253. 

Also, the organic content is expected to influence the dissociation constant of acidic analytes, resulting in an increase in the acidic analyte pKa and this could be described as the acidic analyte pKa shift, which is discussed in Section 4.6. On the other hand, the organic eluent will affect the dissociation of basic analytes in the opposite direction, resulting in a decrease in the basic analyte pKa, and is discussed in the Section 4.6 as the basic analyte pKa shift. 

Liquid chromatography has also been widely used for the determination of dissociation constants [88-92] since it only requires small quantity of compounds, compounds do not need to be pure, and solubility is not a serious concern. However, the effect of an organic eluent modifier on the analyte ionization needs to also be considered. It has been shown that increase of the organic content in hydro-organic mixture leads to suppression of the basic analyte pKa and leads to an increase in the acidic analyte pK compared to their potentiometric pKa values determined in pure water [74]. 

A titration curve of a dibasic add is essentially a composite of the titration curves of an equimolar mixture of two weak acids with dissociation constants and Ki- In the two buffer regions, where Equation (3-30) applies, the shape can be calculated directly if Aj. The hydrogen ion concentration at the first end point is given approximately by Vj, and at the second end point it is nearly the same as that for the titration of a monobasic weak acid with = 2- If is much larger than K2, the above approximations are no longer valid then the precision of the intermediate end point is so low that it is no longer of analytical value. The titration curve is too shallow for quantitative use unless KJK2 exceeds about 10 or 10 . ... 

In LC-ESI-MS, the role of the mobile phase pH is complicated. In practice, often a compromise must be strack between analyte retention and ionization. From the perspective of generating preformed ions in solution, the optimum conditions for the ESI analysis of basic compounds, e.g., amines, would be an acidic mobile phase with a pH at 2 units below the dissociation constant pIQ of the analytes, while for acidic compounds, e.g., carboxylic acid or aromatic phenols, a basic mobile phase with a pH two units above the pK, of the analytes is preferred [97]. These conditions are uirfavourable for an analyte retention in RPLC. The analytes elute virtually umetained. In RPLC, it is important to reduce protolysis of basic and acidic analytes, i.e., to assure that the compounds are... 

Apparent Chemical Deviations Apparent deviations from Beer s law- arise when an analyte dissociates, associates, or reacts with a solvent to produce a prttduct with a different iihsorpti<>n spectrum than the analyte. A common example of this behavior is found with aqueous solutions of acid-base indicators. For example, the color change iissociated with a typical iitdicalor tlln arises from. shifts in the equilibrium... 

If the pollutant is charged and exhibits UV-absorbing properties, the CZE mode is readily recommended. Eor basic pollutants, moderately low to low pH buffers are indicated and the analyte migrates coelectroosmotically as a cation (protonated species) whereas for acidic pollutants, high pH buffers will promote the analyte dissociation and it migrates counterelectroosmotically as an anion. In both cases, buffer pH and concentration are the variables to optimize before the addition of any modifiers is considered. 

The pKa or acid dissociation constant is a measure of the strength of an acid in solution. A larger value for the pKa tells us that the analyte dissociates to a lesser extent and is therefore a weak acid. For basic analytes, the reverse applies, in that a higher value for the pKa means a stronger base. 

Acidic analytes. Buffer pH 2 units lower than p a gives non-dissociated species. Buffer pH 2 units higher than p a gives ionized species (anions). 

An interesting application area for anion exchange chromatography is the analysis of herbicides based on phenoxycarboxylic acids and their derivatives. Generally, these compounds were separated at chemically modified silicas using a methanol/water mixture as an eluant to which acetic acid was added to suppress analyte dissociation. The list of herbicides that may be analyzed with anion exchange chromatography comprises ... 

In any Cl plasma, ions of both polarities, positive and negative, are formed simultaneously, e.g., [M-j-H]" and [M-H]" ions, and it is just a matter of the polarity of the acceleration voltage which ions are extracted from the ion source [71]. Thus, negative-ion chemical ionization (NICI) [72] mass spectra are readily obtained when one of the following processes occurs i) dissociation (deprotonation) of acidic analytes such as carboxylic acids, diimides, or phenols, ii) nucleophilic addition (anion attachment), or Hi) ion-pair formation [73-76] ... 

Another situation in which an inflection point may be missing or difficult to detect occurs when the analyte is a multiprotic weak acid or base whose successive dissociation constants are similar in magnitude. To see why this is true let s consider the titration of a diprotic weak acid, H2A, with NaOH. During the titration the following two reactions occur. 

In the second limiting situation the analyte is a weaker acid or base than the interferent. In this case the volume of titrant needed to reach the analyte s equivalence point is determined by the concentration of both the analyte and the interferent. To account for the contribution from the interferent, an equivalence point for the interferent must be present. Again, if the acid dissociation constants for the analyte and interferent are significantly different, the analyte s determination is possible. If, however, the acid dissociation constants are similar, only a single equivalence point is found, and the analyte s and interferent s contributions to the equivalence point volume cannot be separated. 

In addition to the Hquid—Hquid reaction processes, there are many cases in both analytical and industrial chemistry where the main objective of separation is achieved by extraction using a chemical extractant. The technique of dissociation extraction is very valuable for separating mixtures of weakly acidic or basic organic compounds such as 2,4-dichlorophenol [120-83-2] and 2,5-dichlorophenol [583-78-8] which are difficult to separate by... 

The ionization eonstant should be a function of the intrinsic heterolytic ability (e.g., intrinsic acidity if the solute is an acid HX) and the ionizing power of the solvents, whereas the dissoeiation constant should be primarily determined by the dissociating power of the solvent. Therefore, Ad is expeeted to be under the eontrol of e, the dieleetrie eonstant. As a consequenee, ion pairs are not deteetable in high-e solvents like water, which is why the terms ionization constant and dissociation constant are often used interchangeably. In low-e solvents, however, dissociation constants are very small and ion pairs (and higher aggregates) become important species. For example, in ethylene chloride (e = 10.23), the dissociation constants of substituted phenyltrimethylammonium perchlorate salts are of the order 10 . Overall dissociation constants, expressed as pArx = — log Arx, for some substanees in aeetie acid (e = 6.19) are perchloric acid, 4.87 sulfuric acid, 7.24 sodium acetate, 6.68 sodium perchlorate, 5.48. Aeid-base equilibria in aeetie acid have been earefully studied beeause of the analytical importance of this solvent in titrimetry. 

There is a discrepancy between the cyanide criteria for both aquatic and drinking water standards and the current analytical technology. The criteria are stated for free cyanide (which Includes hydrocyanic acid and the cyanide ion), but the EPA approved analytical methodology for total cyanide measures the free and combined forms (11). This test probably overestimates the potential toxicity. An alternative method (cyanides amenable to chlorination) measures those cyanide complexes which are readily dissociated, but does not measure the iron cyanide complexes which dissociate in sunlight. This method probably tends to underestimate the potential toxicity. Other methods have been proposed, but similar problems exist (12). The Department of Ecology used the EPA-approved APHA procedure which includes a distillation step for the quantification of total cyanide (13,14). A modification of the procedure which omits the distillation step was used for estimation of free cyanide. Later in the study, the Company used a microdiffusion method for free cyanide (15). 

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