Mixtures of weak acids

This approach can be used to sketch titration curves for other acid-base titrations including those involving polyprotic weak acids and bases or mixtures of weak acids and bases (Figure 9.8). Figure 9.8a, for example, shows the titration curve when titrating a diprotic weak acid, H2A, with a strong base. Since the analyte is... 

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... 

Buffering properties are exhibited by mixtures of weak acids (or bases) and their salts with a composition such that the concentration of the weak component and the salt are not too far apart. Thus, for b = sy pH = pK A(HA). The addition of a quite large amount of strong hydroxide or acid to such a mixture results in a small pH change in the solution (see Fig. 1.9). 

E Nelson. Dissolution rate of mixtures of weak acids and tribasic sodium phosphate. J Am Pharm Assoc 47 300-302, 1958. 

Buffers contain mixtures of weak acids and their salts (i.e., the conjugate bases of acids), or mixtures of weak bases and their conjugate acids. Typical buffer systems used in pharmaceutical dosage forms include mixtures of boric acid and sodium borate, acetic acid and sodium acetate, and sodium acid phosphate and disodium phosphate. The reason for the buffering action of a weak acid, HA (e.g., acetic acid) and its ionized salt, A" (e.g., sodium acetate) is that A" ions from the salt combine with the added hydrogen ions, removing them from solution as undissociated weak acid. 

As base is added, a mixture of weak acid and conjugate base is formed. This is a buffer solution and can be treated as one in the calculations. Determine the moles of acid consumed from the moles of titrant added—that will be the moles of conjugate base formed. Then calculate the molar concentration of weak acid and conjugate base, taking into consideration the volume of titrant added. Finally, apply your buffer equations. 

The pH of pure (and also not so pure) water is very sensitive to small concentrations of acids and bases. One drop of concentrated sulphuric acid added to a liter of water will change the pH by 4 pH nnits (from 7 to ca. 3). Solntion pH can be stabilized by a buffer (although there may be cases where a stable pH is not desirable) addition of (not too large) quantities of acid or base to a buffered solution will not affect the pH mnch. Buffers are usually mixtures of weak acids or bases and their salts. A common example in CD is the nse of an ammoninm salt (NH4X ) to control the pH of an ammonia solntion. The equilibrium of ammonia in water is given by... 

Cells and organisms maintain a specific and constant cytosolic pH, keeping biomolecules in their optimal ionic state, usually near pH 7. In multicellular organisms, the pH of extracellular fluids is also tightly regulated. Constancy of pH is achieved primarily by biological buffers mixtures of weak acids and their conjugate bases. 

Buffers Are Mixtures of Weak Acids and Their Conjugate Bases... 

To predict the pH of mixtures of weak acids or bases and their salts quantitatively, we set up an equilibrium table, as described in Toolbox 10.1. Then we use the acidity or basicity constant to calculate the concentration of hydronium ions present in the solution. The only difference is that now the conjugate acid and base are both present initially, so the first line of the table must have their initial concentrations. For instance, in the mixed acetic acid/sodium acetate solution, both acetic acid and its conjugate base, acetate ions, are present initially. In the ammonia/ammo-nium chloride solution, both the base (ammonia) and its conjugate acid (the ammonium ions) are present initially. 

The incorporation of a weak-acid and a weak-base resin into a crosslinked inert matrix or adjustment of the polymerization procedure of a mixture of weak-acid and weak-base monomers (or their derivatives) so that discrete acidic and basic domains would be formed has led to the development of a new class of ion-exchange resins known as thermally regenerable resins which will be discussed in the next section. 

A buffer solution is a solution that resists changes in pH. If acid is added then, within reason, the pH does not fall if base is added, the pH does not rise. Buffers are usually composed of a mixture of weak acids or weak bases and their salts and function best at a pH equal to the pKa of the acid or base involved in the buffer. The equation that predicts the behaviour of buffers is known as the Henderson-Hasselbalch equation (named after chemists Lawrence Joseph Henderson and Karl Albert Hasselbalch), and is another vitally important equation worth committing to memory. It is derived as follows, by considering a weak acid that ionises in solution ... 

Use the search engine Google to locate the Web document The Fall of the Proton Why Acids React with Bases by Stephen Lower. This document explains acid/base behavior in terms of the concept of proton free energy. How is an acid/base titration described in this view In a titration of strong acid with strong base, what is the free energy sink In a complex mixture of weak acid/base systems, such as serum, what happens to protons ... 

Mixture of weak acid (HA) and its conjugate base (A ) in significant concentrations gives a buffer. 

Ion exclusion is nseful for the separation of weak acid anions. The decision to use ion exclusion over ion exchange frequently depends on the matrix of the sample. If a mixture of weak acid anion and strong acids anions is to be analyzed, then ion exchange is a separation tool that will be the most effective. However, if weak acid anions are to be analyzed in the presence of large concentrations of strong acid anions,... 

The pH value which a given mixture of weak acid, HA, and salt NaA (in which Na may be regarded as representative of any univalent positive ion) will produce, may be readily estimated if the ionization constant of the acid, K, is known. In terms of concentrations the K is... 

If the ester is replaced by malonate (51), the specific enol equivalent, the condensation works very well. Malonate (51) enoiises completely under the reaction conditions whilst the aldehyde is only slightly enolised and the most electrophilic carbonyl group is still the aldehyde. A mixture of weak acid and weak base is often used as conditions should be as mild as possible to encourage only the fastest reaction (kinetic control). This is known as the Knoevenagel reaction. The product can be hydrolysed and decarboxylated in the usual way to give TM(49). 

Wines are mixtures of weak acids, combined to form salts to a greater or lesser extent according to their p/f (Table 1.3). The proportion of salts also depends on geographical origin, grape variety, the way the vines are trained, and the types of winepress and winemaking methods used. 

The lower nitrates known as the tetra- and tri-nitrates are formed together when cellulose is treated with a mixture of weak acids, and allowed to remain in contact with them for a very short time (twenty minutes). They cannot be separated from one another, as they all dissolve equally in ether-alcohol, acetic ether, acetic acid, methyl-alcohol, acetone, amyl acetate, c. 

Buffers Substances in solution that resist change in pH as acids or bases are added to or formed in the solution. Buffer solutions usually contain mixtures of weak acids and their salts (conjugate bases) or weak bases and their salts (conjugate acids). 

I Mixtures of weak acids and bases I Amphoteric species... 

The mixtures of weak acids and bases may be conjugate acid-base pairs (such as H2CO3/HCO3 ) or nonconjugate acid-base pairs (such as NH4+/CH3COO ). (For more info about conjugate acid-base pairs, see Give me that proton Bronsted-Lowery acid-base reactions, earlier in this chapter.)... 

Contains mixtures of weak acids (e.g. citric acid or tartaric acid) and sodium bicarbonate or carbonate, which release carbon dioxide when dissolved in water... 

Calculating the pH of Weak Acid Solutions The pH of a Mixture of Weak Acids Percent Dissociation ... 

We have seen that the conjugate base provided by salt dissociation is actually involved in the buffering action. The metal ions (like Na in sodium acetate) are not involved. We should therefore rewrite the definition of buffer solutions. Buffers are mixtures of weak acids and their coryugate bases. 

Electrophoretic techniques are generally used for separation of charged analytes. Charged analytes move in electrolyte solutions when an electrical field is estabhshed. Separation is obtained if the charged analytes have different m ation velocity. The electrolyte solution is most commonly a mixture of weak acids and bases in water. 

Poppi, R. J. and C. Pasquini. 1993. Spectrophotometric determination of a mixture of weak acids using multivariate calibration and flow injection analysis titration. Chemom. Intell. Lab. Syst. 19 243-254. 

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