Weak acids, continued

The analogy to weak acids continues in the calculation of the aqueous equilibria of weak bases, as the following example shows. 

The process may now be continued. Methylarsonic acid, when reduced by sulphur dioxide in concentrated hydrochloric acid, gives dichloromethylarsine, CHjAsCl. If this arsine is added to aqueous sodium hydroxide, it is hydrolysed to the weakly acidic methylarsenous acid, CH3As(OH)j, which in the alkali... 

Ultimately, as the stabilization reactions continue, the metallic salts or soaps are depleted and the by-product metal chlorides result. These metal chlorides are potential Lewis acid catalysts and can greatiy accelerate the undesired dehydrochlorination of PVC. Both zinc chloride and cadmium chloride are particularly strong Lewis acids compared to the weakly acidic organotin chlorides and lead chlorides. This significant complication is effectively dealt with in commercial practice by the co-addition of alkaline-earth soaps or salts, such as calcium stearate or barium stearate, ie, by the use of mixed metal stabilizers. 

Figure 1. Continued. Perrin plots of the two fluorophores (open circles and solid squares) in hydrophobic weak acid (e) and neutral (f) fractions.

Figure 4. Phase-resolved plots, continued. Fractions hydrophobic weak (c) and strong (d) acids. (Continued on nea page.)...

A third approach to buffer solutions is to add strong acid to a solution of a weak base. This produces a buffer solution if the amount of strong acid is about half the amount of weak base. Continuing with our examples of acetic acid-acetate buffers, if a solution of hydrochloric acid is added to a solution of sodium acetate, then hydronium ions react quantitatively with acetate anions ... 

The ionization is reversible. The anion (acting as a weak base) can recombine with the hydrogen ion to reform neutral HA. Both reactions occur continuously in solution, with the extent of ionization dependent on the strength of the acid. Strong acids, such as HC1, ionize completely in dilute aqueous solution. Thus a 0.01 molar (10-2 molar) solution has a pH of 2. Weak acids, such as acetic and other organic acids, ionize only slightly in solution and form solutions with pH from 4 to 6. 

Our goal in this chapter is to help you continue learning about acid-base equilibrium systems and, in particular, buffers and titrations. If you are a little unsure about equilibria and especially weak acid-base equilibria, review Chapters 14 and 15. You will also learn to apply the basic concepts of equilibria to solubility and complex ions. Two things to remember (1) The basic concepts of equilibria apply to all the various types of equilibria, and (2) Practice, Practice, Practice. 

M concentrations, the pH starts and continues to the equivalence point at a higher pH level, in a manner similar to that in the lower concentration of HC1. See Figure 5.1(c). This should not be unexpected, since a solution of a weak acid is being measured, and once again there are fewer H+ ions in the solution. Still weaker acids start at even higher pH values. 

The water formed in the reaction is continuously removed from the reaction mixture by azeotropic distillation, in order to avoid reversible reaction between water and ester. The progress of reaction can be followed either by measuring the amount of water or by determining the amount of unreacted acid in aliquots withdrawn at regular intervals of time. The reaction can be carried out either in presence of a catalyst, i.e. a weak acid like p-toluene sulphonic acid (a strong acid can hydrolyse the polymer, formed) or in absence of the catalyst. 

In the electrowinning of cadmium, nickel is an interfering element that has to be continuously removed from the electrolyte, a weak acidic cadmium sulfate solution. To handle the undesirable buildup of nickel contamination and at the same time obey environmental demands, the industry installed a solvent extraction kidney [8],... 

For a continuous extractive distillation process to be possible there must be adequate enhancement of the nitric acid-water relative volatility, and a system equilibrium which permits virtually complete separation of nitric acid from magnesium nitrate, the latter taking up the water content of the weak acid feedstock. This requires addition to the weak nitric acid of solutions of magnesium nitrate usually containing 60 wt% or more of Mg(NC>3)2. Under these conditions a nitric acid-water relative volatility of greater than 2.0 is obtained at the low end of the liquid phase concentration at a nitric acid mole fraction below 0.05 (4, 7). 

Finally, one of the first continuous ion-exchange plants installed used a weak-acid resin to recover copper from rayon-fibre spinning solutions. In the Bemberg or copper(II) ammonium process,357 the spinning takes place in an addic copper sulfate solution, and the fibre is then washed in ammonia solution. The wash water contains as much as 30% of the copper required for the spinning operation and its recovery is important in economic and environmental terms. The copper is extracted as the cationic amine complex by the weak-acid resin, and is then stripped from the resin with the acidic spinning solution. Zinc is recovered in a similar manner from vicrose rayon-spinning operations. 

Uric acid, cystine, and some other weak acids are relatively insoluble in, and easily reabsorbed from, acidic urine. Renal excretion of these compounds can be enhanced by increasing urinary pH with carbonic anhydrase inhibitors. In the absence of continuous bicarbonate administration, these effects of acetazolamide are of relatively short duration (2-3 days). Prolonged therapy requires bicarbonate administration. 

As we continue to increase pH, the zwitterion concentration reaches a maximum value at pH 6.0. Above this pH, the zwitterion is gradually converted into the anionic form (A"). Once again, we have a buffer system comprised of a weak acid (HA) and its conjugate base (A-). The point where the alpha values for these two species equal 50% corresponds to a pH of 9.6, which is the pRa for the zwitterion. 

Figure 12.25 shows how acetals can be brominated electrophihcally because of the (weakly) acidic reaction conditions. Proper acidity and electrophihcity is ensured by the use of pyri-dinium tribromide (B). This reagent is produced from pyridinium hydrobromide and one equivalent of bromine. Pyridinium tribromide is acidic enough to cleave the acetal A into the enol ether G. This cleavage succeeds by way of an El elimination like the one encountered in Figure 9.32 as an enol ether synthesis. The enol ether G reacts with the tribromide ion via the bromine-containing oxocarbenium ion H and the protonated acetal D to form the finally isolated neutral bromoacetal C. (The reaction can be conducted despite the unfavorable equilibrium between the acetal A and the enol ether G, since G continuously reacts and is thus eliminated from the equilibrium.)... 

This type of reaction is particularly interesting when the salt of the weak acid is sparingly soluble. Ferrous sulphide is a salt of the weak acid H2S. It is very insoluble in water, but it passes completely into solution in dilute HC1. The formation of the un-ionized H2S removes S ions and thus allows the ionization of the ferrous sulphide to continue unchecked... 

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