Which Is the Principal Species

We are often faced with the problem of identifying which species of acid, base, or intermediate is predominant under given conditions. A simple example is, What is the principal form of benzoic acid at pH 8  

For a monoprotic system, the basic species, A-, is the predominant form when pH P a. The acidic species, HA, is the predominant form when pH pKa. The predominant form of benzoic acid at pH 8 is the benzoate anion, C6H5C02. 

What is the predominant form of ammonia in a solution at pH 7.0 Approximately what fraction is in this form  

For polyprotic systems, our reasoning is similar, but there are several values of pKv Consider oxalic acid, H2Ox, with pK, = 1.27 and pK2 = 4.27. At pH = pS, [H2Ox] = [HOx . At pH = pK2, [HOx-] = [Ox2-]. The chart in the margin shows the major species in each pH region. 

Appendix G tells us that the a-ammonium group (at the left) is more acidic than the substituent (at the right]. What is the principal form of arginine at pH 10.0 Approximately what fraction is in this form What is the second most abundant form at this pH  

We sometimes need to identify which species of acid, base, or intermediate is predominant under given conditions. For example, what is the principal form of benzoic acid at pH 8 The pH of 8 is the net result of all reagents in the solution. The pH might be 8 because a phosphate buffer was added or because NaOH was added to benzoic acid. It does not matter how the pH came to be 8. That is just where it happens to be. 

To solve for [A ]/[HA], raise 10 to the power on each side of the equation  

The diagram below shows the major species for a triprotic system and introduces an important extension of what we learned in the previous section. 

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Calcium ion was titrated with EDTA at pH 11. using Cal-magite as indicator (Table 12-3). Which is the principal species of Calmagite at pH 11 What color was observed before the equivalence point After the equivalence point ... 

C. (a) Explain why the change from red to blue in Reaction 13-2 occurs suddenly at the equivalence point instead of gradually throughout the entire titration, (b) EDTA buffered to pH 5 was titrated with standard Pb, with xylenol orange as indicator (Table 13-2). (i) Which is the principal species of the indicator at pH 5 (ii) What color was observed before the equivalence point (iii) What color was observed after the equivalence point (iv) What would the color change be if the titration were conducted at pH 8 instead of pH 5 ... 

Aspartic acid acts as a triprotic acid with successive dissociation constants of 8.0 x 10-3, 1.4 X 10-4, and 1.5 X 10-10. Depending upon pH, aspartic acid can exist in four different forms in water solution. Draw these forms and calculate the pH range over which each form is the principal species. 

We have also carried out preliminary experiments in which we have detected the laser desorption of ethylene, cyanogen, methanol, and benzene from the Pt(s)[7(111) x (100)] surface. These spectra are shown in Figure 9. In the experiments involving ethylene, cyanogen, and methanol only neutral species are desorbed. In the case of benzene we observe the molecular parent ion in the absence of the electron beam. We believe that this is due to resonance multiphoton ionization of the benzene by the laser after desorption (resonance multiphoton ionization of benzene is very efficient with 249 nm radiation). These spectra are in marked contrast to the results of SIMS experiments which produce a wide variety of complex metal-adsorbate cluster ions. In the case of ethylene, our experiments were performed at 140 K, and under these conditions ethylene is known to be a molecular x-bonded species on the surface. In SIMS under these conditions the predominant species is CH (15)t but in the laser desorption FTMS experiments neutral ethylene is the principal species detected at low laser power. 

The chemical reactivity of these two substituted ethylenes is in agreement with the ideas encompassed by both the MO and resonance descriptions. Enamines, as amino-substituted alkenes are called, are very reactive toward electrophilic species and it is the P carbon which is the principal site of reactivity. For example, enamines are protonated on the p carbon. Acrolein is an electrophilic alkene, as predicted, and the nucleophile attacks the p carbon. 

The essential oil of mustard is obtained by distilling the seeds of several species of Brassioa with water. Brassica nigra is the principal species employed, but there is no doubt that the mustard seeds of commerce contain a substantial amount of the seeds of Brassica juncea and Brassica rapa. The seeds contain the glucoside potassium inyronate, which, under the influence of a ferment, myrosin, also present in the seeds, in the presence of water, is decomposed according to the following equation —... 

Modifications of the basic process are undersoftening, spHt recarbonation, and spHt treatment. In undersoftening, the pH is raised to 8.5—8.7 to remove only calcium. No recarbonation is required. SpHt recarbonation involves the use of two units in series. In the first or primary unit, the required lime and soda ash are added and the water is allowed to settie and is recarbonated just to pH 10.3, which is the minimum pH at which the carbonic species are present principally as the carbonate ion. The primary effluent then enters the second or secondary unit, where it contacts recycled sludge from the secondary unit resulting in the precipitation of almost pure calcium carbonate. The effluent setties, is recarbonated to the pH of saturation, and is filtered. The advantages over conventional treatment ate reductions in lime, soda ash, and COg requirements very low alkalinities and reduced maintenance costs because of the stabiUty of the effluent. The main disadvantages are the necessity for very careful pH control and the requirement for twice the normal plant capacity. 

The high capital cost, about 1500/kW, is the principal deterrent to growth of the IGCC concept. The abiUty to remove up to 99% of the sulfur species from the combustion products make the IGCC an environmentally desirable option as make calcium carbide (see Carbides), from which acetjiene is made. Synthesis gas for methanol and ammonia production is also made from gasification of coke. 

C18-0137. The graph shows the titration curve for a solution of formic acid, which is a principal component in the venom of stinging ants. Identify the major species in the solution at equilibrium, identify the dominant equilibrium, and write the equilibrium constant expression for points A, B, C, and D on the curve. 

The failure to observe photosubstitution in the presence of a sensitizer in which the latter is the principal absorber, the invariance of product quantum yield with wavelengths shorter than 350 nm (onset of n -> -n absorption), and the observation that chloride and bromide ions (known to catalyze S-+T intersystem crossing) strongly diminish the quantum yields of these reactions, strongly points to the lowest excited ir- n singlet state as the reactive species in these transformations. Excitation into the n->ir absorption band results in little product formation. A triplet state may, however, be involved in the photoamination of nitrobenzene.a41)... 

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