Principal species acid/base

Quinoxalin-2-one is a very weak base (pK — 1.37) and so the different orientation of substitution in acetic and sulfuric acids may mean that in acetic acid the principal species undergoing nitration is the neutral molecule, and in sulfuric acid, the mono-cation. Treatment of quinoxaline-2,3-dione, or its iViV -dimethyl derivative in sulfuric acid, with 1 equivalent of potassium nitrate, results in nitration at position 6 with 2 equivalents of potassium nitrate, 6,7-dinitro compounds are formed. When quinoxaline is boiled with aqueous nitric acid, 6-... 

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 ... 

Lux—Flood Definition In contrast to the BrpnstedLowry theory, which emphasizes the proton as the principal species in acid-base reactions, the definition proposed by Lux and extended by Flood describes add-base behavior in terms of the oxide ion. This add-base concept was advanced to treat non protonic systems which were not amenable to the Bipnsted-Lowry definition. For example, in high-temperature inorganic melts, reactions such as the following take place ... 

The sequences of all three pieces of RNA in the E. coli ribosomes are known as are those from many other species. These include eukaryotic mitochondrial, plas-tid, and cytosolic rRNA. From the sequences alone, it was clear that these long molecules could fold into a complex series of hairpin loops resembling those in tRNA. For example, the 16S rRNA of E. coli can fold as in Fig. 29-2A and eukaryotic 18S RNA in a similar way (Fig. 29-4).38/39/67 69 The actual secondary structures of 16S and 18S RNAs, within the folded molecules revealed by X-ray crystallography, are very similar to that shown in Fig. 29-2A. Ribosomal RNAs undergo many posttranscriptional alterations. Methylation of 2 -hydroxyls and of the nucleic acid bases as well as conversion to pseudouridines (pp. 1638-1641) predominate over 200 modifications, principally in functionally important locations that have been found in human rRNA.69a... 

In addition to pure thermal stability, if the polymer is to be heated in air, one must also consider oxidative stability. In the carbon series oxidation always leads to more stable species and tends to occur, blit in the silicon series there is a much higher tendency towards reaction with oxygen. This is the principal reason for the low utility of the silane polymer. Finally, a third factor in polymer stability is the ease of attack by solvents, acids, bases, etc. This is largely determined by the ionic nature of the bonds involved. The silica based polymers should be more susceptible to such attack than carbon, since they have a higher percent of ionic nature. 

The oxides and the metals also dissolve in alkali bases as well as in acids. The oxides and hydroxides of In and Tl are, by contrast, basic hydrated T1203 is precipitated from solution even at pH 1 to 2.5. The nature of the aluminate and gallate solutions has been much studied by Raman and nmr spectra. The principal species in solution for Al is [Al(OH)4] only where Na+ is the cation is there evidence for species in solution that may be related to solids that crystallize from aluminate solutions. The latter contain dimeric anions with 4-coordinate Al, [(H0)3A10A1(0H)3]2 , and others with octahedral Al3+. 

Although B(0H)3 and B(OH)4 are monomeric in dilute solutions, at concentrations above about O.lmolL, condensed borate species that are often referred to as polyborates form. Titration of a boric acid solution with one molar equivalent of a strong base leads to formation of the tetrahydroxyborate anion, B(OH)4, as the principal species in solution. Mixtures of boric acid and its conjugate base, the tetrahydroxyborate anion, form what appears to be a classical buffer system where the pH is determined primarily by the acid salt ratio with [H+] = K[B(OH)3]/[B(OH)4 ]. This relationship is approximately correct for sodium and potassium borates with a sodium boron ratio of 1 2. Here the B(0H)3 B(0H)4 ratio equals one, and the solution pH remains near 9 over a wide range. However, for borate solutions with pH values significantly above or below 9,... 

Following these findings, Lavalley and coworkers expanded the data set in a series of papers (505—510) and pursued the idea to use alcohols as probe molecules for identifying the number of cations that can be simultaneously bound to one adsorbed molecule. It was found that methoxy species are formed according to two principal routes. The first route is the dissociative adsorption of methanol on Lewis acid-base pairs ... 

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. 

We shall now describe how activity corrections can be applied to data obtained from the potentio-metric titration of monobasic acids and mono-acidic bases, where the principal term in need of correction is the concentration of ionized species. 

Most of the species listed in Table I are O-acylated derivatives of NANA and NONA, which are readily converted to one or the other of these basic structures by mild base treatment. Location of the ester-linked acetates was accomplished originally by periodate oxidation and more recently by mass spectrometry. N-acetyl-4-O-acetylneuraminic acid (2), the principal species in equine submaxillary mucin, was shown by Blix and Lindberg (1960) to react with 2 moles of periodate, liberating 1 mole each of formaldehyde and formic acid. The N,0-diacetylneura-minic acid isolated from bovine submaxillary mucin (Blix et al., 1956) was concluded to be the 7-O-acetyl derivative (i) from the fact that only 1 mole of periodate was consumed with formation of 1 mole of formaldehyde. The N-acetyl-O-diacetylneuraminic acid from the same source was originally thought to have O-acetyl groups at positions 4 and... 

Starch is a polysaccharide found in many plant species. Com and potatoes are two common sources of industrial starch. The composition of starch varies somewhat in terms of the amount of branching of the polymer chains (11). Its principal use as a flocculant is in the Bayer process for extracting aluminum from bauxite ore. The digestion of bauxite in sodium hydroxide solution produces a suspension of finely divided iron minerals and siUcates, called red mud, in a highly alkaline Hquor. Starch is used to settle the red mud so that relatively pure alumina can be produced from the clarified Hquor. It has been largely replaced by acryHc acid and acrylamide-based (11,12) polymers, although a number of plants stiH add some starch in addition to synthetic polymers to reduce the level of residual suspended soHds in the Hquor. Starch [9005-25-8] can be modified with various reagents to produce semisynthetic polymers. The principal one of these is cationic starch, which is used as a retention aid in paper production as a component of a dual system (13,14) or a microparticle system (15). 

Polyborates and pH Behavior. Whereas bode acid is essentiaHy monomeric ia dilute aqueous solutions, polymeric species may form at concentrations above 0.1 M. The conjugate base of bode acid in aqueous systems is the tetrahydroxyborate [15390-83-7] anion sometimes caHed the metaborate anion, B(OH) 4. This species is also the principal anion in solutions of alkaH metal (1 1) borates such as sodium metaborate,... 

We can predict the pH at any point in the titration of a polyprotic acid with a strong base by using the reaction stoichiometry to recognize what stage we have reached in the titration. We then identify the principal solute species at that point and the principal proton transfer equilibrium that determines the pH. 

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