Fraction protonated

A glance at the periodic table (which will be covered in detail in Chapter 5) shows a list of elements with numbers that are not as neat as those for carbon. Iron, for instance, has an atomic mass of 55.845. Could an atom have a fractional proton or neutron Of course not. An element must have a fixed number of protons. That is what defines it as an element. However, the number of neutrons in the nucleus of an element can vary. Carbon, for instance, has two prominent forms. Carbon 12 has 6 protons and 6 neutrons whereas carbon 14 has 6 protons and 8 neutrons. 

The surface-enhanced Raman spectra (SERS) provide information about the extent of protonation of the species adsorbed at the silver/aqueous solution interface. The compounds investigated were 4-pyridyl-carbinol (1), 4-acetylpyridine (2), 3-pyridine-carboxaldehyde (3), isonicotinic acid (4), isonicotinamide (5), 4-benzoylpyridine (6), 4-(aminomethyl)pyridine (7) and 4-aminopyridine (8). For 1, the fraction of the adsorbed species which was protonated at -0.20 V vs. SCE varied with pH in a manner indicating stronger adsorption of the neutral than the cationic form. The fraction protonated increased at more negative potentials. Similar results were obtained with 3. For all compounds but 4, bands due to the unprotonated species near 1600 cm-1 and for the ring-protonated species near 1640 cm-1 were seen in the SERS spectra. 

At pH 5.5 EH2, EH, CMP, and CMPH species are present with fractional protonations of 12, 119, and CMP being approximately 0.8, 0.6, and 0.6 (pX values 6.1, 5.8, and 5.8). Since the pX values are sensitive to ionic strength, bound anions may be present in some free protein species but less than at pH 4.5. The complex of EHi,4 with CMPHo., would produce EH2-CMP with no release of protons as observed by Hummel and Witzel. There are several important protein species involved at this pH and there is some controversy as to what they are and how significant. 

The fraction protonated will depend on the pH the higher the pH the smaller will be the amount of the ester which is protonated, and hence the smaller the ratio [HE+]actuai/[E]totai- This quotient can be calculated for each pH, and a plot of the second order kobs versus [HE+]actuai/[E]totai should be linear, with intercept k and slope (k2 — ki). 

Acid-II fraction is mainly composed of alkyl substituted monoaromatic phenols. As for Base-II (G) fraction, proton signals are much more widely spread than that of Fraction K particularly at lower magnetic field, as shown in Figure 11. This may suggest that the fraction is composed of heteroaromatic compounds involving nitrogen atom in the ring structure (18). 

If B(aq) is a sufficiently weak base for the fraction protonated to be small, then very little BH" "(aq) will be formed and very little B(aq) will have been removed. 

As with the weak acid, the approximation generally holds for values around 4 or greater. But again, diluting the solution of the weak base will dramatically increase the fraction protonated, so that the approximation will not hold for very low stoichiometric concentrations. For stronger bases, the extent of protonation becomes sufficiently high for the approximation to be invalid. 

For bases with pA), greater than around 10.0 the base is so weak that the fraction protonated is very low. The contribution to the actual concentration of OH (aq) from the weak base is so small that the contribution from the self ionisation of water is comparable to, or even greater than, that from the weak base. There are now two sources of OH (aq). 

However, because the fraction protonated is so low, then the actual concentration of B(aq) will be very close to the stoichiometric concentration of B(aq), and the first approximation is valid, i.e. ... 

The fraction protonated is so low that the assumption of only slight protonation is also fully justified. 

The fraction protonated is also altered. Substitution into the expression for the fraction protonated now gives ... 

This problem illustrates for a weak base just how dramatically the fraction protonated increases with decrease in concentration. This observation is exactly analogous to the corresponding observation for the fraction ionised for a weak acid. 

The procedure for dealing with this situation is outlined in Worked Problem 4.6. Under these conditions the fraction protonated would appear to be equal to unity because of rounding up problems, and the fraction not protonated would then have to be calculated hrst. This is discussed in Section 4.5. 

Likewise the calculated approximate fraction protonated is impossible ... 

The fraction protonated must be calculated taking note that the self ionisation of water must be included. Electrical neutrality gives ... 

Fraction ionised and fraction not ionised for a weak acid fraction protonated and fraction not protonated for a weak base... 

If ionisation is almost complete, then both these expressions become quotients of two approximately equal quantities so that the difference between the quotients and unity is very small. A direct calculation of the fraction ionised will give a result which is to all intents and purposes equal to unity, i.e. it is impossible to discriminate between 100% ionisation and very nearly 100% ionisation, and [A ]. may appear to be equal to [HAjtotai- When this happens another route has to be followed to find out whether the acid is, for example, 99%, 99.5% or 99.9% ionised. Exactly analogous reasoning applies in the case of a weak base, except for the fact that the fraction calculated is more correctly described as a fraction protonated, from which the fraction not protonated can be found. 

When the fraction protonated becomes very close to unity, i.e. [BH" ]actuai becomes very close... 

OH = K /[HsO+lju ojai can be highly accurate. It is straightforward to find whether this comes to 0.0010 or 0.0005 or some other value. This will allow an accurate calculation of the fraction protonated. 

The relations in Section 4.5 can be used to demonstrate that there is a limiting value to the fraction ionised for any acid, or to the fraction protonated for a weak base. Although some weak acids or bases appear to approach a value of unity, it must not be assumed that this is... 

As with the weak acid, these equations can be used to calculate the fraction protonated at a given pH for any weak base, or to hnd the pH at which a given weak base has a given fraction protonated. 

Weak bases exhibit the same dramatic increase in fraction protonated as the stoichiometric concentration decreases, with all cases reaching a limiting value. In the case of fairly strong weak bases this is virtually 100%, but for the weaker bases the fraction ionised becomes progressively smaller. 

Assuming that the self ionisation of water can be ignored and that the fraction protonated is small, calculate the pH of a 0.001 mol dm solution of sodium benzoate. The acid ionisation constant of benzoic acid is 6.76 x 10 moldm , and the ionic product for H2O is 1.00 X 10 " mol dm at 25°C. Check on the validity or otherwise of the assumptions and carry out a more rigorous calculation if necessary. What would happen if the concentration of the salt were 5 x lO mol dm ... 

Dependence of fraction ionised and fraction protonated on ionic strength... 

Calculations show that the fraction ionised and fraction protonated increase as the ionic strength increases. These calculations show that there should never be an automatic assumption of only slight ionisation or slight protonation, especially at moderate or high ionic strengths. 

Ionic strength can have a quite dramatic effect on the pK of an acid or base, and consequently on the fraction ionised or the fraction protonated, even though the stoichiometric concentration remains the same. 

There could be situations where the effect of ionic strength could alter the fraction ionised or fraction protonated to such an extent that the approximation of only slight ionisation or protonation is no longer valid. 

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