Neutralization reactions using mass relations

EXERCISE 4.15 Using Mass Relations in a Neutralization Reaction... 

We use the hydrolysis of A into P and Q as an illustration. Examples are the hydrolysis of benzylpenicillin (pen G) or the enantioselective hydrolysis of L-acetyl amino acids in a DL-mixture, which yields an enantiomerically pure L-amino acid as well as the unhydrolysed D-acetyl amino acid. In concentrated solutions these hydrolysis reactions are incomplete due to the reaction equilibrium. It is evident that for an accurate analysis of weak electrolyte systems, the association-dissociation reactions and the related phase behaviour of the reacting species must be accounted for precisely in the model [42,43]. We have simplified this example to neutral species A, P and Q. The distribution coefficients are Kq = 0.5 and Kp = K = 2. The equilibrium constant for the reaction K =XpXQ/Xj = 0.01, where X is a measure for concentration (mass or mole fractions) compatible with the partition coefficients. The mole fraction of A in the feed (z ) was 0.1, which corresponds to a very high aqueous feed concentration of approximately 5 M. We have simulated the hydrolysis conversion in the fractionating reactor with 50-100 equilibrium stages. A further increase in the number of stages did not improve the conversion or selectivity to a significant extent. Depending on the initial estimate, the calculation requires typically less than five iterations. 

Thus, an analysis method with a much better estimation accuracy is necessary. For a more detailed analysis, a method using the diffusion equation or the geochemical mass transfer analysis must be used. The biggest difference in these two methods is the chemical reaction model. The method is frequently used for predicting neutralization or salt attack of concrete structures in the civil engineering fields. Regarding mass transfer, the law of conservation of mass relating to the solid-phase element concentration Cp and... 

ABSTRACT. The principle of operation of drift tubes and their application to the determination of ion-neutral reaction rate coefficients, k, as a function of the ion/reactant molecule (E ) and the ion/buffer gas (Ej,) centre-of-mass energies are discussed. It is shovm that drift tube data of k versus Ej., for atomic ion/neutral reactions can be used with confidence in modelling the ion chemistry of shocked interstellar gas. However, it is stressed that drift tube data relating to molecular ion reactions must be used with caution since internal excitation of the ions can occur in collisions with the buffer gas. Some consideration is given to the variation with Ej, and Ej. of the rate coefficients, k3, for ternary association reactions and to the relevance of the data in estimating radiative association rate coefficients appropriate to shocked interstellar gas. 

Apart from the proton transfer reactions discussed in Section II, phosphorus species undergo a range of other ion-molecule reactions in the gas phase. The types of instruments which have been used to study ion-molecule reactions of phosphorus species include ion cyclotron resonance (ICR) mass spectrometers and the related FT-ICR instruments, flowing afterglow (FA) instruments and their related selected-ion flow tubes (SIFT) and also more conventional instruments This section is divided into four topics (A) positive ion-molecule reactions (B) negative ion-molecule reactions (C) neutralization-reionization reactions and (D) phosphorus-carbon bond formation reactions. 

Studies of the structure and reactivities of the three ions discussed in this article have continued to bear out the possibility of an ionic mechanism for soot formation. The reaction rate coefficients are certainly rapid and in general larger than those used by Olson and Calcote (10) in their model, at least for certain isomeric forms of the ions and at the temperatures of ca. 325 K used in our work. However, until more sophisticated flamesampling mass spectrometers are employed, the exact isomeric form of the many ions seen in flames cannot be known, and thus cannot be related directly to our work. As is to be expected, details of the reactivity or nonreactivity of various isomers vary as one moves from one ionic species to another. Thus in some cases (C5H5 ) acyclic forms are less reactive, while the cyclic isomers react rapidly, while in others (C H " , C Hc" ) the opposite is the case. Those channels where the cyclic ions are less reactive suggest an opportunity for the formation of cyclic neutrals in... 

The energy Q related to the nuclear reaction is determined from the differences in the masses M of the reactants and the products converted to million electron volts so that, for the example reaction, Q = [M2 ai + Mip — ( 2751 + Min)] x 931.5. The masses are expressed in atomic mass units as neutral atoms and the conversion factor is 931.5, in units of million electron volts per atomic mass unit. A more convenient calculation is to use, instead of M, the commonly tabulated mass excess or defect A. The quantity A is the atomic mass minus the mass number (A) for the nuclide, expressed in million electron volts. These quantities for the individual reactants and products can be substituted in the calculation of Q. For this example, Q = A( Al) - - A( H) — A( Si) — A( n) = —17.194 - -7.289 -I- 12.385 - 8.071 MeV = -5.591 MeV. The negative value of Q shows that the kinetic energy of the proton is required for the reaction. 

In a constant-neutral-loss scan, all precursors that undergo the loss of a specified common neutral are monitored. To obtain this information, both mass analyzers are scanned simultaneously, but with a mass offset that correlates with the mass of the specified neutral. Similar to the precursor-ion scan, this technique is also useful in the selective identification of closely related class of compounds in a mixture. For example, the loss of 44 Da is a common reaction of carboxylic acids. Through the constant-neutral loss scan, the identity of all carboxylic acids present in a complex mixture can be revealed. Similarly, by monitoring the 98-Da neutral loss, the presence of phosphopeptides can be detected in a complex mixture [6]. 

Another technique called selected (onflow tube (SIFT)-MS [188-190] is closely related to PTR-MS. It involves reactions of neutral analytes with selected ions such as H30, NO, or 02". The reagent ions are generated (e.g., using microwaves), and selected by a quadrupole mass filter. They enter a flow tube where they encounter analytes. Following the reaction, the newly formed ions are separated in the second quadrupole downstream from the fiow tube. SIFT-MS has successfully been used in the monitoring of gaseous samples, including air pollutants as well as breath (e.g., [191-195]). 

---