Acid-base titrations simulation

The Acid-Base Titration simulation in eChapter 16.6 allows you to simulate titrating unknown acids with various concentrations of NaOH. 

The Acid-Base Titration activity (eChapter 3.10) allows you to simulate the titration of five acids, A through E. Assuming that the acids can all provide one hydrogen, perform simulated titrations and determine the concentration of all five acids. 

In Figure 10.49 a computer calculation is given, simulating the gigantic acid-base titration that led to the formation of the neutralization products of ocean and sediments. The importance of this calculation is that it demonstrates the essence of equation 10.12 as primary igneous rock minerals react with water and acids, the excess volatiles, by reactions like ... 

Figure 5b. Numerical simulation of the potentiometric acid-base titration based on the SC model [47].

Because of the close analogy between acid-base and redox behavior, it will come as no surprise that one can use redox titrations, and also simulate them on a spreadsheet. In fact, the expressions for redox progress curves are often even simpler than those for acid-base titrations, because they do not take the solvent into account. (Oxidation and reduction of the solvent are almost always kinetically controlled, and therefore do not fit the equilibrium description given here. In the examples given below, they need not be taken... 

Excel simulation of an acid-base titration and generation of a pH curve... 

The TOT program is used in analytical chemistry to simulate acid-base titrations and chelating or precipitating reactions. 

Figure 4 shows a simulated acid-base titration curve for two natural PE chitosan and alginate. It can be seen that in the case of alginate (an acid PE), turbidity starts to increase at pH below 4.0 [23]. In this area of acidic pH, the carboxylic groups of the PE molecule are protonated, which decreases PE... 

List the benefits and disadvantages of using a wet lab acid base titration vs. a simulation of the same experiment. 

In an early work by Mertz and Pettitt, an open system was devised, in which an extended variable, representing the extent of protonation, was used to couple the system to a chemical potential reservoir [67], This method was demonstrated in the simulation of the acid-base reaction of acetic acid with water [67], Recently, PHMD methods based on continuous protonation states have been developed, in which a set of continuous titration coordinates, A, bound between 0 and 1, is propagated simultaneously with the conformational degrees of freedom in explicit or continuum solvent MD simulations. In the acidostat method developed by Borjesson and Hiinenberger for explicit solvent simulations [13], A. is relaxed towards the equilibrium value via a first-order coupling scheme in analogy to Berendsen s thermostat [10]. However, the theoretical basis for the equilibrium condition used in the derivation seems unclear [3], A test using the pKa calculation for several small amines did not yield HH titration behavior [13],... 

Simulation Results. A onc-dimensional simulation model based on the Nernst-Planck and Poisson equations [14, in which all the acid-base reactions occurring in the membrane are taken into account, has been used to give a qualitative description of the pH step titration process. In these simulations, a pH step is applied outside a 2 mm thick stagnant layer, which is assumed to be present in front of an 8 mm thick membrane. Diffusion coefficients in the membrane are assumed to be 4/10 of those in water (this value is based on experience with ion step experiments). Lysozyme, used as a model protein, is assumed to contain 11 carboxylic groups (pKa = 4.4), 2 imidazole groups (pKa = 6.0), and 9 amino groups (pKa = 10.4) per molecule. Concern... 

Use the Titration simulation eChapter 4.6) to determine the concentration of an unknown acid by adding 0.40 M NaOH in increments of 1.0 mL. Repeat the titration adding increments of 0.10 mL of base near the end point. Once more, repeat the titration, adding increments of 0.05 mL of base near the end point If your add is dilute enough, repeat the titration three more times using 0.10 M NaOH in 1.0-mL, 0.50-mL, and 0.05-mL increments, (a) Tabulate the acid concentrations that you calculate from your titration data. Are the values all the same If not, why not (b) Which value do you consider to be most predse and why ... 

To use this method, the sample is dissolved in a system containing two phases (e.g., water and octanol) such that the solution is at least about 5 x 10-4 M. The solution is acidified (or basified) and titrated with base (or acid) under controlled conditions. The shape of the ensuing titration curve is compared with the shape of a simulated curve, which is created in silico. The estimated p0Ka values (together with other variables used to construct the simulated curve such as substance concentration factor, CO2 content of the solution and acidity error) are allowed to vary systematically until the simulated curve fits as closely as possible to the experimental curve. The p0Ka values required to achieve the best fit are assumed to be the correct measured p0Ka values. This computerized calculation technique is called refinement , and is described elsewhere [14, 15]. 

The function Data EqAH2, m simulates the pH-titration of a weak diprotic acid, AH2, in acid excess, with a strong base. The computation of the equilibria is similar to the examples Eql. m and Eq2. m given in the Chapters Example General 3-Component Titration (p.56) and Example pH Titration of Acetic Acid (p.58). From the present point of view, the important aspect is that all variables are collected in one structure s. The model is now stored in s.Model, the logP values in s. log beta, etc. Importantly, all the information contained in s is returned to the invoking programs. 

Mass titration relies on the assumption that there is no acid or base associated with the solid surface, and this assumption is problematic for many specific materials (cf. the discussion concerning Figs. 3.1 and 3.3). Simulations of mass titration taking into account such acidic or basic impurities were carried out by Zalac and Kallay [65]. 

Experimental solutions were prepared from stock solutions in 250-ml volumetric flasks, each to simulate a point in a potentiometric titration. The solutions were prepared as several series, each series representing a total hafnium concentration. Each member of a series was made up to a particular ratio of analytical concentration of acid added to total hafnium, designated as Z. Individual solutions were prepared by adding the calculated volumes of hafnium stock, acid or base stock, and NaCl stock in that order to volumetric flasks. Mixing and dilution to the mark completed the preparation of solutions. All solutions were prepared using borosilicate volumetric ware and were stored in polyethylene bottles at 25°C. 

An Excel spreadsheet can be constructed with appropriate formulas (to include the effects of dilution of the sample by titrant) to simulate the titration of weak and strong acids and bases (Figure 18.21). Some simulations use a master equation to calculate all points on the titration others use separate equations for different regions of the curve, for example before the equivalence point, at the equivalence point and after the equivalence point. The concentration of different species at a particular pH is calculated from [H (aq)l, and the volume of titrant required to produce that amount of each species is calculated. 

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