Titration curve carbonate system

The pH of polyfunctional systems, such as phosphoric acid or sodium carbonate, can be computed rigorously through use of the systematic approach to multiequilibrium problems described in Chapter 11. Solution of the several simultaneous equations that are involved is difficult and time consuming, however. Fortunately, simplifying assumptions can be invoked when the successive equilibrium constants for the acid (or base) differ by a factor of about 10 (or more). With one exception, these assumptions make it possible to compute pH data for titration curves by the techniques we have discussed in earlier chapters. 

The compositions of titrated solutions and titrants used, as inferred from the very condensed information given in the paper, are summarised in Table A-25. In all systems, carbonate was always in large excess of Zr. From the relative offset of the titration curves, the authors determined the number of basic ligands (the sum of OH and CO3" groups) to be close to 4, yielding the generic formula Zr(0H)x(C03)y for the complex, with X 3- y = 4. Unfortunately, in the paper no details are given on these calculations, so that we were not able to reproduce them. 

The constitution of the limiting complex in the Th(lV)-carbonate system has been determined using potentiometric titrations (measuring the free hydrogen ion concentration at different carbonate concentrations). The experiments have been made in a KNO3 ionic medium, and the authors have only determined the equivalence point where the limiting complex is formed, but no equilibrium constants. The authors have tested different models and their titration curves are only consistent with the stoichiometry Th(C03)f the compositions Th(C03)4, Th(C03) " Th(COj >4 (OH)j" were ruled out. There are few experimental details in this short communication and the experimental data can therefore not be reinterpreted by this review. However, the study seems well done and the suggested stoichiometiy is considered reliable by this review. 

Fig. 4-13. Relationship of the pC-pH Diagram to the titration curve for the carbonate system at 25° C.

The first meaningful deconvolution of titration curves with theoretical description of acid-based dissociation on the surface of carbons was descried in the mid-90 s of the last century [213-215]. In this approach it is assumed that the system under study consists of acidic sites characterized ly their acidity constants, Ka. It is also assumed that the population of sites can be described by a continuous pK distribution, flpKa). The experimental data can be transformed into a proton binding isotherm, Q, representing the total amount of protonated sites, which is related to the pKa distribution by the following integral equation (cf. Fig. 10) ... 

Figure 11.10 illustrates some of the characteristics of the recommended adaptive gain pH control system. The set point and the programmed controller response depend on the titration curve of the brine. As explained in Section 7.5.6, one objective of this preliminary acidification is the decomposition of the carbonate value in the treated brine. This takes place in two stages ... 

Fortunately, many pH systems have weak adds and bases that flatten out various portions of the titration curve to provide a buffering effect. The overall difficulty is reduced especially if the set point ends up residing on one of these plateaus as shown in Figure 1 -Id for a weak acid and base [Ref. 1,1]. The natural buffering of surface and ground waters from carbonates can change your mood from suicidal to merely depressed when the actual curves are compared to theoretical titration curves with pure water. Titration curves of fabricated samples will be much steeper than the titration curves of actual process samples, especially if the lab uses deionized water instead of plant water. 

Some titration curves have a long region of relatively flat slope. The addition of reagent in this portion of the titration cun e has little effect on the pH. Tliis flatness of the curve can be due to buffering, which occurs for mixtures of a weak acid or a weak base and its salt. It is also important for keeping solutions used for calibration at a known pH despite contamination from residue on the electrode. Some of the more commonly encountered buffer systems are acetic acid-acetate, carbonic acid-bicarbonates, and citric acid-citrates. 

There is also mixed evidence about the effect of CO2 on the CIP of charging curves. Reference [451] reports an insignificant role of CO2 in the titration of alumina. In contrast, a substantial effect was found at a carbonate-to-aluminum ratio greater than 0.1 [452]. Shifts in the CIP in opposite directions in the presence of CO2 have been reported. Reference [453] reports a shift in the CIP of titania to high pH by 0.4 pH unit in the presence of 0.001 M Nal KX),. On the other hand, the CIP of goethite was shifted to low pH by about 1 pH unit in a system equilibrated with the atmosphere with respect to a CO2-free system [66,454]. 

The acidic groups have different pK values depending on their location on the carbon surface relative to the location of nonacidic groups that can exert an inductive effect on them [46], Then, by a potentiometric titration method it is assumed that the system under study consists of acidic sites characterized by their acidity constants K. The site population can thus be described by a continuous pK, distribution function f(pKJ [54-57]. The experimental titration data are thus transformed into a proton-binding curve from which the distribution of acidity constants is obtained by using, for example, the splines-based numerical procedure SAIEUS suggested by Jagiello [58]. 

Differential titration experiments were carried out on three systems (79). Ferroperoxidase and carbon monoxide ferroperoxidase gave identical curves. The peroxidase thus shows no Bohr effect. Since the two derivatives correspond as to bond types to the analogous hemoglobin and carbon monoxide hemoglobin, it can be stated with fair certainty that the heme-linked groups in horse-radish peroxidase are not imidazole. 

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