Zero-point titration

The values of PZC referred to as cip and pH" in Table 3.1 are obtained by acid-base titration. The experimental procedure is basically the same (although many variants have been described), and the substantial difference is in the method of selection of the zero point. Titration gives very accurate changes in uo from one pH value to another (or at least the part of due to proton adsorption/dissociation) without assumptions, but some assumptions are necessary to obtain the zero point. Existence of CIP does not prove the absence of specifically adsorbed ions. Lyklema [44] showed that in case of specific adsorption of metal cations the shift in the CIP to pH below the pristine PZC is more pronounced for metals having stronger affinity toward the surface (e.g. Pb > Ca) and the do at CIP is also more positive for more strongly bound cations. 

The so-called zero-point titration also gained application in water analysis [21]. In this method, the potential difference between two identical indicator electrodes (e.g., two silver wire electrodes) is followed. One of them is in a half cell containing solution corresponding to the equivalence point of the titration, and the other one is dipped into the sample-containing titration vessel. The silver nitrate reagent (0.01 M) is added as long as potential difference exists. With this zero-point potentiometry a determination limit as low as 1.3 mg/dm can be achieved. 

Zero Point Charge (ZPO measurements. Potentiometric titration of samples (3.0 g) was carried out in an aqueous suspension (500 ml electrolytic KNO3 solution) according to the procedure reported by Parks (24),... 

Potentiometric titration of an aqueous suspension of oxides in the presence of varying concentrations of indifferent electrolyte has been used successfully to determine the zero point of charge (z.p.c.) and the variation in excess surface charge with pH (I, 8). The variation in excess surface charge (rH+-r0H-) with pH and NaCl concentration is shown for goethite in Figure 4. 

Hendershot, W. H. 1978. Measurement technique effects on the value of zero point of charge and its displacement from zero point of titration. Can. J. Soil Sci. 58 438-442,... 

The term lEP has been used even quite recently for a zero point determined by drift method [18]. The principle of the method is as follows. A series of buffer solutions of equal volume and different pH (in this instance chloroacetic acid-sodium chloroacetate for acidic range and NH3-NH4NO3 for basic range) is prepared. The same amount of powder is added to each solution and the pH of the slurry is measured. The instant change in pH (negative or positive) induced by addition of powder is plotted as the function of initial pH. The pH at which this change equals zero is taken as the zero point. This method is in fact a modified potentiometric titration without correction. Consequently such results are referred to as pH in Table 3.1. Moreover, weak acids often adsorb specifically and this affects the obtained zero point, thus pristine value can be only obtained in case of fortuitous coincidence using this method. 

Only the zero point is obtained by inert electrolyte titration (no ao data). 

Rgure 13.12 Mass titration curves for the determination of the zero point of charge of carbons after different severity of oxidation. (Reprinted from Ref. [76] with permission from Elsevier.)... 

Colloid Titration A method for the determination of charge, and the zero point of charge, of colloidal species. The colloid is subjected to a potentiometric titration with acid or base to determine the amounts of acid or base needed to establish equilibrium with various pH values. By titrating the colloid in different, known concentrations of indifferent electrolyte, the point of zero charge can be determined as the pH for which all the isotherms intersect. See also Point of Zero Charge. 

Sakurai, K., Ohdate, Y., Kyuma, K., 1988. Comparison of salt titration and potentiometric titration methods for the determination of zero point of charge. Soil Sci. Plant Nutr. 34, 171-182. 

SO. For good working discussions of potentiometric titration methods, see G. H. Bolt, Determination of the charge density of silica soils, J. Phys. Chem. 61 1166 (1957), and D. E. Yates and T. W, Healy, Titanium dioxide-electrolyte interface. 2. Surface charge (titration) studies, J.C.S. Faraday 1 76 9 (1980), A critical discussion of the uses of potentiometric titration to measure (Th for soil clays is given in J. C. Parker, L. W, Zelazny, S. Sampath, and W. G. Harris, Critical evaluation of the extension of zero point of charge (ZPC) theory to soil systems. Soil Sci. Soc. Am. J. 43 668 (1979). 

The mass balance equations to be used in the evaluation are given in the following. They are based on an exactly neutralized solution as a zero point, i.e., the point in the titration where the amount of acid added equals the initial alkalinity, Ay, which is identical to -Hy in equation (8-17). 

Thus, when titrating iodide with silver nitrate, coagulation occurs as soon as a slight excess of silver ion has been added (so that a point of zero charge has been surpassed). 

Figure V-8 illustrates that there can be a pH of zero potential interpreted as the point of zero charge at the shear plane this is called the isoelectric point (iep). Because of specific ion and Stem layer adsorption, the iep is not necessarily the point of zero surface charge (pzc) at the particle surface. An example of this occurs in a recent study of zircon (ZrSi04), where the pzc measured by titration of natural zircon is 5.9 0.1...

In Fig. 15.7 are presented (a) the part of the experimental titration curve in the vicinity of the equivalence point (b) the first derivative curve, i.e. the slope of the titration curve as a function of V (the equivalence point is indicated by the maximum, which corresponds to the inflexion in the titration curve) and (c) the second derivative curve, i.e. the slope of curve (b) as a function of V (the second derivative becomes zero at the inflexion point and provides a more exact measurement of the equivalence point). 

Dilute solutions of sodium thiosulphate (e.g. 0.001 M) may be titrated with dilute iodine solutions (e.g. 0.005M) at zero applied voltage. For satisfactory results, the thiosulphate solution should be present in a supporting electrolyte which is 0.1 M in potassium chloride and 0.004 M in potassium iodide. Under these conditions no diffusion current is detected until after the equivalence point when excess of iodine is reduced at the electrode a reversed L-type of titration graph is obtained. 

The end point of the reaction is conveniently determined electrometrically using the dead-stop end point procedure. If a small e.m.f. is applied across two platinum electrodes immersed in the reaction mixture a current will flow as long as free iodine is present, to remove hydrogen and depolarise the cathode. When the last trace of iodine has reacted the current will decrease to zero or very close to zero. Conversely, the technique may be combined with a direct titration of the sample with the Karl Fischer reagent here the current in the electrode circuit suddenly increases at the first appearance of unused iodine in the solution. 

Procedure. Charge the titration cell (Fig. 17.24) with 10.00 mL of the copper ion solution, 20 mL of the acetate buffer (pH = 2.2), and about 120mL of water. Position the cell in the spectrophotometer and set the wavelength scale at 745 nm. Adjust the slit width so that the reading on the absorbance scale is zero. Stir the solution and titrate with the standard EDTA record the absorbance every 0.50 mL until the value is about 0.20 and subsequently every 0.20 mL. Continue the titration until about 1.0 mL after the end point the latter occurs when the absorbance readings become fairly constant. Plot absorbance against mL of titrant added the intersection of the two straight lines (see Fig. 17.23 C) is the end point. 

The fact that in HPLC only UV-active components are registered, whereas in titration all basic functional groups are detected constitutes a difference in specificity (quality) and sensitivity (quantity) of these two methods relative to a given impurity. See Fig. 4.17 (left). [Solvent A (water) behaves differently from the other four as can be seen from Fig. 4.17 (right). The material was known to exist in a crystal modification that theoretically contains 3.2% water, and moderate drying will most likely drive off only the excess Indeed, the best-dried batches are all close to the theoretical point (circle, arrow in Figs. 4.16-17), and not near zero. This is only partly reflected in Table 4.15, column A for this reason tabular and graphic information has to be combined. Solvent B, which is an alcohol, behaves more like water... 

Beyond the stoichiometric point, in the final region of the titration curve, the concentration of acetic acid is very close to zero. There are no acid molecules to react with any further hydroxide ions, so excess hydroxide ions are... 

With a low constant current -1 (see Fig. 3.71) one obtains the same type of curve but its position is slightly higher and the potential falls just beyond the equivalence point (see Fig. 3.72, anodic curve -1). In order to minimize the aforementioned deviations from the equivalence point, I should be taken as low as possible. Now, it will be clear that the zero current line (abscissa) in Fig. 3.71 yields the well known non-faradaic potentiometric titration curve (B B in Fig. 2.22) with the correct equivalence point at 1.107 V this means that, when two electroactive redox systems are involved, there is no real need for constant-current potentiometry, whereas this technique becomes of major advantage... 

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