Neutralization titrations electrode

Fig. 5.35 Apparatus for controlled-current coulometry. The case for neutralization titration of an acid by internal cathodic generation of OhT. GE generating electrode CE counter electrode.

Two major types of end points find widespread use in neutralization titrations. The first is a visual end point based on indicators such as those described in Section 14A. The second is a potentiometric end point, in which the potential of a glass/calomel electrode system is determined with a voltage-measuring device. The measured potential is directly proportional to pH. Potentiometric end points are described in Section 21G. 

Ammonium acetate solutions formed by neutralizing acetic acid using ammonium hydroxide are essentially neutral. Thus, these solutions are suitable for standardization of electrodes, and for use as titration standards. Solutions must be used while fresh, however, as they become acidic on standing. 

Alkalinity is measured by acid-base titration with methylorange or phe-nolphthalein as indicator. Phenolphthalein changes color at pH 8.3, whereas methylorange changes color at pH 4.3. At pH 8 the neutralization of the strong alkali ingredients like NaOH is essentially complete. Further reduction of the pH to 4 will also measure carbonates and bicarbonates. Colorimetric tests and glass electrode systems are used to determine pH. 

Ammonium chloride is analyzed by treatment with formaldehyde (neutralized with NaOH) and the product HCl formed is analyzed by titration using an acid-base color indicator such as phenolphthalein. Alternatively, it may be mixed with caustic soda solution and distdled. The distillate may be analyzed for NH3 by titration with H2SO4 or by colorimetric Nesslerization or with an ammonia-selective electrode (APHA, AWWA, WEF. 1995. Standard Methods for the Examination of Water and Wastewater. 19th ed. Washington, DC, American Pubhc Health Association). The presence of ammonia or any other ammonium compound would interfere in the test. The moisture content in NH4CI may be determined by Karl—Fischer method. 

This technique uses both direct and back titrations of weak acids and bases. Values of are obtained directly. In purely aqueous media, over the pH range 2-10, the titration of dilute (0.005 to 0.05 M) solutions of weak monovalent acids and bases with a glass electrode can lead to reliable thermodynamic pKs. Over this pH interval, the activity coefficients of the ionic species can be calculated by means of the Debye-Hiickel equation. Also, the activity coefficients of the neutral species remain essentially constant and... 

In potentiometric titration a voltage is obtained from an electrode that is sensitive to an ionic species such as H-jO+, i.e., the pH of the solution in this case. We will consider the titration of the mixture of a strong acid (HC1) and a weak acid (CJ+jCOOH) with NaOH (ref. 10). As 2 ml volumes of the base are given to the acidic solution, the pH increases and when one of the acids is neutralized the pH changes very rapidly by a small addition of NaOH. We want to find these maximum points of the first derivative of the titration curve. In the following main program the DATA lines contain 32 data pairs, each consisting of the volume of the added NaOH in ml and the measured pH. 

Procedure for Ammonium Nitrate. Transfer a 25-ml aliquot from the bottle of step 6 to 100 ml of the neutralized form aldehyde-water soln in a 400-ml beaker, add 4 drops of 1% phenolphthalein (or thymolphthalein) indicator and titrate to a pink color with. 0.5N NaOH. Add 2ml of NaOH in excess, cover the beaker, and let stand for 5 mins. Back-titrate with 0.1N HCl to pH 8.5 (glass-calomel electrode system)... 

M. Cremer at the Institute of Physiology at Munich discovered in 1906 that a potential difference of 0.2 V developed across a glass membrane with acid on one side and neutral saline solution on the other. The student Klemensiewicz, working with F. Haber in Karlsruhe in 1908, improved the glass electrode and carried out the first acid-base titration to be monitored with a glass electrode.13... 

A cuprimetric method for the determination of ascorbic acid is based on a direct potentiometric titration with CUSO4 using a copper-based mercury film electrode as the indicator electrode. The electrochemical reaction taking place is ascorbic acid reducing Cu(n) ions quantitatively to Cu(I) ions in neutral or acidic aqueous solutions. The linearity... 

Electrometric Titration Precipitation Reactions.—One of the most important practical applications of electrode potentials is to the determination of the end-points of various typos of titration the subject will be treated here from the standpoint of precipitation reactions, while neutralization and oxidation-reduction processes are described more conveniently in later chapters. 

The anode compartment in a coulometric titration cell contains 250 ml of 1 M Na2S04, with the CO2 removed. Suppose that 1.2463 g of pure NajCOj is added and electrolyzed with platinum electrodes to completion of the neutralization. How long will this procedure take if the current is 194.36 mA ... 

An important variation in the application of nonaqueous solvents has been to add various neutral (see below) solvents to the sample solvent, the reagent, or both, or to use mixtures of solvents. Fritz dissolved weakly basic samples in a variety of solvents such as chloroform, nitrobenzene, ethyl acetate, and acetonitrile and then titrated with perchloric acid in acetic acid using a glass electrode or methyl violet as indicator. Dioxane has been substituted for acetic acid as a solvent for titrant perchloric acid, and sharper titration curves often are observed. Such a reagent is no longer strictly anhydrous when no provision is made to remove water from the 72% perchloric acid used to prepare the reagent. 

Figure 6.3. Titration of H30 and Cu aq with ammonia (a) and with tetramine (trien) (b). Equilibrium diagrams for the distribution of NH3-NH4 (c) of the amino coppeifll) complexes (d) and of Cu ", Cu-trien (e). The similarity of titrating with a base and titrating a metal ion with a base (Lewis acid-base interaction) is obvious. Both neutralization reactions are used analytically for the determination of acids and metal ions. A pH or pMe indicator electrode (glass electrode for and copper electrode for Cu " ) can be used for the end point indication.

The dissociation constants of weak acids or weak bases are often determined by monitoring the pH of the solution while the acid or base is being titrated. A pH meter with a glass pH electrode (see Section 21D-3) is used for the measurements. For an acid, the measured pH when the acid is exactly half neutralized is numerically equal to pA a- For a weak base, the pH at half titration must be converted to pOH, which is then equal to pf<(b-... 

The titration curve of iron(II) with cerium(IV) appears as A in Figure 19-3. This plot resembles closely the curves encountered in neutralization, precipitation, and complex-formation titrations, with the equivalence point being signaled by a rapid change in the ordinate function. A titration involving 0.00500 M iron(II) and 0.01000 M cerium(IV) yields a curve that for all practical purposes is identical to the one we have derived, since the electrode potential of the system is independent of dilution. A spreadsheet to calculate iisysiem a function of the volume of Ce(lV) added is shown in Figure 19-4. 

A significant investigation of titrations in ethyl alcohol solutions is that reported by E. R. Bishop, E. B. Kittredge, and J. H. Hildebrand. These workers determined neutralization curves for various acids and bases in ethyl alcohol solutions, using the hydrogen electrode. They observed, in addition, the electromotive force readings between which the interval of the indicator was located. Unfortunately, the constant of the hydro-... 

Preparation of suitable indicator solutions. The prepar on of indicator solutions for use in the colorimetric pH determination has already been described in detail in Chapter Five ( 3). These solutions, however, were suited only for measurements of buffered solutions. Isohydric indicator solutions have to e prepared in another manner. H. T. Stern titrates the indicator with sodium hydroxide and measures the pH during neutralization with the quinhydrone electrode. A similar procedure has been described by Pierre and Fxjdge. Fawcett and Acree keep in stock a large series of neutralized indicator solutions and determine their pH approximately by colorimetric means. 

In the first part of a Karl Fischer reaction, sulfur dioxide and alcohol react to form an ester that is neutralized by an organic base. In the second step of the reaction, the alkylsulfite anion is oxidized by iodine. This second step consumes the water in the sample. The end point is determined using an electrode that provides a small potential across two platinum tips. When all of the water is consumed in the titration cell, the slight excess of titrant introduced into the cell causes an increase in the current. This increase is the indication for the end point. 

Shapoval et al. reported an examination of the acidic properties of metaphosphate ions in the molten KCl-NaCl eutectic at 700 °C [141]. This investigation was performed by the potentiometric method with the use of the gas platinum-oxygen electrode as an indicator. The potentiometric titration with weights of a strong base resulted in the neutralization process occurring in two stages, listed below ... 

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