Acids coulometric titration

Coupling the mediator s oxidation or reduction to an acid-base, precipitation, or complexation reaction involving the analyte allows for the coulometric titration of analytes that are not easily oxidized or reduced. For example, when using H2O as a mediator, oxidation at the anode produces H3O+... 

If the oxidation or reduction of H2O is carried out externally using the generator cell shown in Figure 11.25, then H3O+ or OH can be dispensed selectively into a solution containing a basic or acidic analyte. The resulting reaction is identical to that in an acid-base titration. Coulometric acid-base titrations have been used for... 

Representative Examples of Coulometric Titrations Using Acid-Base, Complexation, and Precipitation Reactions... 

Explain why the coulometric titration must be done in neutral solutions (pH = 7), instead of in strongly acidic solutions (pH<0). 

The principle of coulometric titration. This involves the generation of a titrant by electrolysis and may be illustrated by reference to the titration of iron(II) with electro-generated cerium(IV), A large excess of Ce(III) is added to the solution containing the Fe(II) ion in the presence of, say IM sulphuric acid. Consider what happens at a platinum anode when a solution containing Fe(II) ions alone is electrolysed at constant current. Initially the reaction... 

Coulometric acid-base titration. The method below, described by Dahmen and Bos167, was applied to non-aqueous media (see part B), where precautions should be even more strictly adhered to than in aqueous media. The instrumental set-up, as depicted in Figs. 3.87-3.89, was originally developed by their former colleague Dubbeling together with Van den Enk-van Twillert). 

Coulometric titration. For this technique, often designated controlled-current or amperostatic coulometry, it is useful to distinguish between redox, complex-formation and precipitation titrations on the one hand and acid-base titrations on the other and to discuss each group separately. 

Recently, Mazzocchin et al. have studied several titration methods for determining relatively large amounts of technetium. The most precise results have been obtained by coulometric titration of TcO ions With electrogenerated tin (II) according to the procedure suggested by Bard and Lingaiie . The supporting electrolyte consists of 2.5 M sodium bromide, 0.15 M stannic chloride and 0.2 M hydrochloric acid. The titration reaction is very fast and currents up to 40 mA can readily be employed in the detection of the equivalence point. 

A Chemically mediated spectroelectrochemical titration B Cyclic voltammetry at a 3-mercaptopropionic acid-modified Au electrode C Chemically mediated coulometric titration. 

Clement and Paris [17] have devised a pair of methods for the determination of cobalt in steels, especially materials encountered in the nuclear industry. In the first technique, suitable for the analysis of solutions containing 8 to 160 mM cobalt(II), iron(III) is used to oxidize cobalt(II) in a picolinic acid medium, after which the resulting iron(II) is titrated po-tentiometrically with a standard solution of cerium(IV). An alternative procedure, for concentrations of cobalt(II) below 8 mM, involves a constant-current coulometric titration with electrogenerated cerium (IV) to measure the iron(II) that arises from the original reaction between cobalt(II) and iron(III). 

Controlled-current coulometry is also called coulometric titration. An apparatus for controlled-current coulometry is shown in Fig. 5.35 for the case of determination of an acid. It consists of a constant current source, a timer, an end-point detector (pH meter), and a titration cell, which contains a generating electrode, a counter electrode in a diaphragm, and two electrodes for pH detection. The timer... 

Figure 17-8 Apparatus for coulometric titration of cyclohexene with Br2. The solution contains cyclohexene, 0.15 M KBr, and 3 mM mercuric acetate in a mixed solvent of acetic acid, methanol, and water. Mercuric acetate catalyzes the addition of Br2 to the olefin. [Adapted from D. H. Evans, Coulometric Titration of Cyclohexene with Bromine," J. Chem. Ed. 1968,45,88.]... 

At unit activities of the oxidant and reductant, the potential depends only on pH the slope of the line for a plot of potential versus pH is governed by the ratio m/n. Potential-pH diagrams are a concise means to display the redox properties of a system. We will take uranium as an example. The +6, +5, +4, and + 3 oxidation states are known in aqueous solution. The determination of +6 uranium by coulometric titration has been investigated by many workers and the lower oxidation states have all been used as coulometric titrants. Hydrolyzed uranium species exist in a noncomplexing solution, but the chemistry is simplified considerably if the discussion is limited to solutions more acidic than about pH 4. Some of the half-reactions to be considered are listed next with E° vs. NHE ... 

Some of the reasons for considering coulometric titrations in nonaqueous solvents are that many organic compounds are not soluble in water, metals can exist in oxidation states that are not found in water, and advantage can be taken of the acidity or basicity of the solvent to improve the basic or acidic strength of a base or acid, respectively. 

An assay for the determination of vitamin K3 (2-methyl-1,4-naphthoquinone) by a combination of both constant-potential and constant-current cou-lometry has been reported [13]. The assay requires the two-electron reduction of the compound to the corresponding hydroquinone at a mercury pool electrode (E = -0.60 V vs. SCE) in acetate buffer, pH 5.9, followed by the coulometric titration of the reduction product with electrogenerated Ce(VI). This method is preferable to the standard method requiring preliminary reduction to the hydroquinone by zinc dust in acid medium, followed by titration with standard Ce(IV) solution. It is capable of low-level determination (1-2 mg) of this vitamin in pharmaceuticals, biological fluids, and foods. 

Coulometric titrations have been employed for redox, acid-base, precipitation, and complexation titrations of organics and inorganics in both aqueous... 

The uses of constant-current coulometry for the determination of drugs in biological fluids are few, basically due to sensitivity restriction. Monforte and Purdy [46] have reported an assay for two allylic barbituric acid derivatives, sodium seconal and sodium sandoptal, with electrogenerated bromine as the titrant and biamperometry for endpoint detection. Quantitative bromination required an excess of bromine hence back titration with standard arsenite was performed. The assay required the formation of a protein-free filtrate of serum with tungstic acid, extraction into chloroform, and sample cleanup by back extraction, followed by coulometric titration with electrogenerated bromine. The protein precipitation step resulted in losses of compound due to coprecipitation. The recoveries of sodium seconal and sodium sandoptal carried through the serum assay were approximately 81 and 88%, respectively. Samples in the concentration range 7.5-50 pg/mL serum were analyzed by this procedure. 

The coulometric titration of weak bases with electrogenerated H+ has been successful for nicotinamide, sodium salicylate, metronidazole, sodium veronal, isonicotinic acid, and other compounds, in 0.1 M sodium perchlorate in a mixture of acetic acid and acetic anhydride (1 6) [47]. The endpoint of the titration was noted photometrically using a malachite green indicator. Approximately 5 mg of each compound was determined with an average deviation of 0.3%. 

Adhami and Herlem789 have carried out a coulometric titration at controlled potential of iodine in fluorosulfuric acid and have shown that iodine is quantitatively oxidized to I2+ by removal of one electron per mole of iodine. 

Coulometric titration procedures have been developed for a great number of oxidation-reduction, acid-base, precipitation, and complexation reactions. The sample systems as well as the electrochemical intemediates used for them are summarized in Table 4.1, and indicate the diversity and range of application for the method. An additional specialized form of coulometric titration involves the use of a spent Karl Fischer solution as the electrochemical intermediate for the determination of water at extremely low levels. For such a system the anode reaction regenerates iodine, which is the crucial component of the Karl Fischer titrant. This then reacts with the water in the sample system according to the... 

Another example of the application of the principles of coulometric titrations to a continuous on-stream analyzer is the moisture analyzer developed by Kei-del.18 It illustrates one of the outstanding advantages of the coulometric generation of a titrant namely, an intermediate is produced as a titrant that would not be available in standard solutions. The principle of the moisture analyzer is to place a phosphoric acid solution between two closely spaced platinum electrodes (helically wound in a glass tube). When current is passed between the two electrodes, the water in the phosphoric acid is electrolyzed... 

The material for the actuator has to be selected on the basis of the titrant to be generated. For the titration of an acid or base, a noble metal electrode is usually adopted. A constant current is often used in the coulometric titration, and the quantity of charge is then calculated by the time of the electrolysis. The analyte can either be an acid or a base. If there are no other interfering redox couples, the titrant generated at the electrodes depends on the direction of the applied current ... 

A coulometric titration method was introduced for sequential determination of sulfite, thiosulfate and ascorbic acid (1) in solutions containing sulfite-thiosulfate or sulfite-ascorbic acid couples. Formaldehyde or acetaldehyde can be used to mask the sulfite component. Two sequential measurements of coulometric time, one for both components in the mixture and one for the sample solution in which sulfite is masked, can be used to determine the concentrations of sutfite-thiosulfate and sulfite-ascorbic acid couples. The method is linear for 0.5-60 p,M 1 in the presence of 0.44-13 (xM sulfite, with RSD 0.1-4% and current efficiency of ca 98.0%. The method can be used for determination of the presence of sulfite and 1 in real sample matrices such as mineral waters and vitamin C tablets . 

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