Coulometric titrants

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 ... 

In addition to references already cited, several other publications are recommended for reading. These include works on electron transfer reactions in organic chemistry by Eberson [48] use of Ag(II) as a coulometric titrant by Lingane and Davis [49-50], as well as by Schotherst and Den Boef [51] current efficiency losses by Comninellis, Grissen, and Platner [52-53] and various studies of mediator and radionuclide electrochemistry [54-56]. 

The titrant in a conventional titration is replaced in a coulometric titration by a constant-current source whose current is analogous to the titrant s molarity. The time needed for an exhaustive electrolysis takes the place of the volume of titrant, and the switch for starting and stopping the electrolysis serves the same function as a buret s stopcock. 

The purity of a sample of Na2S203 was determined by a coulometric redox titration using as a mediator, and as the titrant. A sample weighing 0.1342 g is transferred to a 100-mL volumetric flask and diluted to volume with distilled water. A 10.00-mL portion is transferred to an electrochemical cell along with 25 mL of 1 M KI, 75 mL of a pH 7.0 phosphate buffer, and several drops of a starch indicator solution. Electrolysis at a constant current of 36.45 mA required 221.8 s to reach the starch indicator end point. Determine the purity of the sample. 

Description of the Method. The concentration of Cr207 in a sample is determined by a coulometric redox titration using Fe + as a mediator and electrogenerated Fe + as the "titrant." The end point of the coulometric redox titration is determined potentiometrically. 

The concentration of H2S in the drainage from an abandoned mine can be determined by a coulometric titration using KI as a mediator and as the titrant. ... 

One method for the determination of H3ASO3 is by a coulometric titration using as a titrant. The relevant reactions and standard-state potentials are summarized as follows. 

Coulometric determination of Cu(II) and Ni(II) was canied out applying such reagents as 8-hydroxyquinoline and 8-mercaptoquinoline. Electrogenerated bromine was used as a titrant. 

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... 

Figure 14.2(a) is a schematic diagram of a suitable circuit for coulometric titration with internal generation of titrant and using the dead-stop or... 

The limitations of coulometric titration with internal generation of the titrant include the following. 

Verhoef and co-workers suggested omitting the foul smelling pyridine completely and proposed a modified reagent, consisting of a methanolic solution of sulphur dioxide (0.5 M) and sodium acetate (1M) as the solvent for the analyte, and a solution of iodine (0.1 M) in methanol as the titrant the titration proceeds much faster and the end-point can be detected preferably bipoten-tiometrically (constant current of 2 pA), but also biamperometrically (AE about 100 mV) and even visually as only a little of the yellow sulphur dioxide-iodide complex S02r is formed (for the coulometric method see Section 3.5). 

In this automatic system, the authors preferably used coulometric generation of titrant (cf., microcoulometric determination of deviations in the above end-point titration ), e.g., H, OH, Ag, Hg2+, Br2,12, Fe(CN) (cf., Table 1 in ref. 63). The detection method may be potentiometric (logarithmic signal), amperometric (linear signal), biamperometric, conductometric, oscillometric, etc. Moreover, the authors evaluated triangle programmed titration curves by... 

Amperometric titrations have an even wider range of application than polarography. Although the titrant may be added from a burette, in many applications it is electrically generated in a coulometric cell (p. 261). Such an arrangement lends itself to complete automation and is particularly valuable for the titration of very small quantities. For examples of coulometric titrations with amperometric equivalence point detection see Table 6.5. 

As stated previously, the iodine titrant is generated electrochemically in the coulometric method. Electrochemical generation refers to the fact that a needed chemical is a product of either the oxidation halfreaction at an anode or the reduction half-reaction at a cathode. In the Karl Fischer coulometric method, iodine is generated at an anode via the oxidation of the iodide ion ... 

In the volumetric method, the titrant is added from an external reservoir. In the coulometric method, the titrant (iodine) is generated internally via an electrochemical reaction. 

Mediators are occasionally termed mediator titrants or auxiliary redox couples, while mediation is sometimes called indirect coulometry. The efficiency of the reaction shown in equation (5.8) is assumed to be 100%, and so, in practice, the coulometric experiment is performed in the same way as if direct electron transfer occurred - the charge required to reduce (or oxidize) the mediator is determined, and Faraday s laws are then applied. 

The sample concentration can be calculated from (5.62). The handling of the results is more complicated than in normal titrations because two end-points must be determined and thus a desk-top calculator was recommended for the purpose [100]. From the point of view of the accuracy and precision, it is advantageous to generate the titrants coulometrically [98, 99]. 

There are two types of Karl Fisher titrations volumetric and coulometric. Volumetric titration is used to determine relatively large amounts of water (1 to 100. ig) and can be performed using the single- or two-component system. Most commercially available titrators make use of the one-component titrant, which can be purchased in two strengths 2 mg of water per milliliter of titrant and the 5 mg of water per milliliter of titrant. The choice of concentration is dependent on the amount of water in the sample and any sample size limitations. In both cases, the sample is typically dissolved in a methanol solution. The iodine/SCVpyridine (imidazole) required for the reaction is titrated into the sample solution either manually or automatically. The reaction endpoint is generally detected bivoltametrically. 

The Karl Fischer titration,30 which measures traces of water in transformer oil, solvents, foods, polymers, and other substances, is performed half a million times each day.31 The titration is usually performed by delivering titrant from an automated buret or by coulometric generation of titrant. The volumetric procedure tends to be appropriate for larger amounts of water (but can go as low as 1 mg H20) and the coulometric procedure tends to be appropriate for smaller amounts of water. 

Some method of signaling is required to indicate when the amount of titrant generated is equivalent to the amount of unknown present, and all of the endpoint detection methods used in volumetric titrimetry are, in principle, applicable to coulometric titrations. A list that covers most of the published coulo-metric titration procedures is given in Table 25.2. It is beyond our scope here to describe any of these in detail because each of these methods is a subject for discussion in its own right. Discussions of the equations for a number of types of titration curves are found in texts by Lingane [15], Butler [16], and Laitinen and Harris [17]. 

An automated constant-current coulometric system employing electrogenerated iodine for assay of ascorbic acid or sodium ascorbate has been reported [38]. Using this apparatus, 25 samples of 30 mg could be determined in 2.5 h with an accuracy and precision of 0.3%. The automated system demonstrates accuracy and precision that are equivalent to or exceed the manual USP method, is significantly more rapid, and eliminates the need for preparation, standardization, and storage of titrant. 

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. 

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