Precipitation titrations coulometric

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. 

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+... 

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

Discussion. Iodine (or tri-iodide ion Ij" = I2 +1-) is readily generated with 100 per cent efficiency by the oxidation of iodide ion at a platinum anode, and can be used for the coulometric titration of antimony (III). The optimum pH is between 7.5 and 8.5, and a complexing agent (e.g. tartrate ion) must be present to prevent hydrolysis and precipitation of the antimony. In solutions more alkaline than pH of about 8.5, disproportionation of iodine to iodide and iodate(I) (hypoiodite) occurs. The reversible character of the iodine-iodide complex renders equivalence point detection easy by both potentiometric and amperometric techniques for macro titrations, the usual visual detection of the end point with starch is possible. 

Fast solution reactions between analyte and a reagent titration to stoichiometric point by volumetric or coulometric methods end-point detection by visual indicators, precipitation indicators or electrochemical means. 

Nikolic et al. reported the preparation and coulometric determination of quaternary ammonium iodides of procaine and of other local anesthetics [63]. After extraction from 0.33 M NaOH, the quaternary iodide salts were prepared by precipitation with methyl iodide in ethyl ether. The quaternary iodides were then coulometrically determined with the use of a Radiometer titrator. The method used a silver cathode and anode (in electrolytes of 2 M and 0.4 M H2SO4, respectively), and a reference mercurous sulfate electrode. For drug determinations in the range of 0.12 to 0. 96 mg, the standard deviations were typically found to be 4 to 8 pg. 

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. 

Other typical reagents generated for coulometric titrations are hydrogen and hydroxyl ions, redox reagents such as ceric, cuprous, ferrous, chromate, ferric, manganic, stannous, and titanous ions, precipitation reagents such as silver, mercurous, mercuric, and sulfate ions, and complex-formation reagents such as cyanide ion and EDTA [8-10]. 

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... 

A fundamental requirement for all coulometric methods is 100% current efficiency that is, each faraday of electricity must bring about chemical change in the analyte equivalent to one mole of electrons. Note that 100% current efficiency can be achieved without direct participation of the analyte in electron transfer at an electrode. For example, chloride ion may be determined quite easily using poten-tiostatic coulometry or using coulometric titrations with silver ion at a silver anode. Silver ion then reacts with chloride to form a precipitate or deposit of silver chloride. The quantity of electricity required to complete the silver chloride formation serves as the analytical variable. In this instance, 100% current efficiency is realized because the number of moles of electrons is essentially equal to the number of moles of chloride ion in the sample despite the fact that these ions do not react directly at the electrode surface. 

A further advantage of the coulometric procedure is that a single constant-current source provides reagents for precipitation, complex formation, neutralization, or oxidation/reduction titrations. Finally, coulometric titrations are more readily automated, since it is easier to control electrical current than liquid flow. 

Summary of Coulometric Titrations Involving Neutralization, Precipitation, and Complex-Formation Reactions... 

Precipitation and Compiex-Formation Reactions Coulometric titrations with EDTA are carried out by reduction of the ammine mercury(II) EDTA chelate at a mercury cathode ... 

Coulometric Titration of Chloride Ions. The apparatus (Tacussel, Lyon) is composed of a coulometric generator (GCU) and a detection unit (DPA). Ag+ ions are formed continuously from the generator. Chloride ions waiting to be titrated cause the precipitation of these ions. After their total consumption, the current increases to a preselected point of the detector unit which automatically stops the titration reaction. The amount of Ag+ ions consumed is given by the generator unit. 

During heat treatments, zinc and calcium stearates react with hydrogen chloride that has evolved from the polymer or with labile chlorine atoms to liberate zinc and calcium chlorides. These compounds are titrated by a coulometric method in tetrahydrofuran (7) using a generator of Ag+ ions that are precipitated in the presence of chloride ions. When zinc and calcium stearates are used together, both chlorides are titrated at the same time, and they are not differentiated. 

Coulometric titrations can be applied to a number of different types of determinations, including acid-base, precipitation, complexation, and redox titrations. Some typical examples are given in Table 11.4.1 detailed descriptions of the scope and nature of coulometric titrations are given in references 20 and 25-27. 

A single constant current source can be used to generate precipitation, complex formation, oxidation-reduction, or neutralization reageni.s. Furthermore, the coulometric method adapts easily to automatic titrations, because current can be controlled quite easily. 

Coulometric Titrations. An aqueous iodide sample may be titrated with mercurous ion by anodizing a mercury pool electrode. When metallic mercury is oxidized to mercurous ion by a current passing through the system, the mercurous ion reacts directly with the iodide ion to precipitate yellow Hg2l2 ... 

All of the four types of titrations have been implemented coulometrically (i.e., acid-base, precipitation, complexometric, and redox titrations). Acid-base titrations are achieved by generating protons or hydroxide ions from the solvent water by electrolysis (the hydrogen and oxygen evolution reactions). A list of possible methods is given in Table 1. 

Controlled-current coulometry (coulometric titration) can be utilized to determine not-easily oxidizable (or reducible) analytes of different applications via acid-base, precipitation, com-plexation titrations, etc. Furthermore, it benefits short analysis time and small amount determination [2]. Dzudovic et al. [21] reviewed some studies employing acid-base titrations for the determinations of non-aqueous or water-insoluble compounds (organic and inorganic). Typically, acidimetric titrations were undertaken coulometrically based on the EF liberated by the oxidation of the introduced H2O. Coulometric titrations of bases in nonaqueous solvent were performed using anodic depolarizers (titrants) to generate as a source. On the other hand, coulometrically atkalimetric... 

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