The Coulometric Method

The volumetric method is used (rather than the coulometric method) when the water content is higher (greater than about 1%). 

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  

The critical datum is not a buret reading, as it was in the case of the volumetric method. Rather, the amount of iodine used is determined coulometrically by computing the coulombs (total current over time) needed to reach the end point. The coulombs are calculated by multiplying the current applied to the anode-cathode assembly (a constant value) by the total time (seconds) required to reach the end point. The modern coulometric titrator automatically computes the amount of moisture from these data and displays it. 

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Dissolved inorganic carbon is present as three main species which are H2CO3, HCOs and CO. Analytically we have to approach the carbonate system through measurements of pH, total CO2 or DIC, alkalinity (Aik), and PcOj- In an open carbonate system there are six unknown species H", OH , PcOj/ H2CO3, HCOs, and CO . The four equilibrium constants connecting these species are K, Ki, Kh, and fCw. The values of these equilibrium constants vary with T, P, and S (Millero, 1995). To solve for the six rmknowns we need to measure two of the four analytical parameters (Stumm and Morgan, 1996). Direct measurement of Pco is the best approach, but if that is not possible then the most accurate and precise pair (Dickson, 1993) is Total CO2 by the coulometric method Johnson et al., 1993) and pH by the colorimetric method (Clayton et ah, 1995). 

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 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. 

The iodine then reacts with the water that is present. The amount of water titrated is proportional to the total current (according to Faraday s law) used in generating the iodine necessary to react with the water. One mole of iodine reacts quantitatively with 1 mol of water. As a result, 1 mg of water is equivalent to 10.71 C. Based on this principle, the water content of the sample can be determined by the quantity of current that flows during the electrolysis. For this reason, the coulometric method is considered an absolute technique, and no standardization of the reagents is required. 

In the coulometric method, standardization is not necessary, since the current consumed can be measured absolutely. However, a standard with known water content should be checked periodically to ensure that the system is functioning properly. In this case, a certified water standard is generally used, and the amount of water is determined and compared with the amount that is certified to be present. Some coulometric titrators are equipped with an oven for liberating the moisture from samples that are either insoluble in methanol or that react with I2, methanol, or one of the other reagents. Solid standards (e.g., potassium citrate monohydrate) are available for checking the oven, and this check is performed after the coulometer function has been verified. 

At least lOmg of water must be present in the sample to obtain an acceptable precision, given the magnitude of the equivalent water titre T of the commercial reagent needed. When the amount of water is lower than this (as low as 10 pg of water), the coulometric method must be used to increase sensitivity. 

The coulometric method was also applied to the determination of the number of electrons involved in the two-step polarographic reduction of chlor-diazepoxide [18]. The reduction of a 2.5 x 10 5 M solution of the compound in 0.1 N H2S04 yielded n values of 1.97 and 4.06, respectively, at a mercury pool cathode employing working electrode potentials of -0.45 V and -0.90 V vs. Ag/AgCl wire reference electrode. 

The reason for the different number of electrons, and consequently for the differences in products found in the two studies, may be the difference in the applied coulometric methods. The microcoulometric method applied in Reference 64 is not exhaustive as is the coulometric method applied in Reference 65, and the products formed in the former study may therefore be of an intermediate nature. Insoluble products formed in the bulk solution during microcoulometry precipitate and never get in contact with the electrode, whereas precipitation onto the mercury pool electrode in the exhaustive electrolysis allows further reduction of an insoluble intermediate. 

Table 22-2 lists some other separations performed by controlled-potential electrolysis. Because of limited sensitivity and the time required for washing, drying, and weighing the electrodes, many electrogravimetric methods have been replaced by the coulometric methods discussed in the next section. 

Adsorption and desorption isotherms may be determined by monitoring equilibrium moisture uptake or loss in samples stored in desiccators with different relative humidities (saturated salt solution yielding different percentage relative humidity). The amount of water present is determined by loss of drying (LOD), TGA, Karl Fischer titration, the coulometric method, or near-IR spectroscopy. 

Owing to metal chlorides titration by the coulometric method, and carboxylic acid titration by the potentiometric method, it is possible to follow the metal soaps consumption during thermomechanical heat treatments. This new technique provides a better understanding of the stabilization mechanisms of PVC with the calciumr-zinc system, and offers a better explanation of synergistic effects between metal soaps and secondary stabilizers such as epoxidized soya-bean oil, a-phenylindole, and butanediol-p-aminocroto-nate. The influence of these last stabilizers on zinc chloride formation enables us to classify them into short- and longterm stabilizers. 

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. 

Some methods that are specifically for the detection of oxygen have been used the colorimetric method [42] the coulometric method [43] and another based on the phenomenon of radiothermoluminescence of polymers, as a function of oxygen content [44], The latter was used in a transient experiment and hence measured the diffusivity. but if the solubility is known, or can be measured, the permeability can be calculated. 

The 21 formed in the second reaction is determined either by visual chemical titration with a reagent such as sodium thiosulfate in the presence of a suitable endpoint indicator or by amperometric, coulometric, or photometric titration methods. The most sensitive KF methods for the measurement of iodine are coulometric. For both the volumetric-amperometric and coulometric methods the endpoint is detected by a pair of platinum electrodes called the indicator electrodes. An electrical potential (100-400 mV) is applied across the electrodes to balance the circuit and the endpoint is reached when the concentration of I2 ( 50pmoll ) depolarizes the cathode deflecting a galvanometer. The volumetric method measures the amount of standardized reagent necessary to depolarize the platinum electrodes. The coulometric method utilizes, in addition to the indicator electrodes, a second pair of platinum electrodes (generator electrodes) that electrolytically convert the 1 to I2. The current consumed in this process is used to calculate the amount of water using the equation that describes Faraday s laws of electrolysis. 

The advantages of the coulometric method are its simplicity of operation, its increased sensitivity, and the fact that it does not require a standardized reagent, such as water saturated 1-octanol, to calculate the water content but only to assess the accuracy of the instrument. The advantage of the volumetric method is that it permits the use of a wider range of very polar solvents as well as nonpolar solvents for dissolving the sample in the titration vessel. Furthermore, the volumetric titration vessel can be heated to enhance the dissolution of slightly soluble samples. Either the volumetric or the coulometric titration instrument can be joined with an oven (evaporation) or a distillation apparatus (azeotropic distillation). In this configuration, the moisture that is volatilized from the sample can be transported to the titrator with a dry gas and the evaporated water measured either coulometrically or volumetrically. The recent... 

Coulometry. Two methods of coulometry are used coulometry at controlled potential and coulometric titrations. The main advantage of the coulometric method is the elimination of the necessity of standardization as the Faraday constant is a standard. In analysis of complicated samples encountered in environmental analysis the coulometric titrations are more advantageous where 100% current efficiency can be more readily attained by suitable choice of the reagent-solvent system. Coulometric titrations are suitable for determining the amount of substance in the range 0.01 to 100 mg (and sometimes below 1 iJg). Under optimum conditions these titrations can be carried out with a precision and accuracy of 0.01%. Automatic coulometric analyzers for the determination of gaseous pollutants (SO2, O3, NO, etc.) have proven to be useful in environmental chemistry. 

The most commonly used estimator is based on the coulometric method which consists, beginning with a capacity Ceference known and measured in reference conditions, of integrating the / (either for charge or discharge). The available capacity gavauawe in the use conditions is then given by the relation ... 

The silver bromide does not interfere with the neutralization reaction as would the hydrogen ions that are formed at most anodes. Both potentiometric and indicator end points can be used for these titrations. The problems associated with the estimation of the equivalence point are identical with those encountered in a conventional volumetric analysis. A real advantage to the coulometric method, however, is that interference by carbonate ion is far less troublesome. It is only necessary to eliminate carbon dioxide from the solution containing the analyte by aeration with a carbon dioxide free gas before beginning the analysis. The coulometric titration of strong and weak bases can be performed with hydrogen ions generated at a platinum anode. 

The exact nature of the adsorbed intermediate ( COH or COads) can only be ascertained unequivocally using an in situ spectroscopic technique, because the coulometric methods used until now are much too inaccurate to yield the exact number of electrons (three or two, respectively) needed to oxidize the chemisorption residue of CH3OH to CO2. 

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