Silver coulometric generation

Oxidative microcoulometry has become a widely accepted technique for the determination of low concentrations of sulfur in petroleum and petroleum products (ASTM D3120). The method involves combustion of the sample in an oxygen-rich atmosphere followed by microcoulometric generation of a triiodide ion to consume the resulting sulfur dioxide. It is intended to distinguish the technique from reductive microcoulometry, which converts sulfur in the sample to hydrogen sulflde that is titrated with coulometrically generated silver ion. 

A number of instrument manufacturers offer automatic coulometric titrators, most of which employ a potentiometric end point. Some of these instruments are multipurpose and can be used for the determination of a variety of species. Others are designed for a single type of analysis. Examples of the latter are chloride titrators, in which silver ion is generated coulometrically sulfur dioxide monitors, where anodically generated bromine oxidizes the analyte to sulfate ions carbon dioxide monitors, in which the gas, absorbed in monoethanolamine, is titrated with coulometrically generated base and water titrators, in which Karl Fischer reagent (see Section 20C-5) is generated electrolytically. 

The use of a pair of identical metallic electrodes to establish the equivalence point in amperometric titrations offers the advantages of simplicity of equipment and elimination of the need to prepare and maintain a reference electrode. This type of system has been incorporated into equipment designed for the routine automatic determination of a single species, usually with a coulometric generated reagent. An example of this type of system is an instrument for the automatic determination of chloride in samples of serum, sweat, tissue extracts, pesticides, and food products. Here, the reagent is silver ion coulometrically generated from a silver anode. The indicator system consists of a pair of twin silver electrodes that are maintained at a potential of perhaps 0.1 V. Before the equivalence point in the titration of chloride ion, there is essentially no current because no easily reduced species is present in the solution. Consequently, electron transfer at the cathode is precluded and that... 

The coulometric generation of titrants is widely applicable to redox, pre cipitation, acid-base and complexing reactions. Of particular value is the determination of many organic compounds with >romine and of mercaptans and halides with the silver ion. Amperometric equivalence point detection is the most common. An attractive feature of the technique is that the need to store standard and possibly unstable reagent solutions is obviated. In fact many applications involve the use of electrdgenerated reagents such as halogens and chromium(II) which are difficult or impossible to store. The technique is especially useful for the determination of veiy small amounts. 

Some typical important industrial applications of coulometry include the continuous monitoring of mercaptan concentration in the materials used in rubber manufacture. The sample continuously reacts with bromine, which is reduced to bromide. A third electrode measures the potential of B12 vs. Br and, based on the measurement, automatically regulates the coulometric generation of the bromine. Coulometry is used in commercial instruments for the continuous analysis and process control of the production of chlorinated hydrocarbons. The chlorinated hydrocarbons are passed through a hot furnace, which converts the organic chloride to HCl. The latter is dissolved in water and the Cl titrated with Ag" ". The Ag" " is generated by coulometry from a sUver electrode, Ag°. It is necessary for the sample flow rate to be constant at all times. Integration of the coulometric current needed to oxidize the silver to silver ion results in a measurement of the Cl concentration. 

An ECD measures the current generated by electroactive analytes in the HPLC eluent between electrodes in the flow cell. It offers sensitive detection (pg levels) of catecholamines, neurotransmitters, sugars, glycoproteins, and compounds containing phenolic, hydroxyl, amino, diazo, or nitro functional groups. The detector can be the amperometric, pulsed-amperometric, or coulometric type, with the electrodes made from vitreous or glassy carbon, silver, gold, or platinum, operated in the oxidative or reductive mode. Manufacturers include BSA, ESA, and Shimadzu. 

An advantage of the technique is the use of an electrical standard to replace chemical standards and the problems associated with their preparation and stability. The coulometric titration also permits the generation of reagents such as copper(I) or bromine, which are difficult to employ as standard solution, or others such as silver(II) or chlorine, which are virtually impossible to use in any other way. A disadvantage of the coulometric titration is its lack of specificity. 

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]. 

Acids and bases have been standardized coulometrically with standard deviations of 0.003% halides by generation of silver ions to 0.005% potassium dichromate by generation of iron to 0.003% and conditions established under which... 

Chloride can be determined by titration with anodically generated silver ion. Mixtures of bromide and chloride ordinarily cannot be analyzed successfully by this method, but Boyer used a selective oxidation of bromide followed by coulometric titration of chloride, together with a coulometric titration of total halide, to analyze milligram quantities of nonstoichiometric lead bromochloride. 

The reactions in coulometric-amperometric determinations of CT depend on the generation of Ag fi om a silver electrode at a constant rate and on the reaction of Ag with Cl in the sample to form insoluble silver chloride ... 

The accepted reference method for determining chloride in blood serum, plasma, urine, sweat, and other body fluids is the coulometric titration procedure. In this technique, silver ions are generated coulometrically. The silver ions then react with chloride ions to form insoluble silver chloride. The end point is usually detected by amperometry (see Section 23B-4) when a sudden increase in current occurs on the generation of a slight excess of Ag. In principle, the absolute amount of Ag" needed to react quantitatively with Cl can be obtained from application of Faraday s law. In practice, calibration is used. First, the time required to titrate a chloride standard solution with a known number of moles of chloride (nci )s using a constant current I is measured. The same constant current is next used in the titration of the unknown solution, and the time r is measured. The number of moles of chloride in the unknown (ncr)u is then obtained as follows ... 

Challenge Problem. Sulfide ion (S ) is formed in wastewater by the action of anaerobic bacteria on organic matter. Sulfide can be readily proto-nated to form volatile, toxic H2S. In addition to the toxicity and noxious odor, sulfide and H2S cause corrosion problems because they can be easily converted to sulfuric acid when conditions change to aerobic. One common method to determine sulfide is by coulometric titration with generated silver ion. At the generator electrode, the reaction is Ag Ag" " + e. The titration reaction is S + 2Ag —> Ag2S(.t). 

In coulometric titrations, a constant current generates the litrant eleclrolytically. In some analyses, Ihe active electrode process involves only generation of the reagent." An example is the titration of halides by silver ions produced at a silver anode. In other titrations, the analyte may also be directly involved at the generator electrode. An example of this type of titration is the coulometric oxidation of iron(ll)-in part by elec-trolytic.illy generated cerium(l V) and in part by direct electrode reaction (Section 24B-2). Under any circumstance. the net process must approach 100% current efficiency with respect to a single chemical change in the analyte. 

A known volume of seawater is acidified and the carbon dioxide produced is stripped out of the sample by an inert gas. The gas is bubbled through a reagent containing ethanol-amine, which reacts with the carbon dionde to produce hydroxyethylcarbamic add (Eq. (8-26)). The latter is coulometrically titrated by the hydroxide ions generated at the cathode (Eq. (8-27)), and the pH in the reagent solution is monitored colorimetrically through the indicator thymolphthalein. At the anode silver is oddised (Eq. (8-28)). The amount of electrons produced corresponds to the amount of carbon dioxide in the sample and can thus be converted into concentration by dividing by the sample volume. 

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. 

Precipitation and Complex-Formation Titrations A variety of coulometric titrations involving anodi-cally generated silver ions have been developed (sec Table 24-1). A cell, such as that shown in Figure 24-9, can be used with a generator electrode constructed from a length of heavy silver wire. End points are detected potentiometrically or with chemical indicators. Similar analyses, based on the generation of mer-cury(l) ion at a mercury anode, have been described. 

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