Coulometer chemical

Originally, the number of coulombs passed was determined by including a coulometer in the circuit, e.g. a silver, an iodine or a hydrogen-oxygen coulometer. The amount of chemical change taking place in the coulometer can be ascertained, and from this result the number of coulombs passed can be calculated, but with modern equipment an electronic integrator is used to measure the quantity of electricity passed. 

The way in which these alternatives with their particular measuring characteristics are carried out can be best described by (1) controlled-potential coulometry and (2) coulometric titration (controlled-current coulometry). Both methods require an accurate measurement of the number of coulombs consumed, for which the following instrumental possibilities are available (a) chemical coulometers, (b) electrochemical coulometers and (c) electronic coulometers. 

In contrast with the other chemical coulometers, which are fairly accurate especially with high Q values, but cumbersome, the coulometric coulometer is rapid and sensitive, especially with low Q values. 

Ethyl lactate was purchased from Aldrich Chemical Company, Inc., and used without further purification. The water content was 0.8 mg/mL by Karl Fisher titration (Metrohm model 684 KF coulometer). 

Here, m and M are the amount and the molar mass of the analyte. The coul-ometer is usually an electronic one that integrates the current during the electrolysis, although chemical coulometers, e.g. a silver coulometer and a gas coulometer, can also be used. In this method, the deposition of the analyte is not a necessary process. All substances that are electrolyzed with 100% current efficiency can be... 

Portions of the 6 and 15% Florisil eluate were injected into the DC200 and DEGS columns with the coulometer equipped with the sulfur-detecting cell. No peaks were obtained from the 15% eluate injection. Of the several peaks that appeared on the DC200 and DEGS columns from the 6% eluate injection, one was identified as Ronnel DC200 reference 1.00, sample 1.00, DEGS, reference 1.00, sample 1.00, and concentration 0.2 p.p.m. Ronnel is chemically 0,0-dimethyl 0-(2,4,5-trichlorophenyl) phosphorothioate and thus should yield a peak with both the chloride and sulfur detectors. Identification was assured since a peak was found at the proper retention times with both detector cells one column was used for the chloride, and two columns were used for the sulfur detector cells. 

Coulometer A device that permits measurement of the quantity of charge. Electronic coulometers evaluate the integral of the cur-rent/time curve chemical coulometers are based on the extent of reaction in an auxiliary cell. 

Some of the first methods of measuring quantities of electricity involved the use of chemical coulometers. To do this, an electrolytic cell is placed in seri with the sample electrolysis cell so that the same current passes through both. A typical coulometer cell consists of a platinum crucible containing a silver-nitrate solution and a silver anode. Silver metal is deposited on the preweighed platinum crucible and the latter reweighed to determine the amount of electricity passed Q is calculated from Equation 4.10. 

Coulometry. Faraday s laws of electrolysis, enunciated in 183 form the basis of coulometric techniques. By the beginning of the present century the silver coulometer had been shown to provide an accurate means for the measurement of quantities of electricity. An excellent survey of various chemical and other coulometers is available ( ). The electronic digital coulometer, first described in 1962 ( ), was a major practical advance. 

The apparatus which Hittorf employed consisted essentially of a glass cylinder with a cathode near the top and an anode near the bottom. The anode was chosen to be of the same metal as in the salt used (e.g.. copper in work with copper sulphate solutions) in order to maintain the chemical nature of the electrolyte. The cathode was of platinum, gold or silver on which the metal ions plated out. The upper catholyte solution therefore became more dilute during electrolysis and the lower anolyte more concentrated, so ensuring gravitational stability. After the passage of a known quantity of electricity (measured with a silver coulometer) the upper half of the cell was slid sideways by means of a glass plate and the catholyte solution was analysed. In the next paper (2.) Hittorf analysed the anolyte solution also and introduced middle sections but unfortunately separated the compartments by means of intestinal membranes. He did make it plain, however, that the results should be calculated with respect to the mass of water in the final solution. 

The publications of these chemists are so significant that it is worthwhile to reconstruct an example of their thinking, as described by Swift (10) "These workers were interested in the determination of thiocyanate. They either knew or learned that the anodic oxidation of thiocyanate to cyanide and sulfate with 100% current efficiency is difficult if not impossible of attainment. However, it was known that thiocyanate could be quantitatively, stoi-chiometrically, and rapidly oxidized by bromine in acid solutions with 100% current efficiency. Therefore they added a relatively high concentration of a soluble bromide to an acid solution containing the thiocyanate, anodically produced bromine, and allowed this bromine to diffuse into the solution and to oxidize the thiocyanate. By working with relatively large samples of thiocyanate, and by measuring the quantity of electricity involved by means of a chemical coulometer, they demonstrated that an accuracy within 1 ppt could be attained."... 

In order to make a quantitative measure of the amount of electricity required for a reaction to take place, a reliable coulometer is essential. Much of the early work in the field followed Szebelledy and Somogyi in using a chemical coulometer. This amounts to a circular application of the Faraday relation, so that an exact value of F is not required. On the other hand, it merely replaces an inconvenient or difficult chemical measurement with another one, hopefully less inconvenient. 

Another chemical coulometer depends on the production of a gas. Classically, this was the combined H2 and O2 from electrolysis of water containing an indifferent electrolyte such as sodium sulfate. This is fairly sensitive, but involves the inconvenient measurement of a gas volume, which must be corrected to standard conditions of temperature and pressure. The apparatus for collecting and measuring the mixed gases was called a "voltameter," but T. W. Richards (16), noting the possible confusion with "voltmeter," coined the name "coulometer."... 

It was found that a slight negative error inherent in the O2-H2 coulometer could be eliminated by substituting electrolysis of hydrazinium sulfate to give H2 and N2 (17). J. E. Fagel, Jr., in the author s laboratory (18), found that increased sensitivity could be obtained with the use of a slightly modified Warburg monometer a displacement of 1 cm in the manometer corresponded to 0.05 coulomb, or about 6 x 10 milliequivalent of chemical action. 

The development of electronic instrumentation has effectively displaced mechanical devices as the prime element of coulometers. Those in use today almost universally utilize a capacitor charged by the current being measured. The capacitor is placed in the feedback loop of an operational amplifier (op amp), the output of which registers the time-integral of the current, a principle previously used in analog computation circuitry (20). One of the first to use this method for chemical coulometry was Booman in 1957 (21). The difficulty with this approach is that a capacitor of reasonable size (Booman used a 30-yF non-electrolytic capacitor, a giant of its kind) cannot accomodate sufficient charge at the potentials suitable for chemical use. This limitation has been overcome in several ways. 

A complete system providing both a sensor and an actuator would be ideal in the field of process control, but because of a lack of truly reliable chemical sensors on the market the concept has not been widely implemented. One exception relates to the analytical method of coulometry, a technique that offers great potential for delivering chemical compounds to a controlled reaction. Especially attractive in this context is the method of constant- current coulometry, which can be carried out with an end-point sensor and a coulomet-ric actuator for maintaining a generator current until the end-point has been reached. In this case both of the required devices can be miniaturized and constructed with the same technology. 

The quantity of electricity passed through a cell may be determined by measuring the current as a function of time, and determining the area under the current-time curve. A calibrated galvanometer with a short response time is used. Alternatively a chemical coulometer is commonly employed. This consists of an electrolytic cell in series with the experimental cell, the same amount of electricity therefore passing through both the chemical reaction at the cathode or anode (or both) of the coulometer must occur with 100% current efficiency, and should be easily and accurately estimated. The deposition of silver at the cathode of a silver coulometer (q.v.), the dissolution of silver from a silver anode (see Faraday constant) and the reaction 2e + l2 2r of the iodine coulometer (q.v.) all satisfy these conditions, and another common and convenient device is the copper coulometer (q.v.). 

CHEMICAL COULOMETERS Despite the recent emphasis on electromechanical and electronic... 

One of the earliest chemical coulometers to find general acceptance was the so-called silver coulometer. Actually there are two types of silver coulometers, those involving the reduction of silver (I) to the metal and those which employ the electrolytic dissolution of a silver anode. The silver deposition coulometer tends to give high results owing to the deposition of silver dust and inclusion of the mother liquor (15) among many other sources of error. Nevertheless, the silver deposition coulometer continues to be popular to the present day (16). 

Craig and co-workers (17) have recently made a very thorough study of both the silver deposition and dissolution coulometers. They prefer the latter for very precise work and have used it to redetermine the electrochemical equivalent of silver and the faraday as 1.117972 0.000019 mg/coulomb and 96490.0 2.4 coulomb/g-equiv. (chemical scale), respectively. Foley (18) has suggested a silver and thallium oxide coulometer utilizing silver and thallium salts at pH 9.5 to give an overall cell reaction of... 

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