Silver coulometer

It is apparent (Fig. 1.21) that at potentials removed from the equilibrium potential see equation 1.30) the rate of charge transfer of (a) silver cations from the metal to the solution (anodic reaction), (b) silver aquo cations from the solution to the metal (cathodic reaction) and (c) electrons through the metallic circuit from anode to cathode, are equal, so that any one may be used to evaluate the rates of the others. The rate is most conveniently determined from the rate of transfer of electrons in the metallic circuit (the current 1) by means of an ammeter, and if / is maintained constant it can eilso be used to eveduate the extent. A more precise method of determining the quantity of charge transferred is the coulometer, in which the extent of a single well-defined reaction is determined accurately, e.g. by the quantity of metal electrodeposited, by the volume of gas evolved, etc. The reaction Ag (aq.) -t- e = Ag is utilised in the silver coulometer, and provides one of the most accurate methods of determining the extent of charge transfer. 

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. 

C19-0097. Electrochemistry can be used to measure electrical current in a silver coulometer, in which a silver cathode is immersed in a solution containing Ag" " ions. The cathode is weighed before and after passage of current. A silver cathode initially has a mass of 10.77 g, and its mass increases to 12.89 g after current has flowed for 15.0 minutes. Compute the quantity of charge in coulombs and the current in amperes. 

C19-0099. In a silver coulometer (see Problem ), both electrodes are silver metal. Draw a molecular picture that illustrates the reactions that occur during operation of this 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... 

D. N. Craig. J. I. Hoffman. C. A. Law. and W. J. Hamer, Determination of the Value of the Faraday with a Silver-Perchloric Acid Coulometer, J. Res. Natl. Bur. Stds. 1960, 64A, 381 H. Diehl, High-Precision Coulometry and the Value of the Faraday, Anal. Chem. 1979,51, 318A. 

COULOMETER. Also known as cnulombmeier, a device for the measurement of electric currem. Originally developed (1916) by the U.S. National Bureau of Standards, the silver coulometer consists of a small platinum vessel, acting as the cathode, into which a pure silver anode is immersed. An aqueous solution of silver nitrate (15% AgNO<. wl) of very high purity is used as the electrolyte, In use. both the quantity of silver deposited and the lime are carefully noted. These measurements permit a calculation of the average currcnl strength. 

If a silver coulometer had also been inserted in the circuit, what weight of silver would have been deposited on the cathode ... 

One of the most accurate instrument for the measurement of quantities of electricity is the silver coulometer. A solution of purest silver nitrate in distilled water (20 to 40 parts AgN03 to 100 parts H20) is electrolyzed in a platinum crucible which serves as the cathode. An anode of pure silver rod is partly immersed into the solution and enclosed by a ceramic diaphragm so that mechanically separated anode slime cannot sink to the bottom of the crucible. Current density should not exceed 0,02 amp. per sq. cm. on the cathode and 0,2 amp. per sq. cm. on the anode. The level of liquid within the diaphragm should be somewhat lower than in the platinum crucible. When the electrolysis is finished the platinum crucible is washed with pure distilled water, dried and weighed. From the weight increase the quantity of electricity (in coulombs) passed through the solution is then calculated. 

Coulometer — (previously coulombmeter, or also voltameter) A coulometer is an instrument to measure charge, i.e., to perform -> coulometry. Richards and Heimrod [i] suggested in 1902 the name coulometer to replace the previously used term voltameter . Modern coulometers perform electronic integration of - current over time. However, the first coulometers utilized - Faraday s law, e.g., by weighing the amount of silver that has been deposited on a silver electrode in a silver electrolyte solution the charge could be calculated (silver... 

Subsequent investigation by Richards and Heimrod 2 of the silver coulometer employed, proved the silver in the deposits to be too heavy by 0-059 per cent. The corrected ratio is... 

To separate chlorinated organic compounds, a column temperature of 225°C. and a nitrogen carrier gas flow of 75 ml. per minute were used. The coulometer was operated at maximum sensitivity (512 ohms), at a damping position of 4 and a bias of 250. A silver-silver ion cell was used. 

The most accurate determinations of the faraday have been made by means of the silver coulometer in which the amount of pure silver deposited from an aqueous solution of silver nitrate is measured. The first reliable observations with the silver coulometer were those of Kohlrausch in 1886, but the most accurate measurements in recent years were made by Smith, Mather and Lowry (1908) at the National Physical... 

The silver nitrate is purified by repeated crystallization from acidified solutions, followed by fusion. The purity of the salt is proved by the absence of the so-called volume effect, the weight of silver deposited by a given quantity of electricity being independent of the volume of liquid in the coulometer this means that no extraneous impurities are included in the deposit. The solution of silver nitrate employed for the actual measurements should contain between 10 and 20 g. of the salt in 100 cc. it should be neutral or slightly acid to methyl red indicator, after removal of the silver by neutral potassium chloride, both at the beginning and end of the electrolysis. The anode should be of pure silver with an area as large as the apparatus permits the current density at the anode should not exceed 0.2 amp. per sq. cm. To prevent the anode slime... 

The faraday, calculated from the work on the iodine coulometer, is thus 96,514 coulombs compared with 96,494 coulombs from the silver coulometer the agreement is within the limits of accuracy of the known atomic weights of silver and iodine. In view of the small difference between the two values of the faraday given above, the mean figure 96,500 coulombs is probably best for general use. 

In a simplified form of the silver coulometer, which is claimed to give results accurate to within 0.1 per cent, the amount of silver dissolved from the anode into a potassium nitrate solution during the passage of current is determined volumetrically. ... 

In a careful study of the copper coulometer, in which electrolysis was carried out at about 0 in an atmosphere of hydrogen, and allowance made for the. copper dissolved from the cathode by the acid solution, Richards, Collins and Heimrod (1900) found the results to be within 0.03 per cent of those obtained from a silver coulometer in the same circuit. 

A current of 0.050 amp. was passed through a silver titration coulometer, and at the conclusion 23 8 cc. of 0.1 n sodium chloride solution were required to titrate the silver dissolved from the anode. How long was the current flowing ... 

Macinnes and Dole [/. Am, Chem, Soc, 53, 1357 (1931)] electrolyzed a 0.5 N solution of potassium chloride, containing 3.6540 g. of salt per 100 g. solution, at 25 using an anode of silver and a cathode of silver coated with silver chloride. After the passage of a current of about 0.018 amp. for approximately 26 hours, 1.9768 g. of silver were deposited in a coulometer in the circuit and on analysis the 119.48 g. of anode solution were found to contain 3.1151 g. potassium chloride per 100 g. solution, while the 122.93 g. of cathode solution contained 4.1786 g. of salt per 100 g. Calculate the values of the transference number of the potassium ion obtained from the anode and cathode solutions, respectively. 

Jones and Bradshaw [J. Am, Chem, Soc. 54, 138 (1932)] passed a current of approximately 0.025 amp. for 8 hours through a solution of lithium chloride, using a silver anode and a silver chloride cathode 0.73936 g. of silver was deposited in a coulometer. The original electrolyte contained 0.43124 g. of lithium chloride per 100 g. of water, and after electrolysis the anode portion, weighing 128.615 g., contained 0.35941 g. of salt per 100 g. water, while the cathode portion, weighing 123.074 g., contained 0.50797 g. of salt per 100 g. of water. Calculate the transference number of the chloride ion from the separate data for anode and cathode solutions. 

A solution, 100 g. of which contained 2.9359 g. of sodium chloride and 0.58599 g. urea, was electrolyzed with a silver anode and a silver chloride cathode after the passage of current which resulted in the deposition of 4.5025 g. of silver in a coulometer, Taylor and Sawyer [/. Chem. Soc. 2095 (1929)] found 141.984 g. of anode solution to contain 3.2871 g. sodium chloride and 0.84277 g. urea, whereas 57.712 g. of cathode solution contained 2.5775 g. sodium chloride and 0.32872 g. urea. Calculate the true and apparent transference numbers of the ions of sodium chloride in the experimental solution. 

The amount, or quantity, of electricity passing through a circuit is measured in coulombs, which are, in turn, the product of the amperes and time in seconds.2 At an international electrical conference in London (1908) the ampere was adopted as one of the fundamental units. The ampere is defined as the steady current which, when passed through a solution of silver nitrate in water, under definite conditions to be described later, deposits silver at the rate of 0.00111800 gram per second. This value of the ampere is one-tenth of the c.g.s. (electromagnetic) unit within a few parts in one hundred thousand. The instrument used for measuring current in terms of deposited silver is called a silver coulometer or a silver voltameter.8... 

The Silver Coulometer. The silver coulomcter consists, essentially, of a platinum dish or crucible as cathode, and a silver anode with a silver nitrate solution as electrolyte. Surrounding the anode is, in the latest types of the instrument, a porous cup of ceramic material, for reasons to be explained below. A convenient form of silver coulometer is shown diagrammatically in Fig. 3. A dish, Pt, holds a solution of silver nitrate... 

Tabus II. Comparison of the Copper with the Silver Coulometisk. Temperature of Silver Coulometer, 15° to 25°. Temperature ok Copper Coulometer, -2° to 0°. Copper Coulom-eteu in an Atmosphere of Hydrogen... 

Tests of Faraday s Law Tinder Varying Conditions. We have already seen that, if disturbing effects are taken into account, Faraday s law applies to all electrochemical reactions which have been carefully studied. The tests so far mentioned, however, have all been made at ordinary temperature, under atmospheric pressure, and in aqueous solutions. A number of researches have been carried out to find out whether variations in the nature of the solvent, or variations in the physical conditions, such as temperature and pressure, have any influence on the constant in Faraday s law. No real variation in the constant has yet been observed. There are, to be sure, many apparent deviations from the law, such as that observed with the copper coulometcr, which gives a deposit at the cathode which is lighter than the computed value. In this case, as has been seen, the cause of the discrepancy has been found to be the occurrence of a disturbing reaction, In every similar case a simple explanation of the apparent deviation has been readily found. The comparison of the iodine, and of the copper coulometer, with the silver coulometer, as has been described in previous sections, affords precise evidence, for these reactions at least, that Faraday s law is indc-... 

The fact that change of pressure has no influence on Faraday s law is shown by the work of Cohen14 who connected two silver coulometers in series, one as a reference coulometer at atmospheric pressure and the other, in successive experiments, at 500, 1000, and 1500 atmospheres. The results which are given in Table IV show that the same amount of silver was deposited in each coulometer. 

Table IV. Comparison of Silver Coulometers at One Atmosphere and Under High Pressures...

These investigators electrolyzed (without fusion) solid silver iodide, bromide, chloride, and nitrate, between a silver anode and a platinum cathode. The weight of silver deposited in each case was found to agree with that in a reference silver coulometer. 

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