K. A. V a n Lente
Southern Illinois University Corbondole
Coulometry A series experiment
In this experiment Faraday's Laws are applied to three coulometers, connected in series as shown in the accompanying figure. The advantages of the apparatus lie in its simplicity and convenience.
A, silver; 6, iodine; m d C, gar covlometers; D, a rectiRer, or battery;
a
45-v B
M, milliommeter.
The construction and directions for the use of the silver coulometer, A, have been previously described in detail.' The iodine coulometer, B, consists of a thistle tube sealed to a 2-way, 1-mm bore stopcock. The lower tube
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is cut off as close to the barrel of the stopcock as is practical, and the stopcock is roughened with coarse sand paper until there is reasonable electrical conductance when the stopcock is wet with electrolyte and in the closed position. A strip of platinum foil, sealed in the tube as shown, serves as the cathode, while a piece of platinum wire, B.S. No. 18, wrapped around the barrel of the stopcock and taped to the tube, serves as the anode. This makes a convenient, single unit and requires only a 250-ml beaker, a magnetic strirrer with a small stirring bar, and solutions to become operative. The tube is filled with saturated NaaS04 solution, the stopcock thoroughly wet with the solution and! with the stopcock in the closed position, the unit is placed in the 250-ml beaker in such a position that the stirring bar can rotate freely. 50mlof NaI solution (0.1 M ) , 5 drops of a 2% starch solution, and a known volume of standard Na&Os solution are added to the beaker. The experiment is run until the first detectable blue color appears in this beaker. The gas coulometer, C, is made from a 10-ml buret. It has the stopcock and tip cut from the bottom and sealed to the top to make the buret direct reading, and an adapter large enough to carry the two platinum wire electrodes and the outflow tube, sealed to the bottom. 0.3 M KzSOI solution2, is used as the electrolyte and the volume of the combined gases is read directly in the buret after equalizing the liquid levels. The experiment is flexible. The number of coulombs required will depend upon the amount of NaaSzOl used, and the time required may be varied by the current used. Using a known volume of a standard NaaS203 VAN LENTE,K. A,, VANA ~ AR., E., AND WILLARD.H. H.. J. CHEM.EDUC.,30,576, (1959). 1 LEHFELDT, R.A,, Phil.Mag., 15,614 (1908). %~.NGANN, J. J.,J . Am. C h m . Sot., 67, 1916 (1945).
solution, the results of the other two coulometers may be compared and the percent differencesmay he determined, usually assuming that the silver coulometer gives the better value. Most of the resistance in the circuit is caused by the closed stopcock. When, therefore, the K,SOI solution is saturated with hydrogen and oxygen gas prior to starting the experiment, coulometers A and B are bypassed, and the variable resistance, R, is adjusted to give a reasonable evolution of gas in C. Two or three minutes are sufficient for this saturation, and the s t o p cock of C is kept open during this period. The capacity of this system is limited by the 10-ml buret used as the gas coulometer. At room temperature and pressure, about one-half of a milliequivalent of the combined gases can be accommodated [theoretically 0.1741 ml (STP)/coulomb]. This means that 25 ml of 0.02 M NslSzOa, or less, can be used in the iodine coulometer and a deposit of about 50 mg of silver ohtained in the crucible. This relatively small amount of silver deposited is advantageous because it lessens the probability of breaking off some of the silver during washing. A current of about I s 2 0 milliamperes is satisfactory and convenient. Using a known volume of standard N&S203solution,
the results of the silver coulometer show an average deviation from the mean of +O.3 rng of silver. Most of the variance is probably due to the detection of the endpoints by different students. The average deviation from the mean for the gas coulometer, converted to mg of silver, is *0.4 mg, and the values are consistently lower than those from the silver coulometer by about 1.2%. According to previous in~estigators,~ this is postulated as the result, primarily, of the formation of some hydrogen peroxide at the anode, especially at low current densities. Failure of any such hydrogen peroxide to decompose a t the anode and its subsequent reduction a t the cathode would explain the low results for the gas coulometer. The average deviation from the mean for the gas coulometer reflects differences in end-point detection and variable errors due to lack of thermal equilibrium, vapor pressure correct,ions, and solubility factors for different students. If the apparatus is assembled and the solutions are prepared, this experiment can be run conveniently in a two or, a t most, a three-hour laboratory period. PAGE J. A,, (1957).
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LINGARE. J. J.. A n d . Chim. A&.
Volume 43, Number 6, June 1966
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