DECEMBER, 1947
A NEW TYPE OF VOLTAMETER WILLIAM MARSHALL MACNEVIN and GEORGE LLOYD MARTIN* The Ohio State University, Columbus, Ohio
OF
THE various voltameters that have been devised, the silver and iodine voltameters have produced the best results from which the value of the Faraday' may be calculated. Some uncertainty concerning the reliability of the value still exists, particularly in connection with the determination of some of the other constant^.^ Examination of an atomic weight table will show the reason for the present limitation to the values from the silver and iodine voltameters. S i c e the equation for for the calculation of the Faraday is
F
=
equivalent weight electrochemical equivalent
the atomic weight of the element used must be accurately known. This haslimited the possible reactions for use in a voltameter to a very few elements. Many of the systematic errors in the silver voltameter such as solubility of the deposit, effect of acid on the deposit, inclusions in the deposit, etc., are inherent in the handling of a deposit of one metal on another. While the iodine voltameter is not subject to the same sources of error, others such as loss of iodine and reaction of iodine with water are present since iodine is a somewhat reactive free element. During the course of a general review of the various voltameters, certain ideal characteristics have emerged. While it may not be possible to find a reaction which can fulfill all the ideal characteristics, a t least a basis for criticism and selection will be established. BIRGE,R. T., Rev. Modern Phys.,13, 233 (1941).
' BIRGE,R. T., Phys.Rev., (2) 60,766 (1941).
* Monsrtnto Fellow, The Ohio State University, 194G47. Present address: Mallinckrodt Chemical Works, St. Louis, Missouri.
The characteristics of an ideal voltameter reaction are: 1. The reaction should be homogeneous and should involve only ions, no free elements. 2. The atomic weight of the element or ion involQed must be accurately known. 3. The reaction should be absolutely reversible. 4. The .reaction should proceed a t a relatively low potential to reduce the probability of side reactions. 5. Sound and relatively simple analytical methods capable of high accuracy should be applicable to the determinations involved. 6. The preparation of pure materials for use in the voltameter reaction should not be of extreme difficulty. The restriction placed on the choice of a reaction by Condition 2 has resulted in the past in some of the other characteristics being disregarded. It is interesting to note that since the work on the iodme voltameter was done, the value for the Faraday obtained with it has changed twice due to a change in the value for the atomic weight of iodme. Oxygen is the only element whose atomic weight is invariably defined and is therefore in this respect the ideal element for voltameter measurements. However, the oxygen gas voltameter is subject to a large number of systematic errors, which, coupled with the difficulty of manipulation and measurement, make it unsatisfactory for precise use. There is a method by which oxygen may be used as the element involved in a voltameter reaction without using a gas voltameter. Consider, for example, an element M, which exhibits two oxidation states, M" and MIv, for which the corresponding stable oxides are MO and MOz. Assume that conditions for the electrolytic (Contmmd on pogc 601)
DECEMBER, 1947 NEW TYPE OF VOLTAMETER (Continuedfmpage 687)
oxidation of MIr to MIv are known and that precise analytical methods for each have been established by using weights of pure MO and pure MO*. A known weight of MO in solution is placed in a cell and oxidized electrolytically with the current and time being measured by some standard means. The oxidation is run under conditions which so far as is known give 100 per cent current efficiency. At the end of the electrolysis the'remaining weight of MO and the weight of M02 formed by electrolysis are determined. The difference between the MO at the end and the MO originally present is the weight of MO oxidized. The difference between the weight of M02 a t the end and the weight of MO oxidized is the weight of oxygen which is equivalent to the number of coulomhs passed through the cell. Thus the amount of electricity is directly related to a weight of oxygen. The atomic weight of M is not needed at any time. Hence any element that forms soluble oxides may be used. I n this manner, any uncertainty in the value of F due to experimental error in the atomic weieht is removed, since
-
no. of coulombs passed F = 8.MM00 Xweight of oxygen . '
A reaction of this type is an oxygen transfer reaction, and the name, oxygen transjer voltameter, is proposed for a voltameter using such a reaction.
This approach to the determination of the value of the Faraday enables one to choose which of many reactions most closely fits the characteristics of an ideal voltameter reaction. In this way a reaction with the least number of systematic errors can be used, and the resultant value of the F a r d a y will then have a smaller degree of uncertainty. In fact, the value of the Faraday h a l l y determined should be quite accurate since the possible errors will be small enough in number that the determination of their sign and magnitude will not he the tremendous undertaking of the silver and iodine voltameter work. I t is also worth noting that while the atomic weight of the second element involved is not needed in the determination of the Faraday, if the analytical methods are established for the voltameter use and the proper chemical reaction is known, the atomic weight of the element can be very accurately determined. In this way the ratios of the atomic weights of several different elements to oxygen could be determined directly. The oxveen transfer voltameter offers a new auproach to tGe problem of the determination of the valie of the.Faraday. With the increasing importance of s reevaluation of the Faraday, use of an oxygen transfer reaction should result in a value with much smaller uncertainty than the present accepted value of the Faraday.
