Simplified Calibration of Dropping Mercury Electrodes JAMES J. LINGANE, Mallinckrodt Chemical Laboratory, Harvard University, Cambridge, Mass.
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trode during a measured time interval, is drawn for measurement against a millimeter scale. The mercury is collected from the dropping electrode, A , in the cup, B, which is so arranged that it can be rotated easily into or out of the path of the falling drops. The time of collection is noted with a stop watch, beginning and ending with the instant a drop falls. The globule of mercury is drawn up into the calibrated measuring tube by means of gentle oral suction a t a rubber tube attached to tube C until the mercury thread is completely on the scale, and the stopcock is then closed. The length of the mercury thread is measured to 1-0.2 mm. against the scale (30cm. section from a meter stick) and the corresponding weight of mercury is computed from previous calibration data. The mercury is then removed from the measuring tube by suckin8 it over into the weighing bottle, D, and the apparatus is ready immediately for another determination. Pyrex capillary tubing of 0.4- to 0.6-mm. internal diameter and 6- to 8-mm. outside diameter is most suitable for the measuring tube. The tube is calibrated by measuring a thread of mercury in it, and then drawing the mercury over into the weighing bottle and weighing it. The capillary used by the author had an internal diameter of 0.470 mm. and 1 cm. on the scale corresponded to 23.45 mg. of mercury. Before fabrication the capillary should be tested for uniformity of bore by noting the length of about a IO-cm. mercury thread at various positions along the tube; the portion selected should be uniform within about *0.5 per cent over a 20-cm. length. To ensure an accuracy of *0.5 per cent in the value of m the mercury should be collected for a sufficient time to produce a thread a t least 10 cm. long in the measuring tube. With the particular dropping electrode ( m = 2.63 mg. per second) and measuring tube used by the author, only 90 seconds were required to produce a 10-cm. thread. A complete measurement can be made easily in 2 or 3 minutes, which is only a fraction of the time required to determine m by the gravimetric method.
HE diffusion current observed in polarographic measure-
ments depends, among other factors, on the rate of flow of mercury from the dropping electrode, and hence this quantity (usually designated b y m and expressed in the units mg. per second) is a n important characteristic of a dropping electrode. I n theoretical studies a knowledge of m is essential for interpreting diffusion current data by means of the IlkoviE equation ( 1 , 2 ) . In practical analytical work frequent periodic determinations of m (and the drop time) under constant conditions serve as a check on the constancy of behavior of a given dropping electrode. Therefore i t is highly desirable t o have a method for determining m t h a t is quicker and easier of execution than the rather laborious methods heretofore used, which involve weighing mercury collected from the dropping electrode over a measured interval of time ( 2 ) . It was found t h a t the determination of m can be expedited greatly by measuring the volume of mercury, rather than its weight, b y means of the simple apparatus shown in Figure 1. The instrument consists essentially of a calibrated glass capillary tube into which mercury, collected from the dropping elec-
Since m is independent of the medium in which the drops form (2, 3), they can be collected either in water or in air. I n a typical experiment the value of m with collection under water was 2.61 mg. per second, and 2.63 mg. per second when collectedin air. Collection in air in a dry measuring tube is pref-
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erable, because when the measuring tube is wet water tends to be trapped in tiny droplets between the mercury thread and the glass, which makes accurate measurement difficult. The only disadvantage of collection in air is the lack of temperature control. The author found experimentally that the temperature coefficient of m is +0.0031 deg.-l between 0” and 25’ C. Hence the error due t o lack of temperature control will not exceed * l per cent if the ambient temperature is within about *3” of t h a t at which the electrode is t o be used. Incidentally, the temperature coefficient of m calculated b y Kolthoff and Lingane (2, p. 75) is incorrect because of a n error in the value used for the temperature coefficient of the density of mercury. Using the equation given by Kolthoff and Lingane (2, Equation 28, p. 75) and the correct values -0.000181 deg.-’ and -0.0038 deg.-’ for the temperature coefficients of the density and viscosity of mercury, respectively, the theoretical value of the temperature coefficient of m is +0.0036 deg.-l. This agrees well with the foregoing experimental value. ,811
Literature Cited
0.4- To 0.6MM. I.D.
(1) IlkoviE, D.. Collection Czechoslou. Chem. Commun., 6, 498 (1934);
J. Chim. phys., 35, 129 (1938). (2) Kolthoff, I . M., and Lingane, J. J., “Polarography”, New York. Interscience Publishers, 1941. (3) Muller, 0. H., “Polarographic Method of Analysis”, Easton, Penna., Journal of Chemical Education, 1941.
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