ANALYTICAL EDITION
July 15, 1943
stand for a minute or so, before another attempt a t removal is made. After removal from the specimen, the test paper is immersed in 10 per cent hydrochloric acid containing 0.05 er cent potassium ferrocyanide. Upon development a Prussian glue pattern of the rusted surface is obtained. The development time should be relatively short (10 to 15 seconds) in the case of freshly rusted surfaces if an accurate rust pattern is desired. With aged or worn rust it may be necessary to extend the time of development to one minute or more. The pattern is usually more distinct if the paper is dried with heat. The Prussian blue developed in the gelatin has little tendency to diffuse and, contrary to filter paper prints, clear, sharp patterns result. Potassium ferrocyanide is preferred agent since the to potassium as a latter tends to give a blurred and indistinct image.
465
Experimental Tests performed on solvent-cleaned, specially prepared, rusted iron and steel strips using the procedure described gave satisfactory analytical patterns of the rusted surface. Tests made on clean, freshly polished iron strips failed to give a Prussian blue color even upon prolonged development. The test apparently is specific for iron oxide; attempts to remove sulfide films from copper or lead specimens were unsuccessful. This was attributed to the fine structure and close adherence of the sulfide film in contrast to the rather flaky and ~ o o s e ~held layer. v ferric
Literature Cited (1) Crabtree, Eaton. and Muehler. J . Phot. SOC.Am., 6 , 6 (November, 1940).
An Improved Salt Bridge for Polarographic and Potentiometric Measurements DAVID N. HUME AND WALTER E. H4RRlS School of Chemistry, University of JIinnesota, RIinneapolis, Minn.
A
I”; E X T E R K A L reference electrode is frequently used in potentiometric and polarographic measurements. The connection to the solution is by some type of a salt bridge. The ideal salt bridge, in addition to eliminating the junction potential, should have a low resistance (especially in polarography) , should not contaminate the solution being used, and should be easy to handle. A number of bridges have been described in the literature, none of which incorporates all the desired features. The inverted-U bridge of Irving and Smith (a), with ground-glass plugs at the ends, has been widely used in potentiometric work but has a n undesirably high resistance for polarography if the reference electrode is to be used as the anode. A bridge of this type, 40 cm. long, when filled with saturated potassium chloride, was found to have a resistance of about 7000 ohms, some 6500 ohms being
attributable to the plugs. Laitinen (3) has described an inverted-U bridge with sintered-glass ends which has a lower resistance but which, in common with other all-glass designs (1, 4 ) , is rather cumbersome to use. The authora have found the very simple bridge illustrated in Figure I to be highly satisfactory. The saturated calomel electrode, A , is connected to the cell with a piece of ordinary soft rubber tubing. B , 6 mm. in inside diameter, filled with saturated potassium chloride. The rubber tube terminates in a short length of glass tubing filled with 3 per cent agar jell, also saturated with potassium chloride. When it is to be used, the free end of the bridge is simply plugged into the cell. The reference electrode may be kept permanently out of the way and the necessity of simultaneously adjusting two cells in a thermostat to connect \Tith an unbendable glass bridge is eliminated. Figure 1 shows how an intermediate agar plug, D,having the bame composition as the cell liquid, may be used if contamination with potassium chloride is undesirable. If traces of potassium chloride are not objectionable, the free end of the bridqe may be inserted directly into the solution. For this purpose it is convenient to have a thin, coarsely-sintered glass plug at the tip of C. The agar is still desirable to prevent mixing by convection. The end of the bridge is kept in potassium chloride solution R-hen not in use. The resistance of such a bridge 56 em. long was found to be only 600 ohms. For a polarogram showing a diffusion current of 10 microamperes, the error in half-wave potential due to I R drop in the bridge would be about 3 millivolts. This error is negligible for most purposes and may, of course, be decreased b y the use of a shorter bridge or larger tubing. If exact values of half-wave potentials are desired, the resistance of the cell and bridge may be measured and the correction applied whenever necessary. For potentiometric titrations, in which a low bridge resistance is not essential, plug D is conveniently replaced by one of ground glass, as in the bridge of Irving and Smith (9).
Literature Cited
FIGURE 1
(1) Bright, W. M., and Miller, E. L.,IXD.ENG.CHEM.,ANAL.ED.,9, 346 (1937). (2) Irving, G. W., and Smith, N. R., Ibid., 6 , 4 8 0 (1934). (3) Laitinen, H. A., Ibid., 13, 393 (1941). (4) Stern, H. T., J . Phyr C h e m , 29, 1583 (1925).
