ANALYTICAL EDITION
December 15, 1943
concentration of 300 mg. per liter the above ratio drops to 0.0330, at 500 mg. per liter to 0.0318,and at 600 mg. per liter to 0.0308. It was found that high concentrations of sodium chloride lowered the slope of the curve in Figure 3 proportionately to the sodium chloride concentration. One molar sodium chloride lowered thu slope from 0.0340 to 0.0318,and 2 and 3 molar sodium chlhide lowered it to 0.0296 and 0.0276, respectively. This effect has been noted by Peracchio and Meloche ( 5 ) . The polarographic data, plotted on a large scale, were used for the analysis of dilute solutions of bismuth. An extrapolation of this plot (Figure 3) to zero concentration gave an accurate value of the residual current of 0.20 microampere a t -0.500 volt. Values of the diffusion currents were reproducible to an equivalent of about 1 mg. of bismuth per liter at all concentrations. The difTerent values of Ell2 (reported by Suchg, 6, and that above) are explained by the fact that E I , Zis shifted greatly by change of p H of the solutions. Figure 4 shows a plot of E l , , versus p H for solutions having 100 mg. of bismuth per liter. The value of tho difIusion current is not affected by change of pH, if pH < 7, within the limits of experimental error. Changes of pH below 7 only translate the wave along the E axis,
131
Figure 1. For p H > 7 the current voltage curves showed two waves suggesting a stepwise reduction of bismuth but no defhite mechanism is proposed to interpret these two waves. The shift of Elit to grcater negative potentials as the pH valuea increase corresponds to the well known fact that metaltartrate complex ions in general are more stable in alkaline than in acid media.
Acknowledgment The authors wish to express their appreciation to E. I. du Pont de Nemours & Company for the fellowship which made this work possible; this represents an excerpt ,of the research work done with the aid of that fellowhsip.
Literature Cited Clark, “Determination of Hydrogen Ions”, p. 485, Baltimore, Williams & Wilkins Co., 1928. Kolthoff and Lingane, “Polarography”, p. 60, New York, Interscience Publishers, 1941. Lingane and Kolthoff, J . Am. Chem. Soc., 61,825-34 (1939). Page and Robinson, J. SOC.Chem.Id., 61,93 (1942). Peracchio and Meloche, J. Am. Chem. SOC.,60,1770 (1938). Such$, Collection Czechoslm. Chem. Commun.,3, 354-66 (1931).
A Modified Bailey Pressure Regulator GEORGE A. DALIN Plax Corporation, Hartford, Conn.
B
AILEY (1) recently described a .manostat which, under certain circumstances, waa inclined to “bounce”, and explained how the bouncing could be eliminated by grinding a bevel on the tip of the capillary leak. A duplicate was constructed by the author, but it was found that the bevel caused a poor seal between the tip and the rubber seat and prevented attaining a vacuum lower than about 40 mm.
in the apparatus. Furthermore, bouncing set in when attempt was made to set the pressure at 80 mm. Several tips were constructed with various bevels and tried with rubber seats of various hardnesses. As was to be cxpected, increasing the inside diameter of the tip and the angle of the bevel increased the average pressure, a t which the manostat functioned smoothly. Each tip covered a range of about 30 mm. The modification shown in the diagram is, in effect, a variable bevel. The capillary is first drawn down to a very fine tip and then ground down on a steel plate, using c o a m emery and kerosene, until the opening is of such a size that when unobstructed and connected into the apparatus, the pump will take the pressure down to a value which is about 20 mm. higher than the highest pressure required during distillation. To adjust for operation, mercury is placed in the manostat, the block is removed, the stopcock is opened, and the spring tension is adjusted so that the tip will come to rest about 1 cm. below the level of the top of the block. The block is placed in position, the pump is started, and the tip is ushed firmly by hand into the rubber. When the pressure as inxcated on a manometer reaches its lowest value, the hand is removed and the manometer will show the lowest pressure the manostat can maintain. The setscrew is raised, increasing the leak a t the tip until the pressure in the system is about 10 mm. less than the pressure desired. The tip is then lifted free of the seat with one hand and the stopcock is flipped over with the other. This rocedure permits air to enter the chamber over the mercury, aniincreasea the pressure for which the manostat is set. The setting can be decreased by pushing the tip against the seat and flipping the stopcock.
p
With a little practice, the manostat can thus be set within 2 to 3 mm. of the desired pressure. The final adjustment is made with the setscrew and an accuracy of 0.5 mm. in the setting is easily attained. Any drift due to fatigue of the rubber or dirt particlea can be quickly neutralized by means of the setscrew.
Literature Cited MAN0 S TAT
(1) Bailey, A. J., IND.ENQ.CHEM.,ANAL.ED., 15, 283 (1943).
