INDUSTRIAL AND ENGINEERING CHEMISTRY
286
subjected to the same procedure, and an additional yield of pure p-xylene was obtained. Within the distilling tube there exists a dynamic pressure gradient which tends to keep the more volatile component away from the warm end and thus favors separation of the lees volatile material in the pure state. However, i t also facilitates the intermixing of the vapors toward the cold end and is, therefore, the probable cause of the failure to obtain complete separation in one operation. This appears to be an
VOL. 10, NO. 5
inherent shortcoming of the process which may be improved, b u t not completely overcome. Somewhat better separation was obtained in packed tubes, but other disadvantages arose; the distilling rate was slowed down and the visual observation was hampered. I n principle, i t does not seem impossible to obtain pure fractions of the more volatile components, although in our experiments the more volatile component generally contained a few per cent of the less volatile component. RECEIVED Deoember 18, 1937
A Simple Method for Preparing Glass Electrodes M. L. NICHOLS AKD JOHN iM. SCHEMPF, Cornel1 University, Ithaca, N. Y.
W
HILF: investigating the problem of making a glass electrode g i t h a sufficiently low resistance (lo4 to IO5 ohms) to permit its use with an ordinary galvanometer and potentiometer ( I ) > the following procedure was evolved. This method was found to b e easy to carry out, and yielded a sturdy product of high sensitivity. Using 9-mm. Corning 01.5 glass, the parts shown in Figure 1, A , B , C, are prepared. A is formed by breaking off about half of a thin bulb blown on a section of 2- to 3-mm. tubing drawn from the 9-mm. stock. B is a short piece of 9-mm. tubing with an attached spindle to serve as a handle, and C is a section of the original tubing 10 to 12 cm. long. To prepare the electrode, B is heated to softening at the open end and attached lightly to A , following which C is heated and sealed to B , over A , as shown in D and E. The seal is then heated uniformly and gently blown out t o give F . The excess glass is cut off by a hot wire at the positions indicated by the dotted lines in F . If the electrode is sufficiently sensitive, a spot of interference figures 9 to 16 sq. mm. in area ail1 appear in the diaphragm.
The thickness of the bulb blown in preparing A controls the diaphragm thickness for any final electrode size. It is not important or usually possible for the capillary to remain in the center of the diaphragm. The electrode is ready for use after a preliminary soaking in approximately 0.1 A' hydrochloric acid for at least 36 hours. As used in this laboratory, s ~ ~ ~ $ $ $ ~ ~ the electrode is rinsed with cell. distilled water, superficially dried with filter paper, and filled to the level indicated b y the arrow in F with a saturated solution of quinhyY drone in approximately 0.1 i hydrochloric acid. A bare p l a t i n u m w i r e is then ins e r t e d t o m a k e electrical contact. A convenient as- _ _ s e m b l y is s h o w n i n F i g ure 2. T h e asymmetry potential which always develops across the diaphragm FIGURE 2. E L E c T R o D E ASSEMBLY is determined on a solution of known pH value. The precision obtained withathis electrode used in conjunction with a Leeds 8: Northrup enclosed lamp and scale galvanometer and a potentiometer reading to 0.1 millivolt is better than 1millivolt or 0.02 p H unit. The advantages of this electrode are its mechanical stability, sensitivity (permitting its use with apparatus usually available), and ease of manufacture. Obviously, this type of construction can be modified to suit individual requirements.
~
c -
/
6
f
9mm. L
7
C.
9 mdm 10-12
c
m
~
Summary
:** E.
1
A simple procedure for making a very sensitive, durable glass electrode is given. Accurate measurements may be made with this electrode using a portable galvanometer with a sensitivity of the order of 40 megohms and an ordinary galvanometer.
: Literature Cited (1) Mouquin and Garman, IXD. ENQ.CHEM., Anal. Ed., 9,287 (1937) ~
FIGURE 1. PREPARATION OF ELECTRODE
RECEIVED February 28, 1938.
