Cornbined Manometer-Barometer For Distillation G. ROSS ROBERTSON Los Angeles,
University of California,
Calif.
for each millimeter by which the barometer reading deviates from the initial standard setting. Zero errors of similar magnitude would arise from temperature changes. Since a barometer reading in any case must be corrected for temperature, one might readily consolidate all corrections for temperature and change in zero level into one table which would be an elaboration of a standard table (cf. handbooks) for temperature correction with scale having a certain expansion coefficient. If the user is not interested in barometer readings, he may economize by using a narrower bottle with lese mercury. Although scale 76 would then seem superfluous, it is still desirable to provide a mark for future reference when one wishes to know whether vapor may have entered the barometer. In regions of wide barometric fluctuation, unlike the author’s district, it may be desirable to allow 1 or 2 cm. additional length of tubes. I n any case, fluctuations in zero level have no relation to final differential readings of the manometer. Higher precision in measurement of low pressures is obtained by use of larger tubes, as discussed by Zimmerli (1). Before barometer and manometer tubes are fastened to the support, a coat of white lacquer is applied to the back side of the tubes in likeness to the milk-glass coating a t the back of a thermometer. Finally a loose plug of cotton is inserted in the mouth of the bottle for protection. Experience in this laboratory indicates that extraneous vapors or liquids do not readily bridge the gap between the two tubes in
THE
standard U-shaped manometer used commonly in distillation procedures suffers from a serious disadvantage because contaminating vapors readily reach the space normally supposed to be a Torricellian vacuum over the mercury in the closed arm. Modifications in design, such as tube constriction, assembly of two opposed U-tubes (f), etc., are helpful but do not eliminate the trouble, particularly with inexperienced workem. The taller model shown in Figure 1,while virtually the mechanical equivalent of the short U-gage, largely escapes contamination by the interposition of an open mercury well. This modification has given satisfaction for several years in the author’s laboratory in the hands of both skilled and unskilled and careless workers. Its height permits a much wider range of measurable pressure, but makes advisable a permanent center-of-desk or wall mounting, with the convenient assembly of accessories pictured herewith. Incidentally, the device serves as a useful mercurial barometer when the mercury is not standing above zero level in the manometer. The bulb’ with capillary loop connection a t the top of the barometer utilizes the neat method of producing a vacuum over mercury by breaking a mercury column. It also permits restoration of vacuum following contamination without the trouble ofboiling or pumping out. While the procedure is not new, details of technique do not seem to be widely known. The tube should be of Pyrex, including capillary stock of about 1-mm. bore. As first prepared by the glass-blower the tip of-bulb A carries a simple tube extension which at the outset is connected to a vacuum pump capabfe of reducing pressure to 0.1 mm. or below. The barometer tube, already fitted but not attached to its wooden sup ort, is thoroughly cleansed. With a clamp attached a t top of l i t is allowed to stand erect in an ample supply of mercury in the bottle. A Bunsen flame is now played over all of its surface from points near the bottle to bulb A . The vacuum pump is started, and active heating of the tube continued as the mercury is allowed to rise slowly to barometric height. Mercury boils as the pressure becomes very low, with some turbulence, and sudden rise and fall of the column. No harm is done if mercury is distilled into A . When it is judged that adsorbed air and chance bubbles have been expelled from the mercury column, and the emitted air has been removed to the limit of the pump, the top of A is sealed off a t the hand torch. When the barometer tube is cool enough to handle, i t is inclined until the mercury pours over through capillary B into the bulb. Residual air is thus ractically all driven over into A . The barometer is now slowly grought back toward normal erect position, whereupon the mercury in A starts to run back into the main barometer tube. When the end of the retreating mercury column reaches a position a t or near C, the capillary loop is tapped with a dull metal object, which thus causes a break in the flow. A small residual capillary section .of mercury then falls back into the U-position shown in the figure. Perhaps several repetitions of this procedure of inclining, raising, and tapping may be required before a suitable amount of mercury is trapped. One may now assume with negligible error that the space between the two masses of mercury is a vacuum. The sup ort consists of an assembly of five pieces of wood to which are &ted two appropriate sections of a common hardwood meter rule. The two thinnest pieces used in the support should be of thickness equal to that of the rule less half the outside diameter of manometer or barometer tube. The shorter scale marked 76 is only for incidental use of the device as a barometer. Its position is fixed by the setscrews in agreement with the reading of a standard barometer a t the =me altitude and temperature, and on a day when atmospheric conditions are normal.
n i
A barometer so simply constructed without device for zero setting would theoretically give a correct reading only at the pressure and temperature which prevailed a t the time the scale was adjusted. With tube and mercury well of the proportiom shown in the figure, however, an error of only 0.012 mm. appears
Figure 1.
238
Diagram
April, 1945
ANALYTICAL EDITION
the well. If a t any future time, however, there is evidence that contamination has occurred, it is not necessary to reattach the the pump to bulb A . The barometer tube is merely loosened from its mounting, cleansed from white lacquer with acetone on cloth, heated thoroughly, and inclined again to force the new vapors into bulb A . The subsequent operations of return of mercury, trapping of capillary segment, etc., follow as previously described. This time the capillary U-section of mercury wil! probably come to rest with arms a t unequal height, but the barometer should be a t correct position.
In the assembly for evacuation of distilling apparatus SP is a central vertical standpipe, a/s-inch standard iron pipe size, including three T-fittings and an elbow a t the top. The bottom end is plugged gaatight and mounted securely by flange to desk or baseboard. The several accessories are conveniently attached and
239
removed at the respective brass hose nipples in the T-fittings. The elbow fitting, 3/8 to I/,, is connected to a Hoke needle valve, whch may be employed as a simple “bleeder” or air inlet, or may be connected to a manostat not shown in the figure, F is a filter bottle containing granular solid reagent to assist in trapping reactive vapors-for example, soda-lime. T is a vapor trap consisting of a 500-ml. long-necked, round-bottomed flask with a high, short, and sturdy side stem, and central inlet tube leading to the center of the body of the vessel. The flask rests in a dry ice-alcohol bath held in a jar protected by heat insulation, such as that provided by a 1-gallon paint pail lined with mineral wool. One should take care to place the pumpmotor assembly ( P , M ) with belt and pulleys on the inside, thus minimizing hazard of injury to the hands of workers. LITERATURE CITED
(1) Zimmerli, A., IND. ENG. CEEM., ANAL.ED., 10, 283 (1938); U. S. Patent 2,075,326.
Spectrophotometric Determination of Small Amounts OF Copper Using Rubeanic A c i d E. JOHN CENTER AND ROBERT M. MACINTOSH,Battelle M e m o r i a l Institute, Columbus, O h i o A rapid accurate method for small amounts of copper, using rubeanic acid, i s described. Spectral transmittance curves for copper, nickel, cobalt, and iron i n a weak acetic acid solution with rubeanic acid are shown. Fading of the color and maximum permissible amounts of certain elements at 650 millimicrons are indicated. Transmittance vs. copper concentration curves have been prepared for wave lengths of 400 and 650 millimicrons.
R
UBEANIC acid [dithiooxamide, (NH:C.SH)t] has been employed by a number of investigators for microdetection of copper (l-t?),cobalt, nickel, and other ions (8-8,IO). Sccording to Ray (6) copper, nickel, and cobalt are qusntitatively separated in the form of an amorphous, colored precipitate from an ammoniacal solution:
Me(Cu, Xi, Co)
Feigl and Kapulitzss (3)showed that if free acetic acid were present the sensitivity of rubeanic acid to cobalt and nickel w&s greatly depressed, but that copper still gave a reaction. Allport and Skrimshire (I) used a buffered acetic acid solution and reported that copper gave an olive-green color and that lead, manganese, bismuth, tin, and zinc gave no color with the reagent. They quantitatively determined copper in organic materials. According to British Drug Houses ( 8 ) , rubeanic acid may be employed for the colorimetric determination of copper ina solution containing 2% each of free acetic acid and ammonium acetate and 1 cc. of 0.1% alcoholic solution of the reagent in 100-ml. volume. Free mineral acid must be absent, and not more than 0.06 mg. of copper should be present. Willard and Diehl (9) report that the reagent may be used for the quantitative determination of copper at a pH of about 4 with a little gum arabic resent to stabilize the system. They state that manganese anfcinc do not interfere, but that cobalt and nickel give colors with the reagent. Because of the need for more complete quantitative data on the copper-rubeanic acid complex during the analysis of potable water, the following study was made.
EXPERIMENTAL
A spectral transmittance curve of the copper-rubeanic acid complex in a weak acetic acid solution (pH 4.8) is indicated in Figure 1 (conditions given below). Spectral transmittance curves for iron, nickel, and cobalt in a weak acetic acid solution with rubeanic acid are shown in Figure 2. The nickel complex rapidly precipitates at the high concentration indicated. The iron color is due t o reaction with the acetate buffer. A plot of transmittance us. copper concentration a t wave lengths of 400 and 650 millimicrons is given in Figure 3. Standards for the plot were made up from a standard copper acetate solution, and run according to the method given under Procedure. Because it was anticipated that relatively large amounts of iron, nickel, and cobalt might be present in the solution being tested, a working wave length of 650 millimicrons waa selected even though the slope of the line (Figure 3) is far less favorable than a t 400 millimicrons. If no interference from elements absorbing in the blue is expected, or if their concentration is very low, the measurement should be made a t 400 millimicrons. Table I shows the maximum permissible concentration of elementa at 650 millimicrons using a Coleman 10s spectrophotometer with a 5-millimicron slit. The transmittance of the copper-rubeanic acid complex increases on standing, owing to ’precipitation. However, this 100,
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WAVE LENGTH, HILUMICRCNS
Figure 1.
2.6 P.P.M. of
Copper Present as CopperRubeanic A c i d Complex
