July 15, 1929
INDUXTRIAL A N D ENGINEERING CHEMISTRY
water delivered. I n every case it came within the limits given by Treadwell (3) and the Bureau of Standards (1). For 5-ml. pipets the error ran from 0.0 to 0.3 per cent. It is considerably easier to attain a given accuracy with mercury than with water, because of the former's greater specific gravity and surface tension. An ordinary buret stopcock will deliver a drop of the order of 100 mg. of mercury, which is equivalent to 0.007-0.008 ml. of water. By the use of a Luer adapter and hypodermic needle a drop of mercury of 5-10 mg. may be delivered (2). Hence for a 1-ml. pipet a balance with a sensitivity of 50 mg. is accurate, and for a 100-ml. pipet one with a 500 mg. sensitivity may be used. The position of the balance when the weighing flask was visibly heavy has been chosen as the end point and the tared flask thus adjusted. It is equally satisfactory to balance the
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tares and subtract from the weights added the amount necessary to turn the balance. As with the Ostwald calibrating pipet, by this method the required amount is delivered directly. The Ostwald method involves a calibration of the calibrating instrument; the pipet should be fitted with a three-way stopcock and made to contain exactly the desired amount. The method suggested has been found to be as accurate and more convenient. Literature Cited (1) Bur. Standards, Circ. 9 (1916). (2) Shohl, J. A m . Chem. Soc., 50, 417 (1928). (3) Treadwell, "Analytical Chemistry. Vol. 11-Quantitative 522 (1919).
Analysis," p.
Large-Capacity Laboratory Condensers' D. F. O t h m e r EASTMAN KODAECOMPANY, ROCHESTER, N. Y.
I
N T H E operation of several glass distillation units used
for the preparation of small amounts of fine chemicals and for distillation studies, glass condensers of a greater capacity than those ordinarily found in laboratories were necessary. Two types have been developed and their usefulness proved on dozens of operations.2 Figure 1 shows a double-coil condenser with water flowing in both tubes in parallel and vapors condensing on both coils and to a smaller extent on the inner wall of the jacket due to the cooling of the air. Single-coil condensers of this type are also in use, but when a compact unit is necessary the double coil is recommended. In many set-ups where a liquid seal is necessary on the outlet, a vent tube is inserted through the top stopper carrying the vapor inlet and extended along the axis till it touches the bottom coil. The advantages of this type of condenser over the numerous other kinds which have been used in this laboratory are:
length of cooling coil of very small bore, and consequently all the vapors which can enter are cooled in the first few turns and the remainder of the length is wasted. To utilize the whole area, the condenser shown has a coil short in comparison to the bore and consists of two individual helixes screwed together and welded top and bottom. The cross section of the vapor inlet a t the neck is slightly larger than the sum of the cross sections of the two coils. A sheet metal can is formed of a tube with bottom and half of a standard 1-inch I. P. S. coupling soldered on. The can and coil are assembled with a rubber stopper as shown. The dimensions of this particular condenser are: over-all length 410 mm.; outside diameter 85 mm., of tube 10 mm., of inlet 28 mm., and of outlet 10 mm.
(1) It offers the maximum amount of effective cooling surface in the minimum space. The head room necessary for a laboratory distillation unit is usually considerably reduced. (2) There is no back pressure or tendency to prime as in the usual coil condenser. (3) Only one-tenth to one-half as much water is required, since it may usually be discharged within 10" C. of the temperature of the vapor. (4) When only a constant predetermined fraction of the vapors is to be condensed-as, for example, in a dephlegmator condenser-changes may quickly be made, since the water holdup is very small and the control is much more reliable as the uncertainty of convection currents is eliminated.
Relative dimensions vary with the use for which the condenser is intended and those of the one shown are merely suggestive: length over all 330 mm.; outside diameter of jacket 68 mm., of tubing in outer coil 8 mm., in inner coil 6 mm., of vapor inlet 30 mm., and of condensate outlet 10 mm . The size of the condenser described above is limited by the size of the jacket which can be fabricated, and a condenser of still larger capacity is indicated in Figure 2. I n this unit the vapors are condensed inside the coils as in the ordinary coil condenser. Such a condenser usually has a great Received April 12, 1929. These condensers have been satisfactorily constructed and are available in different sizes from the Technical Glass Co., Rochester, N. Y.
Figure 1
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2
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Figure 2
