K. VAN LENTE and E. H. HADLEY Southern Illinois University, Carbondale, Illinois
Tmm are certain advantages in the use of a thermostatic air bath, especially for electrochemical measure ments. These advantages include the low electrical conductivity of air and the elimination of the necessity of cleaning the outside of the apparatus. This cleaning is annoying in oil baths. This article describes such an air bath made from parts which were accessible and not very difficult to assemble. The accompanying figures present a general description of the bath, but some observations may be in order. I n Figure 1 the box covering the 10-in. fan blade is 15'/%in. wide. It is closed, except for a hole in line with the center of the fan blade for an air intake, and an open bottom. The air moves into the bath proper through a 2-in. slit extending the width of the bath (25 in.), between the false bottom and the front of the bath. The approximate air flow, determined by experiment with suspended threads, is shown by the arrows. 1
Presented in part before the Chemistry Section of the Illinois
State Academy of Science, May 7, 1952.
The heater is made of No. 22 B. and S. gage resistance mire, having a total resistance of 82 ohms, wound on a piece of '/4-in. Transite (Johns-Manville Co.) 12 in. long and 6 in. wide. The heater is protected by a fuse made of Wood's metal. The cooling coil consists of about 30 ft. of '/,-in. copper tubing wound back and forth to expose maximum surface. Because the available tap water is too warm to provide adequate cooling during the summer months, the water is kept a t about 13°C. by recirculating it, by means of a pump, through a water cooler. The fan motor is mounted outside the bath to reduce vibration and exclude motor heat from the bath. After trials with several different types and shapes of mercury-in-metal thermoregulators, one was made of ordinary steel tubing, 8 in. long, 19/18in. o. d., with a wall thickness of 0.030 in., and holding 195 ml. of mercury. Attempts to clean ordinary steel tubes with acids followed by r-insing with water-and acetone resulted in the appearance of a yellow film after the mercury was added. This film tended to float on the mercury
JOURNAL OF CHEMICAL EDUCATION
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1.
A* Bath
A. 30 i a X 30 in. X 30 in. wooden boa, oelotea-lined; B. door and cover; C, falas bottom; D, thermoregulator without oomtrJnt pressure deviae; E, heater; F, cooling coil.
surface and foul the contact with the iron wire. Finally, therefore, a length of open steel tubing was mac chined down to proper wall thickness, the inside polished with fine abrasives, the bottom and top pieces
I I 0 V.'
A.C.1
pressed in and welded; and then with no further cleaning, the mercury was added. The mercury was boiled during the filling to drive out dissolved and occluded air. The regulator top is made of Pyrex glass which is cemented to the regulator and which has a metal cap with a finely threaded screw cemented to it. There is a flare in the capillary where the mercury contacts the iron wire, which allows variation in sensitivity. The mercury was cleaned in a mercurous nitratenitric acid column several times, pin-holed, and distilled three times, the last time in a nitrogen atmosphere; but after varying periods of time it still formed a film on cleaned glass. The inside of the capillary, therefore, was coated with General Electric DRI-FILM 9987 to decrease the tendency of mercury to form this film on glass. The regulator was designed to operate a t 25°C. but could be made more versatile by a, plunger attachment such as that described by Parks (1) and Yee and Davis (8). The regulator was given a thin coat of lacquer on the outside to prevent corrosion and the regulator supports were placed outside the bath with the suspending chains hung free of the bath to eliminate vibrational effects. The constant pressure device has been previously described (3). The features of the control unit are shown in Figure 2 and the circuit is a modification of the one proposed by Beaver and Beaver (4). This system will operate without the rectifier, but the resulting chatter of the relay is annoying. While circuits employing thyratron tubes directly (5) and thyratron tubes to trigger relays (6) havebeen described, the above arrangement also operates satisfactorily. The relay is one made by the American Instrument Company (Bulletin No. 921) with the circuit modified as shown in Figure 2. The 110-v. heating circuit is controlled by a mercury switch so that corrosion of contacts is no problem. The bath was operated over a period of months under a variety of room temperatures and atmospheric pressures, and temperature recordings were made. The cooling water ran continuously while the heater operated only when contact was not made in the thermoregulator. A typical set of results over a three-day period showed a temperature of 25.00' with an average deviation of =t0.05°. LITERATURE CITED
A, 0.5-amp. fuses; B, 150-v. selenium rectifier; C. 10 m.f.d.. BOO-v.. d-o. condenser; D, 332 ohms: E. 123 ohme; F. 4 megohma; 8. 71 A-tube; H . thermoregulator; J , 0.4megohm; K, 1 0 0 6 ~ h L. ~ ~235 ; ohma; M.6.8 ohms; N,arm carrying mercury switch for the 110 v., a,-e. heating unit.
(1) PAEKS,W. G.,Znd. Eng. Chem., Anal. Ed., 5, 357 (1933). (2) YEE,J. Y., AND R. 0. E. DAVIS,ibid., 8,477 (1936). A. L., K. VANLENTE,AND R. HITCEENS, ibid., 4, (3) FERGUSON, 2 1. R. Q - llQR21. >----,(4) BEAVER, D. J., AND J. J. BEAVER,Ind. Eng. Chem., 15, 359 11Q?Ri ,-w-v,.
(5) S ~ N E H A RD.T F., , Anal. Chem., 21, 1577 (1949). (6) LEVENS,E., AND R. S. CASS,ibid., 24. 1685-6 (1952)
