JOURNAL O F CHEMICAL EDUCATION
A USEFUL MELTINGPOINT APPARATUS JOSEPH L. WALTER University
THE schematic diagram illustrated here describes a simple, rapid, economical, and accurate melting-point apparatus. Its advantages are that it requires no
A.
Thermometer mounted by s damp on
B
sewrate ring stand
B. Outer test tube C. Inner test tube wound with niohrome wire ( D ) 6. Asbestos ribbon to support tube F. Clamps and eleotric cord to Yariariae
capillary melting point tube, no oil-bath medium, the temperature may be raised a t any desired rate from 30' to 450°C., it may he cooled down within five minutes from 360°C., and it may be cleaned rapidly. The only
of Pittsburgh, Pittsburgh, Pennsylvania
piece of relatively expensive equipment the apparatus requires is a Variac; however, almost every laboratory has one. The remainder of the equipment consists of two test tubes, a few feet of nichrome wire, two battery clips and a foot of asbestos ribbon. There can be several modifications of this apparatus, but here a simple, fast, and economical method for its preparation is described. A 150- X 18-mm. test tube, wound as illustrated with 0.032-in. 20-gage, 0.63-ohm/ft. nichrome wire and moist asbestos ribbon, is forced into a 200- X 25-mm. test tube in which a hole has been blown a t the bottom. The entire chamber is dried in an oven. When dry, the a p paratus is assembled as shown in the figure. To determine a melting point one places a few crystals in the center of the mercury bulb of the thermometer ( A ) with a spatula. The entire thermometer assembly is guided into the heating-chamber assembly until at the position illustrated in the figure. The dial on the Variac is set to about eight volts if the melting point range is below 150°C. If the melting point is above 150°C., the voltage is set proportionately higher. When the temperature approaches the melting range, one may turn the voltage down to about three, thus allowing the temperature to rise very slowly (about two degrees per minute). When the melting point is reached, the thermometer assembly is removed and the residue wiped off the bulb with a cloth moistened with alcohol or acetone. The apparatus is then ready for another determination.
BELVEY W. MUNDY and N. W. ALLEN Virginia Military Institute, Lexington, Virginia
THE problem in any recording polarograph is to apply a uniformly.increasing potential to the electrolysis cell and to then record the current through the cell as a function of this potential. The following is the description of an apparatus that accomplishes this and which can be easily constructed in the average laboratory. A U-tube of glass, inside diameter 12 mm., was constructed. The long arm measured 110 cm., the short arm 70 cm. The two arms were tapered down and joined to 7.5-mm. glass tubimg. A three-way stopcock was located a t the junction of the arms and the third lead, consisting of a short piece of glass tubing, formed a drain for the arms of the U-tube. Details of the apparatus are shown in Figure 1. The U-tube was mounted on a backboard. Withim each of the arms a length of 28 B S chrome1 resistance wire was strung, the terminals leading through glass seals. The length of wire in the short arm was 50 cm. while the wire length in the long arm (above the upper terminal in the short arm) was also 50 cm. The length of wire in the long arm which overlaps the wire in the short arm is immaterial. Thii apparatus can be used to apply a uniformly increasing potential in the following manner. Mercury is placed in the lefthand arm reaching just to the top of the resistance wire, and in the right hand arm reaching just to the bottom of the resistance wire. Initially, the stopcock is set to keep the two columns of mercury separate. A source of potential is then connected to the top terminals of the two resistance wires; and the electrolysis cell circuit, consisting of the electrolysis cell and galvanometer with suitable shunt, is connected across the resistance wire in only the long arm. The stopcock is then opened and mercury is allowed to flow slowly into the short arm until the mercury levels in the two arms are equal. This requires eight to ten minutes. Then, by means of the three-way stopcock, the mercury is removed from the short arm and placed in the long arm for another run. Initially, the potential applied across the electrolysis cell circuit will equal the potential drop across the column of mercury covering the resistance wire in the long arm of the tube. This will be small and it is dependent upon the magnitude of the potential applied. As the mercury drops, uncovering the resistance wire in the long arm and simultaneously covering the short arm resistance wire, an application of a uniformly increasing potential to the electrolysis cell circuit
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Presented before the Annual Meeting of the Virginia Academy of Science, May 17, 1952.
results. This potential will reach its maximum when the levels in the two arms are equal, and will be approximately equal to the potential applied across the terminals of the resistance wire. It should be noted also that the potential applied to the electrolysis cell circuit is a h e a r function of the level of the mercury in the long arm. The initial potential drop corresponding to the column of mercury can be readily determined in each case by a voltmeter. The final maximum potential across the electrolysis cell circuit will be less
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JOURNAL OF CHEMICAL EDUCATION
indicated in Figure 2. The inside projections just cleared the circular wheel by the thickness of photographic paper. The galvanometer in the electrolysis circuit was of the light-reflecting wall type, and light from an ordinary galvanometer lamp was employed. A narrow slit of light v a s obtained by placing a piece of cardboard, with a narrow slit cut in it, just in front of the lamp bulb. To record a polarogram, photographic paper is cut to correspond to the size of the vheel in the camera and it is mounted on the mheel. A thumb tack near the center prevents the paper from slipping during a run, and a piece of tape over the slit keeps the box light tight after removal froin the dark roan. The camera is mounted above the U-tube previously descrihed, with the mercury plummet floating on the column of mer-
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vanometer mirror is adjusted so it plays on the slit of the camera cover near the center of the photographic paper, and is perpendicular to the slit in the camera. With all adjustments made, the tape is removed from the slit about one minute before the figure 2. The Camera start of a run in order to record zero position. To make t,han the potential applied across the terminals of the a run, thc stopcock is opened and the mercury allowed resistance wire by thc potential drop that occurs along to flon. slowly into the short arm. -As the mercury t,he entire column of mercury. The final maximum drops in the long arm, the mercury plummet drops, potential applied ran he determined exactly by means of rotating the wheel upon whichthe photographic paper is mounted. This results in a polar type of polaroa voltmeter. I n order t,o record the current in the electrolysis cell gram,. in which the angle of rotation from t,he initial circuit as a function of t,he potential applied, the follo~v- position is proportional to the applied potential and ing arrangeinent was employed. A camera consisting the distance from the center is a measure of the curof a box 21-cm. square and 5-cm. deep was constructed. rent. This is illustrated in Figure 3. At the compleOne side of the box was hinged and a fine slit was cut tion of a run, thc tape is replaced and the camera rein this side. The edges of the box facing the hinged lid were fared with velvet so as to make the Box light tight. A catch held the lid tight when closed. Mounted in this box was a solid grooved wheel of wood which could he rotated, thc cirrumfcrence of the wheel heing equal to the distance through which the mercury drops (50 em.) in the U-tube previously described. Cord was wrapped around the groove of the wheel, and its ends were brought through ttlo small holes in the bottom of the box. The length of the cord outside the box must he greater than 50 cm. Details of the camera arc shown in the photograph in Figure 2. In this photograph the wheel is shown outside of the box, resting on an iron arm that was used in mounting the camera. Attached to the ends of the cord there were two weights, the heavier consisting of a plummet -LIMITING CURRENT (mercury sealed in glass) and t,he lighter, simply a small weight sufficient to keep the cord under tension when hanging free. The two weights, however, must differ sufficiently in weight so that when they hang free the wheel in the camera rotates easily. The mercury plummet was sufficiently small to fit inside the bore of the U-tube. The whole camera was painted black. The slit in the lid had projections on both sides as Figure 3. Typical P o l u Pola.omam
MARCH. 1953
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moved to the dark room for development of the polarogram. Runs were made in a lighted room with no noticeable fogging of the polarogram. To determine the half-wave potential the angle, 01, from the initial position to the midpoint of the break is measured. Then, if 0 2 is the angle corresponding to the complete uncovering of the resistance wire in the long arm: Or - Span e. rn. f . 0, E4
In exact work, the potential drop corresponding to the column of mercury covering the wire in the long arm should be added to the calculated half-wave potential. Any particular set of apparatus must be calibrated for the current. Then the distance between the
circle corresponding to the residual current and the limiting current can be translated into terms of i and the concentration. In our apparatus a galvanometer with a sensitivity of 0.01r amp./mm. a t 1 meter, and critical damping resistance of 10,000 ohms was employed. It was located a t a distance of 50 cm. from the camera. If a conventional rectangular type of polarogram is desired, this can be accomplished by a camera arrangement consisting of a rotating cylinder mounted in a light tight box of suitable dmensions. In this case, the axis of rotation would coincide with the axis of the cylinder. The whole apparatus can be made less bulky by coiling the resistance wires located in the Utube, and gearing the camera so as to obtain a complete rotation.
