844
ANALYTICAL CHEMISTRY
nitric acid. The residues were filtered out on a Whatman KO.42, 7-cm. paper, washed well with 100 ml of hot water, ignited in air at 650” C., reduced in hydrogen, and weighed. A cupellation blank was subtracted. Spectrographic examination of the iridium residues and the blank showed them t o be badly contaminated. No iridium was detected in the parting acid. Another set of beads (Nos. 4 to 6) were arted with nitric acid and the residue was filtered, washed by fecantation, and then treated with hot aqua regia for 20 minutes. The residues were filtered and washed as outlined above. Spectrographic examination of the recovered indium and the blank showed that some silicon, silver, aluminum, and copper were present as impurities. This was due, no doubt. t o thevaryingamounts of insoluble substances from the cupel itself,
Discussion. When examined under a microscope, all the silveriridium beads had black scales at various places on the surface of the bead; this is a characteristic surface effect ( 5 ) . As the ratio of silver to iridium was increased, the scales appeared only near the base of the bead, just above the cupel. Furthermore, black particles of iridium oxide were scattered about the surface of the cupel, close to the silver bead. Both of these phenomena were observed even when the ratio of silver to iridium was as high as 200 to 1. The mechanically lost iridium was not included in the analyses of the silver bead. CONCLUSIONS
The distribution of iridium in fire assays has been examined. Reabsaying of slags is necessary for a complete collection of iridium and significant losses of iridium to basic slags were observed. The cupellation of lead-iridium buttons leads to serious mechanical losses of iridium, even when the ratio of silver to iridium is high. ACKNOWLEDGMENT
This lvork was supported by search Council (Canada).
5
grant from the Sational Re-
LITERATURE CITED
(1) Allen, IT.F., and Beamish, F. E., ANAL.CHEM.,22, 461 (1950). (2) Barefoot, R. R., McDonnell, W.J . , and Beamish, F. E., Zbid., 23, 514 (1951). (3) Bugbee, E. E., “Textbook of Fire Assaying,” 3rd ed., Sew Tork, John Wiley & Sons, 1940. (4) Davis, C. W., U. S. Bur. Mines, Tech. Paper 270 (1921). (5) Forbes, E. C., and Beamish, F. E., IND.ENG.CHEM., ANAL.ED., 9, 397 (1937). (6) Gilchriat, R., J . Am. Chenz. Soc., 45, 2820 (1923). (7) Gilchrist, R., J . Research N a t l . Bur. Standards, 9, 64i (1932). (8) Ibid., 12, 294 (1934). (9) Ibid., 20, 745 (1938). (10) Ibid., 30, 89 (1943). ( 1 1 ) Gilchrist, R., and Wichers, E., J . A7n. Chena. Soc., 57, 2666 (1935). (12) Hill, M. A . , and Beamish, F. E., . ~ K . A L . CHEM.,22, 590 (1950). (13) Holzer, H., and Zaussinger, E., Z . anal. Chem., 111, 321 (1938). (14) Milazzo, G., and Paoloni, I,., Rend. ist. super. sanitct ( R o m e ) , 12, 693 (1949). (15) Plaksin, N., and Marenkov, E. A , Izrest. A k a d . Natkk S.S.S.R., Otdel. Khim. S a u k , 1948, 209. * (16) Pollard, W. B., Bull. Inst. M i n i n g M e t . , No. 497, 9 (1948). (17) Schoeller, W. R., and Powell, A. R., “Analysis of Minerals and Ores of the Rarer Elements,” London, C. Griffin & CO., 1940. (18) “Scott’s Standard Methods of Chemical Analysis,” Vol. 1 , 5th ed., New York, D. Van Nostrand Co., 1939. ,. 12, (19) Seath, J., and Beamish, F. E., IND.ENG.CHEX,,a h . i ~ED., 169 (1940). (20) Shepard, D. C., and Dietrich, W.F., “Fire Assaying,” Kew Tork, McGraw-Hill Book Co., 1940. . 20, (21) Thiers. R., Graydon, W., and Beamish, F. E., - 4 s . 4 ~CHEM., 831 (1948). RECEIVED for review November 9, 1951.
Accepted January 26, 1952.
Instrument for Automatic Determination of Melting Point RALPH ri. M U L L E R ~m n SEYMOUR T. ZENCHELSKY~ Washington Square College of Arts and Science, New York University, New York 3, N . Y
M
ANY techniques exkt for the determination of the melting points of pure organic compounds (3, IS). In the practical organic chemistry laboratory the capillary method is most frequently employed. Nearly all the literature on the determination of melting points deals with the impmvrment and simplification of this procedure. This method, as well as all others thus far employed, depends upon the subjective decision of the operator for a criterion of melting. Two exceptions may be noted: Lawe employed an electrical indication of melting, and Kardos employed an optical one (3, 5 , 9 ) . Keither of these methods, however, nor any other, has been employed in an instrument for the automatic determination of melting point. The instrument described in this report wae developed for the purposes of the practical organic chenlistry laboratory. It was designed to give rapid and precise melting point data without continuous attention by an operator during the determination, and it employs samples of micro size. INSTRUMENT REQUIREMENTS
Because the definition of melting point involves the existence of an equilibrium between liquid and solid, the ideal measurement would be made while the system is maintained in such a state of dynamic equilibrium. A closed-loop servosystem ~ - 0 u l d supply or remove heat in proportion t o the relative amounts of solid and liquid present, causing the temperature to oscillate 1 Present address, The Los Alamoe Scientific Laboratory,Los Alamos, N. M. 2 Present address, School of Chemistry, Rutgera University, New Brunswick. N. J.
within narrow limits. The mean temperature would be taken as the melting point, provided that the temperature excursions were small enough. Such a state of affairs is impossible to achieve, aa all liquid. supercool, and thus permit smooth servo control from one direction only. A practical instrument would have to approach the melting point in the conventional manner. It would require three basic components: (1) a means for automatically increasing the temperature of the sample a t a predetermined rate, the hot stage assembly; (2) a temperature-indicating device, the thermometer; and (3) a means for automatically determining when melting has occurred as well as for causing the temperature of the melting point to be indicated on the thermometer for as long as is desired after melting has occurred, the sensing and control circuit. The sensing and control circuit requires an objective criterion of melting. The one employed in this instrument makes use of the fact that polycrystalline materials scatter light while shiny metallic surfaces give specular reflection. Thus if a beam of light is incident upon a polished metal surface, it will be reflected in a beam a t an angle that is equal t o the angle of incidence. At that angle a photocell provided with a lens and diaphragm would register maximum light intensity. If a thin film of finely powdered crystalline material is placed on the metallic surface, the light intensity a t the photocell will diminish because the incident light is now scattered in many directions. Heating the metal surface will cause the powder t o melt, leaving a thin transparent film of liquid on the shiny metal surface, Thie system gives specular reflection once more, causing the photocell to register a maximum again.
845
V O L U M E 2 4 , NO. 5, M A Y 1 9 5 2
The practical organic chemistry laboratory requires a means for rapidly and precisely obtaining the melting points of pure organic compounds. An instrument for this purpose is based upon a criterion of melting which is independent of the operator, and it employs samples of micro size. The instrument consists of an electrically heated hot stage with a servo potentiometer temperature indicator. A photoelectric sensing and control circuit arrests the servo motor at the melting point and maintains the temperature indication for as long as is desired. Heating rates as low as 0.5" C. per minute are obtainable,and the temperature may be read to 0.25' C. Melting point determinations on lcnown compounds made with the instrument after calibration give precision and accuracy well within the limits required by the practical organic chemistry laboratory. These results are in agreement with those obtained by the capillary method.
INSTRU\IEYT COMPONENTS
Hot Stage Assembly. The hot stage consists of two brass blocks, each 1 inch (2.5 cni.) square by 0.25 inch high, which lie one over the other, being separated by a Transite spacer, 0.125 inch thick, and are hinged together a t the back (Figure 1). The upper block contains a glass window, 0.75 inch in diameter, in its center; the window is cemented in place with a calcium sulfate-sodium silicate cement. The lower block contains a circular platinum disk, 4 mni. in diameter and 0.25 mm. thick, which is supported on a platform of Transite to provide thermal insulation from the lower block. The Transite platform is cemented to the Transite spacer by means of calcium sulfatesodium silicate cement, and the spacer is fastened to the lower block by means of countersunk screws. The disk has a shiny polished top surface, and one junction of a copper-constantan
thermocouple silver-soldered to the bottom, through a hole in the lower block. Both blocks contain wells, two in each, to accommodate resistance heater wires. The wires in both blocks are 30-gage constantan with glass braid insulation. The resistance of each wire in the upper block-one in each well-is 0.5 ohm, and the two are connected in parallel. The resistance of the mires in the lower block-one in each well-is 0.25 ohm and they are connected in series. The wires in the upper block thus have half the resistance of those in the lower block. This fact ensures that the temperature of the upper block and window is always higher than that of the lower block. In this manner fogging of the window because of condensation of sublimate is avoided. Electrical connection to the heater wires is made on a Transite terminal strip attached to the front of the upper block; a second connection is made to the lower block itself. The two blocks are heated in parallel from a Variac-controlled 12-volt transformer. Both sample and platinum stage, which are in intimate contact, are heated by convection currents in the enclosure between the window and lower block. A snug fitting together of the blocks assures freedom from external drafts. Predetermined heating rates may be made to prevail automatically in the vicinity of the m e l t i n g T i n t by setting the hese Variac settings Variac to the desired heating voltage. are empirically determined, and the hot stage assembly is thus calibrated with respect to the heating rates which will prevail a t the melting point (IO). The hot stage assembly is shielded from ambient temperature effects by the construction of the block and the laboratory temperature is sensibly constant, so that if the voltage input to the Variac is regulated, the calibration is reproducible. Moreover, the heating rate is critical only in that it must not exceed a given maximum value, to ensure adequate heat transfer. Heating rates as Ionr a? 0.5" C. per minute are thus obtainable. T H E R M OCOUPLE INPUT POTENTIOMETER SWITCH
F Figure 2.
E
r\
G
-- - - - - - - - - - - Figure 1. Hot Stage -4ssembly A . Brass block B . Transite insulation spacer C. Heater wells (to contain heater wires) D . Transite platform (to support disk) E. Pl-atinum disk (fastened as a rivet) F. Hinge G. Window H . Transite (insulator for heating wire binding post) I . Binding post (for heater wire) J. Thermocouple junction
Block Diagram of Servo Potentiometer
Sample manipulation is rendered simple by the fact that the hot stage is hinged. To introduce the sample, the upper block is lifted back and a small amount of material is placed on the platinum disk with a spatula. The sample is then covered with a micro cover glass and tamped down to make a thin flat film. It is removed when desired by washing off with solvent. ils the temperature of the stage never exceeds the melting point of the material by more than a few degrees, little carbonization is encountered. To remove carbonized material fine rouge paper is employed.
As the heating rate in the vicinity of the melting point is preset by selecting the appropriate heater voltage, the temperature rise of the sample is of necessity an exponential function of the time. Thus the sample is heated rapidly a t first, and the heating rate is gradually diminished as the temperature of the melting point is attained. This automatic diminution of the heating rate corresponds t o the practice observed in the case of the conventional methods for melting point determination. The arrangement of the hot stage and associated optical assembly permits visual observation during the heating process when desired. Thus the operator may note the various transformations undergone by the sample prior to and during the melting process.
Thermometer. The thermometer consists of a thermocouple and servo millivolt potentiometer. A block diagram is shown in
ANALYTICAL CHEMISTRY
846 Figure 2. Temperature indication is maintained a t the value of the melting point by arresting the rebalancing motor when melting occurs. The temperaturemeasuring junction of the thermocouple is silver-soldered to the underside of the platinum disk of the hot stage assembly, the length of wire passing through the hole in the lower block being insulated from heat losses by the temperature of the block; the hole is packed with calcium sulfatesodium silicate cement. The cold junction is kept a t the freezing point of water by immersion in an ice-water bath. The thermocouple is constructed of No. 30 B. and S. wires in order to minimize its heat capacity.
t150 c3
I Figure 3.
1
111
Rio. Ri2. Ria. Rir. Ria. Ry. Ti. Tz.
A block diagram of the entire instrument is given in Figure 4.
b -150
Schematic Diagram of Schmitt Trigger Circuit
Ci. 50 ppfd. C2, Cs. 1 pfd. Ri, Rd. 10,000 ohms Rx. 56,000ohms Ra, Re, Ru. 100,000 ohms Ra. 4700ohms Ra. 2000-ohm potentiometer R7. 100,000-ohm potentiometer Rs. 1 megohm
the triodes of the Schmitt trigger; this relay controls both the servo motor and the heat input to the hot stage. Thus when the sample is solid the relay is closed; the servo motor functions and heat is supplied to the hot stage assembly. When the sample melts, the increased light intensity to the phototube causes the relay to open; the servo motor is arrested and the current to the hot stage is removed. The “triggering level” or bias setting for the Schmitt trigger depends upon the particular “reflectivity” of the sample. This factor depends upon the nature of the sample itself as well as upon its degree of subdivision and orientation. Consequently, the bias potentiometer must be preset for each new sample. This is accomplished by turning the potentiometer knob until the servo motor is arrested. A neon indicator lamp shows this condition to be in effect. The knob is then turned back approximately 30 degrees; this angle is not critical. The trigger is now set for a melting point determination. In the case of compounds which do not sublime or decompose a t the melting point, so that the reflectivity of the solid does not alter appreciably upon successive remelting, repetitive melting points may be obtained from one sample automatically, without resetting the triggering level, since after the heat is removed, the sample will freeze and the cycle will begin again. To provide for the choice of either this mode of operation or the one involving separate individual samples, the relay may be of the locking type, with electrical reset. Thus if single determinations are desired, the relay will remain locked after melting has occurred until a button is pushed by the operator; and if recycling is desired, the reset button may be locked in position in advance.
2.5 megohnk
120,000 ohms 470,000 ohms 220ohms 500,000 ohms 4700-ohm d.p.d.t. relay 6SJ7 6SN7
The thermocouple output leads go to the servo millivolt potentiometer ( 1 ). This is essentially one of the Brown Electronik type, designed for the particular needs of this problem. The potentiometer slide-wire covers the range from 0 O to 400 O C. The potential is read on a Beckman Helipot Duodial which is mechanically coupled to the rebalancing Helipot. The duodial is graduated in 1000 divisions and can be readily interpolated to division, so that the temperature may be read to 0.25” C. This is also the limiting precision of the servosystem as constructed. A Brown converter and amplifier are employed unmodified, together with a Brown servo motor. Appropriate gearing is provided to the rebalancing Helipot as well as to the slide-wire working current standardization rheostat. The standardization is accomplished by comparison with a standard cell, when desired, by the use of a switch in the potentiometer circuit and a mechanical clutch in the gear train. The servo potentiometer was calibrated against a Leeds and Northrup Student potentiometer, and a table of Helipot Duodial readings versus input potential was prepared. Furthermore, the entire system was calibrated with respect to temperature by comparison with a calibrated reference thermocouple soldered-to the top of the platinum disk of the hot stage assembly. The Helipot Duodial continuously indicates the sample temperature while it is being heated, and the servo motor is automatically arrested when melting occurs by means of a relay which interrupts the current to one field winding of the two-phase motor. Sufficient damping forces are present to maintain the temperature indication within the accuracy limit of &0.25’ C. Sensing and Control Circuit. The sensing and control circuit employs the light reflectance criterion of melting previously described. The light source is a 6-volt automobile headlight lamp mounted behind a 5-cm. focal length lens and diaphragm. -4 beam of light from this source is focused on the sample which covers the platinum stage, through the glass window of the upper block. The reflected beam is focused on a phototube through a 5-cm. focal length lens and diaphragm. The output of the photo‘tube is fed to a direct current amplifier, and the resultant signal to a Schmitt trigger circuit (11, 12). A schematic diagram is shown in Figure 3. The dead zone of the Schmitt trigger is selected to prevent triggering action by stray pickup. h doublepole, double-throw relay is located in the plate circuit of one of
PERFORMANCE
The instrument was subjected to performance tests. Five different organic compounds were selected for this purposeazobeuene, benzoic acid, sulfanilamide, succinic acid, and anthracene-hosen because they are among the purefit codmercially available and their melting points cover a temperature spread from 68” to 215” C. In addition, this group exhibits several common modes of behavior on being heated; three of the substances sublime readily a t temperatures before the melting point, one decomposes noticeably, and one is “well behaved.”
-
HOT STAGE
AMPLIFIER AND TRIGGER
INDICATING SERVO
L_i VAR I AC
Figure 4.
Block Diagram of Melting Point Instrument
Melting points were determined on several different samples of each compound. In the case of sulfanilamide, which neither decomposes nor sublimes on being heated, repeated melting points were also obtained on the same sample. All determinations were made a t heating rates less than 5 ” per minute, as it wm found empirically that higher rates led to lack of reproducibility within the required limits. Table I gives the values obtained. The average deviation from the mean does not in any case exceed the limiting precision
a47
V O L U M E 2 4 , NO. 5, M A Y 1 9 5 2 Table I.
Precision of Results
All melting points were determined by means of t h e instrument. A fresh sample of t h e material was used for each determination; in t h e case of sulfanilamide, repeated determinations were also made automatically on a single sample Anthracene Benzoic Acid Aaobenzene ______ _____ Temp., DeviTemu., DoriTemp., DeviC. ation C. ation C. ation 216.0 -0.2 122.6 -0.1 -0.1 67.1 216.2 0 122.G -0.1 -0.1 67,l 216.2 0 122.8 +0.1 +0.3 67.5 216.2 0 +0.1 +0.2 122.6 67.0 216.2 0 +0.3 -0.1 123.0 67.1 Av. 67.2 122.7 216.2 Sulfanilamide (Single Sample)
Sulfanilamide
Ai‘.
164.6 164.6 164.6 164.6 165.0 164.7
Table 11.
-0.1 -0.1 -0.1 -0.1 f0.3
164.6 164.6 164.6 164.8 164.8 164.7
-0.1 -0.1 -0.1 +0.1 +0.1
Succinic Acid 187.0 187.2 187.2 187.0 187.0 187.1
-0.1
Comparison of Results with Literature
I
I1 67.1
4 (14)
B (4)
...
...
67.2
Benzoicacid
122.7
Sulfanilamide
164 7
164.6
Succinic acid
187.1
186.8 182.3 182.7 182.8
Anthracene
216.2 216.5 216.18 217
1 2 2 . 8 122.30.5 1 2 1 . 4 122.375 122.43
...
... 183
216.4216.7
C (7)
...
ACKNOWLEDGMENT
-0.1 +O.l +O.l -0.1
Column I gives melting points obtained with instrument. Column I1 gives meltin points obtained with modified Hershberg apparatus. Columns A &rough E give melting points obtained from literature Azobenzene
measuring component, at heating rates less than 0.5” C. per minute in the Hershberg apparatus. The melting points so obtained agree very well with those obtained by use of the instrument described here. I n the case of azobenzene, which is deeply colored and darkens before melting, the precision and accuracy of the results are no less than in the case of the other materials studied. This fact would seem t o indicate, and visual observation bears out, that the extreme thinness of the liquid layer on the hot stage suffers no diminished transparency so far rn the sensing element is concerned. There is therefore no reduction in light intensity at the phototube when a sample darkens or chars prior to melting.
D (8) 68
122.45 121.7
E (6) 68 122.5
.., ...
165-66
166
189-90
188
216.5
217-18
215
of the thermometer, nor does the maximum deviation exceed twice this value. Table I1 gives a comparison with the literature values for the compounds employed. Agreement among the literature values themselves is not satisfactory. Accordingly, the melting points of the compounds employed in this investigation were determined by an independent method, using a modified Hershberg apparatus ( 2 , IS). A calibrated thermocouple was used in measuring the temperature of the bath in the Hershberg apparatus, and its output was read on the ~ e r v omillivolt potentiometer of the instrument. All compounds employed melted sharply within the limiting precision of the temperature
The authors wish to thank the Texas Co. for its support of a large part of this work, which was done under the Texas Co. Fellowship in Microchemical Analysis, and Harry Levin of the Beacon Laboratories of the Texas Co. LITERATURE CITED (1) Batcher, R. R., and Moulic, W., “The Electronic Control Handbook,” p. 298, New York, Caldwell-Clements, 1946. CHEM.,ANAL.ED.,8 , 3 1 2 (1936). (2) Hershberg, E. B., IND. ENG. (3) Houben, J., “Die Methoden der organischem Chemie,” 3rd ed., Vol. I, Leipaig, Georg Thieme, 1935. (4) Huntress, E. H., and Mulliken, S. P., “Identification of Pure Organic Compounds,” pp. 115, 146, 517, New York, John Wiley & Sons, 1949. (5) Kardos, F., ANAL.CHEM.,22,1569-70 (1950). (6) Kofler. L., and Kofler, A., “hlikromethoden zur Kennaeichnung organischer Stoffe und Stoffgemische,” pp. 224, 249, 270, 283, 295, Innsbruck, Universitlitsverlag Wagner G.m.b.H., 1948. (7) Landolt-Bornstein, “Physikalisch-chemische Tabellen,” Erg. I11 a, c, Supp. Vol., Berlin, Julius Springer, 1927. (8) Lange, N. A., editor, “Handbook of Chemistry,” 7th ed., pp. 377, 379, 383, 389, 643, Sandusky, Ohio, Handbook Publishers, 1949. (9) Lowe, J., 2.anal. C h a . , 1 1 , 211 (1872). (10) Matthews, F. W., ANAL.CHEM.,20, 1112 (1948). (11) Puckle, 0. S., “Time Bases,’’ p. 57, New York, John Wiley & Sons, 1944. (12) Schmitt, 0. N.. J . Sci. Instruments, 1 5 , 2 4 (1938). (13) Skau. E. L.. and Wakeham. H.. in “Techniaue of Organic
Chemistry,” 2nd ed., Vol. I, Part I, Chap. fII, New York, Interscience Publishers, 1949. (14) Timmermans, J., “Physico-Chemical Constants of Pure Organic Compounds,” pp. 181, 403, 480, New York, Elsevier Publishing Co., 1950. RECEIVEDfor review November 16, 1951.
Accepted February 21, 1952
Precision Semimicro Hydrogenation Apparatus F. A. VANDENHEUVEL Fisheries Research Board of Canada, Atlantic Fisheries Experimental Station, Halifax, N. S., Canada I T H few exceptions ( 4 , 5 ) ,all the laboratory hydrogenation apparatus described in the literature derive from the simple classical device shown in Figure 1. This is composed of a vessel, C , or hydrogenation cell, a graduated buret, B , and a leveling arrangement including a leveling bulb, L. I n the cell is shaken a mixture of unsaturated compound, catalyst, and a solvent when required; the hydrogen contained in the apparatus is absorbed as the reaction progresses. This system allows a measurement of the absorbed hydrogen t o be made and therefore can be used for determination of unsaturation values (absorption a t the completion of the reaction) and of hydrogenation rates. With this type of apparatus, hydrogenation values (a)are obtained with a precision of *3%, provided the proper technique is
used. If accuracy is not the main objective, this is the ideal equipment, being simple t o construct and easy t o handle. Many modifications have been proposed; some lead to somewhat better results but in no case is a precision better than 1% t o be expected. The sources of error are many and no apparatus proposed so far eliminates them all satisfactorily. When, instead of hydrogenation values, hydrogenation rates are required, the difficulties involved are greatly magnified. This type of study not only calls for a much higher precision but also numerous timed readings. The equipment shown on Figure 1 is unsuited for this type of work. The essential drawback is the practical impossibility of making frequent and precise readings when the pressure has t o be
