I*kTDt;STRIdLS.VD ENGINEERIXG CHEMISTRY
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A N2w Microscope Hot Stage" I. Amdur and E. V. Hjort UNIVERSITY OF PITTSBURGH. PITTSBURGH, PA
HE microscope hot stage herfxin described Tvas designed to enable accurate melting-point data to be obtained in in\-estigations where 'ields are so small as to make capillary melting points impowisle or inconvenient. According to Cram ( 1 ) the use of iniicroscope hot stage for melting-point determinations is iractically essential in toxicological investigations. The h( t stage described below, h o w v e r , gives melting points of rt least the same degree of aceuracy as those obtained by the capillary-tube method, instead of the approximations obtaired by Cram's apparatus, and eliminates tlie expense and incolLi-enienceof thermocouple nieasurenieiits as described by Xeth,nimer ( 2 ) .
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strip of tranaite. A similar strip slips over the top in the pegs F , F , F . F . The openings D and E are best seen in Figure 2. Light from the microscope mirror enters through the 6-mni. hole, E , traversing the Pyrex window K , the air space L, the Pyrex sample holder .TI, in which the crystal fragnieiit is placed, t t e air space D , which is 19 nun. in diameter, and the window S.
r%?=+z c
Figure 2-Side
View of Hot Stage
Figure 3 shows a photograph of the hot stage in w e . The range of this hot stage is from room temperature to 600" C. Use of Hot Stage
Prior to use the rotating stage of the microkcope i\ removed and the hot stage placed on the niicroscope. Thermometers are fitted, either with cork stoppers or a.bestos packing into the holes drilled for the purpose. For the preliminary melting-point determination nierelv one thermometer (0" to 360" C.) iq in5ertecl. The stage is then cooled and the proper Aiischutz thermomc+er is inserted in the second thernionieter holes, the itage reheated, and the exact melting point read to *0.1 degree on the Ain>chutz the1nioiiieter.
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t-2 Figure 1-Top
View of Hot Stage w i t h Insulating Corer R e m o i e d
Description of Hot Stage
The hot stage (Figure 1) is iiiade from a solid block of brass, 50 X S i X 13 mni. The thermometer hole; A , -1 are centered 16 mm. from the ends of the block and are 9 nini. in diameter. The holes B, B are centered 29 nim. from the ends of the stage. Each 6-mni. hole is snugly fitted v i t h a re-istance coil made by n-rapping 40 cin. of S o . 24 chrome1 rekistance wire around a thin rod and encasing in a Pyres tube of 6 mm. external diameter. Each wire is led through a hole, I , insulated with porcelain thermocouple tubing to a binding post, G. The binding posts and screws must be insulated from the brass mounting strips, H, H , with liquid porcelain and asbestos. The mounting strips are screwed to the btage through the transite strips, $1,, I . The coils can be heated separately in case there is a variation in their resistance, or in series provided resistances are equal. The 110volt alternating current is varied by inserting a lamp bank and a slide-wire rheostat. The 9-mm. hole, C, is bored to accommodate a micro-electrolytic cell, but is plugged from both sides when melting points are taken. The bottom of the stage is covered with a Received February 10, 1930 Contribution 188 from the Department of Chemistry, Uni%ersityof Pittsburgh 1
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1 Figure 3-HOt
Stage i n Use
Before taking any actual melting points, however, it was found necessary to calibrate the stage by taking melting points of known compounds of high degree of purity and to compare the observed melting point with that of the pure compound. Such a calibration from 60" to 350" C. in the caqe of the authors' instrument gare a constant correction of $2.8" C. It seems highly probable, therefore, that any instrument constructed as described above could be readily calibrated by taking the melting point of any known pure compound and regarding the correction thus obtained as constant for all melting-point temperatures within the range of the hot stage.
AiVA LY TICS L EDITION
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K h e n the temperature of the Ftage is within 10 degrees of the approximate melting point as first determined, the current from the lamp bank is so regulated by the slide-wire rheostat that the temperature rises at the rate of 1 degree per minute until the melting point has been determined. If a camera lucida and magnifying lens are employed, a n enlarged image of the thermometer scale can be projected alongside the image of the crystal fragment under observation, so that the melting point and temperature can be noted simultaneously. When the crystal is anisotropic, polarized light can be used with excellent results. Under crossed Kicols the brilliant polarization colors which are first obtained disappear mhen the crystal melts. I n this way the melting point of the crystal is determined without the inaccuracies due to errors in observation.
Vol. 2, No. 3
The maximum 0r:ected temperature obtainable with a current of 2.80 amper?s was 1%" C.; with a current of 3.28 amperes, 181" C.; with a current of 4.02 amperes, 238" C.; II-ith a current of 5.30 ampnreq, 330" C.; while with a current of 6.08 amperes the te liperature rose abol-e 360" C. I n addition to taking melting points, the hot stage can lie employed to determin" the molccular weight of camphorsoluble substances by a modification of the Rast method ( 3 ) . IJ-ork is noiT- being done on the application of the hot stage to micro-electro ytic reactions, the rewlts of n hicli nil1 be reported in a lat'r paper. L terature Cited (1) Cram, J A m Ckem S o c , 34, 954 (1912) ( 2 ) Niethamrner M z k r o c h s m e , X e u e F o k e 1, 223 (1Q29) (3) Rast, Ber , 55, 1051 (1922)
Providing for Changes of Temperature in Volumetric Analysis' M . G. hlellon PURDUE UNIVERSITY, LAFAYETTE, IND.
KE source of error encountered in volumetric methods
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of analysis is dependent upon changes of temperature in the laboratory. Such changes produce two significant effects. The usual volumetric ware, made of glass, expands with rise of temperature; and the solutions measured in this mare are subject to a similar change over the ordinary working range of temperature. Unfortunately the two changes, the capacity of the glass containers and the volume of the solutions, do not compensate each other. The coefficient of cubical expansion for glass differs from those for solutions in being very much less and in being linear for the range of temperature involved. I n order to comprehend the significance of these changes in their effect on the precision of volumetric determinations, one should consider both the conditions of temperature likely to prevail in chemical laboratories and the analytical practices followed therein. Careful workers calibrate their measuring apparatus (flasks, pipets, and burets) preferably on the basis of the true liter and for our standard temperature of +20" C. One n-onders if all too frequently t h e analyst does not then proceed complacently, applying corrections for defective graduations, but neglecting corrections for changes in the temperature of the solutions being measured. A consideration of some of the possibilities in this direction indicates that the latter correction may be the more significant of the two. Suppose, for example, that a standard solution is prepared on a cold morning when the temperature easily may be 15' C. Many individuals simply dilute the solution to the graduation mark, if a volumetric flask is being used (which is correct only at 20" C., if calibrated as indicated above) and think nothing about the temperature. If the solution is fairly permanent, later it may be used on a hot afternoon in midsummer when the temperature frequently exceeds 30" C. Again the calibration mark for 20" C. (since i t is the only one available) is used in t h e measurements. If one assumes an average change in volume of 0.02 per cent per degree, such a fluctuation means a total error of 0.3 per cent for changes of temperature alone. Although this case might be unusual, it is believed that differences of 10 degrees are not uncommon, and that precise 1
Received M a r c h 20, 1930.
work necessitates a correction if the working temperature differs much from that for TT-hich the measuring vessels were calibrated. Fales (3) states that "If we aim a t a precision of 0.1 per cent in our work our volumetric measurementsmust all be made within a compass of 2" including the standard temperature." Yet in research papers mention is rarely made of such corrections. Correction of Error
Probably the best suggestion for making titrametric analyses of high precision is to use weight burets rather than to rely on measurements of volume. Washburn (18)pointed out the merits of this procedure, and Friedman and LaMer (6) recently proposed a n improved buret. The chief objection to the method is the time required,for making by weight all measurements on the solutions, both in their standardization and use. A second proposal is to perform the analysis volumetrically and then to apply corrections, a procedure recommended a t various times in the past. I n doing this i t is necessary to know the temperature of the solution a t the time of its preparation and also the temperature of all solutions involved in the analysis, measurements on which enter into the calculation of the results of the determination. One also needs to know the change in volume of the solution resulting from a deviation from the temperature of calibration. For making the calculation of the correction, the simplest recommendation (5) is to neglect the effect of change of temperature on the capacity of the glass container and to assume that the effect on the solution being measured is the same as on mater alone. One then uses only the values for the density of water a t the temperatures concerned. I n order to provide for the effect on the container also, tables of correction factors have been calculated (1, 10, 14) from the coefficients of cubical cxpansion of water and glass. To convert measurements for any given temperature to an equivalent basis for 20" C, one needs only the appropriate factors, keeping in mind that the ones mentioned do not apply to solutions more concentrated than 0.2 normal. Unfortunately this simple basis of making correction. IS not strictly applicable, since the coefficient of cubical expan-
