1082
ANALYTICAL CHEMISTRY
method such as that described by Frear (8). Solutionswhiohare strongly acid or alkaline should be neutralized, and any interfering ions present should he removed or complexed. To 1 ml. of the test solution in a 12-ml. centrifuge tube, add 9 ml. of the acetic acid solution. With a mall measuring scoop add 0.3 to 0.5 gram of the powder reagent, stopper, shake far 50 to 60 seconds, and centrifuge until the supernatant liquid is elem (about 4 or 5 minutes a t 4000 r.p.m.). ,Decant the white film on top and pour the clear red solution into an absorption cell. Trausmittancy mrty be read conveniently in a filter photometer with a green filter or in a spectrophotometer at 520 m r . Convert trsnsmittmcy readings to amounts of nitrogen with it standard curve established with known solutions. The standard eume should cover the range from 0.2 to 1.0 p.p,m. in solution in the absorption cell, or 2 to 10 p.p.m. in the sample solution.
to close the tubes during shaking must he washed cmefully. As reported by Warington (9),solutions and reagents undulyexposed to laboratory air may pick up small rtmounts of nitrite. Distilled water must be checked frequently for contamination. LITERATURE CITED
(1) Bray, R. H.. Soil Sci., 60,219 (1945). (2) Frear, D.E., Plant Physiol.. 5, 359 (1930). (3) Griess, Peter, Rer. deut. chem. Ges., 12,426 (1879). (4) Holbourn, A.H. S., and Pattle. It. E., J . Lab. C i h . Med.. 28, 1025 (1943). ( 5 ) Ilosvay, M. L.. Bull. w e . ehim. Paris, 2. 347 (1889) Itt, W. G., Analyst, 63, 655 (1938)
PRECAUTIONS
All glassware, table taps, etc., must be kept SONPU~OUS~Yclean. Reagents should be nitrite- and nitrate-free. X-ray grade barium sulfate is satisfactory if thoroughly dried. Rubber stoppers used
Device for Measuring Change!i of Optical ... I ranemittanrra wttn Iamnaratsre
I
..I
I
ARTHUR FURST, Department of Pharmacology and Therapeutics, Stanford University School of Medicine, San Francisco, Calif., and
JUSTIN J. SHAPIRO. American instrument CO, Silver SI
T.
HE reladive purity i f a compound is invariably associated with a physical constant. A melting point or melting range is customarily used for solids and this value is most oft,en drtwmined by the capillary tube technique. The melting point. oht.ained by this method is not thermodynmieally defined, for it is neither the liquid-solid equilibrium temperature under constant atmospheric pressure nor the triple point, However, it is extremely convenient and universally employed. Reproducible results are obtained and no difficulties are encountered if the sample to he tested is relatively pure, of definite composition, and actually melts below 230" C. Frequent,ly samples are encountered whose melting point cannot be determined reproducibly by the capillary tube method. These solids e m be classified in one or more of the following catogories: hydrates; high melting-Le., above 250' C.; compounds which decompose before or a t their melting point; amorphous waxes, asphalts, or plastics; and vegetable oils or fats, In each of these categories special techniques must bo used. In besting these chemicals, operators may get results that differ as much &R several degrees from each othel'. Morton (8) lists the factors that affeot melting point values. Included among these are rate of heating, thickness of t,hc capillary wall, size of the sample, and stem corrections. If capillary tube technique is employed for t.hese special samples, different manipulative techniques BPP necessary. For compounds which decompose or melt above 250" C. the capillary tube must be either submerged when the temperature of the bath is just a few degrees below t,he melting value, or a6 an slternative, the capillary must he heated at a mtc of 10' to 20" C. per minute, constantly, rather than slowing to thc usually prescribed 1' to 2" C. per minute near the melting point. A ball and ring test ( I ) is often employed to te8t asphalts. Fats present specid problems; an allotropic form may result aftcr the sample is melted and drawn up into the eitpillmtry tube (6); hence the filled tube must be left in the icebox over night. Variations to avoid the capillary tube method were cmployed by Dennis and Shelton (e), who introduced the copper bar, and by Johns (4),who used an aluminum block upon which the sample, placed between two watch glasses, was heated. .4ttcmpts have been made to avoid the human equation by
making this determination automatic. Perhaps the first of thcec w m made by Dubosc (3) and a lstcr one by Wick and Barchfdd (11). At this time true automatic equipment was not available. Kardos ( 6 ) used a Kofler (7) apparatus and measured the melting point by attaching a recordor to a photoelectric cell and noting the change in current in milliitmporees. More recently U.liillcr and Zenchelsky (9) made a fully automatic instrument based upon the sudden increase of rcflceted light from the surface of t.he melted sample to a photoclnetrie eel!. This instrument has becn :able t o attain a precision of +0.3" C. The meltomoter ( I O ) here described reduces to fined and reproducihle values the vnrinhles contributing to the malting point
Figure 1. Meltometer
V O L U M E 2 6 , N O . 6, J U ' N E 1 9 5 4
1083 light through a heated sample to signal that a change of phase has occurred in the sample. This intelligence may be recorded with sample temperature to permit continuous study of the phaw change with temperature or it may be used to lock the pyrometer indicator to permit a fixed value of temperature to he indicated for a particular change in optical transmittance. In the latter operation the instrument is completely automatic. DESCRIPTION OF INSTRUMEXT
Figure 2.
Schematic Diagram of Meltometer
MELT BLOC
The meltometer is self-contained, consisting of a microfurnace attached to a cabinet (Figures 1 and 2). Microfurnace. The microfurnace, which can be tilted, housee the ptabilized light source, the melting point block, filters, shutter, and phototube (Figure 3). The ccver can be removed readily for the purpose of placing the sample (sandwiched between two thin microscope cover slips) into the gold-plated brass block. The heater coil is embedded within the block, and a ChromelConfitantan thermocouple is located on the inner surface of the block adjacent to and in thermal symmetry with the sample. .4n aperture in the block permits a beam from the light source to pass through the sample and to the phototube. The small volume of the melting point block, 0.166 cubic inch, permits rapid approarh to equilibrium temperature at heater currents selected by the heater control knob. A rate of rise of temperature of 0.5" C. per minute a t any temperature within the range of the instrument is reached within 15 minutes from the time the heater is turned on. Cabinet. The cabinet encloses the meters, power supply, and amplifier (Figure 4). The optical transmittance of the sample is indicated on the meter relay and the temperature of the sample is indicated on the locking-type pyrometer. The automatic action of the meltometer is initiated by a pair of relay contacts in the optical transmittance indicating meter relay. The position of the contacts may he manually adjusted by means of knurled knobs projecting through the meter relay window. One is an upper limit and the other a lower limit optical transmittance contact. \\ hen the pointer of the meter relay touches either contact, three actions occur at once: (1) The pyrometer indicator is locked hy electrostatic action a t the temperature indicated a t that instant and remains at that indication until released manually: thuq, no overshoot is noted; (2) the heater is disconnected from the POA er supply and cools rapidly, readying the melting point blork for another determination; and (3) an indicator lamp signals the operator that the determination is completed. OPERATION OF INSTRUMENT
LAMP NO 6S6
Figure 3.
Diagram of 3Iicrofurnace
( ' ~ I Y J ~ S , Thepe variables are sample sin, a n d pate of rise of temperature. The nic,ltonieter is fully automatic, portable, :iiid extremely versatile. I t can be used to measure melting points, decomposition poiritr, softening points, melting ranges, :inti :trbitrary melting points as represcxiitd by amorphous waxes, asphalts, :tnd plastics. Since samples can bc run ( J W ~ again, it is possible to determine iniwtl melting points and allotropir fornis. Studies of eutectic mixtures can be made, as well as molecular weight dtatcrminations. K i t h thc recorder the following invwt igations are possible: opticaI tramniitt.nnce against time and temperatures, wl)limation rate with temperatures, tc.mperature-spectra1 absorpt,ion, and t(,niperature-birefringence. The meltometer utilizes the increase or dtweasr in the transmittance of unfiltc~rod! polarized, or monochromatic.
The meter relay is adjusted a t the start to indicate zero transmittance ( - 5 0 ) and lOOyotransmittance (+50) by means of the zero adjust knob and the adjustable light shutter, respectively. The sample is introduced between t n o clean rover glasses t)?.
l l I1
Figure 4.
Wiring Diagram of Xleltometer
A N A*LY T I C A L C H E M I S T R Y
1084
Table I. Comparison of Meltometer Values with Known Melting Points Compound Naphthalene Naphthalene Benzoic acid Benzoic acid p-Nitrobenzoic acid p-Nitrobenzoic acid Succinic acid
Ambient
% Trans. 15 7 7 10
.,
% Change in Transmittance of Amb. Trans.
Rate ,of Temp. Rise, C./hlin. 2 3 5 5
Known Melting Pt.. C.
.Meltometer >felting Pt..
c.
1
2 2
,,
..
means of a dentist's amalgam carrier, which was found most suitable for transferring fixed small amounts of powdered materials, and placed in the block. The cover is closed and the optical transmittance of the sample is now indicated on the meter relay. This value is noted so that in repeated determinations the sample size (transmittance) may be duplicated, The sensitivity is now adjusted by opening (or closing) the adjustable shutter until the meter relay indicates zero center, The manual setting of the meter relay contacts may now easily be determined.
If the behavior of the sample is known, the heater control may be set for a suitable rate of rise and the meter relay contact may be set to close at a specific per cent change in transmittance from ambient transmittance. Unknowns may be run a t a high rate of rise with contacts placed out of range of scale to determine a suitable rate of rise and a suitable setting for the change-intransmittance contacts. One electrometer tube is used to amplify the phototube current. A melting range may be found by first setting the meter relay contact a t about +20. This permits the instrument to shut off a t the first indication of change in light transmittance. The tem-
perature reading obtained will correspond to the lower value of the range. Without disturbing the sample, the meter relay contact can now be set a t $50 and the heater turned on again. The second temperature reading obtained will be the upper value of the range. The reproducibility and accuracy of the instrument were determined by testing samples of known and unknown melting points. Crystalline Compounds. Table I gives a comparison of hand book values and meltometer values over a range of 160' C. The time-temperature-optical transmittance behavior of naphthalene, a sharp-melting compound, is shown in Figure 5. Each value of Table I1 was obtained by a different operator who had no instructions other than those supplied with the instrument. Nonmelting Crystalline Solids. As the sample decomposed, the percentage transmittance decreased and the indicating needle approached 0% transmittance ( -50). Meltometer values for &methionine were darkening a t 276" and decomposing a t 287"C. (literature values are 278" and 283' C., respectively). Amorphous Solids. Asphalt softening point value could be ob-
Table 11. Automatic Melting Point Determinations Compound
Ambient
% Trans.
Rate of Temp. Rise, ' C./Min.
% Change in Transmittance of Amb. Trans.
Known Melting P t . ,
220 220 220 220
114 114.5
220 220
206
207 207
198a
198 199 197 199 197 197 199 198 195 196 197 199 197 200 200
Acetanilide
8-Aminocaproic acid IJpjohn I
13 19
8 8 8 8 8 8 9 10 12 12 12 12 13 23 25
4 8 0 5 0 5 0 5 0 5 0.5 0 5 0.5 0.5 0 5
...
... ...
... ,..
...
...
E 0 5 0.5 0 5 0.5
C.
..,
...
'
, . .
...
Meltomettr Llelting Pt., C. 114.5 114.5 115 114.5
Upjohn I1
8
0 5
220
Upjohn I11
9 13 16 25 29
0 5 0 5 0 5 0 5 0.5
220 220 220 220 220
195.5a
Upjohn V
8 13
0.5 0.5
230 220
195'
194 196
Upjohn VI
7 9 11 12 14 19
0.5 0.5 0.5
220 220 220 220 220 220
175.5O
173 173 173
0
0.5
0.5 0.5
Capillary method, Upjohn Co., Kalamazoo, 'Mioh.
... , . .
... ...
.... . .
.. ... ... ...
194 196 196 201 202
174
175 175
s
1085
V O L U M E 2 6 , N O . 6, J U N E 1 9 5 4
750
1
ONE MINUTE INTERVALS BETWEEN MARKERS.
"i
t-
i1
,
1
1 20
30
MELTING POINT 80.2
,/
START,
I 40
50
60
70
80
1
were taken as the temperature of the block was raised and lowered between ambient temperature and 290' C. a t a rate of change of 2 " C. per minute. The maximum temperature differential was 1' a t 290' C. with no perceptible differential below 206" C. The reproducibility and accuracy of the instrument were found t o be i l % of scale. The original meltonieter had a range of 300' C., but it was soon apparent that other ranges and greater accuracies were required for some applications. Accordingly, the Illinois Testing Laboratories supplied for further tests pyrometer ranges as follows: 0-lOO', 0-150', 0-300", 0-500', 105-210", and 205-310" C. Accuracies of 0.05' C. with ranges of 15" C. may be obtained by the use of recorders, as supplied by the Brown Instrument Co., division of Minneapolis Honey~vellRegulator
co.
1
90
TEMPERATURE IN DEGREES CENTIGRADE
HEATER COIL
nn
\
Figure 5. Optical Transmittance-TemperatureTime Record of Compound with Sharp Melting Point Melting and solidification points are clearly shown
tained by placing the sample between cover slips just above the aperture. The head was tilted and a t the softening point the sample visrofiity suddenly decreased, filled the apace, and thus shut off the light beam. Values were reproducible to & 1' C. No comparisons with ball and ring values were available. The softening point of fats could also be determined within 1' C. with the apparatus, but no consistent results could be obtained by the capillary method, as the majority of fats would supercool and would not set to a crystalline solid, once melted. Plastics likewise could not be compared, but Figure 6 illustrates the behavior of a resin which discolored near its melting point. The curve clearly shows the phenomenon which might pass unobserved with ordinary methods. 1200
I
Figure 7 .
2 Holes
-n , L l
Diagram of 3lelting Point Block
Temperature-optical transmittance studies conveniently reveal the identifying phase rhanges of most compounds, and, in addition, provide information most usc,ful to the physicist and the chemist. ACKNOWLEDGMEYT
The authors wish to express their appreciation to C. P. Saylor, E. J. Prosen, and L. Barbrow of the Sational Bureau of Standards, W. J. Svirbely of the University of Maryland, and K. J.
I
I
-4 b,. 2,
ONE MINUTE INTERVALS BETWEEN MARKERS
100 o r
Mysels of the University of Southern California for their many helpful suggestions. LITERATURE CITED
I
50
75
IO0
125
I50
,
175
200
225
TEMPERATURE IN DEGREES CENTIGRADE
Figure 6. Optical Transmittance-TemperatureTime Record of Resin Decomposing near the Melting Temperature Softening point i s clearly shown
The periphery of samples melted first, indicating a negative temperature gradient from the heater coil t o the central aperture. To determine the symmetry of this gradient, thermocouples were placed on opposite sides of the aperture (test positions A and B in Figure 7 ) a t the distance which gave closest agreement between known melting points and melting points measured with the instrument. The thermocouples were made of No. 30 copper and constantan wires and were connected to a k e d s and Sorthrup Type K2 potentiometer through a selector switch. Readings
American Society for Testing Materials, "ASTll Standards on Petroleum Products and Lubricants," ASTM Committee D-L, Philadelphia, Pa., ASTM Designation E2839T, 1952. Dennis, L. RI., and Shelton, R. S., J . Am. Chem. Soc., 52, 3128 (1930). Dubosc, A, cited in AIorton, H. d.,"Laboratory Technique in Organic Chemistry," P. 47, Sew York, McGraw-Hill Book Co., 1938. Fisher Scientific Co., Silver Spring. Md., Laboratory, 23, No, 3 (1953). Jamieson, G. S.,"Vegetable Fats and Oils," 2nd ed., p. 380, Yew York, Keinhold Publishing Corp., 1943. Kardos, F., ANAL.CHEM.,22, 1569 (1950). Kofler, L., and Kofler, L., as cited in Weissberger, A . , "Physical Methods of Organic Chemistry," Vol. I, p. 445, Iiew York, Interscience Publishers, 1945. Morton, H. A., "Laboratory Technique in Organic Chemistry," p. 22, New York, hIcGraw-Hill Book Co., 1938. hliiller, R. H., and Zenchelsky, S. T . , A N . ~ LCHEM., . 24, 844 (1952). Shapiro, J. J. (to hmerican Instrument Co., Inc.), U. S. Patent 2,669,863 (Feb. 23, 1954). Wick, G., and Barchfeld, G., as cited in Morton, H. A., "Laboratory Technique in Organic Chemistry," p. 47, New York, McGraw-Hill Book Co., 1938. RECIEYED f o r review M a y
4. 1933. .iccepted February 2 5 , 1954.
