An Automatic Melting Point Record€ LUTHER
F. BERHENKE
Edgar C. Briffon Research Loboratory, The Daw Chemical Co., Midland,
b Automatic recording of the movement of a smo.11 thermocouple piston, supported b y the unmelted solid in a capillary as a function of temperature is the basis of an automatic melting point recorder. It is fast and simple to operate, requires only milligrams of sample, and does not require critic01 control of heating rates. The recorded results compare favorably with those from subjective observotion.
A
N INSTRUMENT to determine the
melting characteristics of a compound objectively and to produce a written record would be most useful. Following Dubosc's (1) apparatus to give more objectivity to such determinations, a number of others have been described (8-6). Of these, the most automatic give only a single temperature, while those giving more information require more attention. Stull (7) described an automatic freezing point recorder which gives objective results, but requires larger samples. Recently Ueberrciter and Orthmann (8) described an instrument, which, using a two-point recorder, automatically gives a combined temperaturesample condition (light transmittance) record and requires only small samples. The present instrument is another solution. DESIGN REQUIREMENTS
The design for such an instrument should provide simplicity of operation with little need for attention during the determination, applicability to milligram amounts of sample, production of a permanent record, and results
Mich.
compatible if not identical with ac cepted methods. TENTATIVE SOLUTIONS
Any scheme which would measure and record sample condition-solid partially melted, or liquid-as a func. tion of temperature should be useful The piston-capillary scheme of Stock and Fill (6) seemed most promising They put a loose-fitting weighed pistor on top of the sample in a capillary tube The motion of the falling piston, sup. ported only by the unmelted solid, could be recorded easily as a function of temperature. It wm chosen foi development. EXPLORATORY MODELS
Because of its simplicity and very nominal cost, the first model (Figure 1) is worth description. The bimetal helix from a CencoDeKhotinsky thermoregulator was the temperatureaensing element. A salve can, 4 inches in diameter, mounted on the shaft carried a strip of graph paper on its periphery. The bimetal and the capillary were immersed (with a thermometer for calibration) in a stirred oil bath. As the temperature changed, the can rotated, carrying the paper past a pen linked to the piston in the capillary.
Sections ahout 1.25 inches wide cut from a regular 0" to 300" C. strip chart, when wrapped around the drum, give a printed, precalihrated temperature scale for the permanent record. One end of the paper strip is anchored on two index pins, while the other is fastened under a brass spring clip. The entire drum is rotated on the shaft until the pen gives the correct temperature reading, then fixed to the shaft. The pen arm, B, and piston arm, C, each about 8 inches long, are mounted on a common shaft, one solidly, the other through a spring-loaded friction clutch so the relative angular positions of the two arms can be adjusted. The shaft itself is mounted in conical pivot bearings. The melting block, D, is a solid copper cylinder, 2.125 inches long by 0.375 inch in diameter with a 0.094 X 2 inch hole drilled down the axis. This block is set in the heater. E. a 100-watt, 110-volt, replacement heate; element for a soldering iron (American Beauty, No. 9273) [with appropriate thermal insulation], The heater current is controlled by a switch, F , and a small variable transformer, G, with the heater current fused for safety. A pilot light indicates when the heater current is on. Heating rate curves established for several settings of the variable transformer make it possible to select appropriate heating rates near the expected melting point. The heating rate is not usually critical, however.
Simple as it was, the system worked; however, the temperature deflection of the bimetal was small (100' = 60 mm.), making it difficult to estimate temperatures to less than 2". A better temperature-sensing device was needed.
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FINAL MODEL
In the final model, a 0' to 300' C. Minneapolis-Honeywell Brown ElectroniK recorder and thermocouple was used as the temperature sensor, modified as detailed below and shown in Figure 2, with certain components mounted on the removable chart backing plate.
Figure 1. Experimental melting point recorder
model
of
Forrimplidty,thevoriov* mountings ore nolshown
The pointer was removed and the chart drive replaced with a bushing to support the front end of the pointer shaft. The chart drum, A , 3.93 inches in diameter, with a 1.25-inch face, was then mounted on the pointer shaft. This 3.93-inch size was calculated so that the 11.0-inch width of a standard Brown strip chart spans exactly the 320" full scale rotation of the drum.
Figure 2.
Final model of recorder
V O L 33, NO. 1 , JANUARY 1961
65
The entire heater assembly is mounted on a swing-out arm (from a wall can opener). so it can be swung back and the recorder case closed. (The recorder assembly was moved back into the case about 1 inch to provide the necessary clearance.) The capillary, H , is standard, 2 inm. in diameter bv 90 mm. long. The temperature is m&ired by an iron-constantan thermocouple, I , enclosed in a stainless steel sheath 0.040 inch in diameter and 6 inches long (Thermocouple Products Co., Inc., Villa Park, Ill.). This serves also as the piston to bear 011 the sample. It hangs freely from the piston arm to enter the capillary n-ith a niinimuin of friction. The couple and connector block weigh about 30 grams. The sample is therefore subjected to a pressure of approximately 50 p.s.i. The estimated effect oii the melting point is less than 0.2" C. The couple itself is a t the very tip of the sheath, eo the temperature measured is that of the solid in contact with its melt. (An earlier d&gn had a plain n-ire piston of the same diameter and an ordinary thermocouple in an adjacent hole in th(J melting block. This is less desirable because it measures the teniperaturc of the block, but this arrangement works fairly n-cll and could be used n-itli a glass piston rod if the samples n-crf too corroqive for use nith nic t a 1s.) OPERATION
The paper 1s attached to tlie chart drum, and the capillary loaded to a depth of 1.5 to 2 m. and placed in the melting point blocak. The thermocouple piston is inscrtcd in tlie caapillary, the iiikcd peii is rr3lcascd to rest on the chart, the hc.nt 10 turned on, and the heating ratels adjustrd the desired ~ a l u e . K h c n th(1 sample i i mc,ltcd. the heat is turned off, tlic tapillarj and chart are remo~.ctl,mid th(, piston i\ clranctl. TYPICAL RESULTS
Figure 3 shows typical melting curves run on the recorder. The results of separate or simultaneous visual determinations are tabulated for comparison. Curve A s h o w the results on five samples commercially offered for melting point thermometer calibration. The capillary values noted are those supplied with the samples. The other records ere made on samples availablc in the laboratory. Curves F and G are from an impure sample of H containing about 5 w i g h t %, of impurity. Curves C and G n-cre run with heating rates of 8" per minute for comparison with B and F run a t 2" pcr minute. Curve D was run on benzoic acid with an earlier design having a plain nire piston and nith the thermocouple in an adjacent hole in the melting block. Curves E and I n r r e run nith a special setup n hich permitted visual 66
b
ANALYTICAL CHEMISTRY
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Figure 3. Curve A I
II 111 IV
V B C
D E F G H I
1'
Typical melting point records
Compound Vanillin Acetphenetidine Sulfanilamide Sulfapyridine Caffeine Benzoic acid (2'/min.) Benzoic acid (8'/min.) Benzoic acid (outside thermocouple) Salicylic acid Impure 5-phenylsalicylic acid (2'/min.) Impure 5-phenylralicylic acid (8'/min.) 5-Phenylralicylic acid 98% salicylic acid-2% benzoic acid
observation of the sample during the run. INTERPRETATION OF RESULTS
As with aiij automatic recording instrument. the intt.rpretation of the record and correlation n ith the results of other techniques or iiicdiods must hc left to the chemist. Generally, the filial tciiiperature on the recorder corresponds most closely to the initial mclting point (meniscus) in a capillary or to the frcezing point of a larger sample. The purer the saniple, thc closer the agrecment. The effect of impuritics in the saniplc is well illustrated by curves F , G, and I , n hich show early shrinking, nider melting range, and lower final temperature. This nil1 occur, of course, only in systems where the several components form eutectics. The relative independence of heating rate can be seen by comparing curves C and G, run at 8" per minute, with
Visual Result, 'C. Hot staie Capillary 81-3 134-6 164.5-5.5 190-3 235-7.5 121-3 1 23.5-4.5
158-1 6 0 21 0-1 3 216-17 155-1 5 8
214-16 21 6 . 5 - 1 8
B aiicl F ruii at 2" per iiiiiiutc'. I t is evident that the thermocouple piston accurately incasures the temperature of the sample, even though the bath may be several dcgrees hotter. This is not true of tho esteriial tlirwnocouple arrangement, such as uscd to produce curve D. Kith don- heating, this could be usable in cases n-here corro-ion problcms required 3 glass piston. To correlate the instrunii,iit results bettcx n-ith thosc from visual capillary detrrminatioris, the niclting block was replaced by a stirred oil bath with a tlicrniometer. This allowed visual observation of thr sample during the detrrniinatioiis n-hich gave curvt's E and I . The t,eiiiperatures are iiiarked a t IJ-Iiicli the observed point,>--netting of the n-all, the meniscus, and all nieltcd-occurred. The thermometer and recorder temperatures agreed very closely during the heating up t o the iiieiiiscus point. During the rest of the melting, the recorder tcniperature
Iaggrtl l~f~hintl the bath, until a t the final nicltiiig point the bath temperature was 1' to 2' higher t'han the recorder. Once thc *ample was all melted, thc recortlcr teniperature rose rapidly and caught up n.ith the bath. This is furth(,r (+tlcnce that this system give's :L fairly wliable record of the sample tempc~r:xtiiri~ during fusion. llcasurcment of the piston t r a w l a t the sewr:iI points indicates that. roughly, 1 0 5 travel corresponds to the "wetting" point and 2;Yc travel t'o the nirniscus point. This may not be true for snniples such as =1 I-. which eshihi t odc I-sliaped curves. Thc~ t,eniperature accuracy of the nicthotl is cssentially that of t l i p recorder (=0.5'). The shape of tlic curve tlrnwn is iiiflueiicetl to sonic' CI-
tent by the packing of the sample, but the results with different operators agree very well, Principal variation nil1 be in the interpolated netting and meniscus points, more with impure than pure samples. The melting point is not so influenced. For a single operator, the results are identical within the limitations of the recorder itself. Because the instrument measures only volume changes, subtle changes such as polymorphic transformations, desolvation, etc., without appreciable volume change nil1 not he recorded. Hon-ever, shrinkages, often unnoticed in a capillary, show up well. The instrument has proved very useful in providing objective data on the melting bchavior of man:- samples,
particularly from repeated preparations of a compound. LITERATURE CITED
(1) Dubosc, -4.,Mat. grasses 6 , 3308-9 (1913); Reo. prod. chim. 28, 115 (1928). (2) Furst, A., Shapiro, J. J., ANAL.CHEM. 26, 1082-5 (1954). (3) MacAIullin, R. B., J . Am. Chem. SOC. 48, 439-43 (1926). (4) Monand, P., Bitll. soc. chini. France 1955. 1601-2. ( 5 ) Paimer, H. F., Kallace, G. H., J . A m . Chem. Soc. 48, 2230-2 (1926). (6) Stock, J. T., Fill, AI. A., Anal. Chim. Acta 2, 282 (1918). ( 7 ) SttIll, D. R., ISD. EXG.CHEX.. ANAL. ED. 18, 234-42 (1946). (8) Ueberreitrr, K , Orthmann. H. J., Kitnststofe 48, 525 (1958). RECEILEDfor review >Iay 2.3, 1960. Accepted A h g u s t 22, 1960. Division of Analytical Chemistry, 138th Meeting, .4CS, S e i r Tork, S . T., September 1960.
Quantitative Determination of Low Atomic Number Elements Using Intensity Ratio of Coherent to Incoherent Scattering of X-Rays Determination of Hydrogen and Carbon C. W. DWIGGINS, Jr. Petroleum Research Center, Bureau o f Mines, U . S. Department o f the Interior, Bartlesville, Okla.
b A new method based on the intensify ratio of coherent to incoherent scattering of x-rays has been developed for the determination of low atomic number elements. The method has been used to determine hydrogen and carbon in hydrocarbons, particularly petroleum, and in a matrix containing additional elements. Corrections for the sulfur and nitrogen content have been developed. Carbon and hydrogen may be determined in quadruplicate in 20 minutes or less. Conventional x-ray spectrographic equipment may be used without modification. The precision and accuracy of the method appear to equal or surpass those of conventional microcombustion methods for many types of samples.
A
h t h cohiwnt (Raylcigh or uiimotiifitd) arid incoherent (C'oin~iton or niotiificd) scattering of x-rays h : i ~ ( >been long known. fenrI
