Measurement of Calibration Standards for Thermometry Vincent G. Colarusso and Michael A. Semon Fisher Scientific Co., Fair Lawn, N.J. An equilibrium can be established at a specific temperature and pressure between the three phases (solid, liquid, vapor) of a substance. Accurate measurement of this triple point by the method described for each of the selected materials can provide readily available, precisely defined reference temperatures by which calibration of thermometers or other temperature measurement devices may be standardized. Utilization of these materials permits the calibration of the temperature sensor under the same conditions of measurement employed during the actual analysis or test. The method described permits measurement of the triple point with a minimum accuracy of ~ 0 . 0 5“C and a precision of =t0.025 “C. T o OBTAIN the maximum accuracy afforded by a temperature measuring device, the calibrating procedure should be conducted under similar, if not identical, conditions of the analytical measurement being made. Standard materials with accurately determined solid-liquid phase transition temperatures can be utilized in this manner for calibration in many analytical procedures. The temperature at which a substance undergoes the transition from solid to liquid is expressed as its melting point, freezing point, or triple point, depending on the conditions of measurement. The melting point or freezing point is the temperature at which the solid and liquid phases are in equilibrium under normal atmospheric pressure ( I , 2). The triple point is the temperature at which solid, liquid, and vapor (gas) phases of a substance are in equilibrium (1,Z). Measurement of the triple point temperature was selected for evaluation of the materials investigated because this temperature could be easily defined and reproducibly measured. The closed, evacuated system eliminates the possible effects of dissolved atmospheric gases (including water vapor) on the precision of the measurement. The gases which tend to dissolve in the liquid phases of an open system may not be completely expelled on freezing. To assure reproducibility of measurement conditions in an open system would be more difficult. Pressure effects on the solid-liquid transition temperature are minimal, in the order of a few hundredths of a degree Celsius (1-3). Because of the lower pressure, the triple point may be slightly lower than the corresponding melting or freezing point. For measurement accuracies within +0.05 “C, however, melting point, freezing point, and triple point temperatures may be used interchangeably ( I ) . The method employed by the authors is based on the thermometric technique and cell design described by Schwab and Wichers in their work with benzoic acid (4). Enagonio, Pearson, and Saylor reported basically the same technique (1) “International Critical Tables,” Vol. IV, 1st ed., McGraw-Hill, New York, N.Y., 1928, p 6 . (2) A. Weissberger, “Physical Methods of Organic Chemistry,” Vol. I, Interscience, New York, N. Y., 1945, pp 1-2. (3) D. Enagonio, Separation and Purification Section, Analytical Chemistry Division, National Bureau of Standards, Washington, D.C., personal communication, 1967. (4) F. W. Schwab and E. Wicherss, J . Res. Nut. Bur. Stand., 34, 3 (1945).
-I
Pt Thermometer
1
Recorder Servo-R iter II
I isolating cabinet
Figure 1. Triple point temperature measuring system for phenol, naphthalene, and phthalic anhydride; but used cells of smaller dimensions (5). Several organic compounds were evaluated by this method as calibration standards to provide reference temperatures at approximately 30” intervals over the range of 50 to 300 “C. EXPERIMENTAL
Apparatus. The system employed for determining the triple point temperature is shown in Figure 1. The body of the thermometric cell is 45-mm o.d., heavy wall glass tubing and is 25.0 cm in length. The thermometer well is 11-mm o.d., standard wall glass tubing. A 2-mm stopcock and capped filling port (TjS 19/22 joint) are incorporated to facilitate preparation and reuse of the cell. Temperature measuring instrumentation is comprised of a standard platinum resistance thermometer (L & N 8163-C), Model G-2 Mueller Bridge (L & N 8069-B), and a dc microvolt indicating amplifier (L & N 9835-B). The amplifier is connected to a Texas Instruments Servo-Riter I1 recorder to obtain the equilibrium temperature curve. A Dewar flask, 65-mm i.d. and 250-mm deep (SGA #JF2450), is used to insulate the thermometric cell during measurement. Cells are heated in a forced draft oven (Fisher #l3-244-2) that was modified with additional heaters to permit operation to 300 “C. Procedure. PREPARATION OF CELLS. Three thermometric cells are prepared for each compound evaluated. The cells are thoroughly cleaned and then each is charged with approximately 350 gm of sample. The material is carefully melted, poured into the cell, and the cell is capped. ,Outgassing is initiated by evacuating the head space through the stopcock after the charge has solidified. The cell charge is reliquified (by heating in the oven) and the head space evacuated on solidification two more times to complete the outgassing procedure. For each evacuation, pumping is continued for (5) D. P. Enagonio, E. G. Pearson, and C. P. Saylor in “Thermometric Cells for Calibration of Liquid-in-Glass Thermometers,” Temperature, Its Measurement and Control, Vol. 3 , Part 1, C. M. Herzfeld, Ed., Reinhold, New York, N. Y., 1962. VOL. 40, NO. 10, AUGUST 1968
1521
5 minutes after cessation of initial pump noise. A Precision Scientific Model 25 vacuum pump was used for outgassing of all. cells prepared. TRIPLE POINT MEASUREMENT. The prepared cells are placed in the oven adjusted to a temperature 10 "C above the melting point of the compound. When all solid has liquefied, one of the cells is removed and shaken axially with the thermometer well until freezing is initiated by the formation of a uniform suspension of fine crystals (a slush). The cell is immediately placed in the Dewar flask of the measuring sys-
~~
Table I. Reproducibility of Temperature: p-Nitrotoluene Average Resistance, Temperature temperaPrecision Cell Rt, ohms "C ture, "C "C FS-4"
30.7467 51.536 30.7450 51.516 30.7461 51.527 30.7467 51.536 51.529 4~0.007 FS-Sa 30.7480 51.547 30.7478 51.545 30.7480 51.547 10.001 30.7480 51.547 51.546 FS-6b 30.7450 51.516 30.7442 51 .SO9 30.7440 51,507 10.003 30.7442 51.509 51.510 FS-6ac 30.7506 51.572 30.7497 51.563 1.0.004 30.7504 51.571 51.569 Preheat period: 30 minutes. a FS-4 & FS-5 are filled with same lot of recrystallized material. * FS-6 is filled with distilled material. c FS-6a is filled with distilled material dried for 24 hours.
Table 11. Reproducibility of Temperature: Naphthalene Average Resistance, Temperature temperaPrecision Cell R t , ohms "C ture, "C "C ~
~
FS-2"
1
33.6148 4 33.6157 33.6161 33.6154 33.6149 33.6155 33.6150 33.6160
80.268 80.277 80.281 80.275 80.269 80.276 80.270 80.280
80.275
10.004
80.274
10.004
Preheat period: 30 minutes. 5 Both cells filled with same lot of purified material. Table 111. Reproducibility of Temperature : Benzoic Acid Average Resistance, Temperature temperaPrecision Cell R t , ohms "C ture, "C "C FS-7"
FS-8'
37.7712 37.7705 37.7705 37.7697 37.7711 37.7701 37.7705 37.7707
122.358 122.351 122.351 122.342 122.357 122.347 122.351 122.353
122.350
4Z0.005
122.352
A0.003
Preheat period: 30 minutes. = Both cells filled with same lot of material.
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ANALYTICAL CHEMISTRY
tem. Cell and flask are insulated and the thermometer is installed. Measurement is not made on the first cell; it is allowed to stand 30-60 minutes (depending on the compound) to preheat the system. Immediately after preheating, the second cell is crystallized (slushed) in the same manner, substituted for the preheat cell and the time-temperature plot is started. A rise in temperature of the undercooled liquid by the latent heat of crystallization will be observed until a maximum value is reached, which is the triple point. The third cell is run in the identical manner immediately after measurement is completed on the second cell. An ice point determination, R,,is made prior to each set of triple point measurements to verify the calibration of the platinum resistance thermometer. A complete series includes: determination of R,,preheat of system, and triple point measurements on two cells. At least three series were run for each compound investigated for a minimum of six measurements with two cells. RESULTS
pNitrotoluene. Crystallization (slush) on initial freezing of this compound is readily obtained. In each cell, the triple point temperature could be maintained within ~ t 0 . 0 0 2"C for at least 3 hours. Reproducibility of the temperature is shown in Table I. Two methods for purifying technical grade material were evaluated: recrystallization (from methanol) and distillation. The triple point for distilled p-nitrotoluene was lower than expected (Cell FS-6). This material was removed from the cell, dried over PzOs for 24 hours and then rerun. An appreciable increase of the triple point temperature resulted. Naphthalene. The desired slush at the start of freezing is not readily obtained with this compound. This difficulty with naphthalene cells was reported by Enagonio, Pearson, and Saylor (5). They also found that the type of crystallization would differ from one freeze-out to another. Premature crystallization (or caking) of the material must be avoided to obtain reproducible freeze-outs. More uniform cooling was effected by insulating the cell with a closely fitted sleeve made from a section of fiber-glass insulation. Delayed by the reduced cooling rate, freezing will initiate with formation of a few fine crystals on shaking the insulated cell. At this point, the cell is immediately removed from the sleeve and continued shaking produces the fine crystalline suspension. Occasionally, partial cell removal from the sleeve for observation would cause immediate caking on the exposed cell wall. To prevent this, the cooling rate is observed with an iron-constantan thermocouple in the thermometer well of the cell and connected to the recorder. When the recorder trace first shows a temperature rise, the cell is removed from the sleeve to obtain the slush with continued shaking. With the technique described, the triple point temperature in the naphthalene cells could be maintained within +0.002 "C for at least 15 minutes. Reproducibility of the temperature is shown in Table 11. Purified grade naphthalene was used without further treatment to prepare the cells. Benzoic Acid. The characteristics of this compound as a reference material have been thoroughly described by Schwab and Wichers (4). The triple point temperature of the cells prepared for this study could be maintained within *0.002 "C for at least 3 hours. Reproducibility of the temperature is shown in Table 111. ACS-certified grade benzoic acid was used without further treatment to prepare the cells. Adipic Acid. This compound does not readily crystallize (slush) on freezing. Employing the insulated cell technique
Table IV. Reproducibility of Temperature: Adipic Acid Average Resistance, Temperature temperaPrecision Cell R t , ohms "C ture, "C OC FS-1" 40.6103 151.421 40.6099 151.417 40.6095 151.413 40.6095 151.413 151.416 10.003 FS-4" 40.6068 151.396 40.6065 151.393 40.6086 151.404 40.6090 151.408 151.400 iz0.005 Preheat period: 40 minutes. Both cells filled with same lot of purified material.
described for naphthalene, reproducible freeze-outs are obtained, The triple point temperature of the adipic acid cells could be maintained within &0.002 "C for at least 20 minutes, Reproducibility of the temperature is shown in Table IV. Certified grade adipic acid was used without further treatment to prepare the cells. Anisic Acid. Because of the thermal stability of this compound, greater care in cell preparation and heating is required to achieve reproducible temperatures. It was found that prolonged heating for a 14-hour period (at 10 "C above melting point) would lower the triple point value approximately 0.01 O C. Heating periods of 4 hours (sufficient to liquefy a cell charge), however, had no appreciable effect on the results. The insulated cell technique as described previously is required to obtain consistent freeze-outs with the anisic acid cells, The triple point temperature could be maintained within 10.002 "C for at least 10 minutes. Reproducibility of the temperature is shown in Table V. Technical grade anisic acid, purified by acid hydrolysis of the recrystallized sodium salt, was used to prepare the cells. 2-Methylanthraquinone and p-Toluic Acid. These compounds were investigated as possible alternates for anisic acid. These materials exhibited better thermal stability, but consistent results could not be obtained with any of the procedures described. Duration of the triple point temperature for both compounds was so short that there is serious doubt an equilibrium was ever achieved. Results obtained for these materials are shown in Table VI. 2-Chloroanthraquinone. This compound readily slushes on freezing for consistent freeze-outs. With these cells, the triple point temperature could be maintained within 10.002 "C for at least 20 minutes. Reproducibility of the temperature is shown in Table VII. Technical grade 2-chloroanthraquinone, purified by sublimation, was used to prepare the cells. Carbazole. The insulated cell procedure is required to obtain reproducible results with this compound. The triple point temperature of the carbazole cells could be maintained within 10.002 "C for at least 10 minutes. Reproducibility of the temperature is shown in Table VIII. The material darkens during prolonged heating periods, but triple point values were not appreciably affected. Certified grade carbazole, purified by sublimation, was used to prepare the cells. Anthraquinone. This compound also requires the insulated cell procedure in order to obtain consistent freeze-outs. The triple point temperature in these cells could be maintained within 10.002 "C for at least 10 minutes. Reproducibility of the temperature is shown in Table IX. Technical grade
Table V. Reproducibility of Temperature: Anisic Acid Average Resistance, Temperature temperaPrecision Cell R t , ohms "C ture, "C "C FS-2" 43.6660 182.969 43.6666 182.975 43.6651 182.959 182.968 10.006 FS-54 43.6682 182.992 43.6661 182.970 43.6678 182.988 182.983 10.009 Preheat period: 40 minutes. a Both cells filled with same lot of purified material. Table VI. Reproducibility of Temperature : p-Toluic Acid and 2-Methylanthraquinone p-Toluic Acid Average Resistance, Temperature temperaPrecision Cell R t , ohms "C ture, 'C "C FS-IO" 43.3084 179.258 43.2990 179.160 43.3066 179.239 179.219 10.036 FS-12" 43.3065 179.235 43.3084 179.258 43.3031 179.204 179.233 10.018 Preheat period: 40 minutes. Both cells filled with same lot of purified material. 2-Methylanthraquinone Average Resistance, Temperature temperaPrecision Cell R t , ohms "C ture, "C "C FS-2' 42.7185 173.053 42.7471 173.468 42.7338 173.330 42.7497 173.495 173.337 iz0. 024 FS-8b 42.7690 173.695 42.7765 173.772 42.7731 173.737 42.7757 173.764 173.742 10.023 Preheat period: 40 minutes. Both cells filled with same lot of purified material. Table VII. Reproducibility of Temperature: 2-Chloroanthraquinone Average Resistance, Temperature temperaPrecision Cell R t ,phms "C ture, "C 'C FS7" 46.1657 209.057 46.1633 209.032 46.1640 209.039 209.043 10.010 FS-9" 46.1630 209.029 46.1625 209.023 46.1627 209.025 209.026 10.004 Preheat period: 50 minutes. a Both cells filled with same lot of purified material.
anthraquinone, purified by sublimation, was used to prepare the cells. Other compounds which were investigated include salicylic acid, succinic acid, hydroquinone, tartaric acid, THAM (trishydroxymethylaminomethane), p-aminobenzoic acid, and p-nitrobenzoic acid. All of these compounds decomposed (to varying degrees) during the heating periods required. VOL. 40, NO. 10, AUGUST 1968
1523
DISCUSSION
Small differences in purity of the compound did not affect the measurement precision of individual cells. For example, the precision for p-nitrotoluene cells (Table I) ranged from &0.001" to lt0.013 "C, even though the quality of material differed. The degree to which average temperatures agree for cells containing the same lot of material is believed to be primarily indicative of the homogeneity. Adequate purity must therefore be maintained to ensure a homogeneous material. Average temperatures obtained for cells of the same prepared compound were in agreement to better than zt0.020 "C. The overall precision, based on all measurements with two cells, is shown in Table X for compounds found to be suitable
Table VIII. Reproducibility of Temperature: Carbazole Average Resistance, Temperature temperaPrecision Cell Rt,ohms "C ture, "C "C FS-12" 49.6164 245.356 49.6130 245.320 49.6143 245.334 245.337 ztO.013 FS-13" 49.6134 245.324 245.341 49.6150 49.6153 245.345 245.337 10.008 Preheat period: 50 minutes. 6 Both cells filled with same lot of purified material. Table
IX. Reproducibility of Temperature : Anthraquinone
Average Resistance, Temperature temperaPrecision Cell Rt, ohms "C ture, "C "C FS-4" 53.2997 284.600 53.2998 284.601 53,2985 284.587 53.2992 284.595 284.596 5~0.006 FS-5' 52.2990 284.593 53.2984 284.586 53.2981 284.583 53.2991 284.594 284.589 10.006 Preheat period: 60 minutes. a Both cells filled with same lot of purified material. Table X. Temperature and Precision of Calibration Standards Triple point Overall temperature" precisionb "C "C 10.009 p-Nitrotoluene 51.537 80.273 10.005 Naphthalene 122.351 f0.004 Benzoic acid 151.423 10.010 Adipic acid 182.976 10.010 Anisic acid 209.034 10.009 2-Chloroanthraquinone 245.337 hO.011 Carbazole 284.592 10.005 Anthraquinone 5 The triple point temperature reported is the average of all measurements made on two cells of the same lot of purified material. b Based on all measurements made on two cells of the same lot of purified material.
1524
ANALYTICAL CHEMISTRY
temperature calibration standards. A minimum overall precision of *0.025 "C could not be obtained with compounds that do not produce the required crystallization or with thermally unstable materials. Inconsistency of material and unpredictable crystallization characteristics may prevent attainment of repeatable measurements with compound mixtures or highly impure materials, but this was not investigated in this study. Duration of equilibrium temperature expectedly decreases as the temperature level increases due to heat leakage from the measuring system. As the differential between ambient and equilibrium temperatures becomes greater, heat losses also increase which shorten equilibrium duration. Maintenance of temperature within zt0.005 "C for a minimum of 5 minutes with repeatability of that temperature was considered sufficient indication of an equilibrium. A measurement sensitivity of "C is possible with the instrumentation specified. For this study, a sensitivity of 5 X "C was employed. The amplifier sensitivity and recorder span were adjusted to present 0.050 "C full scale across the 9.75-inch chart. Maximum noise level observed at this sensitivity was 0.5% (one-half chart division). The thermometer resistance at the observed temperature, R t , is measured after the recorder trace indicates a stable temperature has been attained. To eliminate any possible linearity errors, the bridge is balanced with the recorder out of the circuit during this step. The resistance at the ice point, R,,is determined before each series of measurements. The resistance ratio, R,/R,, is calculated and the observed temperature is determined by interpolation from the calibration table supplied by the manufacturer of the platinum thermometer (manufacturer certified calibration traceable to NBS). The error introduced by the linear interpolation is less than 0.0001 "C (6). All measurements, including R,, are made with a thermometer current of 2 ma. Because the resistance ratio method is applied, correction for the heating effect by the thermometer current is not necessary (7). The prepared compounds listed in Table X are being used in our laboratories for standardization of thermometers employed in melting point determinations. With the reference temperatures provided by the standards, compensation for stem corrections as well as scale corrections is accomplished under the same conditions that an unknown sample would be determined. The standards are also routinely run along with unknown samples to promptly detect any calibration changes which may have occurred. With the precision achieved and the calibration of measuring instrumentation used, minimum accuracy of the reported triple point temperature is better than ~k0.05"C. For greater accuracy with sensitive temperature measuring devices, the correlation of melting and freezing points to triple point values would have to be considered. An accuracy of ~k0.05 "C,however, is usually more than adequate for calibration of measuring devices generally used in most laboratories.
RECEIVED for review November 20, 1967. Accepted May 24,1968. (6) . , Leeds & Northrup Co., Philadelphia, Pa., Technical Publication A 1.2101, 1965.17) "Notes to Supplement Resistance Thermometer Reports," 'National Bureau-of Standards, U. S. Government Printingoffice, Washington, D. C., 1963.
