ment but decreases the duration of constant absorbance. I n the recommended range of acid concentration, the maximum color intensity is attained within 15 minutes and will remain constant a t least 20 additional minutes before fading begins. When samples are processed according to the procedure, sufficient acid accompanies the chromium in the estraction step so that additional acid is not needed. The acid concentration of many sample extracts will fall slightly above the recommended range and, therefore, the absorbance readings should be taken within 10 minutes after adding 1,5-diphenylcarbohydrazide t o the organic extract. Apparently a slight amount of phosphoric acid is estracted by 4methyl-2-pentanone and some hydrochloric acid accompanies the iron(II1) chloride, which is known to esist as solvated HFeCI4 in the organic phase. Interference Studies. From solutions 1N in hydrochloric acid, the extraction of metals other than chromium is slight (7, 8, 10). Amounts of vanadium(V) not exceeding 0.5 mg. per aliquot can be present before sufficient amounts extract and cause interference. The amounts of mercury(II),
molybdenum(VI), and tin(1V) should not exceed 50 y in the aliquot taken for analysis. All these limits can be extended by backwashing the extract. The tolerance limit for iron is only 2 mg. when the extraction from aqueous solutions 1N is conducted in hydrochloric acid and, although much larger than the amount able to be tolerated in aqueous medium (Q), is not large enough for many applications, Inclusion of 1M phosphoric acid as masking agent in the aqueous phase extends the tolerance limit to 8 mg. For larger amounts of iron, a single backwash of the 4-methyl-2-pentanone extract with a fresh portion of aqueous 1N hydrochloric acid removes 975% of the iron. Iron(II1) chloride is quickly stripped from the organic phase and by this modification, as much as 0.320 gram of iron can be present initially in the sample aliquot. Undoubtedly this limit could be extended even further by including phosphoric acid in the backwash solution. The percentages of iron extracted from solutions of various hydrochloric acid concentrations, as determined by flame spectrophotometric measurements on the organic phase, are: at 0.6hT,0.6%; a t 0.8N, 1.2%; a t lN,
2.5%; a t 1.2iV, 5%; and from the literature (10)a t 2N, 25%, and a t 3N,
77%.
LITERATURE CITED
(1) Allen, T. L., ANAL CHEbf. 30, 447 (1958). (2) Ayres, G. H., Ibid., 21, 653 (1949). (3) Bernhardt, H. A., U. S. Atomic Energy Comm. Rept. MDDC1541 (1947).
(4) Bryan, H. A., Dean, J. A,, ANAL. CHEX29, 1289 (1957). (5) Cazeneuve, .4.,Bull. soc. chim. (3), 23, 701 (1900). (6) Mann, C. K., White, J. C., Abstracts of Papers, 132nd Meeting, ACS, New York, 1957, p. 29B. ( 7 ) Morrison, G. H.? Frejser, H., “SoIvent Extraction in Analytical Chemistry,’’ pp. 128-9, Wiley, New York, 1957. (8) Mylius, F., Hiittner, C., Ber. deut. chem. Ges. 44, 1315 (1911). (9) Sandell, E. B., “Colorimetric Determination of Metals,” 2d ed., pp. 262-3, Interscience, New York, 1950. (10) Specker, H., Doll, W., 2. anal. Chem. 152, 178 (1956). (11) Weinhardt, A. E., Hixson, A. N., Ind. Eng. Chem. 43,1676 (1951). RECEIVED for review September 23, 1957. Accepted December 23, 1957. Abstracted from a portion of a M. S.thesis to be submitted by Mary Lee Beverly to the Graduate School of the University of Tennessee, Knoxville, Tenn.
Auto matic Cryoscopic Determination of Molecular Weights EDWARD L. SIMONS General Elecfric Research I aborafory, Schenectady,
b A completely automatic recording cryoscopy apparatus has been assembled for the determination of molecular weights. The temperaturesensing element is a thermistor that forms one arm of a Wheatstone bridge circuit. The cooling curve of the liquid in the freezing cell is traced out by a millivolt recorder which measures the unbalance of the bridge produced by changes in the thermistor temperature. Provision has been made for automatically relieving the supercooling of the liquid.
C
in benzene and cyclohexane are used in this laboratory for the routine determination of molecular weights. The conventional Beckman technique (3) has three operational drawbacks: It requires continual attention by the operator during the entire cooling process; it is difficult t o make accurate temperature readings while the merRYOSCOPIC MEASUREMENTS
N. Y.
cury column in the Beckman thermometer is falling or rising; and manual plotting of the cooling curve of temperature us. time is necessary. These drawbacks can be eliminated by the use of automatic recording cryoscopic equipment. Such equipment has been described by Stull (16, 17) and Witschonke (18), who used platinum resistance thermometry, and by Herington and Handley (7, 8), who used a thermistor as the temperature-sensing element. More recently, Zeffert and Witherspoon (20) described an automatic thermistor temperature recorder that can be used for cryoscopic measurements. Methods based upon such equipment, however, provide no automatic means for the control of supercooling. If supercooling is persistent, as it is for benzene solutions, attention of the operator is required to note the onset of this phenomenon and to take steps to relieve it. The apparatus described, like one
developed earlier in this laboratory by Zemany ( a l ) , uses a thermistor rather than a platinum resistance thermometer as the temperature-sensing element because of the simpler instrumentation involved. Its unique feature lies in the method developed for the automatic control of supercooling. APPLICATION OF THERMISTORS T O CRYOSCOPY
Thermistors are semiconductors whose large temperature coefficients of resistance make them particularly useful for the measurement of small temperature changes (2, fa). I n the reported applications of thermistors to cryoscopic measurements (4, 7,8, IO,11,14, f9-21), the thermistor has been used as one arm of a Wheatstone bridge circuit, and in most cases the resistance of the element has been measured manually, by a null method, as a function of time during the freezing process. VOL. 30, NO. 5, MAY 1958
979
However, Herington and Handley (7, 8) and Zeffert and Witherspoon (20) have pointed out that instead of using a null method with the bridge in balance, the resistance to be measured may be directly related to the unbalance bridge voltage. By the substitution of a millivolt recorder for the galvanometer of the bridge circuit, cooling curves for liquid samples may be obtained in terms of voltage as a function of time. This technique of automatic recording of freezing point curves has been adopted in the apparatus described here. AUTOMATIC CONTROL O F SUPERCOOLING
Most methods of inducing crystallization in a supercooled liquid involve either the introduction into it of a seed crvstal of the solvent or the formation in"it of a seed crystal by the rapid cooling of a small portion of the liquid (6.7.13.15). ., In this apparatus crystallization is induced by allowing a stream of nitrogen gas, precooled by passing through a coil immersed in liquid nitrogen, to impinge upon the outer wall of the cell that contains the liquid to be frozen. The flow of nitrogen gas is automatically controlled by a mercury switch on the recorder that actuates a solenoid valve in the nitrogen line. The switch can be adjusted to operate a t any desired position of the recorder pen. I n practice it is set so that the nitrogen flow starts at a temperature about 0.4" C. below the freezing point of the pure solvent. Crystallization is induced quickly, and, as the temperature rises toward the freezing point of the liquid, the switch automatically reverses and the nitrogen flow is shut off. I n this manner the agent used to relieve supercooling is introduced exactly when it is needed, and only for as long a time as it is needed. I
APPARATUS
Details of the apparatus described below are shown schematically in Figure 1. The freezing point cell is a test tube with a side arm through which a slow stream of nitrogen is passed in order to exclude atmospheric moisture from the sample. The cell is inserted through a rubber collar into a larger tube which serves as an air jacket. The air jacket has a tube extending down its inner wall and terminating a t the lower end of the freezing point cell. The stream of cold nitrogen gas that is used to induce crystallization is admitted through this tube, and leaves the jacket through a V-shaped notch in the supporting collar. The assembled freezing point cell and air jacket are mounted in a large Dewar flask filled with a mixture of chopped ice and water. A two-holed cork stopper in the top of the freezing point cell carries the stirrer and the thermistor well. As recommended by Zeffert and Hormats (19), the glass thermistor well contains an
980
ANALYTICAL CHEMISTRY
4 '
c1 Figure 1. Schematic diagram of cryoscopic apparatus A.
Rheostat for stirrer motor 8. Stirrer motor C. Cam D. Pivot ioint E. Bearing F. Quartz stirrer G. Glass thermistor well
H. Freezing point cell 1.
1. K. I.
Inlet for dry nitrogen stream (ambient temperature) Inlet for cold nltrogen stream Air jacket Dewar flask
amount of mercury sufficient to cover the thermistor bead. The stirring assembly is adapted from the design described by Glasgow, StreB, and Rossini (6). The stirrer is constructed of quartz and is operated by a variable speed electric motor coupled to a vertical stroke mechanism. The coupling is through a clutch that protects the motor from an overload if the contents of the freezing point cell should completely solidify and bind the stirrer. The length of the stirring stroke can be adjusted by changing the point of attachment of the vertical rod to wheel C in Figure 1. A stroke of 0.5 inch and a stirring rate of 100 to 120 strokes per minute were used for the experiments described here. Bridge Circuit. The thermistor is Type 32A1, Victory Engineering Corp., Union, N. J. It has a nominal resistance of 2000 ohms a t 25' C. and a temperature coefficient of resistance a t that point of -3.9%. Its resistance a t the freezing point of benzene (5.5" C.) is about 4000 ohms. The thermistor forms one arm of a Wheatstone bridge and is connected to the circuit by means of a length of shielded single conductor cable. Because this apparatus mas designed solely for the determination of depressions of less than 0.5" C. in the freezing points of benzene (5.5" C.) and cyclohexane (6.5" C.), it was not necessary to use the instrumentation of Zeffert and Witherspoon (WO), whose equipment permits the automatic continuous recording of thermistor temperatures in the range from -80" to 32" C. The two fixed arms of the bridge are 5000-ohm, pre-
cision, wire-wound resistors, and the fourth arm consists of a pair of decade resistance boxes in series. One covers four decades in steps of 0.1, 1, 10, and 100 ohms and the second consists ot steps of 1000 ohms. Power for the bridge is supplied by a 1.5-volt dry cell, The decade boxes are set so that the resistance of the thermistor a t the freezing point of the solvent will produce a bridge unbalance of about 2 mv. The recorder, which replaces the galvanometer in the conventional bridge circuit, is a Brown Model No. 153x12, high impedance (0 to 50,000 ohms) instrument, with a full-scale deflection of 10 mv., a full-scale response time of 3.5 seconds, and a chart speed of 0.5 inch per minute. The recorder is equipped with an auxilliary back-setter control switch, equivalent to a singlepole, double-throw switch, that can be set to operate a t any desired position of the pen. In this equipment it is tripped a t 9.5 mv. Through its operation of two solenoid valves, the switch controls the flow of nitrogen gas that is used to induce crystallization. Nitrogen Supply. One solenoid valve is located between the nitrogen Bource and the coil of 1/4-inch copper tubing that is immersed in liquid nitrogen. This valve is normally closed, except when supercooling of the liquid has brought the pen to 9.5 mv. The second valve is located on a tee, inserted between the exit end of the copper coil and the jacket surrounding the freezing point cell. It is normally open during operation of the bridge, and by venting to the room it ensures that the contents of the nitrogen coil in the Dewar flask remain a t l-atm. pressure. It closes only when the first valve opens to permit cold nitrogen to impinge upon the freezing point cell. A separate nitrogen line supplies the slow stream of ambient temperature gas that maintains an inert atmosphere over the contents of the freezing point cell. MATERIALS
Research grade benzene (Phillips Petroleum Co., 0.06 to 0.07 mole % impurity) was used without further purification as the cryoscopic solvent. Because the freezing point of this material is affected by atmospheric moisture, the solvent was stored in a nitrogen dry box. The freezing point cell was thoroughly flushed with nitrogen before use, and the solvent was transferred to it by pipet in the dry box. The mouth and side arm of the freezing point cell were stoppered whenever the cell was weighed with solvent in it. The cryoscopic constant for benzene was determined through the use of triphenylmethane that had been recrystallized from ethyl alcohol. The solute was weighed directly into the freezing point cell before the solvent was added, or a weighed pellet of the material was transferred to a cell that already contained a known weight of solvent. Approximately 10 ml. of solvent were used.
0
(0
4
30
80
40
00
10
80
90
00
TO
80
80
100
12
11 0
10
30
00
40
I +
100
10
r
t
WITH COLD NITROGEN
-
bring the liquid-solid portion of the cooling curve within the range of the recorder. A series of measurements that had been made with a calibrated resistance box in place of the thermistor had shown that displacements of the recorder pen were proportional to the small changes in resistance that produced them. It was possible, therefore, from the observed gradient of 0.96 division per ohm, to correct the observed freezing points of these two solutions for the change in the bridge setting. The data may be represented by a modified form of the general quadratic equation adopted by McMullan and Corbett (10) for the resistance-concentration relationship in an ideal system:
I
m= a'hd
T 0
10
30
40
50
60
70
80
90
100
I
1
5
I
0
10
20
20
40
so
00
70
80
90
4
I I
WITHOUT COLD NITROGEN
Figure 2. Automatic relief of supercooling of benzene PERFORMANCE
The effectiveness of the cold nitrogen technique for the automatic relief of supercooling is illustrated by the two cooling curves of benzene shown in Figure 2. The width of the chart paper is equivalent to approximately 0.5" C. The arrow on the chart indicates the point in the supercooled region a t which the flow of cold nitrogen begins. To test the performance of the equipment, measurements mere made of the freezing points of a series of solutions of triphenylmethane in benzene. Kraus and coworkers (1, 9) have shown that the freezing point depression in this system is a linear function of the solute concentration up to a t least 0.047 molal. The freezing point of the solution, expressed in the arbitrary scale units of the chart paper, was obtained by extrapolation of the liquid-solid equilibrium portion of the solution cooling curve back to its intersection with the liquid portion. The freezing point of the solvent was given by the position of the cooling curve, which may also be taken as an indication of the purity of the
benzene. A preliminary freezing of the liquid was usually necessary before reproducible freezing points could be obtained on a given sample of solvent or solution, and the freezing point used in calculations was the average of the values obtained on two successive freezings of each liquid. The data obtained on a series of eight solutions of triphenylmethane in benzene are listed in Table I, along with results obtained from the calculations shown below. For solutions 7 and 8 the setting of the decade resistance boxes in the bridge circuit had to be changed before the run in order to Table 1. NO. 1 2 3
- b'(Ad)*
(1)
where Ad is the difference betm-een the freezing points of the solution and the solvent, both expressed as points on the arbitrary scale of the recorder chart paper and a' and b' are the parameters of the equation. The modification of the hIchIullan and Corbett equation is based upon two assumptions: that the resistance of the thermistor a t the freezing point of benzene remains constant during the course of these experiments; and that displacements of the recorder pen are proportional to the changes in resistance that produce them. The first assumption was verified by freezing point determinations made during the series of experiments on three separate samples from the same stock bottle of benzene. The range between the high and the low values was less than 1 ohm out of about 4000, and the average of the three was used in the calculation of the freezing point depressions. The verification for the second assumption has already been presented. The parameters, a' and b', of Equation 1 were computed by the method of least squares, and the data of Table I may be expressed in the form m = 1.073 X lo-' Ad
- 5.638 X lo-' (Ad)* (2)
As shown by the figures in Table I, this equation fits the data with an estimated standard deviation in Ad of
Freezing Point Depressions in Triphenylmethane-BenzeneSystem Molality, Depression, Ad, Ad? m X 103 Ad, Div. Eq. 3 Eq. 4 6.32 19.73 29.16
Std. dev.
6.1 18.3 27.5 34.7 50.5 57.9 74.8 81.0
5.9 18.6 27.6 34.5 50.6 57.6 i4.6 81.2 0.21
VOL. 30, NO. 5, MAY 1958
6.0 18.9 27.9 34.8 50.5 57.5
...
... 0.42
981
0.2 scale division. As each division is equivalent to approximately 0.005 'C. (see below), this corresponds to a standard deviation of 1 X 10-3' C., which is the same as the value reported by McMullan and Corbett (IO). If only the first six solutions of Table I are considered, the data up to 0.06 molal can be represented by the linear equation m = 1.046 X
Ad
(3 )
with an estimated standard deviation of 0.4 scale division or 2 X C. It is from the slope of Equation 3 (molality per scale division) and the cryoscopic constant reported by Kraus and coworkers (1, 9) for benzene (5.07' C. per molality) that the estimate of 0.005"C. per scale division was obtained for this work. In practice the cryoscopic constant for each new bottle of benzene was calculated from the freezing point depression (expressed in scale divisions) measured for an approximately 0.05 molal solution of triphenylmethane in benzene. A 0.05 molal solution of triphenylmethane in benzene has a freezing point about 50 scale divisions below that of the pure solvent (Equation 3). At this concentration the estimated standard deviation, or error, of 0.4 division corresponds to a relative error of 0.8%. Assuming a negligibleerror in the weights of solvent and solute used for a single determination, the estimated relative error for the determination of the cryoscopic constant from the ratio of molality to freezing point depression should also be 0.8%. This was confirmed by sets of replicate determina-
tions of the cryoscopic constant which were made over a period of 10 months on various lots of benzene. The value of 1.4% so obtained, and based upon 23 degrees of freedom, is not significantly different from the estimate of 0.8% based upon 5 degrees of freedom. The data presented here are for an ideal solute-solvent system and were obtained solely to illustrate the performance characteristics of the equipment. When cryoscopic measurements are to be made on nonideal systems, appropriate extrapolations of the data must be employed to correct for the variation with composition of the apparent molecular weight of the solute. ADVANTAGES OF AUTOMATIC CRYOSCOPY
The following advantages characterize the apparatus described in this report. Once the solute and solvent have been weighed into the cryoscopic cell and the cell has been assembled in the freezing bath, no further operator attention is required. A permanent record of the cooling curve is automatically obtained. The extrapolations required for the determination of the freezing points of solutions are more easily made from a continuous curve than from one plotted manually from discrete temperature readings. ACKNOWLEDGMENT
The author gratefully acknowledges the assistance of Harley W. Middleton, Marie DeVito, and Joyce H. Northrop in the construction and testing of the equipment.
LITERATURE CITED
Batson, F. BI., Kraus, C. A., J . Am. Chem. SOC.56, 2017 (1934).
Beck, 4.,J . Scz. Instr. 33, 16 (1956). Daniels, F., Mathew, J. H., Wdliams, J. W., Bender, R., Alberty, R. S., "Experimental Physical Chemistry," 5th ed., p. 68, McGraw-Hill, New York, 1956. Giguhre, P. A., Secco, E. A., Can. J . Chem. 32, 550 (1954).
Glasgow, A. R., Jr., Ross, G., J .
Research Natl. Bur. Standards 57, 137 (1956). Glasgow, A. R., Jr., Streiff, A. J., Rossini, F. D., Ibid., 35,355 (1945). Herington, E. F. G., Handley, R., J . Chem. SOC.1950, 199. Herington, E. F. G., Handley, R., J . Sci. Znslr. 25, 434 (1948). Kraus, C. A., Vingee, R. A., J. Am. Chem. SOC.56, 511 (1934). Mclfullan, R. K.> Corbett, J. D., J . Chem. Educ. 33, 313 (1956). hfikhkelson, V. Ya., J . Anal. Chem. C.S.S.R., 9, 21 (1954). Pvluller, R. H., Stolten, H. J., ANAL. CHEM.25, 1103 (1953). Sewman, M. S., Kuivila, H. G., Garrett, A. B., J . Am. Chem. SOC. 67, 704 (1945). Richards, L. A., Campbell, R. B., Soil Sci. 65. 429 (1948). Spooner, D. d., J . &ci. Instr. 29, 96 (19.52) \ - - - -
Stull, D: R., IND. ENG.CHEY.,ANAL.
ED. 18, 234 (1946). Stull, D. R., Rev. Sci. Instr. 16, 318 (1945): Kitschonke. C. R.. ANAL. CHEJI. 24, 350 (19E i2). Zeffert, B. M. Hormats, S., Ibid., 21, 1420 (19;49,. Zeffert, B. M.,, Withers oon, R. R., Ibid.', 28, 1701 (19567. Zemany, P. D., Ibid., 24, 348 (1952).
RECEIVEDfor review May 9, 1957. Accepted November 23, 1957. Division of Analytical Chemistry, 131st meeting, ACS, Miami, Fla., ApriI 1957.
Drying and Decomposition of Sodium Carbonate ARTHUR E. NEWKIRK and IFlGENlA ALlFERlS General Electric Research laboratory, Schenectady, N. Y.
b Thermobalance curves are given for the drying and decomposition of sodium carbonate in the temperature range from 25" to 1040" C. using different crucible materials and atmospheres. The reaction of sodium carbonate and silica resulted in a weight loss a t t,emperatures as low as 500" C. It is recommended that sodium carbonate for analytical use b e dried in platinum to prevent errors due to its reaction with silica and silicates.
I
more than a coincidence that the accepted maximum temperature for drying sodium carbonate for use as a T IS
982
*
ANALYTICAL CHEMISTRY
primary standard, and the lower limit for the initiation of the reaction of sodium carbonate and silica are the same -Le., 300" C. This fact is not generally recognized. Directions for drying sodium carbonate usually specify porcelain or platinum containers, but glass weighing bottles are also used. In this Iaboratory, satisfactory results have been obtained by drying in glass a t 250' C. (1). The recommended temperature range from 250" to 300' C. is the result of many studies (6), but the upper temperature limit is surprisingly low in view of the several reliable reports of the thermal stability of
sodium carbonate at temperatures close to its melting point, 851'C. Richards and Hoover (8) found that sodium carbonate heated under carbon dioxide for a long time just below the fusion point lost on fusion only about 0.003% in weight. Duval (2) reported that sodium carbonate, when heated in a thermobalance, is stable in the range from 100" to 840' C. Motzfeldt (7), who made a careful study of this decomposition in a platinum cell, concluded that there is no chance for significant decomposition at any temperatures below 800" C. a t least. He attributes previous discordant results to
