Cryoscopic Determination of Purity of Highly Reactive Substances AUGUSTUS R. GLASGOW, Jr., and MILTON TENENBAUM National Bureau
of Standards, Washington, D. C.
A freezing point apparatus was constructed of glass and noble metals for assaying the purity of reactive substances. The sample is stirred by a platinum-iridium stirrer sealed within a closed glass system. An iron armature protected by gold or platinum is attached to the upper portion of the stirrer. A solenoid encased in iron, mounted and guided outside the glass wall of the freezing point tube, holds the armature within its magnetic field, so that niovement of the solenoid results in a corresponding movement of the stirrer within the sealed tube. The tube has openings into the cell for cleaning, introduction and removal of samples, temperature measnrenient, and crystal induction. The apparatus was used for esperiments on titanium tetrachloride.
M
AST compounds of scientific iriteiest have high chemical
reactivities or are poisonous, hygroscopic, or reactive 134th air. The evaluation of purity of such substances from temperature measurements of the solid-liquid equilibria requii es that the system used be inert to these substances and that the operation be performed in a closed system. Because previous apparatus developed in these laboratories for the determination of pni ity from time-temperature freezing and melting curves ( 2 , 4-6)were not satisfactory for such compounds, a n e x freezing point apparatus as constructed of glass and noble metals. In addition to measurements of purity, it is possible t o determine diicctly the difference between the freezing point of the sample
under its o n vapor pressure (saturation pressure) and under the combined pressure and solubility effect of an inert gas at a controlled pressure. This report describes the details of construction of the apparatus and the experiments performed on titanium tetrachloride of different levels of purity. IPPtR4TUS
Essential iedtuiee of the apparatus ale: Chemical inertness. All surfaces that come in contact with the sample are either borosilicate glass or noble metals (platinum, platinum-iridium alloy, or gold) Provision for introduction and removal of volatile or nonvolatile samples without contact mith air and without contamination Provision for initiating crystallization, stirring, and obtaining precise temperature measurements of the sample sealed inside the freezing point cell Provision for rigorous chemical cleaning of the internal surfaces. The apparatus utilizes the reciprocating action of n double helical stirrer to maintain solid-liquid equilibiium. The direct
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b Figure 1.
-4ssembly of Apparatus
A.
M o t o r , pear reduction, 106 r.p.rn. B . Brass wheel, 21/~-inch diameter, 3/16 inch thick C . Support, channel iron D. Axle, 'la-inch diamrter, 4 inches long E . Adjustment screws to permit Il/;-inch \-ariation in position of axle for alignment of solenoid F . Circular disk. brass, i'ljr-inch diarncter with ball bearing inserts G, G I . Circular disk, brass, li./s-inch diameter, 5 , 16 inch thick, slotted and pinned H . Connecting rod steel. '/4-inch diameter pinned a i center Connecting r o d ' t o mires supporting solenoid I. I ' . Connecting rod t o brass counterweight J. Counterweight, 11 pounds K . Brass cylinder srrving a8 guide a n d emergency support for counterweight, holes in bottom and sidcs t o release air I,. Brass bearing t o guide connecting rod H 11. Piano wire S. Thermometer. platinum resistance, 25-ohm 0. Turnbnckle. 8-inrh I'. Steel rod guide~forsolenoid,aia-inch .-lrmature, soft-iron, protected b y gold or platinum Brass spool xorlnd >i.ith magnet wire S. Iron casing f o r solenoid T . Steel plate. 3's inch thick to support guide rods P T'. Angle iron, 1-inch T+. Transite block with tapered hole t o fit freezing point tube 11.. Breding well Ti-ith expansion bellows S. Glass tubing connected t o vacnum p u m p Y. Re-entrant well f o r insertion of thermometer 2 . K a l l of freezing tube o. Platinum-iridium rod, '1s-inch diameter b. Wall of freezing tube jacket SO-mm. diameter c. Stirrer, doublr helical, platiAiim-iridium d. Glass partition P. Glass tube, 3-mm i. Glass tube, 6-n~n1. b. 0 ' . Conducting strips silvrr attached t o each end of heating t a p e ~ v l i i c hcox-ers t o p haif of fre'ezing tube. Top of tube is heated with t h i s t a p e when samples freezing above room temperature are measured h. Turnbuckle, supports spaced 120' a p a r t 2. Brass guide, soldered t o iron casing of solenoid with a,',-inch hole t o center and guide solenoid Threaded flange attached to connecting rod j. X.. Electrical connection for power supply t o solenoid
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ANALYTICAL CHEMISTRY
1908
Section X - X
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4 Figure 2.
Details of freezing point tube
A.
Glass t u b e , facilitates cleaning, sealed before introduction of sample P o i n t of final seal when tube is fabricated Tapered section of t u b e t o fit tapered section of transite hlock D. "Seeding" well for inducing crystallization E Glass tubinn. connected t o vacuum vumw for
B. C.
temperature changes occur Glass partition, separating sample chamber a n d seeding well Glass tube connected to sample chamber 1 J . Sample chamber k'. Thermometer well If.
a l l dimensions i n rnm
Figure 3.
Introduction and removal
of nonvolatile samples , I , J, K . Same a s i n Figure 2 Class wool
lifted with magnet t o permit seal a t R Inlet for inert gas Outlet t o vacuum system E . Point a t which ampoule is sealed after sample h a s been transferred t o U' S. Point at which final seal is made after sample is degassed T. T'. Break-off t i p C. Ampoule containing sample t o be measured L". Bmpoule for receiving sample a f t e r measurement
P.
Q.
1909
V O L U M E 2 8 , NO. 1 2 , D E C E M B E R 1 9 5 6 drive stirrer used in equipment previously described ( 2 , 4,5 ) has been eliminated to permit confinement of the material in a waled system. This makes it possible to measure temperature under triple-point conditions or in a controlled atmosphere at other pressures. The iron armature to which the stirrer is attached is magnetically diiven from outside the vessel. This armature and stirrer unit is Permanently sealed within the freezing point tube. Vertical 1inch strokes of the stirrer are produced by holding the armature i n the magnetic field of a solenoid mounted around the outside of the stationary freezing point tube while lowering and raising the solenoid with a shaft attached to an electric motor. All internal parts of the system are constructed of glass or noble metals, so t h a t measurements of purity can be made on reactive substances. The cell is constructed of borosilicate glass. The shaft and coil of the stirrer are made of an alloy composed of 80% platinum-20% iridium. The surface of the armature is plated n i t h gold in one apparatus and encased in a platinum envelope in another. Because alignment is rather critical, the tube has been permanently mounted and is equipped with five different openings to allow necessary operations to be accomplished without the need for dismantling. A drawing of the entire assembly is shown in Figure 1. The solenoid, S , is attached to the bottom of shaft H and vertical motion is produced by attaching the top of the shaft 0.5 inch off-center on the motor-driven wheel, B. The solenoid is counterbalanced with a brass weight, J , suspended from a yoke a t G. The freezing point tube is supported by a Transite block, T’, machined to fit the tapered part of the tube. The block is split at the center and the two halves are bolted together so that the tube may be detached. Temperature is measured with a 25-ohm platinum resistance thermometer, N , placed in well Y and attached to a resistance bridge (Rlueller type) and a galvanometer (riot shown). The sensitivity of the thermometric system with a 2-ma current flowing through the platinum thermometer is such that 1 mm. on the galvanometer scale is equivalent to 0.0003” C. Details of construction of the freezing point tube which is designed for measuring freezing points of samples of 50-rnl. volume are s h o m in Figure 2.
vent decomposition of a n j vapois from the sample in making this seal, it is important to keep another part of the manifold cooled. This permits the heated vapors to travel toward the cooled parts in both directions away from the point of sealing. Using this procedure for titanium tetrachloride, decomposition a t the seal was eliminated. Sonvolatile samples are introduced by gravity flow from a specially designed ampoule, U (Figure 3), and are removed after completion of the measurement by admitting an inert gas through is an iron cylinder F , which forces the sample out through I. encased in glass attached to glass tube 0. The position of 0 can be adjusted to keep the Tvall a t point R free of the liquid during transfer. Sealing a t R is done with the sample under atmospheric pressure of an inert gas. This prevents sudden collapse of the heated glass, which 13 ould occur during sealing if large pressure difference existed between the inside and outside of the glass walls. Dissolved gas is removed from the sample, using the technique of alternate freezing and melting in vacuum. Final sealing a t S is done by fusion. Heat flow to or from the sample is controlled by evacuating the space b e h e e n the double-walled jacket. I n this evacuation the air is removed through stopcock E (Figure 2). The walls are silvered to minimize heat losses from radiation. Any air admitted into the double jacket space is first freed of carbon dioxide and water vapor by means of a drying column containing Ascarite and magnesium perchlorate. To prevent supercooling of the liquid during the freezing experiment, a small portion of the sample is cooled appreciably below its freezing point. Crystal nucleation is induced a t the glass partition, H (S), by inserting liquid nitrogen or powdered solid carbon dioxide into tube D of Figure 2. 4 wire nith a small flexible coil is used to force the solid refrigerant into contact with H . Nucleation of crvstals by this method is referred to as “seeding” by thermal shock. The solenoid iFigure 4) is made bv winding 5 oounds of KO. 26 B. and S. gage enaheled’copper magnet wire on‘ the brass spool, G , and it is activated with a 110-volt direct current source. The magnetic field strength is increased in the vicinity of the stirrer
Openings A and I are used to flush cleaning agents through the tube. Final rinsing is done with distilled water, A and I are sealed, and the walls are thoroughly dried by pumping the system to a pressure of 10-5 mm. of mercury by means of a vacuum system connected to F . Volatile samples are introduced by distilling them from breakoff tip ampoules sealed to a glass manifold attached to the vacuum system a t F (Figure 2). After completion of the experiment, the sample is distilled back into another ampoule on the manifold. The sample is then cooled and the ampoule sealed off. To pre-
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Figure
4. Details of encased solenoid
ironF
-4.
Turnbuckle s u p p o r t s ( h in Figure 1) B. Threaded flange, a t t a c h i n g solenoid t o connecting rod (jin Figure 1) C. Iron ring, welded to form p a r t of iron lid D. Iron lid, a t t a c h e d t o rest of t h e iron casing b y sir screws E . I r o n casing, covering brass spool, machined from standa r d 61/#-inch 0.d. iron pipe F . Brass guide, soldered t o iron casing (same a s i in Figure 1) G. Brass spool, wound with 5 pounds of No. 26 B and S gage magnet wire. H . Electric outlet, power SUPPIP t o magnet (same a s k in Figure 1)
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all dimensions in inches
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1910
ANALYTICAL CHEMISTRY
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Figure 5 .
Details of stirrer
A.
Part X
...
T h r e a d , to fit J in iron core B. Platinum-iridium rod C. Platinum-iridium weld of helices t o i/a-inch rod D. Inner helix! made b y winding downward o r e r cylinder of appropriate size (diameter of this cylinder depends upon a n y expansion i n helical coil after winding tension is removed). Size specifications in drawing refer t o this expanded condition, v h i c h is permanent set of coil. E . Outer helix, wire continued a t F t o form outer coil, E (D and E are continuation of same s t r a n d of wire). E is made i n s a m e manner as D ,b y winding wire u p . ward over another cylinder slipped o r e r D. T h e two cylinders are removed a n d two coils welded together a t C and to rod B . P a r t X . Gold plated, flashed with copper, plated with 0.005 inch of silver, plated with 0.003 inch of gold. nnlished. a n d burnished H . f r o b cylinder 1. Eight, rods, 80% platinum-20% iridium, screwed into iron cylinder a n d silver-soldered a t points I before plating. Used t o act as bearing surface to prevent wearing of gold plate J . Threaded iron plug, attaching stirrer to iron cylinder, silver-soldered before plating C . Silver-solder joints. Good plating surface is produced bv rnundine these fillets and e l m i n s t i n u s h a m ednea. P ? r t -? (platinum encased) J Plug made of 80% platinum-20% iridium alloy threaded t o a t t a c h stirrer t o iron core. Gold-soldered after assembly K . B a n d , SO% platinum-20(7, iridium alloy, encirclinplatinum foil, '/a inchJhick. acting as bearing surface Protects against wearing of thin platinum foil L. Platinum tubing, fold-soldered t o platinum rasing a t attached t o vacuum pump so t h a t casing m a y be %ed for leaks, later crimped, gold-soldered closed. and c u t off .If P. Platinum disk, 25 m m . thick V,'O. Platinum foil, 15 mm. thick 0. Soldered joint. made with high melting gold-platinum . solder R. Set screw to hold plug J ' S. Platinum fused joint T. Gold-soldered joint
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all dimensions in centimeters
by encasing the spool in an iron jacket, E. Tests shelved that the solenoid could support 4 kg. in addition to the weight of the stirrer. I n freezing experiments stirring can be continued until one third to one half of the sample is frozen. This amount of solid prevents further stirring and there is no danger of breakage, since the armature of the stirrer slips in the magnetic field. Details of the stirrer and the iron armature are shown in Figure
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230476-
vi
titonium tetrachloride purity 9996 mole %
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b y f r e e z i n g point
230474
5. The stirrer is made of an alloy containing 80% platinum and 200/, iridium. This alloy has more elasticity and hardness than pure platinum. The double helical coil ( I ) is made by winding one strand of \Tire on cylinders of the required size. TWOends of the coil are then fused to shaft B a t point C. The armature in one model, as shown in part X , was gold plated by the Electrodeposition Section of this bureau. I n the other model the encasement of the armature in platinum, part Y,rms fabricated to specifications by J. Bishop and Co. Platinum Korks, llalvern, Pa. Both types have been satisfactory in preventing any reaction with the iron.
10
T o test for leaks in the seams of the platinum foil, tube L in art Y is attached to a vacuum pump and the space betveen the roil and the iron c11inder is evacuated. Part Y is then checked with a helium leak detector. Permanent closure at L is made by melting a small piece of gold which is placed in the platinum tube prior to evacuation. The tube is then crimped and cut off about 1 inch above Q.
30
50
40
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TIME IN MINUTES
I
I .24 4 0 ° C
Ln
23 0 2 0 I-
I: L 0
5
2 3 015
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Extreme care must be taken to prevent flaws in the gold plate. The platinum type has the advantage of permitting the glass freezing-point tube t o be annealed after the stirrer is sealed inside. I n the other case this cannot be done, because the gold plate might blister.
20
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;I23 010 E W m
E
1
2 3 005-
T
0I O C
titanium tetrachloride purity 9 9 5 mole %
I
10
20
30
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40
50
' A
-_ 60
70
TIME IN MINUTES
Figure 6.
Freezing curve of titanium tetrachloride at saturation pressure
Srale of ordinates gives resistance i n ohms of platinum resistance thermometer (0.1 ohm is approximately 1.0' C.)
1911
V O L U M E 2 8 , NO. 1 2 , D E C E M B E R 1 9 5 6
point of titanium tetrachloride under saturation pressure n-ith zero impurity, Tjo, was calculated t o be -24.10" & 0.01" C. Figure 6 shows curves of experiments done under saturation pressure. The curve in Figure 7 was obtained ivith the sample under 1 atm. of dry nitrogen. I n other experiments, not shown in the figures, the temperatures of the freezing curve of a sample of titanium tetrachloride a t saturation pressure were observed to be 0.007" C. lower than the corresponding temperatures of the freezing curve of the same sample under a pressure of 1 atm. of nitrogen.
LL
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+
23042
1-
titanium tetrachloride purity 99 9 3 ma!e %
'L (r,
w
23 0 4 0 -
LITERATURE CITED I IO
20
35
40
50
60
1-1 70
T I M E IN M I K L T E S
Figure 7.
Freezing curve of titanium tetrachloride under 1 atm. of dry nitrogen
Grale of ordinates gives resistance in ohms of platinum resistance ttiermometer (0.1 o h m is approximately 1.0' C.).
EXPERIMESTAL DATA
The procedures for determining the purity from time-temperatiire freezing and melting curves and the principles involved are given in earlier publications ( I , 8,4-7). Three typical freezing point curves obtained with titanium tetrachloride samples of different levels of purity was shown in Figures 6 and 7 . From a series of experiments such as these a value for the freezing
(1) Glasgow, A. R., Jr., Krouskop, S . C., Beadle, Joan, hsilrod. G. D., Rossini, F. C., ASAL. CHEX 20, 410 (1948). (2) Glasgow, 8.R., Jr., Krouskop, S . C., Rossini, F. D., I b i d , 22, 1521 (1950). (3) Glasgow, A. R., Jr., Ross, G. S., J . Research SatZ. Bur. Standards 5 6 , 137 (1956). (4) Glasgow, -1.R., Jr., Streiff, A. J., Rossini, F. D., Ibid., 35, 353 (1945). ( 5 ) lIair, B. J., Glasgow, A. R., Jr., Rossini, F. D., Ibid., 26, 591 (1941). (6) Schwab, F. W., Wichers, E., "Temperature, Its IIeasurement and Control in Science and Industry," pp. 256-G4, Reinhold, S e w I'ork, 1941. ( 7 ) Taylor, W.J., Rossini, F. D., J . Reseaich S a t l . Bzar. Standards 32, ,197 (1944). RECEIVED for review M a y 3 , 1966. Accepted August 30, 1 9 X . Division of Analytical Chemistry, 128th Meeting, ACS, Minneapolis, Blinn., September 1955. Work partially supported b y t h e Edgewood h r m y Chemical Center.
Factors Affecting Use of Dielectric Methods in Determination of Sea Water in Navy Special Fuel Oil T. D. CALLINAN, R. M. ROE, and J. B. ROMANS U. S. N a v a l Research Laboratory, Washington 25, D. C. The dielectric constant, per cent power factor, dielectric loss factor, and conductivity of a number of boiler fuels were evaluated as a means of measuring the sea w-ater content of Navy Special fuel oils on shipboard. These properties varied considerably from one fuel oil to another at comparable frequencies from 10 kc. to i 5 RIc. When these oils were emulsified with synthetic sea water, the values so obtained increased with increasing sea water content but remained a function of the characteristics of the original fuel oil. The necessity of using a dry reference sample is emphasized.
I
Ii T H E course of investigating the causes of fuel slagging in
Xavy boilers, it became increasingly evident that information concerning the degree of contamination of Navy Special fuel oil by sea water nould be of considerable value. Present analytical methods, n-hile highly accurate, are timeconsuming and of little assistance under actual operational conditions. It would be desirable, therefore, to have a compact and rugged instrument that would indicate, or record continuouslj variations in the salt water content of Navy Special boiler fuel. S o suitable instruments or devices are available a t the present time, although several methods have been proposed for the determination of sea viater in fuel oil, each based on the measurement of some m-ell-establishedelectrical property of hydrocarbons or of water ( I O ) . Among several methods proposed n-ere those for the determination of salt water content of emulsions by ( a ) determination of the dielcctric constant of the emulsion or
the change in dielectric constant as a function of salt water content, ( b ) change in per cent power factor, ( c ) change in dielectric loss factor, and ( d ) change in conductivity. Although dielectric constant measurements have been used to follow the progress of oxidation in petroleum products, to determine impurities in transformer oils, for the detection of m-ater in hydrocarbons, and the like ( l e ) ,such determinations are made on highly refined, essentially nonpolar materials. The impurity usually represents only a trace or a t least a very minor constituent of the product under examination. In contrast, boiler fuel oils, including Navy Special fuel oils, are complex mixtures of variable composition, chiefly hydrocarbon in nature, containing appreciable amounts of polar compounds. I n addition to aromatic and paraffinic compounds knonm to be present, S a v y Special fuel oil contains large amounts of asphaltic and resinous materials of unknown composition, oxygenated bodies, sulfur and nitrogen compounds, organometallic compounds, and adventitious niaterial picked up in the course of refining or delivery. THEORETICAL COh SIDERLTIOY S
AIaxwell has given a rigorous calculation for the determination of the dielectric ronstant of a dispersion of tn.0 different substances of different dielectric constants. hlaxu-ell's original formula can be replaced by the generalized empirical formula, (13) E= = 2 , 8 , € , K
(1)
u ithout great inaccuracy. The so-called logarithmic mixture rule results when the exponent K above is eliminated. The resultant formula rvhich is used in practice ( 2 , 13) is
