A Simplified Freezing Point Apparatus Gaylon S. Ross and Augustus R. Glasgow, National Bureau of Standards, Washington 25, D. C.
n-c WAY to follow the course of a sepa-
0 ration or purification process is to
determine the freezing point. of the fractions obtained, and an inexpensive and rugged apparatus has been developed for this purpose. It is easily cleaned, allows small samples to be introduced and removed with a minimum of contact with air, and provides for automatic seeding of the sample. The construction details for an 8-ml. capacity cell are shonn in Figure 1. Either a thermocouple or a thermistor may be used as the temperature-sensing element. K h e n a thermocouple is used, the precision and accuracy of the temperature measurement is dependent upon the temperature stabilitv of the reference junction, the type of potentiometer, and the sensitivity of the detector. Likenise, nhen a thermistor is used, the precision and accuracy which is obtained depends upon the stability of the Wheatstone bridge, the stability of the voltage/current source, and the sensitivity of the detector. After the apparatus has been cleaned and thoroughly dried, the cell is filled with dry inert gas and the Serum cap is put in place. The serum cap prevents interchange of the gas within the cell and the atmosphere. The sample can be introduced into the cell in several ways. When a series of samples of differing degrees of purity are to be measured, they can be most easily introduced and removed through a serum cap H by means of a hypodermic syringe of the desired capacity. Pouring techniques can be used by substituting a glass standard taper joint and cap for the serum cap, the sample can be poured or distilled into the cell under vacuum conditions by using a glass manifold and the technique described by Glasgow and Tenenbauni ( 2 ) . K h e n a serie; of relatively impure samples of the same substance are analyzed, the cell does not need to be cleaned thoroughly after each purity analysis. I n this case, nith either serum cap or tapered joint, it nau found useful to mount both band clamps and the stirrer motor on a vertical rod. The entire asfenibly could then be rotated around a single, pivotal mount without changing the alignment. The samples are either poured into the cell or introduced by means of a hypodermic syringe. Removal is accomplished by tilting the entire asqembly. The rubber bellon s are vacuum-tight and t h e apparatus can be dried by evacuation. However, the bello\? s bind against the stirrer when the cell is evacuated and the samples must be analyzed under a pressure of one atmosphere (dry air or inert gas can be used). The freezing bath used with this type of cell is selected so that its temperature
700
ANALYTICAL CHEMISTRY
will be at least fifty degrees below the freezing point of the sample. The seeding loci, L and N , are close to the outer wall of the vacuum jacket, and the sample a t these points has a fifty degree gradient between its coldest portion and the bulk of the sample. Consequently, crystals are first formed at some position in these protrusions; these act as seeds to induce the bulk sample to crystallize. The sample, when seeded in this way, undergoes very little supercooling and therefore attains good temperature equilibrium when only a small amount of solid phare is present. Thi. latter condition is beneficial in that a shorter extrapolation of observed data is required to obtain the freezing point of the sample. Stirring is accomplished with a nickel stirrer moved by means of a vacuumactuated reciprocating motor. Both the motor speed and the stirring stroke length are variable. Seoprene and silicone rubher bellows have been used. Yeither caused contamination of the samples which were being analyzed. However, commercially available polytetrafluoroethvlene bellows could be used to eliminate possible contamination of samples which react with rubber. The time-temperature freezing curves have been both manually and automatically recorded (1, 3 ) . Both the Leeds and Sorthrup student-type potentiometer and the Leeds and Sorthrup Universal K-3 potentiometer have been used. In both cases a Leeds and Korthrup microvolt indicating amplifier was used as the detector. The Kheatstone bridge was constructed so that all four resistance arms, of which the thermistor as one, could be set a t nearly equal resistance. The beadshaped thermistors were chosen so that their resirtance at the desired teniperature \vas between the limits of 200 and 2000 ohms. Again, the Leeds and Korthrup microvolt indicating amplifier was used. In both cases temperature differences of *O.O0lo C. were easily detectable. Manually obtained data were analyzed by the method of Taylor and Rossini (5), but when the freezing curve was automatically recorded, the optical projection method described by Saylor (4) was found to be the most useful. The limits of precision and accuracy obtainable with any type of cryometric apparatus are dependent upon the ability to maintain good thermal and thermodynamic equilibrium within the sample. Regardless of the particular apparatus, this limit is different depending upon the type of material being analyzed and upon its purity. However, the sensitivity of the thermometric equipment is usually the limiting factor in Fimple cryometers. On the other hand, the accuracy with which the
Figure 1.
Freezing point apparatus
A, Brass slirrup attached to reciprocating motor drive; 6 , Thermocouple or thermistor leads; C, Glass cap attached to rubber bellows; D-D‘,Band clamp holding bellows and Elass cap, C; E X ’ , Brass yoke clamped on nickel stirrer ring lhrough rubber bellows; F, Rubber bellows; G-G’, Bond clamp to hold bellows and freezing-point cell; H, Rubber serum cap. This may b e replaced b y other filling devices; I, Nickel stirring rod, terminating in a single J, Silvered vacuum jacket; helical stirrer; K, Dewar for freezing bath; I, Seeding device; M, Platinum blade protruding from glass thermocouple tube. In use, thermal contact is made between temperalure sensing junction and the platinum blade b y a solder connection inside the glass tube; N, Second seeding device; 0, Constant-temperature warming or coaling liquid; P, Thermocouple well; Q, Nickel stirrer ring to which stirring rod i s fastened
melting point can be determined is largely dependent upon the calibration of the thermometric assembly. The cell described here has been used for a large number of samples of the boron hydrides, titanium telrachloride, and derivatives of styrene and ethyldichlorobenzene. I n general the purities of materials in the range of 85 to 99.9 mole per cent can be readily determined using this type of freezing-point cell. When the thermometric equipment was cali-
brated in the manner described in reference ( I ) , the melting points of materials in the general purity range of 99.+ mole per cent were determined to appwxiThe estimated mately *O.0lo C. standard deviation of a typical purity analysis when five analyses were made was 0.01 mole per cent. LITERATURE CITED
(1) Glasgow, A. R., Jr., et al., Anal. Chim. Acta. 17, 54 (1957) and republished
by W. M. Smit, “Purity Control by Thermal Analysis,” Elsevier Publishing Co., Amsterdam, 1957. (2) Glasgow, A. R., Jr., Tenenbaum, M., ANAL.CHEM.28.1907 (1956). (3) ROSS, G. s.,~ i x o nH. , D.,.J. Research NBS 64C. 271 (1960). (4) Saylor, ‘C. P:, Anal. Chim. Acta 17, 35 (1957) and republished by W. M. SGit, “Purity Control by Thermal Analysis,” p. 180, Elsevier Publishing Company, Amsterdam, 1957. (5) Taylor, W. J., Rossini, F. D., J. Research NBS 32, 197 (1944).
Stand-By and Operational Apparatus for Removal of Oxygen from Commercial Nitrogen Paul Arthur, Deportment of Chemistry, Oklahoma State University, Stillwater, Okla.
engaged in routine work or in research, many chemists find frequent need for a convenient method to remove oxygen from commercially available grades of nitrogen and other gases so that these gases can be used for such things as the removal of oxygen from solutions or the providing of inert atmospheres for reactions. The ideal apparatus for this would be ( a ) efficient, convenient, and clean in operation, (6) self-indicating as to the condition of the charge, ( c ) ready to use even after standing f x long periods without being used, (d) of such type as to operate a t room temperature, and ( e ) of a type which would introduce no new components into the gas stream. Alkaline pyrogallol scrubbers lack in characteristics ( a ) , ( b : , and ( e ) , the alkali being corrosive to glass in the scrubbers and freezing glass joints, the reaction introducing traces of carbon monoxide into the gas, and the solution being so dark in color e\ en when in good condition that its chxge cannot be determined visually. Hot copper turnings in glass apparatus (1) or activated copper on kieselguhr a t 180’ C. 115) meet all the requirements except ( d l ; but the need for glass equipment to make the copper visible complicates design since the gases must be cooled quickly before they are used. Many other suggestions have been made including the u.se of vanadous sulfate (4) or chromous sulfate (3) formed in acid solution by reduction of vanadyl sulfate or chromic sulfate, respectively, by amalgamated zinc. For hydrogen, or for nitrogen containing hydrogen (pres :nt normally or added deliberately), passage over a platinum black catalyik (2) has been suggested. Although the vanadyl-vanadous reaction probably would have worked as well, the chromic-chromous reaction was the one originallj and presently employed in the apparatus described here. The reaction iiitroduces small HETHER
amounts of hydrogen into the gas, but fortunately this hydrogen is harmless for many applications. On the other hand, the transparent blue color of the chromous solution contrasts so well with the dark green color of the chromic solution that the condition of the solution is readily apparent, while the formation of a precipitate indicates any need for more acid or for replacing both acid and chromic solution. Although designed originally for polarographic applications, this apparatus has proved to be so good in all respects that it is currently in use, a t Oklahoma State University, not only by polarographers but also by organic chemists needing an inert atmosphere for reactions and by physical and inorganic chemists needing to remove possible oxygen interference in electrode reactions of many types. For the reactions involving these substances, the important equations are Zn(Hg)
+ 2Crf3
-+
Zn+Z
+ + 4&0+
4Cr+2
0 2
+ 2Cr+2 + Hg
+
4Cr+3
+ 6H20
while the principal side reactions are 2Cr+2
+ 2H30+
-+
+ 2Hz0 + H2 Zn+2 + 2H20 + Hz
2Cr+3
Zn
+ 2H30+
-+
II
c
EXPERIMENTAL
The apparatus functions as follows (see Figure 1). The gas passes, in order, through a safety trap, A , a scrubber, B , a second safety trap, A’, and a second scrubber, B‘, then through a spray trap, E , and finally a drying trap, F. Since, when the gas flow is turned off solution usually moves back out of the scrubbers, traps A and A’ are designed so that not only is this solution caught, but when the gas is turned on again the solution is forced back into the scrubbers. The scrubbers themselves are designed so that as the gas to be purified bubbles through the acid-chromous ion solution, the bubbles lift the solution and pump it over amalgamated zinc so the chromic ions formed are rapidly reduced again. Parts C and C‘ are bulbs into which the acid-chromium ion solutions from the scrubbers are forced by nitrogen pressure for storage when the apparatus is put in stand-by condition. Construction Details. Most of the information needed for fabricating the required parts will be evident from Figure 2. All constructed parts are made of borosilicate glass of ordinary thickness, the diameters indicated being outside diameters. Smaller diameter tubes shown, such as all the inlet and outlet tubes, the “TI’ part of D, and the inner tubes of A and D, are 6-mm. 0.d. The reactor-scrubber, B, requires some explanation. The central tube, 20-mm. 0.d. and 16 cm. long, must be
_ _ _ _ _ _ _ _ _ _ _ :1 _ _ _ _ _ _ _ _ _ _ I J
Figure 1.
J
L
Assembly schematic VOL. 36, NO. 3, M A R C H 1964
701
