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
4Cr+2
+ + 4&0+ 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, MARCH 1964
701
Figure 2.
Details of construction
centered and its bottom must be attached, through a perforated glass flare (see cross section S), to the sides of the jacket. Three spaced glass rods, P , sealed to its upper end and to the jacket give added strength. The gas dispersion tube, Q, is centered in the central tube, while amalgamated zinc rods fill the space, R, between the central tube and the jacket. With this arrangement, when bubbles of the gas being scrubbed rise through solution in the central tube, the solution is lifted upward through the length of the tube to spill over its top and flow downward over the amalgamated zinc. The reduced solution passes on down through the perforations and into the bottom of the scrubber to be lifted again in a continuous cycle. Part D , a pressure-relief valve, is filled with mercury to a depth of 35 to 40 mm., a loose cotton plug to prevent spattering of mercury being placed in the vertical tube K . The spray trap, E , is filled loosely with either cotton or fine glass wool, while F is filled with a coarse-e.g., 6- to 10-mesh-solid drying agent such as indicating silica gel, wads of glass wool a t the bottom and top being used to keep the drying agent in place. A11 standard taper ioints should be fastened, either by ;sing a fusible cement or by using stopcock grease lubrication and wire spring holders, to keep the joints from parting under gas pressure. Parts G, H,and J are 2- to 3-mm. stopcocks of the type apparent from the diagram. Two each of parts, -4,B, and C are normally needed (see d and A’, B and B’, and C and C’ in Figure 1) for purifying commercial grades of nitrogen. Assembly and Operation. I n Figure 1 the method of connecting t h e parts is illustrated. Dotted lines represent connecting tubes, 3/16-inch i.d. and of either Tygon or surgical rubber. G, H,and J are as described earlier, while L is a “T”-tube of 4. to 5-mm. i.d. Zinc rods inch in diameter and about 6 inches long, and sufficient in number (usually about 10 rods, or 2l/2 to 3 pounds) to fill the space R, should be placed in B. Since the rods are 702
ANALYTICAL CHEMISTRY
heavy and hard and the glass is fragile, it is best to put the rods, one a t a time, in a glass tube just large enough in diameter to accommodate the rod, using this tube with a plunger to slide the zinc rods into position. A solution about 0.1N with mercuric nitrate and I N with nitric acid should then be poured into B to cover the zinc rods and allowed to stand for 30 minutes or until the rods are completely covered with mercury. Bubbling a slow stream of nitrogen through the solution so the solution will circulate is helpful at this stage. When amalgamation is complete, drain off the spent reagent and wash the rods with distilled water. Finally, put in enough of a solution 0.1M to 0.15M with chromic ion (chromic sulfate or a chrome alum such as potassium chrome alum may be used) and about 3 N with sulfuric acid barely to cover the upper end of the central tube; then add about 50 ml. more. Drain the 50-ml. excess off into the storage vessel C (using nitrogen pressure, if necessary) to be used as volume replacement for evaporation, then close stopcock H and fasten or cement the top of B into place. Repeat with B’ and C’; then turn nitrogen on so it flows slowly through B and B‘. Removal of oxygen will start almost immediately and within a b m t 30 minutes the green color of chromic ion normally will change in B‘ to a blue color indicating that the apparatus has reached top efficiency. If this blue does not appear (for very low percentages of oxygen it may appear even in B ) the percentage of oxygen is unusually high and it may be necessary to add one or more additional scrubberreactors like B to the train. Such, however, would show that inordinately high percentages of osygen were present in the gas-much more than has ever been encountered here. When the apparatus is not to be used for a period of several days or longer, the solution from B and B’ should be stored in C and C’, respectively. This will conserve both the acid and the zinc-the only substances actually used up in the operation of this ap-
paratus. To transfer the solution from B, open stopcock H to connect B to C and use nitrogen to force the solution from B into C. Finally close H to prevent return of the solution to B. By similar action, transfer solution from B’ to C’, closing H’to keep the solution from returning to B’. During such storage, any blue chromous ion will disappear by reaction with the acid and water, the pressure developed by the hydrogen so formed being vented from both C and C’ through D. When the solution is stored in C and C’, all reaction ceases once the solution still wetting the amalgam and the chromous ions in the solutions in C and C‘ are used up. Consequently, even after months of standing the apparatus can be reactivated within moments. To do this, turn G so a slow stream of nitrogen flows to L , and carefully use the following procedure, Open H’ so C’ is connected to B’, and close K and the outlet from F . Nitrogen pressure will force solution from C’ to B’. When enough solution is in B’, close H’and open H to connect C to B, forcing solution from C into B. Finally, close H and at once turn G so nitrogen flows through A . Although usable at once, when the solution in B’ becomes definitely blue, the apparatus will have attained maximum efficiency, The acid is spent when a precipitate appears in quantity in the solution. This normally \+-ill occur in B long before i t occurs in B’; in fact, B often requires recharging several times before recharging is required for B’. To recharge B , open H and J and drain both B and C. Fill B with about 11%-sulfuric acid to dissolue any precipitate, and drain. Finally, although it is possible t o recharge once merely by bringing the acid concentration back to 3 N , it is best to discard the spent solution and refill with fresh chromic ion-acid solution as before. Recharging of B‘ is done similarly. DISCUSSION
The speed and efi.ciency of this apparatus depend upon the cycling action of the solution and the consequent rapid regeneration of chromous ion as it passes over the amalgamated zinc. The actual efficiency of the whole process is evidenced by the fact that polarographic tests in nonaqueous solutions made with nitrogen containing over 1% oxygen originally, showed no indications of oxygen though as little as 0.0001% in the purified gas could readily have been detected. LITERATURE CITED
( I ) Fieser, L. F., J . -4m. Chem. SOC.46, 2639 (1924). ( 2 ) Irving, H., Shelton, R., Evans, R., J . Chem. SOC.1958,3540. (3) Kolthoff, I. ,,M., Lingane, J. J., “Polarography, Tol. I, 2nd ed., p. 396, Interscience, New York, 1952. (4) Meites. L., hleites,, T.,. h A L . CHEM. ‘ 20, 984 (1948). (5) Meyer, F. R., Ronge, G., 2. Alzgew. Chem. 52, 637 (1939).
