in the train and adjust the air flow to 0.725 liter per minute. Raise the water bath to contain tubes E and F. After 1 minute remove the HF inlet stopper at N (with air flow on) and add 0.5 ml. of HF reagent to generator tube E. Replace the stopper quickly. After 15 minutes (with air flow on and to llrevcnt any backwash) remove the H F inlet stopper, disconnect the vacuum pump, loosen the stopper in tube F , and remove absorbing tube G. Rinse the inlet bubbler into tube G with 10 ml. of water. Pour the contents of tube G into a 5O-ml. plastic vial, add 15 ml. of reducing agent, and mix for 15 minutes. Allow this to stand for 2 to 3 hours (99% of the color develops in 2 hours). Remove generator tube E and rinse the inlet bubbler with acetone. Absorber tube G with a clean and dry inlet bu1)l)ler can be replaced and readied for the next analysis. Change the special H2S01 acid in tube F after eight determinations. After 3 hours of development, transfer the samlde from the plastic vial to a 50-in1. volumetric flask and fill to the mark with water. Measure the color intensity on the spectrophotometer between 750 and 850 inp. Plot the a1)sorl)ance at the peak (approximately 812 mp) against the concentration of Si02 in micrograms for the standard curve. The procedure for samples is the same as any one determination above. High concentrations of silica are controlled by diluting the solutions to the 5- to 50-pg. level.
DISCUSSION AND RESULTS
Various solutions containing KOH, NH40H, saturated CaO, Ca(OH)*, Ba(OH)*, BaCL, and distilled water were tried as absorbents for SiF4. Because water proved t o be best, the procedure was simplified and time was saved by using aqueous acid ammonium molybdate as the absorbent. Thus, the SiF4 was simultaneously absorbed and converted to silico molybdic acid during the 15 minute absorption. Experiments proved that absorption periods less than 15 minutes gave low results. Also, a t least 10 minutes was necessary for the conversion of the SiFl to silico molybdic acid ( 3 ) . It was determined from a yield curve that 0.5 ml. of the HF reagent was enough to drive the reaction to completion under the conditions of the experiment. Various temperatures were tried in the water bath. Above 90' C., the polyethylene tubes were too pliable and too hot to handle. A 70' C. constant temperature water bath was therefore adopted. Dry air was the only medium used as a carrier in the experiments. Two separate fusion samples were used to obtain data for the standard curve. One determination at each level of concentration was made on each fusion on 3 separate days (an average of six determinations for each point on the curve). The result was a linear curve.
Table I shows good agreement in the results of the three techniques used in the analysis of the silicate samples despite the fact that they contained other elements in high concentration, that they had to be fused, and that the diffusion and absorption samples mere determined in the microgram rangc. The results obtained by absorption were in the range of 25 to 40 pg. The above technique permits rapid determinations of micro amounts of silicon in aqueous solution, or in samples in which the silicon is made soluble I)y alkali fusion. The success of the absorption technique for the determination of SiOv depends ul)on three important factors: Dry air must be bubbled through the solution in generat'or tube E and in absorption tubes F and G. No condeiisation should be present in the exit tubing from the generator to the absorber lest the SiF, be premat,iircly absorbed. Specially prepared concentrated H2SOl must be used in the generator and in the tube just behind the generator. LITERATURE CITED
( 1 ) Alon, A., Bemas, B., Frankel, lI., Anal. Chzm. n c / a 31, 279 (1964). ( 2 ) Hozdic, C., Ibzd , 33, 567 (196.5). (3) Miillin, J. B., ILiley, J. P., Zbzd., 12, 162 (19%). (4) "Standard Methods of Chemical Analysis," 6th ed., Vol. 1, p. 956, N. 11.
Furman, ed , Van Nostrand, New York, 1062.
Apparatus for Rapid Degassing of liquids Rubin Battino' and F. David Evans, Chemistry Department, Illinois Institute of Technology, Chicago, 111.
arc iiiimerous occasions when Teslic~iincnt~al studies require the )arat>ion of gas-free liquids. HERE
1iiq
alq)aratus for the rapid and efficient degassing of a variety of liquids in quant8itics up to 1 liter is described in t,his I q r r . The apparatus was develolid s~mifically for measureinents of the solubilities of gases in liquids, but it should lirove useful for other pur1)oscs also.
G a m lrave l~ecnrcinoved from liquids :I niinibcr of incthods which can be cat ygoi'izcd as: (a) simple boiling; (b) ev:icuation above thc liquid with or without heatring; and (e) evacuation above the frozen solvent. I3unsen ( 3 ) , follo\retl by many other workers boiled thc solvciit and t.hrn allowed it to cool untlc,r vacuu~n. Leduc ( 7 ) found that evm aTtclr 1)rolongedl)oiling, water gave 111) g : 1)iil ~ )l)lw on fiwzing. Succwsivc fi,ccsziijg ant1 iridting untlrr vacuuni is an cxffrctive i?ietlicitl of degassing, but it is rat1ic.r inconvrnirnt for largr samliles 1)y
and frequently requires more than six phase-change cycles even with small sain1)les. Hihben (6) employed the method of sublimation in vacuo, and Paunov (8) made use of the observation that ultrasonic frequency radiation affects the solubility of gases in liquids. I3aldwin and Daniel ( I ) degassed the oils they were studying by permitting the oil to drip into an evacuated chamber. They reported a n efficiency of 97-98Cr, which is insufficient for Irrcision irork. (:Lever et d.(4) ptirii~)ed on the boiling solvent until 10-2UY0 of the liquid was evaporated. The liquid was t.hen transferred to another vessel and sprayed through a fine nozzle into a n evacuated flask. Although this procedure was effective, the transfer st,rl) and tlic single stage s1)raying could be iin])rovcd upon. Cook ( 5 ) degassed his solvents by evacuating above the boiling solvent and removing about 20y0 of the solvent. He checked for completeness of (le-
6061 6
gassing by two techniques: cushioning the liquid between mercury and looking for undissolved g w buhl,les, and following the degassing by observing a thwmocouple gauge placed with a cold trap between the boiling solvent and the gauge. The solvent was considered to be degassed when the pressure on tlic thermocouple gauge reached the base pressure of the vacuuni system. (Tllc boiling was continucd :rntl some ad(litional qolvciit,reinovcd T:iylor (9) cnil )loyctl a circiiht )I y system coupled with a coiidciisiirg coluinn leading to vacuuin. The circulation was achieved by heating the liquid in a side arm of the main vessel, and a mixture of bubbles and liquid moved from the side arm into the inain vcwel. The disadvaiitnqv of tlii-: l i i ' o ccvl I irc iwre that heat-scti 1-i 1 ivc 1i (11 I i t 1s Present address, Chemistry Department, Wright Stnte College, Dayton, Oliiri
Vr'l
4XIl '! '
11.
0\+'3BER
1966
1627
could not be treated, and that liquids (like water) which have a great tendency to "bump" would behave erratically. However, the condenser does minimize the loss of solvent from the degassing vessel (to a cold trap.) The apparatus to be described incorporates features of the Cook, Clever, et al., and Taylor approaches in a manner calculated to achieve a versatile apparatus for the rapid degassing of liquids.
cross-section about 10 mm. long and 0.5
s-1
t
VAC.
=3 ..*=1
APPARATUS
The apparatus shown in Figure 1, is drawn to scale as indicated, and consists of two parts: the pumping section and a liquid reservoir. The principle of the method is to circulate the liquid through an evacuated space, thus prebnting a continuously renewed large surface area for outgassing. The liquid reservoir section was fabricated from 74-mm. 0.d. tubing, and has a capacity of about 1 liter when the liquid level is 5 cm. below the nozzle. To aid degassing a magnetic stirrer, Ai?, keeps the liquid in motion within the reservoir. The joint near S-1 is used for introducing the solvent, and the stopcock S-4 leads to a storage spiral in the gas solubility apparatus. Connection to the vacuum is past the cold-finger A via stopcock 8-2. Coolant at about -10" C. is circulated through the cold-finger condenser to minimize the loss of vapor and to enhance the degassing process by recycling a certain amount of liquid by evaporation-condensation. (The procedure of introducing a cold-finger into the degassing chamber can certainly be used in other types of degassing apparatuses to minimize loss of solvent.) If necessary, the liquid in the reservoir can be heated by using a heating tape or a heat gun. With volatile solvents the bulk of the liquid cools because of the combined effects of forced evaporation and cool liquid returning from the cold-finger. The solenoid in the pumping section introduces a fair amount of heat into the liquid by heating the iron nails in the piston. The all-glass pumping section is detailed to scale in Figure l. The pump cylinder was constructed from a length of 19-mm. 0.d. tubing. The piston, P , fits fairly snugly in the outer tube while permitting free movement. The hole through the center is 4 mm. in diameter, and the annular space is filled with iron nails (shaded in the
mm. wide. The flat portion is horizontal and this results in a fan shaped
APF?
Figure 1.
Degassing apparatus
figure). The ball valve, V-1, has a rod extending through the length of the piston. To assure a proper seal the ball was hand-ground into the center hole using fine carborundum powder. (The valve, V-1, can also be constructed from a ground ball and socket joint.) The piston was lifted by energizing the solenoid, S (Cat. No. 18891, the Harry Alter Co., Inc., Chicago, Ill,), through a variable transformer which controlled the force of the piston movement (80-90 volts was sufficient to produce smooth action) and an industrial timer (Industrial Timer Corp., Parsippany, N. J., Model CMO) to control the rate of cycling. The timer was operated 25% on and 75% off to minimize the heating effect of the solenoid and to permit sufficient time for the piston to return to rest. The fastest cycling period used was "5 second. A bumper made of Teflon, T,was held in place with vigreux indentations and was used to cushion the fall of the piston. Under these conditions the liquid w&s circulated at a rate of about 1 ml. per second. On the upstroke the liquid moves past the valve, V-2 and through the nozzle, N . This nozzle has a flat
Table I. Comparison of Degassing Rates' for Three Liquids
Liquid Water
Without circulation Prolonged evacuation was very difficult because of severe bumping. The rate of degassjng was much lower than with the circulation method. 2 hours to reduce the pressure of 350 microns. Olive oil 2 hours 10 minutes to reduce the pressure to 400 microns. Benzene 5 minutes a To reduce the pressure to the base pressure (15 microns) of the vacuum system.
1628 *
With circulation 30-40 min. (no bumping)
ANALYTICAL CHEMISTRY
spray moving through the evacuated space and then flowing down the reservoir wall. Although we did not investigate this variation, it should prove possible to substitute a peristaltic pump for the pumping section. V-2 was modified from a commercially available ground float (Cat. No. JV9090, Scientific Glass Apparatus Co., Bloomfield, N. J.). The modification involved sealing an iron nail within the float to permit the float to be manipulated with a magnet. Valve V-2 is limited in its movement by two sets of vigreux indentations. The lower set (see the detail in Figure 1) is only indented slightly to minimize breakage since the movement of V-2 can be quite forceful. I n operation a vapor space forms in the region directly below V-2 and being able to move 11-2 aside allows the pump to avoid the build-up of this vapor or gas space. If the gas space is too large, the pump action fails. The absence of this gas space appears to indicate that the pressure throughout the system has equilibrated, that is, that degassing is complete (to the limits of the pumping pressure). Stopcock 8-3 leads to vacuum, and the thermocouple gauge, TC, is positioned to record the pressure of the noncondensible gases in the system. When 5-3 is closed with S-2 open it is possible to measure the static noncondensible gas pressure in the system and this value is a better measure of the extent of degassing than the pressure determined with S-3 open. When degassing is complete the solvent is transferred to a previously evacuated storage spiral via stopcock 8-4. The liquid's own vapor pressure is used to drive it over into the stjorltge spiral, the liquid in the reservoir being heated where required. In this manner the transfer of the degassed liquid is carried out without any contact with atmospheric gases. COMPARATIVE TESTS
The three liquids used for comparative testing were water, olive oil, and benzene. These are of particular interest to us and also show a great range of behavior in terms of vapor pressure, viscosity, and gas solubility. I n all cases the time required to degas the liquid without circulation was manifestly longer than with circulation. The comparisons were made by first determining the time necessary to reduce the permanent gas pressure above the liquid to the initial base pressure of the vacuum system with circulation of the liquid and, then to repeat the procedure but without circulation. The results of this comparison are presented in Table I. A very good test of efficient degassing is to memure the solubility of a standard gas in a standard liquid ( 5 ) . Battino and Clever (2) have proposed that the solubility of oxygen in water a t 25' C. and 1 atm. should be adopted as stand-
ard; a Bunsen coefficient of 0.02847 is recommended for this system. The average result obtained using water degassed with the apparatus described herein was 0.02834 which is within 0.5% of the recommended standard value, and this is in excellent agreement.
linkage to the point of use; (4) flexibility in heating or cooling so that liquids with a wide range of properties can be degassed, and (5) a fine spray action. Other apparatuses have incorporated one or more of these features but not all five. ACKNOWLEDGMENT
CONCLUSION
The comparative tests of degassing and the solubility determination indicate that the degassing apparatus described in this paper is both rapid and efficient. The particular features of the apparatus which improve its performance over earlier methods are: (1) cyclic operation with constant renewal of the liquid surface to vacuum; (2) constant monk toring of the pressure; (3) an anaerobic
Roman Witt, glassblower, is thanked for suggesting the all-glass pump and subsequently constructing it. We also thank R. C. Kintner for the high-speed motion picture studies he did of the pumping section in action. LITERATURE CITED
(1) Baldwin, R. R.,Daniel, S. G., J. Appl. Chem. (London) 2, 161 (1952).
(2) Battino, R.,Clever, H. L., Chem. Rev. 66, 395 (1966). (3) Bunsen, R. H.,Ann. Chim. et Phys. 43, 496 (1855); Phil. Mag. 9, 116, 181 (1855). (4) Clever H. L., Battino, R., Saylor, J. H., bross, P. M., J. Phys. Chem. 61. 1078 (1957). (5) Cook, M. W., U. S. At. Energy Comm.-UCRG2459, Jan. 14, 1954, p. 116 (Ph.D. Thesis). (6) Hibben, J. H.,Bur. Std. J . Research. 3 , 97 (1929). (7) Leduc, A., Compt. Rend. 142, 149
.--
(1906). ~ - , (8) Paunov, P. G., Ann. Univ. So&, 11, facuUe Phys.-Math., Levre 1. 35, 179 (1938-39); Chem. Abstr. 34, 2225 (1940). (9) Taylor, R. K.,J . Am. C h m . SOC. 52,3576 (1930).
WORK supported by Public Health Service via grant No. GM 12071
Chemical Pumping with Sodium-Potassium in Mass Spectrometer and Other High Vacuum Systems John Roboz, Air Reduction Co., Inc., Central Research Laboratories, Murray Hill, N. J.
OETTERING
PROPERTIES
Of
TFzium-potassium alloy (NaK) can be utilized to remove many troublecausing residual gases in high vacuum systems. Possible applications include the ion source and analyzer sections of spark source mass spectrographs, isotope ratio instruments, and, in general, high vacuum systems where the presence of traces of oxygen, water, hydrocarbons, and carbon monoxide are especially harmful. In a recent paper (3) the author reported the development of a mass spectrometric technique for the analysis of specific impurities in ultrapure oxygen a t the fractional p.p.m. level. In this technique the oxygen matrix is removed from the gas phase with 100% efficiency by employing liquid sodium-potassium alloy for chemical pumping. Those impurities (Nz! Ar, rare gases) which do not chemically react with the NaK are analyzed by the mass spectrometer in a highly concentrated form (impurity enrichment factor >106) and without any oxygen interference--e.g., CO formation in the ion source, etc. It was demonstrated in the course of this work that traces of hydrocarbons, water, carbon monoxide, etc., are quantitatively and permanently removed together with oxy-
gen by the sodium-potassium alloy. NaK may thus be used as a final p u m p ing stage to provide a background-free ion source, etc. The vapor pressure of NaK a t room temperature has not yet been reported, but on the basis of available information (2) it is estimated as less than 1O-Io torr, thus there is no danger of contamination. A specific application suggested is in spark source mass spectrographs where traces of oxygen, water, hydrocarbons, etc., are notoriously present in the background interfering in many types of measurements; indeed several attempts have been made to remove residuals by cathodic etching, cryogenic pumping (4), cryosorption pumping ( I ) , etc. Using the present technique, liquid NaK is evenly distributed on the walls of a glass or metal container which is connected through a suitable valve to the source. After ultimate vacuum is reached with the mass spectrometer pumping system the valve is opened. Exposure time (10-30 min.) must be determined experimentally since there is no visible sign of the completed reaction. Gettering capacity appears enormous on the basis of stoichiometric calculations but actual efficiency is much lower because the oxide layer that
07971 forms always covers some of the unreacted alloy. After saturation the vessel is exposed to moist air and the yellowish solid that forms can simply be removed by rinsing in water. Sodium-potassium alloy is inexpensive and is readily available both in large quantities and in convenient glass ampoules under protective argon filling (or vacuum). The NaK employed in these experiments (44% Na) has been obtained from MSA Research Corp., Callery, Pa. Experimental work is currently in progress for several applications; details will be reported separately.
LITERATURE CITED
(1)Harrington, W. L.,Skogerboe, R. K., Morrison, G. H., ANAL. CHEM. 37, 1480 (1965). (2) Mellor, J. W., “Comprehensive Treatise on Inorganic and Theoretical Chemistry,” Vol. 11, Suppl. I1 and 111, Wile , New York, 1963. (3) RoEoe, J.,. 14th Ann. Conf. Mass Spectr. Allied Topics, Paper 144, Dallas, Texas, May 1966. (4)Socha, A. J., Willardson, R. K. 11th Ann. C o d . Mass Spectr. Allied Topics, Paper 84, San Francisco, Calif., May 1963; 14th Ann. Conf., Paper 37, Dallas, Texas, May 1966.
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