Manometers for Low Pressures SAULDUSHMAN, General Electric Company, Schenectady, N . Y.
G
AGES for measurement of low pressures may be classified in three different groups: (1) direct-reading gages, such as those of the McLeod mercury type; (2) resistance gages, for example, the Pirani-Hale type, which depend upon the change with pressure in heat conductivity of a gas; and (3) ionization gages which involve a measurement of the number of positive ions produced in the residual gas by a definite electron current. Each of these different types of gages has its limitations and particular field of applications. The simplest type is the McLeod gage which reads the pressure directly. It does not indicate mercury vapor or water vapor. Manometers based on the radiometer principle, such as that of Knudsen, have not proved convenient for general use, although they are of interest in special investigation; the same remarks apply to viscosity manometers, such as those of Langmuir and others. The vibrating quartz fiber, suggested by Haber and Kerschbaum, and improved by I. Langmuir, has been found useful in a number of cases where it is desired to measure residual pressures of chemically active vapors in sealed-off devices. As generally useful tools for measurement of low pressures, two types of gages have been developed. The first of these, known as resistance type, makes use of the fact that a t low pressures the heat conductivity of a gas is a linear function of the pressure. The gage consists of a tungsten filament mounted in a bulb, and the pressure may be determined by one of three methods:
The filament is maintained at constant voltage, and the change in current measured with change in pressure. The resistance and, consequently, the temperature of the filament are kept constant and the change in total watts measured. At constant current the change in resistance is a measure of the pressure. The temperature of the filament at constant current may also be determined by a thermocouple attached to the filament. With gages of the resistance type, pressures as low as mm. may be measured quite accurately. The indications vary, of course, with the nature of the gas. The second type of gage used in high-vacuum technic is known as the ionization manometer and involves a measurement of the ionization produced in a gas by a definite electron current. The electrons emitted from a tungsten or oxidecoated filament are accelerated by applying a positive potential, varying between 100 and 250 volts, to an adjacent electrode (the anode), and the positive ions, resulting from collisions between electrons and gas molecules, are collected by a third electrode (the collector), which is maintained a t a negative potential (10 to 50 volts) with respect to the cathode. The ratio of positive ion to electron current is a function of anode voltage and nature of gas, but varies linearly with pressure at the lower range (below 10-8 mm.) and is independent of electron current for relatively low values of the latter. On the whole, this type of gage has proved to be the most useful form in high-vacuum work. R ~ O I C I VSeptember ~D 21, 1931. Preaented under title of “New Qages.”
For more oomplete discussion of this topic, see J . F~onklinInst.,
211, 689
(1931).
.,.ENDOF SYMPONIUM ...
Determination of Isopropyl Alcohol in Ethyl Alcohol F. M. ARCHIBALD AND C. M. BEAMER, Standard Oil Company of New Jersey, Linden, N . J.
T
HE industrial use of mixtures of isopropyl alcohol with ethyl alcohol has given rise to a need for a simple
30 per cent. The exact strength is determined by titrating with a standard acid solution, and additional water is added until the strength, by titration, is 30 per cent. Samples for titration are weighed, About 10 cc. of the alcohol mixture in question are taken in a 50-cc. glass-stoppered graduated cylinder. About 20 cc. of exactly 30 per cent caustic soda are added to the alcohol, and the is placed in a water bath to 250 C. The cylinder is shaken vigorously and then placed in the water bath until the two layers separate completely. The shaking is repeated a second~time,returned to the waterbath to separate] and then shaken a third time. This procedure insures a minimum time of contact and a minimum time in the water bath, which is necessary to give reproducible results. After the third settling period in the water bath, a sample of the supernatant layer of alcohol is withdrawn in a 1-cc. pipet and titrated with 0.1 N sulfuric acid solution using methyl red indicator. The titration volume is referred directly to the graph (Figure 1) for interpretation in terms of 91 per cent isopropyl alcohol present. I n this laboratory a 3-gallon Pyrex battery jar is being used
analytical method to determine the proportion of the two alcohols in such mixtures. A method of analysis has been developed on the basis of the difference in the solubility of caustic soda in the two alcohols. The method of analysis is simple and rapid, and accurate to a fraction of per cent Of isopropyl alcohol a t small concentrations-i, e., less than 20 Per cent isoProPY1 Above 50 Per centbisoProPY1 cohol be with any accuracy diluting the mixture with a known volume Of to bring the concentration below 50 per cent. The analytical technic involves agitating a sample of the alcohol under question with an excess of caustic soda solution of a standard strength and at a definite temperature, The supernatant alcohol is separated and its alkalinity determined by titration. The is referred to as an empirical cuwe which shows percentage of isopropyl alcohol plotted against alkalinity.
METHOD OF ANALYSIS A solution of c. P. caustic soda, 410 grams, is dissolved in 1 liter of water. This gives a solution slightly stronger than 18
January 15, 1932
INDUSTRIAL AND ENGINEERING CHEMISTRY
as a water bath. An initial adjustment of the water temperature is sufficient to complete a given test.
19
Curves A and F were made with 35.5 sodium hydroxide and
25 per cent sodium hydroxide, respectively, and show less sensi-
tivlty for 35.5 er cent soda as regards isopropyl content. Curye F shows compyete miscibility with alcohols below 5 per cent in isopropyl content. By comparing C and D made with 30 and 27.5 per cent caustic soda, respective1 it i s found that a difference of 1 per cent in the strength opthe caustic soda solution used is ca able of introducing an error of about 1 per cent in the amount o f isopropyl alcohol determined. 2. Temperature of contact:
Curves B and E were made b workin at 20" and 40" C., respectively. By comparison witK curve it is found that a deviation of 1" C. in temperature is capabfe of introducing an error of about 0.5 per cent in the amount of isopropyl alcohol determined.
8
3. Proportion of caustic soda solution to alcohol: Two volumes of caustic soda to one of alcohol were used in the work shown in the graphs. Since the ethyl alcohol and the is0 ropy1 alcohol used contained different amounts of water, it 8llows that different blends of these alcohols contained varying amounts of water which, in each case, had to come to equilibrium with the caustic soda solution. The large excess of caustic soda avoided any appreciable change in caustic soda strength because of the transfer of var 'ng quantities of water from one phase to the other. A ratio o& to 1was tried instead of 2 to 1. The results were substantially the same. The effect of changing the caustic soda solution ratio can be seen in the following data, which show the titration obtained when testing various alcohol mixtures by the method described. 2 : l RATIO 39.5 36.6 32.4 28.1
4:1 RATIO 39.1 36.5 32.5 28.1
The divergence is practically within the range of experimental error. From this it is inferred that the amount of caustic soda used need not be measured very accurately.
4. Water content of alcohol: With alcohol mixtures containing 10 per cent of water or less, FIGURE 1. ALKALINITY OF ALCOHOL MIXTURES AT EQUILIBRIUM the analytical method appears t o be accurate. Large concenWITH 30 PER CENT CAUSTIC SODA AT 25" c. The supernatant alcohol is quite free from entrained alkali, as evidenced by the fact that the alkalinity remains unchanged on filtration. EMPIRICAL RELATION The graph of Figure 1 expresses the empirical relation between alcohol composition and the solubility of caustic soda in the various compositions. It is the result of experimental work, according to the method described above, on mixtures of alcohol of known composition. These mixtures were made from commercial grades of alcohol, 91 per cent isopropyl and 95 per cent ethyl alcohol (not denatured). Part of the lower range of concentration is plotted to a larger scale and shown as curve C in Figure 2. It can be seen that the method described above gives sufficiently accurate results so that there is no difficulty in drawing a smooth curve through the experimental points. Different operators were able to check points on the curve.
EFFECT OF VARIABLES The analytical procedure was repeated over the lower range of isopropyl concentration with deliberate variations in one factory a t a time. The results, plotted in Figure 2, show the sensitivity of the analysis to these variables and give an idea of the accuracy that may be expected with ordinary care. Curve C is the standard method given in detail above. The discussion of these curves touches the following points: 1. Concentration of caustic soda solution:
FIGURE 2. ALKALINITY OF ALCOHOL MIXTURES WHENSHAKEN WITH CAUSTIC SODA
20
ANALYTICAL EDITION
trations of water in the alcohol dilute the caustic soda solution and introduce serious error. Dehydrating such alcoholic mixtures does not offer serious difficulty because the two alcohols are very similar in physical and chemical properties. Dehydration with quicklime was found t o be easy and effective. For example, a mixture of 93 per cent of eth 1 alcohol and 7 per cent isopropyl alcohol (technical purity in goth cases) was diluted 1 t o 1 with water and distilled through a bead-packed glass
Vol. 4, No. 1
column, collecting the first 50 per cent cut. This cut was shaken with an excess portion of quicklime and placed in an Engler flask. A small portion was distilled off and collected for analysis. It was found to contain a quantity of alcohol which, calculated on a basis of the original undil4ted sample, showed an allowable error of 0.25 per cent isopropyl alcohol. RECEIVRD November 9, 1931.
Determination of Small Amounts of Ethyl and Butyl Alcohols M. J. JOHNSON,University of Wisconsin, Madison, Wis.
M
ANY m e t h o d s have been devised for the d e t e r m i n a t i o n of ethyl and butyl alcohols in mixtures. A~~~~ the recent t h o s e of Bogin ( 2 ) a n d of D o n k e r (9) make use of the Of
A R A P I D method for the determination of small amounts (3 to 15 mg.) of ethyl or butyl alcohol or mixtures of the two is described. I t is particularly useful in the analysis of bacterial cultures where only small samples are available. The alcohols are oxidized by means of an acid dichromate solution, and the acids produced are distilled directly from the Oxidizing solution according to a procedure which makes use of the Duclaux principle. The whole determination may be completed in 40 minutes.
in water* The methods of W e r k m a n and O s b u r n (6) and of van der Lek (4) involve t h e o x i d a t i o n of the alcohols to acids, which are then determined by a suitable method. In the course of an investigation reported elsewhere, it became necessary to determine ethyl and butyl alcohols in small samples of bacterial culture. Since a large number of determinations were to be made, a rapid method was desired. No method making use of solubility differences could be considered because of the large amount of sample required, and methods involving oxidation of the alcohols, steam distillation of the resulting acids, and determination of acetic and butyric acids in the distillate are likely to be slow and cumbersome. It was thought that a method might be devised whereby the acids could be distilled directly from the oxidizing solution in a modified Duclaux procedure. Such a procedure, if feasible, would eliminate the steam distillation and greatly shorten the time necessary for the determination. . After more than a hundred trials in which the effect of a number of variables was studied, a satisfactory procedure was developed. The method t o be described has been in use for more than a year, during which time approximately 150 routine determinations have been made. It has been used in a study of the acetone-butyl alcohol organism ( 3 ) and has given satisfactory fermentation balances.
EXPERIMENTAL PROCEDURE REAGENTS.The following reagents were used in this method: 1. Oxidizing solution. Mix equal volumes of exact1.y 3 N potassium dichromate solution and exactly 10 N sulfuric acid. Carbon dioxide-free water should be used in making these solutions. 2. Barium hydroxide. Solution 0.1 N o r 0.02 N strength. 3. Indicator solution. To 0.1 gram of phenol red (phenol sulfonphthalein) add 2.85 cc. of 0.1 N sodium hydroxide and make up to 500 cc. APPARATUS. The pieces of apparatus employed were as follows: 1. Alcohol-distillation apparatus. The distilling vessel is a Pyrex test tube, 38 by 200 mm. It is connected to a small
condenser. The lower end of the condenser tube is connected by a short piece of rubber tubing to a glass tube which extends to the bottom of the receiver flask, of 10 cc. capacity, and recalibrated to deliver 10 cc. 2. Acid-distillation apparatus. A drawing of this a paratus is given in Figure 1. &e distilling vessel, A , is a Pyrex test tube, 20 by 200 mm. The condenser tube is made from 8-mm. tubing. The bend at B is made as close above the r u b b e r s t o p p e r as practicable (1 cm.). The end of the condenser tube, C, is drawn out to a tip to facilitate collection of the distillate continuously rather than drop by drop. The receiver is a 10-cc. volumetric flask. Two of these are needed. The flame of the microburner heating the test tube is kept constant at any desired size by means of the bubbling bottle D and the manometer F . The tuhes entering the bottle should be of ample size (7 mm.). The excess gas escaping from the bottle is burned. The jet should not be constricted. The size of the flame of the burner may be varied by adjusting cock E or by changing the head of water in the bubbling bottle. The burner should be so adjusted that the rate of distillation is fairly rapid, but not so rapid as to permit any mechahical carrying over of liquid. This rate is such that 10 cc. of distillate are collected in 7 or 8 minutes. Once the apparatus is adjusted, the reading on the manometer F , and consequently the rate of distillation, should be kept constant. The manometer is filled with paraffin oil or some other nonvolatile liquid. A cylindrical shield, G, protects the flame from air currents. 3. Buret. This should be graduated at 0.01-cc. or at 0.05-cc. intervals, depending upon the strength of alkali used.
A sample of culture containing not more than 15 mg. of total alcohols is placed in the distilling tube of the alcoholdistillation apparatus, made slightly alkaline, and diluted to approximately 20 cc. A few glass beads are added, and the sample is distilled into the 10-cc. receiver Aask. About 1 cc. of carbon dioxide-free water is first placed in the flask so that the end of the condenser tube is below the surface throughout the distillation. The distillation should be carried out a t a constant rate to prevent sucking back of distillate. The last part of the distillate is collected with the end of the condenser tube above the surface of the distillate. The distillation is continued until the receiver Aask is full to the mark. While this distillation is in progress, exactly 10 cc. of oxidizing solution are pipetted into the distilling tube of the aciddistillation apparatus. Four small glass beads are added, and the 10 cc. of alcohol distillate are rinsed into the tube with 5 cc. of carbon dioxide-free water. The tube, which should now contain exactly 25 cc. of reaction mixture, is closed with a rubber stopper and the stopper wired down. The tube is heated in a boiling water bath for exactly 5 minutes, and is
