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V O L U M E 25, NO. 6, J U N E 1 9 5 3 ie indebted to Ole Lamm of Stockholni for having dran-n his attention to Hallwachs’ work (6). LITERATURE CITED
Brice, B. A , , andHalwer, AI., J . Opt. Soc. Anter., 41, 1033 (1951). Cecil, R., and Ogston, A. G., J . Sci. Instr., 28, 263 (1951). Claesson, S., Arkiv. Rem. M i n e d . Geol.. 23 A, S o . 1 (1946). Claesson, S., “The Svedberg 1884 30,’8 1944,” p. 82, Uppsala, -4lmquist & Wiksell, 1944. (5) Debye. P. P., J . A p p l . Phys., 17,392 (1946). (6) Hallwachs, W., Ann. Phys. Chem., X e u e Folge 50,577 (1S93). :7j Jones, H. E., Ashman, L. E., and Stahly, E. E., -1s.4~.CHEM.,
(1) 12) (3) 14)
21,1470 (1949).
(8) Iiegeles, G., J . -4m.Chem. SOC.,69, 1302 (1947). (9) Kegeles, G., and Sober, H. A., ANAL.CHEM.,24,654 (1952). (10) Longsworth, L. G., IND. ENG. CHEW, ANAL.ED., 18, 219 (1946). (11) lliller, E. C., Cran-ford, F. !A7., and Simmons, B. J., -4x.4~. CHEY., 24, 1087 (1952). (12) Muller, R. H., and Frachtman, H. E., Intern. Congr. Pure and A p p l . Chem., p. 33 (1951). (13) Svensson. H.. Acta Chem. Scand.. 6.720 (1952). (14) Thomas, G. R., O’Konski, C. T.,‘and Hurd, C : D., AXIL. CHEY., 22, 1221 (1950). (15) Tiselius, A , , and Claesson, S., A ~ k i vKern. Mineral. Geol., 15 B, S o . 18 (1942). (16) Zaukelies, D., and Frost, A. A., ANAL.CHEW,21,743 (1949). RECEIVED for review l u g u s t 28, 1952. .4ccepted March 11, 1953.
Design and Use of an Electronic Pressure Controller F. J. DIGNEY AND STEPHEN YERAZUKIS Rensselaer Polytechnic Institute, Troy, .V. Y . Apparatus was designed which will maintain a constant pressure within small laboratory equipment such as an equilibrium still or distillation column. The pressure can be set to any desired value from vacuum to slightly above atmospheric pressure. Fluctuations from the control point are of the order of hO.1 mm. of mercury. Once the pressure is set, it will remain constant indefinitely. This equipment will be useful for maintaining constant pressure in small laboratory equipment automatically for hours, weeks, or months. It is especially indicated where very precise pressure control is required for long periods.
THE
successful pursuit of many laboratory problems, such as the determination of vapor-liquid equilibrium, may frequently depend upon a delicate control of pressure. rllthough numerous commercial controllers are available their expense or performance, particularly in so far as the need for continual observation and adjustment are concerned, often leave much to be tleeired. This pressure controller is not only easily and econoniically constructed but conveniently used, requiring a minimum of operator attention.
5.l 2 Y Y STOPCOCK
Sur
TUBING-
3 TO CMITROLLED
4 ELECTROCYTE RESERVE 6 ELECTRODE 5 REFERENCE PRESSURE B
APPAR4TUS
The controller consists of a simple manometer, one side of is connected to a reference pressure; the other is open to the system to be controlled. Any differential between these pressures will unbalance the manometer fluid. This effect is employed to actuate the corrective mechanism to restore the system to the desired control point. The barostat, Figure 1, which is of a size convenient for coniplete submergence in a small constant temperature bath includes the manometer and reference pressure bulb. The temperature as well as the volume and quantity of the gas sealed in the reference pressure bulb by the manometer fluid can be precisely controlled so that any desired reference pressure can be generated and maintained for an indefinite length of time. Since the refel ence pressure against P hich the system is balanced is dependent only on controlled variables, it is evident that atmospheric conditions cannot affect the operation of this pressure controller. The three electrodes provide the necessary contacts. -4s long as the pressure in the controlled system is equal to the reference pressure, the manometer is balanced and neither of the electrodes contacts the liquid, an electrolyte. On the other hand, if the elstem is off the control point, the manometer will be unbalanced with the consequence that one of the electrodes will contact the electrolyte, thus closing an electrical circuit and transmitting a signal to the main controller, Figure 2. This controller in turn operates a relay-powered tire valve in either the pressure or
7
R hich
I / -
I5
c
CI
23 CMI
,qI
/
I
LITER FL4SK
35 CM
A’2rr
lUeNG
4
2 r u CAPlLL4RI
/ L 3 8 CM
4 L
Figure 1. The Barostat
ANALYTICAL CHEMISTRY
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makes it possible to obtain good temperature control despite the on-off characteristic of the circuit employed. T h e m e r c u r y t h e r m o s t a t c a n b e constructed easily and can be made very sensitive through the use of a fine capillary. X sensitivity of 0.75" C. per i n c h of c a p i l l a r y or a c o r r e s p o n d i n g control of 10.002° C. is not at all difficult to obtain. Both the pressure and vacuum tanks, 5-liter glaeq flasks, are provided with constant leaks which serve to minimize the effect of demand for restorative action on the relative pressure and vacuum. h 5 - l i t e r s u r g e tank is also used to smooth out pressure fluctuations. Figure 2.
W-iring Diagram of Pressure Controller
CI. 8 microfarads. 250 volts
30 microfarads, 450 volts Car Cc. 8 microfarads, 450 volts Ce, Ce. 0.1 microfarad Ri,Rz. Variable resistances Ra, Ra. 560,000 ohms Ra, Re. 10.000 ohms Cn.
vacuum tanks, Figure 3, depending on the direction of the manometer unbalance. The motor valve continues to operate until the syetem has been restored to the control point. The action of the main controller, Figure 2, may be described in the following, perhaps oversimplified, manner. The electronic circuits are so designed as to open and close relay Relays 2 and 3, serving as nothing more than on-off switches, are opened and closed on contact of the electrodes with the manometer fluid in the barostat. For example, if the pressure falls below the desired value, electrode B will contact the electrolyte, closing relay 3. Motor valve B in the pressure tank will now be energized and deenergized by the pulsating current generated by thz oscillating relay 1. so that air will be pumped into the system. The pressure of the system will increase until the control point is reached. electrode B breaking contact and alloxing relay 3 to open. If the pressure exceeds the desired value, electrode d will make contact, relay 2 will be closed, and motor valve A will operate, thus decreasing the pressure of the system. 1 about 2 times per second.
The control circuits f u r the water bath are not unusual. A mercury thennostat is employed as the sensing element for a snxill electronic controller, Figure 4, supplying power to a heater, usually an electric light bulb. The low hest capacity of the light bull,
OPERATING CH4RACTERISTICS
R i , R i i . 2 megohms Rs. 82ohms R o . 120,000 ohms Rio. 15,000ohms TI. Transformer RYi, RY2, RYs. 10,000-ohm relays
The inevitable choice twtween sensitivity and stab i l i t y in t h e controllei which has been described must be made. It is evident that a high sensitivity setting, equivalent to leaving very little clearance between the surface of the balanced manometer fluid and the electrodes, will make the entire system prone to excessive cycling. The latter may be inhibited by reducing the relative pressure and vacuum in the supply tanks, although such treatment would preclude a rapid restoration from a severe disturbance. Another means of avoiding cycling would be to dampen the motion of the manometer fluid by locating a constriction in the base of the manometer. -4far more elaborate scheme for
Figure 3. 1. To equilibrium still 2. Calcium chloride bottle 3. Open-end manometer 4. Relative vacuum manometer 5. Relative pressure manometer 6. Surge tank 7. Pressure tank 8. Vacuum tank 9. Pressure vent
Pressure Controller Vacuum vent Pressure supply Vacuum supply Water bath Barostat Mercury switch Main controller 17. Water bath controller
10. 11. 12. 13. 14. 15. 16.
V O L U M E 25, NO. 6, J U N E 1 9 5 3
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LT-7 117 L 7
ro
RELAY
mj.
barostat which would make it possible t o provide a restorative actio’n proportional in some manner to the deviation from control point. Such a scheme, however, arould make necessary additional electronic equipment as well as auxiliary pressure and vacuum sources. It was not found necessary to employ any of the above-mentioned modifications in the controller which was used to maintain constant pressure for the measurement of vapor-liquid equilibrium. A sensitivity of & O . l mm. of mercury without excessive cycling was easily obtained if relative pressures and vacuums of from 5 to 10 mm. of mercury were used.
SWITCH
CONCLUSIONS
Figure 4.
Wiring Diagram of Water Bath Controller
CL. 20 microfarads, 150 v o l t s
Ra.
3400ohms
RI.
Ra.
22,OOO ohms
RI. 60,OOOohmn
600,OOOohms
R,. 10,OOOohms
retaining a high sensitivity within a minimum of cycling would consist of additional electrodes placed in cascade fashion in the
The controller described proved to be reliable and over a period of several months of almost continuous operation, it was found to require but slight adjustment. Although the use of glass as a material of construction limited this controller to pressures in the vicinity of 1 atmosphere, a far wider control of pressure could have been obtained by suitable substitution of metal for glass. RECEIVEDfor review June 2,1952. Accepted March 28, 1953.
Determination of Hydroperoxides in Petroleum Products D. C. WALKER AND H. S. CONWAY Research Department, Standard Oil Co. (Indiana), Whiting, Znd. In a new method, hydroperoxides in petroleum products are determined by extraction and reduction with sodium arsenite. For gasolines and distillate fuel oils the method is more sensitive than the iodometric method, and avoids the dependence on sample size characteristic of the ferrous method. Known concentrations of hydroperoxides in gasolines, fuel oils, and white mineral oils are determined with an accuracy of 0.03 hydroperoxide number (milliequivalents of active oxygen per liter of sample) for numbers less than 1, and within 2% for higher hydroperoxidenumbers.
T
HE determination of low concentrations of peroxides in
petroleum products has been of concern to the petroleum industry for many years. Formation of gum and sediment in refined distillates, development of disagreeable odors of white mineral oil, and bearing corrosion in engines are among the many effects attributed either directly or indirectly to peroxides. These may form whenever hydrocarbons are exposed to air, and the primary products of the reaction are alkyl hydroperosides (ROOH) rather than dialkgl peroxides (ROOR) ( 3 , 4,9 ) . Published methods for the determination of hydroperqxides utilize either ferrous or iodide ions in acid solution as reducing agents. In the ferrous method, the amount of hydroperoxide is determined by measuring either the ferric ion produced or the excess ferrous ion. In t,he iodide methods the liberated iodine is titrated with thiosulfate. The variations of both methods have been adequately summarized by Kolthoff and Medalia ( 6 ) , who concluded that the ferrous method was “inherently much less accurate than iodometric methods.” Several iodometric procedures ( 2 , 6, 7 , 11) are accurate for concentrated hydroperoxides, but no data have been reported on synthetic solutions of hydroperoxide in hydrocarbons more dilute than 0.1 A’. In applying the iodometric methods to petroleum fractions, part of the liberated iodine may be lost by addition to olefinic components or by volatilization ( 8 , 1 2 ) . Arsenious oxide is used as the reducing agent in a method proposed by Siggia ( 1 0 )for the assay of benzoyl peroxide. Because Siggia’s procedure requires complete solution of the sample in aqueous alrohol, the amount of petroleum product that can be
taken for analysis is limited, and low concentrations of hydroperoxide cannot be determined. In a new method that has been developed for the determination of hydroperoxide in petroleum products, aqueous sodium arsenite is used as the reducing agent in a two-phase system, and organic materials are removed by separation and extraction before determination of excess arsenite. Because the sample need not be dissolved in the reagent, there is no limit on the amount of sample that may be taken, and very low hydroperoxide concentrations can be accurately determined. METHOD
The extraction apparatus, a modification of a commercially available tetraethyllead extractor ( I ) , is diagramed in Figure 1. Standard Solutions. Arsenious oxide, 0.1 N . Dissolve 4.9450 grams of pure, dry arsenious oxide in 50 ml. of 1 N sodium hydroxide. Make neutral or very slightly acid to litmus paper with 1‘V sulfuric acid and dilute t o 1liter. Iodine, 0.05 N . Dissolve 12.7 grams of C.P. iodine and 40 grams of C.P. potassium iodide in 25 ml. of water, and dilute to 2 liters. Standardize against the arsenious oxide solution. Procedure. Into the separatory funnel of the extraction apparatus introduce 75 ml. of distilled water, 2.0 ml. of 1 N sodium hydroxide, and 2 drops of phenolphthalein indicator solution. In accordance with the anticipated hydroperoxide content, add the amounts of sample and of 0.1 N arsenious oxide given in Table I. If the amount of sample used is less than 50 ml., add enough petroleum ether to bring the volume of the top phase to between 50 and 100 ml. Drain the mixture into the lower part of the apparatus and rinse the funnel with 25 ml. of 95% ethyl alcohol. Bubble nitrogen through the solution a t a moderate rate for 5
