Combined Absorption and Titration Tube for Volumetric Determination of Carbon Dioxide EARL J. ROBERTS Bureau of Agricultural and Industrial Chemistry, Agricultural Research Administration, U.S . Department of Agriculture, New Orleans 19, La. forations in the lower hemisphere. B is a glass rod with a small three-blade propeller a t one end and a larger three-blade propeller of opposite pitch located at such a distance from the end that it is just above F when the small propeller is in the center of the lower bulb of the absorption tube. The solution in the absorption tube should circulate in a clockwise direction during titration. For operation, a known volume of standard sodium hydroxide solution, sufficient t,o neutralize 1.5 times the amount of carbon dioxide to be absorbed, is placed in the absorption tube and diluted with carbon dioxide-free distilled water to about 110 ml. The bubbler tube is fitted with a one-hole rubber stopper and inserted into arm E of the absorption tube, so that it is 2.5 cm. (1 inch) from the bot'tom. D is protected from atmospheric carbon dioxide by a soda-lime or Ascarite tube. The apparatus in which the carbon dioxide is t'o be generated is then connected to a wash tube containing distilled water and the assembly is freed from carbon dioxide by passing a stream of carbon dioxidefree air through the system under pressure sufficient to cause the passage of air through t'he wash tube at the rat,e of 25 to 30 ml. (180 to 200 bubbles) per minute. The outlet from the wash tube is then connected to the bubbler tube and the generation of carbon dioxide is started. The stream of carbon dioxidefree air is continued throughout the determination. When the reaction is complete and the carbon dioxide has all been removed from the generator and wash tube, the bubbler tube is removed and washed free of the alkaline absorbing solution with carbon dioxide-free Tvater which is allowed to drain into the absorption tube. An excess of saturat,ed barium chloride solution is added and the stirrer inserted into D as described above. The stirrer is started and the excess alkali is t,itrated,to neutrality (phenolphthalein) with hydrochloric acid of convenient strength. For the titration a piece of small-bore glass tubing, extending to the bottom of E , is connected t,o tip of the buret ( 4 ) , so that loss of carbon dioxide during the titration is prevented. Blank titrations should be made t,o correct for carbon dioxide in the solution of alkali.
of work on a quantitative method for aconitic I KacidT H (E5 )course the problem became one of determining carbon dioxide, since the aconitic acid determination is made by decarboxylation. Although many methods have been described for carbon dioxide determinations, most of them are cumbersome and time-consuming. The method of Friedmann and Kendall ( 1 ) has disadvantages in that a large excess of alkali must be used and the apparatus is difficult to wash free of alkali. The improved methods of Wells, May, and Senseman ( 6 ) , McCready, Smenson, nnd JIaclay (Z), and Reid and Reihe ( 4 ) require relatively large volumes of water for washing the apparatus and special precautions against absorption of atmospheric carbon dioxide.
T
.J
T
E
D
i
i B
0 (D
- 4 A
B
c
Figure 1. Absorption and Titration Tube for Carbon Dioxide
1-
The apparatus has been very efficient in a laboratory a t room temperature 25" to 30" C. and in the laboratory of a sugar mill where there is no temperature control. Some typical results obtained with the apparatus described are given in Table I. They show that the carbon dioxide is completely absorbed in 0.4 N sodium hydroxide solution.
The apparatus (Figure 1) permits determinations of carbon dioxide to be made accurately and rapidly. The apparatus consists of an absorption tube, C, a bubbler, A , and a stirrer, B . The absorption tube is constructed in such a way that the gas enters the tube a t the lower end of arm E , passes up through the absorbing solution into the space above connecting tube F and then through the solution again via F into arm D. A is a small glass tube 151th a bulb at one end containing five 0.75-mm. per-
Table I. Determination of Carbon Dioxide (Compounds decomposed by the methpd of Roberts and Ambler, 6, unless otherwise noted) Carbon Carbon Weight Dioxide Dioxide of Compound Sample Found Theoretical Grams Grams Grams 0 044 0.044 0 100 Calcium carbonate= 0,088 0 089 0 200 0 253 0.253 1 00 Aconitic acid 0.506 0 507 2 00 0 0255 0.0253 0 10 0.0500 0.0507 0 20 0.416 0 414 2 00 Xonopotassium aconitate 1.205 1.205 2 00 Acetone dicarboxylic acid (1
616
Decomposed by method of MacIntire and Willis (3).
AUGUST 1947
617 LITERATURE CITED
(1) Friedmann, T. E., and Kendall, A. I., J. B i d . C h m . , 82, 47 (1929). (2) McCready, R. M . , Swenson, H. A., and Maclay, W. D., IND. ENG.CHEM.,ANAL.ED., 18, 290 (1946). (3) MacIntire, W. H., and Willis, L. B., J. I d . Eng. Chem., 7, 227 ( 1915).
(4) Reid, J. D., and Weihe, H. D., IND.ENG.CHEM.,ANAL.ED.,10, 271 (1938). ( 5 ) Roberts, E. J., and Ambler, J. A., Ibid., 19, 118 (1947). (6) Wells, P. A., May, 0 . E., and Senseman, C. E., Ibid., 6 , 369 (1934). AQRICULTURAL Chemical Research Division Contribution 198.
Vacuum Pressure Regulator F. W. MELPOLDER The Atlantic Refining Co.,3144 Passyunk Ave., Philadelphia, Pa.
DESCRIPTION OF REGULATOR
PRESSURE regulator was developed to monitor pressures in the range of 1 to 1000 microns of mercury in vacuum systems comprised predominantly of noncondensable gases. I t was desirable that the regulator indicate absolute pi essures rather than relative pressures such as those mcasured by the Pirani or ionization gages ( I ) , since calibrations of these latter gages are different for various gases and frequently the composition of the gas i n vacuum syqtems is unknoir n. This regulator is an adaptation of the \IcI,eod compression gage, to which automatic controls were added to move mercury alternately up and down the gage in cycles and to operate the vacuum confro1 valve For accurate prrqsure measurement the gas under compression in the gage must conform t o the gas lan s and consequently condensable gases should be removed in a liquid nitrogen trap located between the gage and vacuum system The regulator has been in satisfactory operation in the laboratory for the past t n o years as the pressure controller on a short-path assay distillation unit at 1micron prrssure
VACUUM CONTROL
A McLeod gage of conventional design was fitted with sealed-inglass tungsten contacts a t positions A , B , and C. At position D a short length of 30-gage tungsten wire was inserted in the top of capillary tube E and cemented in place with thermoplastic cement. The operating pressure of the gage was deter -’ ined by calculation of the proper dimensions of bulb F , capillary tube E , and length of the wire contact inside the capillary tube at D . Only the tungsten contacts at A and D required careful positioning. The control circuit shown in the schematic wiring diagram operates on 115 volts 60 cycles. The vacuum control solenoid valve, single-pole SI, having a 0.5-inch pdrt opening, is the controlling valve between the pumps and the vacuum system manostat. KOspecial requirements are necessary for providing a tight seat in this valve, since gas flow through small orifices a t low pressures is relatively slo~v. Two other solenoid valves, Sz and Sd,control the air flow in and out of of the McLeod mercury reservoir. -411 three solenoid valves, SI, Sn,and S3,are opened xvhen energii.ed. Attached to S?and S I are needle valves which restrict the flow of air, thus allowing a smooth rise and fall of mercury in the gage. -4separate vacuum pump war used to pull mercury down in the gage through SI. The volume of the ballast bottle was determined by the total volume of the vacuum system, diameter of port opening in t h e vacuum control solenoid valve, 81, and speed of the pumps. The ballast volume must be large enough to prevent surging or rapid change in pressure. In this case a ballast volume of 5 liters was satisfactory. OPERATION OF REGULATOR
3 SWl
115v 60-
VACUUM
REGULATOR UNIT
R I ,R2. United-Cinephone electronic switches having t h e following numbered post
connections : 1, 3. Power leads 4. Normally closed switch position 5. Center pole o f single-pole double-throw switch 6. hormally open switch position 7, 8. Trip connections which moves switch arm from position 4 to 6 when closed R3. Struthers-Dunn midget latch-in relay, normally spring-closed (as shown) h u t opened w h e n energized and latched i n t h e open position R1. kdvance midget power relay, one pole normally spring-open, one pole normally spring-closed. Relay ehown i n spring-held position SWi. Toggle switch double-pole single-throw SWz. Toggle switch double-pole double-throw SW2. Toggle switch single-pole single-throw S W I . Toggle switch single-pole double-throw, center off posi tion
The solenoid valves, SI,Sz, and Sa,are controlled manually with sw;tches S y s and SW4 upon starting the distillation until the pressure in the manostat is reduced below 10 mm. of mercury. As soon as the pressure is sufficiently lowered SWz is placed in the “automatic” position. Mercury in t h e McLeod gage is then automatically alternately raised and lowered between contacts A and B , showing the progressive decrease of pressure in the system. When the pressure reaches the desired point, mercury in bulb F rises in capillary tube E and touches contact D , thereby actuating relays R, and RBand closing Si. This valve remains closed until a cycle occurs in which mercury fails to touch contact D, owing t o increase in pressure of the system. Relays Rz and Ra a t this instant transmit a momentary impulse to the “trip” coil of RB,causing the contacts t o close which open S1. The “trip” coil in Ra is permitted to actuate the relay only when the mercury fails to touch D. Although the regulator monitors the pressure only once a minute, no significant change in pressure resulted during this interval. The maximum pressure fluctuation a t 1-micron pressure was found to be 0.1 micron. LITERATURE CITED
(1) Pirani, M., and Neumann, R., ElectToiiic Eng., 17, 277 (1944); 17, 322, 367, 422
(1946).
