GAS TITRATIONS SIDNEY K4TZ AND JOHN T. BARR' K-25, Carbide and Carbon Chemicals Co., Oak Ridge, T e n n . AMMONIA AR'D HYDROGES CHLORIDE
4 n instrument for direct gas-with-gas titrations, using measurements to determine titer - pressure _ and end point, has been developed to take advantage of the reactivity of many halogen-bearing gases. The precision of such titrations has been determined to be better than 3% of the whole at the 95% confidence interval in the reaction of ammonia and hydrogen chloride. The reactions of chlorine, chlorine monofluoride, chlorine trifluoride, and fluorine with several gaseous hydrocarbons were studied for possible analytical applications. Fluorine, chlorine-fluorine mixtures, chlorine monofluoridefluorine mixtures, chlorine trifluoride-fluorine mixtures, methane-ethanemixtures, and olefins in hydrocarbon mixtures were determined with precisions similar to that noted for the ammoniahydrogen chloride titrations. Gas-with-gas titrations are a rapid means of determining reactive gases and of investigating gas reaction mechanisms.
Experimental. .~PPARATUS. The gas titration apparatus, presented in Figure 1, consists of the reaction vessel anti magnetic gas mixer, gas-measuring vessels, pressure-measuring dwiers, cshemical traps, and mechanical vacuum pump. The copper reaction vessel, 8 inches in diameter and 6 . 5 iiiclies tall, was fianged a t the top and sealed mith 12 bolts and B Tygon gasket. Connecting valves were attached to the reaction vessel in such a manner that there was a minimum of connecting valve and line volume. The volume (5 liters) of the reaction vessel was made sufficiently large to ensure that the 7-ml. unmixed volume in the connecting valves and lines would not introduce an appreciable error. The pressure range, 0 to 50 nun. of mercury, used in the reaction vessel, was selected as being Ion. enough t,o reduce adsorption effects and high enough so that the maziniuni measurable pressure waR about 100 times as large as t h c vapor pressuie of the precipitate formed in the reaction.
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a few of the many procedures for chemical quantitative gas analysis are based upon the reaction of the unknown gas with a reagent gas. The most comnion of these is the combustion analysis ( 7 ) of hydrocarbons or hydrogen with oxygen. -4nother is the analysis of isobutene ( a ) by the reaction with hydrogen chloride in the condensed phase.
'i Figure 2.
-45 X 2 inch stainless steel gas mixing vane was fastened t o the center of a shaft supported by the bottom of the reaction vessel and held upright in the center of the vessel by loose fitting bearings. A magnet waa attached to the shaft '/a inch from the bottom of the shaft. Just outside the reaction vessel and centered below the internal magnet, a corresponding magnet was positioned a t the top of a drive shaft. The drive shaft was rotated a t 100 r.p.m. by a laboratory stirrer equipped with a reduction gear. The nickel gas-measuring vessels, 2.5 inches in diameter and 8 inches tall, were connected t o the vacuum manifold, the pressure transmitters, the reaction vessel, and the gas supply. The 900-cc. volume was about one fifth the volume of the reaction vessel in order to allow larger pressure increments to be measured. The ratio of the volume of each of the gas-measuring vessels to the volume of the reaction vessel was determined by expansion of air into the reaction vessel; from these ratios the relative volume of the measuring vessels was calculated. The gas pressures in the measuring vessels were determined through Booth-Cromer pressure transmitters ( 1 ) on a mercury manometer (Figures 2 and 3). This transmitter is an electrically balanced pressure-transmitting diaphragm. Because of the requirement for high sensitivity over a relatively short pressure range, a Booth-Cromer pressure transmitter with an iV-butyl phthalate manometer and later a Taylor absolute pressure transmitter ($), calibrated for the pressure range 0 to 50 mm. of mercury, were used for measurements of pressure in the reaction vessel. Although both instruments have equal sensitivity in this range, the Taylor instrument was preferred for greater ease of operation. A small linear volume change accompanies change in gas pressure when using the Taylor instrument whereas the volume of the gas remains unchanged by changes in gas pressures during measurement with the Booth-Cromer pressure transmitter, The manifold and manifold connections were 5/einch internal diameter copper tubing with l/le-inch walls fitted with 3/,-inch
The extension of the analytical utilization of gay-with-gas reactions to other gas pairs or groups of reactive gases has been delayed because of mechanical difficulties in the handling and mixing of the gases, and the measuring of the gas volunies or pressures, although the procedure is theoretically simple and is obviously similar to the conventional solution titrimetry. An instrument for gas titrations has been constructed utilizing such recent developments in equipment as the magnetic stirrer, valves which are resistant to corrosive gases and capable of fine adjustment, and sensitive pressure-measuring devices that may be used with corrosive gases. The gas volumes and temperatures remain constant and pressure measurements are used to determine the titer and titration end point. This instrument makes possible the rapid titration of a gas with a reagent gas for those reactions where the number of moles of product gas is different from the sum of the moles of reactant gases. 1
Booth-Cromer Pressure Transmitter
Present address, Pennsylvania Salt Manufacturing Co., Philadelphia 18,
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619
ANALYTICAL CHEMISTRY
620 Kerotest packless diaphragm valves (OE-75 13-S, Kerotest Manufacturing Co., Pittsburgh, Pa.) equipped with Fluorothene stem washer seats. The connecting lines to the pressure-measuring devices were '/B-inch internal diameter copper tubing with l/16inch wall. The remaining connecting lines were l/4-inch internal diameter copper tubing with l/ls-inch walls fitted with :/,-inch all nickel Hoke-Phoenix diaphragm needle valves (special No. H 13, Hoke, Inc., Englewood, K. J.). A soda-lime trap for the absorption of acidic gases and a magnesium perchlorate trap for the absorption of water and ammonia were placed in series between the manifold and the vacuum pump. > YCG
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vessel was evacuated, nitrogen was flushed through the reaction vessel for a t least 2 minutes. and the reaction vessel was again evacuated. These measures were necessary because of adsorption of the gases on the precipitate. I n the second or visual procedure, the titration end point was detected by visual observation of the pressure indicator for the reaction chamber. At the point where the pressure just began to reverse its direction of change, the addition of reagent gas was halted. The reaction vessel was then prepared for the following titration by evacuation alone. Data obtained by the intersect end point procedure for titrations of 0.5 and 1.4 niillimoles of Pydrogen chloride with ammonia and 0 5 millimole of ammonia with hydrogen chloride are presented in Table I and data for the visual end-point procedure are presented in all the titrations of Tables I and 11. For the titrations of Table I, in which both the visual and intersect procedurrs were used, the reaction vessel was prepared by the method described for the intersect procedure.
Table I.
Comparison of Intersect and Yisual End-Point Titrations (Ammonia aJded t o hydrogen chloridea) Range of IW -P P.P S S I-. -
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APPLICATIOSTO TITR-4TIOX O F ASXXONIA AXD HYDROGEN CHLORIDE.I n the gas phase reaction, NH3 HCI NHdCI, in which a solid is the only product, it may be expected that for any quantity of one of these gases placed in the reaction vessel the second may be added, so that if the pressure inside the reaction vessel is plotted as the ordinate against the quantity of the second gas added as the abscissa a V-shaped curve will result. As illustrated in Figure 4, the first part of the V descends a t a slope of minus 1 and the second ascends a t a slope of plus 1. At the intersection point, the ordinate equals the vapor pressure of ammonium chloride and the abscissa equals the quantity of the second gas needed to reach equivalence. Two procedures have been developed and compared for determining the end point for the titration of each of these gases with the other.
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Increments, End-Point Procedures Cm. Hgb Visual Intersect Hydrogeh Chloride Bias C.1.C Bias C.1.C kO.20 8 9 . 9 7 t 0 1 0 . 2 8 -0 24 + 0 . 0 8 ztO.25 - 0 . 3 8 1 0 . 7 1 0 . 4 2 -0.72=kOo.17 1 0 . 3 4 4 27.66t027.72 -0.56+0.21 a Reaction vessel prepared as for intersect procedure. b Pressure of 1 cm. of mercury equals 0.484 millimole for a 900-cc. vessel a t 250 C. C Confidenceinterval (95%) for single titration.
No. of Titretions
Table 11.
l-isual End-Point Titrations
Range of Presslire Order of Increments of Addition Gas Used, Cm. Hg" Bias C 1.6 NH3toHC1 9 92 t o 10 24 0 00 0 10 + O 33 "3tOHC1 29 87 t o 30 20 -0 11 T 0 26 &O 64 S H a ' t o HClC 9 72 t o 10 16 0 00 + 0 38 k 0 93 0 10 + 0 18 10 08 t o 10 24 1 0 36 4 H C I t o NHs a Pressure of 1 om. of mercury equals 0.484 millimole for a 900-cc. vessel at 25' C. b Confidence interval (95Vc) for single titration. C Hydrogen chloride TT-as diluted with a n equal anionnt of nitrogen before the ammonia addition.
No. of Titrations 10 6 6
DISCUSSION AND CONCLUSIOYS
Figure 4.
Sample Titration Curve
= volume of ammonia measuring vessel volume of reaction vessel
I n the first or intersect procedure, the reagent gas was added t o the sample in the reaction vessel in three or four increments before the end point and three or four increments after the end point. These points were plotted. The intersecting straight lines were constructed and the abscissas a t the point of intersection were taken as the titer as shown a t A in Figure 4. Before starting the next titration, the calculated amount of sample gas was added to return the system to the end point, the reaction
While the precision of the. intersect and the visual end-point procedures are comparable for the titrations of Table I, the visual procedure has the least bias from the theoretical result. Because the bias noted for the intersect titrations were all in the same direction, this bias cannot be due to gas impurity or order of addition. I n the visual end-point titrations of Table I1 there is no significant bias from the theoretical result for the visual endpoint procedure: from this i t has been concluded that the bias of the visual end-point procedure noted for data of Table I is a result of the reaction vessel preparation necessitated by the use of both end-point procedures for these series of titrations. I n addition to bring more accurate, the visual end-point procedure is more rapid than the intersect procedure. The latter procedure has the additional disadvantage for the ammonia and hydrogen chloride reaction that the excess reagent gas adsorbs somewhat on the precipitate, making it difficult to prepare the reaction vessel for subsequent titrations. H4LOGEYS AYD HYDROC4RBONS
The second step in the development of the gas titrator was a study made to help dcfine the scope and precision of this technique. To do this, the reactions of chlorine, chlorine monofluoride, chlorine trifluoridc,and fluorjne with several of the gaseous hydrocar-
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V O L U M E 25, NO. 4, A P R I L 1 9 5 3 bons byere selected because the similarity of the gases provided a rigorous test of t,he resolving power of the instrument. This paper report,s the points of analytical interest which were found during this work. A discussion of the application of this instrument to the st,udy of reaction mechanisnis will be published elsewhere. Experimental Method. ~IATERIALS. The hydrocarbons and chlorine were the purest grade obtainable from the Matheson Co. The chlorine trifluoride was from the Harshaw Chemical Co. and the fluorine was produced a t the I
