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Partial Vapour Pressures and Activity Coefficients of Gb and Gd in Aqueous Solution

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Partial Vapour Pressures and Activity Coefficients of Gb and Gd in Aqueous Solution DMC A NON-CONTROLLED GOODS National Defense I♦ Defence nationale PARTIAL VAPOUR PRESSURES AND ACTIVITY COEFFICIENTS OF GB AND GD IN AQUEOUS SOLUTION by J.M. Preston Chemical Defence Section Protective Sciences Division and V. Starrock Canadian Defence Research Fellow attached from Wehrwissenschaftliche Dienststel/e Bw ABC-Schutz, Munster, Germany DEFENCE RESEARCH ESTABLISHMENT OTTAWA REPORT 893 PCN September 1983 13A Ottawa ABSTRACT The partial vapour pressures and thus the activity co­ efficients of GB and GD and water in aqueous solutions of the agents have been measured through the range of mole fractions in which the agent and water are miscible. The technique used was to measure the total vapour pressure over the agent/water mixture and then to solve numerically for the partial vapour pressures with a computer implementation of the Gibbs-Duhem equation. The agent activity coefficients display the hydrophobic nature of the nerve agents at mole fractions less than one half, but are close to unity otherwise. RESUME On a mesure les tensions de vapeur partielles et, ainsi, les coefficients d'activite du GB, du GD et de l'eau dans des solutions aqueuses de ces agents dans la plage des fractions molaires pour laquelle l'agent et l'eau sont miscibles. La technique utilisee consistait a mesurer la tension de vapeur totale au-dessus du melange agent/eau, puis de calculer la valeur numerique des tensions de vapeur partielles a l'aide d'une version informatisee de l'equation de Gibbs-Duhem. Les coefficients d'activite traduisent la nature l1ydrophobe des agents neurotoxiques a des fractions molaires de mains de 0,5 mais ils sont voisins de l'unite a des fractions molaires superieures. ZUSAMMENFASSUNG Die Partialdruecke, sowie daraus abgeleitet, die Aktivitaetskoeffizienten von GB, GD and H 0 in waesserigen Loesungen dieser Kampfstoffe wurden ueber den gesamten Molenbruch-Bereich,2 in dem diese Kampfstoffe and Wasser sich mischen, bestimmt. Hierbei wurde der sich bei einer gegebenen Temperatur einstellende Gesamt-Dampfdruck ueber den Kampfstoff-Wasser-Mischungen gemessen; mit Hilfe eines Rechnerprogrammes liessen sich bei Zugrundelegung der Gibbs-Duhem'schen Gleichung die entsprechenden Partialdruecke von GB, GD and H 0 ermitteln. Die hydrophoben Eigenschaften der Nervenkampfstoffe kommen bei2 Molenbruechen < 0.5 deutlich <lurch die diesbezueglichen Akitivitaetskoeffizienten zum Ausdruck, waehrend sich fuer Molenbrueche > 0.5 die Aktivitaetskoeffizienten der Nervenkampfstoffe dem Wert 1 naehern. iii 1 INTRODUCTION General When liquid nerve agents are released in the field they come in contact with water, as vapour, liquid, or solid. The purpose of the investigation reported here is to measure the effect of this interaction on the agent vapour pressure. This measurement amounts to a determination of the partial vapour pressures, and therefore the activity coefficients, of GB/water mixtures and GD/water mixtures over the full available range of agent mole fractions. Our motivation for undertaking this study was to enable more accurate models of agent behaviour in humid field conditions to be constructed. Watts (1), in discussing the paucity of data on nerve agent interaction with water, discusses the two effects which the absorption of water will have on a falling drop of agent: to change the agent vapour pressure and hence its rate of evaporation, and to enlarge the drop and hence its terminal velocity. The evaporation rate of agent on vegetation and soil will similarly be affected by interaction with water. These measurements of activity coefficients may also be applicable to biochemical studies of G agents in the bloodstream. Theory of Partial Vapour Pressure and Activity Coefficients An ideal solution is defined as one in which the cohesive forces between molecules are uniform, regardless of the identities of the molecules (2). The partial pressure of each component above such a solution is proportional to its mole fraction in the solution, i.e., (1) pi = x;: p� where P. is the partial vapour pressure of component i, X� is the mole fractioii of component i in the solution, and P? is the va�our pressure of the pure compound. Equation 1 is a statement of R�oult's Law. It is rare, however, to find even binary solutions which agree well with Raoult's Law. More usually the force of attraction or repulsion between the two components is different from that in neat solutions, thereby increasing or decreasing the partial vapour pressures. The differences are often referred to as deviations from Raoult's Law. For these non-ideal solutions one writes 0 = pi ai pi (2) which serves as a definition of the activity, a., of each component, i. Since the activity is the product of the mole £?action and a value proportional to the deviation from ideality, one also writes (3) 2 where y is called the activity coefficient. Thus the activity coefficient is equal to the partial vapour pressure of a compound divided by the partial vapour pressure which that compound would exhibit in an ideal solution. L o y. = P. / X. P. (4) 1. 1. 1. 1. Activity coefficients are widely used when considering physical and chemical equilibria (with cognizance of the appropriate standard state). Reactivity of G-agents In this work we studied solely the physical interaction between agent and water. The effects of chemical reactions were minimized by selection of observation time and of pH of the added water, which greatly affects rates of hydrolysis (3). The requisite pH range, 4-6, may well be encountered in real situations. EXPERIMENTAL Our experimental approach was to measure the total vapour pressure over the agent/water solution, throughout the range of mole fractions, and at various temperatures. The technique used to calculate the partial vapour pressures from these measurements of total vapour pressure will be discussed in the next section. The apparatus is shown diagramatically in Figure 1. The key element is the magnetic reluctance manometer (hereafter called MRM) (Validyne DP7 Variable Reluctance Differential Pressure Transducer, Validyne Engineering Corporation, Northridge, California, and Pace CD25 Transducer Indicator, Pace Engineering Co., North Hollywood, California). In the centre of this manometer is a thin steel diaphragm which deforms according to the pressure differential across it, while its position is sensed by small electromagnets located synnnetrically on either side. In principle, this manometer can be calibrated and used to measure pressure difference directly. In practice, however, it tended to drift or jump in both zero setting and sensitivity. To overcome this we used a conventional manometer, containing di-octyl phthalate. Once the agent/water solution was in equilibrium with its vapour on one side of the MRM (with the stopcock above that side-arm closed), laboratory air was admitted slowly into the other side and the manifold until the MRM read zero. The pressure above the agent/water solution could then be read directly from the di-octyl phthalate manometer by means of a cathetometer. Since the zero setting of the MRM was adjusted just before each measurement, and verified after each run, this was considered to be a technique free of systematic error. The rotary pump was capable of evacuating the system to less than 2.5 Pa (20 mTorr)*. * 1 Torr = 133.3 Pa 7.50 Torr = 1 kPa 3 i_ { ( '< ( \ \ '\ ✓ t l ( ( I I 1,_1_, fl ( Wa:w r-f­ ...l <{w r->-.....1�<{ (.)Io Or-z 0 ..!.I <{ wa:I Oa...:::--== a: � f- � <{ <{ f­ 3:CD (/) � <{ ....J CD<{ wa: (.)(.)w -zf­ wf- .........,. w zr-� c,UO w <{::J z - �- � ....J <{ �� ��IL__ Q. C} Oa. ...J <{ Oa: (.)f- >­ a:a. Q<{� f-r- Oa.::J a: Fig. 1. Apparatus used to measure the total vapour pressure of agent/water mixtures. The water bath surrounding the sample cells, magnetic reluctance manometer, and the connections between them could be removed for freeze-thaw degassing of the samples. 4 The solutions were prepared by weighing typically 250 mg of agent into small vials which could be fitted directly onto the sidearm of the MR.M with greased ground-glass joints. The mass of water required to produce the desired mole fraction was then calculated and added by syringe, after which the solution was reweighed so that the precise mole fraction could be GB which had been distilled within the last year and since then calculated. ° kept at -20 C proved satisfactory, but GD with a similar history had vapour pressures well above the literature values. The GD used in this experiment was synthesized two days before data collection started. Since only one mole fraction was studied each day, aliquots for approximately one week's work were removed from the freezer when required. No stabilizer was used. Distilled water from the DREO still was used. Its pH, measured by pH paper, was 4, so in some experiments trace amounts of ammonium hydroxide were used to raise the pH, as discussed below. The vial containing the solution was placed on a sidearm, with an empty vial on the other sidearm. Four cycles of cooling the solution in a dry ice-acetone bath with pumping and then warming to room temperature with the stopcock closed were used to degas the sample. On the fourth cycle the MR.M was zeroed, then with the stopcock closed the dry ice-acetone bath was replaced by a water bath in which the MRM, its sidearms, the vials, and the tubing up to the stopcocks was immersed. The reading of the MR.M was then monitored to determine when equilibrium had been reached (usually at least 30 min) and to ensure that hydrolysis was not occuring (if the pressure was found to be falling significantly the run was discarded). When the MR.M indicated a steady value the di-octyl phthalate manometer was used to read the pressure, as discussed above.
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