G . H. Fricke, R. L, Carpenter, and R . Battino
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Effect of Various Gases on the pH of Water Gordon H. Fricke, Rick L. Carpenter, and Rubin Battino” Deparfment of Chemistry, Wright State University, Dayton, Ohio 45437
(Received September 25, 7972)
Publicetion costs assisted by The Petroleum Research Fund
Ultrapure water ( X 5 megohm cm, 25”) was saturated with various “inert” gases: Ha, 0 2 , Nz, He, Ar, CHI, and CO. The pH difference between degassed water and water saturated with a gas was found to be k0.06 ( t g o ) for individual measurements and rtO.01 ( t g o ) for the average of a series of measurements. These differences are considered to be within experimental error a t a pH of 7.00. Thus, previously reported “anomalou!P pH effects are either due to impurities in materials or artifacts in the experiment.
Introduction The pH of relatively pure degassed water was found1 to shift from a value (of 7.00 for pure water to values between 7.50 and 8.00 when the water was saturated with supposedly “inert” gases such as He, Nz, Ar, and Ne. In what follows we report on our investigation of this phenomenon. Our early experiments seemed to confirm these results, but we found that each improvement in the purification of water, in the purification of the gases, and in the experimental design resulted in a decrease in the difference between the ppf observed for degassed water and the pW observed for water saturated with one of the gases.2 The result of our most recent experiments using our most refined procedures is that the p H shifts are zero within the experimental error obtainable with our instruments. If the pH shift had been real, i t was proposed that the decrease in the activity of the hydrogen ions may have been due to a n altering of the water structure or to the formation of weak coordinate bonds between the gas and hydrogen ions, causing the gas to act like a Lewis base.132 It has been suggested that cations such as HNe+ are likely to exist in wateir owing to the large electronegativity of neon.3 This would cause an effective decrease in the hydrogen ion concentration which would be observed as a shift to a greater pH value. Although this prediction may be valid, apparently the concentration of the cation present a t atmospheric pressure is too low to be detected by conventional pH measurements such as those employed in this research. Experimental Seetiom T a p distilled water was redistilled in a Corning all Pyrex still and then transferred to a self-contained recirculating “water system.” The water system consisted of four mixed-bed ion exchange columns, an activated charcoal column, a submicron Millipore filter, a conductivity cell, and a peristaltic pump. Tygon tubing was used in the peristaltic pump. The four Illinois Water Treatment Co. ion exchange columns were used in the following order: one Universal Grade, two Research Grade, and one Puritan Grade. All columns were wrapped with aluminium foil to retard degradation of the ion exchange material by light. All connections in the system were made by glass (Pyrex) and Teflon. A procedure similar to this was recommended by Hughes, et u L . , ~ and Iversons for the attainment of ultrapure water. The Journal of Physical Chsrnistry, Vol. 77.
No. 6.
1973
The gases were purified before they were dispersed into the water in the reaction vessel. Each gas was passed through a series of liquid scrubbers which were separated by Kjeldahl traps. The scrubbers were arranged in the following order: base, acid, base, acid, pH 7.00 phosphate buffer, water, and water. The acid was 5% H2SO4 and the base was saturated Ca(0H)z. After the scrubbers, the gas passed through three cold traps (maintained at ca. -78”) packed with glass beads, molecular sieves, and glass beads, respectively. We used three cold traps in series because we found that the microscopic mist created by the scrubbers was not removed by a single trap. The all Pyrex glass 1-1. reaction vessel with Rotaflo Teflon stopcocks was designed so that only a Teflon-coated magnetic stirring bar and the gas dispersion tube came in contact with the water after the water entered the vessel. The reaction function was separated from the measuring function. A stopcock located at the bottom of the reaction vessel was opened to allow water to flow from the vessel so that measurements could be made a t various times without disturbing the reaction. Water flowed through the conductivity cell, through the pH cell, and was then discarded. The reaction vessel was directly attached by Pyrex tubing to the ultrapure water system and to the vacuum system which was used to degas the water. The reaction vessel had been steamed with about 40 1. of water before it was glass blown into the system. Figure I shows the reaction vessel with the location of the flow-through conductivity cell and the flow-through p For most of the runs, the pH of the water and the aqueous solutions was measured with a Sargent Model 3007010 combination electrode which contained a platinum rope junction for the reference electrode. Two nitrogen runs and all of the hydrogen runs were performed with a Fisher Model E-5 combination electrode which contained a ceramic disk junction. The latter electrode was necessary for the hydrogen runs because the platinum rope of the Sargent electrode apparently established a platinumhydrogen electrode.2 The reference potential of the piatinum rope electrode was altered greatly. The ceramic disk electrode eliminated this problem. (1)
(2) (3) (4)
(5)
E M Holleran, J. T. Hennesy, and F R LaPietra, J Phys Chem 71, 3081 (1967) R L Carpenter, M S Thesis, Wright State University Dayton, Ohio, 1972. B Fung, J Phy.? Chem , 69,596 (1965) R C Hughes, P C Murau, and G Gundersen, Anal Chem, 43, 691 (1971). A Iverson,J Phys Chem 68, 515 (1964)
Effect of Gases o n Water pti The combination electrode was located below the level of the liquid in the r e d i o n vessel (see Figure 1). A tygon tube, which was partially filled with water, was connected to the vent in the electrode. This pressure-compensating tube was raised to the proper level to balance the pressure head of t,he liquid in the reaction vessel. The pH readings were obtained with an Orion Ionanaiyzer Model 801 digital pH/mV meter. Simultaneously, the pH was monitored with a Texas Instruments ServoRiter I1 recorder. The pH meter and recorder were adjusted with pH 7.00 and 4.01 buffers before each run. After the buffer adjustment, the electrode was rinsed and soaked in deionized water for a minimum of 20 min before it was placed i n the flow-through pH cell. The electrode was checked with the buffers after each run. At one point, erratic pH readings were observed which seemed to be caused by the magnetic stirring motor used to rotate the stirring bar. The magnet used to turn the magnetic stirring bar in the reaction vessel was then mounted on the end of a 7-in. rod which was driven by a high-torque motor (see Figure 1). No further problems were observed with the motor. Since the resistance of pure water varies from 31.8 megohm em a t 15' to 11.1 megohm cm at 3505,'j and since possible resistance changes were anticipated, the resistance of the water was monitored whenever pH measurements were mstde. The resistance was measured with a new design flow-through conductivity cell7 and an Industrial Instruments ode1 RC-16B2 conductivity bridge. The conduct,ivit.y cell had a cell constant of approximately 0.005 cm-I. The temperature of the water in the reaction vessel was measured with a copper-constantan thermocouple which was attached t o the outside of the reaction vessel. The thermocouple was thermally insulated from the surroundings. The water in the reaction vessel was degassed by the extraction procedure of Battino, et a1.8 The final residual pressure in all cases was less than about 100 p . The pressure was measured with a Consolidated Vacuum Corp. thermocouple vacuum gauge Type GTC-100. Measurements were carried out in the following manner. The reaction vessel was flushed. with gas, and with the gas still flowing into the vessel, the pure water was allowed to i?ow into the vessel through the conductivity cell. The vessel was rinsed twice by this procedure with about $00 m.1 of water each timg before introducing the test water into the vessel. The pH, resistance, and temperature of the final incoming waters were recorded. The water was purged for about 10 min and degassed. The magnetic stirring bar was rotating a t a fairly high speed during this procedure. The purging and degassing procedure were repeated. The second time the extraction was continued until the residual pressure was about 100 p. 'Ithen, with the stirrer stopped, gas was allowed to enter the reaction vessel above the water. The bottom stopcock was opened. The resistance, pH, and temperature of the degassed water were measured, after which the water was purged with the stirrer rotating. At various times about 100 rnl of water were withdrawn from the vessel and the resistance, pH, and temperature were recorded. All gases were purged for a minimum of P hr before the final readings were made. No additional resistance or pH changes were found for water which had been purged for up to 24 hr .
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a DRAIN
Figure 1 . Reaction vessel for measurements showing ptl electrode flow-through conductivity cell, stopcocks, gas dispersion tube, stirring apparatus, and flow patterns.
The water was brought into the reaction vessel from the recirculating water system under an atmosphere of the gas in question. From the time the water entered the reaction vessel until the final measurements were made was a maximum 6 hr. There appears to be negligible conductivit y changes (less than a few per cent) in the water if it remains in contact with Pyrex glass for less than 24 hs. The unstable pH response in an unbuffered flowing water system which was reported by atesQ and Quickenden10 was also observed in this research. The pH differences reported in this paper were for a static condition. The electrode responded with a stable, constant pH value within 1 min of the time the flow was stopped. The reading remained constant for sa minimuni of 2 min. Qccasional drifting was observed for longer time intervals.
Results and Discussion The result of purging ultrapure water with various gases was measured by a change in pH from degassed water to water saturated with a gas: ApH = ppb of saturated water - pH of degassed water. The results for the seven gases are presented in Table I. The reported average deviation of ApH is for absolute values of the deviations The reaction vessel was not thermostated. It was observed that, in some cases, the resistance of the water inG. Otten, Arner. Lab., 41,July, 1972. G. H. Fricke and R. Battino, to be submitted for publication. R. Battino, M. Banzhof, M . Boqan, and E. Wilhelm, Ana/. Chem.. 43,806 (1971). R. G . Bates, "Determination of p H : Theory and Practice," Wiley, New York. N. Y.. 1964. T. I. Quickenden, D. M. Betts, 8 . Cole, and M. Noble, J . Phys. Chem., 75, 2830 (1971). The Journa! of Physical Chemistry, Yo!. 77. No. 6. 1973
