THE PREPARATION OF CONDUCTIVITY WATER BY ISAAC BENCOIVITZ’ AND HENRY T. HOTCHKJSS, JR.
Introduction I n order to obtain accurate conductance data, water of a high degree of purity must be used in the experimental determinations. It has been shown that the successful application of the fundamental principles relating to the conductivity of dilute aqueous solutions is largely dependent on this factor; and froin the extent of the discussion of the question it is evident that the preparation of pure water is a most important and difficult problem. The most reliable method of determining the degree of ionization ( a ) ,of a univalent electrolyte at the concentration C is by means of the relationship A, a = -where A, is the true equivalent conductance of the electrolyte at the A, concentration C, and A, is the true equivalent conductance a t infinite dilution*. Also the mobilities of the ions are accurately obtained by the application of Kohlrausch’s law, given by the expression A, = A, A, from which the mobility of the anion (Ua), and that of the cation (E,), are obtained by dividing the respective ionic conductance A, 01’11, by one faraday. It is then obvious that it is of prime importance to determine the value of the true conductance at infinite dilution with great precision. The accuracy with which this is known will affect to a large degree the error involved in the calculation of the degree of dissociation, and of the mobilities. Sumerous empirical functions3 were applied in the evaluation of d m. All of these expressions however, contain empirical constants which have no scientific foundation, and none mere found to hold outside certain definite ranges of concentration. 1: was Washturn4who suggested a very logical way of obtaining true values for the conductivity at infinite dilution. The method consists in plotting K, the constant of the mass action equation against the concentration, using trial values of A m ,and rejecting all those values where the coordinates cause the point, in the range of dilute solutions, to fall outside the path of a smooth curve. Obviously this method makes use of the assumption that a t sufficient dilution all electrolytes must obey the mass action law. This assumption is a thermodynamic necessity. His procedure can only be followed when the correct values for the conductivity of very dilute solutions (o.oooo5 N. or lower) are known.
+
National Research Fellow in Chemistry. 2\Tashburn: J Am Chem Soc 40, 106 (1918) A moia accurate relationship v a s discovered by RIacInnes. TVashburn. “Principles of Physical Chcrnistry”, 261 (1921). Kohlrausch: Ges ilbh 2,1127,1131, 1132; Noves and Falk: J. Am. Chem. Soc 34,462 (1912); Kendall: J . Chem. Soc. 101, 1275 (1912); Kraus and B J Am Chem. Poc. 35, 1315 (1913); Bates: 35, 5 2 7 (1913); Kendall: 36, 1069 (1914); us: “The Propertles of Electrically Conducting System:,” p 67 (1922) J. Am. Chem. Soc., 40, 122 (1918).
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ISAAC BESCOWITZ AND HENRY T. HOTCHKISS, J R .
“Water Correction” Two methods can be used for obtaining reliable data on the conductivity of dilute aqueous solutions. One requires the use of highly purified water under such conditionf that no contamination takes place during the measurement of conductivity the other involves the determination of the nature and the amount of the impurities present in the conductivity water which is in equilibrium with the atmosphere, and then the application of the metathesis correction obtained from the mass action relation. This “water correction” has been discussed by many investigators1. As a result of a thorough treatment of the problem Iiendall suggests the use of the latter method, which is the only logical procedure where the solutions are in contact with the atmosphere and the measurements are made in soda-glass conductivity cells. The use of glass cells was shown to influence the results to a somewhat greater extent than the impurities present in ordinary conductivity water2. However it was definitely established3 that “equilibrium water” (water saturated with carbon dioxide under atmospheric conditions) has a variable conductivity depending upon local conditions, and the methods of experimentation. The study of this condition showed4that not only the normal atmospheric conditions, but even the presence of a person or a lighted gas burner in the laboratory affects the conductance of the mater solution under investigation. “Equilibrium water,” according to the accepted values has a conductivity of 1.0- 0.8X Io-60hms-’. However, in laboratories where other investigations are being carried on, the best water of this description which can be obtained has a conductivity of 3.0- 2 . 0 x 10-6. But evenwith water of 1.0 x Io-6,the“water correction”for a O.OOOOI K. solution of potassium chloride is 44 per cent, for a 0.0001X. it is 8 per cent. The correction is even higher for lithium chloride5. In order to bring the accuracy of conductance data on dilute solutions to within a hundredth of one per cent it is necessary, therefore, to use “ultra pure conductivity water” (a term introduced by Kendall) ; to carry on all experimental operations in an atmosphere free from conducting impurities; and to employ quartz cells. This technique is now the accepted procedure for precision measurements. It was followed by Weiland6, Kraus and Parker7, Parker8, and is being used in this laboratory.
To maintain “ultra pure conductivity water” a t its original degree of purity is comparatively simple. The preparation of such water however, is attended with a great deal of difficulty. The lowest conductivity recorded for pure 1Kendall: J. Am. Chem. SOC. 38, 1480, 2460 (1916); Washburn: 38, 2431 (1916), Kendall: 39, 9, (1917). Kraus and Parker: J. Am. Chem. SOC.44, 2429 (1922). 3 Washburn: J. Am. Chem SOC.40, 106 (1918). 4 Kohlrausch: Ges. Abh. 2,871; Kendall: J. Am. Chem. SOC.38,2464 (1916); Washburn: 40, I I Z (1918). 6 Washburn: J. -4m. Chem. SOC. 40, 109 (1918). J. Am. Chem. SOC. 40, 131 (1918). J. Am. Chem. SOC. 44, 2429 (1922). * J. Am. Chem. SOC.45, 2017 (1923).
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707
water is 0.043 X IO+, at 18’C.l This value was reached after forty-two back and forth distillations in vacuo a t a low temperature. The glass conductivity cell in which the measurement was made had been standing for ten years. It was kept filled with conductivity water to remove traces of soluble matter from the glass. Only very small quantities of this water were obtained and attempts t o duplicate the experiment have failed2. However, such purity is not necessary since water whose conductivity is 0.06 x 10-6 may be considered to be suficjently pure for most measureinents3. Several methods for the preparation of this kind of water have been suggested by various authors. Previous Work Kendal14 reported that he obtained water having a conductivity as low as 0.9 x 10-6 by a single distillation of ordinary tap water. Eourdillon5,and later
Weiland6 prepared pure water by bubbling air, free from gaseous impurities, (carbon dioxide, ammonia, etc.) through distilled water; a method described by Kohlrausch and others who found that it was an effective way of purifying water. Eourdillon charged a 13 litre copper boiler with ordinary distilled water containing some sodium acid sulphate, and collected conductivity water (O.II~IO-6) half an hour after steam was generated. The condensed watei was allowed to fall in a vertical tube against a stream of carbon dioxide-free air. Weiland used the same principle. Starting with distilled water (0.80.6 x I 0-6), he redistilled from potassium permanganate after having bubbled carbon dioxide-free air through it for twenty hours, while the temperature was kept just below the boiling point. For his still he employed a 13 litre quartz flask, and collected a,bout 3 litres of water having a conductivity of 0.070.05 X 10-6. Xore recently Kraus and Dexter7 built a still whose effectiveness is dependent on the principle of purification by fractional condensation. Three fractions of unequal purity were obtained, and the middle fraction was found to have the lowest Conductivity. This may be taken as additional evidence that carbon dioxide does not comprise the whole of the conducting impurity in conductivity water8. Operating in its final form, steam from a 90 litre boiler, containing alkaline pcrmanganate, mas led through a trap which removed a small amount of condensed steam and any entrained moisture. Then twenty per cent of the vapor was condensed and collected in the conductivity cell, and the remaining steam separated and liquefied in a subsequent stage of the process. Kohlrausch: Ann. Physik, Erganzungsband, 8, I (I878); Kohlrausch and Heydweiller : Ann. Physik, 53, 209 (1894);Kohlrausch: Z.physik. Chem 42, 193 (1903) Taylor: “Treatise on Physical Chemistry,” p j21 (1924). Washburn: J. Am. Chem. Soc. 40, 130 (1918). J. Am. Chem. Soc 38,2465 (1916). J. Chem. Soc. 103, 791 (1913). J. Am. Chem. Soc. 40, 131 (1918). J. Am. Chem Soc. 44, 2468 (1922). Kendall: J. Am. Chem SOC.39, 13 (1917);Kashbuin: 40, 122 (1918).
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ISAAC BENCOWITZ AND HENRY T. HOTCHKISS, J R .
]Ire have experimented with stills similar in type to those discussed. As our work required large quantities of pure water, we found it necessary t o design a new still, incorporating some effective changes in design with the features of these stills which our preliminary work showed to be desirable. The efficiency of a stream of carbon dioxide-free air as an agent for removing the absorbed gases from water cannot be questioned. Rourdillon records the lowest conductance of water obtained from his still as being 0.086 x IO-^. The conductivity value at times was considerably higher. This may be attributed t o the method used in treating the condensed vapor with air which had been scrubbed free of carbon dioxide. Weiland charged his still with pure distilled water having a conductivity of 0.8 X IO+, and since the range of conductivity water distilled and stored in an ordinary laboratory is 3.0-2.0 X ~ o - ~ wcould e not obtain good conductivity watei by a single distillation using a similar still. Kraus and Dexter installed a large go litre boiler because they found that a smaller still did not yield the proper water. I n our experimental work we could riot get good water when a 2 0 litre boiler was used,-which is in agreement with their results. We did not want to make use of a large boiler for several reasons. Aside from the fact that a bulky apparatus was undesirable, we wished to make the operation of' our still practically continuous, that is, we wished to design our still so that it would produce good conductivity water in sufficient quantity for our work whenever required. The fact that this could not be accomplished with a still having a large boiler which required preheating for a much longer period of time was for us an important consideration. Moreover, when we take cognizance of the fact that Kraus and Dexter discarded the first 20 litres of the watcr they distilled over, it is apparent that the yield of conductivity water would of necessity be more or less spasmodic. Our work, however, demonstrated t o us the value of their observation that fractional condensation is a very good method of purification, and we have applied the idea in the final stage of our process. The unit which we have built therefore embodies the principles used by Kraus and Dexter as well as by Weiland. It will yield, in an ordinary laboratory, conductivity water suitable for the investigation of dilute solutions. Our method is briefly this:-ordinary tap water is boiled with alkaline permanganate, and the steam generated is condensed to supply a second boiler with freshly distilled water. Air, freed froni carbon dioxide, is then utilized to sweep out gaseous impurities, and the water so purified is subjected to a fractional condensation upon distillation. Description of the Improved Still Referring to Fig. I , boiler I is charged with about 2 0 litres of tap water, and small amounts of potassium permanganate and sodium hydroxide are added. The water is heated to just below its boiling point. After it has digested for some time with the alkaline permanganate the temperature is raised and the water distilled over into boiler 11. On account of the form of the apparatus, little or no water is carried over mechanically. The first litre of distillate is discarded. The second copper boiler is fitted with a tube H, connected with
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PREPARATION O F CONDUCTIVITY WATER
the source of the carbon dioxide-free air. This air is prepared by drawing air from outside the building and forcing it through an absorption train consisting of suitable containers holding sodium hydroxide and sulphuric acid'. Both the delivery tube (A) and the air supply pipe project almost down to the bottom of the boiler. Thus the steam cannot force its way back through condenser I to the first boiler, and the air which bubbles up constantly through the water in the second boiler stirs the water and remains in contact with it for the maximum period of time. This air, which is forced in by a pump of the Sprengel type, eventually passes up a long vertical tube, fitted with condenser 2 , and escapes through a trap (B). Any water that condenses in z is returned
r
FIG.I
to the boiler. When the water in the second still is boiling vigorously, steam rises and is condensed in 2 forming a column of water which seals the vertical tube so that the steam begins to flow through the connecting tube C. However, a very small amount of water vapor and air is allowed to pass to the trap R. One end of the connecting tube (C) is so attached that the water cannot drain into it from 2 , while the steam jet at the other end is directed down into the air condenser 3 which is drained by the trap D. The diameter of this air condenser is quite large so that the mean velocity of the steam is decreased, allowing any drops of water which are entrained in the steam to be precipitated out. Practically dry steam therefore enters the long condenser 4, the temperature of which is so regulated that it condenses about eighty per cent of the entering water vapor. The condensed water is separated by gravity at E from the steam and air which continues to flow around a gentle curve and is conWeiland: J. Am. Chem. SOC.40, 134 11918).
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ISAAC BEiYCOWITZ AND HEXRY T. HOTCHKIGS, JR.
densed at 6. No part of the steam or air is sucked doirrn the tube from E. This tube delivers the mater to the quartz conductivity cell when flow is occasioned by displacement. There is a water seal due to the bend in the delivery tube below the cooling jacket 5. In order tha#tthe pure water collected in the conductivity cell may not be contaminated by contact with the atmosphere, a constant stream of carbon dioxide-free air, issuing froin tube I prevents the water from meeting the impure air of the laboratory. Throughout the system the water seals, made by proper bends in the pipes, prevent air from the room being drawn into the apparatus if a momentary partial vacuum is created by the condensation of the steam. It has been found in actual practice that it is convenient to disconnect boiler I at the coupling K while the conductivity water is being prepared in the succeeding stages of the apparatus. Boiler I is then filled and the water preheated. When the water in the second boiler becomes low some steam passes back through the delivery tube A and the first condenser, and exhausts into the air a t I(. By this time the digestion in the first boiler is complete, and it is again coupled with the rest of the system. The heat under boiler I is increased and the flame under boiler I1 lowered. The distillation from I to I1 is then allowed to take place over night. A minimum of time is thus expended in refilling the second boiler with fresh distilled water, which is maintained just below its boiling point while air freed from carbon dioxide is bubbled throuhg it. The following morning more heat is supplied beneath the second boiler and the still is flushed with live steam for half an hour to remove iiiipurities which have been absorbed from any air which has entered the apparatus. R e have found it necessary to do this when the distillation is discontinued even teniporarily. The wet interior surfaces seem to adsorb the impurities which enter with the air which the water seals cannot keep out against the back pressure cadusedby the complete condensation and halting of the steam flow. After the still has been flushed with steam, the temperature of the water in condenser 4 is regulated and the conductivity water collected. For each litre of water collected in the cell, zoo cc. are removed from the system at B and D, and 2 0 0 cc. are collected in the receiver G. In building this apparhtus standard sizes of block tin tubing were used in conjunction with the two copper boilers. All joints were soldered with tin, and the entire apparatus can be assembled by a tinsmith, Conclusion Kraus and Dexter found that the middle fraction in their condensation had the lowest conductivity. However, no explanation was offered. Assuming that carbon dioxide is the chief impurity causing the rise in the conductance of the condensed water, we may formulate an explanation of this phenomenon on the basis of certain physical laws governing gas-liquid systems. During the process of distillation, and after the water has been scrubbed for a considerable period of time, it is found that the water still contains an appreciable amount of carbon dioxide. Vigorous boiling, in conjunction with the turbulence caused by the entering stream of carbon dioxide-free air will produce in the
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saturated steam. a mist of minute droplets of water, part of which are carried in the steam flow. The heavier droplets will be removed in the first fraction, while many of the very fine droplets will be carried over into the condenser (4) where they serve as nuclei for further condensation. These minute particles, be they mechanically carried over or formed by condensation may be shown to consist of “ultra pure water.” The rate of absorption of a gas by a liquid is practically always limited by the rate of the diffusion through the medium.’ This diffusion will take place at the liquid-gas junction through two films; one consisting of a layer of liquid in which mixing caused by convection currents is slight as compared with motion in the main body of the liquid, the other being a layer of gas next to the interface in which similar conditions obtain2. The rate of diffusion through the liquid film is proportional to the difference between the concentration of the solute in the liquid at the interface, and its concentration in the main body of the liquid3. In the case under consideration the gas (carbon dioxide) is very insoluble at the temperature employed. Therefore the gas film resistance to absorption may be considered to be negligible since the liquid at the interface is substantially saturated with solute at the existing pressure. Where the diameter of the droplet, constituting the liquid phase, is exceedingly small the liquid film will constitute the greater portion of its volume. Bohr4 found the coefficients of absorption and escape were practically identical for carbon dioxide. It follows from this and the “two film theory” that the contamination of the water depends almost wholly on the slow diffusion of carbon dioxide into the liquid phase. The formation of small droplets at temperatures below the boiling point facilitates the removal of slightly soluble gases from the liquid phase into the gas phase. Furthermore, since the vapor pressure is inversely proportional to the radius of curvature of the surface, we call easily conceive that the solubility of gases in minute particles is smaller than in the bulk of the liquid. In the still which we have designed, a fine mist is created by the ebullition of the water in the second boiler, the minute droplets of which enter the condenser as condensation nuclei, while the air passing through the system sweeps out any gaseous impurities and any excess steam. This excess steam is condensed and forms the third fraction. We have attempted to show that the minute particles of mater carried over into the condenser have n strong tendency to give up any dissolved gases and do not reabsorb gas readily. It stands to reason that these particles which act as nuclei of condensation consist of “ultra pure water,” and that the second fraction, which is made up of steam condensed around these particles, should be the purest fraction, especially under conditions where the gaseous phase is removed continually. ‘Adeney and Becker: Proc. Rcy. Soc. Dublin, 15, 385, 609 (1918); 16, 133, 143 (1920). * Lewis and Whitman: Ind. Eng. Chem. 16, 121j (1924). Whitman: Chem. Met. Eng. 29, 146 (1923). Ivied. Ann. 68, joo (1890).
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ISAAC BENCOWITZ AND HEXRY T. HOTCHKISS, J R .
Summary I . An improved type of still for the production of water for use in the investigation of dilute aqueous solutions has been described. Water, of a conductivity as low as 0.06-0.07 X IO-^ can be readily obtained. The advantages of this still are,-that its operation is practically continuous, and that it can be operated successfully in any ordinary laboratory. 2 . An attempt ~7as m2,de to explain the advantages of fractional condensation as a method of obtaining water of low conductivity. We wish to express our thanks to C r . R. R. Renshaw who has aided in this investigation.
Haoemeyer Chemical Labcratory, New York Uniuersity, January 29, 1925.
