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method for determination of the vapor-pressure curves is probably the easiest. Figure 3 shows the Diihring system of a solution that has two possible solid phases with one transition point as shown. For a one-phase system the saturated-solution line would be a smooth curve from the cryohydric point. In other words, this curve is the vapor-pressure curve of the saturated solution plotted as a Diihring line, that is, of course, not generally straight. At each transition point there is usually a break in the vapor-pressure curve and hence in the Diihring line. The line from 0 ' C., called the F vs. F x curve, is really the freezing point-composition curve plotted as the freezing point us. the temperature of water at the same pressure. The Diihring lines of solutions up to the cryohydric com-
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Vol. 21, No. 2
position run from their respective freezing points on this curve. For compositions above the cryohydric the Diihring lines run from the saturated solution curve at points that correspond to the respective concentrations of the solutions. The use of Diihring's rule for the calculation of activity coefficients does not seem to be generelly known. If the lines are plotted for the system a t different concentrations, it is easy to determine the activity of the water at any temperature and concentration by reading the temperature of the water at the same vapor pressures as the solution. The ratio of the vapor pressure of water at this temperature to that at the temperature of the solution is the activity of the water in the solution. This should be of great use a t very high concentrations.
Viscosity Relationships in the System Sulfuric AcidNitric Acid-Water1 F. H. Rhodes and H. B. Hodge, Jr. CORNBLL UNIVBRSITY, ITHACA.N. Y.
N SPITE of the very great
The viscosities of a large number of binary and terthese rough measurements, nary mixtures of these components have been deterhowever, indicated that the technical importance of mined at temperatures of O", 25", 50°, and 75" C. viscosity relationships in the "mixed acid," we have The results indicate the existence, in liquid form at portion of the system relabut little information as to the ordinary temperatures, of a hydrate of sulfuric acid tively rich in sulfuric acid identity of the compounds of the formula HzSOI.HZOand of a ternary compound presented some very interestwhich may actually exist in which contains SO8 and N2Os in the ratio of lOSO::N2Os. ing features which had been ternary mixtures of sulfuric There is no definite evidence of the existence of a previously overlooked. acid, nitric acid, and water hydrate of nitric acid in liquid form. T h e s e measurements have a t o r d i n a r y temperatures. V a r i o u s investigators have been repeated, using an acstudied the relationships betweem the composition of mixed curate viscometer. The results are described in this report. acid and certain of its ihemical and physicd properties, but Experimental Procedure in general the data thus obtained have not served to identify the compounds which may actually be present. Much of this VISCOMETER-The viscometer used in this work was of the work has been done a t temperatures considerably above or type described by Bingham.a Two such instruments were below those at which the acid is used in nitration, so that the employed, the one with the smaller capillary being used with information that is available may not apply to the constitu- the mixtures which showed viscosities of less than about 10 cention of mixed acid under the normal conditions of use. The tipoises a t 0" C. Connectionsbetween the viscometer tube and results of these various investigations will be considered later, the rest of the apparatus were made by ground-glass joints, in connection with the discussion of the data obtained by since rubber connections might have been attacked by some viscometric methods. of the solutions used. The viscometer tube was set in a Since any change in the nature or the relative amounts of thermostat. A 10-liter bottle served as a pressure reservoir. the molecular species present in a liquid should affect the This bottle was provided with a rubber stopper through which viscosity, and since viscosities, may be measured a t the tem- extended three glass tubes-an inlet tube for water, an outlet peratures a t which mixed acid is ordinarily used, the measure- tube for air, and an outlet tube for water (extending to near ment of the viscosities of mixed acid should afford some in- the bottom of the bottle). The outlet tube for water was formation as to the nature of the compounds actually present connected by rubber tubing to a glass overflow outlet. By and might, perhaps, throw some light on the mechanism of admitting water slowly through the inlet tube, the air in the the nitration reactions, Viscosity relationships in this system bottle was compressed until sufficient pressure was developed have been studied by Bingham and Stone1#*but the number to lift the water to the overflow outlet, a t which point the of individual mixtures of which the viscosities were determined pressure remained constant. By adjusting the height of was rather small and only a few of those containing large this overflow outlet the pressure within the system could be amounts of sulfuric acid and relatively small amounts of water regulated. An empty bottle was connected into the air were investigated. At the time of the publication of these pre- outflow line to provide additional capacity and to minimize vious results B. B. Paul, working in this laboratory, was engaged any fluctuations in pressure. With this system it was possible in themeasurement of the viscosities of mixed acid. Since the to eliminate any detectable variations in pressure within the measurements were being made principally for the purpose of apparatus during a determination. The pressure reservoir obtaining information for use in the design of pumps and was so connected with the viscometer tube that the pressure agitators for handling nitrating acids and nitration mixtures, could be applied to either arm of the tube while the other arm the viscometer used was a rather crude one and no special was vented to the air. To prevent the absorption of moisture pains had been taken to insure extreme accuracy. Even by the acids while a measurement was being made, bulbs containing glass wool moistened with concentrated sulfuric 1 Received June 26, 1928. acid were inserted in the lines between the pressure bottle * Numbers in text refer to bibliography at end of article.
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and the viscometer. The pressure within the system was indicated by a differential manometer connected into the line between the reservoir and the viscometer tube. Viscometer tube No. 1 (with the smaller capillary) was calibrated and its constants were determined from data obtained with standard solutions of sucrose of known viscosity and density.3 Tube No. 2 was calibrated by measurements with acid mixtures of which the viscosities were known by determinations made in viscometer tube No. 1. It so happened that both of the tubes were so made that the time of flow was independent of the direction of the flow, so that it was not necessary to apply a correction for the difference in level of the two bulbs. ACIDSUsm-The sulfuric acid used in this work was from a special lot of pure material. It contained 98 per cent of sulfuric acid and 0.0009 per cent of non-volatile matter and gave no test for either hydrochloric acid or nitric acid. Some of the mixtures were prepared with a special lot of fuming sulfuric acid, which contained 30 per cent of excess sulfur trioxide and was free from hydrochloric acid and from nitric acid. The nitric acid was prepared from the C. P. acid. The concentrated acid was blown with air to remove any oxides of nitrogen and was then redistilled. I n some cases a small amount of a solution of silver nitrate was added to the acid before the redistillation in order to remove any chlorides which might have been present, but usually this was not necessary. When nitric acid of greater than about 65 per cent concentration was required, it was prepared by redistilling the purified nitric acid with about an equal weight of concentrated sulfuric acid, All such distillations were made under reduced pressure to avoid the decomposition of the nitric acid. When nitric acid of especially high concentration was required, the redistilled material was sometimes blown with air and then again distilled over sulfuric acid. DISTILLATION APPAI1ATuS-The apparatus used in making the distillation is shown in Figure 1. Nitric acid or a mixture of nitric and sulfuric acids was introduced through the neck A into the Pyrex flask D. The tube E was then inserted. This was fitted to the opening a t A by a ground-glass connection, and the lower end, which extended almost to the bottom of the flask, was drawn down to a fine capillary. By this means a fine stream of air was drawn through the acid during the distillation. This served to eliminate bumping, to minimize the decomposition of the nitric acid, and to remove any oxides of nitrogen which may have been produced. The nitric acid which was distilled over was collected in the receiver R, which was surrounded with ice to prevent decomposition of the distillate. The connection between the receiver and the vacuum pump was made through a tower packed with pieces of solid sodium hydroxide in order to prevent the vapor of nitric acid from coming in contact with the pump. I n each distillation the first portion of the distillate (10 or 15 cc.) was used to rinse out the receiver and was then discarded. The distillation was always discontinued while a considerable amount of the nitric acid in the still remained yet unvaporized. The distillate was tested for chlorides and sulfates, but in no case was either of these impurities found. PREPARATION OF SoLuTIoNs-In preparing the solutions for use in the viscometric measurements a number of primary mixtures were made up from known weights of each of the three components. From these primary solutions secondary mixtures were made by dilution with known amounts of solutions of sulfuric acid of the same content of sulfuric acid as the primary solutions themselves. For example, by mixing 60 grams of a solution of 50 per cent sulfuric acid and 50 per cent nitric acid with 40 grams of a solution of 50 grams of sulfuric acid in 50 grams of water, there was obtained a
143
secondary solution which contained 30 per cent of nitric acid, 50 per cent of sulfuric acid, and 20 per cent of water. In most cases the solutions were prepared to contain each component in exact multiples of 10 per cent in order to facilitate interpolation from the concentration-viscosity curves. All solutions were analyzed before use. Mixtures which contained only a single acid were simply titrated with a standard solution of sodium hydroxide. I n primary mixtures containing all three components the total acidity was determined by the titration and the content of sulfuric acid was determined by precipitation as barium sulfate. With some of the secondary solutions the composition, as calculated from the amounts and compositions of the primary solutions taken; was simply checked by titration of the total acidity. In many of the secondary mixtures, however, the content of sulfuric acid was also determined independently. VISCOSITYMEASUREMENTS-The viscometric measurements were made in the usual way, except that due precautions were taken to prevent the loss of nitric acid by evaporation and to avoid the absorption of moisture by the concentrated acids. With each mixture a t each temperature, several individual determinations were made and the viscosity was calculated from the weighed average of these results. The measurements were made a t different temperatures, 0 ", 25", 50", and 75" C., in order to obtain information as to the effect of temperature upon the viscosity relationships. Because of the vaporization and the slight decomposition of the nitric acid at 75" C. the results obtained a t this temperature are less accurate than those obtained a t the lower temperatures. Binary System: Sulfuric Acid-Water
Many investigators, using many different methods, have studied the relationships between the composition and the chemical and physical properties in this system. I n almost every case the results have indicated the existence of a
II
-To-sucnom
L Figure 1-Distillation
Apparatus
monohydrate, HzS04.Hz0, containing 84.48 per cent by weight of sulfuric acid. This hydrate appears to be stable in the liquid phase at ordinary temperatures. The existence of this hydrate is indicated by measurements of the electrical conductivity of aqueous solutions of sulfuric acid. K ~ h l r a u s c hKohlrausch ,~ and GrotrianJ6and Crompton6 have shown that the curve representing the change in electrical conductivity with change in concentration has a pronounced minimum a t this point, while Knietsch' found that the rate of change of conductivity shows a maximum a t the composition of this monohydrate. The solubility of sulfur dioxide in aqueous sulfuric acid is a t a minimum when the concentration of the acid is 85.8 per cent.* With hydrogen chloride, however, the solubility is at a minimum when the concentration of the sulfuric acid is 89.3 per ~ e n t . ~ That J ~ this point lies somewhat above the
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composition of the monohydrate is due, presumably, to the of the existence of this hydrate is inconclusive, since none partial dehydration of the hydrate by the hydrogen chloride, of the other methods of investigation reveal its presence. so that the maximum concentration of the monohydrate is According to Coppadoro,lo the failure to find any indication attained only when the solution contains a slight excess of of the presence of this hydrate in liquid sulfuric acid is due to sulfuric acid. the fact that it is stable only in the solid state and decomposes Very complete data as to the densities and the coefficients into water and monohydrate when melted. Pickering, who of expansion of solutions of sulfuric acid are a ~ a i l a b l e . ~ ~did p ~ the ~ - ~most ~ important work in this field, was unable to obThe maximum density is attained when the solution contains tain any crystals from solutions which contained more than about 97.5 per cent of sulfuric acid. This does not corre- 69 per cent and less than 76 per cent of sulfuric acid. When spond to the formula of any probable hydrate. From their such solutions were cooled the viscosity increased continuously results Domke and Beinl' developed formulas by which the until finally a rigid glass was obtained. He found that even densities at 0" and 60" C. may be calculated from the densities the monohydrate and the tetrahydrate could be greatly supermeasured at 15" C. They found that the deviations between cooled, although crystallization did finally occur. Inasmuch the actual densities and the densities as calculated from their as the composition of the dihydrate falls within the region in formulas showed maxima a t 22 and 85 per cent sulfuric acid which Pickering was unable to obtain crystallization, his a t 0" C. and a t 28 and 85 per cent sulfuric acid a t 60" C. work neither proved not disproved the existence of such a hydrate. Biron later obtained definite evidence of the formation of such a hydrate in the solid state. Giran repeated the work of Pickering and extended it through the region in which the original investigator was not able to obtain crystallization. He proved the existence of the dihydrate. Tammann25 maintains the existence of these three hydratesthe monohydrate, the dihydrate, and the tetrahydrate. There has been considerable discussion as to the existence of pure anhydrous sulfuric a t ordinary temperatures. Domke and Bein," from their work on the densities and other properties, conclude that the so-called LLanhydrous"acid contains some monohydrates and some free sulfur trioxide or pyrosulfuric acid. Veley and Manley15 reach the same conclusion from their studies of refractive indices. S a c k u P holds the same opinion. Mendelyeev27 thought that the fact that anhydrous sulfuric acid fumes in the air indicates the presence of some pyrosulfuric acid. On the other hand, LidburyZs Figure 2 found that the rate of crystallization is greatest in 100 per cent sulfuric acid and from this concluded that the dissociaPickering'3 calculated that the curve representing the second tion into monohydrate must be very slight, if, in fact, any such differential of the density in respect to the concentration dissociation takes place a t all. shows a pronounced break a t about 85 per cent acid. The coefficient of expansion has a maximum value a t 84.5 per cent acid. MendelyeevI4 found that the curve obtained by plotting the rate of change of density with concentration against the concentration shows an abrupt break a t the composition of the monohydrate. Veley and Manley16 measured the refractive indices of aqueous sulfuric acid for light of four different wave lengthsHa, D, Hg, and Hy. I n each instance the refractive index attained a maximum value a t the composition corresponding to that of the monohydrate. The specific heats of solutions of sulfuric acid were determined by BiPon,lG who found that the differences between the actual values and the values as calculated on the assumption that the relation between specific heat and concentration is a linear one showed two minima-a very distinct one a t the composition of the monohydrate and a very indistinct one a t the composition of a hypothetical decahydrate. From measurements of the effect of the addition of sulfuric acid upon the freezing points of aqueous acetic acid, Jones" concludes that sulfuric acid forms two hydrates which are stable a t ordinary temperatures, a Figure 3 monohydrate and a dihydrate. The most positive evidence as to the existence of hydrates The surface tensions of mixtures of sulfuric acid and water is afforded by the results obtained in the determination of the freezing points. Such determinations have been made have been determined by Rontgen and Schneider29 and by by Jacquelain,'* Marignac,lg Pfaudler and Schnegglz0Thilo,21 Morgan and Davis.30 These measurements do not give any Pickering,22 Pictet,23 Knietsch,' Giran,24 and others. All indication of the existence of definite hydrates in the liquid these investigators have found that the form of the freezing- state. Aston and Ramsay31 have calculated the molecular point curve proves the existence of a definite monohydrate complexity of anhydrous sulfuric acid, basing their calculawhich is stable a t the melting point. The existence of a tions upon the change of s'urface tension with temperature. tetrahydrate, at least in the solid form, is also generally ad- They found that the molecular complexity is 32 at ordinary mitted, although Knietsch holds that the evidence in favor temperatures, decreasing to 19 a t 184"C. and to 2.8 a t 280" C.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
I n general, then, the experimental evidence indicates that there are three definite hydrates of sulfuric acid which are stable at the melting point-a tetrahydrate, a dihydrate, and a monohydrate. There is no conclusive evidence that the tetrahydrate or the dihydrate exists in liquid sulfuric acid a t ordinary temperatures-and, in fact, the form of the melting-point curve indicates that these two hydrates are probably not stable a t temperatures much above the melting point. The monohydrate is stable, even a t or above room temperatures, and is present in considerable quantities in ordinary liquid sulfuric acid. I n solutions which contain less than enough free water to combine with all the acid to form the monohydrate, some of the excess acid may be decomposed to form monohydrate and pyrosulfuric acid. The anhydrous or nearly anhydrous acid is probably polymerized to a considerable extent. The viscosity relationships in the system water-sulfuric acid have previously been investigated by Graham,32 Knietsch,' Dunstan and W i l ~ o n , ~Bingham ~ * 3 ~ and Stone,' and Rhodes and B a r b ~ u r . ~ 'The results of Graham and Knietsch are given in terms of "outflow" only and not in terms of absolute viscosities. Bingham and Stone made accurate measurements at three temperatures-IO", 20", and 40" C.-but they examined so few of the individual mixtures that the true form of the viscosity-concentration curve is not disclosed. The determinations of Rhodes and Barbour were made with a rather crude type of viscometer and were not extremely accurate, although they do agree very well with those obtained with the more elaborate apparatus used in this present investigation. The experimental results are shown graphically in Figures 2 and 3. At 0" C. the addition of sulfuric acid to water $creases the viscosity, a t first slowly and then more rapidly. The viscosity rises to a maximum a t about 85 per cent sulfuric acid-the composition of the monohydrate-drops to a pronounced minimum at about 95 per cent acid, and rises finally to a second maximum at the anhydrous sulfuric acid. The curve a t 25" C. has the same general form, although both the maximum and the minimum are less pronounced. At 50" C. the maximum is still less pronounced and does not fall exactly a t the point corresponding to the composition of the monohydrate, the mixture with the greatest viscosity contains slightly more than 85 per cent of sulfuric acid. This observation was checked several times in several independent experiments, using different solutions. At 75" C. there is no definite maximum except that a t the composition of the anhydrous acid, and no minimum. The curve does, however, show a "flat" between 90 and 95 per cent sulfuric acid. It is interesting to note that in the determinations made a t 0" C. some of the individual measurements were made with liquids which were a t temperatures below their true freezing points. For example, the pure monohydrate melts a t 8.53" C., so that the viscosity given for the 85 per cent mixture is really that of a supercooled liquid. In the interpretation of these results certain problems are presented. It appears reasonable to assume that the changes in the viscosity with composition are brought about by changes in either the size or the shape of the molecular species present. Any irregularities in the curve of viscosities must be due to changes in molecular structure, such as polymerization or depolymerization of the acid or the water, ionization, or interaction between the acid and the water to form hydrates. Since the maximum in the viscosity curve a t 0" C. is a t the point corresponding to the composition of the monohydrate, and since there is no reason to assume and no evidence to prove that either ionization or polymerization is greatest a t this composition, the most obvious hypothesis is that the maximum in the viscosity curve is due to the fact that in a mixture of this composition the sulfuric acid is present largely
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or partly as the liquid monohydrate. This hypothesis is supported by almost all of the available experimental data. I n almost every case the curve showing the variation of a physical property with variation in composition shows a singular point at this composition. It is known that a solid monohydrate does exist, and the sharpness of the maximum in the melting-point curve a t this concentration indicates a high degree of stability for this compound. The second maximum a t the composition of the anhydrous acid is evidently due to the presence of the anhydrous acid itself. It is to be expected that this material would have a high viscosity, since it is known to be highly polymerized. The very sharp minimum between the composition of the monohydrate and that of the anhydrous sulfuric acid may be due in part to the effect of the addition of small amounts of water (or monohydrate) in depolymerizing the anhydrous material. There is no independent evidence to support this assumption, however. The results a t 25" C. are analogous t o those a t 0" C. That the maximum is less pronounced may be due to simply a high rate of change of viscosity of the monohydrate itself, or it may be caused by a partial decomposition of the monohydrate into water and anhydrous acid at the higher temperature. The pronounced decrease in the viscosity of the sulfuric acid a t the slightly higher temperature may be due in part to the lower degree of polymerization. This would also explain the less marked character of the minimum in the viscosity curve. At 50" C. the maximum is not only less pronounced; it is shifted so that it comes at a point somewhat richer in sulfuric acid than that corresponding to the monohydrate. This may be explained by the hypothesis that at this temperature an a p preciable amount of the monohydrate is dissociated into water and free acid and that these two products affect the viscosity to different extents, the water having the greatest effect. In a mixture containing equimolecular amounts of water and anhydrous acid the viscosity will be lower than that of the pure monohydrate, because of the effects of these two decomposition products. Increasing the amount of acid in the mixture will decrease the amount of free water, so that the viscosity will rise, although the actual percentage of the monohydrates is slightly less. With still further increase in the amount of acid the effect of the free acid itself in lowering the viscosity more than counterbalances its effect in reducing the amount of free water, so that the viscosity begins to drop. The result is that the maximum viscosity is attained with a mixture containing a slight excess of the unhydrated material. The same hypothesis will explain the form of the curve obtained at 75" C. I n the foregoing discussion the conclusions have been based on the form of the viscosity curve and not on the form of the fluidity curve. According to some the interpretation of viscosity measurements in their relation to chemiical composition should be based upon the fluidity relationships and not directly upon the viscosities themselves. The existence of a monohydrate of sulfuric acid should be indicated by a maximum in the deviation between the curve representing the variation of fluidity with volume concentration and a straight line joining the fluidities of the two pure components. If the fluidities, as calculated from the measurements made by the writers, are plotted against either the volume concentrations or the weight concentrations of the components, curves are obtained which lie below the straight lines joining the fluidities of the two pure components and which are strongly convex downward. These curves do not, however, appear to have any particular significance. The point of maximum divergence from the straight lines varies with the temperature a t which the determination is made, but in no case does this point of maximum deviation correspond to the composition of any hydrate of which the existence is
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indicated by other evidence. Even a t 0" C. the point of maximum deviation does not lie anywhere near the composition of the monohydrate, although this hydrate is known to be stable in solid form a t this temperature and its existence in liquid form is indicated by many phenomena. It appears that, in this instance a t least, conclusions drawn from the fluidity-volume concentration curve do not have any particular significance. Binary System: Nitric Acid-Water
The freezing-point curve for mixtures of nitric acid and water has been determined by PickeringZ2and by Kiister and Kremann.37 These investigators agree that the curve shows two maxima, a sharp one a t the composition of the monohydrate and a less pronounced one a t that of the trihydrate. According to Kiister and Kremann, the difference in the sharpness of the two maxima indicates that the trihydrate is less stable than the monohydrate. It appears that these two hydrates, and only these two, may exist in the solid form. On the other hand, the curves shown by plotting the variations of the physical properties of liquid solutions of nitric acid against the composition often show singular points a t about the composition of a hypothetical dihydrate-"03.2HzO. In a great many instances, however, the position of this singular point varies slightly with the temperature or the pressure. KIemenc and NageP found that the only hydrate of which the existence is indicated by measurements of
3
8s k~ IO
ZO 30 40
io
60 70
do
"NO:
Figure 4
vapor pressures is this dihydrate. B~usfield,~Q on the basis of his determinations of densities, postulates that the dihydrate is the only hydrate that can exist a t ordinary temperatures. The maximum contraction on mixing nitric acid and water occurs at the composition of the trihydrate, although there is also a break in the contraction-concentration curve a t the point corresponding to the dihydrate.40,41 The minimum in the vapor-pressure curve lies between 63.5 per cent nitric acid (composition of dihydrate) and 70 per cent nitric acid, and shifts toward the higher concentrations as the temperature rises.42 Erdmann48 claimed to have isolated orthonitric acid of the formula N(OH)5 or HN03.2Hz0. He based his assertion upon the fact that when aqueous nitric acid is evaporated in a current of air at -15' C. the residue approaches the composition of this hydrate. Kiister and Kremann4' vigorously disputed the existence of this hydrate as a definite chemical compound and maintained that the result obtained by Erdmann was due simply to the fact that a mixture of this composition happens to have a minimum vapor pressure a t this particular temperature. They showed that by varying
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the temperature the composition of the mixture having the minimum vapor pressure also varies. They found that the curve for the coefficient of expansion of aqueous nitric acid shows sharp minima a t the composition of the monohydrate and the trihydrate, and that the positions of these minima are not changed by changing the temperature. From measurements of the heats of dilution B e r t h e l ~postulated t~~ the existence of several hydrates of nitric acid in solution, the dihydrate being the most definitely indicated. From measurements of the absorption spectra Schaefer46. 47 concludes that nitric acid in aqueous solution may exist in two forms. The form present in dilute solution gives an absorption spectrum similar to that of the alkali nitrates; concentrated nitric acid has an absorption spectrum resembling that of the alkyl nitrates. He maintains that these observed differences are to be explained by differences, not in the degree of ionization or hydration, but in the actual molecular structure of the acid in the various concentrations. These differences in structure may, however, be brought about by differences in hydration. The addition of sulfuric acid to a dilute solution of nitric acid causes the absorption spectrum to change toward that of concentrated nitric acid. When dissolved in 85 per cent sulfuric acid the nitric acid gives an absorption spectrum practically identical with that of the anhydrous substance. Hartley48 also measured the absorp tion spectra of solutions of nitric acid. His results appeared to offer evidence in support of the existence of the dihydrate and various other hydrates. Other investigators have studied the variation in certain of the physical properties of aqueous nitric acid with composition,49~60*61 but the results throw little light upon the identity of any deiinite hydrates that may be present. It is generally admitted that anhydrous nitric acid tends to decompose a t ordinary temperatures with the formation of water and nitrogen pentoxide. The difficulty of preparing absolutely anhydrous nitric acid is well known. Veley and Manley41found that the refractive index rises to a maximum at about 70 per cent nitric acid, then decreases to a minimum a t 98.68 per cent, and again rises as the concentration approaches 100 per cent. They attribute the final rise to the partial decomposition of the nearly anhydrous acid. The electrical conductivity of aqueous nitric acid increases to a maximum a t about 30 per cent acid, then decreases to a minimum at 96.12 per cent and again increases as the concentration approaches that of the pure acid.4116~*53Aston and R a m ~ a y from , ~ ~ the rate of change of surface tension with temperature, have calculated that pure nitric acid is polymerized to some extent, the degree of polymerization varying from 1.86 to 1.68 a t different temperatures. Our knowledge of the composition of nitric acid in aqueous solution may be summarized as follows: 1-Nitric acid forms two definite hydrates, a monohydrate and a trihydrate, which are stable in solid form at low temperatures and which may be present in small amounts at least in aqueous solutions a t ordinary temperatures. 2-Both water and nitric acid are polymerized to some extent. 3-Pure nitric acid is decomposed t o some extent into nitrogen pentoxide and water. 4-Nitric acid in solution is ionized. 5-There is some indication that nitric acid in solution may exist in two isomeric modifications, the relative amounts of which depend upon the concentration of the solution. 6-A number of the physical properties of the solutions show abnormalities a t between 60 and 70 per cent nitric acid, and in many cases the singular point corresponds rather closely t o the composition of a hypothetical dihydrate. Since, however, the position of the singular point usually changes somewhat with the temperature, and since there is no direct experimental evidence of the existence of a dihydrate in solid form, it is probable that the singular properties of solutions of about the composition of the dihydrate are due to the resultant of the effect of the various changes in structure which may take place on dilution.
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The viscosities of solutions of nitric acid in water have been determined by several investigators.44,54.55156.' Of these results the most accurate are probably those of Bingham and Stone,' although those of Graham32agree very RTell when properly corrected. Bingham and Stone did not, however, measure the viscosities of a very large number of individual mixtures, nor did their determinations cover a wide range of temperatures. These experimental results are shown graphically by Figure 4. At 0 " C. the addition of nitric acid to the water at first decreases the viscosity and then causes it to increase until the concentration of the acid reaches about 63 per cent, beyond which there is a drop to the value for the pure acid. The "negative viscosity effect" of the addition of the first small amounts of acid is usually assumed to be due to the action of the acid in depolymerizing the water. At the nitric acid end of the scale the graph also has a pronounced down-
viscosities against the weight concentrations, we find that the points of maximum deviation between the actual fluidity curves and the straight lines drawn between the fluidities of the pure components shift even more markedly than do the maxima in the viscosity curves and agree even less satisfactorily with the composition of the dihydrate. It will be recalled that increase in temperature also causes a shifting in the minimum point in the vapor pressure-concentration curves. The two effects would appear to be analogous and, perhaps, due to the same cause. The interpretation of viscosity measurements in this binary system is rendered particularly difficult by the multiplicity and complexity of the molecular changes which may occur. The existence of a singular point in the viscosity curves at about the composition of the dihydrate is probably due to the resultant of the effects of the various changes in molecular structure which may take place with change in concentration. Binary System: Ntric Acid-Sulfuric Acid
This binary system has been studied by only a few investigators. Holmes57 found that the freezing-point curve for the mixtures of the anhydrous acids shows a maximum at 89 per cent sulfuric acid and a minimum at 95 per cent. The composition of the mixture with the maximum freezing point corresponds with that of a compound of the formula 5HzS04."Os. Carpenter and Lehrman5*point out that the system H2SOd-HN03 is really not a binary system but a ternary one in which the components are SOS, NzOb, and HZO, and that the presence of a maximum in the freezing-point curves for nitric acid and sulfuric acid does not prove the existence of a binary compound between the acids as such, but may indicate the formation of a ternary compound between the two anhydrides and water. Sapozhnikovsgfound that the mixture of maximum density contains 10.8 per cent of nitric acid and therefore corresponds with the mixture of maximum freezing point. Figure 5
ward curvature. This may be due to depolymerizing effect of the water upon the polymerized nitric acid or it may be
caused by the interaction of the water with the small amount of nitrogen pentoxide, which is assumed to be present in very concentrated nitric acid. At the higher temperatures the viscosity-concentration curves have about the same general form. The downward curvature in the region of low concentrations still persists, but is less pronounced so that no actual minimum is present. This effect of increasing temperature in decreasing the negative viscosity effect is obviously to be explained by the fact that the degree of polymerization of pure water is less at higher temperatures, so that any phenomenon due to depolymerization will be less pronounced. On the other hand, the negative viscosity effect at the nitric acid end appears to be relatively somewhat greater as the temperature rises. This would lend support to the theory that this negative viscosity effect is due to the interaction of the water with small amounts of free nitrogen pentoxide rather than to depolymerization of the acid. As the temperature is increased, the position of the maximum in the viscosity curves shifts slightly toward the nitric acid end of the scale. At 0" C. this maximum lies very close to the composition of the theoretical dihydrate, but the mixture of maximum viscosity a t 75' C. contains considerably more than 65 per cent of nitric acid. If we plot the fluidities against the volume concentrations instead of the
Figure 6
Bingham and Stone' determined the viscosities of a few mixtures of the practically anhydrous acids. The number of individual mixtures examined by them was so small, however, that their results do not indicate the true form of the viscosity-concentration curve. The curves as drawn by them show a single maximum in viscosity in the general neighborhood of 80ger cent sulfuric acid, whereas the present writers have found that there are really two maxima, one at about 88 and the other a t 100 per cent sulfuric acid, with a pronounced minimum between them a t about 97 per cent. The results are shown graphically in Figures 5 and 6. The addition of sulfuric acid to the nitric acid increases the vis-
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V/.xoSlT/ES A T 25°C.
Figure 7
Figure 8
cosity slowly a t first and then more rapidly, until a maximum is reached a t about 88 per cent sulfuric acid. Beyond this point the viscosity decreases to a minimum a t about 97 per cent and then again increases until the value for pure sulfuric acid is reached. I n the determination made at 75' C. no actual minimum occurs, although the curve is distinctly downward in the region between 95 and 100 per cent sulfuric acid. These viscosity curves are of the same form as the freezingpoint curves of Holmes, and the maxima and minima fall a t approximately the same concentrations. The agreement is too good to be due to mere coincidence. We might conceivably assume that the maximum in the viscosity curves for the mixture of the anhydrous acids is due to the same compound that is responsible for the maximum in the sulfuric acid-water curve-i. e., to the monohydrate of sulfuric acid, produced by the partial dehydration of the nitric acid. This hypothesis is hardly tenable in view of the fact that the most viscous of the anhydrous mixtures are even more viscous than the pure monohydrate of sulfuric acid. The evidence obtained by viscometric methods points to the existence of a compound formed by interaction between the anhydrous acids. This may be either the binary compound 5HzS04.HN03 as postulated by Holmes or the ternary compound 10SOa.N206.9H20 of Carpenter and Lehrman. The present writers' results show that this compound is relatively stable in the liquid as well as in the solid condition and is present in relatively large amounts even at temperatures as high as 75" c.
free water contains appreciable quantities of nitrogen pentoxide along with the true nitric acid. It appears that in the presence of unhydrated sulfuric acid the nitric acid itself is partially decomposed. The same investigator found that, of ternary mixtures of the same molecular concentration of nitric acid, those which contain water and sulfuric acid in equimolecular proportions are able to nitrate cellulose to the greatest extent. The addition of sulfuric acid in excess of the amount required to combine with all of the water to form monohydrate decreases the nitrating power to some extent; the addition of excess water decreases the nitrating power very rapidly. An excess of either water or sulfuric acid decreases both the fugacity and the nitrating power of the nitric acid, but the effect upon nitrating power is much more pronounced than the effect on vapor tension. Somewhat similar results were obtained in the study of the nitration of naphthalene.61 On the other hand, Kostevitch62reports that to get nitration of benzene to dinitrobenzene, or to get nitration of mononitrotoluene, it is necessary to have sulfuric acid present in concentrations considerably in excess of that required to combine with the free water. It is possible that in these nitrations the active agent is nitrogen pentoxide or some compound of nitrogen pentoxide with sulfur trioxide and water. gives data as to the densities of mixed acids. Pascal and Berl and Samtleben,65 and Carpenter and B a b o P have investigated the boiling points and the vapor pressures of binary and ternary mixtures, and have studied the progress of the concentration of nitric acid by distillation from sulfuric acid. McDavid6' measured the amounts of heat evolved when various mixtures of sulfuric acid and nitric acid are diluted with water. None of these investigations afford much information as to the identity of the compounds present in the ternary system. The freezing-point relationships in the ternary system were determined by Carpenter and L e h r m a ~ ~who , ~ ~found no evidence of the existence of any ternary compound between the acids as such and water. The freezing-point data do show, however, that a compound of the formula 1oso3.Nz05.9Hz0exists and is stable a t its melting point. The maximum in the freezing-point curve for mixtures of the anhydrous acids which was previously observed by Holmes is attributed to the presence of this compound. Bingham and Stone1 measured the viscosities of various ternary mixtures and from their results plotted graphs showing the changes in viscosity (or fluidity) with changes in composition. The number of mixtures examined by them was so small that their results do not disclose all the interesb ing relationships in this system.
Ternary System: Sulfuric Acid-Nitric Acid-Water
Much of our present knowledge as to the relationships between composition and chemical and physical properties in the ternary system sulfuric acid, nitric acid, and water is derived from the work of Sapozhniko~.~g,60When sulfuric acid is added to a solution of nitric acid in water, the partial pressure of the nitric acid from the mixture increases to a maximum and then decreases. I n any series of mixtures containiig the same molecular percentage of nitric acid, that one in which the sulfuric acid and the water are present in equimolecular proportions has the highest partial pressure of nitric acid vapor. This shows that in ternary mixtures the sulfuric acid reacts with the water to form a stable monohydrate, and that this monohydrate has less solvent power for nitric acid than has either water or anhydrous sulfuric acid. Sapozhnikov also found that the vapor given off from mixtures relatively poor in sulfuric acid contains nitric acid as such, while the vapor from mixtures containing considerably more than enough sulfuric acid to combine with all the
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AT 75'C.
711
Figure 9
The results of the measurements are shown by Figures 7 , 8 , 9 ,and 10. I n these graphs the curves were drawn through
points of equal viscosity. Since the fluidity is the reciprocal of the viscosity, each curve is the locus, not only of points of equal viscosity, but also of points of equal fluidity. In the diagrams the number adjacent to each curve indicates the viscosity, in centipoises, of the mixtures whose compositions are represented by points on that curve. From the diagram representing the relationships a t 0" C. it appears that the viscosity curves for mixtures containing small and constant amounts of sulfuric acid are of the same general form as the curve for the binary system nitric acidwater. The progressive replacement of water by nitric acid in such mixtures results first in a slight decrease in the viscosity, due to the negative viscosity effect, followed by an increase to a maximum a t about 63 per cent nitric acid and a final decrease to the viscosity of the anhydrous mixtures. With increasing amounts of sulfuric acid the negative viscosity effect of small amounts of nitric acid becomes less and less and finally disappears. On the other hand, the negative viscosity effect of the addition of small amounts of water to mixtures very rich in nitric acid becomes more and more pronounced. I n binary mixtures free from sulfuric acid this effect appears only as a slight downward curvature of the graph near the point representing anhydrous nitric acid; but in mixtures containing 20 per cent of sulfuric acid the addition of small amounts of water depresses the viscosity to a definite minimum. It would appear that, even in the presence of mch relatively small amounts of sulfuric acid, a portion of the nitric acid is decomposed to form either nitrogen pentoxide or a compound between nitrogen pentoxide, sulfur trioxide, and water; that the formation of this compound is responsible for the increase in the viscosities of mixtures of the anhydrous acids with increasing content of sulfuric acid; and that this compound is decomposed by the addition of water. With increasing concentration of sulfuric acid the maximum in the viscosity curve shifts and becomes less pronounced, finally disappearing altogether when the content of sulfuric acid reaches about 50 per cent. The viscosity relationships give no indication of the existence of any ternary compound between water, nitric acid, and sulfuric acid. The results obtained a t 25', 50", and 75" C. differ quantitatively, but not qualitatively, from those at 0' C. The same general features are shown by all the graphs. Conclusions
I n the system sulfuric acid-water the viscosity relationships indicate clearly the existence of a monohydrate of sul-
Figure 10
furic acid, which appears to be relatively stable even a t temperatures as high as 75' C. The viscometric evidence as to the existence of this hydrate is supported by a great deal of evidence from other sources. No evidence was obtained to indicate the existence of either a dihydrate or a tetrahydrate of sulfuric acid in the liquid form, although both of these hydrates are known to exist in solid form at low temperatures. I n the binary system nitric acid-water the viscosity curve shows a maximum at about the composition of the dihydrate. This hydrate is not known to exist in the solid form, and the evidence as to the presence of a dihydrate in liquid mixtures is very contradictory and inconclusive. Since the composition of both the mixture which shows a maximum viscosity and of the mixture of which the fluidity deviates most widely from the mean varies with the temperature at which the measurements are made, the evidence obtained by viscometric measurements does not prove the existence of a dihydrate of nitric acid. Small amounts of nitric acid have a negative viscosity effect upon water, due probably to the depolymerization of the water by the acid. The addition of sulfuric acid diminishes and finally eliminates this effect. Small amounts of water have a very slight negative viscosity effect upon nitric acid. The addition of sulfuric acid increases this effect. In the binary system nitric acid-sulfuric acid there is a pronounced maximum in the viscosity curve at the point at which the ratio of so3 to NzOs is 10:l. This may be due to formation of the compound 5HzSOb.HNOs postulated by Holmes, or to the compound 10S03.Nz05.9H20 described by Carpenter and Lehrman. The composition of the mixture of maximum viscosity agrees equally well with both formulas. In the ternary system no new compounds appear. Bibliography 1-Bingham and Stone, J . Phys. Chcm., 27, 701 (1923). 2-Bingham, "Fluidity and Plasticity," p. 76 (1922). 3-Bingham and Jackson, Bur. Standards, Sci. Pa9c7 298 (1917). 4-Kohlrausch, Ann. Phys. Chem., 159, 233 (1876); 121 6, 1 (1879); [21 IT, 69 (1883). bKohlrausch and Grotrian, Ibid., 154, 215 (1875). 6-Crompton, J . Chcm. Soc., 53, 116 (1888). 7-Knietsch, Bcr., 34, 4089 (1901). 8-Miles and Fenton, J . Chcm. SOL.,111, 59 (1920). 9-Cupr, RCC.17UV. chim.. 44, 476 (1925). 10-Coppadoro, Guze. chim. iluZ., 39, 11,618 (1909). 11-Domke and Bein, Z . anoig. Chcm., 43, 125 (1905). 12-Kremann and Ehrlich, Monafsh., 26, 831 (1907). 13--Pickering, J . Chem. Soc., 57, 64 (1890). 14--Mendelyeev, Z . phys. Chcm., 1, 273 (1887). 15-Veley and Manleg Proc. Roy. SOL.(London), A76,469 (1905). l+Biron, J . Russ. Phys.-Chem. Soc., [21 31, 517 (1889). 17-Jones, A m . Chcm. J . , 16, l(1894).
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18-Jacquelain, Ann. ckim. phvs., 131 30, 343 (1850). 19-Marignac, Ibid., [31 39, 184 (1853). 20-Pfaudler and Schnegg, Sitz. Akad. Wiss. Wien, 71, 351 (1875). 21-Thilo, Chcm.-Ztg., 16, 1688 (1892). 22-Pickering, J . Ckem. Soc., 67, 331 (1890); 68,436 (1893). 23-Pictet, Comfit. rend., 119,642 (1894). 24-Giran, Bull. SOC. ckim., 13, 1049 (1913). 25-Tammann, 2. anorg. allgem. Ckem., 161, 363 (1927). 26-Sackur, Z . Elektrockcm., 8, 80 (1902). 27-Mendelyeev, Bcr., 17, 2536 (1884). 28-Lidbury, 2. pkys. Ckem., 89, 453 (1902). 29-Rontgen and Schneider, A n n . Phys. Ckem., 29, 165 (1886). 30-Morgan and Davis, J. A m . Ckem. Soc., 38, 555 (1916). 31-Aston and Ramsay, J . Ckem. Soc., 66, 167 (1894). 32--Graham, Phil. Trans., 161,373 (1861). 33-Dunstan and Wilson, J. Ckem. Soc., 91, 85 (1907). 34-Dunstan, Proc. Ckem. Soc., SO, 104 (1914). ENO. CHEM.,16, 850 (1923). 35-Rhodes and Barbour, IND. 36-Bingham, “Fluidity and Plasticity,” p. 175 (1922). 37-Kiister and Kremann, Z . anorg. Chem., 41, 33 (1904). 38-Klemenc and Nagel, Ibid., 166, 257 (1926). 39-Bousfield, J. Chcm. SOL.,107, 11, 1405 (1915). 40-Kolb, A n n . ckim. pkys., [41 10, 136 (1867). 41-Veley and Manley, Proc. Roy. Soc. (London),A69, 86 (1901). 42-Sproessen and Taylor, J. A m . Ckcm. Soc., 43, 1782 (1921). 43-Erdmann, 2.anorg. Ckem., Sa, 431 (1902); 2. angew. Chem., 16, 1001 (1903).
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44-Kiister and Kremann, Z . anorg. Chem., 41, 1 (1904). 45-Berthelot, Comfit. rend., 78, 769 (1874). 46-Schaefer, Z . anorg. allgem. Chem., 97, 285 (1916). 47-Schaefer, Zbid., 98, 77 (1916). 48--Hartley, J. Ckem. Soc., 83,658 (1903). 49-Veleyand Manley, Phil. Mag., [a] 3, 118 (1902). 50-Gladstone and Hibbert, J. Ckem. SOL.,67, 831 (1895). 5l-Gladstone, Ibid.,69,589 (1891). 52-Kohlrausch and Grotrian, Ann. Pkrs. Ckem., 164, 1 (1875). 53-Kohlrausch. Z . pkrs. Ckem., 1,74 (1887). 54-Chevencau, Compt. rend., 165, 154 (1912). 55-Poiseuille, Ann. chim. pkys., 21, 76 (1847). 56-Bousfield, J. Chem. Soc., 107,II,1781 (1915). 57-Holmes, J. IND. ENG.CHEM.,12, 781 (1920). 58-Carpenter and Lehrman, Trans. A m . Insl. Chem. Eng., 17,35 (1925). 59--Sapozhnikov, Z. Phys. Ckem., 49, 697 (1904); 51, 609 (1905). 6O--Sapozhnikov, J . Russ. Pkys.-Ckem. Soc., 32, 375 (1900); 86, 305, 506, 1098 (1903); 86, 518, 669 (1904); 37, 374 (1905); 88, 1192 (1906); Z . Schiess. Spreng., 453 (1906); 8, 201 (1908); 4, 441 (1909). 61-Sapozhnikov, J. Russ. Pkys.-Ckem. Soc., 46, 1102 (1914). 62-Kostevitch, “T. N. B. and T. N. T.”printed by B. C. Millard, London, 1919. 63-Pascal, Compt. rend., 165,589 (1917). 64-Pascal and Garnier, A n n . chim., 16, 253 (1921). 65-Berl and Samtleben, Z . angew. Ckem., 35, 201 (1922). 6 6 C a r p e n t e r and Babor, Trans. A m . Ins!. Ckem. Eng., 16, 111 (1924). 67-McDavid, J. Soc. Ckem. I n d . , 41,246 (1922).
Sodium Aluminate as a Coagulant in Chemical Treatment of Cannery Waste Waters’ J. A. Holmes and G. J . Fink NATIONAL ALUMISATE CORPORATION, 6216 WEST 66TH PLACE, CHICAGO, ILL.
T ONE of the tomato
Improved results have been obtained at two tomato from this well the wastes are catsup and chili sauce canneries by use of small doses of sodium aluminate pumped by centrifugal pumps plants of the H. J. as a coagulant along with lime. The waste-water treatto the treating plant. Two Heinz Company, s i t u a t e d ing Plant at one of these canneries is described in detail. pumps are available so that j u s t o u t s i d e t h e c i t y of The average chemical treatment at this plant during any increase in volume of Princeton, Ind., it is neces1927 was 2.5 pounds of lime and 0.4 pound of sodium waste due to plant operation sary to discharge a large volaluminate per 1000 gallons of waste water. or rain can be handled. ume of tomato waste sewage into an open ditch, which passes into the storm sewers of Waste-Water Treatment Princeton and then on into another open ditch which passes The waste-water treatment is a continuous flow process. through a portion of the city and then through farms on the other side of the city. Without treatment or after passage The discharge from the Pump passes into a 6 by 10 foot, over coke filters as originally practiced, the organic matter 30 by 40 mesh (0.Ol.l-inch opening) North screen, where the of the sewage rapidly putrefied, producing an offensive odor coarser Particles are removed and discharged into a dump to which the citizens of the town objected and the stock on wagon. From the Screen the waste water Passes into a mechanically agitated mixing tank, where the lime and the farms refused to drink the water. The tomato skins are removed by peeling for chili Sauce sodium aluminate are added. The water then passes into and by scalding for catsup. The peeling table and scalding a 6 by 7’/z foot basin with eighteen ar0und-the-d bdfks. residues are passed through cyclones from which the solid From here it Passes into a series of Six hoppered concrete material is carried by two conveyors to a hoppered storage settling basins, each approximately 10 by 11 by 7.5 feet. tank and the liquid discharged into the plant sewer. The From the hoppered basins the water passes into another series solid material is hauled away in wagons. The waste which of three settling basins, the first two being rather small and requires treatment is made up of discharges from the cyclones the third, 20 by 30 feet. This large basin is baffled to Prevent and water used for genera] cleansing of floors and apparatus short-circuiting. The water then passes into a final settling and for washing and sterilizing bottles. The cannery has a basin approximately 25 by 60 feet, which was formerly the maximum daily capacity of 15,000 bushels of tomatoes and drying bed. All these basins are equipped with sludging approximately 100 gallons of water are required for each lines. The effluent is discharged directly from the last basin bushel of tomatoes handled. During normal operation into the open ditch which runs beside the plant. The sludge (16 hours), however, the volume of waste is approximately from the settling basins is Pumped by means of a centrifugal 500,000 gallons per day. The storm sewers also discharge Pump into a nearby field, where it is allowed to dry- Forinto the waste sewers and during rains the total volume is merly the s l u d g h s were run into a large basin with a sand and greatly increased. The plant sewers empty into a concrete gravel bottom and after drying the material was removed and basin or well of approximately 4000 gallons capacity and hauled to the field. This method worked very well except during rainy weather. The method now used of pumping 1 Presented before the Division of Water, Sewage, and Sanitation Chemistry a t the 76th Meeting of the American Chemical Society, Swampthe to the this as as the necessity for further handling. The dried sludge is scott, Mass., September 10 to 14, 1928.
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