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(5) Chatillon, G. E. (to John Chatillon & Sons), U. S. P a t e n t 1,826,732 (Oct. 13, 1931). (6) Clarvoe, G. W., Proc. Am. SOC.Testing Materials, 32, 11, 689-90 (1932). (7) Couette, M., Ann. chim. phys., [6] 21, 433-505 (1890). (8) Emrnons, W. J., and Anderton, B. A,, Proc. Am. SOC.Testing Materials, 25, 11, 346-55 (1925). (9) Green, H., and Haslam, G. S., IND. ENQ.CHEM.,19, 53-7 (1927). (10) Horsfield. H. T.. J. SOC.Chem. Ind.. 53. 107-15T 11934). (llj Hubbard, Prevost, and Field, F. b.,’Proc. Am, SOC.Testing Materials, 25, 11, 335-45 (1925). (12) Karrer, E., J. Rheology, 1, 290-7 (1930). Materials, 34, 11, (13) M ~ B J. ~ w,, ~proc,~ Am,~ sot. ~ Testing , 591-603 (1934).
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(14) Manning, A. B., Dept. Sci. Ind. Research, London, Fuel Research Paper 39 (1933). (15) Milburn, H. M., Proc. Ana. SOC.Testing Materials, 25, 11,356-64 11925). (16) Mooney, M., and Ewarts, R. H., Physics, 5, 350-4 (1934). (17) Peek, R . L., Jr., J. Rheology, 3, 345-72 (1932). (18) Pittman, C. U., and Traxler, R. N., Physics, 5, 221-4 (1934). (19) Pochettino, A., Nuovo cimento, 8 , 77-92 (1914). (20) Revnolds, Osborne, Phil. Mag., 151 20, 469 (1885). (21) Traxler, R. N., Pittman, C. U., and Burns, E’. B., Physics, 6, 58-60 (1935). RECEIVED November 29, 1936. Revision of a paper presented at the 7th Annual Meeting of the Society of Rheology, New York, N. Y., October 12, 1935.
Preparation and Storage of Standard Chromous Sulfate Solutions HOSMER W. STONE AND CARROL BEESON, University of California at Los Angeles, Calif.
T
HE preparation and storage of standard chromous reducing solutions are of special importance, for this reagent is the most powerful of any of the well-known reducing agents which can be used as an aqueous solution. The change from Cr++ to Cr+++ has a molal oxidationreduction potential of -0.41 volt ( 5 ) . Its reaction with the oxygen of the air is extremely rapid. Hence, like titanous solutions, the chromous reagent must be protected from the atmosphere a t all times. The lack of an entirely satisfactory method of preparation and the difficulty involved in excluding the air from the reagent have greatly curtailed its use. The present work describes both a simple efficient method of
li
I i/
STOD COCK
i STOP COCU
GLASS WOOL 2 x 3 0 CM UPFL ow AEDUC 70Q
3
20 Mf;m
AMAL GAMA TED
/NSULFUDIC AC/D AND
.o/ N CHDOMOUS s +?/NZQCDCLASS
-AM42 GAMA TFD
FIGURE 1. DIAGRAM OF APPARATUS
preparing the chromous sulfate reagent and an improved method of protecting it from the atmosphere. Thornton and Sadusk (9) report preparation of the chromous reagent-by a reduction of an acid solution of potassium dichromate with zinc amalgam in a Jones reductor. A 0.067 N solution of chromous sulfate was obtained by them which was not over 0.18 N in sulfuric acid, representing a 67 per cent yield. The rate of reduction was 40 ml. of solution per minute. Carbon dioxide was used to exclude the air during the preparation. Apparently, the reagent was stored under hydrogen from a Kipp generator (IO). When tthis method was used by the authors, the concentrations of the resulting chromous solutions varied so greatly that it was necessary to start with two or three times the theoretical requirement of dichromate to insure a given concentration of the reagent. During certain experiments, the rate of flow of the solution through the Jones reductor was reduced to a few millilitersper minutebythe buoyant effect of the hydrogen resulting from the action of the acid on the zinc. This effect was dependent on the concentration of the acid and the extent to which the zinc had been amalgamated. A more complete amalgamation of the zinc eliminated the hydrogen but reduced the yield. A considerable concentration of acid was required to enable the zinc to reduce the potassium dichromate satisfactorily. Because of the reaction of the acid with the zinc, there was always an uncertainty as to the acidity of the resulting solution. Solutions of low acidity could not be prepared by this method. The quantity of nonessential ions introduced into the solutions was large. The use of violet chrome alum solutions, in d a c e of the strongly acid Dotassium dichromate solutions in the -;reparation of the chromous reagent, has been found to eliminate the acid difficulties and to reduce the unavoidable ions in the final solutions by a t least 40 per cent. In addition to this, yields of from 90 to 100 per cent of the chromium in the chromous form were obtained a t from two to five times the rate of reduction used by Thornton and Sadusk (9). A list of references on the subject of the preparation and use of chromous solutions in by Brennecke (2). analysis is given -
Influence of Complex Chromic Salts The fact that chromic salts exist in solution in both violet and green modifications is of im-
MAY 1.5, 1936
ANALYTICAL EDITION
portance in this work. At room temperatures these forms are a t equilibrium and when the equilibrium is disturbed it is adjusted rather slowly. The authors found that the violet solutions were more rapidly reduced than the green ones. Three factors which were observed to favor the rapid reduction are also known to promote the formation of the violet complex in the solution. 44chrome alum solution that has been freshly prepared, that is made up in dilute sulfuric acid solution, and that is held a t room temperature or below, is violet in color and rapidly reduced by the zinc amalgam in a Jones reductor. If the solution is allowed t o stand, if acid is not added, or if the solution is heated, there is a shift to the green color and the rate of reduction is less. The effect of the acid on the color has been shown by Orlova and Petin (8). The fact that higher temperatures and aging of the solution tend to favor the green form is proved by the work of Montemartini and Vernazza ( 7 ) . The violet color in chrome alum solutions is thought to be due to complex groups in which six water molecules are coordinated with chromium as in [Cr(Hz0)6]+++,while the change to the green color involves the replacement of two or more of the water molecules in the complex with sulfate ion. [Cr(Hz0)4S04]+represents one of several possible complex ions of the green type. This situation is complicated by the fact that these groups are hydrolyzed in the solution and by the fact that they exist as condensed aggregates. Graham (4) gives a comprehensive bibliography of this sGbject.
Experimental Work The amalgamated zinc mas prepared by stirring 240 grams of 20-mesh c. P. zinc in 100 ml. of 3 N hydrochloric acid for 30 seconds. Then 100 ml. of 0.013 M mercuric chloride solution (5 ml. of a saturated solution at 25' C. diluted to 100 ml.) were added t o the zinc acid mixture. Soon after the addition of the mercuric chloride the evolution of hydrogen practically ceased. The stirring was continued for 3 minutes longer, after which the amalgamated zinc was washed thoroughly with distilled water by decantation. The reductor shown in Figure 1 was then filled with the amalgamated zinc. In this device, the customary Jones reductor was modified so that air pressure was used in place of hydrostatic pressure to force the solution through the reductor. Moreover, the usual direction of flow through the reductor was reversed, causing the liquid t o flow up through the amalgamated zinc. The use of air pressure permitted the investigation of much higher rates of flow. The reversal of the direction of flow took advantage of the buoyancy of any hydro en liberated in the reductor. The hydrogen aided the flow of the kquid instead of hindering it, as is the case in the usual form of reductor. The drawing (Figure 1) of the arrangement used in preparing the chromous reagent, and in protecting it from the air during the determination of its concentration, requires little explanation. Before starting the preparation of the chromom reagent, the air was flushed from the system by a stream of carbon dioxide from the cylinder. The gases escaped through the outlet above the buret. While the stream of gas was maintained, the stopcock above the reductor was opened and the chrome alum solution was foiced through the amalgamated zinc. When sufficient chromous reagent had been prepared, the line from the reductor was cut off, and the chromous solution was thoroughly agitated by a stream of carbon dioxide bubbles. After about 15 minutes of such agitation the outlet above the buret was closed and the reagent was ready for titration. The air was effectively excluded from the stored reagent by means of a small pressure of carbon dioxide. The use of the pressure reducing valve and gage made it possible to reduce the high pressure from the liquid carbon dioxide to the fairly constant low pressure required to exclude the oxygen of the air from the reagent. This low pressure was comparable t o that obtained from a Kipp generator. The pressure could be maintained without the escape of gas and without the building up of pressure in the system. This method was far more satisfactory than the use of a Kipp generator, such as has been recommended by Zintl and Rienacker (11) and
189
Thornton and Wood (10). Moreover, the valve and gage, which cost about $7, were no more expensive than the Kipp generator which they displaced. The cylinder of carbon dioxide involves no extra expense, since in using such a reducing agent a cylinder of inert gas is usually required for flushing out the titration beaker. The usual arrangements for anaerobic work have been modified to meet the requirements of repeated preparation and testing of the concentration of the reagent. Readers desiring details of methods for the storage and use of such reagents are referred to the authors mentioned above (10, I I ) , and particularly to Crowell and Baumbach (5). It would seem that wherever standard solutions are to be protected from the oxygen of the air by the pressure of an inert gas, the use of the pressure reducing valve and cylinder of gas would be desirable. Although liquid carbon dioxide is usually free from oxygen or other impurities which might oxidize the chromous ion, the precaution was taken of purifying the gas used. As shown in Figure 1, the carbon dioxide entered the wash solution through a Jena sintered-glass distribution tube. This insured a thorough mixing of the gas with the chromous sulfate-sulfuric acid solution which is capable of absorbing oxygen very rapidly. The amalgamated zinc, in the bottom of the bottle, served to regenerate any chromous ion which may have been oxidized. Because it has been reported that hydrogen may be liberated from acid solutions by the chromous ions, the authors considered this possibility carefully and found no evidence of such a reaction under the experimental conditions of this work. There can be no doubt as to the stability of chronious sulfate-sulfuric acid solutions with respect to this reaction, in view of the fact that Thornton and Sadusk (9) and Crowell and Baumbach (3) reported no appreciable change in the titer of such solutions over a period of a month. Asmanoff (1) found that chromous sulfate did not liberate hydrogen even from 10 N sulfuric acid, but that it did react slowly with 4 N hydrochloric acid, and that ammonium salts and platinum catalyzed the liberation of hydrogen from sulfuric acid by chromous sulfate. That no change of titer was observed in this work with the solutions containing hydrochloric acid may be explained by the fact that the concentration of the hydrochloric acid never exceeded 0.1 N and that the period of storage was never more than a few hours a t the most. The titer of the chromous reagent was determined by siphoning the solution, under carbon dioxide pressure, into the buret from which it was delivered below the surface of an excess of standard potassium triiodide solution. After dilution and acidification, the excess of triiodide was determined by titration with sodium thiosulfate solution. Each yield in the tables represents an average of two or more closely agreeing duplicate determinations on each sample prepared. Any possible effect of oxygen dissolved in the potassium triiodide solution was neglected. The percentage yield was based on the assumption that the c. P. chrome alum was indeed pure. The authors did not attempt to determine why 100 per cent yields of the chromous reagent were not regularly obtained when the recommended conditions for the preparation were used. Since Lundell and Knowles (6) have already shown that chromium solutions can be quantitatively reduced by zinc amalgam in a Jones reductor, it seems sufficient to state that the values reported are strictly comparable with each other.
Experimental Results The experiments of Table I indicate that with an efficient zinc amalgam one may force the solution through the reductor a t any convenient rate and obtain satisfactory yields of the chromous reagent. The high rate of 200 ml. per minute,
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much less. The explanation lies in the fact that the violet chromium complex gradually changed to the more slowly reduced green complex. The shift from the violet to the green form was retarded by the presence of acid, hence the age of the solution was only a small factor where acid had been added. A solution made from green chromic sulfate crystals was 89 per cent reduced a t a rate of 20 ml. per minute and only 44 per cent reduced a t a rate of 120 ml. per minute. Similar BETWEEK RATEOF FLOW THROUGH REDUCTABLE I. RELATIOX reductions in yield, in cases where the chrome alum solution TOR AND YIELDOF CR++ had been changed to green by heating, have shown that the (Concentration of violet Cr+++,0.liM) rate of reduction is a factor to be considered where the green Age of Rate of Yield Expt. Cr i- f Reducof form is being reduced. Acid Solution tion Crt+ NO. Certain preparations of zinc amalgam, which a t first were Hours Ml./?nin. % entirely satisfactory, suffered a loss of efficiency that could 0.01 N HCl 0.5 40 100 21B 0.5 40 99 0 . 0 1 N HC1 22B not be explained. An example of such an amalgam is found 0.01 N HC1 0.5 150 93 248 25B 0.01 N HC1 0.5 150 91 in one preparation which, after having previously been satisfactory, so decreased in efficiency that a chromous yield of 0.1NHzSOi 2 50 97 37s 120 96 0 . 1 N HzSOI 19 405 only 64 per cent was obtained. By replacing this amalgam 2.26 135 98 0.1NHzSOi 38s 39s 0 . 1 N HzSOa 0.5 200 97 with a fresh preparation and keeping all other factors constant, the yield was returned to nearly 100 per cent. This difficulty of low efficiency on the part of the zinc amalgam was met O F V A 4 R I A T I O S s I N ACID AND CR+++CONTABLB 11. EFFECT only where the amalgamated zinc had been used and stored CENTRATIONS ox YIELDOF CR++ under water for a period of not less than several days. Concn. of Age of Rate of Yield
used in experiment 39S, is about as rapid as it is practicable to force a liquid through such an apparatus. The apparent decrease in yields with the increase in rate, brought out by comparing experiments 21B through 25B, has not been observed in other cases. It might be explained by a decrease in the efficiency of the amalgam, a point discussed later.
Expt. NO.
135 415 39s 22B 28s
TABLE 111.
Acid None None
0.1NHzS04 0 . 0 1 N HC1 0 . 1 N HzSOa
Violet Cr+++ C r + + + Solution M Hours 0.01 0.01 0.1 0.1 0.4
0.5 0.5 0.5 0.5 0.75
NO.
28s 29s 385 40s
41s 43s 44s
,Of++
50 140 200 80 17
90 92 97 99 96
%
E F F E C T O F B C E O F SOLUTION, WITH AND WITHOUT ACID, ON YIELDOF CR +
Expt.
Reduction Ml./min.
Acid 0 . 1 N HzSOI 0 . 1 N H~SOI 0.1NHzSOr 0 . 1 N HsSOI
None None None
*
Concn. of Age of Rate of Yield Vioiet C r + + + Reducof C r + + + Solution tion Cr++ M Hours Ml./min. % 0.4 0.4 0.1 0.1 0.01 0.01 0.01
0.75 27 2.25 19 0.5 4 4.5
17 17 135 120 140 100 150
96
Conclusions Certain difficulties arising in the use of the Thornton and Sadusk method of preparing standard chromous sulfate solutions have been pointed out. A method of eliminating these difficulties by the use of violet chrome alum solutions in place of potassium dichromate solutions has been given. A study of the factors involved in the preparation has shown that those things which tend to change the violet complex in the alum solution to the green complex, also tend to reduce the yield of the desired reagent. An improved method of protecting the reagent from the oxygen of the air has been described.
9.7 .~
98 96 92
84 72
Table 11offers evidence that the yield is less where no acid has been added to the chrome alum so~ution, that satisfactory yields can be obtained with either sulfuric or hydrochloric acid. that a change in concentration of the acid from 0.01 N to 0.1 N does n i t change the yield materially, and that variations in the cr+++ concentration of the chrome alum to Oa4 Cr+ff do not the yield greatlyThough the yield with hydrochloric acid is as good as with sulfuric acid. the hvdrochloric acid solutions are known to be less stable. Experiments 41S, 43S, and 445 show that the longer a chrome Of chromous stands’ the lowerthe ion Obtained upon When acid was added to the chrome alum solutions, this reduction in yield with age was
Literature Cited (1) Asmanoff, 9., 2. anorg. allgem. Chem., 160, 209 (1927). (2) Brennecke, E., “Die chemische Analyze,” by Margosches, Vol. 33, pp. 96-116, Stut.tgart, Ferdinand Enke, 1936. (3) Crowell, W. R., and Baumbach, H. L., J . Am. Chem. Soc., 57, 2607 (1935). (4) Graham: M. A., Am. Chem. J., 48, 145-90 (1912). (5) Grube, G., and Schlecht, L., 2.Elektrochem., 32, 178 (1926). (6) Lundell, G. E. F., and Knowles, H. B., IND.ESG. C H m f , 16, 723 (1924). (7) Montemartini, C., and Vernazza, E., Industria chimica, 7, 857-65 (1932). (8) Orlova, S.I.,and Petin, N. N., J . Gem Chem. (U.S. S. E . ) , 1, 345-58 (1931). (9) Thornton, W. M., and Sadusk, J. F., IND.ENG. CHEM.,Anal. Ed., 4, 240 (1932). (10) Thornton, W. M., and Wood, A. E., IND. ENG.CHEM.,19, 150 (1927). (11) Zintl, E., and RienKcker, G., 2. anorg. allgem. Chem., 155, 85 (1926) I
RECEIVED November 22, 1935. Presented before the Division of Physical and Inorganic Chemistry a t the 90th Meeting of the ilmerican Chemical Society, San Francisco, Calif,, August 19 t o 23, 1936.
