ANALYTICAL EDITION
August, 1945
(6) Frevel. L. K., personal communication. (6) Gaubert, P., Bull, aoc. fraw. mint?ral., 50, 504 (1927).
(7) Hanawalt, J. D., Rinn, H. W., and Frevel, L. K., IND.ENG. CHEM.,ANAL.ED., 10, 457 (1938). (8) Hill and Hendricks, IND. ENQ.CHIM., 28, 440 (1936). (9) Hodge, H. c . , L ~ F M,~ L,, ~ and ~ me, , w. F?, CHEM.,ANAL.ED.,10, 156 (1938). Survey, Bull. 848 H.3 u. s. (10) Larsen, E. and s . 8
(1934).
495
(11) Larson, H. W. E., IND.ENO.CHEM.,ANAL.ED.,7,401 (1935). (12) MacIntire, Winterberg, Marshall, Palmer, and Hatcher, I-. ENO.CHEM.,36,547 (1944); extensive bibliography. (13) Troemel, G., Mitteil. Kaiser-Wilhelm Imt. Eismfmsch. DiLsrrG dwf, 14, 198 (1932). (14) Winchell, A. N., “Microacopic Characters of Artificial Minerds”,New York, John Wiley & Sons, 1931. THISwork wan supported in part by of New York.
Quantitative Absorption
OF
a
grant from the Carnegie Corporation
Oxygen
Critical Factors in the Application of Acid-Chromous Solutions HOSMER W. STONE
AND
EDWIN R. SKAVINSKP
University of California, Lor Angeles, Calif.
Acetic acid solutions of chromous chloride provide an excellent reagent for the quantitative absorption of gaseous oxygen. This paper reporta a study undertaken to clear up controversial literature rtatementq it shows the effect of the concentration of acid hydrogen in the reagent on the evolution of hydrogen gas and sets forth methods of preparation and use that avoid or eliminate factors that have heretofore caused difficulties.
THE
reaction of acidified chromous solutions with molecular oxygen, represented by the equation 4CR++
+ 4 H + + O2 = 2H10 + 4Cr+++
was proposed for oxygen absorption by Pfordten (17) as early as 1885. This reagent interested him particularly because he wished to separate oxygen from hydrogen sulfide and other acid gases. Jannasch and Meyer (16) promptly adopted the reagent for removing oxygen from the nitrogen which they were using in organic combustions and thus the chromous solution took its place as one of the acceptled oxygen absorbents for gas analysis. Difficulties soon developed, with Bertholet ( 4 ) reporting that chromous solutions liberated hydrogen gas when acid hydrogen was present. This reaction is represented by 2Cr++
+ 2H+ = 2Cr+++ + Ht
The evolution of hydrogen gas by this unwelcome side reaction was confirmed in the publications of Doring ( I S ) , Peters (16)) and Dernstadt and Hassler (12). The report of Anderson and Riffe (1) in 1916, that hydrogen was evolved in acid chromous solution even in the presence of an excess of chromous acetate and that the absorption of the oxygen was only 9774 complete, appears to have been particularly discouraging to prospective users of the reagent. Such a n array of evidence might have completely disqualified chromous solutions as a n analytical reagent, but the report of Asmanow (2) in 1927 that chromous sulfate in sulfuric acid even up to 10 N acid yielded no hydrogen gas revived interest in the matter. A number of papers were published about this time by Zintl (26-28)and co-workers and by Brintzinger ( 7 , 8) and co-workers in which use was made of chromous solutions for potentiometric analytical titrations. Later, Thornton and Sadusk (93) reported that 0.067 N chromous sulfate in’ 0.18 N sulfuric acid
-1
Present address, Southern California Gee Co., Loa Angelae, Calif.
showed no change in titer over aperiod of two months. The United States Steel Corporation (24,using a hydrochloric acid solution of chromous chloride for oxygen in gas analysis, stated “if the excess acid is not great and the temperature is kept below 20” C., the amount of hydrogen liberated is not measurable with an ordinary buret”. A commercial oxygen absorbent for gas analysis which contains chromous chloride in acid solution has been available on the market for a number of years. Stone and Dixon (20) investigated and re orted favorably on the application of a chromous sulfate-suyfuric acid reagent for the absorption of oxygen in gas analysis, prepared by assing a sulfuric acid-chrome alum solution over zinc amalgam. %ranham (6)caIled the attention of these workers to the fact that traces of hydrogen sulfide me liberated by the action of the chromous ion on the sulfate. H e indicated that the volume of hydrogen sulfide was not sufficient to be detected in the gas volume measurement but that sulfide formation on the surface of the mercury in the buret made reading of the mercury meniscus difficult. Branham made other important contributions to the chromous situation in pointing out that, under the conditions he used, the error due to the evolution of hydrogen was of the same order as that due to carbon monoxide formation with pyrogallate, and that the heat resultin from the oxygen reaction caused trouble by lowenng the solubiity of the nitrogen carrier gas, remaining 89 residue after each analysis.
A consideration of these contradictory or a t least controversial statements found in the literature with regard to the suitability of this reagent for accurate work suggested to the authors that a clear understanding of the important factors might eliminate the difficulties and make available a much more satisfactory reagent for oxygen absorption than had been previously known. This has been accomplished by showing the effect of the concentration of acid hydrogen in the reagent on the evolution of hydrogen gas, and by setting forth methods of preparation and use which avoid or eliminate factors that have given rise to the various objections appearing in the literature. The determination of the apparent percentage of oxygen in the atmospheric air with the acid-chromous reagent in highly precise apparatus provides data which show the reagent to be satisfactory even where high precision is required. APPARATUS
The apparatus used in the investigation is illustrated in Figure 1. The two-bulb type of buret shonm was designed especially for the analysis of air, being similar to the one described by Carpenter (10, 11). This buret allowed the estimation of volumes t o 0.002 ml. on a 50-mi. sample, the readings being made to approximately 0.004%. The compensating tube, the buret, and the pipet were immersed in water circulating rapidly from a thermostat, so that the temperature did not vary more than 0.05’. It was found unnecessary to use the thermoregulator for analysee if the order of accuracy desired did not exceed 0.01%. Even
INDUSTRIAL AND ENGINEERING CHEMISTRY
496
when the thermoregulator was not used it was thought desirable to keep the apparatus immersed in the water bath because of the effect of the heat evolved in the oxygen absorption. The absorption pipet was equi ped with a sintered-glass bubbler tip prepared as directed by &one and Weiss (2.82)from glass ground to 100- to 150-mesh. The solution in the reservoir arm of the pipet was protected from the air by a rubber balloon. In the determinations tank air, room air, tank nitrogen, or tank oxygen was used to study such effects as the liberation of hydroen and the displacement of and resaturation with nitrogen. !‘he samples were saturated with moisture and a moist buret was used, so that the gases were kept saturated with moisture a t all times. One limitation of the apparatus, which made the work somewhat tedious, resulted from the unusual sensitivity of measurement. After a chosen number of passes of the sample and residual gas had been made through the absorption reagent the pressure was equalized by opening the capillary line to the manometer before making the final volume reading. Even though the capillary diameter was small and the length no greater than necessary, some of the oxygen would diffuse into the measuring pipet during the pressure adjustment, so that additional passes of carrier gas resulted in hi her apparent values for the oxygen. Accordingly, when it was fesired to determine the effect of varying the number of passes i t was necessary to compare the values for different samples of the same gas rather than use the simpler expedient of making additional passes of the residual gas from a single analySlS.
To obtain satisfactory results, hi h quality stopcocks were used and the apparatus was tested for leaks before each day’s determinations. Indeed, considerable practice was required before an operator could obtain consistently satisfactory results. PREPARATION OF CHROMOUS SOLUTIONS
Because of the peculiarities of the various chromium coordination complexes, unexpected difficulties in the preparation of chromous solutions have frequently either discouraged workers from using the reagent or led them to use unnecessarily cumbersome methods of preparation. Important facts in this connection which have bearing on the work are brought out by Stone and Hume (2f). Of particular importance is the fact that ordinary green chromic chloride solutions are reduced rapidly by zinc amalgam while chromic acetate under similar conditions is scarcely reduced a t all.
Table Author
nd Skavinski
1.
Vol. 17, No. 8
The statements by B a n h a m (6) that solutions of green chromic chloride are not appreciably reduced by amalgamated zinc unless an excess of hydrochloric acid is present, that green chromic chloride is reduced more slowly than chromium potassium sulfate in a Jones reductor, and t h a t a precipitate forms clogging the reductor when green chromic chloride is reduced are not supported by the experience of the authors nor by the results obtained by Stone and Hume (81). Doubtless differences which affected the character of the chromium complex or the activity of the zinc amalgam not stated by Branham could explain these discrepancies. I n any case, control of such critical factors may make the difference between a satisfactory and an unsatisfactory reagent.
I
/
Oxygen in Atmosphere Method
Oxygen in Atmosphere, To
Chromous-acetic acid
20.947
For the work reDorted in this DaDer the hvdrochloric acid solutions of chromous reagent werk prepared“ by dissolving green crystals of CrCl,.XH,O, probably Higley’s (14) [Cr(H20),Cl*]C1.2H20 form, in 0.01 to 0.02 M hydrochloric acid. The solution was then forced over the amalgamated zinc by means of air pressure into the absor tion pipet where it was mixed with h drochloric acid t o give t i e desired concentrations of acid and clromous ion. For the chromous-acetic acid solutions, green crystals of chromic chloride were dissolved in 2 M acetic acid and reduced by passage over the amalgamated zinc. Once chromous chloride has been oxidized in the presence of acetic acid it is not reduced by passage through the zinc-amalgam reductor. It is like the chromic acetate mentioned above in this respect. On the other hand, green chromic chloride in 2 M acetic acid solution is as readily reduced by the zinc amalgam as in hydrochloric acid solution. Even after 24 hours’ standing the green chromic chloride in acetic acid solution is readily reduced. Apparently the chromic chloride does not react to form the more stable rtcetie acid complex until some stage of the oxidation process opens the way.
I
Figure 1.
Diagram of Apparatus
In certain preparations of the reagent it was observed that minute particles of the amalgamated zinc were buoyed up by adhering bubbles of hydrogen and some of these passed through the glass wool filter a t the end of the reductor into the pipet. Trouble arose from the slow liberation of hydrogen as a result of the action of the acid of the reagent on the particles of amalgam. The introduction of a tube containing a sintered-glass disk between the reductor and the pipet eliminated this as a powihle source of error. DlXUSSlON OF RESULTS
Table I presents a summary of values published by differell1 authors for the percentage by volume of oxygeu in the atmobphere. It is included so that the value obtained with t h e
ANALYTICAL EDITION
August, 1945
chromous chloride-acetic acid reagent may be compared with those obtained by the widely used alkaline pyrogallate method. Table I1 records the effect on the values obtained of varying the concentration of the hydrochloric acid and of changing to the relatively low hydrogen-ion concentration of 2 M acetic (approximately 1 X 10-2). The concentration of acid and of chromous ion listed in the table is the initial concentration. This decreases as the oxygen absorption reaction consumes both chromous and hydrogen ion. On the basis of the volume of reagent in the pipet and the amount of oxygen absorbed the concentration of each of these substances decreases about 0.011 in M concentration for each 50-ml. sample of air analyzed. It was the observation that when chromous solution had been used to absorb oxygen nearly to the exhaustion of the hydrochloric acid, higher and approximately correct values for the percentage of oxygen were obtained which first suggested to the authors the idea of using a weak acid such as acetic. By replacing the strong hydrochloric acid with 2 M acetic in preparing the chromous solution one is able to provide acid for the entire oxygen capacity of the reagent without making the hydrogen-ion concentration high enough to liberate detrimental quantities of hydrogen gas. The data of Table I1 also show that the effect of the change in the concentration of hydrogen and chromous ions with oxygen absorption is not of sufficient magnitude to appear as a trend in the values for the first ten analyses on each reagent as listed. Accordingly, this change in concentration is not regarded as significant in interpreting the results. The data indicate clearly that when the reagent is made up to contain the higher hydrogenion concentrations the apparent value for the percentage of oxygen in the air becomes less. This results from the fact that when hydrogen-ion concentration is high more hydrogen gas is produced from the chromous ion-hydrogen ion reaction and hence lower apparent values for the oxygen percentages are obtained.
Table 11. Effect of A c i d Concentration on Apparent Percentage of Oxygen in A i r as Determined with P M Chromous Chloride 2 ,M HAC 1 M HCI 0.5 M HCI 2 34 HC1 1 .5M HCI 20.890 20.899 20.905 20.936 20.905 20.911 10.916 20.877 20.891 20.926
20.921 20.930 20.915 20.913 20.933 20.943 20.877 20.922 20.903 20.929
20.917 20.888 20.921 20.935 20.945 20.937 20.943 20.936 20.946 20.936
20.945 20.923 20.938 20.935 20,945
..... ..... ..... ..... .....
20.945 20 958 20.937 20.946 20,954 20.936 20.948 20.940 20.949 20.941
Average Spparent Percentage of Oxygen and Average Deviation 20.906 +=0.013
20.919 t0.015
20.930 t0.013
20.937 10.007
20.945 10.006
.Is further evidence of the correctness of the conclusion that acetic acid-chromous chloride solutions give satisfactory results, a series of sixty determinations was made starting with a solution 2 ,Tf in chromous chloride and 2 ,If in acetic acid. In Table 111, each column represents an average of ten of the sixty determinations in order. These data also give an idea of the Fange of the determination values and of the extent of the deviations observed. Even with acetic acid and its low hydrogen-ion concentration, chromous chloride solutions were found to yield some hydrogen gas on long standing, although the effect of the liberated hydrogen on the results of the oxygen analysis could not be detected unless the reagent had stood for from 10 to 12 hours. The evolution of hydrogen is so slow, even when hydrochloric acid is present, that this acid may be used without detriment in gas analysis if high precision is not required. In fact, hydrochloric
497
acid solutions of 0.5 M or less concentration are satisfactory for gas analysis in so far as hydrogen evolution is concerned. It must have been some such low hydrochloric acid concentration which Branham (5) used when he concluded that the error resulting from the evolution of hydrogen in the chromous solution was of the same order as the error caused by the formation of carbon monoxide in the absorption of oxygen by alkaline pyrogallate.
Table
111.
Percentage of Oxygen in A i r as Determined by Chromous Chloride in P M Acetic A c i d
PM
(Each column is average of ten analyses) 20.945 20.951 20.954 20.943 20.945 20.944 Oxygen, % Average deviation, yo *0.006 =.-0.006 *0.008 t0.005 10.005 t O . 0 0 5 Range of 20.958 20.966 20.966 20.951 20.955 20.965 values, % 20.936 20.939 20.938 20.926 20.927 20.941 Average for sixty analyses, % Average deviation, 70
20.947 ==0.007
The fact that the authors have yet to prepare an acid chromous solution, even with acetic acid, that does not evolve some hydrogen after long idle periods requires that inert gaseous equilibrium be established with the reagent at the start of each day’s analysis. DISPLACEMENT
OF NITROGEN
Branham ( 5 , 6) compared chromous reagents and alkaline pyrogallate with respect to both nitrogen displacement during oxygen absorption and the number of passes required to resaturate these reagents with nitrogen carrier gas after the absorption had been completed. He reported that the chromous i-eagents were less satisfactory in both respects because of the greater solubility of nitrogen in the chromous reagents and because there is a greater rise of temperature in the absorbing solution due to the heat of the absorption reaction in the case of the chromous solution. The practical issue here is the matter of the number of passes of the gas through the reagent required for accurate analyses. Not only is the nature of the absorbing reagent concerned in this matter but also the particular characteristics of the gas analysis equipment used. For example, the size of the pores of the bubbler tip and the length of the gas bubble column in the absorbing liquid are factors in determining the number of passes required. Three passes of the gas through the chromous reagent were required in this apparatus when air was being analyzed. However, when a 50-ml. sample containing 80% oxygen was analyzed it became necessary to use ten passes for results of highest accuracy. The greater quantity of heat liberated by the absorption of so large a volume of oxygen displaced more nitrogen from the reagent and made the larger number of passes necessary. If samples high in oxygen are to be analyzed it is customary to add a certain percentage of carrier gas for satisfactory analysis in such an apparatus. The displacement of carrier gas could be avoided by using a larger quantity of the carrier gas with the sample or by using a smaller sample and thus reducing the quantity of heat evolved and consequent inconvenient displacement of carrier gas. Under conditions of analysis when the oxygen content does not exceed that of the atmosphere the much higher rate of absorption of oxygen by the chromous reagent as compared to other reagents used for this purpose results in this reagent’s requiring a minimum number of passes as compared to pyrogallate and other reagents. The remarkable superiority of chromous solutions with respect to the rate of oxygen absorption is brought out in the work of Stone (18). I n ordinary analyses where an accuracy not to exceed 0.1% is required, the error due to nitrogen displacement is much too small
498
INDUSTRIAL AND ENGINEERING CHEMISTRY
to be of concern. The number of passes required should be d e termined in every case experimentally for the particular apparatus, reagent, and accuracy desired. COMPARISON OF CHROMOUS CHLORIDE ACETIC ACID REAGENT WITH ALKALINE PYROGALLATE FOR OXYGEN ABSORPTION
Although alkaline pyrogallate has been rated as having a capacity of 27 volumes of oxygen per volume of reagent, Branham (6)found that the bubbler tip became clogged with precipitate before 3 volumes had been absorbed. The following comment has been offered by one of the referees of this manuscript: “Clogging of the bubbler tip when oxygen is absorbed by an alkaline pyrogallate solution is a function of the partial pressure of the oxygen. The tip does not clog with air, but does clog with repeated successive adsorptions of nearly pure oxygen. The clogging, precipitate dissolves on standing. Hence, the inferred limit of capacity is not representative of general or even good practice. We always dilute oxygen with known amounts of nitrogen to keep the CO yield low enough to avoid significant error.” The authors would like to add that the clogging is a function of the type of bubbler tip a.s well as of the partial pressure of the oxygen. Chromous chloride, 2 M , in 2 M acetic acid has a capacity of about 11 volumes of oxygen per volume of reagent and is effective as long as there is sufficient chromous ion to react with the oxygen. It absorbs molecular oxygen at many times the rate characteristic alkaline pyrogallate as shown by Stone (18). There is no dropping off in the rate of absorption with decreasing concentration until the reagent is exhausted. Solubilities indicate that a 3.5 to 4.0 M chromous chloride solution is possible with correspondingly higher oxygen capacities, though the authors have not investigated the action of solutions above 2.5 M in chromous chloride. The strongly alkaline pyrogallate tends to etch the glassware and to freeze the stopcocks, a di5culty not encountered in the use of chromous solutions. Chromous solutions tend to liberate hydrogen gas unless the hydrogen-ion concentration is kept low. Though pyrogallate does not liberate any gas on standing, carbon monoxide may be produced during the oxygen absorption reaction. The chromous reagent has the advantage of being useful for the separation of oxygen from acid gases such as carbon dioxide or hydrogen sulfide. The alkaline pyrogallate could not be used with acid gases but would be useful in separating oxygen from ammonia. The alkaline pyrogallate is simpler to prepare, especially when compared to the older methods for chromous solution. However, the ease and rapidity with which the chromous solution may be prepared by passage of the solution through zinc amalgam leave but a very limited advantage to the pyrogallate in this respect. PREPARATION OF CHROMOUS CHLORIDE-ACETIC ACID REAGENT
Weigh the calculated amount of green crystals of chromic chloride hexahydrate to make the desired volume of a 2 M solution. The commercial product bears the formula CrCla.XH20 and one may assume that X represents 6. Dissolve the crystals in 2 M acetic acid without heating and make up to the desired volume with additional 2 M acetic acid. Next force the solution upward by air or other gas pressure through a 0.1 yomercury-zinc amalgam into the absorption pipet, as recommended by Stone and Beeson (19). There should be a compmt glass wool plug in the exit end of the reductor to prevent fine r r t i c l e s of the amalgam, buoyed up by adhering bubbles of ydrogen, from passing into the pipet. AsatisfactoryO.l%mercuryzinc amalgam (21) may be prepared as follows: 240 grams of 20-mesh zinc are cleaned by immersing in 100 ml. of about 3 M hydrochloric acid for half a minute. The acid-zinc mixture is then added to 100 ml. of 0.013 M mercuric chloride solution (5 ml. of mercuric chloride solution saturated a t 25’ diluted to 100 ml.)and stirred rapidly for about 3 minutes. The amalgam is then washed and transferred to the reductor
Vol. 17, No. 8
tube. The air in the reductor tube may be displaced by water and this by the green chromic chloride-acetic acid solution. When a clear blue chromous solution appears a t the exit of the reductor, the latter is connected to the absorption pipet and the solution forced through the reductor a t a rate of 100 to 150 ml. per minute. Improperly amalgamated zinc or exhausted amalgam will not yield the clear blue solution characteristic of the practically 100% yield of the desired chromous solution. One should not conclude from the results reported in this paper that equally accurate values will be obtained when the acid-chromous reagent is used in the conventional unthermostated apparatus with samples of varying composition. However, with the information presented in this paper a t hand the analyst may readily determine the precautions which must be taken with whatever type of gas he may be using in order to obtain the accuracy which is required. CONCLUSIONS
Acetic acid solutions of chromous chloride provide an excellent reagent for the quantitative absorption of gaseous oxygen. The difficulties of preparation, the displacement of nitrogen carrier gas, and the evqlution of hydrogen may be recognized and avoided by applying the principles indicated. This reagent is equal in most respects and superior in others to the widely used alkaline pyrogallate. Its principal advantages lie in its greater rapidity of oxygen absorption, in its freedom from the etching and foaming effects encountered in the use of pyrogallate solution, and in the fact that it is acid in reaction, affording an opportunity for the separation of oxygen from acid gases. A noteworthy value for the average percentage of oxygen in the atmosphere is given and a p paratus and directions for an accurate determination of this value are described. LITERATURE CITED
Anderson, R. P., and Riffe, J., J. IND. ENG.CHEM.,8, 24 (1916). Asmanow, A., 2. anorg. allgem. Chem., 160, 209 (1927). Benedict. F. G.. “ComDosition of the Atmomhere with S~ecial Reference to Its Oxygen Content”. Publ. 166, Carnegie institution of Washington, 1912. Bertholet, M.. Compt. rend., 127, 24-7 (1898). Branham, J. R.. J . Research Natl. Bur. Standards, 21, 45-61 (1938).
Branham, J. R., and Sucher, M., Ibid., 21, 63-77 (1938). Brintzinger, H., and Oschate, F., 2. anorg. allgem. Chem..
165.
221 (1927).
Brinteinger, H., and Rodis, F., Ibid., 166, 53 (1927). Carpenter, T. M., J . Am. Chem. SOC.,59, 358-60 (1937) Carpenter, T. M., J. Metabolic Research, 4, 1-25 (1923). Carpenter, T. M., Fox, L., and Sereque, A. F., J. Biol. Chem 83, 211-30 (1929).
Dernstadt, M., and Hassler, F., Bar., 41, 2780 (1908). Doring, T., J. prakt. Chem. 121, 66, 73 (1902). Higley, G . O., J. Am. Chem. SOC.,26, 615 (1904). Jannasch, P., and Meyer, V., Ann., 233, 375-84 (1886). Peters, R., 2. physik. Chem., 26, 217 (1898). Pfordten, 0. F., Ann., 228, 112 (1885). Stone, H. W., J. Am. Chem. SOC.,58, 2591-5 (1936). Stone, H. W., and Beeson, C., IND.ENO.CHEM.,ANAL. ED.. 8 , 188-90 (1936).
Stone, H. W., and Dixon, C. R., Rochester Meeting, AMERICAN CHEMICAL SOCIETY, September, 1937. Stone, H. W., and Hume, D. N., IND.ENQ.CHEM.,ANAL.ED.. 11, 598-602 (1939).
Stone, H. W., and Weiss, L. C., Ibid., 11, 220 (1939). Thornton, W. M., and Sadusk, J. F., Zbid., 4, 240 (1932). U. S. Steel Corp., “Methods of Chemists of the U. S. Steel Cory for Sampling and Analysis”, 3rd ed., pp. 37-8, 1927. Zintl, E., and Rienacker, G., 2.anorg. allgem. Chem., 161, 37485 (1927).
Zintl, E., Rienacker, G., and Schloffer. F., Ibid., 168, 97-106 (1927).
Zintl, E., and Zaimis, Ph., 2.angeul. Chem., 40, 1286-91 (19271. Ibid., 41, 543-6 (1928). PRESENTED before the Division of Analytical and Micro Chemistry at 107th Meeting of the AM~RICAN CHEMICAL SOCIETY, Cleveland, Ohio.
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