CHEMICAL NEWS, Reduction of Permanganic Acid by Manganese Peroxide.

77

enable them to further test this factor, and also of any, that the evolution of oxygen observed by them is not due data with regard to other methods of determining heating effect.

to the action of the oxide on the permanganic acid, is one of great weight, but, to our minds, it is by no means conclusive. They found that when hydrogen, or carbon monoxide, is absorbed by a 5 per cent solution of potassium permanganate to which has been added 2'5 volume-per cent of concentrated sulphuric acid, the evolution of oxygen which occurs during the absorption is much more rapid than that which takes place after the exhaustion of the gas. This was shown in the following manner: 20 c.c. of the acidified permanganate solution and 40 c.c. of hydrogen were inclosed in a glass-tube and agitated together for twenty-four hours. The tube was then opened

A REDUCTION OF PERMANGANIC ACID BY and the gas in it displaced by carbon dioxide. On closing

MANGANESE PEROXIDE.*

By H. N. MORSE and C. L. REESE.

In a previous article (Am. Chem. Fourn., xviii., 401), one of us, together with Messrs. Hopkins and Walker, described certain experiments on the reduction of permanganic acid by percipitated manganese peroxide. It was there concluded that, whenever the latter is introduced

into, or is formed in the presence of, the former or its salts, a reduction of the acid occurs with an evolution of three-fifths of its active oxygen, in accordance with the following equation

xMnO2 + 2H MnO4 = (x + 2) MnO2 + H2O + 30. It was also shown that to this cause is to be ascribed the observed instability of solutions of permanganate.

the tube again and agitating the contents for another twenty-four hours it was found that only 2'5 c.c. of oxygen had been liberated, whereas, according to the experience of Messrs. Meyer and Von Recklinghausen, about 20 c.c. of oxygen are evolved when, under similar conof hydrogen are agitated for the same length of time, or ditions, 20 c.c. of the permanganate solution and 40 c.c.

even for a shorter time.

It is clear then-and our own results, to be given hereafter, confirm the conclusion-that the evolution of oxygen, which occurs when a gaseous reducing-agent is being ab sorbed by a concentrated acidified solution of permanganate, is much more rapid than that produced by the oxide, which results from the absorption after the gas has disappeared. It is also more rapid than that observed when an equivalent amount of oxide is produced in the permanganic acid by a liquid reducing-agent, like a soluSix months later (Ber. d. Chem. Ges., xxix., 2549), there tion of manganous sulphate. To us, however, the con appeared an article by Messrs. V. Meyer and Max Vonclusion that the evolution of oxygen in the two cases is Recklinghausen in which is described the evolution of due to wholly different causes is not obvious. We should oxygen which takes place when hydrogen or carbon suspect, rather, that the great initial evolution, observed monoxide is absorbed by a nearly saturated (5 per cent) when hydrogen and carbon monoxide are absorbed, is in acidified solution of potassium permanganate. The some way dependent on the gaseous character of these opinion was then expressed by one of us (Ibid., xxx., 48), reducing-agents. that the phenomenon observed by Messrs. Meyer and Von Recklinghausen was due to the peroxide which is formed when reducing gases are absorbed by permanganic acid; in other words, that it is an instance of a kind of reaction which had been under observation in this labora. tory for several years.

After the communication referred to had left the hands of the author, but before it appeared in printed form, an article by Messrs. H. Hirtz and V. Meyer was published (Ber. d. Chem. Ges., xxix., 2828), in which they maintain that the phenomenon observed by Messrs. Meyer and Von Recklinghausen is fundamentally different from that de. scribed by Morse, Hopkins, and Walker. The grounds for this opinion appear to be two-fold. In the first place the authors make the wholly unwarranted assumption that Morse, Hopkins, and Walker experimented with neutral solutions only, that is, with potassium permanganate; while Messrs. Meyer and Von Recklinghausen employed acidified solutions. We cannot interpret otherwise the expressions: "Diese Forscher haben gefunden dass eine Loesung von uebermangans aurem Kali unter gewissen Bedingungen durch fein vertheilten Braunstein unter Entwickelung von Sauerstoff zersetzt wird." "Da nämlich Wasserstoff und Kohlenoxyd als reducirende Koerper aus der Loesung der uebermangansäure Braunstein ausscheiden, und dieser, nach den genannten Autoren, mit Kaliumpermanganat Sauerstoff unter gewissen Bedingungen entwickeln kann." die Morse-Hopkins-Walker'sche Erscheinung bei unseren Säure-loesungen eintrete." . How erroneous must have been the view which we ascribe to Messrs. Hirtz and Meyer, on the basis of the above quotations, will ap pear when we state that, of the forty-eight experiments described by Morse, Hopkins, and Walker, thirty-five were with acidified solutions of permanganate.

"Ob nun

The second ground which the authors have for believing

From the American Chemical Journal, xx., No. 7.

|

We know at present altogether too little regarding the reduction of permanganic acid by the peroxide to give an opinion of any value as to its cause; and we attach very little value to present speculations on the subject, except so far as they may prove useful in giving direction to a further study of the phenomenon. There is, however, one fact which suggests a provisional hypothesis. We have considerable evidence tending to show that the molecules of the ordinary precipitated peroxides of manganese are quite complex. Perhaps the best evidence of this is to be found in the large formulæ which must be assigned to them in view of the small proportions of water, or of alkalis or other bases which they contain. If they are thus complex, it is conceivable that the tendency toward increasing complexity of constitution should be strong enough to effect the decomposition of permanganic acid with liberation of the superfluous oxygen.

If, in the absence of any other explanation the reduction of permanganic acid by the peroxide, we accept for the moment the proposed hypothesis, we should ascribe the greater inital evolution of oxygen which is observed when gaseous re ducing-agents are used, to the greater initial simplicity of the precipitated peroxide molecules. It is interesting in this connection to notice the volume. relations of the hydrogen absorbed and the oxygen which is liberated during or immediately following the absorption. They are approximately 2: 1 in all of the experiments of Meyer and Von Recklinghausen. This relation would be explained in terms of the suggested hypothesis by the following reactions:

=

2HMnO4 + 3H2 = 4H2O + 2MnO2; 2MnO2 + 2HMnO4 = H2O+2(MnO2)2 + 1402. We wish to state again that we have but little confidence in the above speculations. We claim for this merit only, that, inherently, they are not less improbable than the supposition that the evolution of free oxygen in the two cases under consideration is due to entirely distinct and different causes.

78

Reduction of Permanganic Acid by Manganese Peroxide. (OHEMICAL NEWS,

Aug. 12, 1898.

Furthermore, we do not see that the problem can be, solved by any simple experimental demonstration. The experimentum crusis" of Messrs. Hirtz and Meyer (Ber. d. Chem. Ges., xxix., 2829), established but one thing; namely, the fact that the evolution of oxygen is phenomenally rapid during or immediately subsequent to the precipitation of the peroxide when a gaseous reducing agent is employed, and that fact, taken alone, appears to throw no light on the cause of the evolution. Convinced that, with our slight bases of established fact, discussion is premature, if not profitless, we have undertaken a further study of the reduction of permanganic acid by manganese peroxide. The present communication contains the results which were obtained on treating the acid, under equal conditions, with equivalent quantities of hydrogen gas and manganous sulphate. We have thus far experimented with three concentrations of perman-temperatures, provided they were free from oxide. In the ganate solution.

The first (A) contained in Ic.c. 2.822 miligrms. of KMnO4 equivalent to 5 milligrms. of iron. twice as concentrated as A. The second (B) was each c.c. 51 milligrms. of KMnO4, equivalent to 90:38 The third (C) contained in milligrms. of iron. The solutions A and B are of the concentration usually employed in analytical work, while C has about the strength of the 5 per cent solution which was employed by Messrs. Meyer and Von Recklinghausen and later by Messrs. Hirtz and Meyer.

We

when 20 c.c. quantities of their 5 per cent acidified solution of permanganate were agitated with air in closed tubes, they obtained volumes of oxygen ranging from 19 gave them 19 and 2.2 c.c., while in a parallel experiment to 2'9 c.c. An agitation of thirty-three and a half hours hours they obtained only 2'4 c.c. In another instance the in which the agitation was continued for eighty-seven three and a half hours 2.8 and 2'9 c.c., and at the end of volumes of liberated oxygen were, at the end of thirtyeighty-seven hours 2'4 c.c. Since the decomposition was and a half hours the authors were led to believe that there not appreciably greater in eighty-seven than in thirty-three These results were not in accord with our own experience. is a limit to the reaction by which the oxygen is liberated. In the first place, we have always found dilute, moderately acidified solutions of permanganate quite stable at ordinary observation, the decomposition of permanganic acid by second place, in all the cases which have come under our when all of the acid had been reduced to the oxide. the peroxide, which is attended by the liberation of oxygen, has been a continuous reaction, which ceased only had not, however, experimented with solutions as concentrated as that used by Messrs. Meyer and Von Recklinghausen. We therefore inclosed in glass tubes of about 80 c.c. capacity 20 c.c. portions of permanganate Great pains were taken to free these solutions from the solution C. (51 milligrms. of KMnO4 per c.c.) and oxide which is usually found in the commercial salt, and volumes of diluted sulphuric acid, which were equivalent which, as has been shown, is the usual cause of the in- agitated for twenty-four hours and then titrated with to three times the potassium of the salt. These were stability of unfiltered solutions of potassium perman- oxalic acid. The reduction, if any had taken place, was ganate. To this end compact abestos filters were prepared too minute to be detected, nor was any trace of a brown upon perforated porcelain disks placed in the bottom of deposit upon the glass to be discovered. The degree of cylindrical funnels, such as are used in the Gooch method acidity was somewhat greater in our solution than in that of filtration. Two filters, one placed above the other, employed by Messrs. Meyer and Von Recklinghausen. were employed. The lower one was closed with a per- The difference in the results we must ascribe to the abforated stopper through which the stem of the upper one passed. The filter through which the liquid entered the in theirs. Their 5 per cent solution of permanganate was sence of oxide from our solution, and the presence of it flask was thus protected from the dust of the air, and the acidified with concentrated acid, and then boiled ("gut absence of any brown colouration of the lower filter proved ausgekocht"), but it does not appear to have been filtered. the effectiveness of the filtration. We have found these This treatment leads infallibly to the formation of a large precautions necessary in all quantitative experiments on the reduction of permanganic acid by the peroxide; since, added to a 5 per cent solution of the salt, there is a visible quantity of the oxide; for, when strong sulphuric acid is if they are neglected, it is impossible to ascertain how precipitation of the peroxide, and the quantity of it is much of the observed reduction is to be ascribed to the rapidly increased by heating the solution to the boilingoxide purposely added, and how much to that which was point. Our solution, on the other hand, was filtered with already in the solution. great care, and acidified with diluted acid (3 parts of water cleansing the tubes, and to prevent the entrance into to I of acid). Moreover, great pains were taken in them of dust, or any reducing substance.

When solutions of permanganate which contain suspended oxide are heated-especially those which have been acidified-the quantity of the oxide increases very rapidly. We call attention to this fact in connection with the necessity for careful filtration, because it appears to us to have a bearing on some of the results which were obtained by Messrs. Meyer and Von Recklinghausen. Their acidified solutions were." gut ausgekocht," but it does not appear from their published account that they were filtered.

The volume of permanganate solution used was the same in all experiments; namely, 20 c.c. was enclosed with whatever was to be added to it, in tubes This quantity having about 80 c.c. capacity, except in the case of solution C, where, owing to the large volume of hydrogen used in some experiments, larger tubes were employed. Our agitating arrangement was an oscillating table upon which the tubes were held by rawhide lacings. The table was rocked by means of an electric motor and made about 100 strokes per minute. The agitation was continuous, except for occasional interruptions which lasted only a few minutes.

The amount of reduction effected during the agitation was measured by means of standard solutions of oxalic acid and potassium permanganate. This method, with the use of calibrated measuring apparatus, appeared to us quite as accurate as that of determining directly the volume of the liberated oxygen.

Messrs. Meyer and Von Recklinghausen found that

Having failed to establish the fact of decomposition within twenty-four hour periods, we determined to extend and to compare the more dilute solutions A and B with C the time of agitation, using varying proportions of acid; with respect to stability. The results, and the conditions under which they were obtained, are given in tabular form. signify reduction in accordance with the equation The numbers given under the head of "volume reduced of the permanganic acid. (See next column). 2HMnO4 H2O+2MnO2+30, and not total reduction

neutral condition lost a measurable quantity of oxygen. It will be seen that none of the three solutions in the As regards the acidified solutions, it appears that A and B suffered but little, if any, decomposition, whether the acid permanganate. In the case of those acidified portions of was equivalent to once or six times the potassium of the the solution C, in which the acid was equivalent to three times the potassium of the permanganate, there was, after of decomposition. an agitation of one hundred and fifty hours, some evidence could be detected upon the glass. This evidence of reduc tion was wanting in those instances in which the acid A slight deposit of the brown oxide was equivalent to once and twice the potassium.

If we succeeded in our endeavour wholly to exclude the per cent solution of permanganate becomes per se someoxide from our solutions, it must be concluded that a 5

CHEMICAL NEWB, }

Aug. 12. 1898.

Methods of Analysis applied to Silicate Rocks.

Permanganate Solution A ; 1 c.c. = 5 M.grms. Fe.

Acid.

Volume reduced. Oxygen lost.

Time.

Hours.

C.c.

C.c.

what unstable when treated with a quantity of sulphuric acid which is equivalent to three times the potassium of the salt. Our solution C, however, proves itself much more stable than the equally concentrated, but somewhat less acid, solution of Messrs. Meyer and Von Reckling. hausen. The most obvious cause of this difference we have already referred to, namely, the certain presence of suspended oxide in their solution at the beginning of the agitation.

We shall not attempt to explain the fact that Messrs. Meyer and Von Recklinghausen obtained no larger volumes of oxygen after an agitation of eighty-seven hours than were liberated in other experiments during thirty-three and a half hours; but it occurs to us as something which may have a bearing on the case, that unless proper precautions were taken at the time of transferring the liquid to the tubes, to secure a uniform distribution of all the suspended and precipitated oxide, the quantities of oxide introduced into the different tubes would be likely That the liberation of oxygen, which occurs when an acidified solution of permanganate containing suspended oxide, is agitated in closed tubes, does not cease after a few hours will be amply proved by our later experiments. (To be continued).

to vary.

79

their purpose, thus allowing the various separations to be made more perfectly and without the annoying interference of several grms. of foreign fixed salts, which are most troublesome in that part of the analysis devoted to the separation of silica, alumina, iron, lime, and magnesia.

Another of their advantages is that with some of them it is possible to estimate in one portion the alkalies, in addition to those constituents usually determined in the silica portion. Where the material is limited, as it so often is in mineral analysis, this is a most important advantage, sufficient to outweigh all possible objections; but in rock analysis, where the supply of material is usually ample, it is rarely worth considering. A still further point in their favour is, that it is probably more easy to obtain them entirely free from fixed impurities than an alkaline carbonate.

There are, however, objections to their use. With some of them an extraordinary amount of time must be devoted to grinding the mineral to an impalpable powder, and the flux itself may need considerable hand pulverisation. Once introduced, they must be removed before the analysis can be proceeded with, and this removal takes much time and is always a possible source of error. The expulsion of boric acid and drying of the silica in the recent method of Jannasch and Heidenreich (Zeitsch. fur Anorg. Chemie, xii., p. 208, 1896), from their own account requires the almost constant supervision of the chemist for several hours. In mineral analysis these objections are entitled to far less weight than in rock analysis, since the object sought-usually the deduction of a formulawarrants the expenditure of much time and painstaking care. Finally, it has been found that one or more of these fluxes are not available for altogether general use, since certain minerals do not fully succumb to their attack, as andalusite with boric acid and others with lead oxide (Jannasch). Therefore, however well adapted one or the other of these methods may be for the analysis of homogeneous minerals, it is very improbable that the vivid anticipations of Prof. Jannasch (loc. cit., p. 219), to the effect that the boric-acid method will soon supersede the alkaline-carbonate-fusion method in rock as well as mineral analysis, will be speedily realised. Nor can the great saving in time of 50 per cent which is claimed be for a moment admitted. It may be that a chemist attending to only one analysis at a time will finish it somewhat sooner by following Prof. Jannasch's procedure than the one here outlined; but it is quite possible, as previously intimated, for two-or even sometimes portions of three analyses to be carried on in different stages of completion at the same time by the methods herein set forth.

The practice of separating alumina, &c., by the usual methods, after first attacking the rock powder by hydro. fluoric and sulphuric acids-silica being estimated in a separate portion-while attractive in principle, was aban. doned by the writer after fair trial, owing to the disturb. ance sometimes occasioned by incomplete expulsion of fluorine and to a less degree by the presence of sulphates instead of chlorides. With exception of the com. paratively few analyses made thus, the sodium-carbonate method has always been employed. In the case of rocks rich in fluorine strict accuracy would require the separa tion of silica to be made as in the Berzelian method for fluorine estimation; but in practice it is hardly ever necessary to resort to this tedious procedure, since the amount of fluorine is usually small, and it can by no possibility cause a loss of much more than three-fourths its own weight of silica by volatilisation as silicon fluoride when the sodium-carbonate fusion is evaporated directly with hydrochloric acid. Probably the loss is less, since some fluorine perhaps escapes as hydrofluoric acid. How ever this may be, the error is of comparatively slight importance, since it attaches to the constituent always present in greatest amount.

Purity of the Sodium Carbonate used as a Flux.-Not withstanding the most earnest efforts for years, it has been impossible to procure, either in the open market or

80

Methods of Analysis applied to Silicate Rocks.

by special arrangement with manufacturers, an article of sodium carbonate which can be called chemically pure. With special precautions small lots can be prepared in the laboratory that will contain less than 1 m.grm. total impurity in 10 grms.; but such an article cannot be purchased in the market, and rarely will the so-called chemically pure dry sodium carbonate contain as little as I m.grm. in 10 grms. The invariable contamination, aside from sand and straw, which have sometimes been found in large amount, is silica, alumina, iron, lime, and magnesia, all of these going into aqueous solution with the carbonate. The chief of these impurities are usually silica, alumina, and lime. An article of the above degree of purity is satisfactory in almost all imaginable cases, since the use of the usually extravagant amount of 10 grms. for a fusion would introduce an error of but o'r per cent in the analysis, supposing I grm. of mineral to be operated on. This error is undoubtedly fully equalled by the introduction of dust from the air in the various long evaporations.

Precautions in Fusing.-Special directions with regard to the fusion and its first treatment are unnecessary, except to say that the flame should not be directed vertically against the bottom of the crucible, but at an angle against the side and bottom, nor should the flame be allowed to envelop the whole crucible. These precautions apply in all ignitions of reducible substances, and yet they are rarely observed. In neither case, if neglected, will there be the necessary oxidising atmosphere within the crucible; on the contrary, reduction may occur fraught with serious consequences. This is especially true it the rock contains more than traces of pyrite or other sulphide, when, after cleansing and igniting the crucible, there may appear on its interior a darkening due to oxidation of reduced iron which had alloyed with the platinum. This may in exceptional cases amount to several m.grms. in weight, and can be removed only by repeated ignitions, followed each time by scouring or treatment with hydrochloric acid. In order to avoid the use of nitre in case of

pyritiferous rocks, it is well to first roast the weighed powder in the crucible in which the fusion is to be made. It sometimes happens that the cooled flux, and even its solution, will indicate absence of manganese when it is really present in quantity to give normally a strong colouration. Two fusions made side by side or successively, under apparently similar conditions, may in one case show little or no manganese; in the other consider: able. This observation has been frequently made, and therefore the absence of a bluish-green colour in the fusion is not to be taken as proof of the absence of manganese. This difference of behaviour I can ascribe to no other cause than that of a reducing atmosphere in one of the crucibles and an oxidising one in the other, even though the conditions were apparently alike.

Drying and Testing of Silica.-As to the best way of rendering silica insoluble by evaporation, my own predi

lection is for a double evaporation instead of a single one on the water-bath. By fusing with sodium carbonate in the forenoon, the silica is ready for the first filtration in the afternoon. It is quite unnecessary to carry the evaporation beyond approximate dryness. The filtrate is again evaporated, always in platinum, and is ready for final filtration the following morning, when approximately I per cent of silica is recovered and added to the main portion. My experience is that a better separation of silica is effected hereby, and in no more time than by a single long evaporation. That which is subsequently recovered from the ammonia precipitate rarely exceeds a half, or, at the most, I m.grm.

Drying in an air-bath at 110° C. or higher, or on a hot plate or sand-bath, or over a free flame, in order to render Bilica insoluble, offers no advantage unless much magnesium is present, and then the most favourable temper ature, according to Gilbert (Technology Quarterly, iii., p. 61, 1890; Abstract in Fresenius's Zeitschr. fur Anal. Chemie, xxix., p. 688, 1890), is 120° C. The presence of

much calcium chloride seems to facilitate dehydration of the silica, while magnesium chloride above 120° C., by decomposing, forms a silicate which dissolves in hydrochloric acid and increases the amount of silica carried into the filtrate. It does not appear from Gilbert's paper that the blast furnace slags, on which he experimented, contained titanium, phosphorus, or iron in appreciable amounts. Basic magnesian rocks usually do, and in such cases it is doubtful if the employment of a drying temperature of 120° would not materially add to the large impurity always to be expected with the silica. In other cases he confirms the earlier belief that drying temperatures higher than that of the water-bath increase the amount of insoluble impurity, chiefly alumina, in the silica, and that this amount cannot be reduced by long digestion with hydrochloric acid. Further, he confirms Lindo's statement that evaporation with sulphuric acid till the appearance of white fumes gives a higher result in silica than with hydrochloric acid. But for general rock analysis the use of sulphuric acid at this stage must be rejected utterly.

never

Blasting for twenty to thirty minutes is necessary to expel all moisture from the silica. Its weight should always be corrected for impurities, which are absent, by evaporating with hydrofluoric and sulphuric acids and again blasting. If toward the end of evaporation with these acids, when the hydrofluoric acid has been driven off and the sulphates begin to appear in solid form, the residue has a peculiar milky or enamellike appearance, it may be taken as evidence of much phosphorus and titanium. This appearance is so unusual and striking that it is worth while calling attention to it. With basic rocks very rich in titanium and phosphorus the residue may amount to 2 or even 3 per cent of the rock.

The subsequent precipitate of alumina, &c., is usually ignited in the crucible containing the residue from the silica.

It might be supposed that this residue would contain most of the barium of those rocks carrying that element together with sulphur or sulphates, but the reverse is true as a rule. Only when there is a considerable excess of SO, over the BaO will much of the latter be found there, and usually there is none at all. Should some be present, its removal and estimation at this stage is not necessary, as it can be more conveniently recovered later, together with the silica accompanying the alumina, &c., pre. cipitate.

The separation of silica in rocks containing fluorine has been touched upon in commenting on the sodium carbonate method of fusion.

Platinum in Filtrates.-The filtrates from the silica always contain notable amounts of platinum. This arises in very small degree from the crucible fusion, in a larger one from the action of hydrochloric acid on manganate and permanganate, sometimes chromate of sodium, and, if much iron is present, in no small degree from the reduction of ferric chloride to ferrous by the platinum of the dish. This reaction is little known, apparently, but is mentioned in Gmelin-Kraut (Anorg. Chem., iii., p. 359, sixth revised edition), and can be readily demonstrated by evaporation of ferric chloride in platinum.

Metals of the Hydrogen Sulphide Group.-The presence in appreciable amounts of metals precipitable by hydrogen sulphide, except perhaps copper, is of such infrequent occurrence in most rocks that discussion is unnecessary in their connection. In case it is necessary to precipitate them, however, it is always well to bear in mind that some titanium may be thrown down along with them. Separations of the silica should be made in porcelain, to eliminate platinum, or, better still, the quantitative estimation of any of these metals should be made in a separate

It is possible that this appearance is caused by zirconium with the phosphorus and titanium. (See page 81, footnote).

Methods of Analysis applied to Silicate Rocks.

portion of the rock broken up by the action of hydrofluoric | quired. In these cases there is no difficulty in getting all and sulphuric acids. the manganese into the filtrate.