The Colloid Chemistry of Basic Chromic Solutions - American

VEN to-day it is far rines ionize as anions. The from clear whether try of chromic oxide hydrosol. In aiew of the large amount of re- three chlorides ...
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Januarv. 1923

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The Colloid Chemistry of Basic Chromic Solutions' By F. L. Seymour-Jones2 COLUMBIA UNIVERSITY, NBW YORK,N. Y.

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VEN to-day it is far rines ionize as anions. The I t is the purpose of this paper to present a reoiew of the chemistry of chromic oxide hydrosol. In aiew of the large amount of rethree chlorides can be reprefrom clear whether chrome liquors really sented by Werner's system as: search now proceeding on the chemistry of chrome liquors and on contain basic chromic salts, the theory of chrome tanning, such a reoiew from a purely chemical or whether they are simple [Cr(HzO)aClz].C1.2Hz0 standpoint may serae to clear the ground of some misconceptions. colloidal. dispersions of chroGreen a-salt mic oxide. Postulating the existence of colloidally dispersed chromic oxide, it may exist [Cr(HzO)s.Cl].Clz.H20 [Cr(H2O)e].Cl$ Green y-salt Violet @-salt either as a negatively or as a positively charged sol. In the first case the freshly precipitated oxide is dispersed in excess of caustic HYDROUS CHROMIC OXIDE PEPTIZED WITH EXCESS ALKALI alkali, and in the second either the oxide is peptized with excess Herz,lo in 1901, noted that chromium hydroxide, freshly of the chromic salt or alkali is added to a solution of the normal precipitated from chrome alum with alkali, was easily soluble in chromic salt. The positive sol was first prepared as su% by Graham* in excess of the precipitant. After thorough washing and drying 1862, similarly t o the ferric oxide sol, by peptization of the over sulfuric acid in a desiccator, the hydroxide was insoluble. hydrogcl with chromic chloride. Even before this one peculiar These he regarded as allotropic forms of the hydroxide. Hantzsch," in criticizing this, considered the difference in soluproperty of this oxide had been noted by Northcote and Church' in 1855. They found that chromic oxide, when mixed with bility was better explained by the difference in specific surface of certain metallic oxides, such as ferric, manganic, cobalt, and nickel the two forms, due to the hydration in the one case, or, as an alteroxides, which are themselves insoluble in potassium hydroxide, native explanation, that the solubility of one was due to hydramay render these soluble, or may itself be rendered insoluble tion on Werner's theory: by them, according to the proportions used. Cr(OH2)s.(OH)az=*r(OH)s 4-6HzO Schiff6 in 1862 found that chromic acetate gave no precipitate on boiling, and hence that chromium, unlike ferric iron, could Herz, in conjunction with Fischer,lz further studied the prenot be estimated by the basic acetate method. Reinitzer' cipitation of chromium hydroxide by sodium and potassium confirmed this and further found that the presence of chromic hydroxides from chromic chloride solutions. The precipitate ion to a large extent inhibited the precipitation of iron and alu- was soluble in excess of the alkali, yielding a clear green solution. minium by sodium acetate. He also noted that chromium hy- They considered that they had here a case of colloidal solution. droxide, precipitated with potassium or ammonium hydroxide, On long standing, hydrous chromic oxide precipitated, the staetc., always contained some of the precipitating metal which bility of the solution being increased by excess of alkali and by could riot be removed by washing, even with boiling water. low temperatures. It was difficult to redissolve the precipitated Other chemists also studied the chemistry of chromium from chromic oxide in alkali, and hence this was regarded as a case the analytical point of view. The equilibrium between green of peqtization. and violet forms of chromic salts was studied by Richards and They examined the behavior of the peptized chromic hydroxide B ~ n n e t Niels ,~ Bjerrum? Minnie Graham: and many others. under dialysis with an animal membrane. In every case the The structure of chromium compounds from the point of view external water rapidly became alkaline, while the chromic hyof secondary valence has been studied by Werner and his school. droxide precipitated a t the membrane. In this it differs from The peculiar properties of chromic salts in solution may be a similar solution of alumina, where the aluminium passes through conveniently summarized here. Taking the chloride as typical, the membrane. With a similar solution of zinc hydroxide, part we find that it exists in several modifications. The most common of the zinc passes through and part precipitates a t the memform is the hexahydrate, or- and y-green salts and a &violet salt. brane-i. e., part exists as zincate and part is colloidally dispersed In aqueous solution the a-green form is stable a t high temper- as hydroxide or hydrous oxide. atures and the @-violetform a t low temperatures, an ordinary Experiments on precipitation with electrolytes gave indefinite solution being an equilibrium mixture. The change from violet results. They next measured the conductivity. They made to green on heating is rapid, but the reverse change on cooling the assumption that if the chromic hydroxide is a peptized is comparatively slow. The green chloride is easily soluble in colloid, the conductivity will be dependent upon the sodium water, but the violet form is practically insoluble unless a trace hydroxide concentration alone, whereas if a compound is formed, of chiomous chloride be present. The violet solution has a the conductivity should alter. Just sufficient alkali was added greater density than the green solution.9 In the a-salt only to redissolve the precipitate first formed and then the conducone chlorine, in the y, two chlorines, and in the ,9, all three chlo- tivity was measured. The solution was then boiled, the precipitated chromic hydroxide filtered off, and the conductivity again Presented before the Division of Leather Chemistry at the 64th Meetmeasured. They found that it had not changed, and hence coning of the American Chemical Society, Pittsburgh, Pa., September 4 to 8, 1922. cluded that the chromic hydroxide is in colloidal dispersion. 2 lS51 Exhibition Scholar of the University of Leeds. Bancroftla remarks on this that, since it is the hydroxyl ion * Phil. Mag., [41 23 (1862),290. which peptizes the chromic hydroxide, the conductivity should 4 J . Chem. Soc. (London), 6 (1853),54. be altered. When the hydroxide precipitates, it carries down 6 A n n . , 124 (1862), 168. 6 Monatsh., 8 (1882),249. with it some alkali and some water, which should change the

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Proc. A m . Acad. Arrs Sci., 89 (1903), 1, includes a bibliography of 62 references. See also Z . physik. Chem., 47 (1904). 29. 8 2'. physik. Chem., 59 (1907),336; abstracted from Kgl. Danske V i d cnskab. Selskab. Skrifter natzrrwidenskab. math. Afdel., I71 4, 1. * A m . Chcm. J . , 48 (19121,145, includes a bibliography of 53 references.

2.a n o y g . Chem., a8 (1901), 342. I b i d . , SO (1902), 338. 1%Ibid., 31 (1902), 352. I* Trans. A m . Electrochem. Soc., 28 (1916), 351. 10

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conductivity. Probably the large excess of sodium hydroxide present causes the change of conductivity to fall within the limits of experimental error: More recently Chatterji and Dharl* found no appreciable change in conductivity on adding chromium hydroxide to sodium hydroxide solution. Hildebrand16 is reported to have applied the hydrogen electrode to the measurement of the change in hydrion concentration on adding sodium hydroxide to suspensions of alumina and of chromic oxide in water. With the alumina a definite break in the curve occurs, corresponding to the formation of sodium aluminate, NaA102. No break occurs with chromic oxide, rendering the formation of sodium chromite unlikely under the conditions of the experiment. Wood and BlackI6 in 1916 examined chromic oxide from the amphoteric standpoint. Freshly precipitated and well-washed chromic oxide was allowed to stand for two months in solutions of sodium hydroxide. At the end of that time the more concentrated solutions were becoming yellow. This did not occur when all contact with air was carefully excluded. They conclude that minute traces of chromic oxide dissolve to form sodium chromite, which is slowly oxidized by the atmospheric oxygen to form chromate. The acidic dissociation constant of chromium hydroxide was too small to measure, but the results led to the conclusion that it functions as a polybasic acid. The conclusions reached by Herz and Fischer12were criticized by Kremann,” who pointed out that Nernst, in the electrolysis of chromium solutions, found that the colored boundary shifted towards the anode. Kremann concluded that it was probably not a case of colloidal electrophoresis, but rather that chromites of the type NazCnO, were formed, where the colored chromite ion migrated anodically. With some justice he pointed out that Fischer and Herz dialyzed against water, and hence that chromite, if present, might be sufficiently unstable to hydrolyze, and a precipitate of the hydroxide would naturally form a t the membrane. He dialyzed an alkaline solution of chromium hydroxide against dilute alkali, and, after some hours, found that chromite ion had dialyzed through, Fischer’8 replied to these criticisms. Any anodic migration was explained by Bredig’s results, which showed that colloids were able to migrate with the current. Further, in the dialysis experiments Kremann made up his solutions from chrome alum. Fischer and Her212 had previously shown that chrome alum gave very peculiar and indefinite results, which they attributed to the formation of chromium-sulfuric acids, capable of dialysis. Fischer also argued that chrome alum is a salt of the violet variety, while the chlodde he used was the green one. He dialyzed green chromium chloride against sodium hydroxide solution for thirtysix hours, and no chromium passed through the membrane. Traces coming through later were due to the destructive action of the alkali on the membrane. Nagelle ultrafiltered a dispersion of hydrous chromic oxide in alkali through collodion filters. The oxide was completely filtered out, the liquid coming through colorless. Bancroft found on shaking precipitated chromic oxide with water and benzene thac the oxide went to the dineric interface. Nagel therefore tried t o shake out the peptized chromic oxide with benzene or kerosene, but this was useless in alkaline solutions. We can, therefore, conclude that chromic oxide can be colloidally dispersed in alkali solutions, from which on long standing it precipitates out, If these solutions are exposed to the air, some oxidation to chromite and chromate occurs. The nature o€ the “The Physics and Chemistry of Colloids” (Faraday and Physical Societies’ Discussion), 1981, 122. See footnote 13. 16 Discussion following paper by Bancroft. 16 J . Chem. Soc. (London), 109 (1916),164. The reference to Nernst given by 17 Z. anorg. Chem., 38 (1903), 87. Kremann appears to be incorrect. 18 Ibid., 40 (1904), 39. loNsgel, J . Phys. Chem., 19 (1915), 331, 669; Bancroft, Ibid., 19 (1815), 275. 14

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peptizing electrolyte is uncertain. It may be the alkali itselfthe hydroxyl ion, as some would say-or quite possibly a trace of alkali chromite. WITH CHROMIC CHLORIDB HYDROUS CHROMIC OXIDE PEPTIZED

Fischerlz studied the solubility of freshly precipitated and washed chromium hydroxide in violet chromic chloride solutions. The results showed that the solubility of the hydroxide was not directly proportional to the chloride concentration. He could not precipitate the chromium hydroxide from solution, either with electrolytes or by heating for eight days on the water bath. H e concluded that a basic chloride was probably formed by hydrolysis, and that chromium hydroxide was also present as a colloid. However, he abandoned the study of chromium salts, owing to their color and general complexity, and turned to alurninium as providing a simpler system. Neidle and BarabZostudied the dialysis of colloidal solutions of hydrous chromic oxide in chromic chloride, using parchment paper membranes. The results are somewhat confusing, as at first the colloidal particles diffused through the membrane. This ceased later, which they attributed to the growth of the particles. They also studied the diffusion a t high temperatures (around 80” C.), and as a result discovered the more rapid preparation of *purer sols. The examination of these sols showed that the reputed stability of chromium hydroxide sols to electrolytes was in reality due to insufficient dialysis. This is shown by the following table: Hydrous CnO3

Sol 1 2 3

Eq./LiterCrzOx HCI

-G.

0,0693

0,0984 0 . CSS8

Cc.0.1 N KCl Required to Precipitate 10 Cc. of Sol

Trace 0,0020 0.0058

0.37 3.00

30.00 did n o t precipitate

They conclude that ( a ) the higher the temperature, ‘the less electrolyte is necessary for the stability of a sol, and (b) the concentration of electrolyte necessary for stability at a given temperature increases with the concentration of the colloid. The green chromic chloride readily yields sols on dialysis, while only traces are obtained on dialyzing the violet salt. Niels Bjerrum* applied conductivity and hydrogen-ion measurements to the study of the hydrolysis of violet chromic chloride, and these tend to throw some light on the colloidal aspect. His results in adding sodium hydroxide to chromic chloride solution show decreased conductivity until one mol of NaOH per mol CrCls was added; then the conductivity very slowly increased until three mols NaOH per mol CrCls were added, after which the conductivity rapidly increased. This indicates the formation of a definite basic salt, CrOHC12. His results were corroborated by Denham,21 using the electrometric method. Nagel19 tried the ultrafiltration of a “basic” chromic chloride solution, and found that the peptized chromic oxide was filtered out, a normal solution of chromic chloride passing through. He therefore concluded that no appreciable amount of basic salt was formed, which is strange in view of Bjerrum’s results. If Nagel’s chloride solution was more basic than CrOHC12, he may very possibly have had hydrous chromic oxide peptized in a solution of CrOHC12. But this does not explain why normal CrCla solution passed through the filter. HYDROUSCHROMIC OXIDE PEPTIZED

WITH

CHROMIC SULFATS

T. W. Richards and F. Bonnet’ in 1903studied “the changeable hydrolytic equilibrium of dissolved chromic sulfate.” An attempt was made to determine whether any definite basic salt was formed. Shaking out the green solution with an alcoholether mixture, they finally obtained a solution in which the salt appeared to be CrOHSOa. A similar result was obtained by 21

J . Am. Chem. Soc., 38 (1916),1961; 89 (1917), 71. J . Chem. Sac. (London), 93 (1908),41.

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digesting the green solution with the hydroxide on the steam bath for several hours. But this is not the limit of basicity in the cold. By shaking for several days a violet solution with chromium hydroxide, whereby it is rapidly converted into green, they finally obtained Crb(OH),(S04)4. Of course there is no limit to the theoretical number of basic salts. The last one named can be written HO HO HO I

I

I

HO HO I

I

and obviously the process of formula building can be extended ad lib., though practical justification may not be forthcoming. Another test for the existence of a basic salt was applied. When the sulfate ion in chromium sulfate is precipitated as BaS04, the precipitate contains some of the green salt occluded, since the green salt is highly hydrolyzed, but does not occlude any of the violet salt. Richardsz2 had previously shown that such occlusion, which is quite a general phenomenon, is probably due to the distribution of a definite molecular species between the solvent and the precipitate a t the moment of formation of the latter. Consequently, he hoped in this way to get a t the formula of the basic salt. Solutions of sodium sulfate and chromic chloride were mixed, allowed to stand, and then precipitated with barium chloride. The amount of salt occluded increased with thc delay in precipitation, showing that time was necessary for the formation of the more complex sulfates. The salt occluded was a basic chromium sulfate and not a chloride. They also examined the migration on electrolysis of a basic sulfate solution. The migration was entirely cathodic, and there were 19.3 g. Cr per 96,580 coulombs, a surprisingly large figure. They argue that, since the atomic weight of chromium is 52, each atom of chromium cannot be associated with more than two charges and probably with not more than one. Assuming that the sulfate ion alone migrates anodically with a mobility of 70, the mobility of the chromium group, if with one charge, is 41, if with two charges, is 243. The latter figure is improbable, while the former resembles those for zinc and magnesium. They regard it as possible that the cation may be CrO+ or Cr(OH)2+,which SiewerP3 and Whitney24 showed to be the most probable cation in boiled (green) solutions of chromic chloridc: or nitrate, or the cation may be yet more complex, with basic groups attached. In general, Richards and Bonnet concluded that a green basic salt existed, but they were not prepared to assign any definite formula to it, since hydrolysis might very possibly proceed in steps. Further work as to the existence of a colloidal substance in the hydrolyzed solution was promised, but does not appear to have been published. In view of the foregoing and Bjerrum’s results with the chloride, I carried out the ultrafiltration of a basic chromic sulfate solution. This was prepared by reduction of a sodium dichromate solution with sulfur dioxide, the excess sulfur dioxide being removed by boiling. The dark green, concentrated solution contained 269.9 g. CrzOa per liter. This was ultrafiltered through hard filter papers impregnated with 1 and 5 per cent gelatin dispersions, the papers being subsequently hardened in 4 per cent aqueous formaldehyde solution, and through a collodion disk. In every case the solution passed through unchanged, no colloidal particles being retained by the filter. The same result was obtained when the solution, diluted with three volumes of water, was allowed to remain in a collodion bag suspended in air. The concentrated solution and one diluted to ten

volumes with water were dialyzed in collodion bags against water, the water being changed frequently. In less then eighteen hours even the concentrated solution had completely dialyzed through the membrane, the liquid remaining in the bag being colorless. Such a solution as that described above should have a basicity equivalent to CrOHS04, or, according to Bassett,26would contain a mixture of 95 to 96 per cent chromic sulfate and 4 to 5 per cent chromium dithionate. Conductivity titrations of chromic sulfate solutions with barium hydroxide have been carried out by A. W. Thomas and S. B. Foster,Ze and by W. R. Atkin and D. Burton.21 These, however, were for the purpose of determining when all three sulfate ions had been precipitated, and the readings are insufficient to determine definitely whether there is a break in the curve corresponding to the formation of a definite basic salt. In the former’s curves there is evidence of such a salt being formed. I intend to investigate this point by this and other methods as time permits. It seems probable that there may exist a basic salt, CrOHSO‘, analogous to the chloride. The question is complicated by the fact that in certain basic chromic sulfate solutions part of the chromium is found in a complex anion. It is quite possible that such basic solutions and colloidal dispersions of hydrous chromic oxide may exist together. The whole question is certainly a most complicated one, and the pure chemistry of “basic” chromic solutions must be cleared up before any definitive theory of chrome tanning can be elaborated.23 OTHER SOLS

B i l t ~ in , ~ 1902, ~ arguing that in the series of precipitating anions NOa- was the least powerful, proceeded to study the formation of colloidal hydroxides of polyvalent metals by dialysis of their nitrates. Chromium hydroxide hydrosol was prepared by an eight-day dialysis through parchment of a fairly concentrated solution of commercial chromic nitrate, changing the external water three times a day, until it finally gave no test for NO*-. The sol so prepared was dark green by both transmitted and reflected light. It had a neutral reaction, and contained Nos-. Like other chromium sols, it was apparently resistant to electrolytes. Sodium and barium chloride solutions gave no turbidity, nor did a few drops of hydrochloric acid. Sulfuric acid or saturated sodium chloride solution gave decided turbidity and precipitation. Woudstra,ao in 1909, prepared a “red” colloidal sol of chromic hydroxide from chromic acetate solution, both by dialysis and by distilling off the acid in steam. The sol was brownish red by transmitted and a dirty green by reflected light. The sol was electropositive and very resistant to electrolytes. Twenty per cent solutions of MgS04, BaC12, NazCOa, KCN, KSCN, KMnO4, and concentrated solutions of HzSOI, HCI, “03, NaOH, (NH&COa, and (NH&(COO)t gave no coagulation. The acids dissolved the chromic oxide and gave green solutions. CHROMIC OXIDEJELLIES Bunce and Finch81 studied the formation of chromic oxide jellies. If sufficient sodium acetate is added to a chromic sulfate solution, and then an alkali, the solution sets to a jelly which is not completely reversible. The concentrations of reactants employed can vary within wide limits, and both violet and green jellies formed, according to whether ammonia and slight or large excess of alkali be employed. Nagell* also studied this 26 28

I’YOC. A m . Acad. Arts Sci., 35 (1900). 377; Z . anorg. Chem., 93 (1900).

2*

2’

Ann. Chem. Pharm., 116 (1862),86. Z. physik. Chem., 20 (1896),40.

J. Chem. SOC.(Londow), 83 (1903)’692. J. A m . Leather Chem. Assoc., 15 (1920),510.

J. Soc. Leather Trades’ Chem., 6 (1922), 14. Thompson and Atkin, Ibid., 6 (1922),207. 99 B n . , 38 (1902),4431. 80 Kolloid Z . , 5 (lQO9),33. 8 1 J. Phys. Chem., 1’7 (1913),769. 27

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jelly formation. Recently W e i ~ e reviewed r~~ the literature on hydrous chromic oxide, and also showed that jellies of both the positive and negative colloid could be formed.

minium chloride the following transitions to aluminium hydroxide sol: AI(0H)Clz; AI(OH)&I; 2AI(OH)sAI(OH)aCl. Further, with increasing proportions of alumina, the aluminium hydroxy salts incline to the formation of negative aluminate complexes, CONCLUSION in addition to positive ones, a tendency which increases with dilution. These exist for zirconium hydroxide and probably It will be seen that the question of whether chrome liquors also for dilute ferric hydroxide sol, and this may, according to contain basic salts or positively charged chromic oxide sol Pauli, prove to be general for metal oxide sols. From the or both, is as yet undecided. Possibly an explanation may be similarity between aluminium and chromium, something of found in the work of Pauli and A d ~ l f i,n~their ~ research on alu- this sort may also occur with chromium sols,28but the evidence mina hydrosols. They find that the usual aluminium hydroxide so far is not conclusive. hydrosol, as prepared from aluminium acetate, proves to be a The negatively charged sol certainly exists, but is of no applibasic acetate, or aluminium monoacetate [AI(OH)a.CHaCOO], cation in tanning practice. The concentration of alkali neceswhich is largely hydrolyzed and in which the complex ions sary to peptize the chromic oxide is such that it would rapidly 3AI(OH)s.AI(OH)s+ and CHsCOO- have grouped themselves dissolve any hide put into it, apart from the sol being precipitated into colloidal particles. They were able to prepare from alu- by the salts or acid in the hide. I tried such a sol on hide powder and the latter was very rapidly hydrolyzed. The positive sol a* J . Phys. Chem., 26 (1922), 401. I * “The Physics and Chemistry of Colloids” (Faraday and Physical probably exists, but its investigation is complicated by the Societies’ Discussion), 19111, 14. possibility of basic salts and negative chromium complexes.

Earning Power of Research as Demonstrated by t h e Experience of t h e American Rolling Mill Company‘ By D. M. Strickland THEAUERICAN ROLLING MILL Co..MIDDLETOWN, OHIO

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T HAS BEEN SAID with

The experience of many business men is that to make research pay most efficient productive it is only necessary to adequately support a suficient number of methods, rolling Procedure, much truth that many research Programs are and all manufacturing steps studies on worthy projects under competent directidn. One and often more than one problem is sure to be successfully soloed, and be determined scientifically. not undertaken in this country until Some Pressure is this more than repays the entire expense. Conscientious endeavor along brought to bear upon the corthese lines resulted in sheet poration financing it. This steel for electrical equipment pressure may be economic, or it may be the strong arm of the of such perfection that to-day i t is used throughout the land. law, which insists that earth, air, or water be no longer polluted. The peculiar qualities of this steel are such that in transA much brighter picture is that presented by corporations which formers alone it is responsible for a reduction in the cost of set out to attain a definite objective, and who from the begin- electric current wherever it is used a t the present time. ning ally themselves with science in the confident knowledge This saving-one of the dividends on the research undertakingthat with its assistance its ideals can always be realized. can be reckoned in millions of dollars annually. The American Rolling Mill Company was founded by men RUST-RESISTING METAL who foresaw the trend of the times and who recognized the possibilities of organized research. In addition to the research Having accomplished this primary result, research was then staff there are many executives and directors of operating activities who are keenly appreciative of research development directed toward another commercial need-the production of and who cooperate whole-heartedly with the men in the research a rust-resisting sheet metal. The rust problem was scientifically laboratories. In any plant the greatest earnings are produced studied and it was found that governmental investigators and‘ by research when there is this close cooperation between the men testing engineers had already reached the conclusion that the responsible for productive manufacture and the theoretical purer the iron the more enduring it is when exposed to corrosive investigators of the research organization. Mutual confidence conditions. The research organization proceeded a t once to among the technologists and the men who work out in practice the study of producing commercially pure iron. Raw materials the laboratory recommendations insures research accomplish- had to be chosen with exacting care; purification processes, rolling practices, annealing procedures, and other necessitated ment and better commercial products. manufacturing steps were changed before the requirements of commercially pure iron were met. In the end a rust-resisting HIGH-GRADE STEEL OBTAINED iron was developed which has been produced with great uniYears ago the need for high-grade steel for electrical require- formity year after year. The sum of such impurities as sulfur, ments was recognized. Electrical engineers insisted that the phosphorus, carbon, manganese, copper, silicon, hydrcgen, cost of electrical equipment be reduced and its efficiency in- nitrogen, and oxygen is always less than sixteen-hundredths of creased. Here was a problem calling for team work on the one per cent. part of the chemists, the metallurgists, and the electrical engiIn the production of sound metal the occluded gas is a major neers. It was not sufficient that the analytical composition problem. Research investigation proves that the service life of this steel be satisfactory; it was also imperative that the of finished metal sheets was shortened when they were contaminated with appreciable amounts of occluded gases. It was 1 Received Jyne 24, 1922.