JOURNAL OF PHYSICAL SCIENCE Established Proprietor and Editor June 1, 1917 All free from Starch SECURE the best results for your experiments by using the genuine British-made Whatman Filter Papers. Guaranteed free from starch. There are now seven acid-washed grades; four of which are positively ashless, No. 40, 41, 42, 44. No. 43, also Folded Filters and Fat Extraction Thimbles, out shortly. WHATMAN Stocked by all Laboratory Furnishers. In the event of difficulty in obtaining supplies or free H. REEVE ANGEL & COMPANY, 15 NEW BRIDGE STREET, LONDON, E.C. CHEMICAL News, June 1, 1917 THE Volumetric Determination of Sulphur in Pyrites. CHEMICAL VOL. CXV., No. 3001. NEWS 253 Lunge refers to several volumetric methods proposed by various authors, but does not find a satisfactory usefulness in any of them. Wilderstein (Zeit. Anal. Chem., i., 432) was the first to propose that sulphuric acid might be determined by a standard solution of barium chloride. Teschemacher and Smith (CHEMICAL NEWS, xxiv., 61) took up the same idea, especially for the analysis of pyrites. Compare C. and J. VOLUMETRIC DETERMINATION OF SULPHUR also Glendinning and Edgar (Ibid., p. 140). IN PYRITES. By T. J. I. CRAIG, D.Sc. Introduction. MANY analysts have investigated and proposed various methods of ascertaining the precentage of sulphur in pyrites. In cases of any importance pyrites samples are almost invariably tested for sulphur by gravimetric analysis; the sulphur being converted into sulphuric acid and then precipitated and weighed as barium sulphate. . Simple as this method appears to be, it is hedged round by several sources of error, and much controversy has been published by one and another as to this or that modification giving the most accurate result. A reliable and simple volumetric method which at the same time is fairly rapid would be decidedly useful. The author of this paper being in a laboratory where many samples of pyrites are regularly analysed by gravi metric methods, and the reduced staff becoming hard pressed for time owing to the European War, looked about for a suitable accurate volumetric method which would assist in obtaining the results more rapidly and with at least equal reliability. The idea arose that after dissolving the pyrites in aqua regia (or by other suitable method) it might be possible to eliminate all acids other than sulphuric acid and determine the latter by titration with standard alkali. Investigations were carried out, and it was found to be quite feasible, when using aqua regia as solvent, to completely remove in the course of the analysis, first the nitric acid, then the hydrochloric acid, and finally the arsenic acid, leaving only the sulphuric acid, for the determination of which a rapid and accurate method was worked out on the above lines. Subsequent experiments showed that a more convenient method of attacking the pyrites is to treat it with dilute nitric acid, and finish off by adding a few cc. of bromine and boiling until all excess bromine is eliminated. By this means no hydrochloric acid is introduced, all the bromine is boiled out, and the nitric acid is practically completely eliminated on taking to dryness, moistening with water, drying again, and heating for some time at 110° C, Accounts are hereinafter given of experimental investigations carried out with regard to the aqua regia method of oxidation, and also the dilute nitric and bromine method, which latter appears to be a new suggestion. Historical. G. Lunge goes pretty fully into the gravimetric (BaSO4) method, beginning on p. 89 in vol. i. of the 4th (1913) edition of his well-known publication on “Sulphuric Acid and Alkali"; W. S. Allen and H. B. Bishop read a comprehensive paper on this method to the Eighth International Congress of Applied Chemistry (vol. i., Sect. 1, p. 33); and in 1915 H. C. Moore also thoroughly investigated the method and published the results in the Fourn. of Ind. and Eng. Chemistry (vii., No. 7, p. 634) Other investigators who have taken the matter up are:Gladding (Fourn. Am. Chem. Soc., 1896, [5], xviii., 446, and Journ. Soc. Chem. Ind., xv., 617); Pattinson (Journ. Soc. Chem. Ind., xxiv., 7); the Committee of the Chemical Society of North France, who reported to the Fifth Inter national Congress of Applied Chemistry, Berlin (Zeit. Angew. Chem., 1903, [23], xvi., 541); Knorre (Chem. Ind., 1905, xxviii., 2); and E. T. Allen and Johnstone (Fourn. Ind. Eng. Chem. 1910, ii., 196). Beringer introduced the addition of sodium acetate and acetic acid into this titration (CHEMICAL NEWS, lix., 41). Lunge refers to various other methods proposed by Mohr, Clemm, Wilderstein (second method), Schwartz, and Pappenheim, and which are described in the treatise of Fresenius and Mohr. The method proposed by Wilsing (Chem. Ind., 1886, p. 25) has a slight but apparently useful modification of those just mentioned. L. W. Andrews (Am. Chem. Journ., 1889, p. 567) proposed to determine sulphuric acid combined with bases by adding a solution of pure barium chromate in hydrochloric acid to the hot dilute sulphate solution, and then ammonia, and filtering of the liquid now containing chromate equivalent to the SO3. The chromate is determined by means of potassium iodide and decinormal thiosulphate. U. S. Borge and T. Sotgia (Atti. v. ist. Veneto, 1912, lxxi., [2], 1097, and Chem. Abs., x., 1308) publish a modification of Andrew's method. Reuter (Chem. Zeit., 1898. p 357) 2nd Marbourin and Moulinié (Chem. Centr., 1898, i., 218) describe similar analytical processes. Raschig (Zeit. Angew. Chem., 1903, [24], xvi., 818) determines sulphur in pyrites by dissolving in aqua regia, drying, boiling for five minutes with 50 cc. of 1 per cent hydrazine hydrochloride, diluting to 250 cc., pouring 50 cc. into 400 cc. benzidine solution, filtering after five minutes, washing with not more than 20 cc. H2O, and titrating the precipitate with N/10 alkali and phenolphthalein. Zehetmayr (Zeit. Angew. Chem., 1910, xxiii., 1159) proposed to heat pyrites with metallic iron, treat the result with HCI, and titrate the H2S given off by means of iodine and thiosulphate. E. Martin (Monit. Scient., 1913, iii., 686) gives the following method: The ground sample (0·5 grm.) is treated with aqua regia (25 cc.), and after several hours sodium chloride (1 grm.) is added and the mixture evaporated to dryness. The residue is treated with HCI (4 cc.). 50 cc. boiling water are added, and then sodium carbonate (4 grms.) in small portions at a time. The filtered solution is made neutral to methyl-orange, and boiled to expel CO2. 60 cc. of Ba(OH)2 solution (45 grms. per litre), and some phenol. phthalein are added, CO2 is passed through until the colour is just discharged, the precipitate is filtered off, washed with warm water, and the filtrate after cooling is titrated with N/2 HCI. Further details are given of certain precautions necessary to the method (Monit. Scient., 1914, iv., 86). Considerable excess of Na2CO3 must be present, and the CO2 used should be just enough to discharge the phenolphthalein pink colour. Several methods have been proposed for determining the available sulphur; e.g., W. G. Mixter (Am. Chem. Journ., ii., 396) burns the pyrites in a curret of oxygen and passes the vapours into a mixture of bomine and hydrochloric acid. Lunge suggests using neutral hydrogen peroxide to absorb the vapours and then finding the total acidity by titration with alkali. He gives a complete list of methods on these lines, and they appear in his "Sulphuric Acid and Alkali,” 4th edition, p. 100 et. seq. Various methods have been proposed for obtaining pyrites sulphur in the form of SO3. The finely-ground pyrites may be dissolved by means of fuming nitric acid, aqua regia, potassium chlorate and hydrochloric acid, or potassium chlorate and nitric acid. Lunge himself recommends aqua regia (1 HCl to 3 or 4 HNO3), using 50 parts per 1 of pyrites. Some chemists recommend a solution of bromine in hydrochloric acid. Lunge, however, has found this to be unsuitable. Drown (CHEMICAL NEWS, xliii., 89) heats the pyrites with 1.25 sp. gr. NaOH, adds bromine, then HCI, and takes to dryness Noaillon (Zeit. Angew. Chem., 1897, p. 351) uses ÑaCIO3 and HNO3. Allen and Bishop (loc. cit.) recommend oxidation of the pyrites by means of a mixture of 2 parts by volume of liquid bromine and 3 parts of carbon tetrachloride (free from S), then in fifteen minutes 15 cc. HNO3 are added, and after another fifteen minutes gentle heat is applied till all action ceases and the solution is evaporated to dryness and treated with HCl as usual. Fresenius (Zeit. Anal. Chem., xvi., 335) prescribes to decompose pyrites in the dry way by fluxing with 20 parts of a mixture of 2 parts anhydrous Na2CO3 (free from S), and I part KNO3, but according to Lunge this process and its subsequent treatment is much more troublesome and tedious than the wet way oxidation by means of aqua regia. Moreover, the sulphur of the galena and barium sulphate which are useless to the manufacturer are determined with the useful sulphur which is combined with the iron and copper. Gladding describes a comparison of the fusion and aqua regia methods (Fourn. Am. Chem Soc., xvi., 398). To obtain the sulphur of the pyrites in solution the author for the first series of experiments employed the well known method described by Lunge of treating the finely ground ore with aqua regia. The weighed portion of the sample is first treated with the HCl (1 part), then the HNO3 (3 parts) is poured in, and the mixture gently warmed till action commences. It is then set aside away from the heat till the action is over, after which the vessel is returned to the steam-bath and left covered up till all action ceases, and the solution is then allowed to evaporate to dryness. Hydrochloric acid is now added and evaporation to dryness again carried out. Finally, the dried mass is heated to 110° C. for one hour, and then dissolved in distilled water, and the insoluble matter is filtered off and washed For those who prefer it, the solution method of Allen and Bishop may be adopted. The author, however, was not favourably placed for obtaining a suitable sulphur free carbon tetrachloride, and therefore employed the aqua regia method, which was always found quite satisfactory, at all events so far as concerns the dissolving of the sulphur. It was observed in the Allen and Bishop method of attack that no bromine was left in the dried residue, thus leaving only nitric and arsenic acids to be removed in order to obtain the sulphuric acid as the only acid which would be determined by the titration with standard alkali and phenolphthalein as indicator. A few experiments were made to observe the behaviour of nitric acid alone on finely-ground pyrites, and it was found that strong HNO, acted with great violence and much frothing. By diluting the HNO3 with its own volume of distilled water the action went on smoothly and quite energetically enough. A noticeable amount of sulphur was left undissolved. It was found, however, that this sulphur could be easily and completely dissolved by cooling the liquid to about 50° C., adding a few cc. of bromine, and shaking round for a minute or so; the bromine gathered up the sulphur, and by adding a little more HNO3 and bringing to the boil complete oxidation | took place. The solution dried quickly, leaving a residue from which all the nitric acid could be driven out by thoroughly drying on the steam, moistening with water, and drying again, and then heating for an hour in the air-bath at 110° C. This method was finally adopted, and is recommended for dissolving pyrites to obtain a solution containing the useful sulphur in the form of neutralisable sulphuric acid which could then be determined by titration with standard alkali and phenolphthalein as indicator in the manner hereinafter described. It is recommended also as a satisfactory method of dissolving pyrites for gravimetric determination of the sulphur. Pyrites, FeS2, when dissolved in aqua regia yields ferric sulphate and free sulphuric acid : 4FeS2+1502+2H2O2Fe2(SO4)3+2H2SO4. Natural pyrites as used for sulphuric acid manufacture usually contains sulphur in excess of that required to form normal salts with the bases which are present, and thus there is a certain proportion of free sulphuric acid after the oxidation by means of aqua regia or other such method in which no extraneous bases like alkalis have been introduced. This free acid assists in expelling the volatile acids when the solution is taken to dryness. It is well known that all the nitric acid can be eliminated from the aqua regia solution of pyrites by adding HCI and evaporating to dryness. To remove all the hydrochloric acid is not so easy, as even after repeated evaporations with water a small yet appreciable amount clings to the residue. The author finds, however, that this HCl is completely removed from the solution by the addition in the cold of pure moist silver oxide. The resulting silver chloride may be easily filtered off and washed quite free from SO3, and thus solution of pyrites can be obtained containing all the useful sulphur in the form of neutralisable SO3, and free from other acids with the exception. of arsenic acid when arsenic acid is in the pyrites. This arsenic acid (as will be subsequently demonstrated) is carried out of the sphere of action in an insoluble inactive form united with or adhering to the ferric hydrate which is formed when the solution is being neutralised with standard alkali. Lockemann and Paucke (Zeit. Chem. Ind. der Kolloide, 1911, viii., 273) have shown that ferric hydrate precipitated by ammonia completely removes small amounts of arsenic from solution. They state that this property is diminished when soda or potash is used instead of ammonia, and give as their view of the cause that the ferric hydrate takes up a large amount of alkali and thereby the attraction for arsenic is lessened. The author of this paper, however, has investigated this point, and finds that ferric hydrate precipitated in the cold by a small excess of fixed caustic alkali retains only a slight amount of the latter, which is completely dissociated on boiling with water. It would appear to be more likely in the case of Lockemann and Paucke that the weakened adsorptive power for arsenic is due to the greater affinity of the arsenic for the excess of free alkali in solution. The author finds that with a relatively large amount of iron present and no great excess of alkali there is substantially complete removal of the arsenic by the ferric hydrate. In this way the arsenic acid is rendered inert and is not determined with the sulphuric acid. After dissolving the pyrites any sulphur combined with barium and lead (and therefore useless to the manufacturer) will be found in the insoluble matter as barium and lead sulphate, and these are filtered off. If calcium is present in the pyrites it will form calcium sulphate, and a part of it at all events will be soluble, and in gravimetric determinations will increase the BaSO4 to a certain degree. Lunge (Fourn. Soc. Chem. Ind., iv., 31j has given the solubility of CaSO4 in different strengths of HCI; e.g., 100 cc. of a 3 per cent HCl solution dissolve 1.26 grms. CaSO4 at 25° C. By the herein proposed volumetric method, any such sulphuric acid combined with calcium (and therefore representing useless sulphur) will not be determined even although in solution, and therefore this test gives a result fairer from the manufacturer's point of view. (See Lunge, Fourn. Soc. Chem. Ind., iv., 449, 924). Maurice de Lummen in a paper on "The Roasting of Blende" in the Chemical Trade Journal (May 18, 1916, No. 1504, lviii., 255), describes the behaviour of compounds of zinc, lead, iron, calcium, magnesium, barium, and fluorine when mineral sulphides containing such are roasted. With reference to zinc sulphides Lunge states that with well conducted roasting all the S passes off mainly as SO2 and a little as SO3. Lead sulphide goes into sulphate, iron sulphide is entirely decomposed, and all the S comes off. If lime is present it goes into sulphate in the burning of the pyrites; so also with barium and magnesium. Lunge, however ("Sulphuric Acid and Alkali," 1903, i., 77, Part I., Third Edition), reckons that sulphur combined with zinc CHEMICAL NEWS,} June 1, 1917 Main Lines of Advance in Analytical Chemistry. in pyrites is useless for SO, making, as the sulphate of zinc is not decomposed at the temperature of a pyrites burner, and further states that in France half of the S combined with zinc is considered as lost, and for copper they reckon o 505 per cent S per 1 per cent Cu as lost. There would thus appear to be some uncertainty as to what really happens to the sulphur united with zinc and copper in pyrites when the latter is roasted. The sellers of pyrites have therefore some excuse for claiming payment for such sulphur but none for including S equivalent to the lime which may be present. So far as the author has been able to ascertain no one appears to have published a volumetric method of determining sulphur in pyrites on the lines of taking the aqua regia or similar solution and titrating it with standard calcium chloride baths as shown. In b a pale intense yellow liquid condenses, the yield being about 50 per cent of the theoretical quantity. It may be found desirable to cool the necks of the retort and receiver by applying cold wet cloths. The nitrosomethylurethane was purchased from Germany, as the experiment was made some years before the war, after reading Von Peckman's paper (Ber.. 1895, xxviii., 855). Von Peckman refers to the nitroso compound in the following words : "Nitrosomethylurethane.-Klobbie got this combination discovered by him through treating urethane with nitrite and sulphuric acid. For the manipulation of large quanti: ties of this body it is more judicious to conduct the red vapours developed through arsenic and sulphuric acid while cooling the same when pasaing into the rarefied pure urethane with an equal volume of ether till the fluid has assumed a dirty grey colour. Then it is washed with water and soda, dried with sodium sulphate, and finally rectified in a vacuum. The latter is not a necessary pro cedure when preparing diazomethane. In working with the nitroso combination one cannot be too careful, on 255 account of its danger already mentioned by Klobbie, to In the same paper Von Peckman speaks of diazomethane as follows: He says that the etheric solution (substantially as prepared above) is intensely yellow at a strength of from 3 to 5 per cent. "At ordinary temperatures diazomethane is a yellow gas which can be condensed by cold after a preliminary attempt. To this end the procedure was as above, but instead of ether waterfree glycerin was employed, and through the whole system a stream of hydrogen was passed; the disappearing gases left behind in a snow chloride of calcium mixture dark yellow little drops which, when removed from the freezing mixture, were in active boiling at about o° C." Caution. The nitroso compound and the diazomethane are highly poisonous substances, and the vapours therefrom should not be allowed to come into contact with any part of the body or face. One or two whiffs of the vapours from the etheric solution of diazomethane striking the fingers may render them so tender in two days that it will be difficult to pick up a pin. The greatest precautions are therefore necessary when handling such poisonous substances. SOME MAIN LINES OF ADVANCE IN THE By A. CHASTON CHAPMAN. ANALYTICAL chemistry has often been referred to as the handmaiden of the other branches of our science, and whilst this was an entirely unobjectionable description in so far as it implied indispensable assistance, it was a little unfortunate in that it carried with it a certain suggestion of inferiority. From the dawn of scientific chemistry in the seventeenth century to a period within the recollection of a good many chemists who are still happily among us, analytical chemistry was almost synonymous with chemistry itself, and it is only in comparatively recent times that it has become a separate branch of applied chemistry with its own literature, its own aims, and its own specialised practitioners. Whilst the division of chemistry into various separated branches became inevitable with the enormous development of the science, and had its obvious conveniences, the progressive subdivision of work has not been without its drawbacks, and even its dangers. That it has conduced to a narrowness of outlook and to a mental onesidedness is undeniable, and it is becoming increasingly difficult always to maintain a just sense of proportion to view facts in their true perspective and to keep a firm hold on fundamental principles. Of all the various members of the body chemical, perhaps none suffered more at first by this process of subdivision than analytical chemistry. During the first half of the last century, chemical analysis occupied a very high position, since not only was it clearly the foundation-stone on which the whole chemical fabric was built, but almost every chemist of distinction practised it assiduously and devoted much of his time to a study of its problems. One need only recall in this connection such names as those of Berzelius, Gay-Lussac, Marignac, Bunsen, Dumas, Stas, Liebig, and Wöhler. With the birth of modern organic chemistry and its colossal development during the past * A Lecture delivered before the Chemical Society, March 15, 1917. |