ISSN:0003-2654    

DOI:10.1039/AN95681FX019

出版商:RSC    

年代:1956

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN95681BX023

出版商:RSC    

年代:1956

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN95681FP041

出版商:RSC    

年代:1956

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN95681BP045

出版商:RSC    

年代:1956

数据来源: RSC

摘要:

APRIL, 1956 Vol. 81, No. 96t THE ANALYST The Documen tation of Molecular Spectra THE Iiifra-red Absorption Data Joint Committee, which was set ~ i p in 1951 by the Chemical Society to advise on the publication of spectral data, made in 195% an interim report with recommendations designed to raise the quality of infra-red absorption spectra published in scientific journals. These recommendations have since been accepted in principle by most British societies, and the relevant parts were adopted by the Publication Committee so t h a t the format of published spectra should be standardised (see zlkzalysf, 1953, 78, 684). The recommendat ions, however, placed considerable limitations on the publication of infra-red spectra, and in its report the Committee drew attention to a consequential need, namely, the preparation in some form of a register or card index of the spectra of pure compounds ; the Committee has since studied various British, ,4n;erican and European schemes for cataloguing and indexing spectra. A system of Keysort punched cards, in which the holes around the edges are coded and slotted according to the spectral and structural characteristics of the compound and in which the cards can be sorted by manual methods, appeared to offer the best solution.The Committee then learnt of a somewhat similar scheme that was being developed by the Gesellschaft fur Spektrochemie und angewandte Spektro- skopie in Germany. I t became clear that a union of the German and British schemes would be mutually beneficial and might have advantages leading to a collection of universal value.A joint plan has now been.agreed upon and the first cards are about to be distributed (see H. W. Thompson, J . Chem. Soc., 1955, 4501). Two identical editions of Keysort cards will be published in English and German, the publishers being Butterworths Scientific Publications, London, and Verlag Chemie, Weinheirn, respectively. Initially, the cards will deal only with infra-red and Raman spectra, but it is hoped to extend the scheme later to ultra-violet spectra. Two separate cards, namely, the spectral and literature cards, will be issued in each edition. The molecular and structural formulae, the main physical properties and the spectrum of the compound are printed on the spectral cards; literature references that may relate either to spectra described on the spectral cards or to important developments in either technique or theory are recorded on the literature cards. The 203 holes around the edges of the spectral card are coded for the structural features of the compound and for its principal absorption bands; those around the edges of the literature card are used to record names of authors, year of publication, an indication of the nature of the work (e.g., analysis) the spectral range studied, the state of aggregation of the sample and details of either the equipment, method or theoretical aspects discussed in the paper. It is to be hoped that this Anglo-German venture will provide a satisfactory basis for a wide international scheme of cataloguing spectral data. J. E. I’AGE 186

ISSN:0003-2654    

DOI:10.1039/AN9568100185

出版商:RSC    

年代:1956

数据来源: RSC

摘要:

186 A VISIT TO CAMBRIDGE [Vol. 81 A Visit to Cambridge A FEW weeks ago some of us spent a very enjoyable morning revisiting favourite places at Cambridge, such as Trinity Hall, King’s College Chapel, the Backs, the old-fashioned market and Parker’s Piece, where, it is understood, Jack Hobbs first played the game. We then went along to meet a very distinguished crowd of visitors to Cambridge, including Professors of Physical Chemistry, industrial users of the polarograph, Government representatives and lecturers from Universities and Technical Colleges. After this reception we were fortunate to hear a very interesting talk by the Research Director of a particular firm that makes polarographic equipment. This was concerned in the main with the development of the polarograph; one of the original polarographs was on view.We like the story the Research Director told of how, in the early days of the development, a foreign visitor was working there. When he saw the step due to a particular ion being traced and re-traced on the polarograph, he had only one expression for it, this being “what nice.” A new Polarographic Research Laboratory was then opened by Professor J. Heyrovsk3, who made no secret of the fact that for him “all is polarography.” A tour was made of the Polarographic Research Laboratory and the General Research Laboratories of the company and this was followed by a visit to the University Research Laboratories. It seemed to us that an industrial company making chemicals takesit for granted that technical service about its products should be provided for the customer.Here was a company of instrument manufacturers setting up a laboratory to give just such service, ie., to carry out research on improvements of the instrument and, more important for the analyst, to indicate how the polarograph can be used to carry out particular tests. How often have we gone along to instrument manufacturers who can prove to us that they have manufactured a first-class instrument but who seem relatively unconcerned about its use. In short, it seems to us to be a good thing that trained analysts should be finding their way into firms making polaro- graphs and other equipment; it must be noted that other polarographic firms in this country are showing equal keenness. There were other questions that interested us ; for example, what is to be the real function of the polarographs finding their way on to the market, i.e., such instruments as the cathode- ray polarograph, the square-wave polarograph and the polarograph with the Univector attachment. Are we only to gain in sensitivity and specificity by their use, or is the time coming when the polarograph will be so accurate an instrument that same of our traditional volumetric and gravimetric procedures will have to take second place to the polarographic method ? Travelling home after a full day we felt that our time had been well spent. J. HASLAM

ISSN:0003-2654    

DOI:10.1039/AN9568100186

出版商:RSC    

年代:1956

数据来源: RSC

摘要:

April, 19561 PROCEEDINGS 187 PROCEEDINGS OF THE SOCIETY FOR ANALYTICAL CHEMISTRY ORDINARY MEETING AN Ordinary Meeting of the Society was held at 7 p.m. on Wednesday, April 4th, 1956, in the meeting room of the Chemical Society, Burlington House, London, W.l. In the absence of the President, the Chair was taken by Dr. D. W. Kent-Jones, F.R.I.C. The following papers were presented and discussed : “The Determination of 4-Chloro-2- methylphenoxyacetic Acid in MCPA by a Differential Refractometric Method,” by R. Hill, B.Sc., A.R.I.C. ; “Paper Chromatography with Continuous Change in Solvent Composition. Part I: Separation of Fatty Acids. Part 11: Separation of Surface-active Agents,” by F. Franks, B.Sc., A.R.I.C. NEW MEMBERS ORDINARY MEMBERS Aubrey Edward Alldridge, BSc. (Nottingham) ; William Jeffery Beer; Kai-Chih Chou, B.Sc.; Stuart Collings, A.R.I.C. ; William Dunnet, A.R.I.C., A.H.-W.C. ; Thomas Talbot Gorsuch, BSc. (Lond.), A.R.I.C. ; Ernest Guy, B.Sc. (Lond.) ; Frederick Wesley Hares, A.R.I.C. ; Brian Percival Harold James, B.Sc. (Southampton), A.R.I.C. ; Anthony Harold Latimer, B.Sc. (Lond.) ; Robert Donald MacDonald, B.Sc. (Lond.) ; William James Murray; Michael William Robertson, B.Sc. (Lond.) , A.R.I.C. ; Kenneth Charles Sellers, B.Sc. (Lond.), Ph.D. (Cantab.), M.R.C.V.S., D.V.S.M.; Jack Stanley Wright. JUNIOR MEMBER Donald Joseph Coplin, B.Sc. (Nottingham). DEATH David Agnew Griffith. We record with regret the death of NORTH OF ENGLAND SECTION AN Ordinary Meeting of the Section was held at 2.15 p.m. on Saturday, March 24th, 1956, at the City Laboratories, Mount Pleasant, Liverpool.The Chair was taken by the Chairman of the Section, Mr. J. R. Walmsley, A.M.C.T., F.R.I.C., F.P.S. The following paper was presented and discussed : “New Reagents and New Developments in the Fine Chemical Field,” by W. C. Johnson, M.B.E., F.R.I.C. SCOTTISH SECTION AN Ordinary Meeting of the Section was held a t 7.15 p.m. on Thursday, March 22nd, 1956,- at the Central Station Hotel, Glasgow. The Chair was taken by the Chairman of the Section, Dr. F. J. Elliott, F.R.I.C., F.R.S.E. The following papers were presented and discussed: “The Determination of Calcium in Plant Material by Flame Photometry,” by R. G. Hemingway, M.Sc. ; “The Flame Photometer in Silicate Analysis,” by A. J. Shorter, M.Sc., M.S., M.Inst.F., A.R.I.C. ; “A New System of Reporting and Recording Analytical Results,” by A.0. Pearson, B.Sc., F.R.I.C. MIDLANDS SECTION AN Ordinary Meeting of the Section was held at 7 p.m. on Tuesday, March 6th, 1956, in the Main Chemistry Theatre, The University, Edgbaston, Birmingham 15. The Chair was taken by the Chairman of the Section, Mr. J. R. Leech, J.P. A lecture was given on “Modern Qualitative Analysis and Industrial Practice” by Professor Dr. C. J. van Nieuwenburg. MICROCHEMISTRY GROUP THE fourth London Discussion Meeting of the Group was held on Wednesday, February 22nd, 1956, at 6.30 p.m., in “The Feathers,” Tudor Street, London, E.C.4. In the absence of the Chairman of the Group, the Chair was taken by the Honorary Secretary, Rfr. D. W. Wilson, M.Sc., F.R.I.C.188 OBITUARY ;YOL S I Dr.R. Belcher, F.R.I.C., F.Inst.F., and Mr. H. Holness, M.Sc., F.R.I.C., introduced the subject of “Small-scale Qualitative Inorganic Analysis,” after which there was an informal discussion. PHYSICAL METHODS GROUP THE 52nd Ordinary Meeting of the Group was held at 6.30 p.m. on Tuesday, February 14th, 1956, in the meeting room of the Chemical Society, Burlington House, London, W.1. The Chair was taken by the Chairman of the Group, Dr. J. E. Page, F.R.I.C. The subject of the meeting was “Polarography” and the following papers were presented and discussed: “A Comparison of Three Highly Sensitive Polarographs,” by D. J. Ferrett, D.Phil., G. W. C. Milner, M.Sc., F.R.I.C., H. I. Shalgosky, BSc., A.R.I.C., and L. J. Slee, B.Sc. ; “Polarography of the Dithionite (Hydrosulphite) Anion and Some Related Oxyacids of Sulphur,” by W. Furness, B.Sc., Ph.D., F.R.I.C. ; “The Polarographic Determination of Uranium in Ores,” by H. I. Shalgosky, R.Sc., A.R.I.C.

ISSN:0003-2654    

DOI:10.1039/AN9568100187

出版商:RSC    

年代:1956

数据来源: RSC

摘要:

188 OBITUARY Obituary HORATIO BAL1,ANTYNE HOKATIO BALLANTYNE, whose fame was as an expert witness and who was acknowledged as one of the greatest chemical authorities on patents, died at his home near Tadworth, Surrey, on January 25th, at the age of 84. His reputation as a chemist in the Courts is so well known that it seems unnecessary to emphasise this. There were few important cases, patent or otherwise, in which he did not appear. Quiet, unassuming and cool, he was a model for all expert witnesses. I t was therefore not surprising that The Times of January 27th reported a moving appreciation of him by that great patent lawyer, Mr. Justice Lloyd- Jacob. Born in Glasgow in 1871, Ballantyne’s early training as a chemist was in the laboratory of the Glasgow City Analyst. He had further experience.with Wallace, Tatlock and Clark and later with Tatlock, Thomson and Redman. It was in 1896 that Ballantyne, still a young man, started his practice in London, and he soon acquired a deservedly great reputation. Most of us only remember him from the early years of the century and particularly after the first World War, when he was at the height of his fame and greatly in demand, since he had a lawyer’s appreciation and understanding of patents and patent law, in addition to his excep- tionally wide chemical knowledge and experience. Indeed it was partly the width of his knowledge that made him so able and so excellent a witness; fortunately he also had the ability to expound his views clearly and convincingly. It is well known that he gave up his practice in 1928 to join the Board of Unilever Ltd., a Company for whom he had acted and whom he advised on many, almost historic, occasions in court. Ballantyne was a member of many Government Committees; he will be expecially remembered in this phase of his career as a member of the Inter-Departmental Committee dealing with awards for inventions by Government servants and as a member of the Board of Trade Committee on Patent Law and Practice in 1929.What may not always be remembered was that Ballantyne was a keen supporter of the Society and that he was a member of Council three times, namely in 1903-04, 1910-11 and 1933-34. He appreciated to the full the value of the information given by proper chemical analysis. Undoubtedly his early training in the laboratories of public analysts had made a great impression on his mind. It was a privilege to see him as he went round the laboratory of a firm, probably in connection with a pending patent action, quietly and unobtrusively, but never failing to grasp the essentials of the problem in hand. He was a Fellow of the Royal Institute of Chemistry and served as a member of Council twice, 1899-1901 and 1915-1918, and was also twice a Vice-president, 1918-1919 and 1920-23. Naturally the Society had not seen much of him in recent years and indeed he only occasionally visited Unilever House, but he still kept in close touch with chemical literature Many of us are proud simply to recall the privilege we had in working with him on many occasions. D. W. KENT- JONES

ISSN:0003-2654    

DOI:10.1039/AN9568100188

出版商:RSC    

年代:1956

数据来源: RSC

摘要:

April, 19561 HEYROVSK+ 189 The Development of Polarographic Analysis BY J. HEYROVSKY (Shortened version of the lechwe delivered at the meeting of the Society o n TNesday, November 29th, 1955) ALTHOUGH the very first polarogramsl obtained 30 years ago neatly showed the “waves” or diffusion currents due to traces of metals, from which the use of such curves for qualitative and quantative analysis at once became evident, this discovery was not widely known until after 1936. In that year the noted German analyst vc’. Bottger, Professor at Leipzig, published in his textbook “Physikalische Methoden der analytischeit Chemie” the author’s account of the polarographic method. By this publication the polarographic method was acknowledged as an analytical procedure and its description has since been incorporated into analytical textbooks.According to the author’s view, polarography is the science of studying the processes occurring around the dropping-mercury electrode. It includes not only the study of current - voltage curves, but also of other relationships, such as the current - time curves for single drops, potential- time The meaning of the word “polarography” needs to be explained. 12cm l Fig. 1. MaSek’s bent capillarq- Fig. 2 . Smolei’s horizontal capillary curves, electrocapillary phenomena and the streaming of electrolytes, and its tools include, besides the polarograph, the microscope, the string galvanometer and even the cathode-ray oscillograp h. The reason why so much study is concentrated upon the small mercury drop is its unique property of giving exactly reproducible results, i.e., because the mean current passing through the solution is dependent only on the applied potential and is independent of time and of the direction of the polarising voltage, No other electrode has this property, except the streaming-mercury (mercury-j et) electrode. Solid electrodes, e.g., rotating or vibrating platinum electrodes, give a curve that changes when the applied potential is reversed or when the rate at which the voltage increases is varied.As the results are not reproducible, there is no sound experimental basis for a theory; in fact, there are no theoretical considera- tions relating to the electrode processes at solid electrodes. The term “polarography ” is therefore restricted to the mercury capillary electrodes.As regards the shape of capillaries, long capillaries, if connected to the reservoir by flexible tubing, should be bent t o the form (Fig. 1) recommended by Ma3ek2 to prevent kinking of the tube. Recently Smolef’s horizontal capillary3 (Fig. 2) has been found very190 HEYROVSK~ : THE DEVELOPMENT OF [Vol. 81 suitable; the drop is regular, with no instability such as it has when it hangs from a vertical capillary. The curves are consequently very smooth and show no disturbances. At first the only polarographic determinations possible were of substances that were themselves depolarisers, i.e., that were reducible or oxidisable a t the capillary electrode. Many organic compounds that are polarographically inactive may, however, be changed into depolarisers by one of the following methods, the first of which was introduced by Fieser et aZ.4 for some ketosteroids (only ketones with conjugated double bonds are reducible).By applying Girard’s T or D reagents the ketosteroids yield reducible hydrazones. Ketones are condensed-according to Zuman5-with secondary amines to ketoimines, which are reduced at positive potentials. Inactive aromatic compounds, such as benzene or pheno- barbital, are nitrated ; morphine-according to Baggesgaard-Rasmussen6-is converted by nitrous acid into a nitroso derivative. Other compounds are converted into reducible substances when treated with halogens, e.g., methionine when treated with iodine. The growth factor leucovorin yields anodic and cathodic waves after oxidation by air in hydro- chloric acid solution.For chlorates Koryta and ‘I‘engyl’ found a suitable catalyst in an oxalic - sulphuric acid solution of titanous ions. These ions are oxidised by chlorate to the titanic state, but are instantly reduced electrolytically at the cathode. In turn they are oxidised again, and in this cyclic reduction produce a cathodic wave that gives a quantitative measure of the chlorate ions. A number of methods have been found of producing the derivative curve, d i / d E against E , which indicates the half-wave potentials b@.ter than the primitive current - voltage curve itself does. The best is that of Vogel and Kiha,8 who used a circuit in parallel with the electrolytic cell in which a condenser of about 3000 pF capacity and the galvanometer are placed.The highest sensitivity is reached by the Barkerg square-wave polarograph, in which the capacity current is entirely eliminated. This instrument records curves with a pen on paper and reveals traces of some cations in concentrations clown to 10-8M in the presence of a great excess of nobler constituents, e g . , copper or tervalent iron. The increasing demand for continuous and automatic control of solutions during industrial reactions directs attention to continuous-recording indicators. For these a most suitable reference electrode has been found in the dropping-mercury electrode, which in chloride solutions maintains the potential of the calomel electrode. I t also keeps a constant potential in redox mixtures or strong oxidising agents such as ferric ions in dilute nitric acid; the reference electrode may be kept in a different solution from that being analysed and a slow flow has to be maintained.In this way the concentration of metallic ions or nitro bodies has been found from the recorded diffusion current at constant voltage. Such simple continuous recording is especially important when the concentration of carbon monoxide in the atmosphere has to be determined. The gas to be analysed for carbon monoxide is slowly passed over heated iodine pentoxide and the iodine liberated is absorbed in a dilute alkaline solution of bromate, which oxidises iodine to iodate.1° A constant stream of a sodium sulphite solution removes the hypobromite and the atmospheric oxygen, and the final solution flowing around the dropping-mercury cathode and dropping-mercury anode produces the marked wave of iodate under the constant applied potential of 1.3 volts.Thus 0.01 to 0.0001 per cent. v/v of carbon monoxide can be recorded. For continuous recording solid electrodes are also suitable as reference electrodes, provided that under the passage of the current they do not yield solid products, e.g., platinum electrodes surrounded by an acid solution of ferrous and ferric sulphates (0.1 N ) or an alkaline solution of ferrocyanide and ferricyanide. By means of the latter the indicating dropping electrode records concentrations of cyanide ions up to 0.4 M. For physiological researches into the amount of oxygen in living tissues, SerAk’s vessel with the dropping electrode is suitable.As shown in Fig. 3, a piece of the tissue is placed on the cellophane, through which oxygen easily diffuses, and the loss of oxygen from the physiological solution is indicated by the current passing through the dropping-mercury cathode and silver-wire anode. Instead of the silver electrode another dropping-mercury electrode may be used. For example, for the deter- mination of sulphur Harrison and Harveyll introduced ammonium acetate in glacial acetic acid. For the determination of sulphur dioxide in sulphuric acid 100 per cent. sulphuric acid may in practice be used. Non-aqueous solvents are applied mostly to dissolve organic Before readings are taken the solution must be stirred. Non-aqueous solutions are used whenever convenient.190 HEYROVSK~ : THE DEVELOPMENT OF [Vol.81 suitable; the drop is regular, with no instability such as it has when it hangs from a vertical capillary. The curves are consequently very smooth and show no disturbances. At first the only polarographic determinations possible were of substances that were themselves depolarisers, i.e., that were reducible or oxidisable a t the capillary electrode. Many organic compounds that are polarographically inactive may, however, be changed into depolarisers by one of the following methods, the first of which was introduced by Fieser et aZ.4 for some ketosteroids (only ketones with conjugated double bonds are reducible). By applying Girard’s T or D reagents the ketosteroids yield reducible hydrazones. Ketones are condensed-according to Zuman5-with secondary amines to ketoimines, which are reduced at positive potentials. Inactive aromatic compounds, such as benzene or pheno- barbital, are nitrated ; morphine-according to Baggesgaard-Rasmussen6-is converted by nitrous acid into a nitroso derivative.Other compounds are converted into reducible substances when treated with halogens, e.g., methionine when treated with iodine. The growth factor leucovorin yields anodic and cathodic waves after oxidation by air in hydro- chloric acid solution. For chlorates Koryta and ‘I‘engyl’ found a suitable catalyst in an oxalic - sulphuric acid solution of titanous ions. These ions are oxidised by chlorate to the titanic state, but are instantly reduced electrolytically at the cathode. In turn they are oxidised again, and in this cyclic reduction produce a cathodic wave that gives a quantitative measure of the chlorate ions.A number of methods have been found of producing the derivative curve, d i / d E against E , which indicates the half-wave potentials b@.ter than the primitive current - voltage curve itself does. The best is that of Vogel and Kiha,8 who used a circuit in parallel with the electrolytic cell in which a condenser of about 3000 pF capacity and the galvanometer are placed. The highest sensitivity is reached by the Barkerg square-wave polarograph, in which the capacity current is entirely eliminated. This instrument records curves with a pen on paper and reveals traces of some cations in concentrations clown to 10-8M in the presence of a great excess of nobler constituents, e g ., copper or tervalent iron. The increasing demand for continuous and automatic control of solutions during industrial reactions directs attention to continuous-recording indicators. For these a most suitable reference electrode has been found in the dropping-mercury electrode, which in chloride solutions maintains the potential of the calomel electrode. I t also keeps a constant potential in redox mixtures or strong oxidising agents such as ferric ions in dilute nitric acid; the reference electrode may be kept in a different solution from that being analysed and a slow flow has to be maintained. In this way the concentration of metallic ions or nitro bodies has been found from the recorded diffusion current at constant voltage. Such simple continuous recording is especially important when the concentration of carbon monoxide in the atmosphere has to be determined.The gas to be analysed for carbon monoxide is slowly passed over heated iodine pentoxide and the iodine liberated is absorbed in a dilute alkaline solution of bromate, which oxidises iodine to iodate.1° A constant stream of a sodium sulphite solution removes the hypobromite and the atmospheric oxygen, and the final solution flowing around the dropping-mercury cathode and dropping-mercury anode produces the marked wave of iodate under the constant applied potential of 1.3 volts. Thus 0.01 to 0.0001 per cent. v/v of carbon monoxide can be recorded. For continuous recording solid electrodes are also suitable as reference electrodes, provided that under the passage of the current they do not yield solid products, e.g., platinum electrodes surrounded by an acid solution of ferrous and ferric sulphates (0.1 N ) or an alkaline solution of ferrocyanide and ferricyanide.By means of the latter the indicating dropping electrode records concentrations of cyanide ions up to 0.4 M. For physiological researches into the amount of oxygen in living tissues, SerAk’s vessel with the dropping electrode is suitable. As shown in Fig. 3, a piece of the tissue is placed on the cellophane, through which oxygen easily diffuses, and the loss of oxygen from the physiological solution is indicated by the current passing through the dropping-mercury cathode and silver-wire anode. Instead of the silver electrode another dropping-mercury electrode may be used. For example, for the deter- mination of sulphur Harrison and Harveyll introduced ammonium acetate in glacial acetic acid.For the determination of sulphur dioxide in sulphuric acid 100 per cent. sulphuric acid may in practice be used. Non-aqueous solvents are applied mostly to dissolve organic Before readings are taken the solution must be stirred. Non-aqueous solutions are used whenever convenient.April, 19561 POLAROGRAPHIC ANALYSIS 191 substances. A suitable soIvent for a number of hydrocarbons, including naphthalene, anthracene, styrene, stilbene and pyrene, is a mixture of 75 per cent. of dioxan with water or butanol. Another suitable mixture is light petroleum, benzene and methanol containing ammonium nitrate. High polarographic accuracy (0.3 per cent.) is attained in polarometric titrations,l2 in which the limiting current of a depolariser serves as indicator.The construction of the titration graph is simple; two points well before the equivalence point and two after suffice for the drawing of two straight lines, the intersection of which gives the end-point. Sandberg13 used sodium dipicrylaminate for the titration of potassium. A clever procedure was used by Langer14 in titrations with fluoride ions. These ions bind the Th”” or Al”’ ions until the nitrate wave, which appears only in the presence of those cations, disappears. Another ingenious method is the use of a “pilot ion” introduced by Ringbom.15 When aluminium, calcium or magnesium are complexed by fluorides, the end-point is shown on the wave of a ferric salt, which is affected by thesurplus of fluoride.KolthofP calls these titrations “amperometric,” and uses his rotating platinum wire as the indicating electrode. Since 1951, polarographic determinations have been coupled with chromatographic separations in quantitative analysis. Lewis and Griffithsl’ carried out their “paper-strip separations” and reached a high degree of accuracy in the analysis of ores. Later, Kemulal* started his “chromatopolarography,” for which he coupled the chromatographic column directly to the polarographic cell. These polarographic results may be photographically registered. In his separations he either elutes the components or uses the frontal method. Finally a new trend in polarographic methods must be mentioned; this makes use of the cathode-ray oscillograph and may be called “oscillographic polarography.” The cathode- ray oscillograph as developed some 20 years ago reached the sensitivity of a polarographic galvanometer.The charging current in oscillography is greater than the electrolytic current owing to the charging of each drop to the requisite potential being 300 times quicker. To avoid this inconvenience, cathode-ray polarographs are constructed to reproduce the polaro- graphic current - voltage curve, but on one drop only. The procedure is first to polarise one drop of 7 or 8 seconds’ duration at a steady voltage for 5 seconds, in which time the charging current becomes small and depolarisers of more positive potential are exhausted : then the voltage is raised through about 0-6 volt in 2 seconds and the peak due to the depolariser to be determined is shown by the luminous current - voltage curve.When the \voltage is reversed, a more or less reversible anodic peak appears. High accuracy is claimed. To avoid interference by the large charging currents in oscillography, the present author started with J. Forejtlg in 1940 to study potential - time curves, I/ against t, with the dropping-mercury electrode. Alternating current from the 50-cycle mains a t a constant amplitude controlled by a large resistance is so adjusted as to charge the dropping electrode in 0.01 second from zero to -2 volts and in the next 0.01 second back to zero, for which about 1 mA suffices. In this way the fluorescent screen shows curves that reveal depolarisers by time-lags or kinks in the potential - time curves and simultaneously denote their reversibility.Thus each depolariser is qualitatively characterised by two kinks, the cathodic and the anodic one, so that this sort of oscillography gives more information about quality than does polaro- graphy. One can increase the sensitivity somewhat by accelerating the time-base to 150,000 cycles per second, when the kinks give rise to luminous lines similar to those in an emission spectrum. The sensitivity is, however, considerably increased by plotting the derivative curves d V / d t against t, or d V / d t against V . Mathematically it is clear that the short horizontal kink will appear on the derivative curve as an indentation. Physically the indentation is caused by the electrolytic current flowing in the opposite direction to the charging current.For this reason also the area of the indentation is a true measure of the quantity, as it gives the number of coulombs required for the electrolytic process. For analytical measurements in oscillographic polarography with alternating current the “electronic polaroscope,” which shows the curve of d V / d t against V , is most suitable. It might be applied in ore analysis, or in the determination of the purity of samples, especially of pharmaceutical products, such as vitamins, hormones and antibiotics, e.g., penicillin G (Fig. 4).20 Owing to the derivative adjustment, the oscillograms have a great resolving power, and can be used for distinguishing many organic isomers that would coincide polaro- graphically.It also serves for detecting traces of noxious gases in factory atmospheres. For this purpose a simple tube with mercury at the bottom is used; this is filled with a suitable192 HEYROVSKY [Vol. 81 electrolyte through which the air is sucked. Small amounts of carbon disulphide, hydrogen cyanide, hydrogen sulphide, sulphurous acid and even acetylene can be detected in this way. To obtain stable figures, the trace must be suppressed for a constant time after the fall of each drop, and a screen with long afterglow must be used. The streaming-mercury electrode has been especially constructed to give stable oscillograms. In organic analysis the figures produced with the streaming electrode are simpler, as no side-reactions or consecutive reactions are possible.In quantitative work empirical calibration curves have to be constructed by plotting the depth of the indentation against the concentration of the depolariser. This may be done conveniently with the luminous axis of Kalvoda,21 a horizontal trace on the cathode-ray tube that can be moved vertically by adjusting a potentiometer. A higher degree of accuracy can be attained with Kalvoda and Macku’s*2 comparative titration procedure. In addition to the solution containing an unknown concentration of the component, a solution containing the same basic electrolyte is titrated until the oscillo- grams with the indentation overlap. Synchronised twin dropping-mercury electrodes are used and the two oscillograms are shown alternately on the screen every twenty-fifth of a second.The sensitivity of the oscillographic method is often as high as that of polarography, revealing traces of depolarisers of the order of moles per litre, especially when adsorptive substances are present, e.g., nitrobenzene or phenosafranine. In this way a 3 per cent. accuracy is possible. 1. 2. 3. 4. 6 . a. P 8. 9. 10. 11. 12. 13. 14. 25. 16. 17. 18. 19. 20. 21. 22. REFERENCES Heyrovskq, J., and Shikata, M., Rec. Trav. Chim. Pays-Bas, 1925, 44, 496, MaSek, J ., Radiometer Polarographics, 1952, 2, 58. Smolei?, I., Coll. Czech. Chem. Comm., 1954, 19, 238. Wolfe, J. K., Hershberg, E. B., and Fieser, L. F., J . Biol. C h e w , 1940, 136, 653. Zuman, P., Chenz. Listy, 1951, 45, 65; 1952, 46, 516, 521 and 599. Raggesgaard-Rasinussen, H., Hahn, C., and Ilver, I<., Pvoc. Int. Cozgr. P w e Appl. Chem., London, Koryta, J., and Tengvl, J., Coll. Czech. Chenz. Covnun., 1954, 19, 839. Vogel, J., and Riha, J., J . Chim. Phys., 1950, 47, 5. Barker, G. C., and Jenkins, I. L., ,Znalyst, 1952, 77, 685. XovAk, J . V. A., Coll. Czech. Chem. Comm., 1955, 20, 1076. Harrison, S., and Harvey, D., Analyst, 1954, 79, 640. Heyrovskl, J., Bull. Soc. Chim. France, 1927, 41, 1224. Sanclberg, R., PYOC. 1st Int. Polavogv. Comgv., Puague, 1951, Part I, 225. Langer, A., I n d . Eizg. Chem., Anal. Ed., 1940, 12, 511. Ringborn, A., and Willrmaii, H., Acta Chem. Sca.tid., 1949, 3, 22. Kolthoff, I. &I., Trans. Electrochem. Soc., 1940, 78, 191. 1-ewis, J. X., and Griffiths, J. M., -4izalyst, 1951, 76, 388. Kemula, W., Rorzn. Chenz., 1952, 26, 281. Heyrovslrgi, J., and Forejt, J., Z. phvs. Chem., 1943, 193, 77. HeyrovslrS;, J., Coll. Czech. Chem. Comiiz., 1933, 18, 739. Kalvoda, R., Ibid., 1955, 20, 1503. Kalvoda, R., and Macku, J., Ibid., 1955, 20, 255. 1947, 11, 125. POLAROGRAPHIC INSTITUTE PRAGUE CZECHOSLOVAK ACADEMY OF SCIENCES Jantiary 2 2 ~ 1 , 1956

ISSN:0003-2654    

DOI:10.1039/AN9568100189

出版商:RSC    

年代:1956

数据来源: RSC

摘要:

April, 19561 FERRETT AND MILNER 193 Analytical Applications of the Barker Square-wave Polarograph Part IX.* The Analysis of Copper-base Alloys and Steels BY D. J. FERRETT AND G. W. C. MILNER Further applications of the square-wave polarograph to the determination of alloying constituents in metallurgical products are described. This new technique is superior to conventional polarography for the analysis of copper-base alloys and steels. For example, the nickel and zinc contents of bronzes can be determined simultaneously in an ammonia - ammonium chloride base solution without any interference from copper, For the determination of lead, the copper ions must be complexed with cyanide and the lead peak can be recorded from an alkaline cyanide base solution. This procedure is useful for the determination of the lead content of samples of which only small quantities are available, e.g., in the examination of copper alloys of historical importance.Simple and direct procedures have been used for the determination of copper, tin and lead in steels by square-wave polarography. No chemical separations are necessary and the valency state of the iron is unimportant. The determination of chromium is possible after oxidation to the chromate state with ammonium persulphate. The chromate peak is recorded from an alkaline mannitol base solution. THE derivative nature of the square-wave polarographl permits the determination of mixtures of elements that give mutually interfering steps in conventional polarography. Some advantages of this instrument in analysis have already been shown2 and its application to the determination of alloying constituents in light alloys has been rep~rted.~ The major constituents of these alloys (aluminium or magnesium) are reduced at highly negative potentials at the dropping-mercury electrode and do not interfere with the determination of those alloying constituents that give steps at more positive potentials. With this type of alloy the square-wave polarograph can be used to determine directly minor constituents even when these are reduced at potentials more negative than those of other constituents present in much larger concentrations. For example, 0.01 per cent.of lead could readily be determined in aluminium alloys containing 4 per cent. of copper. The application of the square-wave polarograph t o the analysis of alloys with major constituents reducing near to zero applied voltage is a more difficult problem.Even so it has proved possible to use this technique for the determination of some of the constituents of steels and copper-base alloys. For most alloys direct methods have been developed, but in two instances it proved necessary to remove interference from the major alloying constituents by complex-ion formation. THE ANALYSIS OF COPPER-BASE ALLOYS DETERMINATIOK OF ZINC AND NICKEL- The determination of these elements by conventional polarography has been accom- plished in a number of ways. Thus Hohn4 and Milner5 have determined nickel in alkaline cyanide solutions. Copper and zinc form such strong complexes under these conditions that they do not interfere with the nickel step.Zinc may be determined polarographically in an ammonia - ammonium chloride base solution only after the copper has been separated. Miher6 applied a chemical method to do this, whereas Lingane' used controlled-potential electrolysis. In more recent work Pomeroy, White and Gwatkins* have shown that the zinc content of synthetic copper - zinc mixtures can be determined directly, Using the derivative circuit of Leveque and Roth, they obtained resolution of the zinc step for a zinc concentration range of 2 to 8 per cent. * For particulars of Part I of this series, see reference list, p. 203.194 FERRETT AND MILNER : ANALYTICAL APPLICATIONS [Vol. 81 Lingane's work has shown that the nickel and zinc steps are separated in ammonium hydroxide - ammonium chloride base solution.The separation is greatest (0.27 volt) in M ammonium hydroxide - 0-2 M ammonium chloride base solutions. As the sensitivity of the square-wave polarograph depends upon the degree of reversibility of the electrode reduction, we have studied base solutions with different ammonium hydroxide - ammonium chloride ratios to determine optimum conditions for the resolution and use of the nickel and zinc peaks. Table I. Results for ihe step heights in the base solutions studied are shown in TABLE I EFFECT OF THE BASE-ELECTROLYTE COMPOSITION ON THE HEIGHT OF THE NICKEL AND ZINC PEAKS M NaClO, M NH,OH M NH,OH M NH,OH M NH,OH M NH,OH Base solution 0-2M HClO, 0.3M (NH,),CO, 6MNH,Cl 3M NH,Cl 0.2M NH,C1 0.1M NH,Cl Nickel peak height E+, volts against Zinc peak height E4, volts against for 100 pg per ml 17 2 60 370 375 388 464 the S.C.E.- 1.07 - 1.07 - 1.13 - 1.12 - 1.06 - 1.05 for 100 pg per ml 120 340 743 752 761 783 the S.C.E. - 1.01 - 1.25 - 1.38 - 1.36 - 1.33 - 1.30 In the sodium perchlorate - perchloric acid base solution the predominant ions present are [Ni(6H20)]" and [2n(4H2O)]". It can be seen that in ammoniacal solutions, in which the aquo ions are no longer the predominant species, the reversibilities of the two reductions are greatly increased, and the difference in behaviour between zinc and nickel becomes less pronounced. The reversibility of the electrode processes is greater in chloride than in car- bonate solutions, and decreases in these former solutions with increasing chloride concentration. The differences here are not large, however, and as the nickel and zinc steps are most widely separated in 0.2 N ammonium chloride - M ammonium hydroxide solutions, this base solution was chosen for our investigation.Copper gives two peaks with Eg values of -0-25 afid -0-51 volt against the S.C.E. from the ammonium hydroxide - ammonium chloride base electrolyte, these values being more positive than those for the nickel and zinc peaks. With alloys containing 0.1 per cent. of nickel, the copper concentration is a thousand times greater than that of nickel and the copper peak might mask the nickel peak. Experiments with solutions containing 5000 pg of copper per ml, however, showed that the recorder pen of the square-wave polarograph returned to the base line in the region of -0.7 volt against the saturated-calomel electrode (S.C.E.).It therefore proved perfectly feasible to determine nickel and zinc simultaneously in the presence of large amounts of copper. Fig. 1 shows a polarogram of a typical copper-base solution, the resolution of the zinc and nickel steps and the complete absence of any inter- ference from the reduction of copper ions being indicated. This technique appeared to be well suited to the direct determination of the nickel and zinc contents of bronzes. This type of alloy, however, contains several per cent. of tin, which is precipitated in a gelatinous form on the addition of the ammonium hydroxide - ammonium chloride base solution. Fortunately this element can be readily removed as its volatile bromide.The first step in the procedure for these alloys incorporates, therefore, an evaporation to dryness in the presence of bromine and hydrobromic acid. Further details follow. Treatment of sample-Weigh 125 mg of sample, transfer it to a 125-ml conical beaker and add 10ml of hydrobromic acid, sp.gr. 1.46. Warm to dissolve and then cool slightly when solution is complete. Add about 2ml of analytical-reagent grade bromine and evaporate the solution to dryness, taking care to prevent loss by spattering in the later stages of the evaporation. Cool the beaker, and then add a further 5 m l of hydrobromic acid, repeating the evaporation to dryness. Cool the beaker and add 2 to 3 drops of concentrated nitric acid to the residue, followed immediately by about 5 ml of water.Boil the soIution to remove bromine and evaporate to a low volume. Dilute this solution with about 5 ml of water and add ammonium hydroxide, sp.gr. 04380, drop by drop until the solution is alkaline. Then dilute the solution to 100 ml with the M ammonium hydroxide - 0.2 M ammonium chloride base solution.April, 19561 OF THE BARKER SQUARE-WAVE POLAROGRAPH 195 Transfer 5 ml of this sample solution to the polarograph cell, and add 0.1 ml of saturated sodium sulphite solution. Record a polarogram for the solution between -0.8 and -1.5 volts against the S.C.E. and compare the peak height with those obtained from standard solutions. Preparation of calibration gra+--Weigh 110 mg of Specpure copper and dissolve it in 2 ml of diluted hydrochloric acid (1 + 1) and two drops of 100-volume hydrogen peroxide.Remove the excess of peroxide by boiling. Weight 62.5mg of Specpure zinc and dissolve it in 2 ml of diluted hydrochloric acid (1 + 1) and dilute this zinc solution to 500ml. Add 50ml of the zinc solution to the copper solution and evaporate to a small volume. Add ammonium hydroxide, sp.gr. 0.880, until the solution is alkaline and dilute 70 80/ Zn 3 3*400/, I I - 1.3 - 1 . 1 Potential, volts Fig. 1. Square-wave polarogram of bronze solution (100 mg per 100 ml), 1 M in ammonium hydroxide and 0.2 M in ammonium chloride to 25 ml with M ammonium hydroxide - 0.2 M ammonium chloride solution. Transfer 5 ml of this standard solution to the polarograph cell, add 0-1 ml of sodium sulphite solution and record a polarograph as described above.Repeat the procedure, using 40,30, 20, 10 and 0 ml of zinc solution, and plot the height of the zinc peak against the zinc concentration. The same technique was also used for the preparation of a calibration graph for nickel. Linear calibration graphs were obtained. RESULTS- Several typical copper-base alloys were analysed for nickel and zinc by the above procedure. A comparison of these results with the chemical figures is given in Table 11. The agreement shown here between the chemical values for these elements and those obtained with the square-wave polarograph indicates that this technique offers a very rapid and accurate method for the determination of nickel and zinc in bronzes, without pretreatment of the sample to remove copper as is normally necessary.DETERMINATION OF LEAD- The lead content of copper-base alloys cannot be determined directly by conventional polarography. Hence Lingane' removed copper from a solution of the alloy by controlled- potential electrolysis, the lead step then being recorded in a 4 M ammonium chloride - M196 FERRETT AND MILNER : ANALYTICAL APPLICATIONS [Vol. 81 hydrochloric acid base solution. In another procedure, Milnerg removed the polarographic interference of copper ions by adding potassium cyanide to the alloy solution to form non- reducible cuprocyanide ions. The lead step was then recorded from an alkaline cyanide base solution. Unfortunately, the copper and lead peaks from non-complexing base solutions are too close together to be satisfactorily resolved by the square-wave polarograph when the TABLE IT DETERMINATION OF NICKEL AND ZINC CONTENTS OF COPPER-BASE ALLOYS Composition of alloy, % P, 0.25; Sn, 9-7; Pb, 1-83; Sb, 0.24; Cu, remainder (phosphor bronze) Sn, 9.8; Pb, 0.41; Fe, 0.06; P, 0.05; C;; remainder .. .. .. .. . . Sn, 5.3; Sb, 0.2; Pb, 4.28; Cu, remainder Sn, 10.4; Pb, 0.47; Cu, remainder . . Sn, 5 - 8 ; Pb, 2.76; Cu, remainder . . Pb, 2.47; Fe, 0.29; Cu and Sn, remainder . . . . Nickel f A > Square-wave Chemical polarographic value, value, % % 0.04 not determined 0.09 0.093 0.20 0.19 0.295 0.29 0.39 0.37 0.86 0.85 Zinc 1 7 Sqnare-wave Chemical polarographic value, value, % o/ / O 1.86 1-91 2.53 2-54 4-30 4.40 2.15 2.16 3.4 1 3.46 4.92 4.92 ratio of copper to lead is 100 to 1 and greater.In the application of the instrument to this determination, therefore, it proved necessary to complex the copper with cyanide ions and to record the lead peak from an alkaline cyanide base solution. By this means it was possible to determine the lead content of alloys, available only in very small amounts. In a 0.6 M cyanide - 2 M sodium hydroxide - 10 per cent. sodium sulphite base solution lead produces a well defined peak on the square-wave polarograph. The quantitative behaviour of this peak was examined by using synthetic copper-base alloys, each prepared from 100mg of copper dissolved in nitric acid plus a suitable amount of a standard lead solution to give a copper to lead ratio between 100 to 1 and 2000 to 1. The standard lead solution was prepared by dissolving 100 mg of Specpure metal in a few millilitres of nitric acid, and then evaporating t o fumes with 5 ml of 60 per cent.perchloric acid. This solution was finally diluted to 1 litre with water. Each sample solution was evaporated to fumes with 5 ml of perchloric acid before being mixed with 25 ml of base solution and was then diluted to 100ml with water. When the height of each lead peak was plotted against the corresponding lead concentration, a straight-line calibration graph was obtained for lead concentrations in the range 0.5 to 50 mg per ml. On application of this procedure directly to typical copper-base alloys, the tin present in the sample was precipitated as metastannic acid during the initial attack with nitric acid.The solution technique was therefore modified to include evaporation to dryness with brominated hydrochloric acid to remove tin from solution. Dissolve the sample in 10 ml of lead-free hydrochloric acid, sp.gr. 1-16. Then add 5 ml of hydrochloric acid saturated with bromine and evaporate to dryness. Repeat this treatment with a further 5-ml portion of brominated hydrochloric acid to remove the tin completely. Dissolve the residue in 5 ml of 60 per cent. perchloric acid and a few drops of nitric acid, sp.gr. 1.42, and then evaporate to fumes of perchloric acid. Add 25 ml of a 2-4 M potassium cyanide - 8 n/r sodium hydroxide - 40 per cent. w/v sodium sulphite solution and make the volume of the final solution up to 100ml with water. Record the lead peak on a 5-ml portion of this solution.Then add a measured quantity of a suitable standard lead solution under the same base-solution conditions and record the lead peak. From the increase in the height of the lead Deak. calculate the lead content of the allov. Further details of the procedure follow. Weigh out 100mg of alloy and transfer it to a 125-ml conical beaker. This prbcedure has been applied to the B.C.S. stanhard alloys, with the results reported in Table 111. In many types of lead-containing alloy the lead is known to be non-uniformly dis- tributed throughout the material, and with such alloys it is necessary to take a large sample weight for analysis in order to obtain a representative figure for the lead content. In the work of proving the above procedure, therefore, it was considered desirable to take 100-mg quantities for analysis to offset any possibility of the lead being unevenly distributed.ItApril, 19561 OF THE BARKER SQUARE-WAVE POLAROGRAPH 197 was realised, however, that because of the high sensitivity of the square-wave polarograph this analysis could have been accomplished on 5 to 10-mg samples. At the request of the Ashmolean Museum, Oxford, this technique was used for determining the lead content of bronze-age axeheads, samples from which were only available in small quantities. The procedure employed consisted in dissolving 10 mg of sample in 1 ml of concentrated hydro- chloric acid and using 2-ml portions of brominated hydrochloric acid for removing the tin. After treatment with nitric acid, the solution was evaporated with 1 ml of perchloric acid to fumes.Then 2.5 ml of the base solution were added and the solution was finally diluted to 10ml with water. The standard-addition technique was used as before. The results obtained for these samples are given in Table IV, and a typical polarogram is shown in Fig. 2. TABLE I11 ANALYSIS OF STANDARD COPPER-BASE ALLOYS Lead P Square-wave Chemical polarographic Nature of alloy Composition, value, value, % Y O % Manganese brass Cu, 58.8; Zn, 33.9; Mn, 1.03; Fe, 0.91; Al, 1.62; 0-78 0.81 B.C.S. sample No. 179 B.C.S. sample No. 207 Sn, 1.75; Pb, 0.78; Ni, 1-01 Fe, 0.06; Sb, 0.04; As, 0-05; P, 0-055 Bronze “C” Cu, 86.84; Sn, 9-80; Zn, 2-53; Ni, 0.09; Pb, 0.41; 0.41 0.41 TABLE IV ANALYSIS OF BRONZE-AGE AXEHEADS Lead A I -l Square-wave Sample Chemical value,lO polarographic value, O/ 10 0.30 0.1 1 0.29 Yo 1.Artefact from County Cavan, Ireland, 1927. 2848 . . 0-30 2. Artefact from Ciudad Real, Spain, 1927. 2004 . . 0.14 3. Artefact from Ireland, 1431. 2317 . . .. . . 0-20 70 60 50 40 30 20 10 - 1 . 1 - 0.9 Potential. volts Fig. 2. Square-wave polarogram of a solution of Bronze-age artefact No. 3 (see Table IV) in 0-6 M potassium cyanide and 2 M sodium hydroxide198 FERRETT AND MILNER : ANALYTICAL APPLICATIONS [Vol. 81 The chemical figures quoted in Table IV provide only a rough guide to the com- position of the sample, as only very small sample weights were available for the standard chemical procedures. There is no evidence of segregation in these samples to explain the difference between the polarographic and chemical results.Four separate 10-mg portions of sample No. 3, for example, gave lead figures of 0-28, 0-31, 0.30 and 0.28 per cent., respectively. THE ANALYSIS OF STEELS DETERMINATION OF COPPER, LEAD AND TIN As the step due to the reduction of ferric iron starts from zero applied voltage, conven- tional polarography cannot be applied directly to the analysis of steel solutions if any oxidation has occurred. In some recommended procedures the iron interference is removed by chemical precipitation or by chemical reduction to the ferrous state. In other procedures the element to be determined is separated from other alloying constituents before being determined polarographically. Hence copper has been determined polarographically by Thanheiser and Maassenll and by Lingane and Kerlinger12 after precipitation of iron (and chromium) with ammonia and with pyridine, respectively. In another method13 copper and lead are determined simultaneously after most of the iron has been reduced to the ferrous state in a base solution containing hydrazine hydrochloride and sodium formate.In the solution of the sample decomposition of the carbides is carefully carried out with a saturated solution of potassium chlorate to prevent undue oxidation of iron. Haim and Barnes1* have determined lead in steels after dissolving the steel in 12 A1 hydrochloric acid in the absence of oxygen, this technique keeping the iron in the reduced state. This procedure is, however, unsuitable for the determination of copper, since this metal will not dissolve under the reducing conditions.Allsopp and Damerell15 have described a laborious procedure for the polarographic deter- mination of tin in steels. The tin is precipitated (with copper and molybdenum) as the sulphide, which is separated and ignited to the oxide. The oxide residue is fused with potassium bisulphate, the melt is taken into solution with hydrochloric acid and this solution is then made alkaline with ammonium hydroxide. Small amounts of iron pass through to this stage and the ferric hydroxide precipitate acts as a carrier for the small tin precipitate. After solution of the hydroxide precipitate in acid, the iron is reduced to the ferrous state with hydroxylamine. The tin is finally determined polarographically from a M hydro- chloric acid - 4 M ammonium chloride base solution.In spite of the above separations, this method is claimed to be more rapid and convenient than standard chemical methods for tin determination. In addition to being indirect, the polarographic procedures require the addition of other reagents, which may contribute to the final concentration of the element being determined. In this section a simple technique is described for the determination of these elements with the square-wave polarograph. I t is based on solution of the steel in acid, followed by the direct examination of the solution polarographically. The sample is first dissolved in hydro- chloric acid, and insoluble carbides are decomposed with nitric acid. Perchloric acid is added to this solution and it is then evaporated to fumes.For the determination of a normal range of copper, lead and tin contents, the iron concentration in solution would be about 5000 mg per ml. Experiments showed that the peak corresponding to this concentration of fully oxidised ferric iron occurs in the vicinity of zero applied voltage and that the recorder pen of the square-wave polarograph returns to the base line at about -0.15 volt. The actual oxidation state of the iron is therefore unimportant in this technique and control of the amount of oxidant used for the decomposition of the carbides is unnecessary. The complete oxidation of the iron is not necessary, but the successful determination of lead and copper in the presence of this amount of fully oxidised iron demonstrates the superiority of the square-wave polarograph over conventional polarography.Fig. 3 shows a polarogram for a fully oxidised lead steel sample, in which the lack of interference from iron can be seen. Ferric ions are reduced when in contact with a mercury surface, and in consequence a film is rapidly formed on the anode and on the surface of the mercury cup. This does not interfere with the sensitivity of the technique in the time-interval needed to record the polarograms. If the solution is allowed to stand in contact with mercury, however, the resistance of the film may be sufficient to prevent the efficient operation of the instrument. Details of the recommended procedures follow.April, 19561 OF THE BARKER SQUARE-WAVE POLAROGRAPH 199 DETERMINATION OF COPPER- Weigh 100 mg of steel and dissolve it in 10 ml of diluted hydrochloric acid (1 + 1) in a small beaker.When the dissolving action ceases, add 3 ml of nitric acid, sp.gr. 1.42, followed by 2 ml of 60 per cent. perchloric acid, and evaporate until fumes of perchloric acid appear. Cool the mixture slightly, wash down the sides of the beaker with a few millilitres of water and evaporate again until the perchloric acid fumes are seen. Cool, add 10 ml of hydrochloric acid, sp.gr. 1-16, and dilute to 100ml with water. Transfer 5 ml of sample solution to a polarograph cell, de-aerate with nitrogen and record the polarogram between -0-15 and -0.35 volt. Determine the copper concentration by the standard-addition technique, using a solution of copper in M hydrochloric acid.Results for the copper content of typical steels are given in Table V. TABLE V DETERMIXATION OF COPPER IN DIFFERENT TYPES OF STEEL Copper r 1 A Square-wave Sample Chemical value, polarographic value, Carbon sieel- 70 % 0.03% carbon steel (A2) . . 0.065 0.073 B.C.S. No. 150 Low-alloy steels- B.C.S. No. 252 . . .. 0.11 B.C.S. No. 255 . . .. 0.24 B.C.S. No. 253 . . .. 0.49 0.11 0-23 0.495 Alloy steels- B.C.S. No. 212 . . .. 0.1- 0.11 DETERMINATION OF LEAD- The amount of lead normally present in steels is very low, unless the lead has been deliberately added. The analysis of such steels is required, however, and the same procedure as that used for the copper determination suffices for the preparation of the solution. When 100-mg sample weights were taken for analysis, a significant variation in the results for lead on the same sample was observed.This behaviour is undoubtedly due to the heterogeneous nature of lead-bearing steels. This type of discrepancy disappeared, however, when 1-g samples were taken and the solution was diluted to 1 litre before the 5-ml aliquot was taken for analysis. Typical results for lead by this procedure are given in Table VI. TABLE VI DETERMINATION OF THE LEAD CONTENT OF LEAD-BEARING STEELS Lead r 1 Square-wave Sample Chemical value, polarographic value, % % B.C.S. 30. 212 . . .. 0.28 0.285 Molybdenum steel (Mo, 0.276%) 0.15 0.16 DETERMIXATION OF TIN- If the square-wave polarograph is used for the determination of tin in steels, the normal technique15 is considerably improved, as the need for the initial chemical separation of the tin is completely eliminated.This new procedure is similar to that described for copper and lead, and it permits the tin content of steels to be rapidly and accurately determined. In the development of this procedure, attempts were made to record the tin peak from solutions of the steel in 10 per cent. and 20 per cent. hydrochloric acid and in the 4 M am- monium chloride - M hydrochloric acid base solution recommended by Lingane.lG Unfor- tunately, the peaks were somewhat ill defined and the tin results were inconsistent. In 5 M hydrochloric acid solutions, however, well defined peaks were obtained and these conditions were chosen for further investigation. Under these conditions satisfactory results were obtained for the tin content of a series of standard steels containing from 0.0005 to200 FERRETT AND MILNER : ANALYTICAL APPLICSTIONS [Vol.81 0.26 per cent. of tin. The polarogram for B.C.S. steel No. 149 is shown in Fig. 4, the excellent separation of the copper and tin peaks being visible. In this example, it is interesting to note that the ratio of the copper and tin concentrations to that of the ferric iron is 1 to 5000 and 1 to 20,000, respectively. Further details of this determination follow. 80 70 60 50 40 30 20 10 I I I 1 - 0-9 - 0.7 - 0.5 - 0.2 Potential, volts Fig. 3. Square-wave polarogram of a fully oxidised solution of B.C.S. lead steel (100 mg in 100 ml of M hydrochloric acid) Take 100 mg of steel and dissolve it in 50ml of hydrochloric acid, sp.gr.1.16. Cool the solution and dilute it t o 100 ml with water. Transfer 5 ml to the polarograph cell, de-aerate and record the tin step between -0-2 and -0.6 volt against the S.C.E. Determine the tin content by the standard-addition technique, using a tin solution in 5 M hydrochloric acid. A linear relation for tin concentration against peak height was obtained. Typical results for the determination of tin in steels by this procedure are given in Table VII. TABLE VII POLAROGRAPHIC DETERMINATION OF THE TIN CONTENT OF STEELS Tin f A 7 Square-wave Sample Chemical value, polarographic value, % % B.C.S. No. 149 . . . . < 0.002 0.00052 B.C.S. No. 218/1 . . .. 0.040 0.037 B.C.S. No. 239/1 . . .. 0.06- 0.052 S.G. 4676 .. .. .. 0*066* 0.060 S.G. 4677 .. .. .. 0.10* 0.10 S.G.4678 .. .. .. 0*17* 0.158 S.G. 4679 .. .. .. 0.26* 0.28 * Chemical results by the Robertshaw - Bromfield pr0~edure.l~April, 19561 OF THE BARKER SQUARE-WAVE POLAROGRAPH 201 These results show the ease and accuracy with which the tin content of steels can be determined with the square-wave polarograph. Unfortunately, any lead in the sample gives a peak coinciding with the tin peak. However, the lead content of steels is generally very small, with the exception of the lead-bearing type of steel. In the analysis of B.C.S. No. 149, this sample was examined for lead after the preliminary reinova of tin as volatile stannic bromide. No peak was detected for this element. I00 90 80 70 60 50 40 30 20 I I - 0 4 - 0.2 Potential, volts Fig. 4. Square-wave polarogram of a 1 mg per ml solution of B.C.S.steel No. 149 in 5 M hydrochloric acid I00 90 80 70 60 50 40 30 20 0 - 0.80 - 0.60 - 0.40 Potential, volts Fig. 5. Square-wave polarogram of a solution of B.C.S. steel No. 258 in 9 M sodium hydroxide containing 6 per cent. of mannitol, showing chromium peak DETERMINATIOX OF CHROMIUM- The most suitable step for the polarographic determination of chromium is that produced by the reduction of chromate ions from a strongly alkaline base solution. This step is reversible and its half-wave potential value is -0.85 volt against the S.C.E. in 0.1 to 1 M sodium hydroxide solutions. The chromate step has been used by von Stackelberg et aZ.18 and by Thanheiser and Willems19 in procedures for the determination of the chromium content of steels.The recommended methods, however, cannot be regarded as entirely satisfactory, because iron is precipitated by the base solution, and ferric hydroxide precipitates are noted for their ability to remove other elements from solution by adsorption. In later work HeyrovskY2O and co-workers have modified the technique to correct for the chromium202 FERRETT AND MILNER : ANALYTICAL APPLICATIONS [Vol. 81 removed by the ferric hydroxide precipitate. The best approach to this problem, however, is to include a complexing agent in the base solution to prevent the precipitation of ferric hydroxide and to select solution conditions so that the polarographic reduction of the iron complex does not interfere with the chromate step. We have used this approach in applying the square-wave polarograph to the determination of chromium in steels.A study of a sodium hydroxide base solution containing iron complexed with tartrate, salicylate, citrate, mandelate, malonate, glycollate, benzohydroxamate, triethanolamine, diethanolamine, tiron (disodium catechol-3 : 5-disulphonate) or sucrose showed the existence of reversible polarographic steps that unfortunately interfered with the chromate step. The only base solution that gave a satisfactory separation of the iron and chromium steps was the 3 M sodium hydroxide - 3 per cent. mannitol solution suggested by Reynolds and Shalgosky.21 Under these conditions the half-wave potential values for the iron and chromium steps are -0.81 volt and -0.60 volt, respectively, against the S.C.E., but neither step is very reversible.Experiments showed, however, that the reversibility was improved by increasing the hydroxide concentration of the base solution. At 9 M sodium hydroxide - 6 per cent. mannitol both chromium and iron gave suitable well separated peaks on the square-wave polarograph, the chromium peak being free from interference even for iron to chromium ratios of 500 to 1. A study of the effects of the mannitol concentration showed that amounts up to 10 per cent. had no influence on the size and shape of the steps. The mannitol concentration has been fixed at 6 per cent. in this investigation. Calibration graphs for chromium under these conditions were prepared from a solution of AnalaR chromium trioxide (CrO,), a linear calibration graph being obtained over the range 0.8 pg per ml to 100 pg per ml..For a chromium concentration of 0.2 pg per ml a clearly defined peak of 40 divisions (equivalent to 11 cm of recorder chart) was obtained. The fluctuation of the base line at this sensitivity is of the order of f 5 divisions, so that the lowest concentration of chromium detectable with this instrument is about 0.03 pg per ml. The best technique for oxidising the chromium in a steel solution still required investi- gation. The simplest procedure for the oxidation of chromium involves evaporating to fumes of perchloric acid and then heating strongly to give complete oxidation. Slightly low results were always obtained when this technique was applied to synthetic chromium solutions, indicating that the chromium was not in the fully oxidised state.Apparently the fuming process causes a breakdown of the perchloric acid, and this results in a slight reduction of the chromate ions. Parks and Agazzi22 claim to have overcome these difficulties with the perchloric acid oxidation. They cooled the solution rapidly after fuming, diluted slightly with water and then boiled after the addition of a few drops of a dilute potassium perman- ganate solution. When this technique was applied, however, to the polarographic deter- mination of chromium, it was never possible to obtain consistently reliable polarograms and at times high blanks were obtained. The blanks were problably due to the polarographic reduction of breakdown products of the perchloric acid. The ammonium persulphate oxidation procedure is free from the above difficulties.However, silver ions are needed to ensure the complete oxidation of the chromium in a reasonable time. According to Oel~chlagen,~3 the silver ions destroy hydrogen peroxide. However, it proved necessary to keep the amount of silver nitrate down to small proportions in this work because silver hydroxide is precipitated from solution on the addition of the base electrolyte. By keeping the acidity of the sample solution low, a reasonable rate of oxidation was obtained by using one drop only of a 0-15 per cent. solution of silver nitrate. The efficacy of the oxidation under these conditions when applied to steels was shown by the appearance of the red colour of oxidised manganese. The appearance of this colour indicates that the chromium has been completely oxidised.After the oxidation procedure, however, it proved necessary to destroy the remaining persulphate ions before recording the polarogram. It was observed that the decomposition of persulphate is much faster from strongly acid than from weakly acid solutions. After the oxidation of the chromium, the acidity of the sample solution was therefore increased and the solution was boiled to decompose the persulphate. The permanganate ions produced simultaneously with the chromate ions were reduced by the addition of one drop of concentrated hydrochloric acid to the warm sample solution. Then after this solution had been cooled, the base electrolyte was added and the polarogram recorded. Further details of the recommended procedure follows.Add 3 mi of 10 M sulphuric acid and 3 ml of water. Warm until the acid attack ceases, then add 2 or 3 Weigh out 100 mg of sample and place it in a 150-ml conical beaker.April, 19561 OF THE BARKER SQUARE-WAVE POLAROGRAPH 203 drops of nitric acid, sp.gr. 1-42, and warm to decompose undissolved carbides. When solution is complete, evaporate to fumes of sulphuric acid, adding more sulphuric acid if chromic sulphate is precipitated. Transfer 1 ml of sample solution to a 10-ml squat micro-beaker and add 10 M sodium hydroxide until a permanent precipitate just persists after vigorous stirring (usually 6 or 7 drops are required). Then add 0.1 ml of 10 M sulphuric acid and stir to redissolve the precipi- tate. Add 2 ml of 1-5 per cent.ammonium persulphate solution and 1 drop of a 0.15 per cent. silver nitrate solution. Then cover the beaker with a watch-glass, bring the solution slowly to the boil and simmer for 15 minutes (the red colour of oxidised manganese should appear after 5 to 10 minutes with steels containing 0.5 per cent. or more of manganese). Cool slightly, add 0-4ml of 10M sulphuric acid and simmer for a further 10 minutes, allowing the volume of the solution to decrease to about 2 ml. Then add 1 drop of hydrochloric acid, sp.gr. 1-16, and warm to discharge the permanganate colour. Cool the solution and run in rapidly with stirring 7.5 ml of a 12 M sodium hydroxide - 8 per cent. mannitol solution. Make the volume of the solution up to 10 ml and record the polarogram between -0.4 and -0-8 volt.Determine the chromium content by the standard-addition technique. The typical results reported in Table VIII show the accuracy and the levels over which the square-wave polarograph is applicable in the analysis of low-alloy steels. A typical polarogram is shown in Fig. 5. Cool and dilute to 10ml with water. TABLE VIII 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. DETERMINATION OF CHROMIUM IN STEEL Chromium A I -l Square-wave Sample Chemical value, polarographic value, Yo % B.C.S. No. 2 5 2 . . .. .. 0.20 0.197 R.C.S. No. 2 5 4 . . .. .. 0.53 0.50 B.C.S. No. 2 5 5 . . .. .. 0.96 0.96 B.C.S. No. 2 5 6 . . .. .. 2-34 2.36 B.C.S. No. 257 . . . . .. 1.72 1.76 B.C.S. No. 258 . . .. .. 3.07 3-06 REFERENCES Barker, G. C., and Jenkins, I. L., Analyst, 1952, 77, 685. Ferrett, D. J., Milner, G. W. C., and Smales, A. A., Ibid., 1954, 79, 731. Ferrett, D. J., and Milner, G. W. C., Ibid., 1955, 80, 132. Hohn, H., 2. Elektrochem., 1937, 43, 127. Milner, G. W. C., Analyst, 1945, 70, 468. -, Metallurgia, 1947, 35, 265; 307. Lingane, J. J., Ind. Eng. Chem., Anal. Ed., 1946, 18, 429. Pomeroy, P. R., White, R. A., and Gwatkins, G. H. R., Metallurgia, 1952, 46, 157. Milner, G. W. C., Ibid., 1947, 36, 287. ‘Case, H., Man, 1954, 54, 18. Thanheiser, G., and Maassen, G., Arch. Eisenhiittenw., 1937, 10, 441. Lingane, J. J., and Kerlinger, H., I n d . Eng. Chern., Anal. Ed., 1941, 13, 77. Nickelson, A. S., “Polarographic Estimation of Lead and Copper in Steel,” reported in “Standard Haim, G., and Barnes, W. C. E., Ind. Eng. Chem., Anal. Ed., 1942, 14, 867. Allsopp, W. E., and Damerell, V. R., Anal. Chem., 1949, 21, 677. Lingane, J. J., J . Amer. Chem. Soc., 1945, 67, 919. Robertshaw, A., and Bromfield, G. C., Analyst, 1944, 69, 340. Stackelberg, M. v., Klinger, P., Koch, W., and Krath, E., Forschungberichte Tech. Mitt Krupp, Thanheiser, G., and Willems, J., Mitt. K.-Wi1h.-Inst. Eisenfovsch, 1939, 21, 65. Heyrovskg, J., Anal. Chim. Acta, 1948, 2, 533. Reynolds, G. F., and Shalgosky, H. I., Ibid., 1954, 10, 273. Parks, T. D., and Agazzi, E. J., Anal. Chem., 1955, 27, 416. Oelschlagen, W., 2. anal. Chem., 1955, 144, 27. Methods of Analysis,” United Steel Co. Ltd., Sheffield, 1951, p. 138. Essen, 1939, 2, 59. NOTE-ReferenCe 3 constitutes Part I of this series. ANALYTICAL CHEMISTRY GROUP ATOMIC ENERGY RESEARCH ESTABLISHMENT HARWELL, NR. DIDCOT, BERKS. October loth, 1955

ISSN:0003-2654    

DOI:10.1039/AN9568100193

出版商:RSC    

年代:1956

数据来源: RSC