ISSN:0003-2654    

DOI:10.1039/AN95681FX039

出版商:RSC    

年代:1956

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN95681BX041

出版商:RSC    

年代:1956

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN95681FP093

出版商:RSC    

年代:1956

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN95681BP099

出版商:RSC    

年代:1956

数据来源: RSC

摘要:

AUGUST, 1956 Vol. 81, No. 965 THE ANALYST PROCEEDINGS OF THE SOCIETY FOR ANALYTICAL CHEMISTRY THE SOCIETY OF CHEMICAL INDUSTRY ON Tuesday, July loth, 1956, in the theatre of the Royal Institution, London, the President] Dr. K. A. Williams, and the Honorary Treasurer] Dr. J. H. Hamence, presented the following Address to the Society of Chemical Industry on the occasion of the celebration of the 75th Anniversary of its foundation- AN ADDRESS TO THE SOCIETY OF CHEMICAL INDUSTRY On the occasion of the Celebration on July loth, 1956, of the Seventy-Fifth Anniversary of the Foundation of the Society of Chemical Industry] The President Officers, Council and Members of The Society for Analytical Chemistry send Greet- ings to the Officers and Members of the Society of Chemical Industry. The Society sincerely welcomes this opportunity to express its appreciation of the goodwill and friendship that has existed between the Societies for the past three-quarters of a century, pays tribute to the past achievements of the Society of Chemical Industry, which has so signally advanced the study of all matters con- nected with Applied Chemistry, and offers good wishes for its continued prosperity. Signed and Sealed on behalf of THE SOCIETY FOR ANALYTICAL CHEMISTRY. (Signed) K. A. WILLIAMS (President), J. HUBERT HAMENCE (Honorary Treasurer). NOEL L. ALLPORT (Honorary Secretary). 0 Seal of The Society f o r ,4 nalytical Chemistry

ISSN:0003-2654    

DOI:10.1039/AN9568100449

出版商:RSC    

年代:1956

数据来源: RSC

摘要:

450 VAN NIEUWENBURG: MODERN QUALITATIVE [Vol. 81 Modern Qualitative Analysis and Industrial Practice BY C. J. VAN NIEUWENBURG (Lecture delivered at the meeting of the Midlands Section on Tuesday, March 6th, 1956) WHEN speaking about “qualitative analysis,” it might be well first of all for me to call attention to the fact that this expression decidedly has a double meaning. On the one side, it is hardly necessary to say that it is the branch of chemical science that the full-grown chemist applies when he wishes to know what elements or groups or compounds are present in a substance under investigation. I t is perhaps wise to include in the definition that moreover he would like to know what are the major and minor constituents and what constituents are only present in traces.Anyhow, it belongs to the activities of finished chemists, engaged in serious chemical research. But there is quite another qualitative analysis, a set or scheme of operations intended to teach the elements of laboratory technique to the very youngest groups of chemistry students in our universities. As often as not, this is something quite apart, not aiming in the first place at true and completely reliable results, but rather at a didactical end. In a great many chemical circles this ambiguity has caused a very regrettable contempt for qualitative analysis, which one meets even now, although the tempestuous growth of qualitative analysis in these last decades has considerably altered and improved the situation. I hope that this lecture may make a modest contribution towards removing the last remainders of this contempt by showing you that by now qualitative analysis has indeed come into its own.Some thirty years ago it looked as if no branch of chemical science was as dead as qualitative analysis, but as you will know it rose from its ashes like a phoenix, being now full of new life. So, in order to understand what is essential in modern analysis, I should like to give you a short survey of its historical growth. Like gravimetric analysis, qualitative analysis was born about the middle of the sixteenth century from the needs oT metallurgy, About that time Agricola first studied the colours that different mineralogical products imparted to a colourless flame. However, the true father of this branch of analysis was your fellow-countryman Robert Boyle, who about the middle of the seventeenth century introduced the first veritable “reagents,” substances specially intended to show the presence of some element.Of course that was only possible after he had given the first serviceable definition of an element, indeed based on its analytical recognisability throughout a number of compounds. From then on the notions of elements and qualitative analysis were indissolubly connected. Moreover, it is interesting to note that a number of Boyle’s reagents were natural organic dyes, just as in modern analysis. The next essential improvement was due to the Swede Torbern Olaf Bergman, who towards the end of the eighteenth century laid down the principles for separating the then known metals into groups by means of hydrogen sulphide and sodium sulphide.This was the first “systematic” qualitative analysis. The idea has been elaborated by Berzelius and by the German school of nineteenth-century analysts like Klaproth, Rose and Fresenius, and given a more or less definite shape by the Swiss Treadwell. Together they built up the well known system of classical qualitative macro-analysis, which has since then practically remained unchanged. It is this “fixed” system which gave rise to the presumption that as a branch of science qualitative analysis was definitely dead. Let us admit that it was profoundly asleep during the first quarter of the present century; but I regret to say that there are still a great many universities where the preparatory teaching of it never passed this stage.However, the germ of rejuvenation was already present. As long ago as 1679 my fellow-countryman and, in fact, fellow-citizen, Anthonie van Leeuwenhoek, the founder of microbiology, presented a paper to the young Royal Society of London, entitled “On the .Figures of Salts,” in which he showed that it is possible to identify chemical substances by means of their crystal shape under the microscope, the root idea of qualitative microscopic analysis. In the first half of the nineteenth century this principle was worked out by two naturalists, the Frenchman Raspail and the Dutchman Harting, and somewhat later byAugust, 1956 J ANALYSIS AND INDUSTRIAL PRACTICE 451 Boricky of Prague and by Haushofer of Munich University. I t came to full growth about 1890 by the studies of a third fellow-countryman of mine, and again a fellow-citizen, Behrens, who gave us a complete list of reactions and reagents for all the more common elements.I won’t treat of the modern development o€ microscopic analysis. Suffice it to say that in the United States important contributions were made by Chamot and Mason, and in the Argentine by Martini. I was privileged to work from the very beginning of my chemical studies, that is now nearly fifty years ago, in the laboratories of Delft Technical University, where the Behrens technique was assiduously cultivated, and I learnt to appreciate its great merits. Of course it opened up quite a new field of analytical activity, the complete qualitative analysis of minute quantities of a substance, far below one milligram. In a great many cases this is extremely useful, and yet, in my opinion, that is not its greatest merit.I think it €ar more important that, as a rule, the microscopic reactions admit of a degree of certainty which is hardly ever attained by the old technique. They give us a set of excellent identity reactions, which can hardly be surpassed. More and more it came to consist of a limited number of separations, and, superimposed on these and inseparably connected with them, a set of identity reactions for thc different elements within a certain group. In due time the question arose whether these microscopic reactions were the only possible set of identity reactions, and of course they were not. I t is the great merit of Feigl, formerly of Vienna University, now of Kio de Janeiro, to have opened up, if not invented, the large field of what is now known as drop-reactions or spot-tests.Since Feigl started work about 1925, a very great number of organic reagents have been iound to give colour reactions of such a striking nature that all doubt whether a certain element is present, or not, is removed. In many respects these drop-reactions, like the microscopic reactions, provided us with an excellent set of new identity reactions. Indeed, what we really were in need of some twenty years ago was a set of identification reactions of such a glaring and convincing nature that all doubts would be removed. This feeling of uncertainty was the greatest drawback of the old qualitative systems of Fresenius and Treadwell.They said that when at such and such a moment of the systematic course you obtained a white precipitate, then it was aluminium hydroxyde, or barium sulphate. I admit that indeed, if everything went accurately according to schedule, it was. But we now know that because of all sorts of co-precipitations and other complications, things are not so simple as the textbooks of that time told us, and the white precipitate might just as well have been some remnant of a preceding group. Generally speaking, I think that one may say that this feeling of uncertainty is by now quite out of date. The modern procedures, using the old trusted principles of separation into groups, or rather the critically mistrusted separations, but now with a set of fully reliable microscopic and drop-reactions superimposed on them, leave no more room for any reasonable doubt.Both have their own advantages and drawbacks, and I would rather say that they are complementary to one another. As a rule, drop-reactions are easier to carry out than microscopic reactions and require less experience, but, apart from some exceptions, they are definitely less selective. A drop-reaction is hardly ever really specific, whereas a great many microscopic reactions are, especially when combined with careful measurements of details like crystallographic and extinction angles and birefringence. But, of course, this requires proper apparatus, whereas the technique of drop-reactions can be carried out by very simple and cheap means. Perhaps this explains why up to now in many countries drop-reactions have been taken into the regular courses for training young students in qualitative analysis, and the microscopic technique, on the contrary, is restricted to a very limited number of university institutes.This is a pity. In analytical chemistry-in teaching it as well as in its practical application -we must be eclectic and not dogmatic or even fanatic. We must know and be master of as many different techniques as possible, and then, in each separate case, choose the one which seems to be the best suited. Nearly forty years ago in my own institute I started with microscopic reactions as the only set of identity reactions, Then in 1927 I had the occasion to acquaint myself with the drop-reaction technique under the personal guidance of Feigl in Vienna, and I became so enthusiastic that I completely switched over to i t ; but that was a mistake.For a number of years now Under its influence the general character of qualitative analysis changed. It seems futile to me to ask which of the two is the better set. I won’t deny that I fell into the same trap.452 VAN NIEUWENBURG : MODERN QUALITATIVE [Vol. 81 we have practised qualitative analysis, even with very young students, in such a way that, as a rule, a microscopic reaction as well as a spot-test is carried out on each element, and this works to our complete satisfaction. The same idea has been followed in the Reports of the Committee on New Reagents of the International Union of Chemistry. As lorig ago as 1934 it was evident that the number of identity reactions had become so large that it was hardly possible any longer for an inexperienced worker to find his way through the labyrinth.The Union formed a small committee to separate the chaff from the wheat and to collect a very restricted number of really recommendable identity reactions. I had the honour to act as its president from 1934 until 1949, and it is now under the guidance of Professor Gillis of Ghent University in Belgium. In its Second Report in 1945 the Committee discharged the greater part of its duty by giving the requested selections, whenever possible one or more drop-react ions and some microscopic reactions. It is interesting to note that for nearly a hundred years people have tried to abolish hydrogen sulphide from qualitative analysis arid that nevertheless it is still going strong. Apart from its disagreeable smell, it has the dra-wback of being a gas, with all the complica- tions that arise from that.It can be used as an aqueous solution-and is indeed largely used as such in Germany-but this solution is not stable in contact with the air. Among the principles proposed to avoid using it, there are three worth mentioning. In the first place there is the use of mixtures containing sodium sulphide as advocated by Vortmann. This works quite well, but has the disadvantage of wasting tremendous amounts of chemicals, and of producing hydrogen sulphide at the most unexpected moments, which makes the remedy worse than the evil. Then there have been a number of methods in which zinc, cadmium or aluminium was used to reduce the solutions to metallic precipitates. As far as my experience goes, they all give rise to incomplete separations. And finally in these latter years the use of thioacetamide is largely advocated. I won’t deny that it is possible to obtain reasonably good results with it, but that is not so easy, and moreover I think that in the long run the sweetish smell of thioacetamide is even more annoying than that of hydrogen sulphide. Personally I think that hydrogen sulphide is still best and I know by experience that it is quite possible to use it without producing ariy inconvenient smell in the working room.Another problem worth discussing is whether it is really necessary to stick to the old separations.For the time being I should not like to part with them in student courses. Here the teacher knows quite well what is in the mixture he gives out, and the only purpose of the whole course is to teach the students chemistry in general, and very simple analysis in particular. For them the separations are of too great a didactical value to throw them overboard, even if that should be possible. He aims at nothing but finding the constituents in as short a time as possible, always provided that this expedition does not impair the complete reliability of the results. I t is evident that if the work could be done without separations, by simply consecutively applying a series of spot-tests or microscopic reactions, he would be fully justified in doing so. Now such collections of tests have indeed been proposed, for the cations by Charlot and Bkzier of Paris by means of spot-tests and by Steimetz of Nancy by means of microscopic reactions, and for the anions by Tananaev of Kiev.One couldn’t deny that they have been set up with remarkable ingenuity and skill and a profound knowledge of the subject. And yet I am rather afraid of such systems. The influence of other ions on redox potentials and on complex formation in unknown mixtures is SO incalculable that I don’t quite see how it is possible always to foresee all contingencies. Anyhow it seems to me that for the time being extreme caution must be recommended. With all these new identification reactions, both microscopic reactions and spot-tests, which are equally well carried out on the micro or semi-micro scale, and no less because of the complete change in the apparatus used, drop-plates instead of test-tubes, centrifuges instead of funnels and filter-paper, and SO on arid so on, qualitative analysis has now been established on a micro-scale. To my way of thinking the change from macro to micro is here no longer a problem worthy of serious discussion.In qualitative analysis the micro- technique is in all respects preferable, and thisl statement holds good for actual research practice just as well as for preparatory university courses. It does the trick better and considerably faster, and moreover-very import ant for teaching purposes-more elegantly. Perhaps you may think that that is putting it rat her strongly, but indeed I feel quite strongly To my way of thinking the answer depends on what we are aiming at.But for a research worker the situation is quite different.August, 19561 ANALYSIS AND INDUSTRIAL PRACTICE 453 on this point. In the past, teaching of qualitative analysis has been one of the sloppiest and shoddiest parts of experimental chemistry tuition. As often as not it was as if teachers said: let us turn the young students loose in their first term on bottles and test-tubes and stinks, to their heart’s content. They will automatically take a dislike to it, and tidiness and cleanliness will come later on. Maybe sometimes the trick worked, but I think that a great many students were spoiled for ever. Elegance and acquiring a good style are of paramount importance in all scientific work, but more especially during the first years of university tuition, In my Institute we are now giving out 20 mg of substance, amply sufficient for a complete qualitative analysis-10 mg for the cations and 10 mg for the anions-and I can only say that the results are completely satisfactory, far better than thirty years ago with the old technique, and that the room in which it is carried out, and the furniture in it, is more like a drawing-room than like an old laboratory room.Speaking of didactical questions, it may be worth while to remind you of the curious fact that qualitative analysis, both in its classical and in its modern form, is one of the few branches of chemistry which up to now is practically without any sound theoretical founda- tion. Why is mercuric sulphide so much less soluble in water than, say, manganous sulphide? Why are barium sulphate and silver chloride so insoluble and magnesium sulphate and mercuric chloride, both such nearly related compounds, not? Why do copper salts give an intensive colour with, say, dithio-oxamide, and lead salts do not? Up to now these questions are practically unanswerable, so much so that one might say that finding new colour reactions is greatly a matter of luck, at least at the beginning.As a rule, one accidentally finds a colour of some ion with a certain organic compound, and then starts investigating related compounds and substituted derivatives until the best one has been spotted. But even then a great deal of luck comes in. It would be highly desirable and beneficial to the development of qualitative analysis if some day a clever theoretical chemist would give us a clue to guide us in our research. The remarkable evolution of qualitative analysis in these latter years has considerably increased its importance and usefulness for industrial research. It has enabled industry to get a far better insight into the composition of the materials with which it is working, especially with regard to traces and with regard to the presence of the less common elements.Don’t let us forget that in a great many industries traces of some elements are quite essential, either for the success or for the failure of the process involved. Let me only remind you of modern metallurgy and of those industries which are based on fermentation processes, where as often as not the whole course of the process is determined by the presence or absence of trace metals.In the laboratories of these industries and in those of research on foodstuffs for man and beast, modern trace analysis has been developed, which in an amazingly short time has become of tremendous industrial, agricultural and hygienic importance. Now you will perhaps object that I am trespassing. I have been asked to speak of qualitative analysis, and trace analysis is largely a matter of colorimetric quantitative analysis. Of course, in a certain sense you would be right, but on the other hand it is well to remember that by far the greater part of all these modern colorimetric methods has grown out of newly found qualitative colour reactions. Indeed, one might say that one of the first aims of the modern trend in qualitative research is to foster colorimetry, which implies that the sharp boundary- line between qualitative and quantitative analysis is gradually disappearing. It is hardly necessary to point out the importance of trace analysis in a great many existing industries, but I should like to draw your attention to the fact that quite probably it will become of even greater and more essential importance in the near future for all applications of atomic energy.Here, even more than in biological processes, minute traces of foreign elements can have a detrimental effect. Let me remind you of the effect of traces of boron in the graphite of the atomic piles, boron being clearly noxious even when present in less than one part in a million, and of the trouble caused by traces of some metals in the steel walling of the piles, which give rise to a secondary radiation. Radioactive isotopes are supposed to be made in the pile and not on its outer surface! Another instance where industrial practice directly profits by the recent development of qualitative analysis is connected with the less common elements.Up to a short time ago these less common elements were practically neglected-and even worse than that, they were intentionally hushed up, When two analysts met they said: let us not speak of titanium or tantalum, because if we do, we shall have to determine them. This policy has not only been detrimental to the picture we built up of the material world around us, but it has also454 VAN NIEUWENBURG: MODERN QUALITATIVE [Vol.81 deprived industry of a great many chances. Now one of the trends of modern qualitative analysis has been to put the common and the lless common elements on a more equal footing to the benefit of our well balanced conception of the material world and of industry. And in the future it will be even more so, because the fission products in an atomic pile are produced regardless of their geochemical frequency. There is at least one point where up to now qualitative analysis decidedly falls short in its service to industry. We still have the detestable habit of first pulverising and irreparably messing up and destroying the structure of every sample which happens to fall into our hands. Isn’t it about time that, at least in qualitative analysis, we paid more attention to the possi- bility of showing the presence of the elements in sztu, or in loco, in the original sample itself? This type of “topographical analysis” is as yet only in its veriest infancy.Prints have been made from polished metal surfaces on a wet reagent paper in order to locate the impurities, but without much success. It seems to me tlhat finding the right techniques for this sort of work would be highly beneficial to a great many branches of science, and particularly to industry. In regard to the future development of qualitative analysis, there remain three points worth discussing. You will probably be well aware that there are a great many analysts who think it will. I do not share that opinion.I don’t want to slander chromatography; indeed, I am fully aware of its great merits for the separation of organic substances, and I know quite well that even in the domain of inorganic separations it has booked and is continually booking new and striking successes, especially by means of paper-partition chromatography. But notwith- standing all that, I am not very optimistic about its systematic and general introduction into qualitative analysis. Indeed, we knew that, long before chromatography proper had been invented. Separation of the components of a drop of a mixed solution on a disc of filter-paper had been practised as long ago as the middle of the nineteenth century, and we now know incomparably more of it all. But, nevertheless, in my opinion, chromatography at present is at the most one of the valuable but very precarious and fickle resources of qualitative ‘analysis, and fundamentally new things will have to be found before it can be expected to replace integral parts of the present technique reliably. One might even ask whether the flare-up of qualitative analysis in these latter years is not in reality more like the con- vulsions of a moribund.Is there any sense in going on when, according to a great many people, in a few years spectrographic analysis will make it all obsolete and superfluous? Here also I am rather optimistic. Of course I know that even now one must admit that emission spectrography can perform nearly all the tricks of ordinary qualitative analysis of inorganic substances, and that infra- red, visible-light and ultra-violet absorption spectrography are rapidly gaining ground in the domains of gas analysis and the analysis of organic groups.And we may be sure that they are only in their infancy. Undoubtedly spectrographic analysis has some striking advantages over purely chemical methods. In the first place it is far more expedient, especially for routine work. In a few minutes it gives us a photo- graph of the spectrum, which, at least in principle, permits us to establish the complete qualitative composition of the substance under investigation and gives at least a good estimate of the quantities. Moreover, this photograph can be kept for a long time and remains a valuable document for later reference. This is not only very important in case of litigation, but it also enables us to look for the presence of other elements which did not interest us or which we didn’t expect in the first place.And finally, at least for some elements like the alkali metals, the alkaline earths, indium, gallium and many more, spectrography is more sensitive than any of the now known identity reactions, to say nothing of quite a number of less common elements for which it is the only means of detection. Of course it is very expensive and, even worse, it can only be successfully carried out in well equipped laboratories which have at their disposal a specialised and highly skilled laboratory staff. Generally speaking, I think we may aver that spectrographic analysis is now limited, and will be limited for some time to come, to routine work in specialised laboratories. This alone is sufficient reason to claim a lasting right of existence for the old trusted chemical methods.But there is more. Even when carried out in well equipped and expertly staffed laboratories, spectrographic The first is whether chromiitography will radically alter it or not. Of course it is an exceedingly useful tool. Far more serious is the competition of s-pectrography. In this respect is it unsurpassable. On the other hand, it has some unmistakable drawbacks.August, 19561 ANALYSIS AND INDUSTRIAL PRACTICE 455 analysis cannot as yet always attain the same degree of certainty as chemical analysis in the more limited sense of the word. When working with unknown or uncommon products, mistakes remain quite possible.For the time being it wouldn’t do to be without the purely chemical methods, if only as a standby to fall back on in cases of doubt-one might say, as its conscience. For those analytical chemists who are working on the development of qualitative chemical analysis it would be extremely dispiriting to know that they were fighting a losing battle. Prophesying has always been a dangerous job. Nobody can foretell what spectrography will bring us in the next twenty- five years. It is sure to improve and assuredly more and more big laboratories will be going to use it in daily practice, perhaps for ultimate analysis, but certainly for getting a first preliminary notion, just to show the way for a more definite analysis, if and when necessary; and in these big laboratories it will be largely a matter of organisation to ensure an efficient co-operation between spectrography and the other branches of analysis.For the time being its price is prohibitive for small laboratories, but the same can be said of a great deal of the apparatus of modern chemical analysis. It seems to me that in the very near future this will inevitably lead either to a concentration of the smaller units or to co-operation in such a way that at joint expense they keep up some institute where the more expensive apparatus is available. But anyhow, it doesn’t yet look as if spectrography is going to monopolise qualitative analysis. In my opinion the question is not so much whether we ought to go on with studying the purely chemical methods, but rather how to do so, knowing that in some respects spectrography is decidedly superior.So, for instance, I think that we had better not waste too much time on “systematic” courses that pretend to give us the complete analysis of a completely unknown substance. This may be useful and perhaps necessary for training young university students, but in industrial and research practice we do not meet with what Biltz once called “the remains of an exploded drug-store,” where strontium and manganese and tartaric acid and fluorides are to be found together. What we are much more in need of are more selective, if possible truly specific, reactions, and particu- larly selective methods for the isolation or extraction of the various ions from those with which they are commonly associated.If we are to compete successfully with spectrography and with chromatography, we shall have to invent a great many more rapid methods which are nevertheless fully reliable. Let us not be afraid of chromatography and spectrography; we have to use them and accept them as serious and perhaps even dangerous competitors, but anyhow as a stimulus to further research. For the time being purely chemical analysis and spectrography and chromatography each have their own reasons for existence, and it is our duty as conscientious chemists to find a happy synthesis of the three. In conclusion there is the final point I should like to raise in connection with future development of qualitative analysis. Up to now we have always tried to find organic reagents for showing the presence of inorganic ions; that means that practically always we aimed at inorganic analysis. One is inclined to ask whether it wouldn’t be possible to do it the other way round. Couldn’t we find a set of inorganic ions that give selective reactions with certain organic groups, in such a way that we can use them for elucidating the constitution of organic compounds? Of course we know that nickel is a selective reagent for cc-dioximes, that copper shows the presence of cc-hydroxycarboxylic acids, and when we skim the second volume of Feigl’s “Spot Tests,” we find a great many other examples. But all this is purely incidental. I fully realise the difficulties of the problem, if only because the behaviour of organic groups is so often dependent on the presence of other groups in the molecule, but nevertheless it seems to me that systematic studies in this direction might be worth while. Altogether, qualitative chemical analysis is fully alive again, bristling with problems, both didactic and of a purely chemical nature. DELFT TECHNICAL UNIVERSITY, DELFT, THE NETHERLANDS To me it seems abundantly clear that they are not. We had better leave such things to spectrography.

ISSN:0003-2654    

DOI:10.1039/AN9568100450

出版商:RSC    

年代:1956

数据来源: RSC

摘要:

456 BACON AND MILNER: THE DETERMINATION OF URANIUM [Vol. 81 The Determination of Uranium by High-precision Spectrophotometry BY A. BACON AND G. W. C. MILNER A procedure is described for determining the uranium content of relatively pure samples of U,O, and uranium metal with a precision (1 U) of 0.04 per cent. The sample is dissolved in nitric acid and this solution is then con- verted to standard conditions of acidity by evaporating to fumes of sulphuric acid. After dilution with water to a standard volume, the absorbancy of the sample solution is measured a t 430mp, the reference solution being of accurately known uranium content. The uranium content of the sample is then obtained either by referring the absorbancy difference t o a calibration graph or by calculation with use of a factor derived from the calibration graph.The main factors that influence the accuracy of the determination are discussed in detail. THE accuracy required from the analytical chemist can vary very widely, but in recent years there has been an increasing demand for analytical methods that would give an accuracy of better than 99-9 per cent. Since about 1949, spectrophotometry has been developed to such an extent that this technique is now making a valuable contribution in high-accuracy work. Moreover, in specific cases spectrophotometric methods have decided advantages over conventional procedures, since they are rapid and simple in operation and chemical separations can generally be avoided. These methods have resulted from the introduction of the differential system of measurement, which employs reference solutions of high absorbancy. Hiskeyl was among the first to show that the accuracy of the spectrophoto- metric methods can be increased by using reference solutions of high absorbancy. It does not follow, however, that under these conditions maximum accuracy can be attained within the concentration range in which solutions obey the Beer - Lambert law.For solutions failing to obey this law, it is possible to determine experimentally the concentration range for maximum accuracy. It was thought that the differential spectrophotometric technique should be of advantage in the determination of macro-quantities of uranium, especially in uranium-base materials. The volumetric method for uranium is subject to serious interference from other elements, as the reagents used to reduce the uranium 10 the quadrivalent state before titration with ceric sulphate generally reduce many other elements.Moreover, the conventional gravi- metric method for uranium, involving precipitation as ammonium diuranate followed by ignition to U,O,, also leaves much to be desired. Many other elements are precipitated by ammonium hydroxide under the same conditions and so cause contamination of the final U,O,. I t was considered that the absorptiometric technique would be free from many of the above objections and would result in some improvement in the determination of macro-amounts of uranium. The soluble compounds of uranium have been the subject of extensive research in recent years, the absorption spectra being used to postulate the ionic species present in solution.Kaplan, Hildebrant and Ader2 studied the absorption spectra of uranyl nitrate in both aqueous and ketone media. Spectra for uranium in hydrochloric and perchloric acid solutions have been reported by S ~ t t o n . ~ Recently, Rabinowitch4 carried out a comprehensive survey of the absorption spectra of uranyl compounds. Spectra are shown for free uranyl ions and also for hydrolysis products, for uranyl ions complexed with acid anions and for uranyl compounds in organic solvents. Some indication is also given of the absorption spectra for solutions with pH values greater than 5. A study of the available information revealed that the absorption spectra were least affected by changes in the acid concentration when the acidity was high.Further, the most effective control of acidity can be attained by evaporating sample solutions to fumes of sulphuric acid, and investigations were therfore limited to the determination of uranium in sulphuric acid solutions. Uranyl sulphate solutions absorb light at two wavebands, fromAugust, 19561 BY HIGH-PRECISION SPECTROPHOTOMETRY 457 275 to 325 mp and from 400 to 450 mp, the corresponding peak absorbancy indices (g per litre per cm) being 1.1 and 0.055, respectively. Unfortunately light of the first waveband is subject to serious interference from many elements, including iron, molybdenum, niobium, zirconium and so on. Many of these difficulties do not arise with the higher waveband and, since the applicability of this technique was just as important as the best accuracy, investi- gations were limited to the higher waveband.Moreover, since the ultimate accuracy attainable in a determination is dependent on the largest single error from any one operation of the procedure, extensive experiments were carried out to assess the degree of control needed for the attainment of maximum accuracy under these conditions. SYSTEM FOR OBTAINING ABSORBANCY VALUES The system used in this work consisted in reversing the solutions in the two cells reserved for absorbancy determinations, setting the instrument on zero, first by the reference solution in cell 1 and, when the solutions are reversed, by the reference solution in cell 2. Two readings are obtained with this system and, when the same two cells are used, the sum of the readings is directly related to the absorbancy difference ; for a complete mathematical treatment reference should be made to the report on which this paper is based.5 Under standard solution conditions it is shown, for example, that- where b, = path length of reference cell, 1, in cm and b, = path length of second cell, 2, in cm.During the exchange of the solutions, it is imperative that the intrinsic absorbancy error of the two cells should not change. The difference between the two readings is a direct measure of the constancy attained during the reading of any pair of solutions. The reading difference is dependent on the absorbancy of the reference solution, even when the same solution is used in both cells.When a series of reference solutions is used, therefore, a reliable value for each should be obtained, a graph constructed and close agreement to these values ensured before taking readings on test solutions. The use of the same solution in each cell ensures that solution errors are constant and the expressions for the reading differences5 under these conditions are- R, + R, = (4 + b,) ( A , - A,) = - (R, + R,), Rl - R, = (b, - b,) (A, + A,) - 2E,, and R2 - R, = (b2 - b,) (A, + A,) + 2% where El is the intrinsic absorbancy error of cell 2 with respect to cell 1 (positive). Typical values with the same uranium solution in each cell for three separate solutions are given in Table I11 A (see p. 466) and from these the mean values for (b, - b,) and El have been calculated.These results have been used to derive the calculated values for R, - R, and R, - R, shown in the calibration data given in Table 111. Comparison of these values against those obtained experimentally shows the maximum experimental error to be of the order of PRINCIPLE OF DETERMINING MAXIMUM ACCURACY- 0.05 per cent. Let C, = the concentration of the reference solution, C, = the concentration of the test solution, A , = the theoretical absorbancy of the reference solution under standard conditions A , = the theoretical absorbancy of the test solution under standard conditions The concentration error at any point on a spectrophotometric calibration graph is in a 1-0-cm cell, and in a 1-0-cm cell. defined exactly as- . . . . .. .. AC €C = € A x - AA ' ' AC AA where EC is the concentration error, EA is the error in reading AA and __is the calibration factor.The fractional error at any concentration C, is, therefore- (2)458 BACON AND MILNER: THE DETERMINATION OF URANIUM [Vol. 81 Now maximum accuracy is attained when the fractional error is a minimum. It follows, therefore, that maximum accuracy is attained when the following expression is at a maximum- (3) AA Hiskeyl has shown that for solutions obeying the Beer - Lambert law - is at a EA maximum when AA is 0.4343. AA AC Since - is constant, the maximum accuracy is attained when C, is at a maximum and, further, the fraction - is best determined by using a differential system of measurement and a value for AC such that AA is 0.4343. For solutions that do not obey the Beer - Lambert law, however, - decreases when the solutions fail to comply to this law.It follows, there- fore, that a plot of the function - x 2 against C, will show a maximum where the rate AA AC AA AC AA C EA A c AA AC of change of - is equal to that for C,. AA AC Theoretically - should be determined by using small values for AC and AA. Hiskeyl has shown, however, that the accuracy in determining AA decreases rapidly below a value of 0.20; hence AC should be chosen such that AA is approximately 0.20; and EA must be maintained constant, this being attained by using the same value for AC at various values for C,, which results in the reading being obtained at the same place on the logarithmic scale of the instrument, when the differential system of measurement is used.Although for solutions that do not obey the Beer - Lambert law AA changes when AC is constant, for small changes in AA, EA can be still considered constant. When AA is measured differentially, then either a change in the meter sensitivity or in the slit width is necessary to re-balance the instrument when the concentration of the reference solution is changed. In order to maintain constant slit width and yet attain satisfactory meter sensitivity, the sensitivity control should be adjusted against a reference solution of high concentration and a slit width chosen to give satisfactory meter response. Moreover, conditions should be chosen such that the sensitivity control can be used to re-balance the instrument when the concentralion of the reference solution is altered.The range of concentrations used for the reference solution is, therefore, limited and only values obtained on solutions of high concentration are shown in Fig. 1. The values for curve A were obtained by adding the absorbancy differences obtained for each increment of 4 g of uranium per litre to the absorbancy obtained for a solution containing 30g of uranium per litre. The values for any concentration on curve B were calculated from the absorbancy difference obtained by using a reference solution containing 30g of uranium per litre and a second solution containing 34 g of uranium per litre. They are, therefore, the theoretical absorbancies that would be obtained at concentrations of uranium higher than 30 g per litre if the solutions continued to obey the Beer - Lambert law.The deviation of curve A from curve B shows the experimental deviation of the solutions from the Beer - Lambert law. The values for curve C were calculated by using the experi- mental absorbancy difference obtained for each increment of 4 g of uranium per litre, use being made of the following expression- (Cl+ C2) 8 Relative accuracy = (A2 -- A,) x This is obtained from equation (3) by substituting c*2 for C,, A, - A, for A.4 2 and 4 for AC. It can be seen that curve C shows the concentrations at which the solutions fail to comply with the Beer - Lambert law far more clearly than curve A and that maximum accuracy is attained when the concentration of the reference solution is about 48 g of uranium per litre, considerably greater than the concentration at which the Beer - Lambert law fails.August, 19561 BY HIGH-PRECISION SPECTROPHOTOMETRY 459 The corresponding absorbancy for maximum accuracy is about 2.2.When the meter sensi- tivity is such, therefore, that the instrument can be read with certainty to 0.1 per cent. transmission, then for small differences in absorbancy the theoretical relative error is & 0.017 per cent. It can be seen, however, that the slope of the accuracy curve over the range 40 to 60g of uranium per litre is relatively small. Hence the concentration employed for the reference solution is not critical. The use of a reference solution of lower concentration than the optimum favours the use of narrower slit widths and 40 g of uranium per litre was chosen for experimental calibration purposes. 3.0 x m 3 U U u .- u - f! 2’0 b x C m L n 4 D d I ‘0 I 1 I I 20 30 40 50 60 Uranium concentration, g per litre Fig.1. Uranium concentration range for maximum accuracy with a Beckman spectrophotometer, l-cm cell and 0.80-mm slit width (10.4 mp) : curve A, experimental absorbancy ; curve B, theoretical absorbancy ; curve C, relative accuracy From the results obtained by using different slit widths, the generalisations that follow were deduced. Decreasing the slit width results in an increase in the slope of the theoretical curve, A, the deviation of the experimental absorbancy from the theoretical decreases and the maximum accuracy, the optimum concentration for maximum accuracy and the concentration where solutions fail to comply with the Beer - Lambert law, all increase.An approximate estimate of the extent of these changes is as folIows- Concentration at which Beer - Lambert Optimum Relative Slit width, A , law fails, concentration, accuracy mm g per litre per cm g per litre g per litre 0-4 0.048 40 54 2.1 0.8 0-047 32 48 1-9 SOLUTION VARIABLES From a mathematical consideration5 it can be shown that, when solution conditions are identical for each member of a pair of solutions but different from standard conditions, then the individual points of the calibration graph and subsequent measurements referred to this graph will only be affected by the fractional error in each pair of solutions. When, however, solution conditions vary in each member of a pair, the validity of the results is influenced by both the fractional and intrinsic errors of each solution.Therefore, when a reference solution is prepared in bulk and retained as a permanent reference solution, great care should be taken to ensure that any subsequent solutions are prepared from exactly the same reagent solutions. The inclusion of it “control” reference solution amongst each batch of solutions is to be recommended as a check on the validity of the permanent reference solution. Moreover, the inclusion of a second “control” solution, different in concentration,460 BAC,ON AND MILNER THE DETERMINATION OF URANIUM [Vol. 81 permits any errors to be classified as intrinsic or fractional and correction factors to be calculated. The type and size of some of the errors introduced by variations in solution conditions are discussed in detail under appropriate headings.CONTROL OF URANIUM CONCENTRATIONS- Experiments were conducted to determine the precision that could be attained when 50 ml and 100-ml calibrated flasks are used for volume adjustment, and the results are shown in Table I. The values show that the error in adjusting the meniscus was the same for both flasks. Casual inspection of the flasks indicated that 1:he neck bore of the 100-ml flasks was larger than that of the 50-ml flasks, but internal-bore measurements revealed that they were the same. Although the fractional error is considerably smaller for the 100-ml calibrated flasks, the four selected 50-ml calibrated flasks were used, and a coefficient of variation of 0.02 per cent. was accepted as satisfactory.Weighings were made with a balance having a sensitivity of & 0-1 mg and all weights were greater than 1 g. TABLE I COMPARISON OF THE USE OF 50-ml AND 100-ml CALIBRATED FLASKS Number of Deviation from Standard Coefficient Test determinations mean (range) deviation, of variation, ml % With 50-mlflasks at 20" C- The same flask . . .. .. 6 +0.009, -0*011 k 0.007 k 0-014 6 flasks selected a t random . . 6 + 0.046, -0*100 -!- 0.051 +o.102 4 flasks selected from 12 . . .. 4 +0.012, -0*010 0.010 k 0.020 With 100-ml flasks at 20" C- The same flask . . . . .. 6 + 0.010, -0.009 0.007 rfr. 0.007 6 flasks selected at random . . 6 +0.031, -0.039 0-024 k 0-024 4 flasks selected from 12 . . .. 4 +O.OlO, -0.012 0.008 & 0.008 CONTROL OF THE ACIDITY- In the study of the effect of acidity on the uranium absorption spectra, a series of solutions was prepared from analytical-reagent grade uranyl sulphate, U02S0,.3H,0.The uranium concentration was maintained constant at 0.75, g in 50 ml and sulphuric acid additions were made so that the acidity varied over the range 0-5 to 1 8 M Typical spectra are shown in Fig. 2 , from which it can be seen that for solutions less than 9 M in sulphuric acid, large changes in the final acidity do not influence the character of the uranium spectrum appre- ciably. At acidities in the region of 18 M , however, the character of the absorption spectrum is completely changed. When the absorbancy values were plotted against acidity for different wavelengths in the waveband 400 to 450mp, an optimum acidity was found for each wavelength at which the absorbancy is least affected by changes in the acidity.Optimum acidities of 2 and 4 M were obtained, for example, for wavelengths of 410 and 430mp, respectively. The behaviour at 430 mp was examined in greater detail by using the differential system of measurement, the result being shown gyapliically in Fig. 3. I t can be seen that small changes in acidity will produce the least error .when both the reference and the test solution are 4 M with respect to sulphuric acid. Further, when the acidity of the test solution differs from that of a reference solution that is 4 M with respect to sulphuric acid, the resultant error is always negative, irrespective of whether the test solution is greater or less than 4 M with respect to sulphuric acid.Moreover, the size of the error is dependent upon the extent of the difference in acidity between the test arid sample solution. Approximating over the curve in Fig. 3 in the acidity range 3.5 to 4-5 M with respect to sulphuric acid, it is found that a positive or negative change of 12 per cent. in acidity from 4 M results in a negative error of - 0.10 per cent. In consequence the acidity difference between the reference solution and the test solution should not vary by more than 2 per cent. if a reproducibility of 3- 0.02 per cent. is to be attained. With the system of preparing a bulk standard reference solution to be used as required over a long period, the acidity of the stock sulphuric acid used toAugust, 19561 BY HIGH-PRECISION SPECTROPHOTOMETRY 461 prepare this standard and that used a t a later date to prepare sample test solutions should not vary by more than 2 per cent.Experiments were carried out to determine the variations in acidity that occur when the procedure of evaporating to fumes of sulphuric acid is used for the removal of other solvent 0.9 h 0) - 0.8 P 0 -0 u 2 2 0.7 2 v x c 13 0.6 s n 4 0.5 13 12 I I 10 9 8 5 T) c x C 13 L .- s n L m 0 - EI 400 410 420 430 440 Wavelength, mp Fig. 2. Absorption spectra for uranyl sulphate measured on a Beckman spectrophotometer with 1-cm cell and 0-05-mm slit width (0-5 to 0.9 mp), the uranium concentration being 15 g per litre (0.063 M ) : curve A, 3-0 M sulphuric acid; curve B, 0.5 M sulphuric acid; curve C, 9 M sulphuric acid; curve D, 18.0 M sulphuric acid acids.Experimental results in the absence of uranium showed that standard acidity can be attained to about 2 per cent. by using a fuming period of 10 minutes 5 minutes. No difficulty was experienced in removing hydrochloric or perchloric acids. With nitric acid, however, diluting the solution with water after a single fuming resulted in the formation z 2 3 4 5 6 Concentration of sulphuric acid, M Fig. 3. Relative error for deviations from standard acidity with a Beckman spectrophotometer, 1-cm cell and 0.25-mm slit width (3.25 mp), the uranium concentration being 15 g per litre and the temperature 23" C of brown fumes, which indicated the presence of some residual nitrogen compounds. This behaviour is associated with the formation of nitrosylsulphuric acid, and the amount formed seemed to be dependent on the concentrations of the acids when mixed.Although no serious errors could be attributed to traces of residual nitric acid, double fuming was consideredBACON AND MILNER: THE DE.TERMINATION OF URANIUM 462 desirable. In the first set of experiments, several 2-3585-g portio.ns of selected U,O, were taken and dissolved in minimum amounts of nitric acid, sp.gr. 1-42. An accurately measured 20-ml portion of 20 N sulphuric acid was then added to each beaker, and the solutions were evaporated to fumes, the process being allowed to proceed with the beakers uncovered. The solutions were fumed for various times, before being cooled and diluted to 50ml with water. The absorbancy of each solution was measured against that of a solution that had been fumed for 1 minute only.The results show that the increase in error with time of fuming is quite significant under these conditions. On repeating these experiments and using covered beakers during the fuming stage, the loss of acid is very much smaller. CONTROL OF THE TEMPERATURE- resulting in a concentration error, and the solution characteristics may alter. effects can be experimentally determined by the following procedures- [Vol. 81 Results obtained in the presence of uranium are given in Table 11. The effect of a change in temperature is twofold6; the solution volumes are changed, The two (i) Use the same solution in cell 1 and cell 2 at different temperatures and measure the temperature difference and the absorbancy difference.(ii) Use two solutions of differing absorbancy in cell 1 and cell 2 at the same temperature, and measure the absorbancy difference. Repeat the experiment, still maintaining the temperature of the solution in cell 1 and cell 2 the same but a t a different value. The first procedure gives the summation of both the volume and characteristic changes. The second procedure gives only the characteristic changes. TABLE I1 INFLUENCE OF FUMING PERIOD ON URANIUM DETERMINATION Uranium solution contained 40 g per litre. Time of fuming, minutes . . .. .. 1 2 3 5 Fuming in a n open beakey- Error, yo . . .. .. .. .. 0 -0.06 -0.18 -0.30 Fzwzing in a covered beaker- Time of fuming, minutes.. . . .. 2 5 10 30 Error, yo . . .. .. . . .. 0 -0.06 +Om06 +0-09 The first system is simple in experimental operation.The volume change can be assessed from specific-gravity tables or specific-gravity determinations, the error calculated and the characteristic error derived. Temperature errors are of the fractional type and measurements are best conducted, therefore, with a solution of high absorbancy. In the experimental determination of the effect of changes in temperature a solution of uranium (36 g per litre) in 4 A4 sulphuric acid was employed. Cell 1 was filled with this solution at room temperature, whereas cell 2 was filled with the same solution at an elevated temperature. Temperature measurements were taken at intervals, together with the absorbancy readings at a wavelength of 430 mp, with a slit width of 0.70 mm.Cell 2 was then emptied, refilled with the same solution at a lower temperature than that in cell 1 and the reading procedure was repeated. Results showed that an increase in temperature from a standard temperature of 23" C resulted in a positive fractional error in the absorbancy, which was related to the deviation from tho standard temperature. When the temperature of the reference solution or the test solutioii differed by -t 8" C from the standard temperature, the absorbancy error was 1 per cent. of A , or 4 1 per cent. of A,, respectively. However, when the temperature of the reference and test solutions was the same, but differed from the standard temperature by 6" C, the absorbancy error was now only Temperature measurements showed that the gradient along the cell housing of thc Beckman instrument could be as high as 1.5" C and that the temperature in the cell housing was dependent on how long the lamp had been switched on.Both the lamp housing and the cell carriage were, therefore, fitted with water jackets, a circulating pump was installed and the circulating water was thermostatically controlled at a standard temperature. Time was always allowed for the solutions to acquire the same temperature in the cell compartment. A water bath was also thermostatically coiitrolled so that solutions could be diluted to volume at the standard temperature. 1 per cent. of A , - A41.August, 19561 BY HIGH-PRECISION SPECTROPHOTOMETRY SPECTROPHOTOMETER VARIABLES RELATIOXSHIP BETWEEK SLIT WIDTH AND METER SENSITIVITY- 463 It is essential in precision spectrophotometry that the meter sensitivity should be so adjusted that a 0.1 per cent.change in transmission will give a detectable movement of the galvanometer needle. Also the instrument must be sufficiently stable to ensure that fluctua- tions are less than the above deflection. To maintain this meter sensitivity as the absorbancy of the reference solution increases the slit width must be increased to re-balance the instrument. A plot of the minimum slit width that will give the above sensitivity against the absorbancy of the reference solution, for the two ranges available on the Beckman instrument, is given in Fig. 4. .c al .I M 1'4 1'2 1-0 0-8 0 6 0'4 0'2 0.0 0'4 0'8 1'2 1-6 2'0 2'4 Absorbancy of reference solution Fig.4. Relationship between absorbancy and slit width to give adequate sensitivity with a Beck- man spectrophotometer for uranium solutions in 1-cm cells a t 430mp: curl-e A, sensitivity setting 1.0; curve B, sensitivity setting 0.1 - - 14 - 1 3 x 12 .c -0 - x U - I1 2 L 2 - 9 5 1 - 8 - 10 3 L la I I I I 400 410 420 430 440 Wavelength, my Fig. 5. Absorption spectra for uranyl sulphate with a Beckman spectrophotometer and 1-cm cell a t an increased slit width of 0.40 mm (4 to 13 mp), the uranium concentration being 15 g per litre: curve A, 0.5 M sulphuric acid; curve B, 3.0 M sulphuric acid According to Hiskey,' the ratio of the slit widths for the reference solution and for the water blank is related to the intensities of the transmitted light by the expression- slit width for solution T Iw I s = (- slit width for water ) ' where Y is a function of light losses due to setting the mirrors and aligning the optical system.The value for Y should be 2 for correctly adjusted spectrophotometers. When the absorbancy values obtained for uranium solutions were plotted against the logarithm of the ratio of the slit width for the solution against the slit width for water, a straight-line calibration graph was obtained with a slope of 2. This result verified that the Beckman spectrophotometer used was in correct alignment. Further experiments showed that a t a constant slit width the meter sensitivity varied inversely as the transmission of the solution. Thus slit width, transmission and sensitivity can be inter~orrelated,~ as follows- slit width for water transmission for solution - meter sensitivity for solution ( slit width for solution ---I = transmission for water - meter sensitivity for water ' Tj'AVELENGTH AND MrAVEBA4ND SELECTION- With the normal light source of the Beckman instrument wide slit widths are necessary to obtain adequate meter sensitivity when reference solutions of high absorbancy are used, This arises because it is impracticable to increase the intensity of the light source.The effect of increasing the slit width on the plot of the absorption wavelength can be seen by comparing Fig. 5 with Fig. 2. By increasing the slit width eightfold, the irregularity in the absorbancy - wavelength plot at 430 mp (shown in Fig. 2) has entirely disappeared, whereas464 BACON AND MILNER: THE DETERMINATION OF URANIUM [Vol.81 the one at 412mp has been hardly affected. Further, the peak height at 420mp has decreased considerably. Doubling the slit width to 0 4 m m produced little change from the absorbancy values shown in Fig. 5, and it follows that the use of wide slit widths considerably decreases the error introduced by inaccurate setting of the slit width. Errors can arise, however, from inaccurate setting of the wavelength scale and these are at a minimum when the smallest change in absorbancy occurs for unit error in setting the wavelength scale. From Fig. 5 the corresponding wavelengths that satisfy this require- ment are 412 and 422mp. Unfortunately, interference by such elements as niobium and molybdenum (proposed alloying constituents for uranium-base alloys) is significant at these wavelengths.At 430 mp, however, the above interference is less troublesome and this wavelength was chosen for detailed investigation.. It is not claimed, therefore, that ultimate accuracy has been attained in this work. PREPARATION OF THE OXIDES- Uranium oxide, U308, can be readily prodiiced by igniting uranium compounds in the temperature range 800" to 1050" C, under oxidisjng conditions8 (but see later work by Brouns and Millsg). In an examination of this technique weighed quantities of different uranium salts were first carefully ignited at a low temperature and then finally for 3 hours at a tempera- ture of 850" C. After the residue had been cooled and weighed, the high-temperature ignition was continued for a further 30 minutes.In all cases, no change in weight was produced by this further ignition, but the amount of U,O, obtained was sometimes less than the theoretical. The U30, samples were next set aside under normal atmosphere conditions for 60 hours and any change in weight was recorded. The general tendency was for the oxides to gain in weight on standing and full details of the results are as follows- METHOD Deviation of weight of U,O, from weight Gain in weight of U,O, on exposure theoretically to atmosphere Uranium compound expected, for 60 hours, 01 Yo /O UO,(C,H,O,),.BH,O (AnalaR) . . .. .. - 4.64 0.05 U0,(N0,),.6H20 (AnalaR) . . . . . . nil 0.03 U0,S0,.3H20 (laboratory reagent) . . . . - 3.06 0.02 UO, (laboratory prepared) , . . . * . - 3.09 0-012 Further evidence in support of the results in the last column was obtained from the examination of Specpure U308 and a specimen of high-purity U308 prepared by chromato- graphy on cellulose.The full history of these samples was unknown, but the first had a loss in weight of 0.27 per cent. on ignition and the second a loss of 0.71 per cent. All this evidence emphasises the necessity of igniting all U,O, samples to constant weight before use. SOLUTIONS REQUIRED- Sulphuric acid, 20 N, standard stock soZu,tio.n--Cautiously pour 555 ml of AnalaR sulphuric acid, sp.gr. 1-84, into 400ml of water, while cooling the solution. Dilute to 1 litre at roum temperature with water. When a further stoc'k solution is prepared, it should be within 2 per cent of the standard stock solution.Standard uranium solution (primary reference solutz'on)-2.3585 g of the selected U,08 previously ignited to constant weight and stored in a desiccator were weighed for every 50 ml of solution required, transferred to a tall lipped conical flask and dissolved in the minimum amount of nitric acid, sp.gr. 1.42, and 20 ml of the stock sulphuric acid solution were added (from a burette to & 0.1 ml) for every 50 ml of solution required. The solution was boiled down and fumed 10 minutes, cooled, diluted to about four times the volume, evaporated and fumed for a further 5 minutes. The solution was cooled and diluted to the appropriate volume at a standard temperature of 23" C. The 250-ml calibrated flask was standardised against the 50-ml flasks subsequently used for preparing other solutions, and the appropriate weight of U,08 required was calculated by using the following equation- Weight of U30, = 2-3585 x y / x , where x = weight of water contained in standard 50-ml flask at 23" C, and y = weight of water contained in the 260-ml flask at 23" C.August, 19561 BY HIGH-PRECISION SPECTROPHOTOMETRY 465 Reference solution--2.3585 g of U,O, were processed as previously described. The solution was diluted to volume in a 50-ml calibrated flask at 23" C.Test soZzdion-Various weights of U,O, were processed as previously described and the solutions were diluted to volume in selected 50-ml calibrated flasks at 23" C. Sample solution-2-3585 g of the U,O, samples (2 g of the uranium metal) were pro- cessed as previously described and the solutions were diluted to volume in selected 50-ml calibrated flasks at 23" C.PROCEDURE FOR CALIBR.4TION- The accuracy of the calibration graph at any concentration is dependent on the absorbancy difference used to determine its slope. For solutions that obey the Beer - Lambert law the slope can be determined with maximum accuracy when the absorbancy 0'6 0'4 + 0-2 2 0'0 I Q U 0'2 - 0'4 I / ~~ ~~ 28 32 36 40 44 48 52 Uranium concentration, g per litre U n 26.0 5 n 25.0 g L u L 0) .- - L u n 24'0 M i e, U 23-0 0 .- U c n u 22'0 21'0 Fig. 6. Calibration graph for uranium by using 1-cm cells, 0-8 mm slit width (10.4 mp) a t 430 mp and a temperature of 23" C: curve A, absorbancy differences versus the uranium concentration (reference solution containing 40 g of uranium per litre) ; curve B, calibration factor versus the uranium concentration (reference solution containing 40 g of uranium per litre); curve C, calibration factor versus the average uranium concentration (multiple reference solutions) difference between any two solutions is 0.4343.The concentration range covered by one reference solution is dependent on the accuracy required over that range, and the choice of the concentration for the reference solution is governed by whether maximum accuracy is required at the upper, lower or centre part of the calibration range. For solutions that do not obey the Beer - Lambert law, the concentration range used to determine the slope is now governed by the change in slope in addition to the accuracy of the absorbancy measurement.At absorbancy differences less than 0.20, the accuracy of the determination decreases rapidly. A concentration difference of 4 g of uranium per litre was therefore used to determine the slope, and the system of plotting averages was applied to determine the slope at any specific uranium concentration. When the absorbancy of the test solution is less than that of the reference solution, measurements are made by balancing the instrument at zero setting against the test solution, and calibration graphs constructed in this manner show zero absorbancy at the concen- tration of the reference solution. Milner and PhennahlO have shown in detail that positive and negative absorbancies must be taken into account when corrections are applied and also when calculating the concentration of the test solution.Seven solutions were prepared to cover the range of 28 to 52 g of uranium per litre. A slit width was chosen (0.8 mm) such that, when the 52 g of uranium per litre solution was used to balance the instrument at zero,TABLE I11 CALIBRATION DATA Beckinan spectrophotometer ; 1-cm cells; slit width 0.8 mm; 430 mp; 4 M sulphuric acid; temperature 23" C ; +reference cell = 1 cm g Per litre 28 32 30 34 38 36 40 44 48 42 46 5 0 52 Uranium (A2 - A , ) (A2 - A , ) Absorb- concen- for for sub- ancy re- tration, C2 - C , single Factor sidiary Factor ferred Calcu- Calcu- g Per _ _ _ _ ~ Ri f R, R2 $- R4 reference C2 - Ci reference C2 - Ci to lated lated litre R , R2 R, R, R , - R, R, - R, b, + b2 b, + b2 solution A , - A , solution A , - A , water R , - K, R2 - R4 Uranium concentration, g per litre 28 40 52 0 0.563 0 0.375 0 0.189 0.0075 0 -- - L - - - - - - 0-183 0 0.350 0 0.501 0 - - - - 0 - 0.012 - 0.5561 - - - - 0 - 0.012 - 0.3684 - I - - 0 - 0.014 - 0-1817 - - - - 0.0075 - + 0.015 - - - - - 0.169 - + 0.014 - - - - - 0.834 -.+ 0.016 - 0.485 - + 0.016 - - - I - - - 0.5561 - - - - 0.3684 -- - - - 0.1817 - - 0 0 0.1757 -1 0.1757 - - - - 0.3414 + 0.3414 - - 0.4922 -b 0.4922 - 21.58 - - 0.1877 21-72 - - - 0.1867 22.01 - - - 0.1817 - - - + 0.1757 22.77 - 23.43 - 24.38 - - -{- 0.1657 - -1 0.1508 - 1.320 - 0.0130 - + z tl z - 1.508 - 0.0136 - - - 21.31 - lj k! 2.052 - -1- 0.0154 : m 5 - - 21-42 - M w $- 0.0148 * - 1.694 - 0.0142 - 22.01 - .. - - 1.876 - - - - 32.77 - - e x z 2.368 - + 0.0164 2 - - 24.14 - - 2.516 - -}- 0.0159 - - 26.52 - - 'TABLE 111 A CALCULATION OF CELL ERRORS Absorbanc y (referred Mean Mean to water) R, R2 R3 R4 R2 - R4 El b2 - 61 El b2 - b, 1-320 0 + 0.0065 0 - 0.0055 -b 0.011 + 0.0036 0*0011 - - 1.876 0 + 0.0075 0 - 0.0075 + 0.015 - 0.00 13 + 0.0033 + 0.0012 2.368 0 + 0.0090 0 - 0.0090 + 0.018 + 0.0030 0.0013 - -August, 19561 BY HIGH-PRECISION SPECTROPHOTOMETRY 467 a movement of the absorbancy dial of 0.001 resulted in a meter deflection of 1 division and, further, that the meter sensitivity control was capable of re-balancing the instrument when the 40 g of uranium per litre solution was used to re-balance the instrument.The absorbancy differences between the 40 g of uranium per litre solution and the other solutions were then found by means of the solution reversal system and the readings obtained are given in Table 111.The absorbancy differences are shown plotted against the uranium concentra- tion in Fig. 6, curve A. The reciprocal of the slope (calibration factor) was calculated for all differences with respect to the 40 g of uranium per litre solution. These values are shown plotted against the uranium concentration in Fig. 6, curve B. Any absorbancy difference can therefore be directly converted to uranium concentration from curve A or, preferably, the calibration factor can be obtained from curve B and the concentration difference calculated, addition or subtraction of this value from 40 giving the uranium concentration in the test solution in g per litre.For the very small differences obtained for the samples given in Table IV the factor at 40 g of uranium per litre is not critical and was approximated to 24 (* 0-0005 A = &- 0.03 per cent. of uranium). For sample solutions with uranium concentrations differing greatly from 40 g per litre, improved accuracy is attained by preparing a further reference solution with a uranium content almost identical with that of the sample solution. The absorbancy of the sample solution is then measured against this new reference solution. With this system the calibra- tion factor is obtained by reference to curve C of Fig. 6, which is prepared from a consideration of the absorbancy differences of consecutive solutions used in the calibration experiments (see Table 111).This curve is constructed by determining the calibration factor for each concentration range and plotting this value against the corresponding mean concentration value for the range. CALCULATING THE CONCENTRATION OF THE SAMPLE SOLUTION- Single reference solution-The absorbancy of the sample solution is obtained with respect to the 40 g of uranium per litre reference solution; the corresponding factor is then derived from the graph (Fig. 6, curve B) and the uranium concentration of the test solution is calculated by means of the following expression- Uranium concentration, g per litre = 40.0 _+ F , [A, - A , ] , where F, is the corresponding factor on curve B to & [A2 - A,]. Sztbsidinry ye ference solution-The absorbancy of the test solution is obtained with respect t o the subsidiary reference solution, the corresponding factor derived from the graph (Fig.6, curve C) and the uranium concentration of the test solution is calculated by means of the following expression- Uranium concentration, g per litre = C, * F , [ A , - A , ] , where F2 is the corresponding factor on curve C at -____ - (reference C , = zero absorbancy). 2 RESULTS The results for the analysis of samples of U308 prepared from different sources and of a specimen of uranium metal are given in Table IV. They are based on the u308 prepared from AnalaR uranyl acetate as the reference standard taken as 100 per cent. stoicheiometric U308 and can be considered relative to each other. This reference source was chosen because of the availability of large amounts of material from which large volumes of the stock reference solutions could be prepared.The importance of employing as the reference a material with an accurately known content of the constituent being determined has been stressed by h'eal.ll All the samples reported in Table IV were, therefore, analysed spectrographically for 37 elements. The high purity U,08 showed a positive value for total metallic impurities of less than 0.005 per cent. and none of the other samples gave a total impurity content greater than 0.02 per cent. From an examination of the relative values for uranium reported in Table IV, it is seen that a maximum difference of 0.06 per cent. of uranium resulted and it is unlikely that this could be due to some impurity undetected in the spectrographic analysis.In addition, the result for the sample of uranium metal proved to be slightly greater than any value obtained for the oxide samples and these values suggested that the conversion to U30, by ignition at468 BACON AND MILNER: THE DETERMINATION OF URANIUM [Vol. 81 a temperature of 850” C is not quite stoicheiometric. This view is supported by the results of recent work carried out by Brouns and Mills,!3 which became available after the completion of our study. These workers have investigated in some detail the conversion of the higher oxide of uranium, UO,, to the stoicheiometric form of U,O,, and have shown that, depending on the source of the uranium oxide, only 99.94 to 99.98 per cent. of the theoretical yield is obtained by igniting at a temperature of 850” C, irrespective of the time of ignition.This corresponds closely to the oxide values derived from Table IV, in which the metal sample is considered as 100.00 per cent. uranium and leads to the recommendation that all oxides should be ignited to constant weight at 1000” C. It is claimed by Brouns and Mills that at this temperature a recovery of 100.00 per cent. (k 0.01 per cent.) is attained irrespective of the source of the oxide by igniting for 1 hour and that no further change in weight occurs up to a temperature of 1200°C. The sample of U,O, designated as prepared from uranyl acetate in Table IV was the same as that used to prepare the stock primai-y reference solution and was included with the other samples as a “control” sample. The values for the different series in the table were obtained on different days over a period of 1 week and the value obtained for the TABLE IV RESULTS OBTAINED ON SAMPLES OF U,o, AND A SAMPLE OF URANIUM METAL Beckman spectrophotometer ; l-cm cells ; slit vvidth 0-8 mm ; 430 mp ; 4 M sulphuric acid ; temperature 3” C Standard deviation Mean from mean Series percentage percentage Coefficient Source of uranium 1 2 3 uranium uranium variation, of of v----7 of % Primary reference solution (U,O, prepared from uranyl acetate) 100.00 100.00 100.00 100.00 nil nil U308 (high purity) .. . . 99.97 100.06 100.00 100*01 0-046 5 0.046 U308 (acetate) . . .. . . 100.03 100*00 99-97 100.00 & 0.030 & 0.030 U308 (Specpure) . . .. . . 100~00 100~00 99.97 99.99 $-0*017 +0*017 .. . . 100*00 99.97 99.94 99.97 0.030 + 0.030 U (metal as cast) .. . . 100.00 100.06 100.03 100-03 & 0.030 3 0.030 Over-all coefficient of variation = +0.031 per cent, u*os (UQ) * * “control” shows that any instability in the stock primary reference solution was less than the detectability of the method. Satisfactory control of the acidity and other factors had, therefore , been attained during sample analysis. CONCLUSIONS The accuracy attained by the described procedure compares favourably with that attainable by volumetric or gravimetric methods. Moreover, this procedure is more specific. The need to use accurately calibrated volumetric equipment and to place solutions in thermo- statically controlled baths is troublesome when only occasional samples are required to be analysed. The differential spectrophotometric technique is best suited to laboratories having to determine the uranium content of uranium-base materials continuously, since the time taken in setting up apparatus and calibrating equipment is then time spent advan- tageously. In addition, the technique is rapid in operation and is therefore convenient for control analysis. REFERENCES 1. 2 . 3. 4. 5 . Hiskey, C. F., Anal. Chew., 1949, 21, 1440. Kaplan, L., Hildebrant, R. A., and Ader, M., Argonne National Laboratory, Lemont, Illinois, Sutton, J., National Research Council of Cana.da, Chalk River, Ontario, Report CRC 325, 1947. Rabinowitch, E., Argonne National Laboratory, Lemont, Illinois, Report ANL 5173, 1953. Bacon, A., and Milner, G. W. C., Atomic Energy Research Establishment Report C/R 1637, Report ANL 4521, 1950. Harwell, 1955.August, 1956 j BY HIGH-PRECISION SPECTROPHOTOMETRY 469 6. Bastian, R., Anal. Chew., 1953, 25, 259. 7. 8. 9. 10. 11. Hiskey, C. F., Rabinowitz, J., and Young, I. G., Ibid., 1950, 22, 1464. Rodden, C. J . , “Analytical Chemistry of the Manhattan Project,” McGraw-Hill Book Co. Inc., Brouns, R. J., and Mills, W. W., Report HW 39767, Office of Technical Services, U.S. Dept. of Milner, G. W. C., and Phennah, P. J., Analyst, 1954, 79, 414. Neal, W. T. L., Ibid., 1954, 79, 403. New York, 1950, p. 46. Commerce, Washington 25 D.C., November, 1955. ANALYTICAL CHEMISTRY GROUP ATOMIC ENERGY RESEARCH ESTABLISHMENT HARWELL, NR. DIDCOT, RERKS. January 16th, 1956

ISSN:0003-2654    

DOI:10.1039/AN9568100456

出版商:RSC    

年代:1956

数据来源: RSC

摘要:

August, 1956 j BY HIGH-PRECISION SPECTROPHOTOMETRY 469 The Determination of Magnesium Oxide in Magnesium BY H. J. ALLSOPP Magnesium metal is removed from any oxide present by sublimation in vacuum and the residue is dissolved in dilute hydrochloric acid. Any iron or aluminium present is removed as the hydroxide, and the magnesium content of the residue is determined volumetrically by using disodium ethylene- diaminetetra-acetate. THE discovery of remarkable mechanical properties, especially a t elevated temperatures, in extrusions made from fine flake powder of aluminium (S.A.P.) by Swiss workers1 opened up a new field of powder metallurgy, which has now been extended to extrusions made from fine magnesium powder.2 In these products no attempt is made to remove the natural oxide film present on the surface of each powder particle, and indeed, it is the included oxide films in the extruded material that by hindering grain growth, impart the high strength.The oxide content is not a direct measure of the grain size (and therefore strength) of a powder extrusion, since a given oxide content may arise from a coarse powder heavily oxidised, or from a finer powder bearing the minimum film formed in air at room temperature.3 Nevertheless, for development of this type of material, knowledge of the oxide content of any given grade of powder and extruded product is essential. It was desired, therefore, to devise a method for the determination of the oxide content of magnesium, and since chemical methods did not offer a likely field for investigation, the possibility of separating the metal from the oxide by vacuum distillation was explored.EXPERIMENTAL The first attempts at a separation of the magnesium oxide from magnesium were based on the well known method for the determination of oxide in aluminium, i.e., dissolution in bromine - methanol. These failed owing to the solubility of magnesium oxide in the solvents used. Since no other likely chemical method could be envisaged, it became apparent that a gaseous separation would have to be employed. A method has been described* in which the magnesium content of aluminium alloys is determined by vacuum distillation and another" wherein magnesium scrap is refined by vaporisation at extremely low pressure. It was thought likely that a separation could be effected along these lines.The apparatus used, shown in Fig. 1, consisted of a platinum-wound electric furnace, which could be rolled back from a silica tube closed at one end and containing the samples to be processed. The open end of the silica tube was sealed with wax into a brass adaptor containing glass-wool as a filtering medium, the other end of the adaptor being sealed with wax into the low-pressure side of a conventional type of oil-diffusion pump, The high- pressure side of the diffusion pump was backed by a two-stage rotary oil pump. A Pirani gauge head was fitted between the diffusion and backing pumps and an ionisation gauge on the low-pressure side of the diffusion pump. The samples under examination were placed in small mild-steel crucibles and covered with closely fitting lids carrying small holes as gas vents.As a precaution, the lids were held on with wire.470 ALLSOPP: THE DETERMINATION OF [Vol. 81 The first experiments consisted in placing a quantity of ignited magnesium oxide in a hole in a small block of pure magnesium and closing the hole with a small screw made from the same material. This was placed in the crucible, which was then put into the silica tube; the system was evacuated to approximately mm of mercury and the temperature was raised to about 1000" C. On cooling, it was found that the residue of magnesium oxide had retained the shape of the hole in which it was placed in the sample and had not, as was originally expected, mixed with the molten magnesium metal. Hence this procedure did not simulate the conditions existing in a specimen of oxidised metal.I I Control couple LA- Fig. 1. Layout of apparatus Three cold-compacts were then made from magnesium powder that had been well mixed, and these were distilled. The results are shown in Table I. TABLE: I FIRST ATTEMPTS AT DETERMINATION OF OXIDE IN MAGNESIUM POWDER Weight of compact, g . . .. . . . . 2.0133 2.0259 2-0047 Weight of residue, g . . .. .. . . 0.0310 0.0289 0.0290 Residue (magnesium oxide + impurities), % 1.54 1.44 1.45 On examination, the residue was found to be dark-grey in colour, probably owing to impurities in the original sample. Spectrographic examination of the ash showed the presence of traces of zinc, iron, silica, manganese, nickel and aluminium. It was therefore decided to determine the magnesium content of the residue chemically and thus to obtain a more accurate figure for the oxide content.Eight cold-compacts were made from the same magnesium powder as before and vacuum distilled. The magnesium contents of the residues were determined by the well known phosphate method. Table TI shows the results, which are in good agreement. TABLE I1 OXIDE DETERMINATION IN MAGNESIUM POWDER (PHOSPHATE FINISH) Weight of compact, g . . . . 2.0100 2.0272 5!*0350 2.0238 2.0226 2.0306 2.0308 1.9976 Weight of residue, g . . . . 0.0278 0.0284 0-0286 0.0296 0.0298 0.0297 0.0301 0.0300 Residue (magnesium oxide + impurities), yo .. . . 1.39 1.40 1.41 1.46 1.47 1.46 1.48 1-50 Weight of magnesium pyrophos- phate, g .. .. . . 0.0740 0.0735 0.0748 0.0751 ' 0.0747 0.0750 0.0754 0.0754 Magnesium oxide, yo .. . . 1.33 1.31 1.33 1.34 1.34 1.34 1.35 1-36August, 19561 MAGNESIUM OXIDE I N MAGNESIUM 471 It was noticed that in all experiments, the residues from the vacuum distillations had retained the shape of the original compacts, thus showing that the magnesium metal had passed from the solid to the gaseous phase without liquifying, and therefore losses would not be caused by spattering on boiling. In order to prove that the method was reliable it was necessary to make an addition of oxide and so establish the recovery. Zinc oxide was chosen, as it reacts readily with hot magnesium to give magnesium oxide and zinc, and it had already been proved that zinc distils over together with magnesium in v a c ~ u m .~ Table IIIA shows the magnesium oxide contents found on six cold-compacts of a new batch of magnesium powder that was used as a basis for the oxide addition. For these tests twelve 2-g portions of this material were weighed out and to six of these 0.1 g of zinc oxide was added, and to the remainder 0-2 g. The powders were mixed with a spatula, quantitatively compacted and vacuum distilled. The magnesium in the residues was determined volumetrically by using disodiuni ethylenediaminetetra-acetate (EDTA), according to the method outlined by Banks.6 The results obtained are given in Tables IIIB and IIIC and show good recovery. TABLE IIIA MAGNESIUM POWDER USED AS BASIS FOR OXIDE ADDITION Weight of compacts, g . . .. . . 2.0032 2.0000 2.0055 2.0044 2.0089 2.0055 Volume of EDTA solution (1 ml = 0.0004 g of ILIgO), ml .. .. . . . . . . 15.1 15.1 14-9 14.9 15.0 15.1 Magnesium oxide, yo . . .. .. . . 1.51 1.51 1.49 1.49 1-49 1.51 Residues dissolved, diluted to 500 ml, and 100 ml taken TABLE IIIB RECOVERY OF MAGNESIUM OXIDE AFTER 0.1-g ADDITIONS OF ZINC OXIDE Residues dissolved, diluted to 500 ml, and 50 ml taken 1 ml of EDTA = 0.0004 g of MgO Weight of magnesium taken (1.5% of g 2.0021 2.0010 2.0003 1.9995 2.0017 2.001 1 MgO), Weight of zinc oxide added, g 0.1004 0.1027 0*1000 0.1060 0.1004 0*1100 Total weight of sample, g 2.1025 2-1037 2.1003 2.1055 2.1021 2.1111 Weight of magnesium oxide from magnesium powder, g 0.0300 0.0300 0.0300 0.0300 0.0300 0.0300 Weight of magnesium oxide from zinc oxide, g 0-0497 0.0509 0.0496 0.0525 0.0497 0.0545 Magnesium oxide calculated on total sample, g 0.0797 0.0809 0.0796 0.0825 0-0797 0.0845 Magnesium oxide calculated on total sample, % 3.79 3.85 3.79 3.92 3.79 4.00 Volume of EDTA used, ml 20.1 20.2 19.9 20.7 19-8 21.1 Magnesium oxide recovered on total sample, Yo 3.83 3.84 3.79 3.93 3.77 4.00 TABLE IIIC RECOVERY OF MAGNESIUM OXIDE AFTER 0-2-g ADDITIONS OF ZINC OXIDE Residues dissolved, diluted to 500m1, and 50ml taken 1 ml of EDTA = 0.0004 g of MgO Weight of rnagnesium taken (1.59/, of R~gO), g 2.0027 2.0076 2.0019 2.0035 2+0070 6.0047 Wei,pht of zinc oxide added, g 0.2038 0.2006 0-2023 0.2065 0.2013 0.2012 Total weight of sample, g 2.2065 2.2082 2.2042 2.2100 2.2083 2.2059 Weight of magnesium oxide from magnesium powder, g 0.0300 0.0301 0.0300 0.0300 0-0301 0.0301 Weight of magnesium oxide from zinc oxide, g 0.1009 0.0994 0.1002 0.1023 0.0997 0.0997 Magnesium oxide calculated on total sample, g 0.1309 0.1295 0.1302 0-1323 0.1298 0.1298 Magnesium oxide calculated on total ( sample, 5.93 5.87 5-91 5.98 5.88 5-88 Y O Volume i f EDTA used, ml 32.7 32.2 32.7 33.2 32.5 32.65 Magnesium oxide recovered on total sample, 5.93 5.84 5.94 6.01 5-89 5.92 Y O The nitrogen content of residues from six distillations was determined and figures showing between 0.06 and 0-08 per cent.of nitrogen were obtained. This indicated that less than 0.2 per472 ALLSOPP THE DETElRMINATION OF [Vol. 81 cent. of the magnesium in the residue was present as nitride and, since its effect on the final result was negligible, its presence was ignored.TABLE IV TYPICAL RESULTS OBTAINED BY RECOMMENDED METHOD Sample No, 1- Weight of sample, g . . . . . . . . of MgO), ml . . . . . . . . .. Magnesium oxide, % . . . . .. .. Weight of sample, g . . . . .. .. of MgO), ml . . .. . . .. .. Magnesium oxide, % . . . . . . .. Weight of sample, g . . . . . . .. of MgO), ml . . .. . . .. . . Magnesium oxide, "/o . . .. . . .. Weight of sample, g . . . . . . .. of MgO), ml . . . . .. .. . . Magnesium oxide, % . . .. . . .. Weight of sample, g . . .. .. .. of MgO), ml . . . . .. .. . . Magnesium oxide, . . . . .. .. Volume of EDTA solution (1 ml = 0.000399 g Sample No. 2- Volume of EDTA solution (1 ml = 0.000399 g Sample 240 Fines- Volume of EDTA solution (1 ml = 0.000399 g Sample S- Volume of EDTA solution (1 ml = 0.000399 g Sample U1- Volume of EDTA solution (1 ml ZE 0.000399 g 3.8268 3.8441 3.8394 3.8484 3.8408 3.828:) Residues dissolved, diluted to 500 ml, and 50 ml taken 10.95 11.45 11.35 11-50 11-55 11.55 1.14 1.18 1-18 1.19 1.20 1.20 2-3817 2.3947 2.3771 2.3443 2.3688 2.3730 Residues dissolved, diluted t o 500 ml, and 100 ml taken 26.95 27.45 27.20 26.85 27.10 27.65 2.25 2.28 2.28 2.28 2-28 2.32 2.4553 2.4779 Residues dissolved, diluted to 500 ml, and 100 ml taken 23.8 1.93 24.2 1.95 14838 1.4281 Residues dissolved, diluted to 250 i d , and 100 ml taken 20.8 1.40 20.5 1.43 1.5650 1.4995 Residues dissolved, diluted t o 600 ml, and 100 nil takcri 18.5 2.36 17.8 2.37 METHOD REAGEKTS- should be standardised against ignited magnesium oxide before use.of equal parts by volume of the purest available triethanolamine and isopropanol.PROCEDURE- crucible and keep the perforated lid firmly in position by means of wire. tube and connect to the diffusion pump. Slowly raise the temperature to 900" to 1000" C over a period of 2 hours. away from the silica tube and allow the tube to cool to room temperature. Disodium ethylenediaminetetra-acetate (EDTA ) , approximately 0.08 N-The solution Solochrome black W.D.F.A. indicator*-Make a 0.5 per cent. w/v solution in a mixture Ammonium chloride, 25 per cent. w/v solution. Place a suitable weight of sample previously cleaned of surface oxide into a mild-steeI Insert into the silica Evacuate the system to about 10-4 mm of mercury. Roll the furnace Remove the crucible from the apparatus and transfer the residue to a beaker.Add 10 ml of water, 10 ml of concentrated hydrochloric acid and 10 drops of concentrated nitric acid, and evaporate until almost dry. Dilute with about 10 ml of water, add 20 nil of 25 per cent. ammonium chloride solution, a pottion of litmus paper and render just ammoniacal. Bring just to the boil. Filter on a No. 41 Whatman filter-paper and wash the precipitated hydroxides of iron and aluminium with water. Redissolve the precipitate into the original beaker, using 10 ml * Solochrome black W.D.F.A., manufactured by Imperial Chemical Industries Limited, is the equivalent of Eriochrome black T. Add two drops of ammonia solution, sp.gr. 0.880, in excess.August, 19561 MAGNESIUM OXIDE I N MAGNESIUM 473 of 20 per cent. hydrochloric acid.Re-precipitate the hydroxides, using another 20ml of 25 per cent. ammonium chloride solution. Filter and wash as before and combine the filtrates in a 500-ml calibrated flask. Make up to the mark and mix well. Transfer a suitable aliquot to a conical flask, dilute to 100 ml with distilled water (if necessary) and add ammonia solution, sp.gr. 0-880, in accordance with the following table- Volume of ammonia solution, sp.gr. 0.880, ml . . 4 6 10 Bring just to the boil. Volume of aliquot, ml.. .. . . .. .. 20 50 100 Add 10 drops of indicator and titrate with EDTA solution to a blue end-point without any tinge of red. Sometimes it may be necessary to dilute the bulk solution to 250 ml or less, in which event the volumes of the additions of 25 per cent. ammonium chloride solution should be ad j ust ed accordingly. RESULTS Some typical values for various samples are given in Table IV. CONCLUSIONS The magnesium oxide content of magnesium metal can be obtained satisfactorily by sublimation of the metal in vacuum followed by the determination of the magnesium in a solution of the residue by titration with disodium ethylenediaminetetra-acetate. The time taken for a single determination is in the region of 5 hours. I thank Mr. T. R. F. W. Fennell, who determined the nitrogen content of the residues. Reproduced by permission of the Controller, H.M. Stationery Office. REFERENCES 1. 2. 3. 4. 5 . 6. Irmann, R., Metallurgia, 1952, 46, 125. Brown, D. J., “The Powder Metallurgy of Magnesium,” Powder Metallurgy Symposium, 1954, The Iron and Steel Institute, London, Group 3, pp. 100 to 104. Hkenguel, J., and Boghen, J., Revue de Metallurgie, 1954, 51, 265. Urech, P., and Sulzberger, R., Aluminium, 1954, 30, 163. Rogers, R. R., and Viens, G. E., J . Electrochem. SOC., 1950, 97, 419. Banks, J., Analyst, 1952, 77, 484. ROYAL AIRCRAFT ESTABLISHMENT FARNBOROUGH, HANTS. October 17th, 1955

ISSN:0003-2654    

DOI:10.1039/AN9568100469

出版商:RSC    

年代:1956

数据来源: RSC

摘要:

474 SPOREK : DETERMINATION OF MERCURY IN FUNGICIDAL PREPARATIONS [VOl. 81 Determination of Mercury in Fungicidal Preparations Containing Organo-mercury Compounds Part I. The Determination of Organo-mercury Compounds by Direct Titration Procedures BY K. F. SPOREK Several analytical procedures for the determination of mercury in the form of organo-mercury compounds arc described. Details are given of methods suitable for the direct titration of organo-mercury compounds with the organic radical attached to the mercury atom. THE expanding use of mercury compounds, especially those with organically combined mercury, for fungicidal purposes has led to a large and constantly growing number of preparations being sold on the market. These materials are used by farmers and nurserymen for the treatment of seeds, bulbs, tubers, plants, fruit trees and so on. The range of mercury compounds used as fungicides is wide and comprises inorganic and organic mercury derivatives.The formulations containing mercury are also numerous and vary in the character of the mercury compound, its content, the presence or absence of other active agents in the form of insecticides (hexachlorocyclohexane, dieldrin, lead arsenate) and the type of the inert diluent (filler), which may be soluble or insoluble in water. When account is taken of the large number of possible diluents that may be employed and the number of mercury compounds with or without other active or inactive agents, it is easy to imagine the complexity of the final products. The routine analysis of the mercury fungicides normally is limited to the determination of total mercury and only rarely is the determination of other constituents attempted.The determination of total mercury is, however, on its own a difficult problem owing to the following facts- (1) Mercury compounds are volatile, and this calls for special precautions when treat- ment at temperatures above room temperature is applied. Ethylmercuric chloride, for example, even at room temperature is sufficiently volatile to make itself detectable by the smell and taste of its vapour. (2) The difficulty in transforming organo-mercury compounds into inorganic mercury salts, which normally calls for rather drastic treatment, such as boiling with sulphuric - nitric acid mixture or sulphuric acid and potassium permanganate. In the course of such treatment other constituents of the material are also attacked.(3) The fact that the best quantitative determination of mercury, which is by way of the thiocyanate titration in acid solution, with ferric alum as indicator, is seriously affected by even small amounts of halide ions. The titration itself is very accurate and simple, but as it must be conducted in complete absence of chloride ion this makes it less attractive. (4) The mercury content of fungicidal preparations is low (1 to 2 per cent.) and so the size of a sample required for a determination is necessarily large (10 to 20 g). The fillers in such a large quantity of the sample usually contain enough chloride to make a titration with thiocyanate impossible.As the mercury content of fungicides based. on this element is important, not only in view of the high price of the active agent used but also because of dangers of damage to the treated seeds, trees and so on with an overdose of mercury compounds, it is of paramount importance to know the concentration of mercury in these preparations. In the search for a method that would give accurate results, be applicable to as many different products as possible, be reasonably easy 110 carry out and not involve use of expensive and specialised reagents, several existing procediires for the determination of mercury were tried. The details of the tested methods as well as their usefulness when such was found are described in the following section.August, 19561 CONTAINING ORGANO-MERCURY COMPOUNDS. PART I EXPERIMENTAL DETERMINATION OF ORGANO-MERCURY ION BY DIRECT TITRATION WITH STANDARD THIOCYANATE- 475 The titration of mercuric salts, usually in the form of nitrate or sulphate, with ammonium thiocyanate solutions, with ferric alum as indicator, is the most widely used procedure for the determination of this element.When organo-mercury compounds are to be tested they are usually transformed into the inorganic mercury salts by oxidation of the organic radical and then, provided chloride ion is absent, they are titrated with the thiocyanate reagent. In this work an attempt was made to titrate certain organo-mercury compounds with thiocyanate without destroying the organic radical. Aqueous solutions of the compounds shown in Table I were titrated in the presence of nitric acid with 0.1 N ammonium thiocyanate solution, ferric alum being used as indicator.TABLE I DIRECT TITRATION OF ORGANO-MERCURY COMPOUNDS WITH STANDARD 0.1 N AMMONIUM THIOCYANATE Compound Solubility in 0.1 N nitric acid solution* Remarks on titration Ethylmercuric acetate . . . . Soluble Sharp end-point ; precipitate Methoxyethylmercuric acetate . . Very soluble Poor end-point; no precipitation Phenylmercuric acetate . . . . Soluble Sharp end-point ; precipitate formed, Methoxyethylmercuric borate . . Very soluble Poor end-point ; no precipitation Phenylmercuric borate . . . . Sparingly soluble Sharp end-point ; precipitate formed, Phenylmercuric nitrate . , . . Sparingly soluble Sharp end-point ; precipitate formed, Bis(ethoxyethy1mercuric) hydrogen phosphate .. . . .. . . Sparingly soluble Sharp end-point ; no precipitation Ethylmercuric phosphate . . . . Soluble Sharp end-point ; precipitate formed, Hydroxyethylmercuric silicate . . Sparingly soluble Sharp end-point; no precipitation Methoxyethylmercuric silicate . . Soluble Sharp end-point; no precipitation Methoxyethylmercuric tannate . . Soluble Reacts with ferric alum Sodium ethylmercurithiosalicylate . . Soluble Sharp end-point, no precipitation Phenylmercuric urea .. . . Sparingly soluble Sharp end-point ; precipitate formed, soluble in acetone soluble in acetone soluble in acetone soluble in acetone insoluble in acetone * “Soluble” indicates 1 per cent. or greater solubility. The results in Table I show that many organo-mercury compounds can be directly titrated with a standard thiocyanate solution.The end-point was usually of the same quality as that for inorganic mercury compounds. Some organo-mercury compounds formed sparingly soluble thiocyanates, which were precipitated during the titration. This precipitation did not interfere with the end-point or affect the result, and all the precipitates were easily soluble on addition of acetone to the titrated solution. The factor for the thiocyanate solution was twice the value applicable when the reagent was used for the titration of inorganic mercury salts (1 ml of 0-1 N solution _I 0.02006 g of mercury for organo-mercury compounds). DETERMINATION OF ORGANO-MERCURY COMPOUNDS BY NON-AQUEOUS TITRATION- The procedure tested had originally been used for the determination of ethylenic com- pounds,l which were treated with mercuric acetate in the presence of methanol. The reaction product for ethylene was methoxyethylmercuric acetate.The titration, which determines only the ionised mercury, gave a measure of the amount of mercuric acetate left in the solution plus the valency occupied by the acetate radical in methoxyethylmercuric acetate. The acetate radical was titrated with standard hydrochloric acid in a solvent mixture containing chloroform and propylene glycol. The end-point of this titration was extremely sharp and for this reason the procedure was tested for its suitability for the determination of certain organo-mercury compounds. The details of the procedure were as follows-About 0.5 g of the compound was dissolved in 25 ml of a chloroform - propylene glycol mixture (1 + 1) contained in a 100-ml conical flask, 5 drops of 0.2 per cent.thymol blue indicator in ethanol were added, and the solution476 SPOREK : DETERMINATION O F MERCURY I N FUNGICIDAL PREPARATIONS [VOl. 81 was titrated with a 0.1 N hydrochloric acid solution (made by dissolving 9 nil of concentrated acid in 1 litre of the solvent mixture). The end-point was indicated by a sharp change of colour from yellow to pink (1 ml of 0.1 N hydrochloric acid = 0.02006 g of mercury in organo- mercury compounds). From this table it is seen that many organo-mercury compounds art: titratable under the conditions described. The end-points were usually sharp to 0.01 to 0 02 ml of the reagent in about 50 ml of the TABLE I1 Table I1 shows the compounds tested and the results obtained.DETERMINATION OF ORGANO-MERCURY COMPOUNDS BY DIRECT TITRATION I N Compound Methoxyethylmercuric acetate . . Phenylmercuric acetate, pure . . Phenylmercuric acetate, technical . . Mercuric acetate . . . . . . Methoxyethylmercuric borate . . Ethylmercuric acetate . . . . Phenylmercuric borate . . .. Phenylmercuric nitrate . . .. phosphate . . .. . . . . Ethylmercuric phosphate . . .. Methoxyethylmercuric chloride . . Bis(ethoxyethy1mercuric) hydrogen Ethoxyethylmercuric silicate . . Hydroxyethylmercuric silicate . . Methoxyethylmercuric silicate . . Methoxyethylmercuric tannate . . Sodium ethylmercurithiosalicylate . . Phenylmercuric urea . . . . NON-AQUEOIJS MEDIUM Solubility in the solvent mixture* Remarks on titration Very soluble Very soluble Very soluble Very soluble Sparingly soluble Very soluble Very soluble Soluble Insoluble Insoluble Insoluble Insoluble Insoluble Insoluble Insoluble Very soluble Insoluble Good end-point Good end-point Good end-point Good end-point Good end-point Good end-point Poor end-point, but possible to Not titratable Not titratable titrate Good end-point when titrated in hot solution Not tit ra table Good end-point when titrated in hot solution Good end-point when titrated in hot solution Good end-point when titrated in hot solution Good end-point when titrated in hot solution Good end-point Good end-point when titrated in hot solution * “Soluble” indicates 1 per cent.or greater solubility. TABLE I11 DETERMINATION OF ORGANO-MERCURY COMPOUNDS BY DIRECT TITRATION WITH STANDARD ACID Compound Methoxyethylmercuric acetate .. Mercuric acetate . . .. .. Methoxyethylmercuric borate . . Ethoxyethylmercuric chloride . . Hydroxyethylmercuric chloride . Methoxyethylmercuric chloride . . Bis(ethoxyethy1mercuric) hydrogen Ethoxyethylmercuric silicate , . Hydroxyethylmercuric silicate . . Methoxyethylmercuric silicate . . Methoxyethylmercuric tannate . . Sodium ethylmercurithiosalicylate . . Ethylmercuric acetate . . .. Phenylmercuric acetate . . .. Phenylmercuric borate . . .. Ethylmercuric chloride . . .. Phenylmercuric chloride . . .. Phenylmercuric nitrate . . . . phosphate . . . . .. .. Ethylmercuric phosphate . . .. Phenylmercuric urea .. .. Solubility in the reaction mixture* Soluble Very solu blt: Soluble Soluble Very soluble Soluble Soluble Soluble Soluble Soluble Soluble Sparingly schble Insoluble Sparingly soluble Insoluble Insoluble Insoluble Insoluble Soluble Sparingly soluble Remarks on titration Good end-point ; precipitate with KI Good end-point Good end-point ; precipitate with KI Good end-point ; precipitate with KI Good end-point Good end-point ; precipitate with KI Poor end-point ; precipitate with KI Good end-point Good end-point Good end-point Good end-point ; precipitate with KI Good end-point ; precipitate with KI Not titratable Good end-point ; precipitate with KI Not titratable Not titratable Not titratable Not titratable Not titratable Good end-point; precipitate w i t h ICI * “Soluble” indicates 1 per cent.or greater solubility.August, 1956: CONTAINING ORGANO-MERCURY COMPOUNDS. PART I 477 final solution. mixture and near the end-point the reagent was added dropwise. Sparingly soluble compounds were titrated at the boiling point of the solvent DETERMINATION OF ORGANO-MERCURY COMPOUNDS BY TITRATION WITH STANDARD ACID- A procedure described in the literature2y3 and based on the fact that solutions of mercuric oxide yield hydroxyl ion when treated with potassium iodide was stated to be unaffected by the chloride ion. In this work the method was tested on organo-mercury compounds. About 0.5 g of the sample was dissolved in 50 ml of 50 per cent. aqueous acetone, 3 g of urea were added, followed by a few drops of phenolphthalein indicator and 0.1 N sodium hydroxide solution until the colour turned to faint pink.The mixture was then treated with 5 g of potassium iodide, and the liberated alkali was titrated with 0.1 N perchloric acid solution to the same faint pink colour of the indicator. The compounds tested and the results obtained are shown in Table 111, from which it is seen that organo-mercury chlorides are amongst the compounds titratable under the described conditions. This is the only procedure capable of determining this type of com- pound and is therefore especially useful for rapid determinations of phenylmercuric chloride, ethylmercuric chloride and ethoxyethylmercuric chloride, which are amongst the most commonly used organo-mercury fungicides. CONCLUSIONS The procedures described above involving direct titrations of organo-mercury compounds are especially useful in assessing the purity of raw materials and testing mixtures from which these compounds can easily be extracted with a suitable solvent. The thiocyanate titration will account for almost all types of compounds except the chlorides, the non-aqueous titration for all organo-mercury salts of weak acids and direct titration with acid for almost all soluble compounds, including chlorides. The various com- binations of the above procedures are capable therefore of dealing with any type of compound. Singly, however, none of them could be applied to the testing of the extremely wide range of organo-mercury fungicides and so their usefulness in this respect is limited. I express my thanks to Plant Protection Ltd., for permission to publish this paper, to J. M. Winchester, Chief Chemist, for his helpful criticism and to Miss A. Butler for assistance in the experimental work. REFERENCES 1. Das, M. N., Axal. Chew., 1954, 26, 1086. 2. - , Ibid., 1953, 25, 1406. 3. Palit, S. R., and Somayajulu, G. R. S., Ibid., 1955, 27, 1331. TECHNICAL DEPARTMENT PLANT PROTECTION LIMITED YALDING, KENT March 7th, 1956

ISSN:0003-2654    

DOI:10.1039/AN9568100474

出版商:RSC    

年代:1956

数据来源: RSC

摘要:

478 SPOREK : DETERMINATION OF MERCURY IN FUNGICIDAL PREPARATIONS [Vol. 81 Determination of Mercury in Fungicidal Preparations Containing Organo-mercury Compounds Part 11. The Determination of Organo-mercury Compounds after Decomposition taj Mercuric Sulphide BY K. F. SPOREK A new technique based on the formation of water-soluble complexes of certain organo-mercury compounds with sodium sulphide is described. This technique was found suitable for the determination of mercury in complex commercial preparations containing various organo-mercury compounds, fillers, chlorinated organic insecticides, dyes, pigments and so on. THE fact that the direct titration procedures described in Part I, although useful for some purposes, were of rather limited value for testing the wide range of organo-mercury fungicides, suggested that it would usually be necessary to separate the organo-mercury compound from the rest of the material (insoluble fillers, organlo-chlorine insecticides, dyes, pigments, oils, etc.) and then carry out the determination on the extract.Experiments were therefore carried out in order to find a suitable medium for this purpose. EXPERIMEXTAL A number of solvents and solutions of compounds known to form complexes with mercury was tested in order to find one that would be suitable for extracting organo-mercury compounds from fungicidal preparations. After several at tempts it was observed that concentrated sodium sulphide solutions had extraordinary slolvent properties for organo-mercury com- pounds. Complete dissolution could usually be effected by shaking the compound with an excess of the aqueous sulphide. Water-soluble mercury compounds on treatment with the reagent would first produce an insoluble olrgano-mercury sulphide, which redissolved in an excess of the sodium sulphide solution.The compounds tested and the relative ease in getting them into solution are shown in Table I. At this point it may be appropriate to note that organo-mercury compounds could be divided into the following four groups, according to their solubility- (i) compounds soluble in organic solvents (phenylmercuric chloride), (ii) compounds soluble in organic solvents and in water (phenylmercuric acetate), (iii) compounds very soluble in water (me thoxyethylmercuric acetate), and (iv) compounds insoluble in organic solvents and in water (ethoxyethylmercuric silicate).The fact that all these types of organo-mercury compounds were easily soluble in the sodium sulphide reagent made it possible to devise a general procedure for their quantitative extraction from fungicidal preparations. The sulphide reagent had no solvent action on the chlorinated organic insecticides, oils, pigments and dyes usually found in these materials. The extracts were therefore free from the substances that in further stages of this deter- mination would yield large quantities of chloride ion. The extraction of organo-mercury compounds is expedited by heating the suspension with the sulphide reagent and so this procedure was finally adopted. A further observation was the easy decomposition of organo-mercury compounds to inorganic mercury sulphide when the extracts were acidified with dilute sulphuric acid.This decomposition was also affected by the temperature of the solution and proceeded at different rates with different organo-mercury compounds. Sulphide solutions of phenylmercuric compounds when acidified in the cold first produced a white precipitate (phenylmercuric sulphide) , which on standing at room temperature slowly darkened and after a few hours was completely changed into the inorganic mercury sulphide. The decomposition proceeded rapidly when the same operation was performed in a hot solution. On boiling the solution for i t few minutes all organo-mercury sulphides were quantitatively transformed into mercuric sulphide, which could be filtered off andAugust, 19561 CONTAINING ORGANO-MERCURY COMPOUNDS.PART I1 479 either weighed or after oxidation titrated with thiocyanate. These facts led to the develop- ment of a method for determining total mercury in a wide range of products containing organo-mercury compounds. In the technique finally adopted a 15 per cent. aqueous sodium sulphide solution (nearly saturated solution) is used for extracting the organo-mercury compound from the tested SOLUBILITIES Compound Ethylmercuric acetate . . Methoxyeth ylmercuric acetate . . .. Phenylmercuric acetate . . Mercuric acetate . . .. Methox yethylmercuric borate . . .. . . Phenylmercuric borate . . Ethylmercuric chloride . . Ethoxyethylmercuric chloride .. .. H ydroxyethylmercuric chloride .. .. Methoxy ethylmercuric chloride .. .. . . Phenylmercuric chloride, . Phenylmercuric nitrate , . Bis (ethoxyethylmercuric) hydrogcn phosphate . . Ethylmercury phosphate Ethoxyethylmercuric Hydrox yethylmercuric silicate . . .. .. silicate . . .. .. Methoxyethylmercuric Methox yethylmercuric Sodium ethylmercuric silicate . . .. .. tannate . . .. .. thiosalicylate . . . . Phenylmercuric urea . . TABLE I OF ORGANO-MERCURY COMPOUNDS IN 15 PER CENT. SODIUM SULPHIDE (Na,S) SOLUTIONS Solubility in water Insoluble Very soluble Sparingly soluble Soluble Very soluble Insoluble Insoluble Insoluble Soluble Soluble Insoluble Insoluble Insoluble Insoluble Insoluble Insoluble Insoluble Insoluble Sparingly soluble Insoluble Solubility in excess of sodium sulphide Soluble Very soluble Very soluble Very soluble Very soluble Soluble Soh ble Very soluble Very soluble Very soluble Soluble Soluble Soluble Soluble Soluble Sparingly sol- uble in cold Soluble Soluble Very soluble Soluble Behaviour on acidification with dilute sulphuric acid White precipitate rapidly changing to White precipitate changing to black White precipitate changing to black Black precipitate White precipitate changing slowly to black in cold White precipitate changing to black on boiling White precipitate changing to black on boiling black (HgS) on heating in cold on boiling Black precipitate Black precipitate Black precipitate White precipitate changing to black White precipitate changing to black on boiling on boiling Black precipitate White precipitate changing to black on boiling Black precipitate Black precipitate Black precipitate Black precipitate White precipitate changing to black White precipitate changing to black on boiling on boiling * “Soluble” indicates 1 per cent.or greater solubility. material. The mixture is filtered to remove the insoluble matter comprising mainly inert diluents and chlorinated organic insecticides, and the filtrate is heated and then acidified with sulphuric acid. The precipitated mercuric sulphide is filtered off and either weighed or titrated after oxidation with a mixture of sulphuric acid and potassium nitrate. The latter procedure is, however, preferable, owing to its higher specificity for mercury. The advantages in going through the above set of operations are- (1) The separation of organo-mercury compounds from inert diluents such as china clay, talc and chalk, and from insecticides such as hexachlorocydohexane and dieldrin, which are incorporated in some dual-purpose fungicide preparations.Many dyes, pigments and oils are also insoluble in the aqueous reagent. The480 (2) (3) SPOREK : DETERMINATION OF MERCURY I N FUNGICIDAL PREPARATIONS [VOl. 81 extract therefore contains the mercury compounds in a comparatively pure state. When the diluents are soluble in water, eg., sodium carbonate or sodium bicar- bonate, the sulphide reagent ensures complete dissolution of the organo-mercury compound (most organo-mercury compounds are sparingly soluble in water) so that subsequent precipitation of the sulphide is carried out from homogeneous solution.On acidification of the solution containing the organo-mercury compound only mercuric sulphide is precipitated; it is easy to filter off and wash free from any traces of organo-chlorine compounds and chlorides and so after oxidation it gives a solution suitable for the thiocyanate titration. This makes the procedure specific for mercury. The procedure makes it possible to determine organo-mercury compounds in solutions containing as little as 0.01 per cent. of mercury, owing to the very high insolubility and non-volatility of mercuric sulphide. Two procedures were finally devised, one: suitable for materials containing organo- niercury compounds in water-soluble diluents, the other for materials made with water- insoluble diluents and containing organic insect-icides, dyes, pigments, oils, etc.Fungicides containing the organo-mercury compound in an insoluble diluent and no other constituent could also be tested by the first procedure, which is simpler as it does not involve filtration of the sulphide solution. In adopting this step it is assumed, however, that the insoluble filler is washed free from any chloride-containing impurities during the filtration and washing of mercuric sulphide and will produce no chloride ion on subsequent treatment with the nitration mixture. METHOD REAGEKTS- All reagents should be of recognised analytical grade. Sodium sulphide solution-Dissolve 50 g o E sodium sulphide crystals, Na,S.SH,O, in SuZphuric acid, diluted-Dilute 500 ml of concentrated sulphuric acid to 1 litre with water. Sulphuric acid, concentrated.Potassium nitrate, crystalline. Potassium permangunate-A saturated aqueous solution. Ferrous ammonium sulphate-A 5 per cent. aqueous solution. Ferric alum indicator solution-Dissolve 1410 g of ferric ammonium sulphate in 1 litre of water, and add sufficient nitric acid, sp.gr. 1.4.2, to change the colour of the solution from reddish-brown to yellow. Ammonium thiocyanate, 0.1 N-Weigh 8 g of ammonium thiocyanate for each litre of solution required, dissolve the salt in water, transfer the solution to a calibrated flask, dilute it to the mark, and shake it well. Standardise the solution against standard silver nitrate, using ferric alum as indicator. PROCEDURE 1- Weigh into a 500-ml flask with a B34 ground-glass socket sufficient of the sample to provide 0.2 to 0.3 g of mercury and add 150 ml of water and 20 ml of sodium sulphide reagent.Attach an 18-inch long air-condenser with a B34. ground-glass cone, mix the contents of the flask well and heat the solution to boiling. Add slowly to the hot solution enough diluted sulphuric acid to produce a permanent precipitate, and then another 30ml in excess and boil for 10 minutes. Filter the mixture with suction through a small glass crucible having a sintered-glass disc of porosity No. 3 and containing a thin pad of asbestos. Wash the flask and the precipitate with a small quantity of distilled water. Place the crucible inside the flask, attach the air-condenser, add l o g of potassium nitrate and 25 ml of concentrated sulphuric acid, mix well and boil until the evolution of brown fumes ceases.Cool the flask to room temperature, carefully dilute with water to about 150ml and boil again to remove nitrous acid. Cool to room temperature, add an excess of saturated potassium permanganate solution and set aside for 5 minutes. Remove the excess with a few drops of ferrous ammonium sulphate solution, treat with 5 ml of ferric alum indicator and titrate with 0.1 N ammonium thiocyanate solution. 1 ml of 0.1 N ammonium thiocyanate = 0.01003 g of mercury. 50 ml of water. If necessary, heat slightly to facilitate solution. Use 1 ml of this solution for every 50 ml of liquid.August, 19561 CONTAINING ORGANO-MERCURY COMPOUNDS. PART I1 48 1 PROCEDURE 2- Weigh into a 500-ml conical flask with a B34 ground-glass socket sufficient of the sample to provide 0.2 to 0.3 g of mercury, add 50 ml of sodium sulphide solution, mix the contents of the flask by swirling and then attach the 18-inch long air-condenser with a B34 ground- glass cone.Keep boiling for a few minutes and then dilute the mixture by pouring 160 ml of cold water through the condenser. Filter the mixture using suction through a sintered-glass filter funnel of porosity No. 2 or 3 Heat the flask to boiling while swirling constantly. TABLE Ir DETERMINATION OF MERCURY IN ORGANO-MERCURY FUNGICIDES BY THE SULPHIDE METHOD Sample BY procedure 1- Phenylmercuric acetate - ethoxyethylmercuric silicate mixture (30 per cent. of mercury) on insoluble filler . . i Methoxyethylmercuric acetate, dilute aqueous solutions i L Mercuric chloride, AnalaR, dried .. . . I By procedure 2- r mixture (3 per cent. of mercury) with y-hexachloro- cyclohexane (40 per cent.) on insoluble filler with oil, i Phenylmercuric acetate - ethoxyethylmercuric I * * I L - . c dye and pigments .. . . .. . . Phenylmercuric acetate - ethoxyethylmercuric silicate mixture (1.5 per cent. of mercury) on insoluble filler[ Phenylmercuric acetate - ethylmercuric chloride mixture (2 per cent. of mercury) with y-hexachlorocyclohexane [ (40 per cent.) on insoluble filler with oil and pigment Phenylmercuric acetate - ethylmercuric chloride mixture r ( 2 per cent. of mercury) with y-hexachlorocyclohexane (40 per cent.) on insoluble filler with oil, pigment with oil and pigment . . . . . . . . c and dye . . . . . . . . . . . . .. with pigment and oil .. . . . . . . . . Phenylmercuric urea (1 per cent. of mercury) with y- r hexachlorocyclohexane (40 per cent.) on insoluble filler I Phenylmercuric urea (4 per cent. of mercury) on insoluble filler, with oil and pigment . . . . .. . .{ r i Phenylmercuric chloride with dieldrin (40 per cent.) on insoluble filler . . .. .. .. .. .. Mercury found, Remarks Yo 29.1 29.0 0.0132 0.0245 0.0648 0.118 0-130 0.240 0.261 0.471 1-299 2.360 100.1 99.9 3.45 3.42 3.43 3.45 3.50 3.45 3.46 1.52 2.23 1.94 1.96 1.04 0.95 4.11 3.94 4.04 4-06 1.61 1 *59 1.56 1.59 1.57 1-58 29.2 by direct nitration and thiocyanate titration 0.01 30 theoretical 0.0237 by dilution 0-0650 0.118 0-130 0.237 0-260 0.473 1.300 2.370 3.48 theoretical 2.24 theoretical 1.60 theoreticaI provided with a thin asbestos pad and wash the residue with 50 ml of water.Transfer the filtrate quantitatively into a clean 500-ml Quickfit flask, €334 neck, attach an air-condenser, heat to boiling and add slowly enough diluted sulphuric acid to form a permanent precipitate,482 SPORISK wol. 81 then add another 30ml in excess and boil for 10 minutes. Continue in exactly the same way as described for procedure 1. RESULTS Many organo-mercury fungicides were tested by the above procedures and the results The range of products tested covers almost all types of this material The results are satisfactory, comparing well with the theoretical are shown in Table 11. made at the present time. values and values obtained by other methods when available. DISCUSSION OF RESULTS The time required for a determination was a.bout 1 hour for procedure 1 and 18 to 2 hours for procedure 2.It was possible, however, to run two and even three experiments in parallel, with big savings in time per determination. The acidification of the sodium sulphide extract and the nitration of the mercuric sulphide could be carried out in open vessels without loss of mercury by volatilisation. However, to avoid losses by spray during the evolution of hydrogen sulphide and heating the nitration mixture, the flasks were provided with air-condensers. In the procedures described above the decomposition of the organo-mercury sulphides into mercuric sulphide was effected by treatment with dilute acid under comparatively mild conditions, which did not affect the organic radical apart from detaching it from the mercury atom. The same effect was achieved, however, by treatment with, for example, hydrogen peroxide, but in this case no hydrogen sulphide was produced. In this way benzene was easily detected by its smell when any of the phenylmercuric compounds was tested and organo-mercury compounds with aliphatic radicals produced mercaptans. Different organo- mercury compounds could therefore be identified and also easily distinguished from inorganic mercury. Work on the above problem is still in progress and it is hoped to give a fuller account of it at some later date. I express my thanks to Plant Protection ]Ad., for permission to publish this paper, to J. M. Winchester, Chief Chemist, for his helpful criticism and to Miss A. Butler for assistance in the experimental work. TECHNICAL DEPARTMENT PLANT PROTECTION LIMITED YXLDING, KENT March 7th, 1956

ISSN:0003-2654    

DOI:10.1039/AN9568100478

出版商:RSC    

年代:1956

数据来源: RSC