ISSN:0003-2654    

DOI:10.1039/AN95378FX029

出版商:RSC    

年代:1953

数据来源: RSC

摘要:

at any point when recording the polarogram. Two polarograms are discussed in detail in order to illustrate the application of the instrument to polarographic problems. A multipurpose polarographic cell, which allows determination of the pH of the solution and of m and t for the electrode without removal of the solution from the cell, is also described. The dimensions of this cell allow for it to be used in a substantially unmodified Cambridge thermostat bath. NOTICES Second International Congress on Rheology A PROVISIONAL programnie for the Second International Congress on Rheology, to be held in Oxford between July 26th and 31st, 1953, has been prepared. All intending participants are asked to make their reservations as soon as possible (and in any case not later than May lst, 1953) by completing a Final Form of Application, which is attached to the provisional programme.The programme, giving full details and the form of application, can be obtained from the Hon. Organising Secretary, Dr. G. W. Scott-Blair, The University, Reading, England. The Association of Clinical Biochemists THE Inaugural General Meeting of the Association of Clinical Bi0chemist.s was held at the Postgraduate Medical School of London on March 28th, 1953. The Association will be both scientific and professional in the scope of its activities. The interim committee is acting as a provisional council, with Dr. A. L. Tgrnoky, Royal Berkshire Hospital, Reading, as Honorary Secretary, from whom further details of the Association’s future activities can be obtained. MEETINGS OF THE ROYAL SANITARY INSTITUTE Wigan Sessional Meeting, Friday, June 5th, 1953 AT this meeting the following papers will be presented: “Staphylococcal Food Poisoning in the Manchester Area,” by M.T. Parker, M.B., B.Ch., Dipl-Bact., Director, Public Health Laboratory, Manchester, and “The Changing Pattern of Refuse Disposal and its Effect on Vehicle Design,” by Clive Walker, Director, Walker Bros. Ltd., Engineers. Walsall Sessional Meeting, Thursday, July 2nd, 1953 AT this meeting the following papers will be presented: “Land Use in Walsall, with Special Reference to Slum Clearance and/Reclamation of Derelict Land,” by M. E. Habershon, O.B.E., M.Eng., M.I.C.E., M.I.Mun.E., Borough Engineer and Surveyor, Walsall, and ‘Some -4venues to a Better Environment,” by James Green, Deputy Chief Sanitary Inspector, Walsall.. London Sessional Meeting, Wednesday, July 15th, 1953 AT this meeting, to be held at 2.30 p.m. at the Royal Sanitary Institute, the following papers forming a Symposium on “Salvage and Utilisation of Food Waste for Animal Feeding” will be presented: (a) “Collection and Processing,” by John Stephen, M.Inst.P.C., Director of Public CZeansing, Luton, and (b) “Distribution and Utilisation,” by Major A. McD. Livingstone, C.I.E., M.C., M.A., B.Sc., Adviser on Agricultural Matters to the Waste Foods Branch, Ministry of Agriculture and Fisheries. Enquiries about these meetings should be addressed to the Secretary, The Royal Sanitary Institute, go, Buckingham Palace Road, London, S.W.1. - PRINTED BY W. HEFFER & SONS LTD.. CAMBRIDGE

ISSN:0003-2654    

DOI:10.1039/AN953780X031

出版商:RSC    

年代:1953

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN95378BX032

出版商:RSC    

年代:1953

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN95378BP069

出版商:RSC    

年代:1953

数据来源: RSC

摘要:

JUNE, 1953 THE ANALYST PROCEEDINGS OF THE SOCIETY OF PUBLIC ANALYSTS AND OTHER ANALYTICAL CHEMISTS Ax Ordinary Meeting of the Society was held at 7 p.m. on Wednesday, April lst, 1953, in the Meeting Room of the Chemical Society, Burlington House, London, W.1. This was the first meeting under the Chairmanship of the new President, Dr. D. W. Kent-Jones, F.R.I.C., and about 170 members and visitors were present. The subject of the meeting was “The Determination of Small Amounts of Lead in Foods and Biological Materials” and the following papers were presented and discussed: “A Rever- sion Method for the Absorptiometric Determination of Traces of Lead with Dithizone,” by H. M. Irving, M.A., D.Phil., F.R.I.C., L.R.A.M., and E. J. Butler, B.A., BSc., D.Phil., A.R.I.C. ; “Preparation of Samples of Foodstuffs and Biological Materials for the Determina- tion of Lead,” by R.F. Milton, B.Sc., Ph.D., F.R.I.C., (presented by K. J. Jarrett, BSc., A.R.I.C.) ; “Sample Preparation for Determination of Lead in Foodstuffs,” by D. A. Elvidge, B.Sc., and D. C. Garratt, BSc., Ph.D., F.R.I.C. NEW MEMBERS Frank Banyard : William Robert Charles Crinimin, BSc. (Wales) ; Charles Brian Dennis ; Edward Frank Hancock, Ph.C. ; Alan Frederick Hulme, A.R.I.C. ; Louis Francis McCallum, F.R.I.C. ; Joel Alfred Henry Totterdell Rosewarne, BSc. (Lond.), A.R.I.C. ; Arthur George Sansome, B.Sc. (Lond.), A.R.I.C. ; Frederick Clarence Saville, A.R.I.C. DEATH \!‘E regret to record tlic cleatli of Walter Thorp. SCOTTISH SECTION AN Ordinary Meeting of the Section was held at 7.15 p.m.on Friday, April loth, 1953, in the George Hotel, George Street, Edinburgh, 2. A lecture on “Modern Methods of Analysis in the Training of the Student” was given by Miss Christina C. Miller, Ph.D., D.Sc., F.R.S.E., F.H.-W.C. PHYSICAL METHODS GROUP T H E Fortieth Ordinary Meeting of the Group was held at 6.30 p.m. on Tuesday, April 14th, 1953, in the Meeting Room of the Chemical Society, Burlington House, London, W.l. This meeting was organised by the Polarographic Discussion Panel. The Chairman of the Group, Dr. J. Haslam, F.R.I.C., opened the meeting and invited Mr. G. W. C. Milner, BSc., F.R.I.C., A.Inst.P., Honorary Secretary of the Panel, to occupy the Chair for the rest of the meeting. The following papers on “Polarography” were presented and discussed: “The Polaro- graphic Determination of Fluoride,” by B. J. MacNulty, B.Sc., Ph.D., F.R.I.C., G. F. Rey- nolds, B.Sc., A.R.I.C., and E. A. Terry; “The Amperometric Titration of Zinc and its Application to the Determination of Zinc in Lubricating Oils,” by D. Pickles, BSc., A.R.I.C., and C. C. Washbrook, A.R.I.C. ; “A Tentative Method for the Determination of Calcium by Means of the Polarograph,” by Mrs. Bertha Lamb, BSc. 333

ISSN:0003-2654    

DOI:10.1039/AN9537800333

出版商:RSC    

年代:1953

数据来源: RSC

摘要:

334 GASKIX ASD HASDS: THE PRECIPITATE ERROR IK [\.ol. 78 The Precipitate Error in the Determination of Sugar Polarisations HY J . G. Y. GASKIZ; AND G. C. HAXDS ’The final volumes, nominally 100 ml, of sugar solutions defecated by wet or dry methods have been theoretically compared. In both methods of defecation it has been shown that the final volume of 100 ml is in the first place diminished by the liquid volume of the obscuring matter (a term defined in the text) and that this figure is further modified in opposite directions according as wet or dry defecation is used. I n making this theoretical comparison it is assumed that minimum amounts of lead salt or solution of similar basicity are used in the defecation and that no changes of volume occur a t the moment of precipitation. The assumptions have been tested and experiments are described, including some with a special type of flask designed for use in the determination of any volume changes during the precipitation.Finally the value of a “standard volume” has been proposed and the method by which this can be most nearly achieved in practice is indicated. AT the tenth session, in 1949, of the International Commission for Uniform Methods of Sugar Analysis, the British National Committee recommended that the continued use of the wet method for the clarification of raw-sugar solutions was justified. Both American and Austra- lian interests disagreed with this and advocated the official use of dry lead coupled with the use of a correcting factor to be applied to wet-lead clarification. The implication that the figure found by the dry-lead method is necessarily correct is not acceptable to the sugar chemists in this country, and this difference of opinion remains to be resolved.Before this is possible, additional information is required on a number of points, and it has been our pur- pose to provide this information and derive therefrom a theoretical approach to the problem of precipitation. Polarisation figures found by the dry-lead method are known to be lower than similar figures found by wet methods, but the value of this difference has not been satisfactorily determined, and advocacy of the merits of either figure has been based on rather vague assess- ments of errors associated with such factors as the volume of the precipitate, the “salt effect” of the excess of lead, the varying basicity of lead solutions, the amount of lead used, and so on.Further, the increasing discrepancy between the two figures with decreasing purity of the raw sugar has been assigned wholly to errors involved in the wet method, which may not be justifiable. It would be true to say that the appeal of the dry-lead method rests on the unproved assumption first claimed by Hornel that “whereas in the usual procedure the error is proportional to the volume of the total precipitate, in this dry defecation the error is only proportional to the difference in volume of acetic acid and the precipitated radicals involved.” 1. The changes in polarisation produced by the use of lead sub-acetate solutions of different basicities.2. The value of the critical minimum amount of lead defecant. 3. The value of the volume changes, if any, that occur at the moment of precipitation. 4. The value of any other volume changes occurring during the defecation process. Hence it is apparent that the following information is required- EXPERIMENTAL About 5 lb of Cuban raw sugar having a polarisation of about 97” was thoroughly mixed and bottled. A series of standard solutions of this sugar was clarified by the wet method, two different lead solutions, each of known basicity and density, being used. One of the lead solutions was the stock basic lead acetate used in this laboratory; it had a density, of 1.25 and a ratio of basic lead to total lead of 0.332. The other was a solution of Horne’s dry lead, which had a density of 1.25 and a ratio of basic lead to total lead of 0.434, the latter figure approximately corre- sponding to the basicity of Horne’s dry lead.The information required under headings 1 and 2 above was obtained as follows.June, 19531 THE UETERMISATION OF SUGAR POLARISATIONS 335 The defecated solutions were made up to 100 ml and filtered, and the polarisations of the clarified solutions were determined; all of these operations were carried out at 20" C. The full results of these experiments are shown in Table I. The emliiation of the volume changes on precipitation of the obscuring matter involved bringing together in a calibrated flask, but without mixing, solutions of the sugar and defecant, together with water to make the total up to a known volume, then mixing by some suitable ~'ol~lme of 1,asic ka(' solution used, nil 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 TABLE I CLARIFICATION OF STANDARD SOLUTIONS OF CUBAN SUGAR Basic Lead A Character of filtration Poor Poor Poor Slow Moderate Fairly rapid Fairly rapid Good Good Good Good Good Good Good Good Good Good Average polarisation Unreadable Unreadable .Unreadable 96.83 97.01 97.01 97.02 97.01 97.05 97.09 97.10 97.15 97.12 97.04 97.1 1 97.11 97.11 Basic Lead B v r__-_-h.Character of Average filtration polarisation Poor Poor Slow Moderate Moderate Good Good Good Good Good Good Good Good Good Good Good Good Unreadable Unreadable 96.91, 96.86* 96.85 96.88 96.94 96.95 96.96 97.03 96.98 97.01 97.04 97.06 97.05 97.11 97.10 Differences, A - I 3 - - - - 0.03 + 0.16 + 0.13 + 0.08 + 0.06 + 0.09 + 0.06 + 0.12 + 0.14 + 0.08 - 0.02 + 0.06 0.00 - 0.01 * The polarsiation of these solutions was difficult to determine.Defeccants-Basic Lead A: sp.gr. 1.25; ratio of basic lead to total lead 0,332. Basic Lead B : sp.gr. 1.25; ratio of basic lead to total lead 0.434. method and finally re-measuring the total volume, all measurements being made at 20" C. For this purpose a special flask of approximately 100ml capacity was designed. About 10 cm of the stem of this flask consisted of tubing with an internal diameter of 3*5mm, and the capacity of the flask to a point about midway along this stem was accurately deter- mined. The mixing experiment was carried out as follows. TABLE I1 VOLUME CHANGES ox PRECIPITATION OF OBSCURING MATTER Particulars of Jask- Internal diameter of stem, 3.5 mm.Capacity to mark, 98.448 ml. 1 linear millimeter on stem equivalent to 0.0096 ml. Defecant used, Basic Lead B (see Table I). Average depression of Defecant meniscus with Sugar used added, defecant, ml mm Cuban (Pol. 56.95) . . 0.7 6.1 Mauritius (Pol. 58.70) 0.3 8.0 Barbados (Pol. 95.33) 0.9 7.5 Average depression of meniscus without defecant, mm 6.4 (=0.061 ml) 9.4 ( ~ 0 . 0 9 0 ml) 8.6 (=0*082 ml) Volume represented Difference by difference between between depressions, depressions, mm ml 0.3 0.003 1.4 0.014 1.1 0.01 1 no The above experiments were carried out on a sugar solution containing, before dilution, 26 g in 53 ml. Similar experiments carried out on a sugar solution containing, before dilution, 20 g in 90 ml, showed measurable depression of the meniscus level when finally mixed.3.36, CASKIN AND HANDS: THE PRECIPITATE ERROR I N [Vol.78 A standard weight, 26 g (or the equivalent proportion when the calibrated volume of the flask differed from 100 ml), of Cuban sugar was dissolved in the minimum amount of water and transferred to the flask, the flow into the flask being facilitated by inserting into the stem of the flask a finely drawn glass tube through which air could escape or be gently viithdrawn. A layer of water was floated on top of this sugar solution, and then the requisite amount of basic lead acetate solution, and the flask and contents, without mixing, were pkced in a thermostatically controlled bath at 20" C.The flask was finally filled to the mark at 20" C, after which the solutions were mixed by rotating the flask by means of a mechanical stirrer. When it was certain that mixing was complete, the flask was returned to the 20" C bath and, when the temperature of the contents of the flask had again reached 20" C, the position of the liquid level was measured. This experiment was repeated four times, both with and without defecant, on the Cuban sugar described above, and later with sugar from Mauritius and Barbados. The corrplete results are shown in Table 11. In further experiments, the polarisations of these three types of sugar were determlnedj both dry and wet defecants being used and the stage at which the solutions were mixed tei% varied. These results can be seen in Table 111.TABLE I11 DEPENDENCE OF POLARISATION ON TYPE OF DEFECAKT ASD WERE MIXED Basic Lead B (see Table I) A I \ 7 Polarisation Polarisation of solution of solution mixed before not mixed Volume making up before making IVeight Sugar used, to volume up to volume used, Mauritius . . 0.4 98.68 98.77 0.132 Cuban . . 0.7 97.05 97.15 0.231 Barbados . . 0.9 95.29 95.38 0.306 ml 6 STAGE AT WHICH S O L U T I O ~ ~ Home's Dry Lead Polarisation of solution mixed before making up to volume 98.60 96.95 95.15 Polarisation of solution not mixed before making up to volume 98.74 97.00 95.24 RESULTS THE EFFECT OF VARYING THE BASICITY OF THE LEAD DEFECANT (TABLE 1)- It is evident that solutions of Cuban sugar clarified with the lead solution of lower basicity have for the most part higher polarisations than the solutions clarified with the lead solution of higher basicity.This difference is apparent almost from the first readable solution and is maintained until more than 1.6 ml of lead solution have been added, after which the two defecants produce almost identical figures. A statistical analysis of all the figures shows- (i) That the differences observed are significant, i.e., they cannot be explained by random error. (ii) That the degree of correlation between the two sets of figures is such as to make it reasonably certain that a variation in some factor common to both series of reading is the cause of the differences. It appears that this variable is in fact the different basicity of the two lead solutions. The disappearance of the differences in the results after the addition of more than 1.6 ml of lead is susceptible to a number of interpretations and needs further investigation. It can also be seen that the amount of lead solution required to defecate a sugar solution adequately can be determined within narrow limits, that this amount is generally less than 1 ml, and that it is smaller for the solution of higher basicity.Similar results have been found for sugars from Mauritius and Barbados. Hence it is possible, for accurate comparisons of the two methods, to select comparable minimum amounts of lead sub-acetate solutions, such that most of the lead will be used in the precipitation of the obscuring matter and there- fore the "salt effect" of the excess of lead on the polarisation will be reduced to a minimum and made strictly comparable.Similar considerations can equally well be extended t o dry-lead defecations.June, 19531 THE DETERMINATIOX OF SUGAR POLARISATIONS 337 I’OLUME CHANGES OK PRECIPITATION (TABLE 11)- The control experiment in which no lead defecant was used revealed a measurable diminution in volume when a known volume of concentrated sugar solution was diluted with a volume of water. This diminution, amounting to about 0.1 ml, is of the same order as the differencs under investigation, and it is of the utmost importance that it should be avoided in all determinations of sugar polarisation. Fortunately this can be done by mixing after dilutior, to 90 ml, when further dilution to 100 ml produces no measurable diminution of volume. The effects of this error on the recorded polarisations are shown in Table I11 to be sigiificant for the three different types of sugar examined.Because of this effect it was neces;ary to ensure that the volumes of the concentrated sugar solution were the same in all expel iments. IVhen this contraction is allowed for, it will be seen from the figures in Table I1 that any change in total volume of the sugar solution caused by precipitation of the obscuring matter 1s Very small (of the order of 0.01 nil), and hence it can be stated that the solid volume of the Precipitate is not sensibly different from the liquid volume of the obscuring matter (defined b e l h ) together with the liquid volume of that part of the lead salt involved in the precipi- tat.,,.THEORETICAL CONCEKTRATION OF A DEFECATED SUGAR SOLUTION The view that one method of determining polarisation is more accurate than another niplies the existence of some standard to which both are referred. The nature of this standard ;an only be determined by a study of the generally accepted methods for determining polarisations. These are essentially the dissolution of a standard weight of sugar, 26 g, in water, the adjustment of the volume of this solution to 100 ml and the subsequent “testing by the polariscope.” If the accuracies of the initial weighing, of making up to volume and of the final reading are assumed, the value of the figure indicated by the polariscope is obviously dependent on the concentration of optically active substances in the final clear solution.It is this concentration for which some standard value is required, and it is variations in this concentration, caused by the need to clarify the sugar solution, that give rise to the differences of opinion mentioned in the first paragraph of this paper and that are now being considered. The solution made by dissolving raw sugar in water is, with few exceptions, optically opaque and needs clarification before the polarisation value can be determined. This clarification is achieved by the addition of a suitable defecant, usually some form of basic lead acetate, and filtration to remove thc precipitated matter. In the wet method this clarification is performed before dilution to 100 ml by the addition of a solution of the lcad salt, and in the dry method clarification is performed after dilution by the addition of dry lead salt, In both methods the separation and removal of the precipitate may modify the concentration of the optically active substances remaining in the clear solution and also disturb their optical activity.The exact nature of the precipated material and the extent of the optical disturbance are both irrelevant to the comparison to be made, provided that all precautions are taken to ensure that the same precipitate and effects are produced by both methods; that is to say, that minimum amounts of lead salt or solution, of similar basicity, are used. It has already been shown that there is no sensible change in total volume caused by the separation of the precipitate (Table 11). The theoretical approach to this problem can be made as follows.A solution of a particular raw sugar, of volume 100 ml, can be regarded as consisting of three parts, of volumes V,, Vo, V,, their total volume being- .. * ’ (1) v, 4- v, 4- vw .. .. .. .. a . . . where V, is the liquid volume ( i e . , the volume occupied by the molecules of the dissolved material) of the optically active substances, mostly sugars that would not be removed by clarification, Vo is the liquid volume of the material that would be removed were the solution clarified, and V, is the volume of water necessary to make 100 ml.[Vol. 78 The nature of V, is such that this solution cannot be examined in the polarimeter, and to do this requires the elimination of V,. Were this possible without addition to the solution, the remaining liquid would have a volume- which is numerically equal to 100 - V,.It will be shown that this volume, V, + V,, with additives or subtractives, is common, for a given sugar, to both methods of defecation under consideration ; further, it contains all the optically active substances not removed by clarification. It is, therefore, a Fuitable standard volume with which to compare the two defecated solutions. DEFECATION BY THE WET METHOD- The practical elimination of V, is not achieved by eliminating V, alone, but by ;ddiW something to it and removing the precipitate so produced. The additive is usually basG lead acetate, which may be added as a solution before making up to volume (wet method), Or as the dry salt after making up to volume (dry method).In either method the liquid vdume of the lead salt has to be considered, and this can be represented by two parts, an electriPllY positive part, VL+, and an electrically negative part, VL-, and the amount added can b:. So adjusted that VL+ will just precipitate Vo. Hence, for the wet method, the 100 ml of solufon before the precipitation will consists of- where Vs, V,, VL+, V,- have the meanings already assigned, and V,, is the volume of waer necessary to bring the total volume to 100 ml. On precipitation, V, and VL+ become associated as a solid and, as already shown, thi association is without significant change of volume. On filtration they will be eliminated, and the remaining clear solution will have the volume- v, + V,- 4- V W 1 .. . ... .. .. .. The expressions (1) and (3) both represent a total volume of 100 ml, and we can therefore derive from them a value for V,,, which is- V, - V,- - v,+ and, substituting this value in expression (4), we have as the final volume after wet-lead defecation- V, + v, - v,+ . . .. . . .. .. . . 338 GASKIN ASD HASDS: THE PRECIPITATE ERROR IN .. * * (2) v, + v w .. . . .. .. I . . . .. V, + Vo + v,+ + v,- + V W 1 . . .. . . .. .. . . ?) .. * * (4) .. * * ( 5 ) Hence, in the wet method, the final volume as compared with the standard volume is diminished by an amount equal to the liquid volume of the electrically positive part of the lead salt added, and therefore the polarisation of this solution will be greater than that of the standard sugar solution. DEFECATION BY THE DRY METHOD- of the dry salt will have a volume- To this is added an mount of dry lead such that the electrically positive part will just pre- cipitate V,.VS + Vo + V, + v,+ + v,- .. .. .. .. .. After precipitation and filtration, V, and V,+ are associated as a solid without change in volume and the remaining volume is- A similar treatment can be applied to the dry method. The solution before the addition .. ' . (1) * . (6) v, + Vo + v, * . * . .. .. .. .. Hence the volume becomes- .. .. * - (7) V8 -k V, + v,- .. * . .. .. .. Hence, in the dry method of defecation, the final volume as compared with the standard volume is increased by an amount equal to the liquid volume of the electrically negative part of the lead salt added, and therefore the polarisation of this solution will be less than that of the standard.June, 19531 THE DETERMIXATION OF SUGAR POLARISATIOKS DEFINITION OF POLARISATION- The numerical values of the volumes shown in expressions (2), (5) (2) 100 - vo the standard volume, ( 5 ) 100 - VO - VLt wet defecation, (7) 100 - Vo + VL- dry defecation. 339 and (7) are- On reference to the standard volume, the amounts and directions of the errors are apparent, The necessity for a definition of polarisation that indicates the final volume to be attained is obvious.The temptation to use 100 ml for this volume must, however, be avoided as un- realistic. Polarisation is an empirical term universally associated with the quality of raw sugar and the determination of its value equally universally involves some form of defecation, It would be manifestly unreal to define it with reference to a pure substance dissolved in a volume that, in practice, is unattainable by the wet method and only accidentally attainable by the dry method.For example, defecation by the wet method will never produce a final volume greater than 100 - Vo - VL+, although it may always be equal to this value whatever the amount of lead added. By the dry method, the final volume can be between 100 - V, and 100, can be equal to 100, or can exceed 100, depending on the respective values of V, and VL- for the sugar under examination: and the final volume will increase with increasing amounts of added lead. To choose between the two methods is difficult; the dry method gives a volume nearer to the academically desired 100 ml, but this volume is more susceptible to error due to an excess of lead; the wet method avoids this error, but gives a volume slightly more remote from 100 ml.It would appear, therefore, that at present the standard volume after clarification should have the numerical value 100 - V,, and this will be most nearly achieved in practice by taking the mean of the figures found by the dry and wet methods of clarification. COSCLUSIONS Under critical conditions it has been shown that differences in the basicity of the lead salt used in the defecation of sugar solutions produce differences in the polarisations subse- quently determined. There is a critical value for the amount of a defecant that will just clarify a given sugar solution. Small volume changes, which occur when a concentrated sugar solution is diluted, are sufficient to affect polarisation figures and must be avoided in practice. A theoretical comparison of the concentration of optically active substances in the solu- tions obtained after wet and dry defecation shows both to be different, in opposite directions, from the concentration of a suggested standard solution. The magnitude of the difference is a function of the amount of defecant added and not of the total volume of the precipitate. Within the limits of existing knowledge, it is suggested that in the critical determination of the polarisation of a raw sugar the most satisfactory value is that given by the mean value of the figures found by wet and dry defecation methods, provided that these determinations have been performed with corresponding minimum amounts of lead salt or solution of similar basicity. The authors wish to thank Mr. A. H. Rheinlander, MSc., F.R.I.C., for criticism and suggestions, Mr. J. W. Stiles for practical assistance, and the Government Chemist, Dr. G. M. Bennett, C.B., F.R.S., for permission to publish this paper. REFERENCE 1. GOVERNMENT CHEMIST’S DEPARTMENT Horne, W. D., J . Amer. Chem. Sac., 1904, 26, 186. THE LABORATORY CUSTOM HOUSE LONDON, E.C.3 November 24th, 1962

ISSN:0003-2654    

DOI:10.1039/AN9537800334

出版商:RSC    

年代:1953

数据来源: RSC

摘要:

340 HASLAM, WHETTEM AXD NElVLANDS : THE DETERMINATION [Vol. 78 The Determination of the Amount and Composition of Free Phenols in Phenol - Fornialdehyde and Cresol - Formaldehyde Resins and Moulding Powders BY J. HASLAM, S. M. A. WHETTEM AND G. NEWLANDS A method has been evolved for the determination of total free phenols in phenol - formaldehyde, cresol - formaldehyde and phenol - cresol - form- aldehyde moulding powders. In addition, tests have been devised whereby the ratio of free phenol to free cresol in the total free phenols contained in a moulding powder can be determined. It has also been shown that the ratio of m-cresol to p-cresol can be determined in the total free cresols present in a cresol - formaldehyde moulding powder prepared from nz-cresol and p-cresol only.It has not been possible to extend the principles of this method to the corresponding combination of o-cresol and m-cresol or to that of o-cresol and p-cresol. PHENOL - FORMALDEHYDE resins are made by the condensation of phenol or crcsols or mixtures of phenol and cresols with formaldehyde. These resins invariably contain a certain amount of the free phenols from which they are prepared, and it has been found that the amount of free phenols in the resins affects the properties of the moulding powders prepared from them. I t is important, therefore, to know the amounts of these free phenols present in the resins, and to have as much information as possible about their comFosition. I t is also possible by the same methods to get a useful indication of the proportions of phenolic and cresylic resins used in the manufacture of a moulding powder of unknown composition.This can be done by determining the free phenol and cresols in moulding powders of known composition, and then finding the relationship between the free phenols and cresols remaining in the powders and the proportions of phenolic and cresylic resins used in the manufacture of the powders. The method that we have used for some years for the isolation of the total free phenols from a resin or moulding powder is based on dissolving the sample in aqueous sodium hydroxide, precipitating the resin by neutralisation of the solution to pH 4.5, filtering off the resin (and fillers where moulding powders are concerned) and steam distilling the phenols in the filtrate.The phenols in the steam distillate are then determined by modifications of the Koppeschaarl bromination method. But for resins prepared from a mixture of phenol and cresols, however, it is often desirable to know the proportions of phenol and cresols in the free phenols. I t has not been possible to resolve chemically all mixtures of phenol and o-, m- and 9-cresols when they occur in low concentra- tions in aqueous solution, although there is little doubt that modern methods of chromato- graphic testing are of considerable value for this resolution. I t is well known that, when a resin is made from a mixture of phenol and commercial cresylic acid containing all three isomers, then, because of the very high reactivity of m-cresol with formaldehyde, no free m-cresol is likely to be present in the free phenols remaining in the resin.These free phenols will therefore consist of phenol, p-cresol (the least reactive) and possibly a small amount of o-cresol. The proportion of phenol and cresols in this important combination can be deter- mined. The method described in full later is based on- (i) A knowledge of the behaviour o phenol and p-cresol towards bromination by potassium bromide - potassium bromate mixture under the conditions of the test. (ii) Application of Chapin’s method2 for the determination of phenol in the presence of certain other phenols to an aliquot of the steam distillate, the bromination value of which is known. Chapin’s method is based on the knowledge that, under certain conditions, Millon’s reagent gives a red colour with phenol but not with certain other phenols.For simple phenol - formaldehyde resins, this is all that need be done. This, however, is not so important as it might appear.June, 19531 OF FREE PHESOLS IK FORMALDEHYDE RESISS 341 Another combination that has been resolved is that of free m-cresol and p-cresol in resins prepared solely from m- and p-cresols. In this method too a knowledge of the bromination value of the steam distillate is the first requirement. The determination then depends on the difference in colour intensity of the nitrosamine solutions prepared from the two isomers by a modification of the method of Savitt, Goldberg and Othmer? BEHAVIOUR OF PHENOL AND CRESOLS ON BROMIXATIOX- I t has been shown by Sprung4 that when phenols are brominated by Koppeschaar’s method, i.e., with aqueous bromine liberated by acid from potassium bromide - potassium bromate mixture, some phenols behave normally, whereas others behave abnormally as regards the amount of bromine they absorb.Phenol and the cresols, for instance, are said to react according to the following scheme- Bromine atoms combining with 1 molecule of the phenol Phenol . . . . . . . . . . . . 2.98 (theory 3.0) nz-Crcsol . . . . . . . . .. , . 2.98 (theory 3.0) o-Cresol . . . . . . . . . . . . 2.20 (theory 2.0) p-Cresol . . . . . . . . . . . . 2.27 (theory 2.0) Sprung concludes from these and other results that the presence of one primary alkyl group in the ortho or para position to the hydroxyl group of a phenol results in the absorption of from 0.1 to 0.3 molecule of bromine beyond that required by the simple theory of ortho and para substitution. -4s our interest lay, in the first place, in the determination of free phenol in phenol- formaldehyde resins and free cresols (known to be mainly P-cresol) in cresol - formaldehyde resins, known amounts of phenol and p-cresol in the concentrations found in our steam distillate were brominated and titrated by our usual procedure.The factors found for these phenols were as follows- 1 nil of 0.1 A’ sodium thiosulphate = 0.00157 g of phcnol 1 ml of 0.1 A‘ sodium thiosulphate = 0.00236 g of p-cresol The figures are in good agreement with those found by Sprung, showing nearly theoretical bromination for phenol and 2.29 atoms of bromine combining with 1 molecule for p-cresol.BEHAVIOUR OF PHESOL AX11 CRESOLS TOWARDS MILLON’S REAGEYT- Chapin found that if he mixed solutions of phenols (4 mg in 6 ml of water) with 5 ml of Millon’s reagent and heated the mixtures in a bath of boiling water for 30 minutes, the red colour produced with phenol became much intensified, whereas the red colours first produced with the cresols soon gave place to various shades of yellow. He found that, after acidification with a definite amount of nitric acid, the solutions underwent no appreciable change in colour for many hours. The colours with phenol and the cresols are as follows- Colour of solution Phenol . . . . . . . . . . . . Deep red o-Cresol . . . . . . . . . . . . Faintly orange nz-Creaol . . .. . . . . . . . . Strong yellow p-Cresol .. . . . . . . . . , . Greenish yellow To develop this test into a quantitative method for determining phenol in the presence of other phenols, he describes three different procedures. The procedure that we have found applicable to the steam distillates obtained from resins is that described in “Standard Methods for Testing Tar and its Products.”2 APPLIC-4TION TO THE DETERMIXATIOS OF FREE PHEKO1.S AND CRESOLS IN RESISS AKD 310171~1lTSG POIVDERS- For resins and moulding powders that are known to have been prepared from phenol only or cresols only, it is necessary first to isolate the free phenols by steam distillation of the filtrate from the filtration of the resin precipitated at pH 4.5 from a sodium hydroxide solution of the sample. It is then only necessary to brominate an aliquot of the steam distillate and to calculate the amount of free phenol or free cresols from the bromine absorbed in terms of 0.1 K potassium bromide - potassium bromate.342 HASLAM, WHETTEM AND NEWLANDS : THE DETERMISATION [Vol.78 For mixed phenol - cresol resins, the free phenols are isolated and an aliquot of the steam distillate is brominated as before. A note is made of the bromine absorbed in terms of 0.1 N potassium bromide - potassium bromate. A volume of the steam distillate calculated to give a titration of 3.00 ml of 0.1 N potassium bromide - potassium bromate is taken and diluted to 100 ml with water. The phenol is determined in this solution with Millon’s reagent by the procedure described in “Standard Methods of Testing Tar and its Products.”z By dividing the weight in grams of phenol found in this solution by 0.00157, the number of millilitres of 0.1 N potassium bromide - potassium bromate required to account for the phenol can be found. Then 3-00 nil minus this volume will be the proportion of the 0.1 N potassium bromide - potassium bromate titre accounted for by the cresols in this solution. From these results the proportions of the bromide - bromate titre due to phenol and to cresols in the original steam distillate can be calculated and, by using the respective factors for phenol and cresol, the amounts of free phenol and cresols in the sample can be found.The method is described in full below. METHOD FOR DETERMIXIXG TOTAL FREE PHENOLS SAMPLE- The size of the sample to be taken depends on the amount of free phenol expected to be present.For resins containing from 10 to 20 per cent. of free phenols, 2 g is sufficient. For moulding powders, 5 g is generally sufficient, but sometimes it may be necessary to take as much as 10 g of sample. PROCEDURE- Place the weighed sample in a 500-ml beaker and add 50 to 200 nil of 10 per cent. sodium hydroxide (according to the size of sample). Boil the contents of the beaker for about 10 minutes to bring everything except the fillers, if present, into solution. Cool the solution and dilute to about 200 ml. Adjust the pH of the solution to 4.5 with 20 per cent. sulphuric acid a t first, and then with N sulphuric acid, using B.D.H. “4.5” indicator externally. Filter off the precipitated resin and wash it well with cold water.Transfer the filtrate and washings, which contain the free phenols, to a 500-ml flask, dilute to the mark and shake thoroughly. Steam distil until 350 to 400 nil of distillate have been collected in a 500-ml calibrated flask. Dilute the distillate to the mark with water and mix well. Transfer 250 ml to a 600-in1 “Iodine” flask for brom- ination. Add 50 ml of an approximately 0.1 N solution of potassium bromide - potassium bromate mixture and 10ml of 20 per cent. v/v sulphuric acid. Stopper the flask and put a few millilitres of 10 per cent. w/v aqueous potassium iodide solution into the trough formed by the stopper and the mouth of the flask so that no bromine escapes. Carry out a blank determination simultaneously and allow both flasks to stand in the dark for 1 hour.Ease the stoppers and allow the potassium iodide to run into the flasks and add a further l 5 m l of 10 per cent. potassium iodide solution to each, taking the usual prccautions to prevent loss of bromine. Titrate the liberated iodine with 0.1 3’ sodium thiosulphate solution until the iodine colour has been discharged to a pale straw colour, add a few rnillilitres of chloroform to dissolve the very voluminous precipitate of tri-bromophenol, as this absorbs appreciable quantities of iodine, and complete the titration after adding starch as indicator. The amount of phenol (or cresol) is calculated from the difference between the blank and sample titres. Transfer 250 ml of this solution to a steam distillation apparatus. 1 ml of 0.1 S sodium thiosulphate = 0 00157 g of phenol 1 ml of 0.1 ,V sodium thiosulphate = 0.00236 g of cresol 31ETHOD FOR DETERMIXING THE RATIO OF FREE PHESOL TO FREE CRESOL SAMPLE- From the steam distillate prepared as described above, an amount is taken that would give a titre of 3.0 ml of 0.1 -1; potassium bromide - potassium brornate.This amount of distillate is measured from a burette into a 100-ml calibrated flask, diluted to the mark with water and mived thoroughly. This constitutes the samplc solution for the method to be described.June, 19531 OF FREE PHENOLS I N FORMALDEHYDE RESINS 343 REAGENTS- Nitric acid, diluted-Prepare this by bubbling air through pure concentrated nitric acid until the acid is colourless and then dilute one volume of the acid with four volumes of water. Measure 2 ml of mercury from a burette into a 100-ml conical flask and add 20 ml of pure concentrated nitric acid.Hasten the resulting reaction by occasional shaking. When all action has ceased (after about 15 minutes), add 35 ml of water. If any separation of basic mercury nitrate occurs, add diluted nitric acid reagent drop by drop until the solution clears. Add 10 per cent. wjv aqueous sodium hydroxide drop by drop with thorough mixing until the curdy precipitate that forms no longer redissolves but just produces a slight permanent turbidity. Add 5 m l of dilute nitric acid reagent and thoroughly mix the contents of the flask. Millon’s reagent so prepared will remain stable for 2 days. Standard $hen01 solutioiz-Prepare a stock 1 per cent.w/v solution of phenol in water. Prepare the standard phenol solution from this on the day it is used by taking 2.5 ml of stock solution and making up to 100 ml with water, so that the solution contains exactly 0.025 g of phenol per 100ml. Formaldehyde solution-Dilute 2 ml of 40 per cent. v/v formaldehyde solution with water to make 100 ml of solution. PROCEDURE- Measure 5 m l of the sample solution into each of two 6 x 1-inch test tubes and mark them A and B, respectively. Into each of two similar tubes measure 5 mi of the standard phenol solution and mark them C and D. Measure from a burette 5 ml of Millon’s reagent into each of the four tubes and mix thoroughly. Place the tubes in a water-bath maintained a t 100” C for 30 minutes. Remove the tubes and cool them immediately in a bath of cold water for 10 minutes.Remove them from the cold water and add 5 ml of dilute nitric acid reagent to each tube and gently shake to mix the contents. To tubes A and C add 3 ml of the formaldehyde solution. Dilute the contents of the four tubes to 25 ml with water (the tubes should be previously marked with a file scratch a t the 25-ml level), shake them thoroughly and set them aside overnight. The contents of tubes A and C will then be found to be yellow because of the bleaching action of the formaldehyde; these are used as blanks in the colorimetric procedure that follows. Measure 20ml of the contents of tubes C and D by means of a pipette into separate 100-ml calibrated flasks and mark these C and D, respectively. Add 5 ml of diluted nitric acid reagent to each and make up to the 100-ml mark with water.Fill two burettes, C and D, from the flasks C and D, respectively, so that C contains the yellow “phenol blank” solution and D the “phenol standard” solution. The strength of the latter is now such that 1 ml contains 0.00001 g of phenol. Place 10 ml of the contents of the test tubes A and €3 (i.e., “sample blank” and “sample”) into 50-ml Nessler cylinders, marking these A and B, respectively. Run, from burette D, a small quantity of the “phenol standard” solution into Xessler cylinder A containing the “sample blank,” and run an equal volume from burette C of the ”phenol blank” solution into Nessler cylinder B containing the “sample” solution. Continue adding successive equal volumes of the “phenol standard” and “phenol blank” solutions to the appropriate Nessler cylinders, with intermediate shaking, until the contents of the cylinders are identical in colour.Yote the volume in millilitres of “phenol standard” solution required for the colour matching. CALCL-LATIOS- Millon’s reagent-This reagent is prepared as follows. NOTE -It i? essential that the colour matching operations are performed speedily. Strength of “phenol standard” solution in burette D == 5;lOO x 0.025 x 20/25 = 0.001 g per 100 ml, I in1 of solution = 0.00001 g of phenol. or Volunie of sample solution matched in Nessler cylinder = 10 25 5 x - = 2 ml.344 HASLAM, WHETTEM AND SEU’LASDS THE DETERMIXATION [Vol. 78 (Volume of phenol standard solution required to match sample solution) x 0.00001 x 50 = weight in grams of phenol per 100 ml of sample solution = P Then P/0.00157 = bromine absorbed by phenol in terms of 0.1 N potassium bromide - potassium bromate = x .Then total bromine absorption of 100 ml of sample solution in terms of 0.1 N potassium bromide - potassium bromate - x = bromine absorbed by cresol = y . y x 0.00236 = weight in grams of cresol per 100 ml of sample solution = C. With P and C found, the percentage composition of the free phenols is easily calculated. EXAMPLE- Volume of “phenol standard” solution required to match sample = 8.0 ml. P = 8.0 x 0.00001 x 50 = 0.004 g of phenol per 100 ml of sample solution. 0.004 x = - n.0015, = 2.55 ml. y = 3.00 - 2.55 = 0.45 ml. C = 0.45 x 0.00236 = 0.001 g of cresol per 100 ml of sample solution.Hatio of free phenol to free cresol = 4 to 1, or percentage composition of free phenols = 80 per cent. of phenol, 20 per cent. of cresol. RESIXS PREPARED SOLELY FROM m- ATD ~-CRESOLS The special class of resins prepared solely from m-cresol and $-cresol has been investigated. This requires rather different treatment from the more general class of resins prepared from phenol and cresylic acid. The problem here is to differentiate between free m-cresol and free p-cresol, whereas previously the problem was to differentiate between phenol and 9-cresol. The initial stages, that is solution of the sample in sodium hydroxide solution, precipita- tion of the resin by neutralisation to pH 4.5, filtration, and steam distillation of the filtrate, can be carried out as described in the previous method.The steam distillate then contains the free cresols. A method for determining m-cresol and p-cresol has been described by Savitt, Goldberg and Othmer.3 It depends on the formation of the nitrosamine by the action of sulphuric acid and sodium nitrite on a solution of the cresols in acetic acid - potassium acetate buffer, followed by the addition of an excess of alcoholic ammonia to give coloured solutions. In their work known weights of the isolated cresols were dissolved in the buffer solution and aliquots were taken for the colour development. In the problem under consideration, the cresols are in aqueous solution a t a relatively low concentration. However, it has been shown that it is possible to find the concentration of the cresol solution in terms of each isomer by amperometric titration with 0.1 N bromide - bromate solution.By combining this knowledge with a modification of the nitrosamine method, results have been satisfac lory. AMPEROMETRIC TITRATION OF CRESOLS- Previous experience of the determination of styrene by amperometric titration with bromide - bromate solution by Kolthoff and Bovey’s method5 led us to consider applying a modification of this method to the determination of cresols in aqueous solution. The reason for over-bromination of 0- and +-cresols when determined by Koppeschaar’s method is con- sidered t o be side-chain substitution. As the reaction rate for nuclear substitution is very much greater than that for the side-chain reaction, it was considered that the small excess of bromine and the short time of standing involved in an amperometric titration should make possible the detection of an end-point a t the completion of the nuclear substitution.It was found that amperometric titration of solutions of the ortho, meta and para isomers did in fact give values that agreed well with the theoretical values for the nuclear bromination of the individual isomers. Satisfactory results were also achieved with mixtures of meta and payn isomers and of nieta and ortho isomers. COJIBISATIOX OF AMPEROMETRIC TITRATION AND NITROSAMISE COLORIMETRIC METHOD FOR THE DETERMINATIOP; OF UNKNOWN MIXTURES OF WZ- AND $-CRESOLS Ihr AQUEOUS SOLUTION- The concentration of the steam distillate containing the m-cresol and $-cresol was determined by amperometric titration and the results were calculated in terms of m-cresolJune, 19532 OF FREE PHENOLS I N FORMALDEHYDE RESINS 345 and p-cresol. This gave the limits between which the concentration of the cresols must lie, even if present as all m-cresol or all p-cresol.A suitable aliquot of the steam distillate was then evaporated to dryness with an excess of potassium hydroxide solution, the amount of which was such that on dissolving the residue in acetic acid, the resulting solution had the same proportions of acetic acid, potassium acetate and water as used in the original method. The nitrosamine colour was then developed and the ratio and amount of the isomers determined from a previously prepared calibration chart. An attempt to apply the method to mixtures of 0- and m-cresols was unsuccessful, partly owing to difficulty in obtaining reproducible figures for the calibration graphs for the ortho isomer and partly because the o-cresol and m-cresol curves were more closely related and gave less satisfactory differentiation between the isomers.The results of a number of determinations are shown in Table I (p. 347). These conditions would also apply to mixtures of the ortho and para isomers. METHOD FOR DETERllIh I N G TOTAL FREE CRCSOLS BY AMPERORIETRIC TITRATIOX REAGENTS- Potasszzim bromidc - potassimz Eromate, 0.1 N solutioiz-Dissolve 2.780 g of analytical reagent grade potassium bromate and 15.0 g of analytical reagent grade potassium bromide in water and dilute the solution to 1 litre. Methanol-Add about 10ml of bromine to 2 litres of methanol and set aside for a t least half an hour.Then add zinc dust in small amounts and shake the mixture thoroughly until the excess of bromine has been used up and the methanol is colourless. Filter off the excess of zinc and distil the methanol. Hydrochloric acid-Analytical reagent grade. Potassium bromide-Analytical reagent grade. o-Cresol; m-cresol ; p-cresol-B.D.H. Laboratory Reagent grade, redistilled before use. PROCEIXRE- Transfer an aliquot of the steam distillate containing the cresols and prepared as described on p. 342 to a 400-ml tall beaker containing 100ml of methanol, 10ml of concentrated hydrochloric acid and about ? g of potassium bromide. Add sufficient water to make a total of 100 rnl with the aliquot of cresol solution used. Then raise the beaker t o immerse the platinum electrode and the salt bridge from the calomel cell and start the rotating platinum electrode.Yote the ammeter reading and add 0.1 N potassium bromide - potassium bromate solution from a burette. Take ammeter readings at intervals during the titration and, when the end-point is approached, as shown by the slower rate of decrease of current after additions of titrant, make the additions in increments of 0.5 ml and take the ammeter readings 1 minute after each addition. When a number of additions have been made with increasing ammeter readings, plot the readings against volume of titrant added; the end-point is at the intersection of the straight lines through points before and after the end-point.If the volume of 0.1 iz; potassium bromide - potassium broinate required = V ml and the volume of steam distillate = v ml, then, total cresols calculated to $-cresol = - mg per ml and total cresols calculated to m-cresol = - mg per ml. 2.7 V V 1.8V v RESULTS- isomers are shown in Table I. The results for a number of titrations of the individual isomers and for mixtures of METHOD FOR DETERXISISG THE RATIO OF FREE p-CRESOL TO FREE W-CRESOL REAGENTS- Potassium hydroxide, N. Acetic acid, 80 per cent. v/v-Dilute 800 ml of glacial acetic acid to 1 litre with water. Sul9hwic acid, coizceiztrated-Analytical reagent grade.346 HASLAM, WHETTEM AND XEU'LANDS THE DETERMINATIOS [Vol. 78 Sodizim nitrate solution-A saturated aqueous solution. Alcoholic ammonium hydroxide-Mix 300 ml of ammonium hydroxide, sp.gr.0.880, o-Cresol ; m-cresol ; p-cresol-B.D.H. Laboratory Reagent grade, distilled before use. Cresol stock solutions, 1 per cent.-Weigh accurately about 2.5 g of each cresol isomer Cresol standard solutions-Dilute an aliquot of the stock solution to 100 ml in a graduated 250 ml of water and 450 ml of ethyl alcohol. into a 250-ml calibrated flask, dissolve each in water and dilute to the mark. flask to give a solution of which 1 ml contains 1 mg of cresol. PREPARATIOS OF CALIBRATION GRAPHS- From a micro-burette put volumes of 0.6, 0.8, 1.0, 1.2 and 1.4 ml of the standard cresol solution into 50-ml calibrated flasks. Add 1.4 ml of iV potassium hydroxide and evaporate the solution to dryness by heating it on a water-bath and drawing a current of air through the flask by means of a glass tube inserted in the flask and connected to a water pump. Dissolve the residue in 5 ml of 80 per cent.acetic acid solution. To this solution add 5 drops of concentrated sulphuric acid and then 2 drops of saturated sodium nitrite solution and mix by swirling. Set the mixture aside for 20 minutes. Then cool the flask in an ice-water mixture and slowly add alcoholic ammonium hydroxide from a burette until the volume is slightly less than 50 ml. Set the solution aside overnight, then dilute it to the 50-ml mark with alcoholic ammonium hydroxide solution and measure the optical density on a Spekker absorptiometer with Ilford KO. 602 spectrum blue filters and 1-cm cells and with water as the reference liquid.In our work the following Spekker absorptiometer readings were recorded- Indicator drum reading for 0.6 mg 0.8 mg 1.0 mg 1.2 mg 1.4 mg p-Cresol . . * . . . 0,206 0.279 0.336 0.396 0,478 m-Cresol . . .. . . 0.022 0,039 0.051 0.067 0465 o-Cresol . . . . . . 0.168 0.209 0.267 0.315 0.356 The straight line graphs for the para and meta isomers relating indicator drum readings to milligrams of cresol per 50ml of solution were drawn. Intermediate straight lines were drawn between the two graphs to give 10 equal intercepts; these graphs represent mixtures of the isomers containing 10 per cent., 20 per cent. . . . 80 per cent. and 90 per cent. of 6-cresol. PROCEDURE- Having found the volume of 0.1 N potassium bromide - potassium bromate, V , required to brominate a volume of the steam distillate by amperometric titration, measure an aliquot of the steam distillate equal to about v/2*25 Vml (see Kote below) into a 50-ml calibrated flask.Add 1-4 ml of IV potassium hydroxide solution and evaporate the solution to dryness on a water-bath, assisting the evaporation by drawing a current of air through the flask. Develop the nitrosamine colour and measure the optical density in terms of Spekker absorptiometer readings as described in the preparation of the calibration graphs. From the concentration of the solution in terms of m- and p-cresols as calculated from the 0.1 N bromide - bromate titration and the volume of solution taken, calculate the weight of cresol in terms of the meta and para isomers in the aliquot of the steam distillate taken, Draw a straight line joining the points on the m-cresol and p-cresol graphs corresponding to these weights of m- and p-cresol, respectively.Mark on this line the point corresponding to the indicator drum reading. Calculate the percentage of p-cresol from the position of the point in relation to the intermediate graphs. Also read from the graph the total cresols present and calculate the concentrations of m-cresol and p-cresol in the steam distillate. From the weight of sample taken, calculate the percentage of free m- and p-cresol present in the sample and the ratio of free m-cresol to free 9-cresol. NOTE-AS the concentration of cresols calculated t o p-cresol = 2.7 V / v mg per ml and the concentration of cresols calculated t o m-cresol = 1.8 V / v mg per ml, the mean value for this expression becomes 2-25 V / v and therefore 1 mg of a 1 + 1 mixture of m-cresol and p-cresol would be present in a volume of v/2.25 V ml.June, 19531 OF FREE PHESOLS IN FORMALDEHYDE RESINS 347 RESULTS- are shown in Table I.Some results of amperometric titrations of aqueous solutions of 0-, m- and p-cresols TABLE I AMPEROMETRIC TITRATION OF AQUEOUS SOLUTIONS OF 0-, m- AND +-CRESOLS Isomer ortho meta para para Mixture { ~i:i >Mixture { Mixture { etz Mixture {:$: Mixture {z: Weight of cresol g 050300 0.0404 0.0400 . . 0.0605 . . 0.0200 . . 0.0071 . . 0.0150 . . 0.0152 . . 0.0226 . . 0.0202 . . 0.0265 . . 0.0186 . . 0.0545 . . 0.0061 0.1 N potassium bromide - bromate required P found, calculated, ml ml 11.11 22.22 22.30 22.33 22.34 14.78 14.70 14.82 14.68 22.37 22.41 11.11 11.34 11.03 13.61 14.02 13.58 19.20 19.60 19.30 20.04 20.14 20.06 23.12 23.59 23.16 Bromine atoms combining with 1 molecule of cresol F found calculated 1.98 2.00 1.98 2.97 3.00 2.98 2.98 1.99 1.98 2.00 1.98 2.00 2.21 2.26 2.20 2.43 2.50 2.43 2.42 2.47 2.44 2.40 2.41 2.40 2.06 2.08 2.07 TOTAL CRESOLS AND PERCEXTAGE OF +-CRESOL FOUND IN AQUEOUS SOLUTION CONTAINING A series of solutions containing $-cresol and m-cresol were prepared by diluting known volumes of stock solutions of individual cresols to 500 ml, and the ratio of the isomers and the total concentration were determined in each solution. The results are shown in Table 11. TABLE I1 KNOWN AMOUNTS OF m- AND +-CRESOLS- DETERMISATION OF RATIO OF THE ISOMERS AND TOTAL CONCENTRATION IN AQUEOUS SOLUTIONS OF MIXED +- AND 7%-CRESOLS Amount of cresol taken in 500 ml of solution Calculated Total cresols Proportion of L I \ proportion of found, per 500 ml p-cresol found g g 6 % g % 0.3025 nil 0.3025 100 0.3042 102 0.3040 101 0.2950 0.1083 0.4033 73 0,3960 73 0.4020 76 0.1514 0,1532 0.3046 50 0.2964 49 0.3029 45 0.0757 0.2298 0.3055 25 0.3167 22 0.3207 27 p-cresol, m-cresol, total, p-cresol, of solution, in mixture, REFERENCES 1. 2. 3. 4. 6. Koppeschaar, W., 2. anal. Chim., 1876, 15, 233. Standardisation of Tar Products Test Committee, “Standard Methods for Testing Tar and its Savitt, S. A., Goldberg, A. M., and Othmer, D. F., Anal. Chem., 1949, 21, 516. Sprung, M. M., Ind. Eng. Chem., Anal. Ed., 1941, 13, 36. Kolthoff, I. M., and Bovey, F. A,, Anal. Chem., 1947, 19, 498. Products,” Third Edition, London, 1950, p. 217. IMPERIAL CHEMICAL INDUSTRIES LIMITED PLASTICS DIVISION WELWYN GARDEN CITY, HERTS. September 23rd, 1952

ISSN:0003-2654    

DOI:10.1039/AN9537800340

出版商:RSC    

年代:1953

数据来源: RSC

摘要:

348 KIRKPATRICK THE DETERMISATION OF [Vol. ’is The Determination of Iodine in Blood Seruni BY H. F. W. KIRKPATRICK The practical aspects of the micro-determination of iodine in serum by the method originally proposed by Chaney in 1940 are discussed, and certain modifications are suggested t o ensure smooth working and good recovery of iodine. The complete procedure is described and experimental results bearing on reproducibility and accuracy, together with the iodine levels in the sera of normal individuals, are recorded. IT is now generally accepted that thyroid activity is directly related to the iodine level in the blood serum and that the relationship is more specific for the protein-bound organic fraction than for the total iodine. Consequently a reliable method ought to be available for the determination of iodine.Among the methods proposed for this purpose that of Chaneyl has attracted most atten- tion, by reason of the introduction of a still designed for isolating the iodine by volatilisation in a minimum volume of distillate and the use of the catalytic action of iodide on the reduction of ceric sulphate by arsenious acid for the final colorimetric step. The latter is a sensitive means of determining iodide, especially suited to the micro-amounts met with in serum, whilst the Chaney Still greatly enhanced the practicability of the comparatively rapid wet digestion method. In Chaney’s procedure organic matter is destroyed and iodine, in all forms, is oxidised to iodate by digestion with a mixture of chromic and sulphuric acids. The iodine is volatilised by reducing the digest a t boilicg point with phosphorous acid, is trapped in an absorbing solution and is finally determined colorirnetrically by the reaction mentioned above.It has become obvious that, in practice, even a relatively simple procedure must be rigidly standardised in order to obtair\, reliable results; the best technique is still a matter of conflicting opinion. I t seems relevant, therefore, to make some contribution to the practical aspects of the determination from observations made in the course of four years’ experience oi its application in clinical work. The method discussed and described here is similar to that of Barker,2 but has some additions and modifications, that, it is believed, remove most of the causes of erratic results and tend to make its routine use practicable.PRACTICAL COXSIDERATIONS DIGESTIOS AND ABSORPTIOS- Chaney’s method was elaborated in detail by Taurog and Chaik~ff,~ but their procedure was criticised by especially for its use of sodium hydroxide solution to absorb the volatilised iodine. Barker stated that absorption in a reducing medium is essential for ensuring good recovery of iodine ; he used a solution of sodium sulphite, subsequently removing sulphur dioxide by acidifying and aerating the distillate. He later eliminated the aeration step by using a sodium hydroxide solution containing added arsenious The addition of arsenious oxide has been approved by Thomas, Shinn, \{‘iseman and but MoranG has rejected it upon the ground that it causes a lion-specific reduction of ceric sulphate.Moran reverted to sodium hydroxide by itself; he obtained a good recovery of iodine by care- fully controlled heating during digestion and by adding hydrogen peroxide to the phosphorous acid used for reduction. We have found sodium hydroxide to be a generally unsatisfactory absorbent, but potassium hydroxide has given distinctly better results. It is interesting to note that Connor, Curtis and Swenson7 have apparently obtained satisfactory results with potassium hydroxide. Sodium sulphite alone and potassium hydroxide solution containing added arsenious oxide have also given uniformly good recoveries ; the latter has been used exclusively for a considerable time. Although a small non-specific reduction of the ceric sulphate occurs, it is never serious enough to jeopardise accuracy, and any slight disadvantage from this effect is considerably outweighed by its beneficial influence upon recovery of iodine.RIoran’s finding that the manner and degree of heating are of paramount importance to the successful use of sodium hydroxide suggests that the addition of the reducing agent also introduces a certain degree of much-needed flexibility in technique, for it is then only necessary to avoidJune, 19531 IODISE I S BLOOD SERUM 349 excessive heating, as denoted by the appearance of dense white fumes, which, as pointed out by Barker, are especially disastrous during the second heating. I t is difficult to stanclardise heating, but it has been found sufiicient, both for adequacy of heating and for provision of a safety factor against loss, to discontinue heating when the flask is distinctly, but not densely, filled with fumes.Besides this moderation in heating, it is important to provide an adequate excess of chromic acid for digestion, as complete reduction of the chromic acid leads inevitably to loss of iodine. REAGESTS- The main difficulty likely to be encountered in setting up and in long term use of the method is that of obtaining reagents of sufficient purity. The sensitivity to iodide of the rate of the ceric sulphate - arsenious acid reaction decreases rapidly under the test conditions when amounts of iodide in excess of 0.15 pg are used, so that accuracy and working range are dependent upon a low initial blank value. Little progress appears to have been made in overcoming this difficulty, the general procedure being to test specimens of chemicals from various manufacturers until a satisfactory combination is found, without any regard to the suitability of subsequent deliveries from the same sources.It is generally agreed that ordinary distilled water can be freed from iodine by re-distilla- tion from alkali in an all-glass still; the water used in the determination and in the preparation of the reagents should be purified in this manner, although the apparatus may be rinsed with distilled water as ordinarily prepared. Concentrated sulphuric acid can be purified if neces- sary by boiling it with a little hydrochloric acid for one to two hours in a fume ~ h a m b e r , ~ but this procedure lias fortunately so far proved unnecessary. Most trouble has been experienced with chromic acid, phosphorous acid and arsenious oxide ; chromic acid is always contaminated, usually to the extent of being unusable, whilst the quality of the other two reagents varies according to source and even to batch.Boiling the solution of phosphorous acid for a period of a half to one hour is usually recommended for removing contamination, but we have found this to be of no value, unusable specimens of phosphorous acid showing no improvement after boiling for several hours. Experimental work has shown that it is possible by simple purification procedures to eliminate uncertainty in the use of phosphorous acid and arsenious oxide, but the only satis- factory solution of the problem created by the general impurity of chromium trioxide has been to replace it with sodium dichromate.The technical grade of sodium dichromate is usually sufficiently free from contaminants to be used directly, but a poor specimen can be improved by washing the powdered solid with absolute industrial alcohol, drying at 120" C and using 87.5 instead of 100 g as described for the preparation of the reagent (p. 351). Phosphorous acid is purified by treatment with activated charcoal under specified conditions, and arsenious oxide is re-sublimed before use. Details of the methods are included in th'e reagent section of the procedure (p. 350). The use of these methods has contributed largely to the uniformly low blank value obtained during the past two years with various batches of materials.STILL- The alternative of prolonged distillation followed by evaporation, besides being tedious, may also cause chromium com- pounds in the distillate to be present to such an extent that it is difficult to prevent interference with the colorimetric reaction. The design of the still has inevitably provoked some rather specialist apparatus, but there is no evidence that efficiency must be divorced from simplicity. The original Chaney still was awkward to manipulate, but was improved by Talbot, Butler, Saltzman and Rodrigne9, who provided a tap for removing the absorption liquid. A further modified form of still* has been used in this laboratory (Fig. l), the modification including the addition of a stoppered inlet to the trap to permit introduction of the absorption solution with the apparatus completely assembled and replacement of the splash baffle-plate and return tube of tlie original still by a closely fitting internal funnel that performs both functions.In this form the still is simpler to construct and manipulate and is no more trouble to set up and dismantle than a small Soxhlet apparatus. It is important to note with regard to the construction that the glass stopper and stopcock must bc made of the same type of glass The use of a special form of still is essential to success. * The still is supplied by Messrs. Baird and Tatlock (London) Ltd., and was designed in collaboration with their technical staff.350 KIRKPATRICK THE DETERMINATION OF [Vol. 78 as the rest of the still if sticking is to be avoided.When completely assembled the top socket of the still carries a reflux condenser and the bottom joint is plugged into the socket of a two or three-necked 500-ml round-bottomed flask, which also carries a dropping funnel. The three-necked flask is suggested because it is a standard product and consequently more easily obtainable; the third neck is closed with a glass stopper. This flask also serves as the digestion flask. B I L. Fig. 1 . Modified Chaney still. A, combined splash baffle and return tube; B, stoppered inlet to trap; C, capillary for regulating reflux action: D, outlet tap from trap COLORIMETRIC DETERMIXATION- The precise practical details for carrying out this final step in the determination apparently vary from one laboratory to another, the essential factors being accurate measurement of volumes and strict control of temperature and time of reaction.Volumes of distillate and reagents and the time of reaction have been chosen in this laboratory to enable absorption measurements to be made in a standard 1-cm glass cell over a useful working range at a temperature of 37" C by means of either a Spekker or a Biochem absorptiometer. Provision has also been made for the following considerations- 1. Addition of sodium chloride to the reaction mixture, for enhancing the catalytic activity of the iodide and eliminating the random effect of traces of chlorides that may appear in the distillate, can be made with advantage, as recommended by Barker.2 2. The modification suggested by Carr, Graham, Ober and RigsB has a marked beneficial effect upon the stability of the blank reading and merits more general attention.This consists in adding sulphuric acid up to a concentra- tion of approximately 2.3 N in the final reaction mixture in order to inhibit the catalytic effect of traces of chromium compounds that may have collected in the distillate. The necessary acid is incorporated in the arsenious acid reagent. 3. A standard graph is prepared from the readings given by addition of standard iodide solution to blank distillates. This procedure was used by Taurog and Chaikof13 and is preferable to the addition of iodide to water, used by Barker,2 as deviations of up to 10 per cent. may occur, notably with the larger amounts of iodide, when the readings given by similar amounts of iodide by the two methods are compared.METHOD COLLECTIO?: OF BLOOD- Venous blood is allowed to clot and the serum separated by centrifugation in the usual manner, but it is advantageous to maintain aseptic conditions throughout, as the serum will then keep unchanged for at least a fortnight if stored at 4" to 6" C. REAGENTS- marble chips to prevent bumping, and distil from an all-glass apparatus. for all reagents and in the procedure. Williams Ltd. AnalaR) cautiously, with cooling, to 450 ml water. and 100 ml water by warming in a beaker on a water-bath. stirring, and set aside with frequent stirring for about 10 minutes. No. 54 filter-paper, returning the first portions of filtrate until clear and colourless. Water-Make ordinary distilled water alkaline with potassium hydi oxide, add some Use the distillate Sulphuric acid, 70 $er cent.-Add 600 ml of concentrated sulphuric acid (Hopltin and Phosphorous acid, 50 per cent.w/v-Melt together 500 g of crystalline phosphorous acid Add 5 g of "Norit" charcoal with Filter through a Whatman AllowJune, 19531 IODIKE I N BLOOD SERUM 351 the filter to drain at the end of filtration; do not wash it. Dilute the filtrate t o 1 litre with water. Sodium dichromate solutioiz-Dissolve 100 g of commercial sodium dichromate (Hopkin and Williams Ltd. AnalaR) in a mixture of 200 ml of water and 5 ml of 70 per cent. sul- phuric acid. Arsenious acid reagent-Heat about 10 g of arsenious oxide in a porcelain dish, covered with a clock glass, over the free flame of a micro-burner in the fume chamber, until the major part has sublimed and condensed on the sides of the dish, remove and store in a clean bottle.Dissolve 3.71 g of the re-sublimed arsenious oxide in 50 ml of N sodium hydroxide solution in a 1-litre calibrated flask and dilute with about 400 ml of water. Add slowly with shaking and cooling 425 ml of 70 per cent. sulphuric acid, make up to the mark with water and mix. Dissolve 3.125 g of pure sodium chloride in this solution. Absorption solution-Dissolve 0.75 g of the re-sublimed arsenious oxide in 100 ml of water containing 2.5 g of potassium hydroxide. Ceric ammonium sulphate reagent-Mix 17 g of ceric ammonium sulphate with about 300 ml of water in a 500-ml calibrated flask and add, slowly, with mixing, 80 ml of 70 per cent.sulphuric acid. Set aside with occasional mixing until solution is complete, make up to the mark with water and mix. Sodium hydroxide solution, 0.5 N. Zinc sulphate solutionlo-Prepare a 10 per cent. w/v solution of crystalline zinc sulphate and titrate 10 ml of it with the 0.5 N sodium hydroxide solution. From 10.8 to 11.2 ml of the latter should be required to produce a pink colour with phenolphthalein. Standard solutions-Dissolve 0.131 g of pure potassium iodide in water and make up to 100 ml. Dilute 5 ml of this solution to 500 ml to provide a stock solution containing 10 pg of iodine per ml. Dilute 5 ml of stock solution to 500 ml to obtain a convenient working solution containing 0.1 pg of iodine per ml. These solutions are reasonably stable if kept away from light in closely stoppered bottles.PROCEDGRE- Precipitation of protein-bound iodine-To 2 ml of serum in a 50-ml centrifuge tube add 20 to 30 ml of water, 2 ml of zinc sulphate solution and 2 ml of 0.5 N sodium hydroxide; mix after each addition. Centrifuge, discard the supernatant liquid, stir the precipitate thoroughly with about 30 ml of water, centrifuge again and discard the water. Make two further washings with water. Dissolve the precipitate in about 5 ml of 70 per cent. sulphuric acid and transfer the solution to the digestion flask. Wash the centrifuge tube with several further portions of acid until a total volume of 25 ml has been used. Total iodine-If it is required to determine the total iodine, transfer 2 ml of serum directly into the digestion flask by means of a pipette and add 26 ml of 70 per cent.sulphuric acid. Digestion-To the acid contents of the flask add 5 ml of sodium dichromate solution and some glass beads, and heat over a moderately hot bunsen burner on an asbestos-centred gauze in the fume chamber until fumes (not dense, see p. 349) of sulphuric acid fill the flask. Allow to cool for a short period, then add 15 ml of water and repeat the heating. Allow to cool. Distillation-To the digest add 30 ml of water and assemble the still completely, placing 6 ml of phosphorous acid in the dropping funnel. Heat to brisk boiling and, when the whole of the still is heated by steam (this condenses freely from the reflux condenser), run out the condensate from the trap and discard it.Unstopper the trap inlet and introduce 1.0ml of absorption solution, replace the stopper and immediately run the phosphorous acid rapidly dropwise into the boiling digest. Allow the distillation to continue for 10 minutes, remove the burner and run off the distillate into a glass-stoppered tube calibrated at 20 ml. Remove the reflux condenser and wash the sides of the still with threc successive portions of about 2 to 3 ml of water delivered conveniently from a pipette, adding the washings to the distillate. Cool, adjust the volume of distillate to 20 ml with water and mix. Coloriinetry-Transfer 5-ml aliquots of distillate into test tubes approximately 1.5 cm in diameter by means of a pipette, add 2 ml of water and 2 ml of arsenious acid reagent, mix and place in a water-bath at 37" i 0.1" C.Allow about 10 minutes for the temperature of the test soliltions to reach that of the bath and then add 1.0 ml of ceric ammonium sulphate solution to each tube, allowing an interval of 46 to 60 seconds after each addition. After exactly 1.5 minutes from the first addition measure the optical density of the first tube and then the others, at the appropriate intervals, in a 1-cm glass cell and with a violet filter. From352 KIRKPATRICK : THE DETERMINATIOS OF [Vol. 78 the readings, calculate the amount of iodine present in the test solution by means of a standard graph. Prepare a standard graph as follows: pool the distillates of two blank determinations, made by omitting the serum, and to six 5-ml aliquots add 0, 0.2, 0.4, 0.6, 0.8 or 1.0 ml of standard iodide solution containing 0.1 pg iodine per ml. To each tube add sufficient water to bring the volume to exactly 7 ml.Add 2 ml of arsenious acid reagent to each tube and proceed as above with additions of ceric ammonium sulphate. Plot the readings against micrograms of added iodine. Note-il’ith a Spekker absorptiometer, Ilford 601 filters are used, but as this filter cannot be used with a Biochem absorptiometer, a similar working range with a Chance OB 1 filter can only be obtained by increasing the concentration of ceric ammonium sulphate. This concen- tration may be increased up to double the original strength, without seriously affecting accuracy, by using 34 g instead of 17 g in the preparation of the reagent described above. RE s ULTS REP~~ODUCIBILITY AND RECOVERY- The few available reports from other laboratories have dealt in somewhat vague terms with the order of accuracy to be expected from this type of method.Barker2 gives experi- mental means showing a recovery averaging 90 per cent. with a range of 87 to 95 per cent. Thomas et a1.j recover 94 to 100 per cent. of added iodide, but give no details. Moran6 reports 88 to 94 per cent. recoveries of iodide added to serum and concludes that 100 per cent. recovery is unattainable. Evaluation of the method proposed has been made by adding iodine to serum in the form of iodide and pure sodium L-thyroxine pentahydrate. The pooled serum had an iodine content just below the normal range, by reason of inclusion of some serum from hypothyroid patients. As it is our practice to carry out duplicate determinations on each specimen, the variation in the means of duplicates has been recorded here, ten duplicate determinations being made on different occasions a t each of three iodine levels giving a range embracing those values found in practice except in severe hypothyroidism and thyrotoxicosis.The amounts of iodine found in 5-ml aliquots of distillate before and after additions to the serum are shown in Table I ; the highest, lowest and mean values are the means of simultaneous duplicate determinations, and the standard deviations are for the means of the ten replicates. TABLE I ADDITION OF IODISE TO SERUM Iodine found Iodine A -, Standard Mean Pg Pg 1% Pg Pg Yo N i l . . .. . . Nil 0.017 0,013 0.0155 * 0.0012 - Potassium iodide .. 0.025 0.043 0.039 0.0415 & 0*0011 104 0.050 0.068 0.062 0.0650 i. 0.0016 98 Sodium L-thyroxine 0.025 0.042 0.037 0.0395 & 0.0014 96 0.050 0.070 0.062 0.0665 & 0.0018 102 The over-all mean recovery of iodine in 40 duplicate determinations is thus 100 per cent.; the comparatively small rise in the standard deviation with increase in the iodine level suggests that variations are due to a constant experimental error rather than to fluctua- tions in recovery. On expressing the results as micrograms of iodine per 100 ml of serum, the means of duplicate determinations show a standard deviation of j0.25 over the range 3 t o 8, increasing to k0.35 a t 13. Increased accuracy in determining the lower levels of iodine, such as occur in hypothyroidism, can be attained by using 3-ml instead of 2-ml quantities of serum and precipitating the proteins with 3 ml each of the zinc sulphate and 0.5 N sodium hydroxide solutions.At the higher levels any advantagc gained by working with greater amounts of iodine is largely offset by the decrease in sensitivity of the colorimetric reaction. Sormal values-Determination of the protein-bound iodine has been made upon the sera of 40 normal healthy individuals. The values obtained ranged from 3.9 to 7.8 pg per 100 ml with a mean value of 5.9 and a standard deviation of f0.85. The suggested normal range is 3.5 to 8.5 pg per 100 ml. r Substance added added, Highest, Lowest, Mean, deviation, recovery,June, 19531 IODINE I N BLOOD SERUM 353 The author expresses his thanks to the Trustees of the London Clinic for providing facilities for this work, and to Glaxo Laboratories Ltd. for a gift of pure sodium L-thyroxine pent ahydrate. REFERENCES 1. 2. Barker, S. B., J . Biol. Chem., 1948, 173, 715. 3. 4. 5. 6. 7. 8. 9. 10. Chaney, A. L., I n d . Eng. Chem., Anal. Ed., 1940, 12, 179. Taurog, A., and Chailroff, I. L., Ibid., 1946, 163, 313. Barker, S. B., and Lipner, H. J., Science, 1948, 108, 539. Thomas, J. W., Shinn, L. A., Wiseman, H. G., and Moore, L. A., Anal. C h e w , 1950, 22, 726. Moran, J. J., Ibid., 1952, 24, 378. Connor, A. C., Curtis, G. M., and Swenson, R. E., J . Clin. Endocrinol., 1949, 9, 1185. Talbot, N. B., Butler, A. M., Saltzman, A. H., and Rodrignez, P. M., J . Biol. Chem., 1944, 153, 479. Carr, E. A., Graham, I). E., Ober, S., and Riggs, D. S., Science, 1950, 111, 552. Somogyi, M., J . Biol. Chem., 1930, 86, 656. DEPARTMENT OF CLINICAL I NVE STI GAT1 ON THE LONDON CLINIC 20, DEVONSHIRE PLACE, LONDON, W.l November 14th, 1952

ISSN:0003-2654    

DOI:10.1039/AN9537800348

出版商:RSC    

年代:1953

数据来源: RSC

摘要:

June, 19531 IODINE I N BLOOD SERUM 353 A Colorimetric Method for the Micro-de termina tion of Potassium in Serum BY S. BAAR Potassium in serum is precipitated with sodium cobaltinitrite without prior de-proteinisation. The precipitated complex salt is dissolved in sulphamic acid, which destroys the excess of nitrous acid. The 8-hydroxy- quinoline complex of cobalt is formed and extracted into chloroform, the absorption maximum at 403 m p being used for the colorimetric determination. The coloured complex is stable, and its absorption obeys Beer - Lambert's law over a fairly wide range. Recovery experiments and comparison of a series of replicate determinations with a widely used colorimetric method show a maximum deviation of 1 per cent. from expected values. THE potassium of biological fluids is most commonly precipitated as potassium sodium cobaltinitrite, as suggested by Kramer and Tisdal1,l or as potassium silver cobaltinitrite as suggested by Breh and Gaebler., The composition of the silver complex is affected by the sodium chloride content of the medium, so that it seemed preferable to precipitate the potassium by the method described by Kramer and Tisdalll and Ab~l-Fadl.~ The com- position of the complex, which should be I<,NaCo(NO,),.H,O, is affected by the potassium concentration,* which is consequently kept within a narrow range.The potassium in the precipitate is determined indirectly. Methods have been described in which nitrous acid is liberated and estimated, but these are unsatisfactory owing to the volatility of the nitrous acid.In the most reliable methods, the cobalt in the precipitate is determined colorimetrically, as, for example, by its reaction with nitroso-R-salt in hot acetic acid s o l ~ t i o n , ~ , ~ but the red colour so formed fades with time. Another method has been described by Ab~l-Fadl,~ but the maximal intensity of the reaction product is only attained after 10 to 15 minutes at 37" C. In this paper is described a sensitive colorimetric method that overcomes some of the difficulties mentioned above. The cobalt content of the precipitated potassium sodium cobaltinitrite is determined by its reaction with 8-hydroxyquinoline. With cobalt a stable chelate compound is formed, which is readily soluble in polar solvents but almost insoluble in water. The complex is formed at pH values between 4.3 and 11.6, and the reagent is scnsitive to 1 part in 100,000, according to Welcher.' METHOD REAGENTS- of distilled water.CobaZtinitrite reagent-Dissolve 120 g of analytical reagent grade sodium nitrite in 180 ml Add 210 ml of this solution to 50 ml of distilled water containing 25 g354 BA.4R: A COLORIMETRIC METHOD FOR THE [Vol. 78 of hydrated cobalt nitrate and 12.5 in1 of glacial acetic acid. Aspirate air through the reagent until it is free from nitrous oxide fumes. Centrifuge the reagent before use and keep it in a refrigerator. I t can be used for 2 months. Ethyl alcohol-Add 30 ml of distilled water to 70 ml of absolute ethyl alcohol. Ammonium hydroxide, 2.2 N-Make an approximately 13 per cent. aqueous dilution of SuLplzamzc acid-Dissolve 5 g of sulphamic acid (amino-sulphuric acid) in 100 ml of 8-Hydroxyq.t~i~zoline-Dissolve 2 g of analytical reagent grade 8-hydroxyquinoline in Chlorojorm-C.P. ammonium hydroxide, sp.gr.0.880. distilled water. 100 ml of absolute ethyl alcohol. PROCEDURE- Add 1.0 ml of the cobaltinitrite reagent and set the mixture aside a t room temperature for 1 hour. Then add 1.0 ml of distilled water and centrifuge the tube and contents a t 1800 r.p.m. a t a radius of 15 cm for 5 minutes. Carefully decant the supernatant fluid and wash the precipitate thrice with 1-ml portions of 70 per cent. ethyl alcohol, the final centrifugation being a t 3000 r.p.m. for 10 minutes. Then remove the supernatant fluid as completely as possible by draining the tube mouth-downwards on filter-paper. Finally dry the walls of the tube with filter-paper.Dissolve the precipitate in 2 ml of the sulphamic acid solution, which destroys the excess of nitrous acid. LVarm the tube for 30 seconds on a bath of boiling water, and then add 4 ml of distilled water, thoroughly mix the contents of the tube and transfer 5 ml to another centrifuge tube. Add 1 in1 of 2.2 S ammonium hydroxide, swirl the solution, and then add 0.5 ml of 8-hydroxyquinoline reagent. Extract the complex by shaking this solution with 4 m l of pure chloroform. Siphon off the aqueous layer and filter the turbid chloroform layer through a 4.23-cm LVhatrnan KO. 1 filter-paper folded and placed over the mouth of a tube without insertion into a funnel. The solution L1.ash 0.1 in1 of serum into 0.8 ml of distilled water in a wide centrifuge tube.The final pH value is 9.5. “ O l 300 340 380 420 460 500 Wavelength, mp Fig. 1 . Absorption graph of cobalt hydroxyquinolate Amount of potassium, mg Fig, 2. Typical calibration graph for serum potassiumJune, 19531 MICRO-DETERMIXATION OF POTASSIUM I S SERUM 355 is then ready for colorimetric examination. The absorption maximum of the cobalt hydroxyquinolate lies a t 403 mp, as determined with a Unicam quartz spectrophotometer (Fig. 1). Throughout this study a Spekker photo-electric absorptiometer was used with the standard projector-type lamp and a combination of Ilford 601 and Kodak Wratten 35 filters. The Kodak filter was used in place of the usual Chance OK 20 heat-absorption filter.CALIBRATION GRAPH- Analytical reagent grade potassium sulphate was dried to constant weight and its purity was found to be 99.35 per cent. by gravimetric determination as potassium platinichloride. From this material a standard solution containing 0.2 mg of potassium per millilitre was made and used in the preparation of the typical calibration graph shown in Fig. 2 . EXPERIMENTAL Efect of dilution-The effect of dilution on precipitation was investigated by adding to a constant amount of standard potassium solution various amounts of distilled water. The final concentration of the cobaltinitrite reagent was kept in the range of 50 to 77 per cent. The coloriineter readings over this range were within the experimental error. The mean concentration, 67 per cent., was chosen for the working conditions.Stability-The stability of the coloured complex was investigated by recording the colorimeter readings a t intervals of 15 minutes. After 2 hours, the readings deviated by not more than 1 per cent. from those recorded initially. KPprodzicibilit3,-The reproducibility of the values recorded and their agreement with the results by an accepted colorimetric method were investigated. A specimen of venous whole blood was taken and the serum was separated in the usual way. Twelve replicate determinations were made by the method described and the results were compared with those of twelve replicate determinations made by the method described by King.8 The proposed method showed a range of 17.50 to 17.85 mg per 100 ml, with a mean of 17.70 mg, compared with a range of 17.50 to 18.00 mg per 100 ml and a mean of 17.82 mg found by King’s method.The standard deviation of the author’s method was found to be 0.154 mg per 100 ml, compared with 0.104 mg for King’s method. The standard error of the difference was 0.185 mg per 100 ml (P = W5), which showed no significant difference. RECOVERY OF ADDED POTASSIUM- The initial potassium content of a 10-ml sample of serum was established by triplicate estimations; 4.45 mg of dried potassium sulphate were then dissolved completely in 5 ml of the serum. TABLE I RECOVERY OF POTASSIUM ADDED TO SERUM AS POTASSIUM SULPHATE The results of the recovery experiment are shown in Table I. Potassium in original serum, mg per 100 ml: 15.0, 15.2, 15.0 (average, 15.07) Potassium in serum to which 20 mg per 100 ml had been added A r -7 Found, Expected, Deviation from expected, mg per 100 in1 mg per 100 In1 Yo 35.02 35.07 -0.14 35.10 35.07 + 0.08 35.08 35.07 f 0.03 COMPARISOS WITH KIKG’S METHOD- Twelve samples of venous blood were taken from twelve patients chosen a t random.Determinations in duplicate were made on each sample both by the proposed method and by King’s method. The results were the same in four out of twelve samples, whilst the percentage deviation of the remainder ranged from -0.905 to +0.620. The percentage recovery of added potassium sulphate, as indicated by Table I, was within 0.2 per cent. of the expected figure. Comparison of a series of determinations showed a maximum deviation of 1.0 per cent. from the expected value.The proposed method canHOLNESS AND LAWRENCE: A SCHEME OF SEMI-MICRO [Vol. 78 356 therefore be recommended for the determination of potassium in serum. It has the advantage over existing chemical methods of being more economical of serum, and the stability of the colour is high without undue complication of the working procedure. I should like to thank the Unit Director, Dr. J. P. Bull, for permission to carry out this work and for his help in the statistical treatment of the data. Thanks are also tendered to Mr. D. M. Jackson for permission to take samples of venous blood from patients in his care. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. Kramer, B., and Tisdall, F. F., J . Biol. Chenz., 1921, 46, 339. Rreh, H., and Gaebler, G., I b i d . , 1930, 87, 81. Abul-Fadl, M. A. M., Biochem. J . , 1949, 44, 282. Piper, C. S., J . SOC. Chenz. I n d . , 1934, 53, 3 9 2 ~ . Sideris, C. P., I n d . Eng. Chem., Anal. Ed., 1937, 9, 145. -, I b i d . , 1942, 14, 821. Welcher, F. J., “Organic Analytical Reagents,” D. Van Nostrand Co., Inc., New York, and King, E. J., “Microanalysis in Medical Biochemistry,” Second Edition, J. & A. Churchill Ltd., Macmillan & Co., Ltd., London, 1947, Volume I, p. 266. London, 1951, p. 87. MEDICAL RESEARCH COUNCIL INDUSTRIAL INJURIES AND BURNS RESEARCH UNIT BIRMINGHAM ACCIDENT HOSPITAL BATH Row, BIRMINGHAM, 15 October 151h, 1952

ISSN:0003-2654    

DOI:10.1039/AN9537800353

出版商:RSC    

年代:1953

数据来源: RSC

摘要:

356 HOLNESS AND LAWRENCE: A SCHEME OF SEMI-MICRO [Vol. 78 A Scheme of Semi-micro Qualitative Analysis for Thir ty-nine Elements BY H. HOLNESS AKD K. R. LAWRENCE The classical hydrogen sulphide scheme of qualitative analysis is modified to include all the elements likely to be encountered in modern analytical practice. The behaviour of tungsten in forming complex acids is recognised and allowed for. An existing method of phosphate removal is modified and expedited to deal both with phosphates and complex tungstates. The details given apply only to the semi-micro technique for 2 to 5 mg of material in about 3 ml of solution. IN the past two decades the greatly increased use of the less familiar elements has brought about a gradual change in our conception of a “rare” element. Titanium, zirconium, molybdenum and tungsten are examples of elements still rare by textbook standards but in common use.The usual systematic scheme of qualitative analysis, as taught in schools and universities, has a background of tradition that ensures its continuance. Nevertheless, with modifications, it can be made to include most of the recently developed elements. The classical scheme of Noyes and Bray,l although comprehensive as regards the number of elements, departs entirely from the traditional elementary system. The same criticism can be levelled at other, less comprehensive, schemes that have appeared since2; no scheme appears to take into consideration, and allow for, the behaviour of tungsten in forming heteropoly acids. The modifications described in detail below are those necessary to convert a simple classical scheme, such as one recently p ~ b l i s h e d , ~ into a scheme embracing all the elements likely to be encountered in modern analytical practice.It makes provision for the complex poly-acids of tungsten and requires only those reagents that are normally found in an analytical laboratory. For 2 to 3 ml of solution containing from 2 to 5 mg of solid, the scheme, in the hands of senior students, has given good results with mixtures containing as many as tenJune, 19531 QUALITATIVE ANALYSIS FOR THIRTY-SINE ELEMENTS 357 elements. There is reason to believe that the scheme will be equally applicable to work carried out on the macro scale, but so far only the semi-micro technique has been used.Except for the silver group of metals-the traditional group 1-the groups remain as in the elementary scheme; they are merely extended to accommodate the additional elements. This means that the group separations have been modified to allow for the extra elements. The silver group has been distributed between the hydrogen sulphide group (group 2) and the “insolubles”; its place is taken by a new group 1, which includes metals easily reduced to their elemental form by hydrazine in hydrochloric acid solution. . Filtrate. Add 2 to 3 drops of dilute hydro- chloric acid, warm and METHOD carbon dioxide and test the gas for hydrogen sulphide. Xeutralise to litmus with ammonium hydroxide, boil and filter. - Precipitate. Boil with I Filtrate. Examine for A. PREPARATION OF SOLUTION AND TREATMEST OF INSOLUBLES- hydrochloric acid and bromine water until the excess of bromine is removed.volume of water, boil and remove any insoluble material. group separations (see B below). it to a piece of nickel foil, add half a pellet of sodium hydroxide and fuse. filter. Boil 2 to 5 mg of the substance with 1 ml of a mixture of equal volumes of concentrated Add an equal Reserve the hot solution for the Wash the insoluble portion with water, dry it and transfer Cool the melt, extract by boiling with a 5 per cent. solution of sodium carbonate, and diluie hydrochloric acid and filter. -- Residue. Boil with dilute nitric acid. filter and 1 Filtrate. Acidifv with dilute nitric acid. Boil off anions. wash the residue. Filtrates. Evaporate just to dryness, take up in 2 N hydrochloric acid and examine for metals.Residuz. Dry and fuse in silica with potassium hydrogen sulphate. Leach with dilute sul- phuric acid and examine for metals. Residue. Dry and fuse in silica with potassium hydrogen sulphate. Leach with a hot saturated solution of ammonium oxalate and filter. Precipitate. A Filtrate. Add a 1 gelatinous pre- per cent. solution cipitate of silica. of tannin and ex- amine for tantalum and n i o b i ~ m . ~ filter. Precipitate. Test for silver. B. SEPARATION INTO GROUPS- Group 1-Add 2 to 3 crystals of hydrazine hydrochloride to the solution and boil until the reaction ceases. At this stage any precipitate is removed as group 1 and contains gold, selenium, tellurium and possibly platinum. This reducing treatment also breaks up chromates and vanadates; it does not break up arsenates or reduce ferric iron.Group 2-Add several drops of concentrated hydrochloric acid to the solution and boil both to reduce the volume to 1 ml and to produce a constant-boiling acid mixture. Pass hydrogen sulphide into the boiling solution, add an equal volume of water and then 2 to 3 drops of a saturated solution of ammonium chloride to precipitate the ammonium salts of certain complex tungstic acids. Boil away the hydrogen sulphide, add as much 2 N ammonium hydroxide as there is liquid in the tube, boil and again pass in hydrogen sulphide; the solution will then be about 0.25N with respect to hydrochloric acid. Remove the precipitated sulphides of mercury, bismuth, copper, cadmium, lead, silver, platinum, vanadium, arsenic, antimony, tin and molybdenum, thallous chloride and the ammonium salts of certain heteropoly acids of tungsten.Saturate the solution with hydrogen sulphide358 HOLNESS ASD LAWREXCE: .4 SCHEME OF SEMI-MICRO [Vol. 78 and place it over a bath of boiling water for a t least 15 minutes to ensure complete precipita- tion of all molybdenum sulphide. Remove any additional precipitate that forms on standing and include it with the main group 2 precipitate. Group 3-Boil off hydrogen sulphide, add 1 to 2 drops of concentrated nitric acid and boil. Withdraw two small portions and test one for phosphate with nitric acid and ammonium molybdate and the other for complex tungstates with tannin as described by Holness5; any phosphate or tungstates present must be removed by special procedures before continuing (see under F, p.361). Similar strictures obtain if organic radicals, fluorides, borates, and so on, are present. Add several drops of a 1 per cent. solution of ferric chloride to ensure complete precipitation of any vanadium or thallium, then add a small amount of a saturated solution of ammonium chloride and boil the solution. Add diluted ammonium hydroxide (1 + 1) dropwise to the gently boiling solution until it is just neutral to litmus, when the precipitated hydroxides and hydrated oxides of iron, manganese, chromium, aluminium, zirconium, titanium, indium, thallium, uranium, beryllium, vanadium, thorium, cerium and the rare earths are removed as group 3. It is inadvisable to use an excess of ammonium hydroxide to precipitate this group. The presence of dissolved carbon dioxide in the ammonium solution can result in considerable precipitation of the alkaline earth metal carbonates, as well as a loss, by solution, of some uranium and rare earths.This fact is seldom commented on in works on qualitative analysis, although it usually finds mention in books on quantitative analysk6 A modified Procedwe foy gyoq5 3-Because of the possible loss, by precipitation as car- bonates, of some of the alkaline earth metals in group 3 and of the tedious nature of the removal procedure when organic radicals, fluorides, borates, silicates, and so on, are present, the follo\ving alternative procedure, which can be used after phosphate and complex tungstate have been removed, has much to commend it.Add 2 to 3 drops of concentrated nitric acid, then 5 drops of concentrated sulphuric acid and evaporate until fumes of sulphur trioxide appear; on cooling, gelatinous particles of silica will be seen adhering to the sides of the beaker. Dilute with water and remove the insoluble portion, which contains silica, barium sulphate, strontium sulphate and some calcium sulphate. Oxidise tlie soluble portion with 1 drop of concentrated nitric acid, test for iron, add ferric chloride and precipitate group 3 as described above. Fuse the insoluble portion with sodium hydroxide as in section A above, but dissolve the precipitated carbonates in acetic acid to provide a solution for the separation of the alkaline earth metals (group 5), which can be added to that obtained by the norinal method.GYOLLPS 4, 5 and 6-With the exception of lithium, rubidium and caesium, only the elements usually included in the classical scheme now remain in solution. The additional alkali metals find their way into group 6, unless there is present an excess of lithium, in which event some will be precipitated in group 5 along with calcium. When all interference is removed, withdraw a further portion and test it for iron. C. SEPARATION OF GROUP 1- Dissolve the precipitated' elements by boiling them with a mixture of equal parts of concentrated hydrochloric acid and bromine water, boil away the excess of bromine, add an equal volume of a saturated solution of ammonium chloride and evaporate just to dryness. Leach the residue with water and remove any precipitated ammonium cliloroplatinate. Boil the solution with oxalic acid to reduce to the metal any gold present ; remove the metallic gold.Next add sufficient concentrated hydrochloric acid to make the solution about 2 S and then add 2 to 3 crystals of hydroxylamine hydrochloride; on boiling, any selenium present is precipitated. Remove the selenium and precipitate the tellurium left in solution by boiling the solution with 2 to 3 crystals of liydrazine hydrochloride. D. SEPilliATION OF GROUP 2- Divide this group into the copper and arsenic subgroups by heating just to boiling with an excess of the 1 per cent. lithium hydroxide and 5 per cent. potassium nitrate reagent,June, 1953: QUALITATIVE ASALYSIS FOR THIRTY-NINE ELEMESTS 359 as described by Holness and Trewick.' Separation is clean, with the sulphides of arsenic, molybdenum, antimony, tin and vanadium remaining in solution, the insoluble ammonium complex tungstates being converted into soluble lithium ones and the sulphides of the copper group being unaffected, in which form they can be removed.The arsenic sub-grou$-Just acidify the filtrate from the lithium hydroxide extraction with dilute hydrochloric acid, warm, filter and wash the precipitate thoroughly with hot dilute ammonium chloride solution. Pvecipitate. Dark brown precipitate denotes tungsten. Pvecipztate. Add concentrated hydrochloric acid and Add magnesium carbonate t o expel hydrogen boil. sulphide, dilute and filter. ___ Filti,ate. Destroy the excess of tannin by boiling with concen- trated nitric acid, then test for silicon and phosphorus by the usual methods.Residue. Shake with a cold ammonium carbonate solution and filter. Residue. IIOS,. Filtvate. Acidifv with dilute hpdro- chloric acid Yellow 1 precipitate I of xs,s,. Precip itate. Test for antimony. Filtrate. .4dd an excess of 1 per cent. chloric acid and bromine water. Boil off the excess of bromine. Add a saturated solution of ammonium chloride and evaporate just to dryness. tannin solution. wise until neutral t o Congo Red. acid medium denotes tin. 1 medium denotes vanadium. 13031 and add ammonium hydroxide drop- A white precipitate in A blue-black precipitate in neutral evaporate until fumes of sulplinr trioxide appear. Cool, dilute and filter. Pvecipitate.Dis- The cofiper stib-grollp--\;C.arm the residue from the lithium hydroxide extraction with diluted nitric acid (1 - l ) , dilute and filter. Pwcipitnte. Yellow I Fil2~atc. Add dilute crystals of am- Iiytlrochloric acid monium platino- and stannous chloride. chloride to coil- firm mercury. i I test for lead. PvxipitaSe. A yellow precipitate of silver iodide. Fillrate. Add concentrated hydrochloric acid and 1 drop of bromine water, and boil. Add a crystal of hydrazine hydro- chloride and boil. -\dd a small amount of ferric chloride solution and tlien make ammoniacal. Boil and filter. Filtrate. Test for copper and cad- mium. Pvecip ita tr . Dissolve in dilute nitric acid, divide into 3 por- tions and test separ- ately for bis- muth, tlial- liuin and vanadium.Filtvate. Add 2 drops of potassiiim iodide solu- tion, warm and filter.E. SEPARATION OF GROUP 3- acid, digest hot and filter. Dissolve the ammonia precipitate in dilute hydrochloric acid, add an excess of oxalic hot dilute nitric acid. Boil off hy- drogen sul- phide and divide into 3 parts. Test separ- a t e l y f o r manganese, thallium and indium. Precipitate. Cover with water; add solid ammon- ium oxalate. Boil, digest hot and filter. - Precipitate. Add a 10 per cent. solution of sodium bicarbonate. Warm and filter. Precipitate. Filtrate. Test Test for for uranium. beryllium. P'recipitate. Cerium an< rare earths. Precipitate. Yellow barium chromate. Filtrate. Test for thorium. Filtrate, Test for vanadium. Filtrate. Add ammonium hydroxide to re-precipitate hydroxides, filter and wash the precipitate with Dissolve the pre- Pass a mixture of ammonium chloride and ammonium hydroxide; reject the filtrate.cipitate in dilute hydrochloric acid, add an excess of ammonium tartrate; make ammoniacal. in hydrogen sulphide, warm and filter. ~~~~ Precipitate. Orange com- plex of titanium. 'recipitate. I Dissolve in Filtrate. Add a 5 per cent. solution of di-sodium hydrogen phosphate, warm and filter Filtrate. Test for zirconium. Filtrate. Acidify with dilute acetic acid, digest hot and filter. Ignore filtrate after first testing it for vanadium. Suspend residue in water; add solid sodium peroxide. Boil until effervescence ceases. Filter. Filtrate. Divide into 2 por- tions. (1) Test for alu- minium. (2) Add barium chloride solution.Filter, reject the filtrate. Boil the precipitate with dilute acetic acid and filter. m 0 M s m m > z U > m 0 X M 5 0 hlJune, 19531 QUALITATIVE ANALYSIS FOR THIRTY-NIXE ELEMENTS 361 F. SPECIAL PROCEDURE FOR REMOVING PHOSPHATES AND COMPLEX TUYGSTATES- This procedure is a simplification of the method described by Holness and Mattock* for phosphate removal ; it effects the rapid removal of interferences from both phosphates and tungstates in one operation. Add 2 drops of a 5 per cent. aqueous solution of zirconyl chloride, 2 drops of a saturated solution of ammonium chloride, boil and add a large excess, 10 to 15 drops, of a 1 per cent. aqueous solution of tannin. To the gently boiling solution, slowly add N ammonium hydroxide solution dropwise until the solution is just alkaline to Congo Red, then add a further 10 drops of tannin solution and continue to boil for 3 to 4 minutes.Make slightly acid with N hydrochloric acid, add 5 drops of 5 per cent. cinchonine solution, boil, remove and discard the precipitated tannin complexes of zirconium (white), tungsten (brown) and tantalum (yellow) ; niobium (red) and titanium (orange) will also be partly removed here, but their presence should have been noted during the preliminary testing for complex tungstates. Destroy the excess of tannin left in the solution by boiling with several drops of con- centrated nitric acid or, better, by following the modified procedure described under section B above. FEATURES OF THE PROCEDURE- The tan& reagent-In extending the scheme of qualitative analysis to a more realistic selection of elements there is one reagent that proves to be of great importance, namely, a 1 per cent.aqueous solution of tannin. This reagent, forming as it does coloured complexes with a large number of elements, was, under the guise of “tincture of galls,” a recognised reagent in qualitative analysis more than 150 years ago; yet to-day, modern textbooks make little mention of its use. As a routine preliminary test on an acid extract of the original substance its use has been de~cribed.~ It gives clear indications and, occasionally, positive proof of the presence in an analysis of certain of the less familiar elements. Apart from its use as a confirmatory test for tin and vanadium, its main use in the above work lies in its behaviour with the heteropoly acids of tungsten.Tungsten forms complex acids with many elements; these acids are stable in acid but not in strongly alkaline solution, although they are soluble in both media. In order to break up these complexes and identify the central complexing element, it is necessary to find a reagent that will combine with the tungsten in alkaline solution, in which the complex is unstable, and remain combined with it when the solution is acidified. By boiling it with the complex in alkaline solution the mustard-yellow tungsten - tannin complex is slowly precipitated, and this, on acidification, turns brown and remains precipitated. Hence the tungsten can be largely removed and the complexing element, frequently silicon or phosphorus, searched for in the solution by the usual tests adopted for their identification.Acid radicals-The extension of the classical scheme described above does not affect greatly the number of acid radicals to be identified. Apart from the behaviour of tungsten, all the additional anions, such as vanadate, molybdate, selenite, selenate, tellurite and tellurate are formed from elements recognised in the systematic scheme. Hence only their initial states of oxidation need be determined for their complete identification. With tungsten present as a simple tungstate ion, this state can readily be identified, either by acidifying a neutral solution and precipitating the characteristic tungstic acid, or by using tannin in alkaline solution and then acidifying. Tungsten present as a complex is identified in the systematic scheme. As certain of the elements dealt with in the above scheme appear in more than one of the tables of separation, the following notes are appended.Silver-The solution for systematic analysis is essentially prepared through hydrochloric acid; consequently any silver present will be converted to silver chloride and most of it is removed as an insoluble substance, but a. small amount is soluble in the acid and finds its way into group 2, where it is precipitated as silver sulphide. Platinum-Only when selenium is present in the analysis is platinum precipitated in group 1 ; a t other times it is precipitated as platinum sulphide in group 2. I t is completely insoluble in the lithium hydroxide reagent and so finds its way into the copper sub-group of sulphides.Valzadium-This element, reduced by hydrazine to the vanadyl state, spreads itself generously throughout groups 2 and 3. It appears in group 2 through the agency of carriers: Tannin does just this.362 HOLNESS ASD LAWRESCE [Vol. 78 if no group 2 elements are present then no vanadium is found there; if members of the arsenic sub-group are present then some vanadium accompanies them; if members of the copper sub-group only are present then vanadium is found there too; if members of both sub-groups are present then vanadium is found with both members. In spite of this, some vanadium remains in solution and passes into group 3, from which it is completely precipitated only if sufficient iron is present to carry it down as hypovanadic acid.Once precipitated, it is separated along with chromium and aluminium. Thallium-When present in the original solution in the tervalent state, thallium salts are reduced by hydrogen sulphide in group 2 and are then largely precipitated as thallous chloride. The thallous chloride still in solution passes to group 3, from which it is completely precipitated as thallic hydroxide, provided some carrier, such as iron, is present. Coyblex tungstntes-Each of the many elements that can form complex acids with tungsten was examined in binary mixtures under the conditions of the analysis ; the following were found to do so, either wholly or partly: phosphorus, silicon, zirconium, tin, iodine, iron”’, titanium, platinum, tantalum and niobium.Those whose ammonium salts are insoluble or only partly soluble in hydrochloric acid are precipitated in group 2 ; this applies mainly to phosphotungstates and to a limited extent to silico- and zirconyl-tungstates. The remaining complexes are tested for and destroyed before group 3 is precipitated. The chief innovations on this systematic scheme of analysis are- (1) The introduction of a new group 1, which includes the elements selenium, tellurium, gold and platinum, by reduction with hydrazine. ( 2 ) The treatment of silver partly as an insoluble and partly in the hydrogen sulphide group. (3) Recognition of the formation by tungsten, during the initial stnges of an analysis, of heteropoly acids. (4) Modification of a scheme of phosphate removal to include removal of complex tungstates. \Ye wish to place on record our thanks to Mr, A. R. Powell for his interest in our work and for his kindly criticism of this paper during the course of its preparation. REFEREXCES 1. 2. I\.ilson, C. L., AmZ. CAiwz, Acfa, 1950, 4, 440. 3. 4. 5 . Holness, €I., A n d . Ciiisn. Acfn, 1949, 3, 290. 6. Xoyes, A . A,, and Bray, \V, C., ‘I.\ System ol Qualitative Analysis for the Rarer Elements,” 3Iacmillan & Cn., New York, 1927. Holness, H., “Qualitative Inorganic Analysis,” Sir Isaac Pitman & Sons Ltd., London, 1951. Schorller, lf‘. R., and Powell, A . K., ”The Analysis of Minerals and Ores of the Rarer Elemcnts,” Second Edition, Charles Griffin & Co. Ltd., London, 1950, p. 150. Scott, \\‘. \Y., and Furman, N. II., “Standard 3lethods of Chemical Analvsis,” Fifth Editioii, D. Van h-ostrand Co. Inc., Kew York, and The Technical Press Ltd., London, 1939, Volume I, P. 9. 7. Holnms, H., and Trewick, R. G., AWL S. Holness, H., and IIattock, G., I b i d . , 1940, 74, 43. SOUTI-I-JYBST ESSEX TECHNICAL COLLEGE CHE>fISTRY DEP.AR1.MENT \\’AL.THAblSTOW, LOSDOK, E. 17 Octobev 21st, 1952

ISSN:0003-2654    

DOI:10.1039/AN9537800356

出版商:RSC    

年代:1953

数据来源: RSC