ISSN:0003-2654    

DOI:10.1039/AN96186FX033

出版商:RSC    

年代:1961

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN96186BX035

出版商:RSC    

年代:1961

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN96186FP161

出版商:RSC    

年代:1961

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN96186BP171

出版商:RSC    

年代:1961

数据来源: RSC

摘要:

AUGUST, 1961 Vol. 86, No. 1025 THE ANALYST PROCEEDINGS OF THE SOCIETY FOR ANALYTICAL CHEMISTRY NEW MEMBERS ORDINARY MEMBERS David Voil Anders, B.S.Ch.E. (Indiana) ; John Waite Atkinson, B.Sc. (Leeds) ; Geoffrey David Brown, B.Sc. (Lond.), A.R.I.C. ; Lewis Edwin Coles, B.Pharm. (Wales), Ph.D. (Lond.), F.P.S., F.R.I.C. ; Geoffrey Leonard Cook; Rommary Donnellan, B.Sc. ; Philip Richard Falkner, M.A. (Cantab.), A.R.I.C. ; Desmond John Ferrett, M.A., D.Phi1. (Oxon.) ; Kenneth Field, M.Sc., Ph.D. (Manc.) ; Phyllis Marjorie Ganvood, B.Sc. (Wales), A.R.I.C.; Eric Stanley Goodwin, A.R.I.C. ; George Rowland Edwin Constantin Gregory; Roland Sydney Hannan, M.A., Ph.D. (Cantab.) ; Denis Hardcastle, M.Sc.Tech. (Manc.) ; Peter James Kipping, B.Sc. (Lond.); Barry Lightfoot; Chin Kuan Lim, M.Sc.(Malaya), P.A.I.W.E., F.R.I.C.; Gaston Lennox Stephen Pawan, B.Sc. (Lond.), M.I.Bio1.; Robert John Wale Powell, B.Sc. (Lond.) ; John Roburn, BSc. (Lond.), BSc. (Nott.) ; Norman Mark Silverstone; David Cyril Thomas, M.Sc. (Wales) ; Joseph Francis Charles Tyler, B.Sc. (Lond.), A.R.I.C. ; John Albert George Vidler, A.R.I.C. ; Keith Hastings Wall, B.Sc., A.R.I.C. ; John Reeve White, B.Sc. (Lond.) ; John Patterson Whitworth; Paul Woodford, BSc. (Lond.). JUNIOR MEMBERS David William Allen; David Betteridge, B.Sc., Ph.D. (Birm.). DEATH WE record with regret the death of Charles Harcourt Wordsworth. NORTH OF ENGLAND SECTION THE twenty-fourth Summer Meeting of the Section was held at the Prince of Wales Hotel, Scarborough, from Friday, June 9th, to Monday, June 12th, 1961.The Chairman of the Section, Mr. J. Markland, B.Sc., F.R.I.C., presided over an Ordinary Meeting at 10.15 a.m. on Saturday, June loth, at which Dr. H. A. Thomas gave a lecture (illustrated by a sound film) entitled “Automation.” On the Saturday evening, the party saw the show at the Futurist Theatre, Scarborough, and on the Sunday afternoon made a coach tour, taking tea at Ravenscar. WESTERN AND MIDLANDS SECTION A JOINT Summer Meeting of the Western and Midlands Sections was held on Friday and Saturday, May 26th and 27th, 1961, in Hereford. On the afternoon of Friday, May 26th, a visit was paid to the Cider Factory of Messrs. H. P. Bulmer & Co. Ltd. At 7 p.m. a meeting was held in the Town Hall, at which the Chair was taken by the Chairman of the Western Section, Dr.G. V. James, M.B.E., M.Sc., Ph.D., F.R.I.C. A film on “An Introduction to Ion Exchange’’ was shown and the following paper was presented: “The Application of Ion Exchange Resins to Metallurgical Analysis,” by J. R. Millar, M.A., F.R.I.C. On Saturday there was a coach tour of the Vale of Evesham and an informal dinner at the Green Dragon Hotel in the evening. 493494 PROCEEDINGS [Vol. 86 MICROCHEMISTRY GROUP THE thirtieth London Discussion Meeting of the Group was held at 6.30 p.m. on Wednesday, June Zlst, 1961, at “The Feathers,” Tudor Street, London, E.C.4. The Chair was taken by the Chairman of the Group, Mr. C. Whalley, BSc., F.R.I.C. The subject for discussion was “Quantitative Paper Chromatography,” which was opened as follows: “Inorganic,” by E. C. Hunt, B.Sc.; “Organic,” by D. Gross, Ph.D. BIOLOGICAL METHODS GROUP THE Summer Meeting of the Group was held on Friday, June 9th, 1961, and took the form of a visit to Boots Pure Drug Co. Ltd., Nottingham. In the morning a tour was made of the Pharmaceutical Manufacturing Factories and after lunch a visit was made to the Biological Laboratories. Twenty-nine members and friends attended and their thanks to the Company were proffered by Mr. J. S. Simpson, F.I.M.L.T., Chairman of the Group.

ISSN:0003-2654    

DOI:10.1039/AN9618600493

出版商:RSC    

年代:1961

数据来源: RSC

摘要:

494 PROCEEDINGS [Vol. 86 Coulometric Methods in Analysis A Review” BY D. T. LEWIS (D.S.I.R., Laboratory of the Government Chemist, Clement’s Inn Passage, London, W.C.2) Modern applications of the principles of coulometry to analytical problems are reviewed. The types of electrolytic apparatus required for the electro- generation of reactant materials are described, special attention being paid to the various electrical indicator systems now being employed for titrimetric end-point determinations. Alternative chemical or photometric indicator methods receive general mention. The recent applications of oxygen and adsorption electrode devices are illustrated ; electrolytic hygrometry is discussed, and some examples are given of the use of coulometry for the quantitative analysis of pesticide residues on agricultural crops, the determination of oxide tarnish on metals and the determination of tin-alloy coatings on iron.The principles of the com- mercial “fuel cells” and their possible analytical applications are briefly indicated. 1. INTRODUCTION The general electrolytic principles governing the applications of coulometry to analysis have remained unchanged since their enunciation by Michael Faraday in 1833. These fundamental laws may be concisely represented by the equation- . Eit .. - - (1) w = E l t = - .. .. .. F where w is the weight in grams of an element formed by a primary electrode process produced by the passage of i amperes of current for a period of t seconds, E is the electrochemical * Reprints of this paper will be available shortly.For details, please see p. 556.August , 19611 LEWIS : COULOMETRIC METHODS IN ANALYSIS. A REVIEW 495 equivalent and is obviously the weight of the particular element produced by one coulomb, E is the gram equivalent weight of the element and F is the Faraday unit expressed in coulombs. The application of equation (1) to any analytical problem can only become possible if physical instrumentation is available that can measure in a precise quantitative manner the three connected variables, weight, current and time, or which will give related physical infomation from which the magnitudes of these variables may be determined. Detennina- tions that can often be made reproducibly to a precision of 0.1 per cent. and an equivalent accuracy will usually satisfy the demands of the most critical analyst, and, if modem techniques of measurement are employed, the coulometric method will invariably satisfy this criterion, not only in the macro range, but in the micro range as well.2. CHEMICAL AND ELECTRICAL STANDARDS The International System of 1908 defined the ampere as that current which would deposit 0.001118 grams of silver per second in a voltameter of special design. This definition became obsolete in 1948 when the Ninth General Conference of Weights and Measures decided that the more fundamental absolute units were becoming known with a certainty exceeding that attending the older “international units.” The theoretical re-definition of the ampere employing Neumann’s integral has resulted in the relationship- 1 International ampere = 0.99985 absolute ampere.From the analyst’s point of view, this change is of insignificant character, although the new value of the coulomb is also affected to a similar extent. The most recent physical measurements of the basic natural constants have yielded the following values1- (60) Avogadro’s Number (Chemical) = 6.02308 x atoms per mole. (45) Electronic charge = e = 1.60207 x e.m.u. (3) Velocity of light = C = 2.997929 x lofo cm per second. (26) Faraday =- = 9649.4 e.m.u. per gram equivalent = 96,494 coulombs per gram equivalent. Ne C The figures in parenthesis preceding the name of each constant give the probable errors in parts per million of the numerical factors. Since the value for e may be expressed as 1.60207 x 10-19 coulombs, it is obvious that an ampere of current is also associated with the transfer of 6.24 x l0ls electrons per second.Silver is employed quite widely in coulometry as an electrogravimetric standard. It is appreciated that it consists of two isotopes of atomic masses 106.950 and 108.949, present in amounts equivalent to 51.9 and 48.1 per cent., respectively. The migration velocities in aqueous solution of these two ionic forms must be exactly equivalent, because there have been no reports of isotopic enrichment during electrolysis.2 Such enrichment can occur during the electrolysis of fused salts at high temperatures. Shields, Craig and Diebeler2 at the U.S. Bureau of Standards, in a study of the absolute abundance ratio of silver to be used for the accurate electrochemical determination of the Faraday, have discussed the various conflicting results for the chemical atomic weight of this element, the 1957 International Value being Ag = 107.88.On the basis of their mass- spectrometric observations, these workers suggest that the chemical atomic weight (0 = 16) is more correctly given by Ag = 107-8731 0.0020, a result differing significantly from the usually accepted value. The position of silver in the electrochemical series makes it particularly useful as a coulometric standard. If we consider a neutral decimolar solution of silver nitrate of ionic strength 0.1 and of activity coefficient fAg+ = 0.77, we have, at 25” C, the following Nernst equation for the reduction Ag+ + e = Ag. At the inert anode, the evolution of oxygen proceeds according to the scheme- where aH+ x aOH- = K, = lO-l* in neutral solution, i.e.E, = EO, + 0.05915 log fAg+ CAg+ = 0.7990 - 0.0659 = 0.7331 volt. 20H- + &02 + H,O + 2e Eo, = Eoo, + 0.02957 log a20d- = - 0-402 + 0.02957 log = - 0.8159.496 LEWIS: COULOMETRIC METHODS IN ANALYSIS. A REVIEW [Vol. 86 The theoretical reversible decomposition potential of silver nitrate is thus quite small, ie., Moreover, if we consider the presence of 0.01 M copper as an impurity in our silver solution, fCUP+ = 0.5 when the ionic strength is 0.03, and the reduction potential of cupric ion is given by- Neglecting polarisation effects, the concentration of silver would have to fall to about 10-9 gram ions per litre before copper impurity became deposited on the silver cathode, i.e., when EAg+ becomes 0.2761 volt. Even lower concentrations of lead and tin would obviously be tolerated, because of their more positive oxidation potentials, and at a controlled-cathode potential, virtually pure silver is deposited.3. MEASUREMENT (a) MEASUREMENT OF CURRENT AND TIME- A precision stopwatch is used by many laboratories for the exact measurement of electrolysis time, the main objection to its use being that it is difficult to switch on the current and start the watch with complete simultaneity. For this reason, electrical or electronic timing units are generally preferred. They can form part of the electrolytic circuitry and give the advantage of being under the single switch control that also controls the current. Times accurate to one-hundredth of a second are possible with modern types of electric chronometers.For the accurate independent measurement of current, the preferred method involves the use of a calibrated resistor incorporated directly into the series circuit of the coulometric cell. The potential drop across the resistor is determined with a precision vernier potentio- meter. Such instruments are available in the United Kingdom and will measure from 2 volts to microvolts with an error of 0.002 per cent. If the electrolysis is carried out at constant current, the total number of coulombs consumed is given by q = it coulombs. If, however, an electrolysis is carried out at controlled potential, the current will usually decrease logarithmically with time according to the expression- where i, is the initial current, the total number of coulombs being given theoretically by the integral- Erev* = E,, + Eo, = 0.7331 - 0.8159 - 0.0828 volt.Ecua+ = 0.3441 + 0.02957 log f ~ u * + C ~ u z + = 0.3441 - 0.0680 = 0.2761 Volt. . . - - (2) .. * . . . i = i e-kt . . . . (3) It is obviously impossible to carry out an experiment to infinite time, and in practical analysis it will suffice if the electrolysis is followed graphically until the final observed current i is Time, t Fig. 1. Plot of current against time 1 Time, t Fig. 2. Plot of log, i = log, i, - ktAugust, 19611 LEWIS: COULOMETRIC METHODS IN ANALYSIS. A REVIEW 497 of the order of 0.1 per cent. of the original i,. This gives a curve of the type shown in Fig. 1, which may be integrated by counting the graphical squares or by cutting out the segment and comparing its weight with a square of the same graph paper of known area.Alternatively, MacNevin and Baker3 have plotted (Fig. 2) the linear equation log& = log,io - kt, and the intercept log& and the slope, -k, are found from the graph. A few points will establish the straight line, and the integral, equation (3), is then readily evaluated. The previous authors employed this method for determining the oxidation of the lower valency states of iron and arsenic by graphically following the coulometric titration over a period of only 10 minutes. (b) DIRECT MEASUREMENT OF A NUMBER OF COULOMBS- This method employs a standard chemical coulometer in series with the electrolysis cell and, provided the coulometric reaction proceeds with 100 per cent.efficiency, it will give an absolute integrated measurement regardless of any irregularities in the magnitude of the current. Silver and copper coulometers are widely employed, the increase in weight of the cathode being measured. Lingane4 describes a thermostatically controlled gas coulometer with potassium sulphate as electrolyte. Correcting for vapour pressure , atmospheric pressure and temperature, 16,811 ml of gas at S.T.P. = 1 Faraday. Many variants of electrolyte formulation are possible in such gas c~ulometers.~ p6 Ehlers and Sease6 have described the construction of a constant-potential coulometer for use in the 0 to 10 coulomb range. Copper is first deposited by the current to be measured from a copper sulphate solution, and the metal is then subjected to a quantitative anodic- stripping process.Lingane and Small7 make use of electrolytically generated acid or base to provide a coulometer accurate to 0.1 per cent. at 10 coulombs and to 1 per cent. at 1 coulomb, In addition to the chemical coulometers, many ingenious electromechanical and electronic devices have been described for determining the “q” function. Thus Booman* has developed an electronic potentiostat and integrator circuit with a 10-microsecond response to current changes and usable in the 10 pA to 10 mA range. Bett, Nock and Morrisg have described a low-inertia integrating motor whose speed of shaft rotation is a linear function of the applied voltage. This sytem has also been studied by Furman and Fentonlo who show that an empirical relation of simplifying character exists between the motor-calibration factor and the value of the series resistor that controls the voltage across the motor terminals.The rotation of the armature shaft is followed by a simple mechanical counter. Strip-chart recorders have been used to study coulomb changes, which are followed mechanically by ball-and-disc integrating systems.ll Coulometric devices of this nature are not commercially available in the United Kingdom. Potentiostats and automatically controlled cathode electrolysers may, however, be purchased, the cathode potential being pre-set and controlled automatically during the electrolytic deposition. These devices are also useful in polarographic measurements for the preliminary removal of large amounts of interfering ions by coulometric deposition.4. ANALYTICAL PRINCIPLES (a) PRINCIPLES OF ANALYTICAL COULOMETRY- In coulometry the electrode processes may obviously be carried out (a) at controlled potential or (b) at constant current, the voltage being applied from an external source, e g . , an accumulator. Such processes are examples of “external electrolysis.” In some unique instances quantitative electrode processes can occur without use of an external battery, e.g., by the short-circuiting through a resistor of a simple chemical cell. The applications in coulometric analyses of such systems are referred to under “Internal Electrolysis” (see Section 7). (b) ELECTROGRAVIMETRIC ANALYSIS- It is not proposed in this review to deal in any detail with external electrolysis of the electrogravimetric type involving the weighing of cathode deposits.The phenomena of “over-potential” and “concentration polarisation” are important in all applications of coulo- metry, hydrogen over-potential being greatest at mercury cathodes (0.8 volt) and oxygen[Vol. 86 over-potential being most pronounced for the noble metals (0.5 volt). Such phenomena permit the prior deposition of cadmium (EO == 0.402 volt), whose normal deposition would have been expected to follow that of the more reducible hydroxonium ion (H,O+). The electrogravimetric deposition of silver has already been discussed in Section 2. 498 LEWIS: COULOMETRIC METHODS IN ANALYSIS. A REVIEW (cj COULOMETRIC TITRATIONS AT CONSTANT CURRENT- Such titrations differ from volumetric titrations in that the titrant is generated electro- lytically, the electron being the standard reagent.The accuracy of these titrations is such that Tutundzic12 has suggested that the coulomb would be a preferable standard in titrimetric analysis, rather than the conventional chemical standards. The coulometric method is par- ticularly useful for studies in the microgram to milligram range and is capable of great accuracy. In primary coulometric titrations the substance to be determined reacts directly at the electrode. In secondary coulometric titrations, a reactive intermediate is first generated quantitatively by the electrode process, and this must then react directly with the substance to be determined. It is obviously a sine qua non consideration that the coulometric reactions must occur with 100 per cent.current efficiency, and this demands a completely inert electrode system, absence of reducible gases, such as oxygen, and also complete absence of electroactive solvents, impurities, etc. It is common practice to purge the atmosphere of the electrolyte cell system with pure nitrogen or argon. The stability of gold, platinum and palladium electrodes in strong oxidising solutions has been critically examined by Lee, Adams and Bricker,13 who conclude that none of these electrodes are truly inert. Various investigations have shown, however, that platinum anodes may be successfully used for the generation of halogen titrants according to the scheme- 2 C1- + C1, + 2e The classical papers of Szebelledy and Somogyi14 have described the internal generation of halogen and subsequent oxidation of thiocyanate, hydrazine, sulphite, etc., and this work has been considerably extended by Swift et Arthur and Donahue16 have also demon- strated that electrolysis of titanic chloride a t a gold cathode will produce titanous ions, which may be employed successfully as a reductimetric titrant.Tutundzic and Mladenovicl7 have similarly used a platinum anode for the generation of permanganate ion in an acidulated manganous sulphate anolyte. It is therefore obvious that noble metal electrodes can be employed under fairly adverse oxidising - reducing conditions. Two types of titrations are usually defined. (a) APPARATUS FOR CONSTANT-CURRENT TITRATIONS- Most titrimetric coulometry is carried out under constant-current conditions, particular attention being paid to the stabilisation of the current supply.A typical circuit is shown in Fig. 3, the switching on of the current being synchronised with the operation of an electric chronometer, although a stopwatch can obviously be used. A chemical coulometer can also replace both the precision resistor and the chronometer if this is preferred. With the circuit shown in Fig. 3, the constant current may be supplied from a 24-volt battery in series with a large resistance or, alternatively, the ax. mains supply may be employed via a constant-voltage transformer with full-wave rectification and appropriate stabilisation in the output circuit. Currents of 10 to 100 mA would generate from 1 x 10-7 to 1 x 10-6 equivalents of acid, base, metal ion, etc., per second during the electrolysis.The coulometric reaction can occur at either of the generating electrodes, and in general the cathode and anode compartments may have to be separated by a porous frit or agar gel, or both, to form an almost impermeable partition and prevent interaction of the electrode products. The electrolyte in either compartment may be agitated with a mechanical or a magnetic stirrer, and it is an advantage, particularly in controlled-potential coulometry, to keep the rate of stirring as smooth and constant as possible. The indicator-electrode system shown diagrammatically in Fig. 3 deserves special mention, because it is representative of those special devices employed in coulometry to denote the end-point of a particular titration. Such systems are described in detail in Section 4 (e).Sometimes, only one electrode or indicator half-cell is employed, this being coupled potentio- metrically with one of the working electrodes.August, 19611 LEWIS: COULOMETRIC METHODS IN ANALYSIS. A REVIEW 499 (e) INDICATOR SYSTEMS- In coulometric acid - base titrations, e.g., the liberation of hydroxyl radical at the segre- gated cathode of a potassium chloride electrolyte, the normal volumetric indicators, such as phenolphthalein, may be used to observe the end-point. Alternatively, in such instances, one could employ the normal glass-electrode system and find the equivalence point from pH observations. Conductimetric systems also have an obvious application in these instances.Spectrophotometric methods have also been widely used for end-point indication, the coulometric cell being suitably disposed in relation to a spectrophotometer and the,end-point A = Potentiometer B = Constant-current supply C = Rheostat E = Working electrodes e = Indicator electrodes or half-cells F = Permeable partition or frit I = Indicator meter, e.g., pH meter or M = Magnetic stirrer P = Potentiometric resistor R = Precision resistor S = Double-pole switch T = D.C. or A.C. operated timer microammeter Fig 3. Diagram of coulometric circuit noted photometrically at that wavelength at which excess of the generated titrant ion absorbs most strongly. For accurate work, measurements of the optical density are made at fixed intervals over the sensitive region of the titration.As an alternative to these preceding techniques, the so-called “amperometric” indicator- electrode system is much employed. A dry battery impresses through a suitable resistor a constant small potential across the two noble electrodes. The variation in current as recorded on a calibrated microammeter will yield a current - time graph showing marked inflexions in the end-point region. In experiments in the 10- to 100-pg range, the electrolysis occurring at the indicator electrodes must be of negligible order or it will interfere with the accurate determination of the amount of titrant generated at the working electrode. LeiseylS has shown that the determination of mercaptans in petroleum oil in alcoholic solution with electrically generated silver ion to precipitate the mercaptide may be fully automated by causing the current increase at the amperometric detector electrodes to energise a relay that stops both the current and an electrical timer. The widely known “dead-stop” indicator system of Foulk and Bawden has been much used for detecting the Fischer reagent end-point in water determinations. A related indicator system, depending on the rate of change of potential and known as the “derivative polaro- graphic method” has been introduced for redox titrations by Reilley, Cooke and F~rman.~e500 LEWIS: COULOMETRIC METHODS I N ANALYSIS.A REVIEW [Vol. 86 Two small platinum or gold electrodes are polarised with a minute constant current. The indicator electrodes show a varying potential difference during the progress of the deter- mination, the rate of change being particularly well marked at the titrimetric end-point.Such a system has been employed by Carson, Vandenvater and Gile20 in their study of the interferences produced by other substances during the reduction of plutoniumV1 with electro- generated ferrous titrant. They used a constant current of 0.1 pA produced by a 3-volt battery in series with a 30-megohm resistor. The value of the polarising current is critical in such applications and requires careful adjustment. Buck, Farrington and Swift21 have made use of a similar system for the titration of monovalent thallium with bromine. Potentiometric methods have been widely used for end-point detection in coulometric, acidimetric or redox determinations and their variations are so well known as to need no further comment.One recent interesting example, however, is given by the work of Mather and Anson,22 who utilised a non-aqueous solvent system consisting of an acetic anhydride - acetic acid mixture containing a perchlorate salt for the coulometric determination of milligram amounts of fluoride ion (error 0.3 per cent.) by titrating it as a base with electrogenerated perchloric acid. This acid was formed at a working mercury electrode according to the scheme- 2Hg + 2CH3COOH -+ Hg,(OOC.CH,), + 2H+ + 2e Most anions interfere, but for solutions of individual ions in acetic acid the method applies, e.g., nitrate or sulphate ions may be similarly determined. End-points were determined potentiometrically with a mercury - mercurous acetate electrode coupled with a glass electrode, sharp voltage inflexions being produced at the stoicheiometric end-point.Furman and ad am^^^ express the view that, for satisfactory end-point detection in the microgram range, pride of place must be given to the “Sensitive end-point method.” This involves the restoration of a pre-determined potential to an indicator cell system, the potential being itself determined by the end-point characteristics of the reaction being studied. This is essentially a null-point method, no current flowing ih the indicator circuit when this known potential is achieved. It is thus capable of great accuracy, and the previous workers em- ployed a high-sensitivity galvanometer as their end-point detector.They have applied the technique to the coulometric titration of p-methylaminophenol and hydroquinone with electrogenerated ceriumIv, the electrolyte consisting of a cerous sulphate solution in 2 M sulphuric acid. The indicator system was composed of a platinum - iridium anode coupled to a lead - lead amalgam - 2 M sulphuric acid half-cell. The potential of the anode may obviously be adjusted to any value by passing a constant generating current and allowing ceriumIv to form at the working electrode in the well stirred solution. The current is passed until the solution potential becomes equal to the indicator-electrode potential and no current flows in the indicator circuit. A known amount of the organic reductant is then added, the solution potential falls and is then again restored to the pre-set potential by electrogenerating ceric ions until zero current is observed in the indicator circuit.The number of coulombs employed to reach this stage can be most accurately determined. Ce3+ -+ Ce4+ + e 5. CONTROLLED-POTENTIAL METHODS If the coulometric electrolysis is carried out at controlled-potential, no indicating- electrode system may be necessary, the magnitude of the final current being sufficient indica- tion of the degree of completion of the reaction. S h ~ l t s ~ ~ has employed this method in the indirect determination of plutoniumv1 with excess of ferrous ion produced at a platinum electrode in sulphuric acid electrolyte, the excess of reductant being determined by coulometric back-titration.4H+ + PuO2,+ + 2Fe2+ --+ Pu4+ + 2Fe3+ + 2H,O. Methods of the controlled-potential type can suffer from the disadvantage of requiring long electrolysis times , but direct indication of the optimum eonditions for the successive electro- gravimetric deposition of cations can be obtained directly from the polarographic curves for the metal salts. Lord, O’Neill and Rogers25 have demonstrated the extreme sensitivity possible by the controlled-potential method and have determined amounts of silver in the millimicrogram range by first electro-depositing the metal and then subjecting it to an anodic strippingAugust, 19611 LEWIS : COULOMETRIC METHODS IN ANALYSIS. A REVIEW 501 procedure. The current - time curve was automatically produced by a recording potentio- meter of known chart speed and the area under the curve assessed with a planimeter (Section 3).A correction is applied for the background-current characteristics of the coulometer. 6. EXTERNAL GENERATION OF TITRANT One major difficulty attending the internal generation of a titrant in a solution is that other substances or impurities may be present which produce electrolytic interference. These difficulties have been neatly circumvented by Pitts et aZ.26327 and Bett, Nock and Morris,S who have described external generator cells wherein the titrant is formed externally and allowed to flow via capillary tubes into the solution to be titrated. Typical single-arm and bifurcate assemblies are shown in Figs. 4 and 5. Zinc anode /-L Generator Sodium chloride I Membrane i Copper sulphate Platinum (saturated) I 1 solution dish cathode Figs.4 and 5. External generators of titrant Chlorine and bromine may be produced at platinum electrodes Iodine is said to be best into reducing solutions of arsenites, etc. solution. which neutralises the alkali moduced at the cathode. in this way and passed generated in boric acid502 LEWIS: COULOMETRIC METHODS IN ANALYSIS. A REVIEW [Vol. 86 A most interesting application of the internal electrolysis technique is due to Hers~h,~2333 and this has been elaborated by KeideP into an automatic system for determining trace amounts of oxygen in inert gases. The apparatus consists essentially of the galvanic-cell system shown in Fig. 6. \ m Cadmium electroee Fig. 6. Coulometric oxygen meter The inert gas containing parts per million of oxygen is bubbled through a porous silver electrode into a 25 per cent.solution of potassium hydroxide containing a cadmium anode. The oxygen is quantitatively and instantaneously reduced and the cell current generated may be measured in the usual manner- 0, + 2H,O + 4e --+ 40H- (Reduction). Cd -+ Cd2+ + 2e (Oxidation). 2Cd + 0, + 2H,O + 2Cd2+ + 40H- (Cell reaction). If one accepts that one Faraday of current reduces 5603.6ml of oxygen at S.T.P., then, if the flow rate of inert gas containing C p.p-m- of oxygen is f ml per minute, it is readily deducible that the cell current, i, is given by- i = 0.287 f c P (?) PA where P is the pressure in atmospheres and T the temperature in degrees Kelvin of the inert gas under the conditions of the experiment.With a flow rate of 100 ml per minute and a temperature of 25" C, oxygen impurity equivalent to 1 p,p.m. would theoretically yield a current of 26.7 PA, and this is almost exactly the value obtained in practice. The method thus offers a most sensitive method for determining oxygen in nitrogen, helium, argon, etc., at levels well below 1 P.P.m., because current magnitudes of unit micro- ampere order are readily measurable. K ~ y a m a ~ ~ discusses the application of the Hersch cell system to other gases, e.g., carbon monoxide and carbon dioxide, and replaces the solution of potassium hydroxide in the galvanic cell by potassium hydrogen carbonate. It is obvious that if a de-oxygenated inert carrier gas is used to remove oxygen from liquids, then the Hersch method can also be applied to determinations in liquid media. Modifications of his original method have also been described by Hersch36 to determine traces of hydrogen in carrier gas.The gas stream is diluted with a known amount of pure oxygen and allowed to react at 500" C on silica wool, the excess of oxygen being measured galvanically. Other applications, e.g., studies of the corrosion of metals by acids, are referred to in the same paper.August, 19611 LEWIS: COUOLOMETRIC METHODS IN ANALYSIS. A REVIEW 503 8. ABSORPTION COULOMETRY Voorhies and Davis3' have recently described a constant-current coulometric method that involves the initial quantitative adsorption of the electroactive species to be determined on to a layer of compressed acetylene black.This adsorbent then forms one of the working electrodes of a generator cell in which the reducible adsorbed substances are subjected to a cathodic reduction process. Milligram amounts of anthraquinone and cupric ion have been determined in this way after adsorption from solutions. Acetylene black may be obtained in a state of high purity, it possesses a low redox blank and exhibits a high over-voltage property for both hydrogen and oxygen. Moreover, I kg weight 1 Fig. 7. Adsorption electrode its contamination by adsorbed atmospheric oxygen is consilered of negligible order. It is probable that organic impurities in gaseous systems may be isolated from the gas phase and determined in this manner. The general design of the working acetylene black electrode is shown in Fig.7. 9. COULOMETRIC HYGROMETRY The electrolysis of water in the presence of any suitable acidic electrolyte proceeds in accordance with Faraday's law, oxygen and hydrogen gases being produced, following the consumption of 2 Faradays, according to the scheme- 2H+ + 20H-+H,(g) + +O,(g) + H,O (liq.). Since the dissociation constant for water, k, = lO-l*, the thermodynamic equation for the reversible decomposition potential at 25" C of an electrolysis cell giving these gases is- log k: = -0.403 - 0,826 = -1.229 volts. 0-059 E=E:,+ - 2 In practice, because of polarkation effects, the decomposition potential as measured experi- mentally is usually about 0.5 volt higher for molar solutions. Wexler and Keide13g have developed an hygrometer based on the principle that, when a water vapour-air mixture is contacted with solid phosphorus pentoxide in an electrolysis cell composed of the inert plastic Teflon, the wet solid becomes electrically conducting.The pentoxide film is contacted with two spirally wound platinum-wire electrodes, and the absorbed water is quantitatively decomposed by a measured current. The magnitude of this resultant electrolysis current permits the determination of water vapour in the p.p.m. range in air.[Vol. 86 At a flow rate of 100 ml per minute of air through the Teflon cell under normal room conditions, 1 p.p.m. of water vapour produces an electrolysis current of about 13 micro- amperes. The instrument responds in a few minutes to humidity changes of the order of 50 to 100 per cent.of the original value. 504 LEWIS COULOMETRIC METHODS I N ANALYSIS. A REVIEW 10. COULOMETRIC APPLICATIONS OF PARTICULAR INTEREST (a) The extent to which foodstuffs are contaminated by residues of toxic agricultural pesticides is a matter of world-wide interest, and in some countries, e.g., the U.S.A., legislation restricts these residue levels to the p.p.m. range. Normal methods of analysis are time- consuming, but Coulson et have recently developed a method for determining BHC, aldrin, dieldrin, DDT and other chlorinated organic pesticides in 1 hour. This method depends on a gas-chromatographic separation of the pesticide followed by a combustion, the effluent gases yielding chloride ions, which are absorbed in water and coulometrically titrated with silver.The method is applicable to mixtures of chlorinated pesticides and to the thiophosphates. Schwabe and Seide14* have also examined the direct determination of gamma-BHC by controlled electrolysis. (b) Reilley and Porterfield41 have employed the electrogenerated complex ion of the soluble mercuric salt of ethylenediaminetetra-acetic acid to titrate milligram concentrations of calcium, lead, zinc, manganese, etc., in strongly ammoniacal solution. The reaction sequence is as follows, the complex ion being produced by reduction at a large mercury cathode : 2e + HgE2- -+ Hg + E4- Ca2+ + E4- -+ CaE2- A pre-titration is necessary to find the blank value due to contaminating ions, the end-points being observed with a potentiometric system. The method is undoubtedly applicable to microgram amounts also.(c) Devanathan and Fernando42 describe an unusual coulometric method wherein a multivibrator circuit gives constant pulses of electrolysing current to a titration cell, these pulses being accurately counted via a relay operating a Post Office mechanical counter. Minute amounts of reagent may be quantitatively produced in this way, and the method is considered superior to the constant-current method when the amounts to be determined are of the order of 10-4g or less. (d) Wilson and othersm have employed the method of controlled-potential reduction to study the number, n, of electrons involved in the reduction stages of the aminoacridines. They make use of a time factor, t+, determined graphically, which represents the time for the initial current to fall to half its original value. The total number of coulombs passed during the reduction is given by q=o.693 id, a n d n = - qM Fw where w is the weight or reducible compound of molecular weight M.( e ) Kunze and Willey44 have examined the anodic dissolution of tin and tin alloy, FeSn,, from tinplate anodes immersed in a decinormal hydrochloric acid electrolyte, a graphite rod being used as cathode. A high-speed recording potentiometer was used to determine the potential differences between the anode specimen and a silver - silver chloride indicator electrode. Steps in the potential - time curve correspond to the dissolution of pure tin and tin alloy. (f) Campbell and Thomasg5have employed the coulometric method to determine T, the thickness of oxide tarnish (in Angstrom units) on a metal film reduced slowly with a small measured current.The cathode potential rises sharply to the hydrogen discharge potential when reduction is complete. The following equation applies- lo5 itM T=- A nFd where i is expressed in milliamperes, t in seconds, A is the film area in sq- cm, d is the density of the film and M is the molecular weight of the oxide composing the film.August, 19611 LEWIS : COULOMETRIC METHODS IN ANALYSIS. A REVIEW 505 Similar methods have been described by the Tin Research Institute46 for the deterrnina- tion of the relative amounts of stannous and stannic oxides on a tinplate surface. (g) Gierst and Mechelynck4’ have made a unique mathematical and experimental study of constant-current coulometry as applied to unstirred, homogeneous solutions and claim that certain determinations can be carried out in times of less than 1 minute with a precision of the order of 0-2 per cent,.An equation is developed, based on the laws of electrolysis and of ionic diffusion, which relates the extent of the electrode reaction to a function called “transition time,” which is indicative of the time interval between the establishment of a capacity charge on the stationary electrode and the f a l l to zero of the concentration of electro- active substance in its vicinity. A detailed schematic diagram of the electronic “transito- meter” coulometer is given by the authors. (12) Considerable international commercial interest is being shown in the development of “fuel cells,”48 which are essentially coulometric devices for the direct conversion of chemical into electrical energy with consequent improvement in the thermodynamic efficiency.Typical of these devices is the Sondes Place cell, which operates at 500” to 700” C and has porous electrodes of zinc oxide and metallic silver, which are contacted with fused carbonate electrolyte. These electrodes, when contacted with air or oxygen and combustible sludge gas, respectively, produce electrical current according to the scheme- Air electrode: 0, + 2C0, + 4e -+ 2CO;- L 5 2 Fuel electrode: 2CO;- -+ + CH, + -CO, + H,O + 4e The corresponding Bacon cell utilises porous nickel electrodes and employs hydrogen as fuel; it operates at a temperature of 200” C and at a pressure of 400 lb per sq.inch.49 These fuel cells suggest an application of coulometry that has not yet been fully explored in the analytical field. There are obviously possibilities of utilising such systems for deter- mining combustible impurities in inert gas phases, e.g., as laboratory indicator cells for gas-chromatographic effluent gases. It is agreed that for such purposes it would be advan- tageous to develop a fuel cell that would operate satisfactorily at room temperature, but some cells are already known that can function at approximately 100” C, and it may be possible by employing catalytically active porous electrodes to lower this temperature yet further. I express my sincere thanks to Mr. W. H. Scates of the Laboratory of the Government Chemist, who has kindly drawn all the diagrammatic illustrations included in this review.1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. REFERENCES Kaye, G. W. C., and Laby, T. H., “Tables of Physical and Chemical Constants,” Twelfth Edition, Shields, W., Craig, D., and Diebeler, V. H., J . Amer. Chem. SOC., 1960, 82, 5033. MacNevin, W., and Baker, B., Anal. Chern., 1952, 24, 987. Lingane, J, J., J. Amer. Chem. Soc., 1945, 67, 1916. Page, J . A., and Lingane, J. J., Anal. Chim. Acta, 1957, 16, 176. Ehlers, V. B., and Sease, J. W., Anal. Chem., 1954, 26, 513. Lingane, J. J., and Small, L. A., Ibid., 1949, 21, 1119. Booman, G. L., Ibid., 1957, 29, 213. Bett, N., Nock, W., and Morris, G., Analyst, 1954, 79, 607. Furman, N. H., and Fenton, A. J., Anal. Chem., 1959, 29, 1213. Lingane, J.J., “Electroanalytical Chemistry,” Interscience Publishers Inc., New York, 1953. Tutundzic, P. S., Anal. Chim. Acta, 1953, 8, 182. Lee, J. K., Adams, R. N., and Rricker, C. E., Ibid., 1957, 17, 321. Szebelledy, L., and Somogyi, Z., 2. anal. Chem., 1938, 112, 313, 323, 332, 385 and 400. Buck, R. P., Farrington, P. S., and Swift, E. H., Anal. Chern., 1952, 24, 1155; Farrington, P. S., Mier, D. J., and Swift, E. H., Anal. Chem., 1953, 25, 59L; Brown, R. A., and Swift, E. H., J . Amer. Chem. SOC., 1949, 71, 2717. Longmans, Green and Co. Ltd., London, 1959. Arthur, P., and Donahue, J. F., Anal. Chern., 1952, 24, 1612. Tutundzic, P. S., and Mladenovic, S., Anal. Chirn. Acta, 1955, 12, 382 and 390. Leisey, F. A., Anal. Chem., 1954, 26, 1607.Reilley, C. N., Cooke, W. D., and Furman, N. H., Ibid., 1951, 23, 1223. Carson, W. N., Vanderwater, J. W., and Gile, H. S., Ibid., 1957, 29, 1417. Buck, R. P., Farrington, P. S., and Swift, E., Ibid., 1952, 24, 1195. Mather, W. B., jun., and Anson, F. C., Ibid., 1961, 33, 132. Furman, N. H., and Adams, R. N., Ibid., 1953, 25, 1565. Shults, W. D., Ibid., 1961, 33, 15.506 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. BOWEN AND CAWSE: DETERMINATION OF SODIUM, POTASSIUM AND [VOl. 86 Lord, S. S., jun., O’Neill, R. C., and Rogers, L. B., Ibid., 1952, 24, 209. DeFord, D. D., Johns, C. J., and Pitts, J. N., Ibid., 1951, 23, 938. Pitts, J. N., jun., DeFord, D. D., Martin, T. W., and Schmall, E. A., Ibid., 1954, 26, 628. Ullgren, K., 2. anal. Chem., 1868, 7, 442. Clarke, B., and Wooten, L., Trans. Electrochem. Soc., 1939, 76, 33. Todt, F., Compt. rend. reunion intern. therm et cin. electrochem., 1950, 232. Oelsen, W., Graue, G., and Haase, H., Angew. Chem., 1952, 64, 76. Hersch, P., Nature, 1952, 169, 793. -, Instrum. Practice, 1957, 11, 817 and 937. Keidel, F. A., Ind. Eng. Chem., 1960, 52, 491. Koyama, K., Anal. Chem., 1960, 32, 1053. Hersch, P. , in “Proceedings of the International Symposium on Microchemistry, 1958, I’ Pergamon Voorhies, J. D., and Davis, S. M., Anal. Chem., 1960, 32, 1855. Wexler, A., and Keidel, F. A., National Bureau of Standards Circular, Washington, 1957, p. 586. Coulson, D. M., Cavanagh, L., de Vries, J., and Walther, B., J . Agric. Food Chem., 1960, 8, 399. Schwabe, K., and Seidel, H., 2. anorg. Chem., 1953, 272, 147. Reilley, C. N., and Porterfield, W., Anal. Chem., 1956, 28, 443. Devanathan, M., and Fernando, Q., J . Sci. Instrum., 1956, 33, 323. Wilson, F. P., Butler, C. G., Ingle, P. H. B., and Taylor, H., J . Pharm. Pharmacol., 1960, 12, 220~. Kunze, C. T., and Willey, A., J , Electrochem. Soc., 1952, 99, 354. Campbell, W. E., and Thomas, U. B., Trans. Electrochem. Soc., 1939, 76, 305. “Tinplate Testing,” Tin Research Institute, Greenford, Middlesex, 1960, p. 37. Gierst, L., and Mechelynck, Ph., Anal. Chim. Acta, 1955, 12, 79. Chemical Age, 1960, 84, 21. Times Science Review, 1960, 36, 14. Press, Oxford, London, New York and Paris, 1960. Received Murch 37d, 1961

ISSN:0003-2654    

DOI:10.1039/AN9618600494

出版商:RSC    

年代:1961

数据来源: RSC

摘要:

506 BOWEN AND CAWSE: DETERMINATION OF SODIUM, POTASSIUM AND [VOl. 86 Determination of Sodium, Potassium and Phosphorus in Biological Material by Radioactivation BY H. J. M. BOWEN AND P. A. CAWSE ( U . K . Atomic Energy Authority, Wantage Research Laboratory, Wantage, Berks.) Neutron-activation analysis has been applied to the determination of sodium, potassium and phosphorus in biological material. When a flux of 1012 neutrons per sq. cm per second for activation and an anti-coincidence counting unit were used, the ultimate limits of sensitivity for the three elements were approximately 10-lo, and 10-lo g, respectively. Radio- chemical separation procedures were used, and it was possible to analyse eight samples for all three elements in an 8-hour working day. WHEN biological material is exposed to thermal neutrons, a large percentage of the induced radioactivity is produced by four nuclides, viz., sodium-24, phosphorus-32, chlorine-38 and potassium-42 ; the half-lives and characteristic radiations of these nuclides are shown in Table I. The only other nuclide frequently contributing a major proportion of the induced TABLE I HALF-LIVES AND RADIATIONS OF "a, "P, 38c1 AND 42K Maximum Nuclide Half-life beta energy, Gamma energy, MeV MeV . . 15 hours 1.39 1-37 and 2.76 Sodium-24 . . .. Chlorine-38 . . .. . . 35 minutes 4.81 1.60 and 2.15 Potassium-42 .. . . 12.4 hours 3-60 1.53 Phosphorus-32 . . .. 14 days 1.71 - activity is manganese46 (half-life 2.6 hours), the gamma spectrum of which can often be observed after plant material has been exposed to neutrons.The gamma spectra of activatedAugust, 19611 PHOSPHORUS IN BIOLOGICAL MATERIAL BY RADIOACTIVATION 507 tomato seeds at three different times after activation are shown in Fig. 1, in which the decay of the peak caused by manganese-56 at 0.85 MeV can clearly be seen; the peaks caused by sodium-24 and potassium-42 are also prominent. Several methods are available for determining the relative contributions of these nuclides to the total activity. Keynes and Lewis1 and Reiffel and Stone2 have utilised simple forms of beta spectroscopy. Chlorine-38 was allowed to decay away, and the hard P-rays from the potassium-42 were determined by counting through a filter sufficiently thick to eliminate the softer /?-rays from the other nuclides. Phosphorus-32 was then determined as the residual beta-activity after decay for 1 week, and sodium-24 was found by difference.Gamma ~pectroscopy3~~~~ is another possible method of analysis, but, unfortunately, it cannot be used 1 I I 1 1 1 0 I .o 2.0 3. Gamma energy, MeV Fig. 1. Gamma spectra of activated tomato seeds: curve A, 3.5 hours after activation; curve B, 7.5 hours after activation; curve C, 25 hours after activation to measure phosphorus-32 (a pure beta-emitter), and the main gamma energies of sodium-24 and potassium-42 are so close together that they are not well resolved by most gamma spectrometers. A third technique, used by Salmon,B involves radiochemical separation of the three nuclides after the addition of carriers. We have used this principle in the work described here; it requires more time and effort than do the other techniques, but is necessary to attain maximum accuracy and sensitivity.It is therefore suitable for analysing small samples of biological material, but offers no obvious advantages when large samples are available. A rapid radiochemical separation of chlorine-38 has been described elsewhere? and will not be discussed here. IRRADIATION- Liquid standards were sealed into 6-cm lengths of polythene tubing (0.5 mm bore), and samples of seeds were sealed in polythene film and then packed in polythene bags in 3-inch x 1-inch aluminium cans. (Before it was filled, the polythene tubing was washed with 6 N hydrochloric acid and then with water distilled from quartz apparatus; it was subse- quently handled with clean Perspex forceps).Each can was irradiated for 15 hours in a flux of about 1012 thermal neutrons per sq. cm per second in the Hanvell reactor BEPO. This period of irradiation activates about half the theoretical maximum number of sodium and potassium atoms, but only 3 per cent. of the corresponding number of phosphorus atoms. In order to attain higher sensitivity for phosphorus, activation should be for about 14 days, and all samples should then be sealed in silica rather than in polythene, which degrades after several days inside the reactor. EXPERIMENTAL508 [Vol. 86 STANDARDS- The standards used consisted of 0:005-ml aliquots of a solution containing 0.5 pg of sodium and 1Opg each of potassium and phosphorus. The standards were prepared by dissolving the calculated amounts of Specpure sodium hydrogen carbonate and potassium carbonate and analytical-reagent grade ammonium dihydrogen orthophosphate in water distilled from quartz apparatus ; they were stored in polythene bottles.Self-shielding during activation was negligible. Some standards were also prepared by absorbing aliquots of the solution in small squares of Whatman No. 541 filter-paper. This technique was satisfactory for amounts of potassium and phosphorus down to lO-'g, but was useless for sodium because of the amount of this element in the paper. Attempts to wash this residual sodium out of the paper were un- successful. It was estimated that the filter-paper contained about 0.35pg of sodium per sq. cm, which is within the range of values measured by Born and Stark.* Possibly, the sodium-24 found was produced by an (n,a) reaction from aluminium present in the paper.PRELIMINARY TREATMENT AFTER ACTIVATION- After irradiation, the exterior of each polythene tube was washed with 6 N hydrochloric acid containing sodium as carrier and then with distilled water to remove active contaminants ; if this precaution was neglected, the results for sodium were often erratic. The tube was then cut open at each end, and its contents were washed into a 50-ml centrifuge tube with water from a hypodermic syringe, about 2 ml of water being ample for this purpose. Each centrifuge tube contained 1 ml of a carrier solution containing sodium, potassium and phos- phorus. Biological material was wet ashed in 1 ml of nitric acid on a sand-bath at 180" to 200" C, 1 ml of sulphuric acid was then added, and heating was continued for 1 hour, by which time fumes of sulphur trioxide were visible.(Any active halogens were expelled during the wet ashing, which was therefore carried out in a fume cupboard.) The tubes were then cooled to room temperature, and chemical separation was begun. BOWEN AND CAWSE: DETERMINATION OF SODIUM, POTASSIUM AND METHOD REAGENTS- All reagents were of recognised analytical grade. Sulphuric acid, 36 N. Nitric acid, 16 N. Hydrochloric acid, 12 N. Perchloric acid, 70 per cent. Ammonia solution, 15 N. Magnesium chloride solution, 50 $er cent. w/v-Prepared from magnesium chloride Barium chloride solution, 30 per cent. w/v-Prepared from barium chloride dihydrate.Ammonium carbonate solution, 10 per cent. w/v. Sulphuric acid, 5 per cent. w/v, in diethyl ether. n-Butyl alcohol-Saturated with hydrogen chloride gas. n-Bzctyl alcohol - ethyl acetate mixture (1 + 1 v/v). Wash solution-A solution 6 N in hydrochloric acid and 1 N in sodium chloride. Combined carrier solution-Prepared by dissolving 37.1304 g of ammonium dihydrogen orthophosphate, 44.5652 g of potassium sulphate and 61.7689 g of anhydrous sodium sulphate in distilled water and diluting to 1 litre. 1 ml = 20 mg each of sodium and potassium and 10 mg of phosphorus. hexah y dr at e . Potassium carrier solution-Prepared by dissolving 38.1381 g of potassium chloride in distilled water and diluting to 1 litre. 1 ml 3 20 mg of potassium. Sodium carrier solution-Prepared by dissolving 225-4164 g of sodium chloride in distilled water and diluting to 1 litre.1 ml = 10 mg of sodium. Combined standard solzction-Prepared by dissolving 7.4261 g of ammonium dihydrogen orthophosphate, 3.5345 g of Specpure potassium carbonate and 0.3653 g of Specpure sodiumAugust, 196 11 PHOSPHORUS IK BIOLOGICAL MATERIAL BY RADIOACTIVATIOK 509 hydrogen carbonate in water distilled from quartz apparatus and diluting to 1 litre in polythene apparatus. 0-005 ml 3 10 pg each of phosphorus and potassium and 0.5 pg of sodium. SEPARATION OF PHOSPHORUS- The separation of phosphorus was based on the solubility of dry orthophosphoric acid in diethyl etherg; salts of alkali metals are insoluble in ether. The phosphate was purified and then finally precipitated as ammonium magnesium phosphate.A 15-ml portion of diethyl ether, previously dried over calcium chloride, was added to the contents of each centrifuge tube, and the tube was spun in a centrifuge to coagulate the metal sulphates. The supernatant ether was transferred to a clean tube, and the precipitate was washed twice with 4 ml of ether containing 5 per cent. of sulphuric acid. The precipitate was retained for determining the alkali metals, and the supernatant ether and washings were evaporated by means of a current of air at room temperature. The pH of the residual acid was brought to 9 by adding ammonia solution, and, when cool, 1 ml of magnesium chloride solution was added; the solution was then diluted to 10ml and swirled. After 30 minutes in a water bath at 20" C, the precipitate was separated by centrifugation and washed three times with water.Finally, it was transferred, as a slurry with acetone, to a weighed aluminium counting tray, dried, weighed and counted; the chemical yield averaged 90 per cent. SEPARATION OF POTASSIUM- This separation of potassium was based on the insolubility of its perchlorate, so that the sulphates present had first to be removed. The precipitate from the extraction with ether was dissolved in water, 2-5 ml of barium chloride solution were added, and the volume was made up to 10ml. After the tube had been set aside for 10 minutes in a bath of boiling water, the precipitated barium sulphate was separated by centrifugation and rejected, and the supernatant liquid was poured into another tube containing 5ml of perchloric acid.After being cooled for 15 minutes in a bath of ice, the precipitated potassium perchlorate was separated by centrifugation, and the supernatant liquid was reserved for determining sodium. The potassium perchlorate was freed from traces of sodium by recrystallisation, 0.1 ml of sodium carrier solution and 1 ml of perchloric acid were added to it, and the solution was diluted to 10ml with water. The precipitate was dissolved by heating at 90" C for 10 minutes and was then re-precipitated by cooling at 0" C for 20 minutes. It was then separated by centrifugation and washed three times with the n-butyl alcohol - ethyl acetate mixture. Finally, it was transferred, as a slurry with ether, to a weighed aluminium counting tray, dried, weighed and counted; the mean chemical yield was 40 per cent.SEPARATION OF SODIUM- This separation was based on the insolubility of sodium chloride in n-butyl alcohol saturated with hydrogen chloride; it was rendered somewhat tedious by the necessity for removing traces of active potassium and inactive barium left from the previous steps. A 1-ml portion of potassium carrier solution was added to the supernatant liquid from the first precipitation of potassium perchlorate, and the solution was evaporated to dryness in a 100-ml beaker on a hot-plate at 150" C. When cool, the sodium perchlorate was dissolved by boiling with 15ml of hot n-butyl alcohol, the solution was cooled, and the residue of potassium perchlorate was separated by centrifugation. The supernatant liquid was poured into a fresh tube containing 5 ml of n-butyl alcohol saturated with hydrogen chloride and was kept at 100" C for 10 minutes.The precipitated sodium chloride was separated by centrifugation, and the supernatant liquid was rejected. The sodium chloride was dissolved in 2 ml of water, 10 ml of ammonium carbonate solution were added, and the solution was boiled for a further 10 minutes. Some barium carbonate was precipitated at this stage, and this was separated by centrifugation. The remaining solution was transferred to a 100-ml beaker, acidified with 0.5 ml of hydrochloric acid, covered with a watch-glass and evaporated to dryness on a hot-plate. Ammonium chloride was removed by heating strongly with a bunsen burner for 5 minutes, and the residue of sodium chloride was cooled, transferred, as a slurry with acetone, to a weighed aluminium counting tray, dried, weighed and counted.510 DETERMINATION O F RADIOACTIVITY- ROWEN AND CAWSE: DETERMINATION OF SODIUM, POTASSIUM AND [VOl.86 The beta-activities of the precipitates were counted with either a 2B2 end-window Geiger counter (efficiency, about 40 per cent.; background, 30 counts per minute) or an anti-coincidence counter (efficiency, about 10 per cent. ; background, 1.5 counts per minute). The low-background counter was suitable only for low count rates and was not often used. Radiochemical purity was checked by counting at intervals for several half-lives (and by gamma spectrometry for phosphorus). The entire procedure, including counting, could be carried out on eight samples by a single individual in 8 hours.DISCUSSION OF THE METHOD COMPARISON WITH OTHER TECHNIQUES- The method was compared with a flame-photometric technique for sodium and potassium and with the molybdophosphate colorimetric technique for phosphorus ; these techniques have been described elsewhere.1° Three replicate determinations were made by each technique, and, as shown in Table 11, good agreement was found. TABLE I1 COMPARISON BETWEEN RESULTS BY ACTIVATION AND OTHER TECHNIQUES Amount of element found by- Amount of , A \ Element element present, activation, conventional technique, tLg P.g tLg 0.0 0.1 0.2 0.4 0.6 0 . . { : 8 Sodium . . Potassium . . 10 Phosphorus . . 4 0.0006 0.112 0.193 0.399 0.620 0-0035 2-12 4.32 8.0 1 9.80 0.01 2-14 4.20 8.2 1 10.7 <0*012 0.088 0.202 0.428 0.587 <0-013 1-92 4.23 8-13 9.97 <Om15 2.06 4.00 7.87 10.1 SENSITIVITY AND ACCURACY- When 0-l-pg portions of sodium, potassium and phosphorus were activated for 15 hours, the respective count rates were approximately 3500, 250 and 200 counts per minute on the 2B2 counter or 1000, 80 and 50 counts per minute on the anti-coincidence counter.These figures are practical and allow for the low chemical yields in precipitating the alkali metals and for decay during the separation. The minimum detectable amounts of the three elements (sufficient to double the background of the anti-coincidence counter) were therefore 0-086 mpg of sodium, 1-2mpg of potassium and 16mpug of phosphorus. I t is possible to attain such sensitivity for sodium and potassium by flame photometry, although not with the simple instrument used by us.However, it is doubtful if any colorimetric method for phosphorus is so sensitive, and the limit can be lowered by a factor of 15 by increasing the period of activation to 1 week. In practice, it is seldom required to work in the millimicrogram range with these common elements, and the sensitivity is further limited by the magnitude of the blank value. Even after thorough washing, the clean polythene tubes yielded about 0.6 rnpg of sodium, 3.5 mpg of potassium and 10mpg of phosphorus, and for work of the highest sensitivity this blank correction would have to be decreased. The accuracy of the technique is determined partly by the homogeneity of the neutron flux (&2 per cent.in BEPO) and by errors in weighing the final precipitates and in the trans- ference of standards by pipette; counting errors can be decreased to 1 per cent. by counting for 10,000 counts. The total error is probably less than +_5 per cent.August, 19611 PHOSPHORUS IN BIOLOGICAL MATERIAL BY RADIOACTIVATION 51 1 TESTING THE RADIOCHEMICAL FROCEDURES- The chemical procedures described above were tested with aliquots of radiochemically pure sodium-24, phosphorus-32, sulphur-35, potassium-42 and manganese-54. Sulphur is a major element in biological material, although it is not activated to a great extent, whereas manganese is much less abundant, but has a very high cross-section for thermal neutrons.ll The results of the tests are shown in Table 111, from which it can be seen that contamination is so small as to be negligible in practice.Chlorine-38 was not used in this test because it is volatilised in the preliminary ashing step and because it has decayed almost completely after 8 hours. Rubidium-86 was not tested either; it would be expected to follow potassium closely, but under our conditions it should not contribute more than 0.2 per cent. to the measured activity for potassium. TABLE I11 CONTAMINATION OF PRECIPITATES BY VARIOUS ELEMENTS Content of nuclide found in precipitate of- ammonium sodium potassium magnesium Nuclide tested chloride, perchlorate, phosphate, % % % Phosphorus-32 . . .. . . (0.05 < 0.05 90 Sulphur-35 . . .. .. - 1.0 Sodium-24 . . . . .. 40 ~ 0 . 1 4 < 0.07 - Potassium-42 .. . . . . <0*04 40 < 0-07 Manganese-54 . . . . . . 0.27 0.1 1 0.35 INTERFERING NUCLEAR REACTIONS- Several nuclear reactions could interfere with the determinations described above, but all could be eliminated by carrying out the activation in a thermal-neutron column having no fast-neutron contaminants. Fast neutrons can give rise to interference from the reactions listed below- (;) 24Mg (ng) 24Na (ii) 27Al (%,a) 24Na (v) 42Ca (rt,p) 42K ( v i ) 4 5 S ~ (%,a) 42K (iii) 32s (n,p) 32P (iv) 35ci (n,a) 3 2 ~ Reactions (ii) and (vi) were of no importance in this work, as the amounts of aluminium and scandium present in biological material are extremely low. The cross-sections for the rest of the reactions are not well known, but appear to be greatest for reactions (i) and (iiz).The magnitude of the interference can only be calculated for specific samples in which the contents of the interfering elements are known. For example, the seeds studied by us contained 0.6 mg of sulphur per g. This would give rise to an amount of phosphorus-32 corresponding to 0.033 mg of phosphorus per g of the original seeds, which is only 0.4 per cent. of the observed value. The magnesium content of the seeds was 0.3 mg per g, but when 0-3 mg of Specpure magnesium was activated, it yielded an amount of sodium-24 corre- sponding to 0.42 pg of sodium per g of seeds (0.2 per cent. of the observed value). We con- clude that these interferences are negligible. RESULTS- Tomato seeds were collected under clean conditions from glasshouse plants grown in sand culture on Long Ashton complete nutrient solution.They were found to contain, per gram, 0-19mg of sodium, 6-96mg of potassium and 7-75mg of phosphorus. These results are the means of four replicate determinations and are similar to the values 0-21, 7.0 and 7.0 mg, respectively, found by other methods.1° CONCLUSIONS The neutron-activation method described is a remarkably sensitive technique for deter- mining sodium, potassium and phosphorus. I t should be particularly useful for determining traces of phosphorus in biological material, as flame photometry is already established as512 KAKABADSE AND MAXOHIN RAPID MICRO-DETERMINATION [Vol. 86 a satisfactory method for determining the alkali metals on the micro scale. As far as the biologist is concerned, however, the method is not sufficiently sensitive for the determination of these elements in most individual cells or parts of cells. For example, a single yeast cells weighs 5 x 10-10 g, and its potassium content is of the order of 5 x 10-l2 g. Even if fluxes of lOI4 neutrons per sq. cm per second were used for activation, this amount of potassium would not be detectable, much less determinable. It is therefore evident that the analytical challenge of biological research is not being fully met. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. REFERENCES Keynes, R. D., and Lewis, P. R., Nature, 1950, 165, 809; J . Physiol., 1951, 114, 151. Reiffel, L., and Stone, C. A., J . Lab. Clin. Med., 1957, 49, 286. Spencer, R. P., Mitchell, T. G., and King, E. R., Ibid., 1957, 50, 646. Druyan, R., Mitchell, T. G., and King, E. R., Ibid., 1958, 52, 384. Hutchinson, W. P., U.K. Atomic Energy Research Establishment Report Med/R2317, Harwell, Salmon, L., U. K. Atomic Energy Research Establishment Report C/M323, Harwell, 1957. Bowen, H. J. M., Biochem. J., 1959, 73, 381. Born, H. J., and Stark, H., Atomkernenergie, 1959, 4, 286. Cripps, F. H., D.S.I.R. Report CRL/AE49, Teddington, 1950. Bowen, H. J. M., and Cawse, P. A., U.K. Atomic Energy Research Establishment Report R2925, Bowen, H. J. M., J . Nucleav Energy, 1966, 3, 18. Received February 3rd, 1961 1960. Harwell, 1959.

ISSN:0003-2654    

DOI:10.1039/AN9618600506

出版商:RSC    

年代:1961

数据来源: RSC

摘要:

512 KAKABADSE AND MAXOHIN RAPID MICRO-DETERMINATION Rapid Micro-determination of Nitrogen [Vol. 86 in Fluorine-containing Compounds BY G. KAKABADSE AND B. MANOHIN (Department of Chemistry, College of Science and Technology, Mamhester 1) A modified Dumas combustion train for the rapid determination of nitrogen in fluorine-containing compounds and those difficult to combust is described. The tube does not contain a temporary filling, and hydrogen peroxide is used as an intermittent source of oxygen. THE conventional Dumas method for determining nitrogen in organic compounds1 requires considerable modification when analysing compounds that contain fluorine2 or are difficult to burn, leaving nitrogenous chars,, or when attempting to combust rapidly.4 With fluorine- containing compounds, tetraflu~romethane~ often appears to cause the difficulty, and it is possible that CF, radicals formed by initial C-C cleavage have time to react with each other instead of with copper oxide, so forming hexafluoroethane. This suggests that the normal pyrolysis temperature of the Dumas train, 600" to 700" C, is too low.The formation of fluorocarbons, which are not absorbed by the potassium hydroxide solution in the nitrometer, will cause high results for nitrogen. According to Sidgwick,g the decomposition of hexa- fluoroethane vapour begins at temperatures in excess of 800" C, so that the first requirement when dealing with fluorine-containing compounds would be appreciably to raise the tem- perature of the ordinary Dumas train.' A satisfactory technique has been developed in the Microanalytical Laboratory at Cambridge (personal communication from Professor R.N. Haszeldine) in which are used a gas-heated combustion train and a silica tube about 70 cm long containing a permanent filling of copper oxide and copper and, in the "beak" end, a layer of sodium fluoride for absorbing silicon tetrafluoride; the sample is contained in a platinum boat. The replacement of the copper oxide - copper mixture with nickel oxide permitted Kirsten8 to use a much higher pyrolysis temperature, 1050" C; although this method and itsAugust, 19611 OF NITROGEN I N FLUORINE-CONTAINING COMPOUNDS 513 modification by Belcher and Macdonald9 proved to be successful for determining nitrogen in fluorine-containing compounds and those difficult to combust, its disadvantage is high wear of the silica tube, nickel oxide apparently catalysing the crystallisation of silica.For substances that are difficult to burn and leave nitrogenous chars, so causing low results far nitrogen, many remedies have been suggested, based on a combined pyrolytic and oxidative attackl0?l1 as well as on Kirsten's method. Special reference must be made to techniques devised by Unterzaucher,12 involving oxygen generated by the catalytic decom- position of hydrogen peroxide, Swift and M ~ r t o n , ~ who used oxygen from a cylinder, and Cropper, Reed and Rothwell,13 who generated oxygen electrolytically in small amounts at a known rate. Finally, rapid methods for determining nitrogen based on modifications of the Dumas method have been advocated by many investigators, these modifications involving, for example, increase in temperature14 and in length of high temperature combined with oxygen injection16y17 and the use of a pre-combustion techniquels and a specially designed nitrometer.19 By increasing the length of the tube to 100 cm and using two high- and one medium- temperature electrically heated furnaces for that part of the tube filled with copper oxide and copper and by inserting hydrogen peroxide as an intermittent source of oxygen in the path of the carbon dioxide, we have been able to combust the classes of compounds mentioned above quantitatively and rapidly.The procedure described in this paper is recommended after much investigation. COMBUSTION TUBE AND HEATING UNITS- is equipped with a side-arm a t the rear end (mouth) for admission of carbon dioxide.DESCRIPTION OF APPARATUS The quartz tube (see Fig. 1) is approximately 100 cm long and 11 mm in diameter and The Silver wool -- -Carbon dioxide in Fig. 1. Combustion tube and heating units rear portion of the tube (25 cm) is empty and serves for the insertion of the sample in a platinum boat; it is heated by a movable furnace of the split type. The remaining portion of the tube is filled permanently and is heated by furnaces A, B, C and D, the first three being close to each other. The lengths of the furnaces and the sequences of the layers surrounded by them are: A, 16 cm long, filled with equal lengths of M.A.R., powdered cupric oxide, copper wire prepared by reducing M.A.R.cupric oxide wire with hydrogen, and cupric oxide powder; B, 28 cm long, filled with 18 cm of cupric oxide wire and 8 cm of copper wire; C, 10 cm long, filled with cupric oxide wire; D, 9 cm long, filled with 10- to 14-mesh granules of sodium fluoride puri~s.~o The gap between C and D, which is at least 4 cm (although there was no detrimental effect on the results when it was increased up to 15 cm), is filled with cupric oxide wire. The "beak" of the combustion tube is packed with M.A.R. silver wool. All layers are held in place by plugs of silica wool, the sodium fluoride layer being held rather loosely. The temperatures of furnaces A, B, C and D are adjusted to 750" (or between 750" and S O O O ) , 850", 600" and 180" C, respectively. When analysing compounds not containing fluorine and those easy to combust, furnace B can be used at 700" C.Furnace A is of the split type; B, C and D are tubular. OXYGEN GENERATOR- A conical 75-ml suction flask (see Fig. 2) is placed in an inclined position between the bubbler (adjoining the carbon dioxide generator) and the combustion tube, and carbon dioxide passes over the surface of 50-volume hydrogen peroxide (M.A.R.). A piece of 60-mesh silver gauze suspended from a platinum hook at the end of a bent glass rod can be made to dip into or merely touch the surface of the hydrogen peroxide by turning the rod. Interruption514 KAKABADSE AND MANOHIN RAPID MICRO-DETERMINATION [Vol. 86 of the contact causes almost instant stoppage of the flow of oxygen. The supply of oxygen is regulated visually, i.e., by observing the disappearance of the nitrogenous char in the combustion tube.Oxygen is produced intermittently, rather than continuously, to avoid excess of oxygen in the combustion train. NITROMETER- We found that the introduction of a few milligrams of black selenium powder into the 50 per cent. solution of potassium hydroxide prevents bubbles of gas from adhering to the surface of the mercury and breaks down foam at the potassium hydroxide meniscus; selenium partly dissolves in potassium hydroxide, forming a dark-reddish solution. Also effective for this purpose is Pornatti’s method, which involves agitation of a steel needle inside the nitrometer by means of a magnet.21 The addition of mercuric oxide also prevents bubbles of gas from adhering to the surface of the mercury.An ordinary semi-micro nitrometer has been used throughout. CARBON DIOXIDE GENERATOR- The source of carbon dioxide is a Tucker generator,22 which is connected to a bubbler containing a saturated solution of potassium carbonate to retain any drops of liquid carried over from the generator. The flow of gas is adjusted with a precision screw-clamp between tube Carbon dioxide in \ \ Hydrogen peroxide Fig. 2. Oxygen generator the bubbler and the oxygen generator. The bubbler also serves as a useful detector of leakage ; when the nitrometer is isolated by rotating the tail stopcock and the precision screw-clamp is fully opened, the appearance of bubbles would indicate leakage. METHOD PROCEDUFE- The mouth of the combustion tube is closed by a rubber stopper, and air is swept out with a fast stream of carbon dioxide, the gas escaping through the tail stopcock into the atmosphere, passing first through a small volume of water in a conical flask; a short piece of rubber tubing attached to the tail stopcock dips into the water. This simple arrangement acts as a hydraulic valve and prevents air from diffusing into the combustion tube.(After a determination, it is advisable to remove the conical flask and to close the stopcock, so as to prevent any water from being sucked into the tube as it cools.) The furnaces are switched on and allowed to attain the required temperatures. When small bubbles can be observed in the nitrometer, the platinum boat, containing between 10 and 20 mg of sample, is inserted in the combustion tube 5 to 10 cm from furnace A, with the stream of carbon dioxide still passing through the apparatus.The mouth of the combustion tube is closed, and carbon dioxide is passed until small bubbles again appear in the nitrometer. The flow of gas is then adjusted so that one or two bubbles rise in the nitrometer per second. The movable furnace is then pulled into position round the combustion tube. In the presence of nitrogenous chars, the rate of combustion when the oxygen-injection technique is used is governed by the rate of disappearance of the char; as oxygen is generated,August, 19611 OF NITROGEN I N FLUORINE-CONTAINING COMPOUNDS 515 the flow of carbon dioxide is appreciably slowed down (one or two bubbles per 2 seconds). When all the char has disappeared, generation of oxygen is stopped at once, and the slow stream of carbon dioxide is maintained for 3 to 5 minutes to ensure complete absorption of the excess of oxygen by metallic copper.At the same time, the movable heater is brought back to its starting position, and the empty part of the tube is re-traversed with it. After this, the tube is swept out with a fast stream (approximately 9 ml per minute) of carbon dioxide for 1 to 2 minutes, and the rate of flow is then decreased. When small bubbles appear once more, the movable heater is switched off and pushed clear of the combustion tube. With a 10-mg sample of p-nitroaniline (calculated nitrogen content 20.28 per cent.) the entire operation took 15 minutes (nitrogen content found 20.1 and 20.2 per cent.).Some typical results are shown in Table I. The nitrometer is isolated, and, after 5 minutes, the volume of gas in it is read. TABLE I RESULTS AFTER COMBUSTION BY PROPOSED PROCEDURE Unless otherwise stated under "Remarks," the temperatures of furnaces A, C and D were maintained at 750", 600" and 180" C, respectively Experi- No. ment Sample Trifluoroacetanilide (M.A. R.) Approximately (1 + 1) mixture of Teflon and Fn-dinitrobenzene (M.A.R.) 10 $-Nitroaniline (M.A.S.) 11 Sulphanilic acid (A.R.) 12 8-Hydroxyquinoline (M.A.S.) Tempera- Nitrogen Nitrogen ture of content content furnace B, found, calculated, Remarks "C 700 860 600 850 700 800 850 700 10.50 7.41 20.29 20.28 9-59 9.65 { 8.11 8-09) Slow combustion Rapid combustion Furnace C at 800" C Oxygen injected; rapid combustion No oxygen injected; slow combustion No oxygen injected; boat placed in close contact with permanent CuO filling Oxygen injected ; rapid combustion Oxygen injected ; very slow combustion I BLANK TEST- A check was carried out under rather artificial conditions by passing a very rapid stream of carbon dioxide through the combustion train for 30 minutes and keeping the temperature of furnace B at 850" C.With the normal Dumas temperature setting (furnace B at 700" C), the blank was hardly perceptible, even with a magnifying lens, i.e., it was practically zero. DISCUSSION OF THE METHOD A blank of less than 0.02 ml was obtained in the nitrometer. According to Clark and R e e ~ , ~ , ~ ~ the ordinary Dumas micro-determination of nitrogen can be used without modification for fluorine-containing compounds.In our view, an increase in the temperature of combustion over that ordinarily used is imperative. Obviously, the temperature requirements will vary somewhat with the nature of the compound, e.g., whether it is partly or completely fluorinated' ; for example, we found that heptafluorobutyramide was more resistant to pyrolytic attack than was trifluoroacetanilide. As a general rule, we established that the lower limit of temperature for complete combustion was about 800°C; the upper limit was 900" C or less, being determined by the extent to which the copper oxide attacked the silica tube.l* Not infrequently, this produced cracks in the tube, and we therefore recommend that the temperature of furnace B be maintained at 850°C.First, the pre-combustion zone (heated by furnace A), containing powdered copper oxide to assist combustion. Here, most substances bum quantitatively when combusted slowly, but fluorine-containing compounds There are three combustion zones in our apparatus.516 KAKABADSE AND MANOHIN : RAPID MICRO-DETERMINATION [Vol. 86 tend to behave differently (see experiment No. 3, in Table I). The temperature setting of A is not altered throughout. Second, the “finishing” zone (heated by furnace B), the function of which is two-fold; it completes the combustion of “awkward” compounds (with B at the high-temperature setting) and helps to carry out combustions rapidly for all compounds (with B at the high- or middle-temperature setting). Third, the “eliminating” zone (heated by furnace C), which helps to restore the carbon dioxide balance prevailing in the normal Dumas method,l6J7 so eliminating carbon monoxide24 to 29 produced in the hot “finishing” zone.30 Experiment 6 (see Table I) emphasises the well known fact that under high- temperature conditions the Dumas method is not very satisfactory.6 As furnaces A and B differ relatively little from each other functionally they could conceivably be amalgamated into one large high-temperature heater.We found it more practicable to have them separate, especially as A is of the split type and hence easy to inspect visually. The introduction of oxygen into the Dumas train has, in our experience, no effect on C-F cleavage.31 The presence of hydrogen, however, according to Milton and Waters:e seems to be necessary for the satisfactory analysis of fluorine-containing compounds. Pre- sumably, fluorine formed by initial C-F cleavage combines with hydrogen to form hydrogen fluoride, which, with silica, gives silicon tetrafluoride and water vapour.Only traces of moisture, always present in the Dumas system, would therefore be required to initiate this chain reaction. If it reaches the nitro- meter undecomposed, it will be quantitatively absorbed by the potassium hydroxide solution, but we found that it might be partly hydrolysed by traces of moisture, resulting in deposition of silica near or in the stopcock and so blocking the flow of gas. This is effectively prevented by a layer of sodium fluoride% in the “beak” end of the combustion tube (maintained at 180” C by furnace D).After having carried out determinations of nitrogen both with and without this sodium fluoride, we strongly recommend its use in the routine analysis of fluorine- containing compounds. For substances difficult to combust, giving nitrogenous chars, the addition of controlled amounts of oxygen to the cupric oxide - copper-filled tube is, in our experience, satisfactory. Our oxygen generator differs from that proposed by Unterzaucher12 in that carbon dioxide does not bubble through a solution of hydrogen peroxide; the flow of oxygen is therefore independent of the flow of carbon dioxide, and oxygen is produced intermittently instead of continuously. Further, we use silver instead of platinum, the 60-mesh silver gauze being relatively cheap and readily available. The catalytic activity of silver is similar to that of platinum; the rate of decomposition of hydrogen peroxide on silver is lo7 times that on an inert material such as polythene.a Experiment No.9 in Table I shows a rather interesting observation: when no external oxygen is available, errors due to the formation of nitrogenous chars can often be decreased by placing the platinum boat containing the sample close to the cupric oxide powder of the permanent filling. This technique, although not a complete remedy, is significantly simple. The achieved increase in the rate of combustion we attribute mainly to increases in length of tube and temperature, both of which favour more thorough combustion ; the greatest danger in any rapid method is the occurrence of incompletely burnt gaseous products of decomposition in the nitr0meter.l’ The active life of the combustion tube in our apparatus is about sixty determinations, with an average weight of sample of 15mg.This raises the problem of the disposal of silicon tetrafluoride. We thank Professor R. N. Haszeldine for the communication referred to on p. 512 and Mr. B. Woodbridge for carrying out various timing operations. REFERENCES 1. 2. 3. 4. 5. Macdonald, A. M. G., I n d . Chem., 1957, 33, 360. 6. Belcher, R., and Godbert, A. L., “Semi-micro Quantitative Organic Analysis,” Longmans, Green Clark, S. J., “Quantitative Methods of Organic Microanalysis,” Butterworths Scientific Publica- Swift, H., and Morton, E. S., Analyst, 1952, 77, 392.Colson, A. F., Ibid., 1950, 75, 264. Sidgwick, N. V., “The Chemical Elements and Their Compounds,” The Clarendon Press, Oxford, and Co. Ltd., London, 1946, p. 72. tions, London, 1956, p, 87. 1950, p. 1128.August, 196 11 OF NITROGEN I N FLUORINE-CONTAIKING COMPOUNDS 517 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. Rush, C. A., Cruikshank, S. S., and Rhodes, E. J. H., Mikrochim. Acta, 1956, 858. Kirsten, W., Anal. Chem., 1947, 19, 925. Belcher, R., and Macdonald, A. M. G., Mikrochim. Acta, 1956, 1111. Belcher, R., and Godbert, A. L., 09. cit., p. 84. Spies, J. R., and Harries, T. H., Ind. Eng. Chem., Anal. Ed., 1937, 9, 304. Unterzaucher, J., Mikrochem. Mikrochim. Ada, 1951, 36/37, 706. Cropper, F. R., Reed, R. H., and Rothwell, R., Mikrochim. Acta, 1954, 223. Levy, R., and Cousin, B., Ibid., 1960, 854. Eder, K., Ibid., 1959, 631. Ingram, G., Ibid., 1953, 131. Trutnovsky, H., Ibid., 1960, 157. Schliniger, W., Mikrochem. Mikrochim. Acta, 1952, 39, 229. Gustin, G. M., Mikrochim. Acta, 1958, 581. Milton, R. F., and Waters, W. A., “Methods of Quantitative Micro-analysis,” Second Edition Pomatti, R., Ind. Eng. Chem., Anal. Ed., 1946, 18, 63. Clark, S. J., op. cit., p. 80. Clark, H. S., and Rees, 0. W., Illinois State Geological Survey, Report of Investigation No. 109, Pagel, H. A., and Oita, I. J., Anal. Chem., 1952, 24, 756. Kao, S., and Woodland, W. C., Mikrochern. Mikrochim. Acta, 1951, 36/37, 309. Kirsten, W., Ibid., 1953, 40, 121. Hozumi, K., and Amako, S., Mikrochim. Ada, 1959, 230. Gore, T. S., and Kulkarni, A. S., Ibid., 1960, 558. Belcher, R., and Godbert, A. L., op. cit., p. 82. Charlton, F. E., Analyst, 1957, 82, 643. Sidgwick, N. V., op. cit., p. 1126. Milton, R. F., and Waters, W. A., op. cit., p. 72. Belcher, R., and Goulden, R., Mikrochem. Mikrochim. Acta, 1951, 36/37, 679. “Hydrogen Peroxide Data Manual,” Laporte Chemicals Ltd., Luton, Beds., 1960, p. 5. Edward Arnold (Publishers) Ltd., London, 1955, p. 71. 1954. Received Novenzbev 15th, 1960

ISSN:0003-2654    

DOI:10.1039/AN9618600512

出版商:RSC    

年代:1961

数据来源: RSC

摘要:

August, 196 11 OF NITROGEN I N FLUORINE-CONTAIKING COMPOUNDS Determination of Added Borates in Mixed 517 Fertilisers BY H. N. WILSON AND G. U. M. PELLEGRINI (Imperial Chemical Industries Ltd., Billingham Division, P.O. Box No. 6, Billingham, Co. Duvham) A method for determining borates in fertilisers in the range 0-1 to 1 per cent. of boron is described. Phosphate is removed by adding bismuth nitrate solution, and the “identical-pH” method is used to detect the end-point of the subsequent mannitol - boron titration. INTEREST in the manufacture of “compound” fertilisers to which borates have been added is increasing, and this has focused attention on the analytical problem of accurately deter- mining the borate present. The amount added is more than a “trace” and may generally be within the range 0.1 to 1 per cent.of boron. Current methods of determining borates are either photometric or volumetric. Most of the photometric methods are perhaps more suitable for “traces” than for larger amounts, and it is thought that a volumetric method will be preferable. These methods depend on the formation and titration of glyceroboric acid or, better, mannitoboric acid, and this necessitates prior separation of the borate from interfering substances, including phosphate. Distillation as methyl borate and hydrolysis of the distillate is well known, but it is time-consuming and difficult to carry out quantitatively. The American Association of Agricultural Chemists1 recommend two methods, one for acid- soluble and the other for water-soluble boron. In the first, phosphate is removed with lead nitrate, the excess of lead (and also calcium) being removed by sodium hydrogen carbonate.Finally, the boric acid is titrated by adjustment of the pH to 6.3, addition of mannitol and titration to the same pH value with sodium hydroxide solution (“identical-pH” method). The water-soluble boron is extracted by hot water, phosphate is removed with barium hydroxide solution, and ammonia is removed by boiling for at least 1 hour. After filtration and removal of carbon dioxide, the solution is made neutral to methyl red, mannitol is added, and the mannitoboric acid titrated to the phenolphthalein end-point.518 WILSON AND PELLEGRINI : DETERMINATION OF [Vol. 88 These methods can produce precipitates that are difficult to handle (even if “filter aids” are used), and in our hands the results were not as reproducible as one could wish.We therefore considered the possibility of using some other reagent to remove phosphate from the solution, Among the known precipitants for phosphate, bismuth nitrate in dilute acid solution seemed promising. As long ago as 1860, Chancelz determined phosphate by precipi- tation as bismuth phosphate and noticed that the crystalline precipitate settled and filtered well. Rathje3 had proposed to titrate phosphate with bismuth nitrate solution, locating the end-point by the orange colour of bismuth iodide, but pointed out that chlorides and sulphates interfere through formation of basic salts. It occurred to us that the use of bismuth would TABLE I BORATE FOUND I N COMMERCIAL FERTILISER Borate content found, as boron, by- Sample A No.Lnalyst A, analyst B, analyst E, % % % 0.171 0.173 0.177 0.169 0.174 0.172 0.176 0.172 0.177 0.160 0.157 0.159 0.158 0.161 0,162 l { also have the advantage that the excess could be readily removed as basic salts by increasing the pH and diluting the solution. A few experiments showed that a separation was possible, but also showed that an excess of bismuth was necessary over that equivalent to the phosphate present, presumably because of the formation of basic salts. EXPERIMENTAL Each of a series of 2.5-g portions of a commercial “compound” fertiliser containing 10 per cent. of P,05 (based on superphosphate) was dissolved in diluted nitric acid, and the resulting solutions were treated with different volumes of a 22 per cent.w/v solution of bismuth nitrate (see “Method”). The amount of phosphate remaining in solution was then determined spectrophotometrically ; the results were- Volume of bismuth nitrate solution used, ml. . 50 45 40 35 30 25 20 Proportion of original P,O, left in solution, yo 0.2 2.0 3.5 9.5 25.0 54-0 76.5 These results show that only with a volume of the bismuth nitrate solution equivalent to 5 ml for each 1 per cent. of P,05 in the fertiliser is removal of phosphate virtually complete. To obtain a crystalline precipitate that settles well, the bismuth nitrate solution must be added slowly to the hot acid solution of the fertiliser, with frequent agitation; after the addition is complete, the pH of the solution is about 1.7.It was found most convenient to allow the solution to cool, dilute it to a given volume and then filter an aliquot part rather than to wash the bismuth phosphate. That this does not lead to loss of borate is shownby the results on p. 519. Excess of bismuth in the aliquot is removed by addition of a small excess of sodium hydroxide. This precipitate (basic salts of bismuth) is washed, and the borate is then titrated in the filtrate by the “identical-pH” method after removal of carbon dioxide. For a fertiliser containing 0.15 to 0.2 per cent. of boron, the titre of 0.02 N sodium hydroxide is in the range 7 to 10 ml. There is a slight blank value; if possible, this should be determined by analysing a similar fertiliser to which no borate has been added.If this is not possible, an average blank value of 0.15 ml of 0-02 N sodium hydroxide can be used. I t is doubtful what significance would attach to water-soluble borate, and one or two preliminary experi- ments suggested that the method of extraction influenced the results. Although the “identical-pH” method has often been treated as empirical, our results justify the use of the stoicheiometric factor under our conditions. We have only concerned ourselves with total or acid-soluble borate. REPRODUCIBILITY OF RESULTS To two samples of fertiliser (A based on superphosphate and B based on ammoniumAugust, 19611 ADDED BORATES IN MIXED FERTILISERS 51 9 phosphate) known amounts of sodium tetraborate were added, and the boron contents were determined as described under “Procedure” ; the results were- Sample ... . A B && Boron added, yo . . 0.517 0.319 0.165 0.958 0.451 0.229 Boron found, yo . . 0-517 0.315 0.162 0-962 0.450 0.231 Two samples of a commercial borated fertiliser (based on superphosphate) were analysed by three analysts, the third of whom made only one determination on each sample and had no previous knowledge of the method; the results are shown in Table I. These results, together with those above, show satisfactory reproducibility. The time required for a determination in duplicate is about 3 hours. METHOD REAGENT- acid, with slight warming if necessary, and dilute to 100ml with water. PROCEDURE- Weigh out 2.5g of sample, transfer to a 400-ml beaker, add 2ml of nitric acid and 50 ml of water, stir, warm, and dilute to 100 ml with water.Warm the solution to 80” or 90” C (do not boil, as boric acid is volatile in steam), and slowly add from a burette, with continuous stirring, 5 ml of the bismuth nitrate solution for each 1 per cent. of P20, present in the fertiliser. Allow the precipitate to settle, cool, wash into a 250-ml calibrated flask, and dilute to the mark. Filter through a dry Whatman No. 40 filter-paper, rejecting the first few millilitres, and, by pipette, place 100 ml of the filtrate in a beaker. Add a few drops of bromothymol blue indicator solution and then a 10 per cent. solution of sodium hydroxide, with thorough stirring, until the indicator turns blue. Separate the precipitate on a Whatman No. 541 filter-paper, carefully wash it several times with cold water, and combine the washings with the filtrate (the total volume should be about 150 to 200 ml).Adjust the pH to about 5 by adding 5 per cent. nitric acid, heat to about 90” C (do not boil), and stir vigorously to remove carbon dioxide. Cool, place in the solution the electrodes of a suitable pH meter, and adjust the pH to 6.3; first use 10 per cent. sodium hydroxide solution, and finally bring the pH exactly to 6-3 with carbon dioxide-free 0.02 N sodium hydroxide. Add l o g of mannitol, and again bring the pH to 6.3 with the 0-02 N sodium hydroxide. Continue to add 10-g portions of mannitol and to re-adjust the pH to 6.3 until, after the final addition of mannitol, the pH remains constant at 6.3. (For samples containing up to 0-5 per cent. of boron, 20g of mannitol are usually sufficient.) The total amount of 0.02 N sodium hydroxide used after the additions of mannitol corresponds to the amount of boron present in the solution. Carry out a blank determination on a similar type of fertiliser to which borate has not been added, and subtract the blank titre from the titre previously obtained; if a “blank” sample is not available, deduct 0.15 ml from the titre of 0.02 N sodium hydroxide. Calculate the borate content, as boron, of the sample from the equation- Boron content, yo = (A - B) x 0.000216 x 100 in which A and B are the titres of 0.02 N sodium hydroxide in the sample and blank deter- minations, respectively. Bismuth nitrate solutim-Dissolve 22 g of Bi(N03)3.5H20 in S ml of concentrated nitric Keep the solution hot during precipitation. REFERENCES 1. 2. 3. Horwitz, W., Editor, “Official Methods of Analysis,” Eighth Edition, The Association of Official Chancel, G., Compt. Rend., 1860, 50, 416. Rathje, W., Angew. Chem., 1938, 257. Agricultural Chemists, Washington, D.C., 1955, p. 18. Received Februavy 24& 1961

ISSN:0003-2654    

DOI:10.1039/AN9618600517

出版商:RSC    

年代:1961

数据来源: RSC

摘要:

520 YOUNG: THE DETERMINATION OF DIHYDROXYBENZENES [Vol. 86 The Determination Liquid of Dihydroxybenzenes by Par ti tion Chromatography Liquid - BY JOHN H. YOUNG [ T h Midland Tar Distillers Limited, Research and Development Department, Four Ashes, nr. Wolverhamnpton) Liquid - liquid partition chromatography is slow, largely because of the time involved in identifying and determining components in the eluate. With the apparatus described, most of these time-consuming operations are avoided by continuously recording the percentage of ultra-violet light of fixed wavelength transmitted by the eluate. By this means, the emergence from the column of separated components is indicated by the appearance of peaks on the recorder chart. Some components can be identified and deter- mined directly from the trace by the relative positions of and areas under the peaks.For others, an automatic fraction collector is used, and the trace serves as a guide for blending fractions for subsequent ultra-violet analysis. The method has been applied mainly, but not exclusively, to the determination of dihydric phenols in aqueous and organic solutions. THE phenolic fraction extracted from coal-tar distillates boiling over the range 230" to 270" C is essentially a complex mixture of monohydroxy aromatics containing minor amounts of dihydroxybenzenes. As the latter may have some bearing on the properties and utilisation of such a fraction, a method was required for determining individual dihydroxybenzenes in the presence of much larger amounts of monohydric phenols. Such a method would also be of value in classifying aqueous phenolic effluents and in evaluating procedures for treating them; for this purpose, the method should be applicable to a dilute aqueous solution of the sample.Chromatographic procedures were clearly suggested, and liquid - liquid partition was preferred to paper chromatography because it could more easily be made quantitative. Such a method had already been successfully devisedl and appeared to be suitable for our purpose. The chief disadvantages of the column procedure were the slowness of operation and the time required for identifying and determining the eluted components. In view of this, a semi-automatic apparatus was designed to work without attention overnight and to record a trace showing the eluted components.The amounts of individual dihydric phenols could then be determined after blending fractions (the trace being used as a guide) or some- times directly from the trace. Although the apparatus has been constructed for determining dihydric phenols, it can also be used for other chromatographic determinations, with a suitable choice of phases, provided that a wavelength can be found in the visible or ultra-violet region at which the solvent transmits light appreciably more than do the components to be measured. DESCRIFTION OF APPARATUS SOLVENT HANDLING- The eluting agent (cyclohexane) is supplied from a 1-litre flask, A, equipped with a bottom off-take leading to a tap and a male spherical joint (14 mm diameter). When gradient elution is used, the more polar solution (20 per cent.v/v of n-butyl alcohol in cyclohexane) is contained in a second 1-litre flask, B, and is continuously supplied to flask A via siphon tube C (2mm i.d.). A stirrer in flask A ensures mixing, and a side-tube is provided to prime the siphon by slight air pressure. I t has been found that the siphon gives a more linear change in concentration with volume of eluting solution than does the more conventional arrangement of a separating funnel mounted on the flask so that the flask remains full. When analysing phenolic mixtures it is often desirable to use only pure cyclohexane as eluting agent during the early stages of elution. A delay before the siphon begins to operate can be achieved by fitting a non-return valve of the type shown at D in Fig.1. A B10 air leak is cut off close to the joint, and a 3/16-inch ball-bearing is ground in with emery powder. Final grinding is done with very fine emery, a second identical ball being used, The device for supplying the eluting agent is shown in Fig. 1.August, 19611 BY LIQUID - LIQUID PARTITION CHROMATOGRAPHY 521 4 f Fig. 1. Details of supply of eluting solution - Spherical joint ( I 4 mm) U Teflon tube (2.5 mm i.d.) Fig. 2. Details of pre-column522 YOUNG : THE DETERMINATION OF DIHXDROXYBENZENES [Vol. 86 and this ball is used in the completed valve. The valve opens only under a positive hydrostatic head of about 40mm of the butyl alcohol-cyclohexane solution, so that the delay before the gradient is applied can be adjusted by varying the relative heights of the flasks and the liquids in them.It is important that eluting agent passing to the column is saturated with the stationary phase (water) at the temperature of the column, otherwise each component may give rise to two partly resolved peaks. When an eluting solution of changing composition is used, water is added to the more polar solution to the extent of about two-thirds of the amount required for saturation. Final adjustments to the water content are then made by passing the eluting agent through a small pre-column (see Fig. 2) packed with the same stationary phase as the analytical column. The eluting solution attains equilibrium by accepting or losing water in this pre-column. INTRODUCTION OF SAMPLES- Aqueous sam$Zes-Portions (0.5 to Eiml) are acidified and then mixed with twice their The resulting free-flowing powder is put into an weight of dry 60- to 80-mesh silica gel.adapter (see Fig. 3) that fits on to the top of the column. Spherical joint (14 mrn)--\ 4 024 B24 NO. I s i n t e r - d Spherical ,'joint (14 mm) Fig. 3. Adapter for aqueous samples Fig. 4. Sample injector Non-aqueous sampEes-Portions (0.5 ml) of samples dissolved in a cyclohexane - butyl alcohol mixture are introduced into the side-limb of the injection device shown in Fig. 4. A three-way tap is set to by-pass the sample limb until the flow has been adjusted. The tap is then turned, and the flow of eluting agent is diverted to wash the sample on to the column. PREPARATION OF COLUMN- The chromatographic tube (see Fig.5) has an internal diameter of about 14mm and is 640mm long. It is fitted at the top with a ground-glass B24 socket and at the bottom with a B10 cone having a No. 1 sinter below it. This position for the sinter decreases the space in which fractions of eluate can diffuse. For the analysis of dihydric phenols, the tube is packed to a depth of 570 mm with 55 per cent. w/w of water on acid-washed 60- to SO-mesh silica gel. This system is based on the original work of Blackburn, Barker, Catchpole and Hollingworthl and is similar to that used more recently by Barker and Hollingworth2ps concurrent with the developments described here.August, 19611 BY LIQUID - LIQUID PARTITION CHROMATOGRAPHY 523 The most satisfactory method of packing the column is to add the correct proportion of water to a stirred slurry of dry silica gel in a large volume of cyclohexane and to continue the stirring for a short time.The slurry obtained can be packed to form a column showing no “bands” by the technique described below. The chromatographic tube is filled with cyclohexane and connected via a siphon tube (see Fig. 6) to a 250-ml flask containing about 60g of the prepared gel in about 150ml of cyclohexane. A micro pump (obtainable from the Distillers Company Ltd.) is then used to pump cyclohexane into the flask at 1500ml per hour so as to force the #24 i.d. mm sinter Fig. 5. Details of column Teflon tube (3 mm i.d.) I Cyclohexane in at 1500 ml per hour Fig. 6. Column-packing device slurry through the siphon tube and into the column; the surplus cyclohexane running from the bottom of the column can be re-cycled.It is sometimes necessary to tap the 250-ml flask lightly to ensure an even flow of slurry. When packing has been completed, the top surface of the column is protected from disturbance by a disc of filter-paper, and the level of cyclohexane is never allowed to fall below this disc. COLLECTION AND MEASUREMENT OF ELUATE- The eluate from the column passes through a Teflon tube (16mm i.d.) to a cell con- structed as shown in Fig. 7 from silica plates secured with Araldite adhesive to a brass plate 4mm thick. This assembly fits in the cell-well of a Unicam SP500 spectrophotometer modified for automatic scanning as described by Shrewsbury.* In this work, the instrument is set at a fixed wavelength (2800 A), and the percentage transmission is recorded on a chart moving at 1 inch per hour.(The cyclohexane used must have a low optical density at all wavelengths accessible to the instrument ; commercial grades must generally be purified before use by passage through dry silica gel.) From the cell, the eluate passes into a Teflon tube (1-5 mm i.d.) coupled by a B10 joint to a flow controller (see Fig. 8). Accurate control is obtained by adjusting the height of a stainless-steel wire (17 s.w.g.) and so varying the restriction to flow imposed by it. It is an advantage to file the wire so that its cross-sectional area is decreased towards its lower tip. The rate of flow is measured by a simple calibrated flowmeter of the type shown in Fig. 9, which indicates the hydrostatic head of eluting solution required for a corresponding rate of flow through a fixed capillary restriction.The flow is normally in the range 50 to 80ml per hour.524 YOUNG : THE DETERMINATION OF DIHYDKOXYBENZEMES Teflon tube [Vol. 86 Brass lid 111 11 Adjustable bracket mm X , 4 mm) L / I Brass pl& 4 mm thick Fig. 7. Details of cell The eluate from the flowmeter is usually collected with a Shandon fraction collector equipped with fifty 15-ml tubes. This type of fraction collector has a scoop that will take the eluate to waste if the mains electricity supply is interrupted, and use is made of this by controlling the mains input in two ways. A time switch stops and starts the collection, and a Simmerstat working on a time cycle of just over 1 minute will direct a fixed proportion of the eluate to waste.This means that a known proportion of each component can be r Loose-fi t t i ng,l ad j ustab I e stainless-steel wire Wide- bore capillary \ Fig. 8. Details of flow controller Overflow tube Fig. 9. Details of flowmeterAugust, 19611 BY LIQUID - LIQUID PARTITION CHROMATOGRAPHY 526 collected during a long run without exceeding the volumetric capacity of the fraction collector. Used in conjunction, the two controls permit complete collection of part of the eluate and partial collection of the remainder. Each change of fraction is marked on the trace by a small “spike.” This is achieved by connecting a micro switch to a pawl that keeps contact with the notched rim of the turn-table. During each change of fraction, the signal to the recorder can then be partly shorted by a 9000-ohm resistor. OPERATION As the spectrophotometer is used for other purposes during the day, it has been con- venient to operate the column overnight as described below.During the day, the pre-column is re-packed, and butyl alcohol is washed from the analytical column with about 500 ml of purified cyclohexane; the rate of flow may be as high as 200 to 300 ml per hour and no air pressure is required. The flow is then adjusted to about 70ml per hour, and, during the last half-hour of the afternoon, the recorder is started, set to zero for dark current and then set at about 90 per cent. transmission at 2800 A with a slit width of 0.46mrn. The sample is introduced, the fraction collector is switched on, and the gradient siphon is primed (both flasks being full).The apparatus is then left without attention until the following morning. RESULTS In order to illustrate the procedure, the trace for a prepared mixture is shown in Fig. 10, the amounts of individual phenols present being in the range 0.2 to 2 mg. It has been found that the butyl alcohol is not eluted evenly, but builds up on the column and, during the run, suddenly “breaks through” at a concentration of just over 1 per cent. At this point on the trace a spurious peak occurs, the nature of which is not known. It may be produced by traces of impurities adsorbed on the silica gel and then displaced as a sharp band by the butyl alcohol. QUANTITATIVE ANALYSIS- The amount of a component present in a peak is given by the expression- kAtq Weight present, mg = - El in which k is a constant, A is the maximum height of the peak expressed as an optical density, t is the time in hours for the elution of the peak (the intercept along the base-line between the tangents to the sides of the Deak), q is the rate of flow in millilitres per hour, E is the extinct& coefficient (Ei&) at <he centimetres.wavelength used and 1 is the thickness of the cell in I 4- Time, hours ( I division = 2 hours) Fig. 10. Trace recorded for a prepared mixture526 YOUNG THE DETERMINATION OF DIHYDROXYBENZENES [Vol. 86 By calibration with prepared mixtures, the value of k was found to be about 5.7. For a true Gaussian peak: k should be equal to 6-27; the lower value is a purely empirical factor, which takes account of the flow conditions in the cell.In practice, it is convenient to tabulate values of k/El for the series of compounds to be determined, and the values used at 2800 A for some common phenols are shown in Table I. Phenol Phenol .. . . o-Cresol . . . . m-Cresol . . .. p-Cresol . . .. 2,3-Xylenol . . .. 2,4-Xylenol , . . . 2,5-Xylenol . . . . 2,g-Xylenol . . .. 3,4-Xylenol . . . . 3,5-Xylenol . . . . 2-Ethylphenol . . 3-Ethylphenol . . 4-Ethylphenol . . TABLE I VALUES OF k/El FOR VARIOUS PHENOLS Value of k/El, mg per ml Phenol . . 0.142 Catechol . . . . . . 0.108 3-Methylcatechol . . . . 0.088 4-Methylcatechol . . . . 0.075 3,4-Dimethylcatechol . . 0-114 3,6-Dimethylcatechol .. 0.087 4,5-Dimethylcatechol . . 0.111 3-Ethylcatechol .. .. 0.163 4-Ethylcatechol . . . . 0.098 Resorcinol . . . . . . 0.135 2-Methylresorcinol . . . . 0.105 4-Methylresorcinol . . . . 0.097 5-Methylresorcinol . . . . 0.103 2,4-Dimethylresorcinol 4-Ethylresorcinol . . Quinol .. . . Value of k/El, mg per ml .. 0.070 . . 0.113 . . 0.077 . . 0.060 a . 0-1 12 . . 0.097 . . 0.118 . . 0.085 . . 0.089 . . 0-186 . . 0-072 , . 0.127 .. 0.257 . . 0,079 . . 0.095 As an alternative quantitative method, ultra-violet analysis of collected fractions is slower, but slightly more accurate; it is essential when the identity of a peak is in doubt. In applying methods of quantitative analysis (especially by measurement of areas) it is important to allow for the polarity of the solvent used. The presence of butyl alcohol is likely to produce a considerable change in the spectrum of a phenol in cyclohexane and TABLE I1 RESULTS FOUND FOR PREPARED MIXTURES Mixture No.1 is that for which the trace is shown in Fig. 10. Average values for extinction coefficient were used in the two instances when the components were not resolved Total amount of collected Amount of dihydroxybenzenes- Amount component found by- A > Component o-Cresol , . .. m- and p-Cresols . . Phenol .. .. 3-Methylcatechol . . 4-Methylcatechol . . Catechol . . .. 4-Methylresorcinol . . 2- and 5-Methyl- resorcinols .. Resorcinol . . .. Total . . .. 3-Methylcatechol . . 4-Methylcatechol . . Catechol . . .. 4-Methylresorcinol . . 5-Methylresorcinol . . Resorcinol . . .. Mixture No. 1- Quinol .. . . Mixture No. 2- Quinol .. .. of --+-A, component ultra-violet measurement present, mg 0.34 0.65 1-62 0.30 0.32 1.86 0.3 1 0.62 1.67 1.68 9.27 0.57 0.15 1.60 0.46 0.26 1.79 1-68 analysis, mg -* -* -* -* 0.34 2-00 0.28 0.71 1.76 1.64 - 0.61 0.19 1.60 0.49 0-42 1.78 1-54 of area, mg 0.33 0.64 1-26 0.31 0.33 0.67 1.85 1-57 9.41 0.57 0.16 0-27 1.80 1.59 found by found by ultra-violet measurement present, analysis, of area, mg mg mg 6.46 6-51 6.73 6-63 6-87 6.57 * Fraction not collected.August, 19611 BY LIQUID - LIQUID PARTITION CHROMATOGRAPHY 527 will therefore affect the extinction coefficient at the wavelength used during the run. The extinction coefficient of the component in the correct solvent must always be used. Quantitative results for two prepared mixtures are shown in Table I1 to indicate the accuracy to be expected from the method. I thank the Directors of the Midland Tar Distillers Ltd. for permission to publish this paper and Mr. D. D. Shrewsbury for designing and constructing the cell and carrying out the necessary ultra-violet analyses. REFERENCES 1. 2. 3. 4. 5. Blackburn, W. H., Barker, L., Catchpole, J. R., and Hollingworth, N. W., Gus Councd Res. Comm. Barker, L., and Hollingworth, N. W., Gas Council Res. Comm. No. 52, 1958. -,-- , J . Appl. Chem., 1959, 9, 16. Shrewsbury, D. D., Analyst, 1959, 84, 68. Burrows, G., Trans. Inst. Chem. Engrs., 1957, 35, 245. No. 24, 1955. Received Januavy 24th, 1961

ISSN:0003-2654    

DOI:10.1039/AN9618600520

出版商:RSC    

年代:1961

数据来源: RSC