ISSN:0003-2654    

DOI:10.1039/AN96691FX001

出版商:RSC    

年代:1966

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN96691BX003

出版商:RSC    

年代:1966

数据来源: RSC

摘要:

10 GREENFIELD, MOULE AND PERRY : CONDUCTIMETRIC [Analyst, 'C'Ol. 91 The Conductimetric Determination of Microgram Amounts of Phosphine in Air* BY S. GREENFIELD, H. A. MOULE AND R. PERRY (Albright & Wilson (Mfg.) Ltd., Research Department, Oldbury, Birmingham) A conductance cell is proposed as a sensitive means of detecting and determining phosphine in air at the part-per-million level. The contaminated air is passed through mercuric chloride solution in the cell, where reaction occurs between the phosphine and the mercuric chloride, liberating hydrogen chloride. This causes a rise in conductance proportional to the amount of hydrogen chloride, and hence to the amount of phosphine. Suggestions are made for the determination of phosphine in discrete samples of air, and for the continuous monitoring of contaminated air.VARIOUS methods have been proposed for detecting and determining phosphine. For example, a known volume of air can be passed through a standard silver nitrate solution, the residual silver nitrate then being titrated with standard potassium thi0cyanate.l For determining phosphine in acetylene, the gas has been passed through a solution containing mercuric sulphate and potassium chloride. The acetylene is passed until the solution is exhausted, as indicated by a strip of silver nitrate paper a t the gas exit; the phosphine content can then be calculated.2 A sensitive detector consists of a glass tube containing either a paper strip impregnated with silver 11itrate,~9* or silica-gel particles, with silver nitrate,5 a copper salt and a mercury complex,6 or a mercury salt and auric chloride' as impregnant. A known volume of con- taminated air is pumped through the tube, and the length of stain is compared with standards.It is claimed that such tubes will detect phosphine down to 0.01 p.p.m. I t has also been claimed that phosphine can be determined spectrophotometrically by the colour produced by reaction with silver diethyldithiocarbamate.8 In any process in which phosphine is used there is always present the possibility of a leakage, with the accompanying toxicity hazard, and it was considered that some form of continuous monitoring of the atmosphere near the plant was desirable. Kone of the above methods lends itself to automation, or to continuous recording, whereas the conductance cell to be described can be used, not only to examine discrete samples of air, but also, in conjunction with a suitable self-balancing bridge and recorder, for monitoring purposes.The cell contains mercuric chloride solution through which the air is passed. Hydrogen chloride is liberated by reaction of the phosphine with the mercuric chloride, causing a sharp rise in conductance, proportional to the amount of phosphine absorbed. EXPERIMENTAL THE CELL- The design of the conductance cell is similar to those already describedgi10711 for the determination of carbon and hydrogen in organic compounds, except that the electrodes are much larger, and the working volume is 11 ml. The electrode system consists of a cylindrical inner electrode of platinum gauze, surrounded by two half-cylindrical outer electrodes, also of platinum gauze.Possible leakage round the sealed-in wires is prevented by a coating of Araldite resin. A steady flow of air is essential in order to give a uniform circulation of solution and hence a steady conductance reading. (The conductance of a flowing solution is not the same as that of a stationary one,) To achieve this steady flow the jet size must be carefully chosen, since if it is too large the bubbles will emerge in bursts. This not only causes inter- mittent circulation of the solution, but also allows the solution to rise up inside the jet. This must be avoided, since this solution is absorbing phosphine but not contributing to the conductance. On the other hand, too small a jet can cause considerable back-pressure in the air-inlet system, which can give rise to loss of phosphine by leakage through the joints.The cell is illustrated in Fig. 1. * Presented a t the meeting of the Society on Wednesday, November 4th, 1964.January, 19661 DETERMINATION OF MICROGRAM AMOUNTS OF PHOSPHINE IN AIR 11 The optimum size of jet is best found by trial and error, but as a guide the jet used in these investigations was approximately 0.1 mm in diameter. The easiest way to obtain such a jet is to draw a capillary with a slow taper and then to cut or grind this back until the desired results are obtained. n Fig. 1. Conductance cell THE MEASURING APPARATUS AKD TEMPERATURE COMPENSATION- The conductance meter was made to a Guest, Keen and Nettlefold design,l2?l3 and is similar to those already de~cribed.~ *lo *l1 Temperature compensation is again by thermistor, but since mercuric chloride solutions, being non-electrolytes, do not have the necessary temperature characteristics, a small amount of hydrochloric acid solution has to be added.Fig. 2 shows the degree of compensation achieved for an unused solution of mercuric chloride with 0-00046 per cent. of added hydrogen chloride, and for a solution with 0-00053 per cent. of added hydrogen chloride, with a particular value of shunt resistor. The graphs are of the percentage change in conductance as the temperature is changed by 0.5" C above and below the mean temperature of 25" C. I I U 3 - V s " -0.10- I 1 I I I I I -0.5 -0.3 -0. I 0. I 0.3 0 Temperature change, @C (Mean temperature, 25@C) 5 Fig.2. Degree of colnpcnsation for part-used (curve A) and fresh (curve B) solutions12 GREENFIELD, MOULE AND PERRY : CONDUCTIMETRIC [AndySt, VOl. 91 Since the thermostatically-controlled bath holds the temperature constant to -t 0.03" C, reference to Fig. 2 will show that the variation in meter reading over this range is +0.0007 per cent. ( + O . O l ohm in 1500) for unused solution and +0.006 per cent. (k0.09 ohm in 1500) for partially used solution. Both these are obviously within the limits of accuracy of reading the meter. CHOICE OF ELECTROLYTE- The requirements of a satisfactory electrolyte are ( a ) that it shall absorb all the phosphine from the sample; (b) that it shall react with the phosphine to give a product that will change the conductance of the solution, and (c) that this change shall be as large as possible.A number of possible reagents, including copper sulphate , cuprous chloride, silver nitrate, mercuric and mercurous nitrate solutions, has been tried, but each failed to satisfy one or more of these requirements. Mercuric chloride, however, absorbs phosphine well, even at low concentrations, e.g., 0.1 per cent. At the same time, being virtually un-ionised, it has a very low conductance. This entails the use of large electrodes to give conductances within the optimum working range of the meter, which in turn gives high sensitivity. In addition, the reaction with phosphine yields hydrogen and chloride ions in solution, which gives a large change in conductance for a small amount of phosphine.A disadvantage of mercuric chloride is that solutions of it do not have the desired negative coefficient of resistance, but this can be conferred by addition of a trace of hydro- chloric acid to make, say, a 0.00048 per cent. solution. Three reactions at least are possible, all giving rise to hydrogen Chloride, but one giving phosphorous acid in addition- The question arises of the reaction between phosphine and mercuric chloride. PH3 + 3HgC12 -+ P(HgC1)S + 3HCl . . . . * * (1) 2PH3 + 3HgC1, -+ P2Hg3 + 6HC1 . . . . * (2) * - (3) PH3 + 6HgC12 + 3H2O -+ HcJPO~ + 3Hg2C12 + 6HCl Reactions (1) and (2) both have the same yield of hydrogen chloride per mole of phos- phine, but reaction (3) not only gives twice as much hydrogen chloride, but also gives 1 mole of phosphorous acid.The conductance under these conditions would be much greater than for reactions (1) or (2). Small increments of 0-002 N hydrochloric acid were added to the cell, and the conductance change was noted each time. A graph was plotted of conductance change against weight of added hydrogen chloride, after correction for dilution of the absorbing solution. A sample of phosphine was diluted suitably with air and a known volume of this was injected by means of a gas-tight syringe into the carrier gas at such a rate that the concentration of phosphine in the gas entering the cell was 2.5 p.p.m. The conductance change was noted and compared with the calibration graph to give the weight of hydrogen chloride released in the solution. This was then converted to weight of phosphine according to each of the three equations.By this means the recovery of phosphine was 88 per cent. if equation (1) or (2) held, but only 33 per cent. if equation (3) held. From this it was concluded that equation (1) held, since the mercuric chloride was present in large excess. From previous experience, 88 per cent. purity was regarded as a reasonable figure; it does not represent low recovery, since a silver nitrate detector on the exit tube of the cell indicated no loss of phosphine over many determinations. The reaction that in fact takes place was found by the following technique. CALIBRATION PROCEDURE- Since the reaction between phosphine and mercuric chloride gives only hydrogen chloride , the cell can be calibrated by adding small known amounts of a suitable dilute hydrochloric acid solution and measuring the resulting conductance changes.It is obviously a simpler and more accurate procedure to add hydrochloric acid for calibration purposes than to use phosphine. A graph is then drawn of conductance change against weight of hydrogen chloride used; this should be a straight line. This permits a factor to be calculated for weight of hydrogen chloride in pg per ohm and hence weight of phosphine in pg per ohm from the equation- PH, + 3HgC12 -+ P(HgCl), + 3HCl.January, 19661 DETERMINATION OF MICROGRAM AMOUKTS OF PHOSPHINE I N AIR 13 It must be noted that the addition of hydrochloric acid not only increases the concen- tration of the hydrogen chloride, but also increases the volume of the solution.In practice, when phosphine is being determined, the solution volume is not increased, so the effective addition of hydrogen chloride, with respect to the original solution volume, must be calculated for each addition. It is this effective addition- where x = initial volume of solution in ml; Cy = sum of the increments of hydrochloric acid solutions in ml; and C = concentration of the added hydrochloric acid solution in pg per ml, that is plotted against conductance meter-readings. hydrogen chloride. DRIFT- As substantially dry gas (e.g., air or nitrogen) is passed through the cell, the conductance is found to increase at a constant rate. Most of the time this conductance drift is attributable to the slow reduction in volume of the solution as water is removed by the gas, but there remain occasions when the drift is somewhat higher owing to factors at present unknown.Correction is made for the drift by noting the change in conductance over a known period, with air or nitrogen passing through the cell at the normal rate (20 ml per minute). This permits a figure for drift in ohms per minute to be calculated, usually less than 0.1 ohm per minute. SENSITIVITY- The factors that might be expected to influence the sensitivity include (a) the initiaI concentration of the hydrogen chloride in the mercuric chloride solution, ( b ) the volume of the solution, (c) the electrode configuration, and (d) the measuring apparatus. The initial concentration of the hydrogen chloride can be shown theoretically to have no effect on the conductance change for a given amount of phosphine.Since, however, the measuring apparatus is at its most sensitive when working between 1000 and 3000 ohms, the initial concentration of hydrogen chloride must be chosen, having regard also to the electrode configuration, so as to give an initial conductance within this range. The volume of solution is, on the other hand, critically important, since, for a given amount of phosphine and a given electrode assembly, the conductance change is greater the smaller the solution volume. I t follows, therefore, that the initial volume of solution must always be measured as accurately as possible, so as to be identical with the volume used in the calibration. As regards the electrode configuration, the electrodes should be as large as possible, and as close together as possible, since this gives the maximum conductance change for a given amount of phosphine.The steady drift at any time can be calculated from the conductance-bridge readings over a suitable period. These readings have an uncertainty at the 95 per cent. confidence level of 50.3 ohm, which corresponds in our particular cell to 0.005 pg of phosphine. Since an upward displacement by 0.3 ohm of the drift due to phosphine can be detected, it follows that an air sample containing, say, 0.05 p.p.m. of phosphine could be analysed by taking a discrete sample of 66.6 ml. A continuously monitored sample of the same phosphine level would be detected, at 20ml per minute, after 3 minutes 20 seconds. INTERFERENCES FROM OTHER GASES- Any gas that reacts with mercuric chloride to give hydrogen chloride, or which dissolves in water to give ions, or which reacts with the hydrogen chloride, will give a conductance change and thereby interfere.For instance, ammonia causes a drop in conductance, but can be removed from the sample gas by passing it through granular calcium chloride. Sulphur dioxide, hydrogen sulphide, arsine and stibine cause increases in conductance; all these can be removed, without affecting the phosphine, by passage through a tube of potassium hydroxide pellets. The graph should be a straight line, and has been found to be so up to 12 pg of added Some slight curvature may be apparent at higher concentrations. By timing the sample determinations, corrections for drift can be made.14 GREENFIELD, MOULE AND PERRY [Analyst, Vol.91 METHOD The first time the cell is used, set the temperature compensation as previously described.9 Also linearise the conductance meter by the following technique. Substitute a decade box for the electrode assembly, and note conductance readings, S, for various values of resistance, R, on the decade box. Plot R values against 1/S values, when it will be found that the graph cuts the R axis close to, and below, the origin, at a point that may be called RL. This repre- sents the resistance of a choke in the cell arm of the bridge, which can be compensated for by a suitable shunt, S’, across the measuring arm of the bridge. To calculate the value of S’, substitute the value of RL in the formula: S’ = PQ/RL, where P and Q are the fixed arms of the bridge.Empty the cell and wash it well with water until the conductance is less than 50 ohms. Again empty the cell and blow it dry with filtered air; do not attempt to dry it with organic solvents. Fill the cell with exactly 11 ml of absorbing solution, with nitrogen flowing at 20ml per minute. Note the conductance when the temperature has stabilised, as denoted by a steady drift. Add successive 20-4 portions of 0.002 N hydrochloric acid, preferably by means of a Microcap disposable capillary, and note the steady conductance readings after each addition. Correct the conductance readings for drift and plot the conductance change for each effective addition against the weight of hydrogen chloride added. From the graph calculate a factor for the conductance change per pg of phosphine from the equation- PH, + 3HgC1, -+ P(HgCl), + 3HC1.The cell is now ready for use. USE OF THE CELL- Discrete samples of air containing small amounts of phosphine-A pressure sample con- tainer, as used by the National Coal Board, is suitable. Ensure that the two absorber tubes are not exhausted. Pass purified nitrogen at 20 ml per minute and note conductances over a period until a steady drift is indicated. Calculate the value of the drift in ohms per minute. Pass the contaminated air through the cell via a pressure-reducing valve and a flowmeter at 20 ml per minute for a timed period and note the conductance change. Correct for drift and find the weight of phosphine from the calculated factor.Discrete samples of air containing higher concentrations of phosphine-Fit a T-tube, having a serum cap on the leg of the T, into the line before the two absorber tubes. Pass purified nitrogen through the cell at 20 ml per minute and note conductances over a period until a steady drift is indicated. Calculate the value of the drift in ohms per minute. Inject a suitable volume of sample in small increments from a gas-tight syringe into the nitrogen stream via the serum cap, and note the conductance change. Correct for drift and find the weight of phosphine from the calculated factor as before. Continuous monitoring of contaminated air-The cell can be used as it stands for this purpose, but the conductivity bridge needs constant attention. One of us (S.G.), however, has used the bridge in a continuously recording form, which permits the cell to be used for continuous-monitoring purposes, although the cell must be recharged periodically with absorbing solution. Alternatively, a flow-through cell can be designed, which will give the phosphine content of the sample as a conductance reading, rather than as a change in con- ductance. Such a cell, however, would have a reduced sensitivity, and calibration with hydrochloric acid would be difficult. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. REFERENCES Filz, W., Mitt. chem. Forsch.Inst. Wirt. (?st., 1954, 8, 61. Strizhevskii, I. I., and Zaitseva, V. P., Zau. Lab., 1956, 22, 546. Lugg, G. A., Commonzeiealth of Australia Departwent of Supply, Defence Standards Laboratory, Hughes, J . G., and Jones, 4. T., Amer. Ind. Hyg. Assoc. J . , 1963, 24, 164. Nelson, J. P., and Milun, A. J., Analyt. Chew., 1957, 29, 1665. Kitagawa, T., and Ogawa, T., .J. Electrochem. Soc. Japan, 1951, 19, 258. German Patent 1,129,731, 1962; Chew. Abstr., 1962, 57, 3142. VaSAk, V., Chemicke‘ Jisty, 1956, 50, 1116. Greenfield, S., Analyst, 1960, 85, 486. Greenfield, S., and Smith, R. D., Ibid., 1962, 87, 875. -- , Ibid., 1963, 88, 886. Moneypenny, H. K., J . Scient. Instrum., 1949, 26, 10. -, G.K.N. Group Research Report No. 261. Report No. 258 (1962). Received February 12th, 1965

ISSN:0003-2654    

DOI:10.1039/AN9669100010

出版商:RSC    

年代:1966

数据来源: RSC

摘要:

January, 19661 THE ANALYST XVCLASSIFIED ADVERTISEMENTSThe rate for classijed advertisements is 7s. a line (or spaceequivalent of a line), with a n extra charge of 2s. for theuse of a Box Number. Semi-displayed classijiedadvertisements are 80s. for single-column inch.Copy required not later than the 20th of the month pre-ceding date of pzlblication which is on the 16th of eachmonth. Advertisements should be addressed to TheAnalyst, 47 Gresham Street, London, E.C.Z. Tel.:MONarch 7644.II I1BRITISH PHARMACOPOEIA COMMISSIONSCIENTIFIC ASSISTANTApplications are invited from graduates (pharmacy orLhemistry) with experience in pharmaceutical analysis forappointment to the staff of the Commission’s laboratoryworking on the investigation of analytical methods andspecifications for drugs.Commencing salary within theIange k950 to L1500 according to qualifications and experience.Applications, marked “Appointment”, giving full detail,to The Secretary, British Pharmacopoeia CommissionsGeneral Medical Council, 44 Hallam Street, London, W.1.UNIVERSITY OF BRISTOLDepartment of Inorganic ChemistryM.Sc. COURSE IN ANALYTICAL CHEMISTRY(With Special Reference to Instrumental Methods)October, 19M-67Applications are invited from persons who hold or expectto be awarded, an appropriate Honours degree or equivalentqualification. The Science Research Council has acceptedthe above course as suitable for the tenure of its AdvancedCourse Studentships.The aim is to develop a research approach to AnalyticalChemistry and investigations of certain problems will form amajor part of the course.Applications and enquiries should be sent to the Registrar,The University, Senate House, Bristol, 2, as soon as possible.UNIVERSITY OF SYDNEYLECTURSHIP IN PHARMACEUTICAL CHEMISTRYIN THE DEPARTMENT OF PHARMACYApplications are invited for the above-mentioned post.Applicants should have specialised in theoretical andpractical aspects of physical methods of analysis, particularlyin relation to drugs.Salary for a Lecturer is within the range LA2400 x 110-L3170 per annum.Information concerning superannuation, Housing Scheme,sabbatical leave etc., and method of application is obtainablefrom the Association of Commonwealth Universities (BranchOffice), Marlborough House, Pall Mall, London, S.W.l.Applications close in Australia and London on January 31st,1966.NALYST REQUIRED for Laboratory engaged inAFerrous and non-ferrous analysis Applicants preferablywith experience should be of O.N.C.Standard. ’Age 18-22years. Apply-Personnel Manager, Kent Alloys Limited,Temple Manor Works, Priory Road, Strood, Kent.CITY OF EDINBURGH PUBLIC HEALTHDEPARTMENTAPPOINTMENT OF DEPUTE CITY ANALYSTApplications are invited from persons qualified inaccordance with Public Analysts (Scotland) Regulations1956, for the post of Depute City Analyst and DeputeAgricultural Analyst. The post is permanent and pension-able subject to a medical examination by the CorporationMedical Referee. Salary scale A2190-f;2455 p.a.Applica-tions stating full personal details, qualifications and experiencetogether with the names andaddresses of three referees toThe Medical Officer of Health, Johnston Terrace, Edinburgh,1, within 14 days of this advertisement.MINISTRY OF AVIATIONEXPLOSIVES RESEARCH AND DEVELOPMENTESTABLISHMENTWaltham Abbey, EssexPHYSICIST or PHYSICAL CHEMIST required to takecharge of a polymer physics group concerned with themechanical and engineering properties of polymers. Thereis ample opportunity for fundamental research particularlyin the field of relating molecular structure to macroscopicproper ties.Appointment in the grade of:SENIOR SCIENTIFIC OFFICER &1809-L2220 (min. age 2G)SCIENTIFIC OFFICER L971-LlG39Superannuation under F.S.S.U.Starting salary according toexperience. Opportunities to compete for permanentpensionable posts.QUALIFICATIONS AND EXPERIENCE 1st or 2nd classhonours degree, Dip. Tech., or equivalent or higher qualifica-tion and, for S.S.O., a t least 3 years’ relevant postgraduateexperience.APPLICATION FORMS AND BOOKLET from S.M.(R)Room 301, Savoy Hill House, London, W.C.2, quotingreference HC/2D/1207.Closing date January 31st, 1966.DURHAM COUNTY COUNCILAPPOINTMENT OF DEPUTY COUNTY ANALYSTSalary Scale L1860-@145Applications are invited for the above appointment fromanalysts holding the Branch “E” Diploma.The County Analyst’s Laboratory is housed in newpremises a t the County Hall on the outskirts of DurhamCity. The County Council operate a five-day week; workingconditions are excellent.Rented housing accommodation isavailable in the New Towns of Newton Aycliffe and Peterlee,both of which are within easy reach. Assistance withremoval expenses will be given.Details of the appointment can be obtained from theCounty Analyst, County Hall, Durham, to whom applica-tions giving full dctails and names of two referees must besent by January 31st, 1966.J. T. BROCKBANK,Clerk of the County Council.RADIOACTIVATION ANALYSISDEPARTMENT OF PHYSICS,Borough Polytechnic, Borough Road, S.E. 1.A course of ten lectures on Tuesday evenings,commencing January 1 lth, 1966, will coverthe principles and practice of radioactivationanalysis.Specialist lecturers will discuss the scope andlimitations of the method, the special tech-niques involved and their applications indifferent fields.Leaflets giving details of the course andregistration forms are available from theSecretary.1959-65 RUN OF ANALYSTS.Offers Box 4083.Green’s lens cleaning Tissueis specially made for the jobWrite for sample 43 6105J. BARCHAM GREEN LTD.Hayle Mill, Maidstone, Kens v i THE ANALYST [January, 19661 Applications should be made in the first instance to:ANALYTICAL CHEMIST1 U N I I FVERR ES EA RC HThe Staff Officer, (ref. D2/ FFF),Unilever Research Laboratory,The Frythe, Welwyn, Herts.for Microbiology GroupThe successful candidate will be based in the Analytical Section of the Laboratoriesbut the work will be mainly with a Group studying the effects of micro-organisms onthe fertility of pulverised fuel ash.It will include the development of methods ofanalysis of ash and plant material with particular reference to the nitrogen cycle andthe production of carbohydrates. The Laboratories, which are located at Leatherhead,Surrey, are well equipped, and there will be opportunities to use modern methodsincluding radioactive tracer techniques. Staff are encouraged t o publish the resultsof their research.A n Honours Degree, or equivalent, is preferred.Salary, according to qualifications and experience, within ranges rising tomaxima of E1,675 or E2,035 p.a. (including allowances) with good promotionprospects.The successful candidate will be invited to discuss housing problems and may receiveassistance with removal or house purchase expenses.Applications stating age, qualifications, experience, present position and salary t o N .Berryman, Per-sonnel Officer (Headquarters), Central Electricity Generating Board, Sudbury House, I S Newgote Street,London, E . C . I . , as soon as possible. Quote Ref. A:446.Analytical Services ManagerA trained analyst, preferably with industrial experience in analyticalcontrol, is required to lead and develop a unit which will provide acomprehensive service. Applicants should have a degree or equiva-lent qualification and, as this is essentially a food research organisa-tion, the Diploma in Branch ‘E’ of the Royal Institute of Chemistrywould be desirable, but is not essential.The successful applicant will be directly responsible to the Head ofthe Laboratory for the provision of whatever chemical and physicalanalytical services are required by the research groups.He will alsobe responsible for the training of junior staff and for the introductionof new methods and techniques. He will be particularly concernedwith automatic methods of analysis and should have an interest indeveloping these where they do not at present exist. He will also berequired to take part in or initiate collaborative testing in co-opera-tion with other Unilever and non-Unilever Laboratories.UR98-314January, 19661 THE ANALYST xviiWOOLWICH POLYTECHNICLondon, S.E.18Department of ChemistryRECENT DEVELOPMENTS IN ANALYTICAL CHEMISTRYA special short course of six lectures on Thursday evenings at 7 p.m.January 20th-February 24th, 196620th January “A New Approach to Analysis-The Small Mass Spectrometer”Mr.R. D. Craig, A.E.I. Limited.27th January “Inorganic Chromatography”Dr. G. Nickless, University of Bristol.3rd February “Differential Thermal Analysis”Dr. R. C. Mackenzie, Macauley Institute for Soil Research.10th February “Dispersive and Non-Dispersive X-ray Fluorescence Spectrometry”Mr. J. B. Baird, Hilger and Watts Ltd.17th February “Recent Developments in Polarography”Dr. G. F. Reynolds, Shandon Scientific Co., Ltd.24th February “Infrared Spectroscopy”Dr. L. J. Bellamy, Ministry of Aviation.Fee for the course: El 5s. Od.Further details and forms of application may be obtained from the Secretary, AdmissionsDept.16, Woolwich Polytechnic, Wellington Street, London, S.E. 18.Allen & HanburysLimited, WarePharmaceutical AnalysisThe Control Division of Allen & Hanburys Ltd. wish to recruit graduate stafffor their Analytical Research and Analytical Control Departments.The Analytical Research Department provides a complete quantitative andanalytical service for the Company’s Research Division, and its work is closelybound up with the demand for, and investigation of, new drugs and pharma-ceutical products.The Analytical Control Department provides a complete analytical service forthe Company’s pharmaceutical and chemical production in which highly exactingstandards are required.The posts offer varied and interesting work for which both Departments arewell equipped, and would suit keen young graduates in chemistry or pharmacy,who wish to embark on a satisfying career in the pharmaceutical industry.Applications giving details of qualifications, etc., should be addressed to thePersonnel Manager, quoting reference no.PA.5xviii THE ANALYST [January, 1966CHEMISTAlbright & Wilson (Mfg) Ltd., Stratford, London E.15., require a Chemist in theQuality Control Laboratory. The position involves analytical work, supervision ofJunior Chemists and liaison with Production Departments.Applicants, aged 25 to 30, should have at least a Higher National Certificate in Chemistry,and must have some relevant industrial experience.A non-contributory Superannuation and Life Assurance Scheme is in operation.Please apply, in the first instance, to The Group Staff Officer, Albright & Wilson Limited,1 Knightsbridge Green, London S.W.l.(Ref. 495).The Chemistry and Materials Section of the AEI Harlow ResearchLaboratory requires the following senior staff :-with the use of Hilger Medium and Large instrunk&. 'His workwill include the development of spectrographic techniques applied toa wide range of metallurgical materials. Minimum qualification: HNC. SEN IOR ANALYST A senior analyst isneeded to work on - -_ ~ ~~development and applications of modern techniques. Experiencein metallurgical or inorganic analysis is preferred, but considerationwill be given to newly qualified persons (minimum qualification HNC)who have a genuine interest in analytical chemistry.These posts carry the usual staff benefits.Rented accommodationmay be made available in appropriate cases.Please write or telephone for application form to :-The Administrative Officer,Harlow Research Laboratory,ASSOCIATED ELECTRICAL INDUSTRIES LTD.West Road, Temple Fields, Harlow, Essex.Tel: Harlow 25293ANALYTICAL CHEMISTThis vacancy in our Development Laboratories a t Gravesend, Kent, requires a youngman up to the age of 25 years with H.N.C. qualifications.The job consists of non-routine analytical work covering a wide range of materials, e.g.metals, rubbers and plastics, and involves personal initiative and the use of moderninstrumental techniques.Suitable analytical experience would be preferred but we are prepared to train someonewith the required outlook.Excellent prospects and a realistic salary awaits the successful candidate.Applications to be made in writing, giving full particulars of education, experience, ageand salary required to:-TSIAdministrative Officer,Associated Electrical Industries Limited,Cable Group,Gravesend, Kent.A E January, 19661 THE ANALYST xixTECHNICON INSTRUMENTS COMPANY LTD.requireMICRO ANALYSTto take complete charge of sales, demonstrations, customer queries andafter-sales service of the new ULTRA MICRO C-H-N ANALYZER, in theUNITED STATES.The successful applicant will be based at the Company’s New YorkHeadquarters where a full sales organization, laboratory facilities andafter-sales staff are available.He will be trained in England and in Germany.Candidates should be under 40 years, with experience in elementalanalysis. A university degree or equivalent is essential.For the right man, this position is an opportunity to get in at the beginningof an exciting and fast expanding field. Continuing research will enlargethe present applications.Salary : Good-to be negotiated-plus benefits.Write, in confidence, with full details to :-Dr. H. W. Holy,Technicon Instruments Co., Ltd.,Hanworth Lane,Chertsey, Surrey.RESEARCHANALYSTAn analyst is required with experience of the application of modern instrumentaland classical methods of analysis in relation to foodstuffs and biochemistry.A goodunderstanding of spectroscopic methods (U.V. and I.R.) is essential and a knowledge ofchromatographic methods of separation would be an advantage. The work willinvolve both the application of standard methods of analysis and development of newmethods .The vacancy is in the Company’s Biological Research Division, Grangemouth,Scotland. Applicants should preferably be 25-30 years of age, and have an Honoursdegree or equivalent.Write in the first instance, giving brief personal details and quoting reference BR.9A,to J. K. Todd, Recruitment and Placement Branch, The British Petroleum CompanyLimited, BP House, Ropemaker Street, London, E.C.2xxiv THE ANALYST [January, 1966Official, Standardised and RecommendedMethods of AnalysisCompiled and Edited forTHE ANALYTICAL METHODS COMMITTEEOF THE SOCIETY FOR ANALYTICAL CHEMISTRYby S.C. JOLLY, €3. Pharm., B.Sc., A.R.I.C., M.P.S.Price S6 6s. netFOR more than 30 years The Society for Analytical Chemistry has been instru-mental in organising collaborative investigations into available methods ofanalysis. The reports of the various Sub-committees of the Analytical MethodsC.ommittee that have done this work have been published from time to time inThe Analyst. The Society has also actively participated in, and sometimesinstigated, collaborative investigations in association with government depart-ments, trade associations, and other learned societies, and the results havelikewise been reported in The Analyst. One group of methods-those for theAnalysis of Trade Effluents-was later republished in book form.Now all the methods of analysis devised and tested in these collaborativetrials have been collected together into the first British book of RecommendedMethods. The detailed procedures form the first part of this book.In 1951 the Society published a Bibliography of Standard, Tentative, andRecommended or Recognised Methods of Analysis, which has been out of printfor some time. A completely revised and considerably expanded new editionof this bibliography makes up the second part of this book.Members of the Society for Analytical Chemistry are entitled tobuy copies a t the special Members’ price of L4 4s. provided theyorder direct from:The Editor, The Analyst, 14, Belgrave Square, London, S.W. I .Remittances made out to “Society for Analytical Chemistry”must accompany Members’ orders.Published for the Society for Analytical ChemistrybyW. HEFFER & SONS LTD., PETTY CURY, CAMBRIDG

ISSN:0003-2654    

DOI:10.1039/AN96691BP013

出版商:RSC    

年代:1966

数据来源: RSC

摘要:

January, 19661 FORD 15 The Quantitative Determination of Benzoic Acid in Soft Drinks by Ion-Exchange Chromatography* BY M. A. FORD (Product Reseavch Department, Beecham Food and Drink Division, Coleford, Gloucestershire) A quantitative method for the determination of benzoic acid in soft drinks and their associated bases and compounds is described. The method consists essentially in the isolation of benzoic acid from the sample by means of ion-exchange chromatography on De-Acidite FF anion-exchange resin, and the subsequent estimation of the isolated preservative by ultraviolet spectrophotometry. The separation of benzoic acid from other constituents is discussed and a simple three-point correction formula is proposed to correct for non-specific background absorbance. Recovery of benzoic acid was satisfactory when the method was applied to a wide range of ready-to-drink beverages, concentrated soft drinks, and fruit bases and compounds.THE analysis of foodstuffs for benzoic acid has been the subject of many papers. The first authoritative monograph was published by Monier-Williams in 1927 and has long been accepted as the standard and recommended method of analysis2 for this preservative. Succeeding workers have omitted the initial “clean-up” stages recommended by Monier- Williams, and have simply extracted the benzoic acid with immiscible ~olvents.~ ,4 l5 l6 Deter- mination of the isolated acid has been made gravimetrically, after sublimation,l97 titri- metrically with a standard base4 and colorimetrically.8 9 9 Ultraviolet spectrophotometry was preferred by some worker^,^,^ $lo since it was more specific than simple titrimetric methods.However, this technique is vitiated when saccharin is present and diethyl ether has been used as the extractant. Recently, newer analytical techniques have been applied to the problem, and papers have appeared in which column chromatography,ll paper chromatography and electro- phoresis12 ?l3,l4 Since immiscible solvents tend to extract interfering materials and both Monier-\iilliams’s method and the more recent chromatographic techniques did not lend themselves to the type of rapid routine estimation envisaged, a suitable alternative was sought. In 1956, Davies and 0wenl8 reported the separation of benzoic acid from acetic and phenylacetic acids on a strongly basic anion-exchange resin, and 0.1 N hydrochloric acid in aqueous dioxan as the eluting solution.Preliminary experiments with a similar type of resin, De-Acidite FF, yielded promising results, and therefore a more detailed investigation was made with this resin. EXPERIMENTAL CHOICE OF ION-EXCHANGE RESIN- Two ion-exchange resins were examined during the investigation, namely, De-Acidite FF and Uowex 1 - X10. The former resin was preferred because of the slow flow-rate through the Dowes resin, apparently due to the high proportion of fine particles present, and because of an artifact that was eluted from this resin, causing an error in the spectrophotometric determination o f benzoic acid. As Davies and Owen did not specify the degree of cross- linking in the resin that they used, it was necessary to espcriment with the 3 grades that were available] viz., 2 to 3 per cent., 3 to 5 per cent.and 7 to 9 per cent. of cross-linking. I t was found that none of these variants affected the position at which benzoic acid was eluted, although cross-linking has a more pronounced effect on larger molecules. It was there- fore decided to adopt De-Acidite FF resin with 3 to 5 per cent. cross-linking and of 100 to 200 mesh, the latter being an acceptable compromise yielding maximum flow-rate for minimum tailing of the eluted peaks. Early experiments were conducted in the conventional manner, in which the resin was converted to the free-base form and the column developed with solutions of chloride ions either as hydrochloric acid or as sodium chloride. It was found that the use of the resin in the free-base form necessitated the use of reagents free from carbon dioxide because of and gas chr~matographyl~ 9 1 7 have been used.* Presented a t the meeting of the Society on Wednesday, March 31st, 1965.16 FORD : QUANTITATIVE DETERMIXATION OF BENZOIC ACID [.d IZdySt, i r O l . 91 its rapid absorption by strongly basic resins to yield the carbonate form of the resin. In addition to proving inconvenient, use of solutions of sodium chloride for elution led to the formation of a precipitate of sodium carbonate if the effluent from the column was allowed to stand for a short period. Strongly basic ion-exchange resins have the following ionic affinities in dilute solutionlg s20- Sulphate > citrate > bisulphite > chloride > carbonate > hydroxide.Exchange will only occur if the ion on the resin has a lower affinity for the resin than the ion in solution. If the resin is used in the free-base form, i.e., the hydroxide form, all anions will be exchanged. If however, the resin is used in the chloride form, then all anions except the carbonate and the hydroxide ions will be exchanged. The use of the resin in the chloride form also obviates the need for reagents free from carbon dioxide and the lengthy regeneration procedure that was previously necessary. CHOICE OF ELUTING SOLUTIOS- The preliminary experiments were carried out with Davies and Owen’s eluting solution, i.e., 0.1 ?j hydrochloric acid - 35 per cent. dioxan. This proved unsatisfactory for two reasons however, first dioxan is unsuitable as a spectrophotometric solvent as it absorbs light strongly at all wavelengths below 260 nm, and secondly, the use of 0.1 x hydrochloric acid caused marked disturbance of the resin bed owing to the formation of pockets of vapour.Reduction in the strength of the hydrochloric acid solution and elimination of the dioxan overcame these objections and recoveries of benzoic acid were satisfactory. However, when elution curves of the major constituents of soft drinks were prepared, it was found that, while most of the compounds were separated from benzoic acid, citric acid and malic acid failed to separate completely. Although neither of these acids possesses any appreciable absorption in the ultraviolet region, both are potential sources of error and should be excluded if possible.Early work had shown that sodium chloride solutions could also be used to develop the column. Satisfactory separation could be achieved with a 0.1 M solution of sodium chloride, but was accompanied by considerable tailing of the benzoic acid peak and a reduction in the amount of benzoic acid recovered. I t was thought that the low recoveries could be attributed to molecular adsorption effects, since the inclusion of an aromatic substituent in a molecule causes an acid to be retained more strongly than its aliphatic analogue. navies and Owen found that the use of mixed solvents considerably reduced this effect, and as ethanol is a more suitable spectrophotometric solvent than most, the effect of adding this solvent to the sodium chloride solution was investigated.In addition to the improvement in the average recovery of benzoic acid, shown in Table I, the tailing of the benzoic acid peak was reduced. TABLE I THE EFFECT OF ETHANOL CONCENTRATION ON THE RECOVERY OF BEKZOIC ACID Ethanol added, per ccnt. . . . . 0 10 20 40 50 75 I t was also found that the position at which benzoic acid emerged was altered, the acid appearing earlier with progressively increasing concentrations of ethanol. This is because solubility also has an effect and those acids that are more soluble in alcohol will emerge earlier, while the emergence of those that are more soluble in water will be retarded. I t was found necessary therefore to adjust the ethanol concentration of the sodium chloride solution, not only to yield satisfactory recoveries of benzoic acid, but also to achieve optimum separation from interfering compounds, and while a concentration of 50 per cent.ethanol gave satis- factory recoveries of benzoic acid, a concentration of 75 per cent. ethanol was necessary to effect complete separation from the other acids. CHOICE OF CONDITIONS FOR THE COLUMN- If amounts of citric acid of the same order as those found in soft drinks were added to the ion-exchange column, the breakthrough capacity of the resin was exceeded and citrate ions appeared in the early fractions of the effluent. In order to separate the relatively large amounts of citric acid from benzoic acid, a large column would be necessary with the resultant Benzoic acid recovered, per cent.. . 91.3 92.8 96.1 95.4 99.0 99.4January, 19661 I X SOFT DRINKS BY ION-EXCHANGE CHROMATOGRAPHY 17 decrease in flow-rate and increased time of analysis. This could be avoided by the prior removal of the bulk of the citrate ions, by making use of the fact that the calcium salts of citric and malic acids are insoluble in alcoholic solution while that of benzoic acid is soluble. Addition of calcium carbonate to a solution containing both citric acid and benzoic acid, and subsequent addition of ethanol to yield a final ethanolic concentration of 50 per cent., resulted in the precipitation of approximately 80 to 90 per cent. of the interfering acids, while an a\.erage of only 2-4 per cent. of the benzoic acid was lost. This loss is due to the cumulative errors caused bj- molecular-adsorption effects, sorption on the calcium citrate precipitate and manipulative losses.I t is therefore recommended that the calibration graph should be prepared under the same conditions. R y using the calcium precipitation technique, the size of the column could be reduced, and a colunin formed by transferring 5 g of moist resin to a chromatography column 1 cm in internal diameter gave satisfactory results. Under these conditions, with a constant-head reservoir of the type described by the author,21 a flow-rate of 1 ml per minute was readily attainable. The exchange capacity of De-Acidite FF is largely independent of pH, and although the pH of the sample is raised from about 3 to between 7 and 8 by the calcium precipitation technique, this has no effect on the exchange capacity of the resin.SPECTROPHOTOMETRY- The benzoic acid is eluted from the column in the form of its sodium salt, and it was decided to acidify the solutions of sodium benzoate in order to utilise the bathochromic displacement of the position o f the peak into the region in which the background absorbance is linear. The difference in the positions of A,. of sodium benzoate solutions in neutral and in acid solutions is shown in Fig.1. The use of acid concentrations from 0.2 N to 2 x for spectrophotometry had little effect on the spectrum of benzoic acid, but an increase in the hydrochloric acid concentration to 5 N caused a further shift in the position of Amax. with a concomitant decrease in intensity. ,4 final concentration of 1 N was therefore used in the method.Fig. 1. Ultraviolet spectra of solutions of sodium benzoate in neutral and acid media: curve A, acidified to N with hydrochloric acid; curve B, acidified to 5~ with hydrochloric acid; curve C, neutral solution The presence of variable background absorbance gave insufficiently precise results. Since the irrelevant absorbance was found to be linear over that portion of the spectrum in which the benzoic acid peak occurred, it was found possible to use a correction of the type proposed by Morton and Stubbs,22 and expanded by Allen.23 The general equation is-18 [Analyst, Vol. 91 For the proposed method the wavelengths of 220 and 245 nm were chosen, together with A,,,. of 230nm, since this is the range over which the background absorbance is virtually linear.Therefore- FORD : QUANTITATIVE DETERMINATION OF BENZOIC ACID 245 - 230 = o.6 245 - 220 K = and substituting this value in equation (1) and re-arranging, it becomes where E,,, E,,, and E,,, represent the observed absorbances at these wavelengths. Because of the necessity for the background absorbance to be linear, it is recommended that the “blank” spectrum should be determined initially, wherever practicable, although the irrelevant absorbance of a wide range of fruit products is in fact linear over this narrow range. During a study of product blanks it was found that certain of them exhibited selective absorbance at 245 nm, found to be due to ascorbic acid, part of which is eluted in the same fraction as benzoic acid.When the ion-exchange resin had been used in the free-base form, the ascorbic acid was broken down on the column and could not be detected in the effluent. However, with the use of the resin in the chloride form, this no longer occurred and, since ascorbic acid has a high molecular extinction coefficient (log E = 4-03), its potential inter- ference was great. Although the correction formula given above would eliminate most of the interference from the ascorbic acid, it would be more satisfactory if this interference could be eliminated entirely. If ascorbic acid is oxidised to dehydroascorbic acid, the absorb- ance at 245 nm disappears. This oxidation is readily accomplished by the addition of a small amount of copper ions to the solution, to catalyse the aerial oxidation of ascorbic acid.With this technique, the interference from this source was completely eliminated and the recovery of benzoic acid from products containing ascorbic acid improved. METHOD APPARATUS- Chromatography columns-All-glass chromatography columns, 1 cm in internal diameter, 30 cm in length, provided with a sintered-glass disc and a stopcock at the lower end, and a B14 standard ground-glass joint at the upper end. Constant-head reservoir-Details of the construction and use have been described by Ford.21 The reservoir should have a capacity of about 250 ml and be provided with a B14 standard ground-glass joint. Sintered-glass discs-1 cm in diameter, porosity 0. For a diagram of the complete apparatus, see Fig. 2. Spectrophotometer-A Unicam SP500 spectrophotometer was used throughout this ..(2) Eiir = E2, - (0.6 E,, + 0.4 E2,J .. investigation. REAGENTS- Analytical-grade reagents should be used wherever practicable. Ion-exchange resins-Permutit De-Acidite FF ion-exchange resin*, chromatographic grade, chloride form, 100 to 200 mesh, 3 to 5 per cent. average cross-linking. Prepare the resin by transferring it to a large sintered-glass funnel and washing it with distilled water until any foreign matter, which may be present, is removed. Remove the bulk of the water by suction and store the resin in a moist state. . Calcium carbonate. Ethanol, absolute, B.P. quality. Ethanol, 75 per cent. v/v. Eluting solution-Dissolve 5.85 g of sodium chloride in 250 ml of water, and when Hydrochloric acid, 5 N.Cupric sulphate, 0.2 $er cent. w l v . Standard benzoic acid solution-Dissolve 100 mg of benzoic acid in water and dilute * The Permutit Company Ltd. now produce De-acidite FF resin in an isoporous form in addition to the previously available macroporous type that was used for the investigation. The isoporous form of the resin is now denoted by the letters ‘I.P.’ after the code number, and it is necessary to specify De-acidite FF anion exchange resin ‘old type’ when ordering this material for use with this method. dissolved dilute to 1 litre with absolute alcohol and mix. the solution to 100ml with water.January, 19661 IN SOFT DRINKS BY ION-EXCHANGE CHROMATOGRAPHY 19 Ion-exchange resin Sintered glass discs 9 Fig. 2. Chromatographic apparatus PREPARATION OF ION-EXCHANGE COLUMN- Transfer 5 g of the moist resin to the chromatography column, and allow to settle.Then back-flush the resin bed with distilled water, stirring the resin with a glass rod to disperse any air bubbles that may be present. Again allow the resin bed to settle and pass 50mI of distilled water through the resin. When the level of the water has almost reached the resin, place a sintered-glass disc on the top of the resin bed to prevent disturbance of the column during additions. Finally allow the level of the water to drain to the top of the resin bed, and the column is then ready for use. The resin must be kept moist at all times, and a fresh column of resin must be used for each test. PREPARATION OF CALIBRATION GRAPH Transfer 0.5, 1.0, 2-0, 3.0, 4.0 and 5-0-ml aliquots of the standard benzoic acid solution (1.0 mg per ml), to 15-ml graduated centrifuge tubes containing 0-2 g calcium carbonate.Add water as necessary to adjust the volume of each to 5 ml, mix, then dilute to 9.6 to 9.8 ml with ethanol and again mix thoroughly with a small stirring rod. Wash the stirring rod and the sides of the tube with a small amount of 75 per cent. ethanol and adjust the final volume to about 10ml. Spin the sample in a centrifuge at 3000 r.p.m. for 2 to 3 minutes. Place a 25-ml measuring cylinder under the column and transfer the supernatant liquid from the centrifuge tube to the ion-exchange column prepared as detailed above, by means of a dropping pipette. Wash the precipitate successively with two 4-ml portions of 75 per cent.ethanol. Wash the stirring rod and sides of the tube with the diluted ethanol as before; ensure that the final volume is about 5ml on each occasion. Transfer the supernatant liquid from each washing, obtained by centrifuging the suspensions, to the column. When the final wash solution has drained to the level of the resin, elute the benzoic acid by adding 75ml of the eluting solution to the column, using the constant-head reservoir, and at a flow-rate of about 1 ml per minute. Continue until 25 ml of the effluent are collected, then replace the cylinder with a 50-ml graduated flask containing 15 ml of eluting solution, and continue collecting until the graduation is reached; in this way the succeeding 35ml of effluent are collected, and retained for analysis.Mix this portion of the eluate; transfer a 5-ml aliquot to a 50-ml graduated flask, and add 10 ml of 5 N hydrochloric acid and 1 ml of 0.2 per cent. cupric sulphate solution. Mix, and allow it to stand for 10 minutes, then dilute to 50 ml with water; simultaneously, prepare a blank by mixing 5 ml of eluting solution, 10 ml of 5 N hydrochloric acid, 1 ml of 0.2 per cent cupric sulphate solution and dilute the whole to 50 ml with water. Measure the absorbance20 FORD QUANTITATIVE DETERMINATIOK OF BENZOIC ACID [A?Zd$St, 1’01. 91 of the solutions at 220, 230 and 245 nm, in 1-cm fused-silica cells and with the blank as a reference solution. Substitute the observed absorbances in equation (2), i.e., EiiY = E,,, - (0.6 E220 + 0.4 E,,,) From the corrected readings construct a calibration graph relating corrected absorbance at 230 nm to concentration of benzoic acid in pg per 50 ml of solution.Calibration graphs prepared as described above have been found to be remarkably constant and only need checking at infrequent intervals. PROCEDURE- For soft drinks containing 100 to 800 p.p.m. of benzoic acid, transfer a 5-g aliquot of a well mixed sample to a 15-ml graduated centrifuge tube containing 0.2 g calcium carbonate and continue as described above, under “Preparation of Calibration Graph.” For bases and compounds containing 800 to 2000 p.p.m. of benzoic acid, dilute 20 g t o 40 g of a well mixed sample to 200 g with water, and mix thoroughly. Then transfer a 5-g aliquot of the diluted base or compound to a 15-ml graduated centrifuge tube containing 0.2 g of calcium carbonate and continue as described above, under “Preparation of Cali- bration Graph.” Similarly, correct the absorbance of the sample solutions as described above by using equation (2), and by reference to the calibration graph, determine the concentration of benzoic acid in the measured solution.Designate this concentration A pg per 50 ml of solution. CALC u LATION- For soft drinks containing 100 to 800 p.p.m. of benzoic acid, Benzoic acid content of sample in parts per million A x Volume of eluate x lo6 Aliquot of eluate x Weight of sample x lo6 A x 50 x lo6 5 x Weight of sample x lo6 10 A Weight of sample - - - - I For bases and compounds containing 800 to 2000 p.p.m. of benzoic acid, Benzoic acid content of sample in parts per million - A x Volume of eluate x Weight of diluted base x 106 Aliquot of eluate x Weight of aliquot of diluted base x Weight of base x lo6 5 x Weight of aliquot of diluted base x Weight of base x lo6 10 A x Weight of diluted base Weight of aliquot of diluted base x Weight of base - - A x 50 x Weight of diluted base x lo6 - - - RESULTS AXD DISCUSSION In order to test the accuracy and precision of the proposed method, recovery experiments were carried out on 12 differing types of fruit juice products and the results of those analyses are summarised in Table IT.For the method to be viable it is necessary for most of the principal constituents of fruit juice products to be separated from the benzoic acid, because the background absorbance must be linear.A study of the ultraviolet absorption spectra of citric acid, malic acid, lactic acid, ascorbic acid, saccharin, cyclohexylsulphamic acid (cyclamate), sugars, quinine, naringin, artificial and natural colours and inorganic ions, revealed that all except cyclo- hexylsulphamic acid, the sugars and inorganic ions possessed appreciable absorbance in the region under examination. Reference to Table 111 shows that the substances that would cause the most serious interference are ascorbic acid, saccharin, quinine and naringin, while citric acid, malic acid and, to a lesser extent, lactic acid would cause slight disproportionate errors at 220 and 230nm.January, 19661 IN SOFT DRINKS BY ION-EXCHANGE CHROMATOGRAPHY 21 TABLE I1 THE RECOVERY OF BENZOIC ACID ADDED TO SOFT DRINKS, FRUIT BASES AND COMPOUNDS Product p.p.m.p.p.m. Orange base . . . . . . 1796 1779-1807 Orange drink . . . . . . 558 548-562 Orange drink . . . . 154 141-1 60 Carbonated orange drink . . 100 90-96 Benzoic Benzoic acid acid Number Average Standard added, recovered, of deter- recovery, deviation, :r cent. p.p.m. 99.7 9.7 99.6 4.0 00 4-0 93 1.8 Lemon base . . .. . . 1799 Lemon drink . . .. .. 441 Bitter lemon drink .. . . 425 Lemon barley drink . . . . 423 Carbonated bitter lemon drink . . 100 Glucose drink . , . . . . 216 Apple compound . . . . . . 601 Blackcurrant juice drink . . . . 160 1 768-1 8 1 7 428-432 405-4 14 405-413 90-94 2 10-2 14 593-600 150-152 minations I 12 12 6 12 6 6 6 6 6 6 6 6 99.6 97.6 96.3 96.9 92 98.5 99.1 94.3 19-8 2.1 3.5 3-0 2.0 1.5 2.5 0.8 TABLE I11 MOLECULAR EXTIKCTION COEFFICIENTS OF THE PRINCIPAL CONSTITUENTS OF SOFT DRINKS Molecular extinction Substance coefficient Benzoic acid .. . . 11350 Citric acid . . . . . . 2 00 Malic acid . . . . .. 160 Lactic acid . . . . . . 70 ,Ascorbic acid . . . . 10640 Saccharin . . . . . . 29670 Quinine . . . . . . 31540 29920 Naringin . . . . .. 26820 16350 Wavelength, nm 230 209 209 209 245 205 208 250 212 286 The elution diagrams of these principal constituents were determined and are as shown in Fig. 3. Most of the compounds were separated satisfactorily with the exception of lactic acid and naringin, while malic acid and ascorbic acid were incompletely separated. 2.0 Fll.jTl 0.2 \ - ; I 2. rl 0. 0 .- U e + - L I00 I00 100 I00 100 - 0 E M .- E g 1.0 ~ o ~ ~ ~ U Y E 8 5 C U Volume of eluate, ml Fig.3. Elution curves of the principal constituents of soft drinks: (u) benzoic acid; ( h ) lactic acid; (c) ascorbic acid; (a) quinine; (e) naringin; (f) citric acid; (g) malic acid; (h) saccharin The interference from ascorbic acid is overcome by oxidising this compound to dehydro- ascorbic acid, and since both malic acid and lactic acid have low molecular extinction coeffi- cients, concentrations of these two acids several times higher than those encountered in practice can be tolerated without causing interference. Similarly, in the case of nuingin,22 FORD [Analyst, VOl. 91 those concentrations which are normally present in grapefruit drinks do not cause any error in the spectrophotometric estimation of benzoic acid, although it is possible that samples containing an inordinately high concentration of this flavonoid may be subject to erratic results.CONCLUSIONS It has been shown that the use of ion-exchange chromatography with mixed solvents, together with the use of ultraviolet spectrophotometry provides a rapid and precise method of analysis for benzoic acid, the time required being approximately 2 to Z& hours. The accuracy and precision that is obtainable is considered satisfactory for the routine type of analysis required, and the basic method can be used for the estimation of benzoic acid in soft drinks and their associated products in the range of 100 to 2000 p.p.m. I thank the Directors of Beecham Food and Drink Division for permission to publish this paper, and also my colleagues, Mr.A. G. Wells for his advice and encouragement, and Mr. D. C. Cook for his valuable technical assistance. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. REFERENCES Monier-Williams, G. W., “The Determination of Benzoic Acid in Foodstuffs,” H.M. Stationery Office, London, 1927. Jolly, S., Editor, “Official, Standardised and Recommended Methods of Analysis,” The Analytical Methods Committee of the Society for Analytical Chemistry, London, 1963. Horwicz, W., Editor, “Official Methods of Analysis of the Association of Official Agricultural Chemists,” Ninth Edition, The Association of Official Agricultural Chemists, Washington, D.C., 1960, p. 384. Hadorn, H., Mitt. Lebensml Hyg., Bern., 1951, 42, 226; Chem. Abstr., 1951, 45, 9195a. Englis, D. T., Burnett, B. B., Schreiber, K. A., and Miles, J. W., J . Agric. Fd Chem., 1955,3, 964. Stanley, R. L., J . Ass. 08. Agric. Chem., 1960, 43, 587. Rathenasinkam, E., Analyst, 1962, 87, 298. Davey, W., and Gwilt, J. R., J . Appl. Chem., 1954, 4, 413. Spanyar, P., Kevei, E., and Kiszel, M., 2. LebensmittUntersuch., 1958, 107, 118; J . Sci. Fd Jaulmes, P., Mestres, R., and Mandrou, B., Ann. Falsif., 1961, 54, 84; Anal. Abstr., 1961, 8, 3942. Van Dame, H. C . , J . Ass. 08. Agric. Chem., 1960, 43, 593. Komoda, T., and Takeshita, R., Shokuhin Eiseigaku Zasshi, 1961, 2, 72; Chem. Abstr., 1962, 57, -- , Ibid., 1962, 3, 374; Chem. Abstr., 1964, 60, 6130d. Goddijn, J. P., 2. LebensmittUntersuch., 1961, 115, 534; Anal. Abstr., 1962, 9, 3446. Fellegiova, M., Ibid., 1963, 120, 17; J . Brit. Fd Mfg Inds. Res. Ass. Abstr., 1963, 16, 1087. Kovacs, A. S., Denke, P., Wolf, H. O., Ind. Obst-Gemueseverwert, 1962, 47, 547; Chem. Abstr., 1963, 59, 3256c. Goddijn, J. P., van Praag, M., and Hardorn, H. J., 2. LebensmittUntersuch., 1963, 123, 300; J . Brat. Fd Mfg Inds. Res. Ass, Abstr., 1964, 17, 553. Davies, C. W., and Owen, D. R., J . Chem. Soc., 1956, 1681. Samuelson, O., “Ion Exchangers in Analytical Chemistry,” John Wiley and Sons Inc., New York, “Properties of, and Instructions for using Zeo-Karb 225 and De-Acidite FF,” The Permutit Co. Ford, M. A., Lab. Practice, 1963, 12, 1093. Morton, R. A., and Stubbs, A. L., Analyst, 1946, 71, 348. Allen, W. M., J . Clin. Endocrinol., 1950, 10, 71. Agric., 1958, 9, ii-202. 6369g. 1953. Ltd., London, 1961. Received June 30th, 1965

ISSN:0003-2654    

DOI:10.1039/AN9669100015

出版商:RSC    

年代:1966

数据来源: RSC

摘要:

January, 19661 KIRKBRIGHT, WEST AND WOODWARD 23 Spectrofluorimetric Determination of Microgram Amounts of Scandium Part 11. Separation by Solvent Extraction BY G. F. KIRKBRIGHT, T. S. WEST AND C. WOODWARD (Chemistry Department, Imperial College, London, S. W.7) The spectrofluorimetric method previously described for determining scandium and based on the use of salicylaldehyde semicarbazone as reagent has been modified by incorporation of a solvent extraction separation pro- cedure to permit the determination of scandium in the presence of the few other ions that also produce fluorescent species, z.e., aluminium, yttrium, etc. Chromium(m), titanium(1v) and zirconium produce low results, while tellurium(1v) and tin(1v) completely suppress the fluorescence of the scan- dium complex.IN a previous communication to this Society1 we have described the spectrofluorimetric determination of microgram amounts of scandium by the use of salicylaldehyde semicarbazone. This reagent forms a blue fluorescent complex with scandium (excitation maximum, 370 mp; fluorescence maximum 455 mp) ; the reagent shows a less intense blue-green fluorescence (325 mp; 470 mp). At pH 6.0 this reagent may be applied to the determination of scandium in test solutions down to 0.002 p.p.m. (0.2 pg). Maximum complex fluorescence is developed within a few minutes, and is then constant for more than 24 hours. Continuous variations, mole-ratio and slope-ratio studies indicate the formation of a 1 to 1 metal-to-reagent complex with a conditional stability constant of lo5.Although the sensitivity of this method cannot be compared directly with that of other methods unless similar instrumentation is used, the determination of scandium with this reagent appears to be as sensitive as with morinJ2 the most widely used fluorimetric reagent for scandium, and considerably more sensitive than any available absorptiometric method^.^ 9 4 s5 The method, as described previously, was subject to several interferences that could not be eliminated by the use of common masking agents. It was, therefore, necessary to devise a procedure for separating scandium from the more important of these interfering ions and for getting it into a state suitable for determination by the proposed method. Separation procedures involving co-precipitation of scandium hydroxide on cadmium hydroxide or extraction of the thiocyanate into ether were investigated, but were found to be unsatisfactory.The double-extraction procedure finally adopted is based on the work of Eberle and Lernel3 and can be used for microgram amounts with a recovery of about 90 per cent. in the range 4.5 to 22.5 pg of scandium. Removal of iron(m), vanadium(v), etc., by extraction of their cupferrates from 10 per cent. hydrochloric acid into chloroform is followed by extraction of scandium from concentrated hydrochloric acid into tri-n-butyl orthophosphate and back- extraction into water. This procedure provides an efficient separation of scandium from aluminium and yttrium which, like scandium, form blue fluorescent complexes with salicyl- aldehyde semicarbazone.METHOD APPARATUS- Fluorescence measurements were made with a Farrand spectrofluorimeter, provided with a 150-W Xenon lamp, in which grating monochromators were used to isolate both the exciting and fluorescent radiation. Slits of 20-mp and 10-mp bandwidth were used in the excitation and analysing monochromators, respectively, and fused-quartz cells (10 x 20 x 50 mm) were used throughout. A 7-54 filter (transmittance greater than 75 per cent. over the range 275 to 375 mp) was used for the excitation radiation. The adsorption of scandium on the calibrated flasks used in these experiments was inhibited by treatment with Repelcote as described previous1y.l24 KIRKBRIGHT, WEST AND WOODWARD SPECTROFLUORIMETRIC [Analyst, Vol. 91 REAGENTS- Standard scandium solution-Boil 0.0690 g of scandium oxide, Sc,03, (high purity grade, Johnson, Matthey and Co.Ltd.) with 30 ml of concentrated hydrochloric acid, analytical- reagent grade, until fully dissolved. Cool and dilute to 1 litre with distilled water. This stock solution is 1 0 - 3 ~ in scandium and may be diluted to l o - 4 ~ when required. 1 ml M solution = 4-50 pg of scandium. Solutions may be standar&sed by EDTA titration at pH 3 to 5 with xylenol orange as indicator.6 Solutions of scandium more dilute than 1 0 - 4 ~ should not be stored because of losses due to adsorption on the glass-ware. M solution, dissolve 0-1788 g of salicylaldehyde semicarbazone (prepared by condensation of salicylaldehyde with semicarbazide hydro- chloride1) in 1 litre of analytical-reagent grade ethanol.Bz@er solution, pH 6-0-Dissolve 100 g of hexamine (general-purpose reagent grade) in 800 ml of water and add hydrochloric acid until the pH reaches 6.0. Dilute to 1 litre with water. Diverse ions-10-2 M solutions of analytical-reagent grade salts. Cz$ ferron (analytical-reagent grade)-Freshly prepared 1 per cent. w/v aqueous solution. Hydrochloric acid (analytical-reagent grade), sp.gr. 1.18. Chloroform (analytical-reagent grade). Tri-n-butyl orthophosphate (general-purpose reagent grade). Ammonia solution (analytical-reagent grade), sp.gr. 0.88. PROCEDURE- To an aliquot of solution containing 4.5 to 22.5 pg of scandium in a 250-ml separating funnel, add 2.5 ml of concentrated hydrochloric acid and 5 ml of 1 per cent. aqueous cupferron solution.Dilute to 25 ml with water, extract twice by shaking the funnel for 1 minute with 25 ml of chloroform and discard the organic extracts. Transfer the aqueous phase to a 100-rnl beaker, rinse the funnel with distilled water, add the washings to the beaker and then evaporate to a volume of approximately 5 ml. Cool the solution and transfer it to a 250-ml separating funnel containing 25 ml of tri-n-butyl orthophosphate. Rinse the beaker with 25 ml of concentrated hydrochloric acid and put this into the separating funnel. Shake the funnel for 1 minute, allow the phases to separate and discard the aqueous layer. Wash the organic phase twice with 25 ml of concentrated hydrochloric acid and discard the washings. Back- extract the scandium into 25 ml of water containing 1 ml of ammonia solution (see Note 1). Separate the phases and shake the aqueous phase twice with 25 ml of chloroform to remove any tri-n-butyl orthophosphate that remains ; discard the chloroform extracts.Transfer the aqueous layer to a 100-ml beaker, add the washings from the funnel and 10 ml of ammonia, cover the beaker with a watch-glass and evaporate the contents to approximately 20ml. Cool, re-dissolve the crystalline ammonium chloride in water and transfer it to a 100-ml calibrated flask containing 2ml of 10 per cent. hexamine buffer (see.Note 2). Add 25ml of 10-3 M reagent solution, dilute to 100 ml with water, and measure the fluorescence intensity after 30 minutes at 455 mp, with an excitation wavelength of 370 mp. The graph of fluorescence intensity against the amount of scandium is linear from 4.5 to 22.5 pg of scandium and passes slightly above the origin (see Fig.1). Prepare and measure two standards with each group of samples, by taking 0 and 22.5 pg (5 ml of M solution) of scandium through the procedure described above. Reagent solution-To prepare a The complete separation and determination of a set of 8 samples takes 6 to 7 hours. NOTE 1. NOTE 2. The aqueous extract should still be acid to litmus a t this stage. pH adjustment is extremely critical in this method. This combination of ammonium The pH tolerance as established chloride and hexamine consistently produces pH 6-00 & 0.02. previously' is 10.05 units. DISCUSSION The more important interfering cations found previously1 were taken, normally in 100-fold molar excess over 22.5 pg of scandium, through the recommended extraction pro- cedure.This investigation showed that the following ions do not interfere appreciably under the conditions described previously : Al, Bi, Ce(Iv), CU(II), Fe(m), Ga, U(VI), V(v) and Y. The only ions investigated that still interfere are Cr(m), Results are shown in Table I.January, 19661 DETERMINATION OF MICROGRAM AMOUNTS OF SCANDIUM 25 Sn(Iv), Te(Iv), Ti(1v) and Zr. I t has already been shown1 that Ba, Re, Ca, Cd, Ce(m), Hg(II), K, Li, Mg, Mn(II), Na, NH,, Pb, Pt(Iv), Sr and T~(I) do not interfere, and the interference of Ag, CU(II) and Ni can be eliminated by masking with cyanide, and that of Zn by masking with o-phenan t hroline. Scandium, pg Fig. 1. Calibration graph of fluorescence intensity (scale reading) against weight of scandium, after estraction procedure TABLE I DETERMINATION OF 22.5 pg OF SCANDIUM AFTER SEPAKATION FROM VARIOUS Foreign ions present, te Iron(II1) (2,790) Iron(m) (2,790) + Ce(;v) (7,000) Yttrium (890) .. . . . . Yttrium (890) . . . . . . Aluminium (270) . . . . Aluminium (270) . . . . Bismuth (10,400) . . . . Copper(I1) (3,170) . . . . Uranium(v1) (11,900) . . . . Vanadium(v) (2,550) . . . . Gallium (3,500) . . . . Chromium (111) (2,600) . . Titaniuni(1v) (2,400) . . . . Zirconium (4,500) . . .. Tellurium (IV) (6,400) . . Tin(1v) (5,900) . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . IONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Scandium found, CLg 21.4 23.1 23.1 22.8 21.4 22.7 21.7 22.1 21.7 21.8 22.2 18.2 19.1 12.6 0 0 Error, per cent.+ 2.7 + 2.7 + 1.3 - 5.0 + 1.1 - 3.5 - 1.8 - 3.5 - 3.0 - 0.7 - 19 - 15 - 44 - 100 - 100 - 4.8 PRECISION- The standard deviation of the combined separation and determination procedures was determined by taking 8 solutions, each containing 22-5 pg of scandium without any foreign ions, through the proposed method. I t was found to be equivalent to &6-2 per cent. of the response due to the scandium complex. The standard deviation calculated from the first 11 results in Table I is & 3.1 per cent. However, all of these determinations were carried out in duplicate and compared with standards prepared simultaneously, and sometimes an obviously discrepant result was rejected. MAGNITUDE AND ORIGIN OF BLANK FLUORESCENCE- It can be seen from Fig.1 that the fluorescence of the blank is approximately equivalent to that of 4 pg of scandium. This blank result is caused by pick-up of an unidentified sub- stance during the extraction procedure, leading to the production of a purple fluorescence on addition of the reagent (excitation maximum, 350 mp; fluorescence maximum, 440 mp), which is unlike that produced by any metal ion investigated. The magnitude of this blank26 KIRKBRIGHT, WEST AND WOODWARD [Analyst, VOl. 91 fluorescence governs the range of concentrations over which scandium can be determined with a satisfactory signal-to-blank fluorescence ratio. Below 4.5 pg of scandium the relative intensity of this blank vitiates results. The most likely source of this impurity is the tri- n-butyl orthophosphate, which cannot be obtained in analytical-reagent grade purity, and is not readily purified by redistillation. The final aqueous solution was also found to contain a small amount of phosphate ion, probably from hydrolysis of the solvent during the extraction procedure, but the amount was not enough to produce significant interference in the deter- mination of scandium. We are grateful to the Science Research Council for the provision of a research student- ship for one of us (C.W.) and also for the award of a special research grant (to T.S.W.) for the purchase of the spectrofluorimeter. REFERENCES 1. Kirkbright, G. F., West, T. S., and Woodward, C., “Proceedings of the SAC Conference, Not- 2. Sandell, E. B., “Colorimetric Determination of Traces of Metals,” Third Edition, Interscience 3. 4. 5. 6. tingham,” W. Heffer & Sons Ltd., Cambridge, 1965, p. 474. Publishers Inc., New York, 1959, p. 800. Eberle, A. R., and Lerner, M. W., Analyt. Chem., 1955, 27, 1561. Umland, F., and Puchelt, H., Analytica Chim. A d a , 1957, 16, 334. Macdonald, J. C., and Yoe, J. H., Ibid., 1963, 28, 264. Korbl, J., and PPibil, R., Chemist-Analyst, 1956, 45, 102. Received June 14th, 1965

ISSN:0003-2654    

DOI:10.1039/AN9669100023

出版商:RSC    

年代:1966

数据来源: RSC

摘要:

January, 19661 STOCK 27 The Amperometric Titration of Submillinormal Concentrations of Copperh) with Mercuryh) Perchlorate BY JOHN T. STOCK (Department of Chemistry, The University of Connecticut, Stows, Connecticut, 7J.S.A .) Although quite precise under rigidly controlled conditions, the mercury(1) amperometric titration, a t a rotating platinum electrode, of submillinormal concentrations of copper (11) in potassium thiocyanate - perchloric acid medium gives results that vary with the concentrations of thiocyanate and of copper(r1). When this variation is eliminated by the addition of potassium iodide, the titration of 1 0 - 5 ~ to ~ O - * N copper(I1) is precise and accurate to within & 1.5 per cent. The titration of N copper(r1) is precise to about 5 per cent. CENTIGRAM amounts of copper(1r) have been determined by titration in acidified thiocyanate medium with mercury(1) nitrate to a vi~ual~,~1~1* or a potentiometricl 9 2 9 4 end-point.Iron(II1) at a concentration as low as N can be determined by mercury(1) amperometric titration at a rotating platinum ele~trode.~ The present work concerns the titration by this ampero- metric method of submillinormal concentrations of copper(I1). VOLTAMMETRY- A deoxygenated 0.3 N potassium thiocyanatc - 0.02 N perchloric acid medium that had been freshly made by mixing solutions of these reagents gave a residual current of less than 0.1 pA at a new rotating electrode held at a potential of zero (unless otherwise stated, all potentials are with respect to the saturated calomel electrode, S.C.E.). The introduction of potassium iodide up to a concentration of approximately 0.06 N had no significant effect on this current.Both in the presence and absence of potassium iodide, the limiting current of copper(II), measured at zero potential, was found to be proportional to the concentration of this ion. The small anodic current given by mercury(1) in acid thiocyanate medium5 was also obtained in the presence of potassium iodide. Fig. 1 shows current - voltage curves obtained at various stages in the amperometric titration of 5-8 x N copper(I1) in acid thiocyanate - iodide medium with mercury(1) perchlorate solution. The anodic current due to an excess of titrant often increased markedly with time. However, when observations were completed within a few minutes, the anodic current was usually almost unaffected by increasing the mercury@) concentration. AMPEROMETRIC TITRATIONS- Initially, the platinum electrode was stored overnight in concentrated nitric acid and then pre-conditioned by immersing in de- oxygenated 0.1 PI; perchloric acid and short-circuiting to a saturated calomel electrode." After some weeks, a low residual current became difficult to attain.When the daily treatment of a new electrode with nitric acid was reduced to a brief immersion, the residual current remained low and was essentially unchanged after 4 months. to 1 0 - 4 ~ copper(I1) in acid thiocyanate medium by the pre-addition technique, similar to that used in the titration of iron(111),~ gave results that were precise to within about 3 per cent.However, the stoicheiometry of the titration reaction appeared to change when the concentration of either copper(I1) or thiocyanate was altered. Such a change was not observed when potassium iodide, up to a concentration of about 0.05 N, was introduced into the titration medium. METHOD REAGENTS- EXPERIMENTAL All titrations were carried out at zero potential. Preliminary titrations of Use analytical-grade reagents and distilled or demineralised water throughout. Mercury(1) perchlorate, approximately 0.01 N in N perchloric acid-Prepare this as des- Store over cribed by Berka, Vulterin and Z3ka,4 but dilute 10-fold with N perchloric acid.28 STOCK: AMPEROMETRIC TITRATION OF SUBMILLINORMAL [Analyst, Vol. 91 metallic mercury. Standardise by titration with copper(I1) sulphate solution as described in the Procedure.Prepare this from the metal by dissolution in the minimum amount of nitric acid, elimination of nitrogen oxides, and suitable dilution with approximately 0.1 N sulphuric acid. Copper(I1) sulphate, 0.01000 N (=0-01000 M) in 0.1 N sulphuric acid. Perchloric acid, approximately 0.6 N (Procedure A ) or N (Procedure B). Potassium thiocyanate, approximately 0.6 N (Procedure A) or saturated (Procedure B) . Potassium iodide, approximately N in boiled-out water. APPARATUS- Except for an agar - potassium nitrate salt bridge, use conventional apparatus for amperometric titration at a rotating platinum electr~de.~?~ At the end of each day, rinse the platinum electrode with water, rotate it in concentrated nitric acid for approximately 1 minute, rinse it again, then remove most of the water with a filter-paper strip and leave to dry.At the beginning of the day, pre-condition the electrode by running a preliminary titration of copper(I1) until the titrant is present in slight excess. I t then had a sensitivity of 146 pA per millimole of silver(1) per litre, measured at zero potential in deoxy- genated 0.1 N perchloric acid at 23" C. Titrations were carried out at room temperature (in the range 23" to 25" C). Electrical measurements were made with a Cambridge Voltamo- scope. Since all titrations are carried out at zero potential, this instrument can be replaced by a Cambridge Spot galvanometer or similar instrument. In the present work, the platinum electrode was rotated at 500 r.p.m.6 5 4 ; 3 c E 3 2 I C - +O +0.3 +0.2 + O . I 0 - 0.1 -0.2 -0.3 Po te t i t ial, volts Fig. 1 . Current - voltage curves a t stages in the titration of 58 p~ copper(I1) in 0.3 N potassium thiocy- anate - 0.02 N perchloric acid - 0.02 N potassium iodide. Percentage equivalent of mercury(1) pmchlorate added : curve A, 0; curve B, 50; curve C, approximately 100; curve D, 150 PROCEDURE- (A) Transfer 25 ml each of 0.04 N perchloric acid and 0-6 N potassium thiocyanate to the titration cell, then add 1 ml of N potassium iodide. Insert the platinum electrode and salt bridge, set the applied e.m.f. at zero, deoxygenate with a stream of nitrogen, then stop the gas stream. Inject 0.01000 N copper(I1) sulphate so that the amount of copper(I1) intro- duced is about 20 per cent.of that contained in the sample solution. After 2 minutes, note the current reading, P, then inject the sample solution. Read the current after aJanuary, 19661 CONCENTRATIONS OF COPPER(II) WITH MERCURY (I) PERCHLORATE 29 further 2 minutes, then titrate with 0.01 N mercury(1) perchlorate until the current has fallen nearly to zero. Allow an interval of 1 minute between a titrant addition and the reading of the current. Find the end-point graphically as the intersection of the linear portion of the titration curve and the line: current = P. (B) Transfer the sample solution to the titration cell and make it approximately 0.3 N in potassium thiocyanate and 0.02 N in perchloric acid by adding saturated and N reagent solutions respectively. Insert the platinum electrode and salt bridge, but do not close the circuit until the solution has been deoxygenated.Then stop the gas stream and at once make the solution 0.02 N in potassium iodide by adding to it a N solution of this reagent. Read the current after 2 minutes, then titrate as in (A). Determine the residual current, R, in a deoxygenated 0-3 N potassium thiocyanate - 0-02 N perchloric acid - 0.02 N potassium iodide medium freshly made from the concentrated solutions. Find the end-point graphically as the intersection of the linear portion of the titration curve and the line : current = R. RESULTS N copper(I1) ion in a given iodide-free potassium thiocyanate - perchloric acid medium showed that Procedure A gave results that were precise to within &2 per cent.Results obtained by a conventional method involving extrapolation of the arms of an L-shaped titration curve8 were less precise than, and signifi- cantly different from, those given by Procedure A. Since current readings in the post- equivalence region were time-dependent, all observations within this region were completed within a total time of about 4 minutes. Under conditions of small and essentially constant residual current, end-point determination, by producing the linear portion of the titration curve to cut the residual-current line,g gave results similar to, but slightly less precise than, those of Procedure A. Typical sets of 6 results, here expressed as the average apparent normality and standard deviation of the mercury(1) titrant solution, obtained at thiocyanate and perchloric acid concentrations of 0.3 and 0.02 N, respectively, were 0.0129 0.00014 by Procedure A, 0.0127 0.00019 by the L-curve method, and 0-0128 &- 0.00016 by the residual-current method. At this concentration of thiocyanate, titrations at various acidities up to 0.08 N gave similar results.Although the results were insensitive to changes in acidity, they were markedly dependent upon the concentration of thiocyanate. This is illustrated by the average results of the titra- tion of 5 x N copper(r1) that are listed in Table I. Titrations at high thiocyanate con- centration were run at low acidity to avoid rapid decomposition of the medium. As deter- A systematic examination of the titration of TABLE I EFFECT OF THIOCYANATE CONCENTRATION IN THE TITRATION OF 5 x 1 0 - 5 ~ COPPER(II) Apparent mercury (I j normality Thioc yanate concentration, 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.5 N Perchloric acid concentration, 0.02-0.1 ti 0-02-0*08 0-02-0-06 0.02-0-05 0.02-0-05 N 0*02-0.05 0*02-0.05 0.0 1-0.03 0.01-0.03 0.01 Number of runs 7 5 3 2 3 3 3 3 3 1 method A 0.0122 0.0123 0.0125 0.0127 0.0133 0.0135 0.0138 0.0150 0.0153 0.0 195 L-curve method 0.01 15 0.0118 0.01 19 0.0121 0.01 24 0.01 29 0.0132 0.0141 0.0143 0.01 80 1 Residual-current method 0.0120 0.0123 0-0123 0-0127 0.01 3 1 0.01 34 0.0137 0.0147 0.0149 0.01 87 mined by the dichromate - iodide method described by Berka, Vulterin and Zyka,4 the normality of this batch of titrant was 0.0124,.Sets of six titrations of 1 0 - 4 ~ copper(I1) in 0.3 N potassium thiocyanate - 0-02 N perchloric acid medium gave, as average apparent normalities of this same batch of titrant, 0.0129, 0.0127 and 0.0128, by Procedure A, L-curve and residual-current methods, respectively.Precise titration is therefore impossible unless the concentrations of both copper(r1) and thiocyanate are essentially constant.30 STOCK : AMPEROMETRIC TITRATION OF SUBMILLINORMAL [AlZdySt, VOl. 91 I I I I 0 0.1 0'2 0.3 0.4 Volume of titrant, ml Fig. 2. Titration of 50 ml of 50 ~ L N copper(r1) with 0.0131 N mercury(1) per- chlorate after arbitrary prc-addition of 0.5 microequivalcnts of copper(I1). P and R are yre-addition and residual-current lines, respectively The titrations were repeated and extended in acid thiocyanate media that also contained potassium iodide.Fig. 2 shows a typical composite titration curve. The results, all obtained at a perchloric acid concentration of 0.02 x, are summarised in Table 11. In each set of 3 experiments, the successive titrations were carried out at potassium iodide concentrations of 0.02 N, 0.04 N and 0.06 N, respectively. At least within these limits, the concentration of this halide is obviously unimportant. As determined by the method described by Berka, Vulterin and Z9ka,4 the normality of the titrant solution was 0.0131,. The results are essen- tially unaffected by 10-fold changes in the concentrations of either thiocyanate or copper(I1) ion, or by the presence of chloride or bromide. In the concentration range to 1 0 - 4 ~ copper(I1) and in the presence of iodide ion, end-point location by Procedure '4 is therefore precise and accurate to within &1-5 per cent.Although less precise, the residual-current method gives acceptable results. The conventional L-curve method8 is not to be recom- mended for this titration, nor is it applicable at the lower end of the copper(I1) concentration range. TABLE I1 TITRATION OF COPPER(II) IN THE PRESENCE OF IODIDE ION Thiocyanate Copper (11) concentration, concentration, N P 0.1 50 0.3 10 0.3 50 0.3 50 0.3 100 0.8 50 1.4 50 Average millinormality and standard deviation . . . . . . . . Apparent mercury(1) millinormality L -~ 7 7 Residual-current method A L-curve method method 13.0, 13.4, 13.2 12-3, 12.8, 12.6 12.6, 13.3, 13.1 13.1, 13.3, 13.3 Failed 13.3, 13.3, 13.1 13.1, 13.1, 13.0 12.9, 12.7, 12.5 13-1, 12.9, 12.9 13.3, 13.2, 13.3 13.1, 13-2, 12.9 13.3, 13.0, 13.3 13.5, 13-2, 13.0 13.3, 12.9, 12.6 13.5, 13.0, 12.8 13.3, 13.2, 13.2 13.0, 13.0, 13.1 13.5, 13.3, 13.3 13.2,* 13.0,t 13.2: - - 13.2, & 0-14 12.8, & 0.28 13.1, + 0.25 * 0.02 N chloride present.t 0.02 N bromide present. 3 0.02 N each chloride and bromide present.January, 19861 COXCENTRATIONS OF COPPER(I1) WITH MERCURY(1) PERCHLORATE 31 End-point location by Procedure A or, less satisfactorily, by the residual-current method, was found to be applicable in the titration of micronormal concentrations of copper(I1). Fig. 3 shows some typical titration curves obtained with the titrant diluted 10-fold with N perchloric acid. At a concentration of approximately p ~ " , copper(r1) can be titrated with a precision of about 5 per cent.An end-point is recognisable at a copper(I1) concentration of 0.6 p~ (curve D); in this titration, the residual current accounted for more than 60 per cent. of the initial current. Standardisation of the titrant against known amounts of copper(rI), at concentrations similar to those to be determined, is recommended for submillinormal titrations and is essential when the concentration of copper(I1) is in the micronormal range. Method B can be used when the copper sample is presented as a highly dilute solution. Further dilution is minimised by the use of concentrated reagents and the end-point is located by the residual-current method. Results obtained with a 10 p~ copper(I1) solution were precise to within 5 per cent.Volume of titrant, mI Fig. 3. Pre-addition titration of 50 ml of highly dilute copper(I1) solution with approximately 0.0013 N mercury(1) per- chlorate. The upper and lower bars across each titration curve are the pre-addition and residual-current lines, respectively. Copper(I1) concentration: curve A , 6 p ~ ; curve R, 2.4 p ~ ; curve C, 1.2 p ~ ; curve I>, 0.6 p~ D I sc u s SION CU(TI) + Hg(r) -+ CU(I) + Hg(I1). The marked depression of the formal potential of the mercury(I1) - mercury(1) couple by thiocyanate and the voltammetry of this couple in thiocyanate medium have been discussed el~ewhere.~ Initially, complexation by thiocyanate will tend to lower the formal potential of the copper(r1) - copper( I) couple. However, if the reaction proceeds normally, copper(1) will be produced as soon as the titration is started.Although the solubility productlo of copper(1) thiocyanate is only about 10-14, this compound is appreciably soluble when the excess of thiocyanate ion is large. On the assumption that the solubility equation given by Fridman and Sarbaevll holds approximately outside the stated range of 0.7 to 5 N thiocyanate, the solubilities of copper(1) thiocyanate at 25" C are calculated to be 4 x N, 2 X N and 8 X N, in 0.1 N, 0.3 x and N potassium thiocyanate solutions, In this titration, the underlying reaction is32 STOCK [Analyst, Vol. 91 respectively. Since the total copper concentration in the present work was always less than 2 x ~ O - * N , copper(1) will remain in solution.A consideration of the values of the formation constantslo of the various thiocyanate complexes of copper(I1) and copper(1) leads to the conclusion that the formal potential of the copper(I1) - copper(1) couple will be raised by the presence of an excess of thiocyanate ion. At 25” C, a potential shift of from +Ova7 to +0.49 volt is calculated for thiocyanate concentrations of 0.1 N to N. The approximate formal potential under the titration conditions should therefore be +0.63 volt (hydrogen scale) or $0.39 volt (with respect to the S.C.E.). The calculated formal potential (25.C) of the mercury(I1) - mercury(1) couple in 0.1 N to N thiocyanate ranges from -0.09 to -0.35 volt (hydrogen scale). Since the formal potential of the copper couple is a t least 0.7 volt more positive than the formal potential of the mercury couple, the titration reaction should go to analytical completion unless the kinetics are unfavourable.Although quick and quite precise under rigidly controlled conditions, the titration exhibits stoicheiometry that is dependent upon the concentrations of thiocyanate and of copper(I1) ions. High titrant normalities, obtained at high concentrations of thiocyanate, are almost certainly due to the partial reduction of copper(I1) by the medium- 2Cu2+ + (2% + 2)SCN- + ~CU(SCK)(F’~ + (SCT\jI2 . . * - (1) 3(SCX), + 4H20 -+ 5SCN- + SO,2- + 7H+ + HCiK , . - (2) where n has integral values of 1 to 4. This reduction has been recently studied by Kolthoff and Okinaka,12 who observed a large hydrogen wave even in freshly prepared 0.001 K copper(I1) in N potassium thiocyanate solution. No copper(I1) remained in the solution after overnight standing.However, the reduction is much slower in 0.1 N potassium thiocyanate; about 55 per cent. of the original copper(I1) remained after 2 days. Since thiocyanogen oxidises iodide,13 the addition of an excess of this ion suppresses reaction (2) and may suppress reaction (1). Any iodine produced will be titrated4 with mercury(1) along with the remaining copper(II), so that the apparent normality of the titrant should become independent of the concentrations of both thiocyanate and copper( 11) ions. Although iodide can form complexes with both copper(I1) and copper(1) ions,14 these effects, and the resulting change in the formal potential of the copper(I1) - copper(1) couple, are likely to be small in the presence of an excess of thiocyanate ion.However, iodide com- plexes mercury(I1) ion very strongly,15 and should cause the formal potential of the mercury(I1)- mercury(1) couple to become more negative than in an iodide-free thiocyanate medium. This work was carried out with the partial support of the United States Atomic Energy Commission (Contract AT(30-1) - 1977) and was completed at the Imperial College of Science and Technology, London. The facilities afforded by the College authorities, in particular by Professors R. M. Barrer and T. S. West, are gratefully acknowledged. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. REFERENCES Tarayan, V. M., and Arutyunyan, A. A., Zavod. Lab., 1953, 19, 900. Tarayan, V. M., “Merkuroreduktrometriya,” Izdatel’stro Erevanskogo Universiteta, Erevan, Matsuo, T., J . Chem. SOG. Japan, Ind. Chem. Sect., 1955, 58, 962. Berka, L4., Vulterin, J., and Zgka, J., Chemisl-Analyst, 1963, 52, 122. Stock, J. T., and Heath, P., Analyst, 1965, 90, 403. Kolthoff, I. M., and Tanaka, N., Analyt. Chem., 1954, 26, 632. Stock, J . T., “hmperomctric Titrations,” Interscience Publishers, a division of John \;Z’iley and Stock, J . T., op. cit., chapter 1 . -, op. cit., pp. 520 and 558. SillCn, L. G., and Martell, A. E., “Stability Constants of Metal-Ion Complexes,” The Chemical Fridman, Ya. D., and Sarbaev, Dzh. S., Russian J . Inorg. Chem., 1959, 4, 835. Kolthoff, I. M., and Okinaka, Y . , Rec. Trav. Chim. Pays-Bas., 1960, 79, 551. Kaufmann, H. P., and Gaertncr, P., Ber. dtsch. chem. Ges., 1924, 57, 928. Sillen, L. G., and Martell, A. E., op. cit., p. 338. U.S.S.K., 1958, p. 144. Sons Tnc., New York, 1965, chapters 7, 9 and 10. Society, London, 1964, p. 121. 3 , op. cit., p. 341. -- Received June 24th. 1965

ISSN:0003-2654    

DOI:10.1039/AN9669100027

出版商:RSC    

年代:1966

数据来源: RSC

摘要:

January, 19661 ASPINAL 33 Vacuum Fusion Analysis with a Mass Spectrometer BY M. L. ASPINAL (Associated Electrical Industries Limited, Central Research Laboratory, Rugby) The suitability of a mass spectrometer for vacuum fusion analysis is discussed; the equipment and its mode of operation and calibration are described. The objections to using an oil pump for extracting the gases liberated from the metal samples have been overcome with this apparatus. Results for oxygen determination in steels, molybdenum and zirconium have been independently checked by using a fast neutron-activation tech- nique, and the results of two methods are shown to be in good agreement. Oxygen levels of 116 p.p.m. have a standard deviation of 6 p.p.m. and coefficient of variation of 5 per cent. Although most of the work has been carried out on oxygen determinations, nitrogen results are quoted for two standard iron samples showing that the method is also acceptable for nitrogen determinations.Kitrogen levels of 31 p.p.m. have a standard deviation of 3 p.p.m. and a coefficient of variation of 10 per cent. The limits of detection for the equipment are 0.1 p g of oxygen, 0.1 ,ug of nitrogen and 0.01 pg of hydrogen. VACGUM fusion analysis is a well known method for the determination of oxygen, nitrogen and hydrogen in metals, and a comprehensive, up-to-date review of the literature has been given by James.l Most papers, however, describe equipment capable of determining oxygen down to 10 p.p.m. This is no longer adequate as the metallurgical requirements for metals sucl as copper, molybdenum, tantalum and tungsten may be below this level; for example, coppir that is to be used in vacuum devices is required to have an oxygen content of less than 0.5 p.p.m.The detection limit of some equipments could be lowered by increasing the sample weights, but large samples are not always available, and an equipment capable of determining gas contents on small samples, in particular oxygen contents below 10 p.p.m., was required. Techniques for the analysis of small amounts of gas were considered, such as gas chromatography and mass spectrometry, and mass spectrometry was thought to be most suitable for vacuum fusion analysis. Mass spectrometers have been used by Taylor2 for the analysis of gas obtained by vacuum fusion; in this method the gas was collected in a bottle and then transferred to the spectro- meter.Martin et aL3 used a mass spectrometer directly coupled to their vacuum fusion equipment. This equipment, however, required large sample weights, 6 to 14 g, and blanks had to be carried out for 1 hour so that sufficient blank gas was present for analysis. The object of the work described in this paper was to produce an equipment suitable for routine analysis of materials with high gas contents, and in particular capable of analysing material with low gas contents for specialised purposes. EXPERIMENTAL DISCUSSION ( a ) THE SUITABILITY OF A MASS SPECTROMETER FOR VACUUM FUSION ANALYSIS There are three main reasons why a mass spectrometer was chosen for this work. First, because it is capable of measuring small amounts of gas quantitatively.Secondly, because it allows positive identification of each gas, whereas some of the earlier equipments could only label the residue gas as nitrogen after the extraction of carbon monoxide and hydrogen. The spectrometer, for example, would give differentiation between argon and nitrogen if this was required. Thirdly (an important advance on some earlier equipments), by coupling the output from the spectrometer to a recorder, a complete record of the rate at which one gaseous component is liberated from the sample, and also the time needed to restore the original blank rate can be observed. The gas liberated from the sample therefore appears as a step on the blank rate, the height of which can be measured from the trace, so making it unnecessary to carry out blanks at regular intervals between sample additions. One difficulty that arises from using a mass spectrometer is caused because nitrogen and carbon monoxide both have the same mass number.Gregory, Mapper and Woodward4 suggested that this problem could be overcome, but with considerable loss of accuracy, by measuring the mass 29 peak that is principally the monoxide of 13C, and multiplying this34 ASPINAL: VACUUM FUSION ANALYSIS WITH A MASS SPECTROMETER [A?Z&?J.’St, VOl. 91 by the 12C/13C ratio to obtain an estimate of the total monoxide present in the mass 28 peak. The presence of organic compounds that also produce a peak at mass 29 made this method unsatisfactory. If the spectrum of a gas is studied it is found that it consists of a parent peak at the mass equivalent of the molecule, a major peak that may be the parent peak and some small peaks that bear fixed ratios for a particular instrument and source condition to the major peak.The characteristic cracking patterns of carbon monoxide and nitrogen are shown in Table I, in which it is seen that the parent peak is also the major peak. It will also be seen TABLE I CHARACTERISTIC CRACKING PATTERNS Carbon monoxide- m/e . . 12 13 14 14.5 15 16 28 29 30 Relative intensity 4.49 0.048 0.61 0-007 0.001 0.95 100.00 1-13 0.21 Nitrogen- 14 - 15 - 28 29 30 m / e . . - - Relative intensity - - 7.18 - 0.021 - 100.00 0-77 0.002 that mass 12, the second largest peak, could be used to measure carbon monoxide] and similarly mass 14 could be used to measure nitrogen.Carbon monoxide also produces a significant mass 14 peak due to the presence of (C0)2+, but allowance is made for this by calibrating the spectrometer with carbon monoxide and measuring the mass 12 and 14 peaks. The amount of mass 14 coming from the carbon monoxide can be deducted from the “total” mass 14 peak before calculating the amount of nitrogen present. Bottles of “Specpure” carbon monoxide, nitrogen and hydrogen are used for calibrp ,ing the spectrometer. Small amounts of these gases are introduced into the gas-handling section, their pressure is measured by a McLeod gauge and the appropriate peak height by the spectrometer. The calibration of the spectrometer does not change by more than 2 per cent. over several months of continuous operation] and a calibration check once a week is satisfactory.The filaments of the spectrometer, which are replaceable] have a life of 6 months under normal operating conditions. Replacement of these filaments can be accom- plished with no appreciable change to the calibration. In order to check that mixtures of carbon monoxide and nitrogen could be satisfactorily separated with the spectrometer, varying amounts of the two gases were mixed in the gas-handling section and then analysed with the spectrometer. Table I1 shows the amounts added and found, from which it can be seen that satisfactory separation has taken place. TABLE I1 SEPARATION OF CARBON MONOXIDE AND NITROGEN Analysis of mixture with 3I.S. 10 Gas mixture added & ---7 Carbon Carbon CO added Nitrogen, monoxide, Nitrogen, monoxide, Nz added per cent.per cent. per cent. per cent. N, found CO found 15.4 84.6 15.5 84.5 0.99 0.99 9.4 90.6 9.8 90.2 0.98 1.01 13.0 87.0 12.7 87.3 1.02 0.99 53.2 47.8 49.5 50.5 0.98 1.08 80.0 20.0 79.0 21-0 1.02 0.95 71.0 29.0 70.0 30.0 1.02 0.97 (b) THE USE OF OIL-DIFFUSION PUMPS FOR VACUUM FUSION ANALYSIS- One criterion for good vacuum fusion is that a high-speed pump is used to extract the gases from the furnace section. This is to remove the gases quickly so they are less likely to be “gettered” by the metal films that form around the cooler parts of the furnace walls. A pumping speed of 30 litres second-l at the mouth of the crucible is usually acceptable. Many vacuum fusion equipments use a mercury-diffusion pump to transfer the gases from the furnace section to the analysing section.When a mercury-diffusion pump is used, the pressure in the furnace section is limited by the vapour pressure of mercury, which isJanuary, 19661 ASPINAL: VACUUM FUSION ANALYSIS WITH A MASS SPECTROMETER 35 3 x 10-3mm (of mercury) at room temperature. At this pressure the mean free path in the furnace section will be about 1 cm and there is a greater chance that “gettering” will occur. A cold trap could be introduced between the diffusion pump and furnace section to reduce the pressure in the furnace section, but this is not desirable as it will also reduce the pumping speed at the crucible. Fig. 1. Comparison of carbon monoxide blanks against temperature; curve -1 = filer- cury pump and low-pressure analyser (I’alladium thimble and “Hopcalite”) ; curve I3 = Oil pump and mass spectrometer One commercial equipment manufactured by Balzers incorporates an oil-diffusion pump.5 This is claimed to produce a pressure of mm of mercury above the crucible, and reduce “gettering” of the liberated gas as the films formed around the cooler parts of the furnace walls are now less porous than those formed with a mercury system.The main objections to the use of an oil pump are that any back-streaming of the oil could increase the blank rate by being cracked on the hot graphite crucible, and also that some of the gas liberated from the sample might be absorbed in the pump oil. Neither of these objections has been found to applv in this investigation, and the use of an oil pump has in fact provided significantly lower blank rates.This is shown in Fig. 1, which compares carbon monoxide blank rates when using an 033C oil-diffusion pump with values obtained over several years with a mercury pump and a low-pressure analyser. There has also been no evidence for “gettering.” TIIE VACUCM FUSION EQUIPMENT The general arrangement of the equipment is shown in Fig. 2 and can be divided into three parts ( a ) furnace, (b) gas-handling and (c) mass spectrometer sections. ( a ) FUKNACE SECTION- This is shown in Fig. 3. A small silica crucible and pedestal are supported by the silica furnace tube. The graphite crucible (14 in. x 8 in. ad., wall thickness & in.) is placed in the silica crucible and the space between the two crucibles is packed with graphite powder that has passed through a 100-mesh sieve.A slotted graphite funnel is placed on top of the graphite crucible. All the graphite parts and powder are made from Morganite EYSA graphite. The top rim of the silica crucible is turned in so that there is the minimum gap between the silica and graphite funnel that prevents graphite powder from being blown out of the crucible during evacuation or outgassing. The silica furnace tube is joined to a stainless-steel manifold by means of a viton O-ring fitting into a tapered groove and compressed with a screwed ring.36 ASPINAL: VACUUM FUSION ANALYSIS WITH A MASS SPECTROMETER [Analyst, VOl. 91 F = o = c = L = E = s = v = P = T = E M.S. Furnace Oil diffusion pump Cold trap Leaks Expansion volumes Spectrometer vacuum pumps Vacuum pumps Pirani gauge Three-way taD 10 M.S.10 = Mass spec;rograph Fig. 2. General arrangement of the apparatus A h L = Vacuum lock S = Sample arm M = Stainless-steel manifold J = Water jacket F = Quartz furnace tube C = RF Coil 0 = 033C Oil-diffusion pump Fig. 3. Furnace sectionJanuary, 19661 ASPINAL: VACUUM FUSION AKALYSIS WITH A MASS SPECTROMETER 37 A Pye 3-1<W, 2-Mc second-l induction heater is used to heat the graphite assembly, and the bottom of the stainless-steel manifold is cooled by water to reduce the temperature of the O-ring seal during operation. The 2B-in. bore stainless-steel manifold connects together the furnace tube, sample-handling system and an A.E.I. O33C oil-diffusion pump, that has an unbaffled speed of 130 litres second-l.The pump uses 704 silicone oil as the pump fluid, and has a simple copper-foil baffle inserted on top of it to prevent back-streaming. The manifold also supports the funnel for guiding samples into the crucible. The top flange of the manifold supports the mechanism for operating the stopper. This consists of a graphite stopper connected to an iron slug with fine molybdenum wire. The slug is moved along the side arm with a magnet, so raising or lowering the stopper which is guided on to the crucible by means of the sample-guide tube. During sample additions and temperature measurements the stopper can be drawn out to one side of the guide tube to give an unobstructed view of the crucible. The temperature is measured with the optical flat on top of the glass-ware, a prism and optical pyrometer.A vacuum lock is fitted to the sample side arm to allow samples to be introduced while the system is under vacuum. ( b ) GAS-HANDLING SECTION- The gas-handling section is connected through a liquid-nitrogen cold trap to the backing side of the oil-diffusion pump, and also to the mass spectrometer by capillary leaks. The trap is used to remove small amounts of unwanted condensable gases such as water and organic compounds that complicate the mass spectrum, and are liberated from the graphite and the cooler parts of the furnace section. The section is pumped by a cold-trapped glass - mercury diffusion pump and A.E.I., D.R.l rotary pump. These pumps can either be connected in series with the oil-diffusion pump during outgassing, isolated from the gas-handling section during analysis or can be used to evacuate the gas-handling section.The three-way tap T provides the necessary control. In addition the section consists of three expansion volumes used to adjust the pressure of the extracted gases to a suitable value and a McLeod gauge that is used during calibration. Litre bottles of "Specpure" carbon monoxide, nitrogen and hydrogen that are obtained from the British Oxygen Company Ltd., are used during calibration, and connected to the system through double &-in. A .E. I. polyethylene-diaphragm valves so that small amounts of gas can be introduced for calibration. There are two capillary leaks in parallel connecting this section to the mass spectrometer.The leaks are made from precision-bore glass tubing and have conductances of 1.7 x litres second-l and 0-5 x The leak with the lower conductance is used for the hydrogen determinations. Greased taps are always a possible source of leaks and only one tap, T, is used in this equipment, the remainder are either metal - polyethylene diaphragm valves or greaseless glass taps with a viton diaphragm. These latter taps have a higher conductance than the &in. metal valves and are to be preferred where a high conductance is required. litres second-l. (C) 1'rASS SPECTROMETER SECTION- The mass spectrometer used is a small gas mass-spectrometer, type M.S. 10, manufactured by A.E.I. Instrumentation Division, and has mass ranges 2 to 4, 12 to 45 and 18 to 200.The output from the analyser tube is passed through electronic circuits where it is amplified and displayed either on the output meter or pen recorder. The spectrum can be scanned by varying the accelerating voltage, The spectrometer tube is mounted horizontally and pumped by its own vacuum pumps, which consist of a cold-trapped A.E.I. 033C oil-diffusion pump backed bv an A.E.I., D.R.l rotary pump. The oil used in the diffusion pump is Apiezon B.W. as there is some danger of coating the source unit with silica if silicone oils are used. Provision is made for baking the analyser tube while outgassing by using band heaters clamped around the flanges. These raise the temperature to 200" C with an unlagged system. Higher baking temperatures up to 400" C can be achieved with an oven, but for this particular application of the spectrometer baking at 200" C is adequate.The magnet, which weighs 35 lb, is easily removed during baking and can be replaced in the same position after baking by using guides. The spectrometer is connected to the capillary leaks by a small inlet flange mounted on the side of the spectrometer away from the source unit.3s ASPINAL: VACUUM FUSION ANALYSIS WITH A MASS SPECTROMETER [AWalyd, Vol. 91 METHOD OF OPERATIOX- The graphite crucible is placed in the silica crucible and the space between the two crucibles is loosely packed with graphite powder. The assembly is placed on its support in the furnace tube that is fitted to the furnace manifold. All the taps in the gas-handling section, which has been under vacuum from the previous analysis, are closed.The three-way tap, T, is turned so that the whole system can be slowly evacuated to avoid graphite powder from being blown out of the crucible. \$'hen the Pirani gauge, P, shows that the pressure has fallen to below 100 microns the oil-diffusion pump can be switched on. Pressure continues to fall in the gas-handling section until it has reached a few microns. Outgassing of the crucible can now start by raising the temperature in about ten steps within half an hour up to a maximum of 2100" C ; at higher temperatures there is a tendency for the stopper to stick to the crucible. When the temperature has reached the maximum of 2100" C it is maintained for 1 hour. This has been found to be sufficient to outgas the crucible to a suitably low level for use with samples with oxygen contents greater than 50 p.p.m.If, however, the sample is expected to have a lower gas content, the outgassing procedure is continued for a second hour. This produces slightlq? lo\ver blank rates; typical values are 19 pl hour-1 at 1900" C (carbon monoxide, 32.5 per cent.; nitrogen, 13.5 per cent.; hydrogen, 54 per cent.), and 8 pl hour-' at 1700" C (carbon monoxide, 23.7 per cent.; nitrogen, 25.7 per cent.; hydrogen, 50.6 per cent.). The outgassing rate can be monitored with the spectrometer set at mass 12, and the minimum outgassing time can be determined. Once this has been determined it is usual to allow the spectrometer to cool from its overnight bake, and prepare the samples while the outgassing of the crucible is taking place.The sample weights can vary in the range 4 g to 80mg depending on whether the expected oxygen content is below 1 p.p.m. of oxygen or above 1000 p.p.m. of oxygen, respectively . The preparation of the sample's surface is usually carried out by abrading it with a file and then by vapour de-greasing it with carbon tetrachloride. The optimum size of sample for low gas contents and ease of manipulation is one, or more, 4-mm cubes. If a suitable etch is available this is preferred to the abrasion, for example, copper samples can be etched in 1 to 1 nitric acid, then by 1 to 1 hydrochloric acid, washed with distilled water and dried with acetone and ether. After the samples have been weighed, they are placed as quickly as possible in the sample arm of the furnace section via the vacuum lock.If a bath material is being used, such as iron for molybdenum or platinum for zirconium, this is also stored in the sample arm. Liquid nitrogen is placed around the cold traps and the blank rate is checked for the equipment. This should be less than 20 pl hour-l at 1900" C. The temperature is lowered to the operating temperature, for example, 1900' C for materials such as zirconium requiring a platinum bath, 1750" C for materials requiring an iron bath. If a bath material is to be used it is introduced and outgassed. Tap T is turned to connect the gas-handling section to the furnace section and a sample is introduced to the crucible. The extracted gases are collected in the gas-handling section and adjusted to a suitable pressure with the expansion volumes.These are used to keep the spectrometer reading on scale when large amounts of gas are collected. The gas is monitored during collection by the spectrometer set either at mass 12 for carbon monoxide or mass 14 for nitrogen. The rate at which the gas leaks from the gas-handling section into the spectro- meter does not have any appreciable effect on the total volume of gas collected. The mass spectrometer is coupled to the recorder which shows one of the extracted gases as a step on the blank rate, Fig. 4. The collection is continued until the blank rate has returned to its original rate shown by the recorder. This usually takes between 2 to 8 minutes. The step height is measured and converted to pressure with the appropriate calibration graph and hence to oxygen, nitrogen or hydrogen depending on whether mass 12, 14 or 2 has been recorded.The gases which are not recorded are measured after completion of the extraction by scanning the spectrum manually. DISCUSSION Peaks at masses 12, 13, 14, 15, 27, 43, 44 and 2 were measured manually on the early analyses for both samples and blanks. It was found that in addition to the peaks at masses 12, 14 and 2, corresponding to carbon monoxide, nitrogen and hydrogen, only mass 15 (which is derived from methane) is producedJanuary, 19661 ASPINAL : VACUUM FUSION ANALYSIS WITH A MASS SPECTROMETER 39 Time, minutes Fig. 4. Typical extractions of carbon monoxide: ( a ) sample wcight 2.27 g, expansion volume 1170 ml ; (b) sample weight 2.96 g, expansion volume 740 ml in significant amounts by the sample.The other peaks 27, 43 and 44 were all present in the blank and sample to an equal extent and are attributed to the presence of hydrocarbons. All of the mass 13 could be attributed to the methane. As a result for routine analysis only masses 12, 14, 15 and 2 are measured. With samples containing large amounts of oxygen the mass 12 correction from mass 15 is small, but for samples with low oxygen contents it becomes significant. For nitrogen calculations two correction factors are applied, one is the mass 14 contri- bution from carbon monoxide determined during calibration, and the other is the mass 14 contribution from methane. Both correction factors for masses 12 and 14 from methane are taken from the manufacturer's results and correspond to 2.8 per cent.and 18.2 per cent. of the mass 15 peak, respectively. Although the correction factors obtained from the cracking patterns of methane will vary slightly between spectrometers, no appreciable error will occur in the results unless the variation is greater than 20 per cent. of the quoted results. A run of twelve steel samples can be analysed in 8 hours, including assembly and outgassing times. RESULTS Most of this work has been carried out on oxygen determinations only, as it is difficult to obtain material with a known nitrogen content. There is also some doubt expressed in the literature as to whether nitrogen from certain materials, for example, zirconium, is liberated quantitatively. The standard iron samples BOL 16, BPL 8 and CRL 17 were obtained from the British Iron and Steel Research Association. TABLE I11 OXYGEN RESULTS O F STANDARD IRON M.S.10 A r \ Number Mean of oxygen Standard deter- content, deviation, Sample minations pp.m. p.p.nl. BOL 18 . . 29 116 6 BPL 8 . . 8 52 8 CRL 17 . . 8 54 3 (100 p.p.m.) (40 p.p.m.) (35 p.p.m.) * BPL 15 B.I.S.K.A. Radiochernical c I Number Mean of oxygen Standard deter- content, deviation, ininations p.p.m. p.p.m. 6 119 4 6 62 8 6 84 5 quote 30 p.p.m. Coleman, Mean oxygen content, p.p.m. 120 (BOL 25) 37 (BPL 15)* 54 (CRL 15)40 ASPINAL: VACUUM FUSION ANALYSIS WITH A MASS SPECTROMETER [Analyst, VOl. 91 They had also been analysed by ColemanJ6 who used a fast-neutron activation technique. The numbers on these samples denote the bar used.It will be seen that Coleman used different bars of the iron, but all the bars are supplied with the same nominal gas content, except BPL 15. Analysis was also carried out on three samples of stainless steel, one sample of molybdenum requiring the use of an iron bath and four samples of zirconium that required the use of a platinum bath. The results for these materials are shown in Tables I11 and IV. I .oo Blank 0.75 - Sample Mol ybdenum-disc Stainless s teel-C, c3 C6 Zirconium-Zr 66 Zr 83 Zr 92 Zr 115 Copper sample Ig _- B TABLE IV OXYGEN RESULTS ON OTHER MATERIALS M.S. 10 Radiochemical h r 7 r .-h------, Number Mean Number Mean of oxygen Standard of oxygen Standard deter- content, deviation, deter- content, deviation, minations p.p.m.p.p.m. minations p.p.m. p.p.m. .. 6 32 2 4 38 3 .. 2 36 - 2 34 - . . 2 44 - 2 39 - . . 5 58 4 2 48 - . . 2 850 - 2 870 - . . 2 780 - 2 770 - . . 2 720 - 2 750 - .. 2 1440 - 2 1350 - ,4n independent check has also been carried out by this laboratory’s radiochemistry department on these samples by using the fast-neutron source at Wantage. The results obtained by use of this method have been quoted for comparison, and it can be seen that in general the agreement between the two methods is good. TABLE V LOW OXYGEN LEVELS IN COPPER Kumber of Mean oxygen Sample determinations content, p.p.m. O.F.H.C. copper . . .. 6 1.27 Copper - 1 per cent. silver . . 4 0.38 Copper - iron alloy . . . . 4 0.24 Vacuum-melted copper . . 6 0-07 ALLOYS Standard Coefficient deviation, of variation p.p.m.per cent. 0.126 9.9 0.048 12.5 0.042 17.5 0.013 18.6 One main advantage of this method lies in the determination of oxygen contents below 2 p.p.m. in metals. Table V shows oxygen levels found in a number of copper and copper- alloy samples. Fig. 5 shows the trace obtained for the lowest of these materials, from which it can be seen that the step that represents 0-15 pg of oxygen is still well above the background. n N ll p W Fig.January, 19661 41 These show a greater variation in the reproducibility, and although little work has been carried out at present on nitrogen determinations the results show that this method can be used for its determination. However, Table VII shows a range of hydrogen results obtained for various materials with this equipment.ASPINAL: VACUUM FUSION ANALYSIS WITH A MASS SPECTROMETER Nitrogen results are quoted for two standard iron samples (Table VI). No standard samples for the determination of hydrogen were available. Sample BNL 15 .. .. BOL 16 . . . . (160 p.p.m.) (45 p.p.m.) TABLE VI NITROGEN RESULTS Number of Mean determinations nitrogen content, Standard deviation, p.p.m. p.p.m. 15 180 30 29 31 3 TABLE VII HYDROGEN RESULTS Number of Mean hydrogen Standard Coefficient of Sample determinations content, deviation, variation, p.p.m. p.p.m. per cent. O.F.H.C. copper . . .. 6 0.7 1 0.10 14 Tantalum sheet . . . . 3 86.6 1.5 1.7 Zirconium . . . . .. 2 346 5 1.4 Vacuum-melted copper . . 6 0.01 0.01 100 CONCLUSIONS The method of vacuum fusion with a mass spectrometer, M.S.10, as the analyser has been used in this laboratory for the past 12 months. During this time the method has been completely reliable and the operation of the equipment is simple and rapid. The method can be applied satisfactorily over a large range of gas contents, ranging from 1400 to 0.05 p.p.m. of oxygen, 150 to 20 p.p.m. of nitrogen and 400 to 0.02 p.p.m. of hydrogen. The main advantage of this method is in determining oxygen contents of metals below 2 p.p.m., and the display of the gas-evolution rate on a recorder. The limits of detection of this equipment are at present 0.1 pg of oxygen, 0.1 pg of nitrogen and 0.01 pg of hydrogen. These limits can be improved if necessary by reducing the pumping speed of the spectrometer, but a major source of error will be surface con- tamination in the form of surface films, which must be removed before any lowering of the detection limit is justified. I am indebted to Mr. P. Jones, who carried out the fast neutron-activation analysis and to Mr. J. A. James for reading the manuscript and making helpful suggestions and criticisms, REFERENCES 1 . James, J . h., Metall. Rev., 1964, 9, 93. 2. 3. 4. 5. Kraus, T., Arch. Eisenhuttenw., 1962, 33, 527. 6. Taylor, K. E., Analylica Chim. A d a , 1959, 21, 6, 549. Martin, J . F., Friedline, J . E., hlclnick, L. M., and Pellissicr, G. E., Trans. Metall. Soc. A.I.M.E., Gregory, J. N., Mapper, D., and Woodward, J. A., Analyst, 1953, 78, 414. Coleman, R. F., Analyst, 1962, 87, 590. 1958, 212, 514. Received January lst, 1965

ISSN:0003-2654    

DOI:10.1039/AN9669100033

出版商:RSC    

年代:1966

数据来源: RSC

摘要:

42 MARTEN AND CATAKZARO : FCSDAMENTAL STUDIES [Analyst, VOl. 91 Fundamental Studies in Automatic Nitrogen Digestion BY J. F. MARTEN AND G. CATANZARO (Technicon Instvumenis Corpovation Inc., Chauncey, New York, U.S.A .) Little work has been reported on the inter-relationship between digestion temperature, media and catalysts in the AutoAnalyzer digestor system. Although classical results could be used to some extent, many phenomena observed could not be explained by means of current theory. The volume of acid in the digestor helix has been found to be critical, and failure to realise this has resulted in loss of nitrogen. Hypotheses are advanced to explain where this nitrogen loss takes place, and subsidiary investigations of catalyst and digestion-media systems were undertaken. The final digestor system is shown to give 99-5 per cent.recovery for ring compounds such as nicotinaniide, when compared with ammonium sulphate standards taken through the same system. ‘l’heories are also elucidated that point to the reasons for the short digestion times, normally niinu tes, in which digestion is completed. THE AutoAnalyzer digestor is now probably sufficiently well known to warrant only a brief introduction. Samples are taken in turn from a sampler unit, mixed with a digestion acid and segmented with air, before being introduced into a helically-grooved heated glass cylinder in which digestion takes place. This is illustrated in Fig. 1. Fully digested samples are diluted, and ,,.,,I I..., Waste bottles Vacuum I Drnln p u m p Fig. 1 .Schematic diagram of the digestor - Auto,%nalyzer system passed on to a mixing chamber irom which portions are continually aspirated into the analytical system, the remainder being rejected as waste. Fig. 2 shows the digestor in detail, digestion being carried out in a glass vessel that rotates about its axis over a series of bar- heaters. One of the most striking features of the digestor is the speed with which it completes its task. Even the fact that rates of digestion are increased %fold for an increase in tem- perature of 10” C does not provide the complete answer. Rather, it is because much of the breakdown occurs in thin films moving over the surface of the helix, which is quickly removed from the direct source of heat. Also, the acid volume is relatively large compared with the sample volume.The increased rate of digestion observed from macro to micro Kjeldahl determinations is extended by means of this technique, use being made of the fact that an increase in acid volume over sample volume accelerates charring and the final breakdown of the nitrogenous compounds. When digestions are carried out in a helical tube, many of the problems that arise in manual digestion are avoided. For example, it is well known that[to face page 42January, 19661 I N AUTOMATIC NITROGEN DIGESTION 43 loss of ammonia may occur owing to local over-heating in macro determinations. In this instance, however, the rotation of the digestion tube prevents the formation of so-called "hot-spots," even at temperatures exceeding 370" C.Also, the by-products of the breakdown, such as carbon dioxide and water, are removed as soon as they are formed. Since Ferraril described the new concept of continuous nitrogen determination, investi- gations have been under way on the optimum operating conditions for this technique. Certain parameters were described last year,2 but little work has been reported on the complex inter-relationship between digestion temperature, media and catalysts. Although literally hundreds of papers have been published, and in fact are still being published, on the classical Kjeldahl technique, since it was first described in 1883,3 much of this information could not be directly applied. We had, therefore, but a relatively short time in which to establish the parameters of what is surely a much more complex set of conditions, having multiphase and, probably, multi-order characteristics (see Fig.3). Flow-rate. Water ml/min Sample Air Digest Waste trap and vacuum pump Air Water Sodium hydroxide Alkaline phenol Sodium hypochlorite = 625 4 mm ml; fjc Fig. 3. Flow diagram for the continuous determination of nitrogen The manifold shown here is generally accepted for routine use for amounts in the range 100 to 1OOOpg of nitrogen per ml, the input volume being 0.9ml per minute. Thus, for 20 samples per hour 1.8 ml of liquid sample are used, giving 180 to 1800 pg of nitrogen for digestion. The amount of acid used for this digestion is 14 ml. On the other hand, for solids, with the AutoAnalyzer solid preparative techniques, 3 to 5 g of, for example, barley, are homo- genised in 200 ml of liquid, and the same amount of suspension is used for the analysis. It was recognised that a limitation of the digestion temperature was necessary to achieve quantitative results.Certain refractory compounds, including nicotinamide, were therefore, not amenable to this technique (see Fig. 4). Three standard materials were chosen for a thorough examination of the effects of tem- perature on the recovery of nitrogen. They were ammonium sulphate, urea and nicotinamide, and represented extremes of resistance to breakdown. Each was run on the digestor a t four temperature settings, the optical densities of the solutions being then plotted against energy input. From the cold state, ammonium sulphate gave a gradually decreasing final optical density up to approximately 270" C in the first heat stage, which is run at the higher temperature to clarify the sample.From then on, what appeared to be a rapid loss of ammonia was observed until the temperature reached 475" C, the loss being approximately 50 per cent. The relationship between energy input and temperature in the two stages of the digestor is shown below. These standards contained 800 pg of nitrogen per ml. Energy input, watts . . . . . . 720 960 1210 1310 1440 Temperature First zone, "C . . .. 100 200 360 350 350 { Second zone, "C . . 200 270 350 415 475 The curve for urea increased initially to a maximum at 270" C, then paralleled the ammonium sulphate curve. The nicotinamide curve increased in value over a wide44 MARTEN AND CATANZARO: FUNDAMENTAL STUDIES [Analyst, VOl.91 I I I I I Energy, watts 700 900 I 100 I300 - I! 00 Fig. 4. Effect of temperature on the recovery of nitrogen (flow-rate, 2-5 ml per minute) from: curve A, ammonium sulphate; curve B, urea; curve C, nicotinamide temperature range, most closely approaching the other two curves at approximately 415" C, and subsequently running parallel to them. Neither urea nor nicotinamide reached the maximum shown by ammonium sulphate at lower temperatures. It is also interesting to note the near coincidence of the three curves beyond the nicotinamide maximum, the recovery as compared with that of ammonium sulphate or urea being approximately 96 per cent. This is the reason for the satisfactory performance of the digestor, even at temperatures of 415" C, and in spite of what appear to be nitrogen losses.Obviously, at this stage it was necessary to ascertain whether this nitrogen loss was apparent or real. In other words, it was necessary to establish whether the analysis of ammonium sulphate by the indo-phenol reaction was affected by the digestion temperature or media. Ammonium sulphate was continuously introduced directly into the colorimetric system via the diluent line, no sample being taken in the normal manner. No change was noted in the final optical density obtained with the indo-phenol reaction under these conditions with increasing temperatures of digestion. These results were further substantiated by collecting digestant at different digestion temperatures, and quantitating the ammonium sulphate by the indo-phenol reaction and the ninhydrin reaction of J a ~ o b s .~ The cause of the nitrogen loss still had to be established, and several possibilities were considered- (i) Physical carry-over of the sample and digestion mixture into the vapour phase. (ii) Volatilisation of an ammonium salt, possibly a perchlorate. (iii) Oxidation of ammonia to nitrogen. (iv) Oxidation to nitrogen oxides. Accordingly, the system was modified to allow scrubbing of the vapour phase. Neither ammonia nor nitrogen oxides were found to be present in the scrubbing solutions. The remaining possibility was thus the direct conversion of ammonium salts to nitrogen. As may be imagined, this phenomenon is difficult to verify experimentally. Grunbaum5p6 has found, however, that ammonia is lost in sealed-tube digestions at high temperatures, oxidation of ammonia to nitrogen by sulphur trioxide and oxygen being postulated.+ -+ -+ --+ --+ -+ t H,SO, 0 2 . SO, 6H,SO, 18H20January, 19661 I N AUTOMATIC NITROGEN DIGESTlON 45 There still remained the problem of how to eliminate, or at least reduce, the nitrogen loss. Increased pressure, which created a partly closed system, had no effect, except for some discomfort to the onlookers. I t was thought that it might be possible to use more efficient catalysts or different digestion media, so that comparable results could be obtained at a lower temperature. Baker7 has investigated several catalyst combinations, and has concluded that mercury is the most effective. Selenium, he found, caused decomposition of ammonia above approxi- mately 387" C.However, Baker is not supported by many others, and there are considerable discrepancies in the observations from one paper to another. I 030 x .- 5 020 0" -0 - m .- U 010 C 700 900 I100 I300 Energy, watts Fig. 5 . Effect of temperature on the recovery of nitrogen (flow-rate, 9 ml per minute) from: curve A, ammonium sulphate; curve B, nicotinamide Our results have shown that there is no significant difference in the nitrogen losses obtained in the presence of mercury or selenium catalysts, although it must be remembered that the conditions used were different from those of the classical method for decomposing nitrogenous compounds. It was, however, established that a mixed mercury - selenium catalyst was most efficient.Differences in the perchloric and sulphuric acid contents and in the addition of potassium sulphate had little or no effect. During the experiments at the highest temperature used, it was noted that evaporation almost to dryness of the digestant at the exit of the digestor resulted in the evolution of sulphur trioxide fumes. If these extreme conditions reflected a deficiency in the technique as a whole, then, conceivably, a shortage of digestion fluid would be conducive to the formation of sulphur trioxide, and the subsequent loss of ammonia by oxidation. The delivery of the digestion mixture was therefore increased from the accepted 2-5 ml per minute in steps to 9 ml per minute (see Fig. 5 ) . Also, a graph of the effect of the digestion temperature on the optical density was plotted, and gave a different curve.I t shows that after an initial drop of approximately 10 per cent. there is no further decrease in the optical density over the remainder of the temperature range, k., up to 475" C, in the first heating zone. Also, the curves of all standards so far examined have, after having reached a maximum, levelled-off close to the curve of the ammonium salt. Thus, severe temperature conditions may now be used for cracking the most refractory of compounds rapidly, without the necessity of the results being based on the steeply descending portion of the curve, with a subsequent loss in accuracy. These results were obtained with the standard selenium-catalyst digestion mixture. Under these conditions, nicotinamide gave recoveries of 96 per cent ., when compared with ammonium sulphate standards treated similarly (see Fig.6).46 MARTEN AND CATAKZARO : FUNDAMENTAL STUDIES [Analyst, k'ol. 91 Flow-rate, W AS Waste NaOH Digested sample Sodium hydroxide Alkaline phenol Sodium hypochlorite Debubbler 8 mm tub f/c. 630 mp Fig. 6. Modified flow diagram for the determination of nitrogen in compounds not readily digested TABLE I RECOVERIES OF NITROGEN FROM AMMONIUM SULPHATE AND NICOTINAMIDE STANDARDS Temperature conditions : first stage, 350" C; second stage, 475" C. Flow-rate : 1.3 ml per minute. Digestant: 90 per cent. of sulphuric acid, 2 per cent. of perchloric acid, 0.5 per cent. of mercuric sulphate and 0.02 per cent. of selenium dioxide Nitrogen recovered from- I A 1 Nitrogen present, ammonium sulphate, nicotinamide, /G Per ml % % 200 99.9 100.2 400 99.1 99.6 600 99-1, 99.5 99.1 800 99.1 98.6 1000 99.9 99.9 0.2 x C 0) -0 m U .- v) - .- g 0.1 0 0 Nitrogen present, p.p.m, Fig.7. Calibration curves for the determination of nitrogen in: curve A, sulphate; curve B, nicotinamide 00 ammoniumJanuary, 19661 I N AUTOMATIC NITROGEN DIGESTION 47 The manifold was then modified in the light of the work done on catalyst systems to introduce not only an increased volume of acid but also the required concentration of mercury catalyst. The digestion mixture finally used was 90 per cent. of sulphuric acid, 2 per cent. of perchloric acid, 0.5 per cent. of mercuric sulphate and 0.02 per cent. of selenium dioxide. Since mercuric sulphate is insoluble in concentrated acid, it was introduced separately.The digestion efficiency was improved such that the nicotinamide recoveries obtained over a wide temperature range were 98.6 to 100.2 per cent. This is aptly illustrated in Fig. 7, which shows the coincidence of the two calibration plots. The results used for plotting these curves are shown in Table I. Obviously, the techniques must be modified to suit the sample to be digested, e.g., when the organic content of a sample is high, the perchloric acid content of the digestant must be increased. Experiments have shown, however, that up to 50 per cent. of perchloric acid can be used without invalidation of the principles discussed here, e.g., when digestion of 8 per cent. solutions of sucrose has been successfully accomplished. This work, therefore, has provided a basis on which these modifications can be made. REFERENCES 1. Ferrari, A., Ann. N.Y. Acad. Sci., 1960, 87, 792. 2. 3. 4. 5. 6. Grunbaum, B. W., Schaffer, F. L., and Kirk, P. L., Ibid., 1952, 24, 1487. 7. Marten, J . F., and Ferrari, A., “Proceedings of the 1963 Technicon Symposium on Automatic Kjeldahl, J., 2. anal. Chem., 1883, 22, 366. Jacobs, S., Analyst, 1964, 89, 489. Grunbaum, B. W., Kirk, P. L., Green, L. G., and Koch, C . W., Analyt. Chem., 1955, 27, 384. Baker, P. R. W., Talanta, 1961, 8, 57. Analytical Chemistry,” Technicon Instruments Co. Ltd., 1964, p. 20. Received January 4th, 1966

ISSN:0003-2654    

DOI:10.1039/AN9669100042

出版商:RSC    

年代:1966

数据来源: RSC

摘要:

48 PRISCOTT, HAND AND YOUNG: ANALYSIS OF [Analyst, VOl. 91 The Analysis of Electrolytic Capacitor Electrolyte The Determination of Chloride and Sulphate in the p.p.m. Range BY B. H. PRISCOTT, T. G. HAND AND E. J. YOUNG Fordrough Lane, Birmingham, 9) (Birmingham Materials Section, Test and Inspection Branch, Post O$ce Engineering Department, A method is described for the determination of chloride and sulphate ions in ethylene glycol - ammonium borate based electrolytes from electrolytic capacitors. The chloride concentration is determined by titration with electrogenerated silver ions by differential electrolytic potentiometry, and the sulphate concentration by titration with barium in the presence of Thorin with a photoelectric titrator. Concentrations of 0.3 p.p.m. of chloride and 0.5 p.p.m.of sulphate in the final solution have been satisfactorily determined. Both procedures have applications in a wide range of analyses. The electrolyte is extracted with 80 per cent. ethanol. MODERN telecommunications practice makes high demands on the complex equipment used, and high-reliability, long-life components are therefore imperative for inaccessible equipment such as submerged repeaters in transoceanic cables. The electrolytic capacitors used in such systems are of the conventional type in which aluminium-foil electrodes are separated by paper impregnated with a solution of ammonium borate in ethylene glycol.lY2 It is known that sulphate and chloride ions seriously reduce the reliability of such capacitors when present in concentrations exceeding 10 to 20 p.p.m.As the amount of electrolyte in one capacitor may be quite small (about 1 g) a procedure capable of determining 3 pg or less of each ion is therefore required. The use of barium and mercurous chloranilates3s4 was found to give high blanks and erratic results at these low levels, although they were found satisfactory for other applications in which higher concentrations were encountered. A procedure based on the chloride - mercury - diphenylcarbazone system has been proposed by KemulaJ5 but was not studied in detail. Gravimetric and nephelometric procedures are not applicable as the solubilities of the usual precipitates of silver chloride (1 pg ml-l) and barium sulphate (2 pg ml-1) are too high, and the procedures presented here rely on ion association in solution rather than in a precipitate.EXPERIMENTAL DETERMINATION OF SULPHATE- For this determination the procedure due to Fritz and Yamamura6 was studied, in which the sulphate ions are titrated with barium ions in 80 per cent. ethanolie solution with Thorin as a visual indicator. An E.E.L. photoelectric titrator is used to detect the end-point. Initial trials with 0.01 N barium chloride solution in an Agla micrometer syringe showed that the maximum response was obtained with an Ilford 604 green filter (with peak transmission at 520 mp). During the titration the added barium ions first associated with the sulphate ions and when all of the sulphate had reacted, the barium ions formed a coloured complex with the indicator, producing a change in instrument reading, the end-point being the point of inflection of the curve of instrument reading plotted against volume added shown in Fig.1 (a) and ( b ) . When aliquots of standard sodium sulphate solution were added to 80 per cent. ethanol it was possible to make reproducible titrations down to concentrations of 0.5 p.p.m. in each of the two vessels supplied with the instrument, representing total sulphate contents of 1.5 and 10 pg in the small and large vessels, respectively. At the lower concentration levels there was considerable delay before equilibrium was reached and in order to prevent the titration from becoming too lengthy, readings were taken at 60-second intervals followed immediately by an addition of a further aliquot of titrant to the solution.With similar titrations made in methanol solutions no end-point was detected. Titrations were next made with sulphate added to a synthetic capacitor electrolyte consisting of 20 g of ammonium borate, 45 ml of ethylene glycol and 15 ml of distilled water.January, 19661 ELECTROLYTIC CAPACITOR ELECTROLYTE 49 The ammonium borate was prepared by distilling analytical-reagent grade ammonium hydroxide into a solution of analytical-reagent grade boric acid until strongly alkaline, and then evaporating and crystallising it. Aliquots of sulphate-containing electrolyte were made 80 per cent. with respect to ethanol, and cations were removed by passage through a small column of Amberlite IR 120 resin in the hydrogen form when it was found possible to make titrations down to 0-5 p.p.m.of sulphate. When, however, attempts were made to cover smaller total contents by concentrating the solution by evaporation, poor results were obtained, as shown in Fig. 1 (c) because of the high glycol content that was obtained. This was overcome by evaporating the solution nearly to dryness (after the addition of a few milligrams of potassium carbonate) to remove the bulk of the glycol and taking up the residue in 80 per cent. ethanol. It was then found possible to titrate down to a total content of 1 to 2 pg as shown in Fig. 1 (b) and Table I. t I 1 1 1 1 1 1 ' 1 " 0. 0.0 I 0.02 0.01 N barium chloride solution, ml Fig. 1. Titration of 0.01 ml of 0.01 N sulphate ions with barium chloride: curve A, in 80 per cent. ethanol; curve R, after the removal of glycol; curve C, in the presence of glycol THE DETERMINATION OF CHLORIDE- For the determination of chloride, the application of the technique of differential elec- trolytic potentiometry (D.E.P.) due to B i ~ h o p ~ , * ~ ~ ~ ~ O was studied, in which he has shown that titrations are feasible several orders below the levels concerned here.The theory and practice of this technique have been adequately described7,* and only the basis of the procedure will be given here. If in the titration of chloride ion in a suitable solvent by silver ions, two silver electrodes are immersed in the solution and a small current is passed between them from a constant- current high-impedance source, only a small potential difference exists between them, before and after, the end-point.A t the end-point, however, potentials of several hundred millivolts can be developed and the curve obtained, by plotting titrant added against potential difference, resembles the first derivative of a normal titration curve. This technique has been satis- factorily used for the determination of chloride ions of much lower concentrations than those at which a normal potentiometric titration would give an erratic response, and Bishop has shown that the electrode equilibrium is attained more rapidly when the titrant is electro- generated than when discrete additions are made from a microburette. The electrodes were prepared from 16-s.w.~. silver wires cast in pairs in epoxy resin, cut off square, ground and polished flat so that only the cross-sectional area of the wire was exposed.Silver and silver chloride electrodes were used in the initial trials and both were found to give satisfactory titration curves, the silver chloride electrode giving the higher response. Under the conditions required in the presence of ammonium salts the chloride coating spalled off fairly rapidly and plain silver electrodes were adopted as the standard. These electrodes require activating before use and cleaning with fine emery, only immersing then in 50 per cent. nitric acid for 30 seconds, washing and storing them in distilled water until required for use was found satisfactory.50 PRISCOTT, HAND AND YOUKG: ANALYSIS OF [Analyst, Vol. 91 The silver ions were electrogenerated from a ring anode of 16-s.w.~. silver wire placed symmetrically around the indicating electrodes.The cathode was a copper wire - copper nitrate half-cell separated from the analytical solution by an agar bridge. A current of 45.3 pA, equivalent to 1.0 pg minute-l, was used. As there is no common point between the generation and measuring circuits, high insulation of the generator battery and control equipment from the measuring circuits is imperative, and the battery was placed on rubber bungs to meet this requirement. The electrical circuits are shown in Fig. 2. A = Copper nitrate half-cell B = Indicator electrodes C = Silver anode M = Meter, 0 to 50 pA R, = 5000-megohm resistor R, = I-megohm resistor VR = I-megohm variable resistor Fig. 2. Differential electrolytic potentiometry circuits Trials showed that the optimum differentiating conditions were obtained by using a 112.5 volt supply with a ballast resistor of 5 x lo9 ohms giving a current density of 1.08 x amp cm-2.The electrode potentials were measured with a Pye Dynacap pH meter and recorded on an Evershed and Vignoles recording milliammeter with a 34-inch wide scale and a chart speed of 0.1 inch per minute. With this procedure it was found that good results were obtained down to at least 0.3 p.p.m. in 80 per cent. ethanol made 0.001 N in nitric acid, and that a greater electrode response was given in methanol, as reported by Bishop. In order to obtain a procedure compatible with that for the sulphate determination, the use of ethanol was adopted for the work reported here. It is possible to determine smaller concentrations of chloride by varying the differentiating current, but the level reached was adequate for the present purpose.Additions of boric acid and ethylene glycol in the amounts expected from capacitors were found to have no measurable effect on the titration. With ammonium borate, however, the nitric acid concentration had to be increased to 0-1 N before satisfactory curves were obtained. Under these conditions spalling of the surface of one electrode was found t o occur, but this was minimised by reversing the polarity of the electrodes after each deter- mination. EXTRACTION OF SAMPLE FROM CAPACITOR- As the concentration of sulphate and chloride ions in the electrolyte is required, the complete extraction of the liquid material from the capacitor is not essential as long as no selective extraction occurs.It was found that 80 per cent. ethanol was a satisfactory solvent and that three extractions removed substantially all the titratable ions. Sulphate determinations on each of three extracts were found to give concentrations of 2.80, 3-00 and 3.03 p.p.m., the electrolyte extracts being 4.0, 3.0 and 1.95 g, respectively.January, 19661 ELECTROLYTIC CAPACITOR ELECTROLYTE 51 C o N s E c r T I J- E D E T E K M I K AT I o N o F c H L o R I D E AN D s u L P HATE- V'hile large capacitors yield sufficient material for the two determinations to be carried out in duplicate on separate aliquots of the extract, small capacitors require that both the determinations be made on a single aliquot, the silver that was added during the chloride titration having been removed with other cations by the ion-exchange step prior to the sulphate determination.A solution containing 2.0 pmoles of sulphate gave a titre of 0.0195 ml of 0.01 x barium chloride and a similar solution after a chloride titration corresponding to 50 pg of chloride gave a titre of 0.0197 ml, showing the consecutive determination of the two 10115 to be satisfactory. REAGENTS- 1IJZlHOD Etliaiiol, 80 per ceitt. Bariziin chloride, 0-01 s-Dissolve 1.2215 g of analytical-reagent grade barium chloride in distilled water and make up the volume to 1000 ml. Thoriiz idicntor--A 0.05 per cent. w/v solution of Thorin [2(Hydrox?.-3,6-disulpho- 1-naphthylazo) - phenylarsonic acid sodium salt: in distilled water. This solution is unstable and .hould be prepared daily.Sitvic acid-B.D.H. transktor grade or equivalent. A irzberlite 111120 resi7i-Aiial3.tical-rea~ent grade. APP.IRATVS- Photoelectric titrntor-That supplied by Evans Electroselenium Ltd. was found to be satisfactory. The glass vessels supplied have working volumes of 3 and 20 ml. pH wzetev-A Pye Dynacap meter was found satisfactory but, any pH meter or valve voltmeter with an input impedance of more than Chart recorder-An Evershed and l'ignoles hfurday recorder with a range of 1 mA full- scale deflection, 3000-ohms resistance and a chart speed of 0-1 inch minute-l was satisfactor!-. Agla micrometev s y i q e or similar equipment. Ion-exclzaiige coZzinz~z-A glass tube approximately 5 inches long by Q inch diameter with a tap at the lower end is suitable for this column.A small piece of quartz wool placed above the tap retains the resin within the column, and a suspension of Amberlite IR120H analytical-grade resin in water is poured in to a depth of about 8 inch. Wash the resin twice with water, twice with 80 per cent. ethanol and allow to drain. With care in construction the bed volume and dead space can be kept to a minimum so that washing volumes can be small. The resin is re-activated by stirring it with N hydrochloric acid for 5 minutes, then filtering and washing it until it is free from chloride. Store it under water until required. DiferePztiating circz~it-~4 battery of approximately 100 volts and stable resistance of 5 x lo9 ohms are used. Megistors obtained from the Morgan Crucible Co.are suitable. The circuit is built into a metal screening container and all external connections are made with co-axial cable, the outer conductor being used as a screen. Insulation of the ballast resistor from the core is critical as many materials regarded as insulators are of the same order of conductivity as the ballast resistors. Similarly, the insulation of the change-over switch must be good. Electrugeszeratiw circuit-Any batter>. - resistance combination giving a constant electro- generative current may be used. A high potential and a high resistance provide niore stable conditions. The effective insulation of all parts of this circuit (except the electrodes in the solution with the indicator electrodes) from the differential circuit is essential to prevent spurious potentials from being indicated on the pH meter.The circuit is shown in Fig. 2. DiffeerentiaL electrodes-Two 1-inch lengths of 16-s.w.g. silver wire are soldered on to lengths of tinned-copper wire, and cemented by means of Araldite AVlOO resin into two shallow grooves cut longitudinally in a 3 x 3 x +-inch piece of Perspex such that the free end of the silver wire slightly projects past the edge of the strip. When the resin is cured, the wires are covered with a layer of resin 9-inch deep, free from air bubbles, and the resin is allowed to cure fully. The end of the assembly is then ground-off square and polished wet, up to 500-mesh carborundum paper. The copper wires at the other end of the strip are used to make connection to the differentiating circuit through coaxial cable. The electrodes are activated by immersing them in 50 per cent.nitric acid for 30 seconds, then washing and storing them in distilled water until required. When the electrodes become inactive in use they are polished on fine abrasive paper and re-activated as described. ohms may be used. The circuit is shown in Fig. 2.52 PRISCOTT, HAND AND YOUNG: ANALYSIS OF [Analyst, VOl. 91 Electrogenerative electrodes-A glass tube 3 x & inches is half filled with a hot 10 per cent. solution of agar containing 5 per cent. of analytical-reagent grade potassium nitrate. When the agar is cold and set, a 20 per cent. solution of analytical-reagent grade copper nitrate is placed in the empty end until this is two-thirds full.A copper wire is immersed in the copper nitrate solution, and the whole mounted vertically in a suitable holder. This forms the cathode half-cell with its connecting agar - salt bridge. The anode is made by bending a length of 16-s.w.~. silver wire into the form of a horizontal loop with a vertical connecting length. Electrode stand-The electrodes are mounted on a suitable stand constructed from Perspex sheet. A symmetrical arrangement is essential to minimise any potential induced in the measuring circuit by the generating current. PROCEDURE- Extraction of eZectroZyte-Cut, or grind away the metal case of the capacitor, carefully extract the inner coil and weigh it on a watch glass. The coil is then loosened to allow ready ingress of solvent, and the capacitor coil is extracted with the minimum amount of 80 per cent.ethanol in a suitable vessel. Three extractions are generally sufficient, after which the coil is dried on the watch-glass in an air oven at a temperature of 105" C, cooled and re-weighed. The loss in weight is taken as the weight of the electrolyte. Determination of chloride-A suitable portion of the capacitor electrolyte in 80 per cent. ethanol is placed in a small squat beaker together with a stirrer magnet. Sufficient N nitric acid is added to the solution to make it 0.1 N with respect to nitric acid. The electrode assembly is introduced, the stirrer started and the differential circuit switched on. After about one minute, during which the potential settles down, the electrogenerative circuit is switched on at 45.3 WA and the chart recorder is started simultaneously. The recorder now plots the course of t'he titration and a characteristic peaked curve I 0 4 8 1 2 1 6 ; Time, minutes 0 I " ' 1 " 1 " is produced, as shown i& Fig.3. 1 Fig. 3. Differential clcctrolytic potentiometry titration curve (titra- tion of 9 pg chloride ions at 45.3 FA) The comparatively high nitric acid concentration used causes spalling-off of the surface film on one electrode after prolonged use, which gives rise to a high differential-electrolytic poten- tiometric base potential. This can be avoided by reversing the indicator electrode polarity between determinations. The time taken to reach the peak of the curve is measured. The peak is rather rounded and the best value is obtained by considering the mean position of the comparatively flat portion at the top.At 45.3 pA, each minute represents 1 pg of chloride ion. Determination of sul$hate-A suitable volume of the extracted electrolyte obtained is taken and a few milligrams of potassium carbonate are added. The solution is carefully evaporated nearly to dryness in a small squat beaker and then cooled. A small amount of water is added to the solid and boiled to dissolve the residue and to rinse the sides of theJanuary, 19661 ELECTROLYTIC CAPACITOR ELECTROLYTE 53 beaker. When the solid is dissolved, the solution is cooled and made 80 per cent. ethanolic. The solution is then passed through the cation-exchange column, the effluent being collected in the small titration vessel. The column is washed twice with 80 per cent.ethanol. Since the column is not working as a true chromatographic column it can be drained before the sample is introduced and after each washing. The solution is then placed in the titrator and an Ilford No. 604 filter is inserted in the instrument; the stirrer is started and Thorin indicator is added to the solution until the instrument, with the sensitivity turned to maximum and the zero control turned almost fully clockwise (i.e., the condition of maximum sensitivity of the instrument), reads 20 divisions on the upper red logarithmic scale. The microburette is filled with 0.01 N barium chloride solution and fitted so that the tip of the jet is just above the surface and can be just dipped into the surface to introduce each increment of titrant.Titration is carried out by introducing equal increments, usually 0.001 ml, at fixed-time intervals, normally of one minute, and reading the galvanometer at the end of each minute until the reading has risen to a high value (about 50 to 80). A graph is then constructed of galvanometer reading against titration, and the end-point is determined from the point of inflection of the curve. Consecutive determination of chloride and sulphate-The chloride concentration is deter- mined by the method described, and after completion of the titration the electrodes and magnetic stirrer are removed, washed down into the vessel with 80 per cent. ethanol andthe solution is used for the determination of sulphate as described before. REPRODUCIBILITY A synthetic capacitor electrolyte was prepared containing 20 g of ammonium borate in 100 ml of ethylene glycol.Additions of chloride or sulphate were made to 10-ml aliquots of the electrolyte which were then diluted to 100 ml with 80 per cent. ethanol and the additions were determined by the procedures described above. The results obtained are shown in Table I. TABLE I DETERMINATION OF CHLORIDE AND SULPHATE IN A SYNTHETIC CAPACITOR ELECTROLYTE Concentration, p.p.m. Coefficient of r A \ variation, Addition Added Found mean per cent. Sulphate . . , . 2.88 1.68, 1.78, 1-94, 1.94 1-84 6-8 Sulphate . . . . 0.94 1.019, 0.966, 0.966, 0.815, 0.938 9.4 0.815, 0.966, 1.019 Chloride . . . . 0.70 0.98, 0-95, 0.97, 1.04, 0.99 3.2 Chloride . . . . 0.35 0.60, 0-68, 0.64, 0.62, 0.62 6.1 0.99, 1.01 0.60, 0.57 The results show that the reproducibility is adequate for the present application, the difference between the two chloride figures of 0.37 p.p.m.agreeing with the difference of 0.35 p.p.m. in the amounts added. CONCLVSIONS It has been shown that the electrolyte can be extracted from capacitors with 80 per cent. ethanol and good yields were obtained by three extractions, the contaminant-to-elec- trolyte ratio remaining constant. The sulphate can be satisfactorily titrated in 80 per cent. ethanol by barium chloride with Thorin indicator on a photoelectric titrator. In methanol solutions or in ethanol solutions containing more than 5 per cent. ethylene glycol, poor results are obtained. The interference from glycol can be removed by evaporating the solution nearly to dryness in the presence of a few milligrams of potassium carbonate and taking up the residue in 80 per cent.ethanol. The interference from the cations is removed by passage through Amberlite IR120 resin in the hydrogen form. The chloride can be determined by differential electrolytic potentiometry titration by electrogenerated silver, by using the electrical conditions described above. Ethylene glycol and boric acid have no effect on the titration but in the presence of ammonium salts the nitric acid concentration must be increased from 0.001 to 0.1 N. This higher acidity causes spalling of silver chloride electrodes and plain silver electrodes are to be preferred. The54 PRISCOTT, HAND AND YOUNG [Analyst, Vol. 91 slight surface spalling encountered was obviated by reversing the electrode polarity between each determination.The consecutive determination of chloride and sulphate has been achieved with good results and the reproducibility is found adequate for the present purpose. These procedures are of general application and have been used for other determinations of halides and sulphate at low concentration levels (the latter preferably in solutions half saturated with boric acid) such as sulphur in nickel alloys, residues on printed-circuit boards and water analysis. Acknowledgement is made to the Engineer-in-Chief of the General Post Office and to the Controller of Her Majesty’s Stationery Office for permission to publish this paper. REFERENCES 1. 2. 3. 4. 5. 6. 7 . 8. -,- , Ibid., 1962, 87, 845. 9. Dunimer, G. TV. A., “Fixed Capacitors,” Pitnian, 1956, p. 125, 3lcKnight Deely, P., “Electrolytic Capacitors,” Cornell - Dubilier Electric Corpn., New Jersey, Brrtaloccini, K. J., and Barney, J . E., .I>zalyf. Clzcnl., 1958, 30, 202. Klipp, R. \V., and Barney, J. E., Ibid., 1959, 31, 596. Kemula, TV., Hulanicki, A., and Janowski, A., Talanta, 1960, 7, 65. Fritz, J . S., and Yamamura, J. S., drzalyt. Climz., 1955, 27, 1461. Bishop, E., and Dhaneshwar, R. G., -4i?nl~~st, 1962, 87, 207. Bishop, E., Dhaneshwar, R. G., and Short, G. D., i n \Test, P. \V., lfacdonald, A\. nl. G., and \T7est, T. S., “Ahalytical Chemistry 1962 : The Proceedings of the lnternational Symposium, Birmingham, in Honour of Fritz Fcigl,” Elsevier Publishing Company, -41nsterdan1, London and Sew I’ork, 1963, p. 236, 1938, p. 68. 10. Bishop, E., and Ilhaneshwar, I<. G., Analyt. Chrtn., 1964, 36, 726. Received iwavch 21id, 1965

ISSN:0003-2654    

DOI:10.1039/AN9669100048

出版商:RSC    

年代:1966

数据来源: RSC