ISSN:0003-2654    

DOI:10.1039/AN96691FX021

出版商:RSC    

年代:1966

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN96691BX023

出版商:RSC    

年代:1966

数据来源: RSC

摘要:

iv SUMMARIES OF PAPERS IN THIS ISSUE [June, 1966Summaries of Papers in this IssueThe Determination of Phenol, o-Cresol and p-Cresol in AqueousSolution by a Kinetic MethodCertain phenols can be determined in aqueous solution by a kineticallycontrolled bromination in which the only measurement is the time taken forthe bleaching of an indicator. This time is proportional to the concentrationof the indicator. Since no specialised apparatus is needed, the method is notrestricted to the laboratory.A. E. BURGESS and J. L. LATHAMDepartment of Chemistry and Biology, Harris College, Corporation Street,Preston, Lancashire.Analyst, 1966, 91, 343-346.A Continuous Monitor for Hydrogen in GasesThe construction and use of an instrument for the continuous monitoringof hydrogen in gas streams is described.The principles upon which theinstrument is based are the catalytic oxidation of hydrogen to water, and thesubsequent determination of the water with an electrolytic hygrometer.Factors relevant to the efficient operation of the instrument for laboratory andplant conditions are discussed. Results indicate a coefficient of variation ofthe order of &Fi per cent. for the 80 to 1000 v.p.m. hydrogen range.J. A. J. WALKER and P. CAMPIONU.K.A.E.A., Reactor Materials Laboratory, Culcheth, Warrington, Lancashire.Analyst, 1966, 91, 347-349.The Determination of Helium-3 in Argon at Levels of 10-l2The helium in the argon is first concentrated by removing the argon onan activated charcoal trap at -190” C.The helium is then transferred to amodified A.E.I. Ltd., MS2 mass spectrometer equipped with a Vibronamplifier, in which the volume of the helium-3 (and helium-4) is measured.With 1-litre samples the limit of detection is approximately 2 x p.p.m.by volume.Current experience on establishing the level of helium-3 in the argonblanket gas of the Dounreay Fast Reactor is outlined.K. R. MELHUISH and H. CHAPMANDounreay Experimental Reactor Establishment, Thurso, Caithness.Analyst, 1966, 91, 350-354.A Precise CoulometerThis paper is a contribution to the application of coulometry to accurateanalysis, and describes an apparatus which can be used for the accurateassay of “pure” sodium carbonate. An instrument has been built whichmeasures coulombs with a probable error of &25 p.p.m.as the product ofa constant current and its time of flow. The current is maintained constantby an electric servo-system, and it is adjusted so that the voltage drop acrossa precise resistor is equal to the e.m.f. of a standard cell. Time is measuredby a quartz-crystal clock. The resistor and cell are checked against localstandards, which in turn have been calibrated against international standardsby the National Physical Laboratory. The clock is checked against a broad-cast frequency and the General Post Office Speaking Clock. Thus the quantityof electricity for a titration is referred ultimately to the fundamental standardsof mass, length and time; the titre is independent of knowledge of the purityof any chemical substance.J. C.QUAYLE and F. A. COOPERImperial Chemical Industries Ltd., Heavy Organic Chemicals Division, P.O. BoxNo. 2, Billingham, Co. Durham.A~~aEyst, 1966, 91, 355-362vi SUMMARIES OF PAPERS IN THIS ISSUEPrecise Coulometry : The Titration of Pure Sodium CarbonateA method is given for the precise and accurate titration of sodiumcarbonate with hydrogen ions generated by the coulometer described in thepreceding paper. This coulometer maintains automatically a constantcurrent for a measured length of time. The results are in good agreementwith those obtained by titration with standard acid referred to pure silveras the ultimate standard, and support proposals to establish the coulomb asa standard in volumetric analysis.Factors are discussed that affect theaccuracy and precision of analysis by controlled current coulometry.F. A. COOPER and J. C. QUAYLEImperial Chemical Industries T,td., Agricultural Division, P.O. Box No. 6, Billingham,Co. Durham.Analyst, 1966, 91,. 363- 373.[June, 1966Iodimetric Determination of Organo-aluminium CompoundsThe alkyl groups in various types of organo-aluminium compounds havebeen shown to react with iodine in hydrocarbon solution, and the stoicheio-metry has becn determined of the reactions occurring between iodine andtrialkylaluminium, dialkylaluminium chloride and dialkylaluminium alkoxidecompounds.Based on these reactions a reasonably rapid and accurate iodimetricmethod has been devised for the determination of low concentrations oforgano-aluminium compounds in various hydrocarbon solvents.Themethod is applicable to the analysis of the hydrocarbon solutions of organo-aluminium catalysts used for the polymerisation of ethylene and propene.Good agreement is obtained between the iodimetric procedure and aprocedure based on conductiometric titration with a standard solution ofisoquinoline for trialkylaluminium compounds and dialkylaluminium chlorides.The iodimetric procedure is also applicable to dialkylaluminium alkoxidecompounds, which cannot be determined by isoquinoline titration.T. R. CROMPTONCarrington Plastics Laboratory, Shell Chemical Company Limited, Carrington,Cheshire.Analyst, 1966, 91, 374-382.A Flame-photometric Method for Determining Traces of Calciumin Lithium ChlorideThe determination of calcium in solutions of lithium chloride (1-5 percent.w/v) containing up to 2 ,u,g per ml of calcium, and up to 1 pg per ml ofaluminium has been investigated. The use of different organic solvents hasbeen studied, and the sensitivity of the determination has been increased3-fold by the use of an aqueous methanol - butanol mixture. It has beenshown that the only serious interference effect, that arising from the presenceof the aluminium content, can be prevented by the addition of trans-1,2-diaminocyclohexane-NNN’N’-tetra-acetic acid (CDTA) . Recoveries fromsynthetic samples showed no appreciable bias, and replicate results indicateda satisfactory precision and a sensitivity of about 0-02 ,u,g per ml.P.EMMOTT and G. LAWM.G.O. Inspectorates, Chemical Inspectorate, Headquarters Building, RoyalArsenal, Woolwich, S.E.18.Analyst, 1966, 91, 383-387viii SUMMARIES OF PAPERS I N THIS ISSUEThe Colorimetric Determination of Hydroxamic AcidsN-Hydroxycarbamates, mono-hydroxyureas and di-hydroxyureas intissue extracts have been determined by diazotising sulphanilamide with thenitrite produced on oxidation and coupling with N - 1 -naphthylethylene-diamine. Mixtures of hydroxylamine and hydroxamic acids were determined( a ) , by selective oxidation a t pH 3.5 and pH 8.0, and ( b ) , after separation ofthe components by thin-layer chromatography.R. NERYChester Reatty Research Institute, Institute of Cancer Research, Royal CancerHospital, Fulham Road, London, S.W.3.[June, 1966Analyst, 1966, 91, 388-394.Plant Mineral Analysis by X-ray Fluorescence SpectrometryShort PaperR.JENKINS, P. W. HURLEYM.E.L. Equipment Co. Ltd., London.and V. M. SHORROCKSHill Farming Research Organisation, Edinburgh.Analyst, 1966, 91, 395-397.The Determination of Total Available Oxygen in Di- tertiaryShort PaperButyl PeroxideD. B. ADAMSLaporte Chemicals Ltd., Luton, Bedfordshire.Analyst, 1966, 91, 397-399.Modification of a Simple and Rapid Titrimetric Method forDetermining Carbon in Iron and SteelShort PaperR. F. JONES, P. GALE, P. HOPKINS and L. N. POWELLThe Steel Company of Wales Limited, Abbey Works, Port Talbot, Clamorgan.Analyst, 1966, 91, 399-400.A Simple Method of Preserving Thin-layer ChromatogramsShort PaperH. A. FONERCeramics Department, The Houldsworth School of Applied Science, The Uni-versity, Leeds 2 .Analyst, 1966, 91, 400-401.The Detection of Cashew-nut Shell Liquid by Thin- layerChromatographyShort PaperT. W. HAMMONDSTropical Products Institute, Gray’s Inn Road, London, W.C. 1.Analyst, 1966, 91, 401-402

ISSN:0003-2654    

DOI:10.1039/AN96691FP117

出版商:RSC    

年代:1966

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN96691BP125

出版商:RSC    

年代:1966

数据来源: RSC

摘要:

JUNE, 1966 THE ANALYST Vol. 91, No. 1803 The Determination of Phenol, o-Cresol and p-Cresol in Aqueous Solution by a Kinetic Method BY A. E. BURGESS AND J. L. LATHAM (Department of Chemistrjl and Biology, Harris College, Corporation Street, Preston, Lancashire) Certain phenols can be determined in aqueous solution by a kinetically controlled bromination in which the only measurement is the time taken for the bleaching of an indicator. This time is proportional to the concentration of the indicator. Since no specialised apparatus is needed, the method is not restricted to the laboratory. PHENOLIC substances are often determined by bromination in aqueous so1ution.l y 2 9, One method is to add an excess of a standard solution of bromate and bromide ions to an acidified solution of the phenol.After a suitable interval of time, during which the bromination reaction proceeds to completion, an excess of iodide ions is added, and the iodine liberated is back-titrated with standard thiosulphate solution. Phenol has also been determined in micro amounts by using the electrochemical generation of b r ~ m i n e . ~ This paper shows that phenol, o-cresol and 9-cresol can be determined bromometrically in dilute aqueous solution, without the necessity of using a titration or an electrochemical technique. Analytical use is made of the kinetic characteristics of the following two reac- tions- (a) the production of bromine in an aqueous solution containing bromide, bromate and hydrogen ions ; ( b ) the nuclear mono-bromination of the phenol. The observed variable is the time of bleaching of an indicator, and the conditions are chosen so that the observed time is directly proportional to the concentration of phenol used.The theoretical basis of the method is that when a mixture of bromide and bromate ions to which phenol has been added is acidified, a stationary-state concentration of bromine is set up in accordance with the following equations- * . (1) . . * * (2) HfBr0,- -+ H+Br- -% products (ultimately bromine) . . ArH, + Br, -% ArH,Br + H+Br- * . . . . . where Ar represents an aromatic nucleus. The rate constant for reaction (2) is large, as would be expected from the fact that the addition of bromine water to aqueous phenol produces an immediate precipitate of tri- bromophenol. Bell and Rawlinson5 found the value of k , for phenol, at 25" C, to be 1.8 x lo5 litre per mole per second.Because of the large rate constant for nuclear bromination, the concentration of bromine is so low that the stationary-state hypothesis may be applied to the system until the point is reached at which the phenol is completely converted into tribromophenol. Consequently, at all stages of the reaction, both before and after the mono-bromination stage, the rate of nuclear bromination equals the rate of generation of bromine from bromide and bromate ions. Under these stationary-state conditions, the rate of nuclear bromination is independent of the chemical nature and the concentration of the phenol, for it is determined only by the rate of reaction (1). A further consequence of the stationary state is that, during mono-bromination, the concentration of bromine is so low that an azo dye, such as methyl orange, is not bleached significantly in the course of a few minutes.The essential feature of the analytical method proposed is that the de-activating effect of the bromine atom on the aromatic nucleus causes the rate constant for di-bromination to 343344 [Analyst, Vol. 91 be much less than that for mono-bromination. As a result, the stationary-state concentration of the bromine rises rapidly as the mono-bromination stage is approached, when it reaches a level at which methyl orange is rapidly bleached. This is confirmed potentiometrically for, at the time when the methyl orange bleaches, there is a sharp increase in the slope of the curve of redox potential against time.The time of bleaching for given initial concentrations of bromate, bromide and hydrogen ions a t a given temperature depends only on the concen- tration of the phenol. The over-all stoicheiometry for the mono-bromination of a phenolic molecule (ArH) by an acidified aqueous solution of bromate and bromide ions, is given by the equation- As has been shown above, the rate of nuclear bromination is independent of the concentration of phenol, and so the reaction shown in equation (3) obeys the kinetic rate law6 for the bro- mide - bromate reaction, namely- SELECTION OF OPTIMUM INITIAL COKCENTRATIOE- Let a Ee the initial concentration of bromate in the reaction mixture, b, the initial concentration of the phenol, R,, the initial rate of the bromate - bromide reaction, A,, the rate of the bromate - bromide reaction when the indicator is bleached, k , the fourth order rate constant of the bromate - bromide reaction (equation 4), and t,, the time of bleaching of the indicator.To obtain a linear calibration graph of phenol concentration against time of bleaching, the rate of the bromate - bromide reaction (given by equation 4) must be effectively constant. Mathematical analysis of equations (3) and (4) shows that deviations from linearity are minimised if the initial concentrations of bromate, bromide and hydrogen ions are in the ratio in which they are consumed in the bromination reaction, namely 1 : 2 : 3. If this is so, then- As 3 molecules of phenol are consumed for each bromate ion consumed, then- BURGESS AND LATHAM : DETERMINATION OF PHENOL, 0-CRESOL BrO,- + 2Rr- + 3H+ + 3ArH -+ 3ArBr + 3H,O .. . . * - (3) - d[BrO,-]/dt = R = K[BrO,-] [Br-j [H+I2 . . . . - * (4) . . * * (5) * * (6) . . - - (7) Hence R,/R, = (1 - b / 3 ~ ) ~ . . . . a.e. R,/R,= 1 - 4b/3a, if b < 3a . . .. - - ( 8 ) R, = 18ka4 . . . . . . R, = 18k(a - b/3)4 where b < 3a . . Equation (8) shows that the rate at the time of bleaching is always less than the initial rate, but, providing that b < a , the rate is effectively constant and the approximation Rl = R, is nearly exact. If this is so, then, since b mole per litre of the phenol are mono- brominated in time t,- R, = 18ka4 c= b/3t, . . . . * . * * (9) i.e. Substitution of a known value of k in equation (10) enables the initial value of a to be calculated for any desired value of t,.Examination of equation (8) shows that the condition for the rate to be constant up to the time of bleaching is that b / a must be so small that K J R , fi 1. However, for a pre-selected time of bleaching, a and b are related by equation (10). This equation shows that for a given time (t,) and concentration of phenol (b),"the value of b/a decreases as the rate constant ( k ) decreases. Lowering the temperature reduces k and, consequently, also reduces b/a. In practice, a value of t, of up to 200 seconds at 0" C proves to be convenient, as this enables an ice-bath to be used for temperature control. METHOD REAGENTS- potassium bromate, and 0.2 M with respect to potassium bromide. respect to sulphuric acid, containing 10 mg of methyl orange per litre of solution.Bromate - bromide stock solzttion-Prepare an aqueous solution, 0.1 M with respect to Sulfihziric acid - methyl orange solzctioqi-Prepare an aqueous solution, 0-15 M (0.3 x) withJune, 19661 AND @-CRESOL IX AQUEOUS SOLUTIONS BY A KINETIC METHOD 345 PROCEDURE- Place 25 ml of the sulphuric acid - methyl orange solution in a 50-ml calibrated flask and add an aliquot of the unknown phenol solution. Dilute the contents of the flask to the mark with water. The resulting sample solution should be not more than 0.004 M with respect to the phenol. Place 10 ml of the bromate - bromide stock solution and 20 ml of the sample solution prepared above, in separate clean, dry boiling-tubes and allow the solutions to cool to 0" C in an ice-bath.Start a stop-clock, and after noting the time, pour the contents of one tube quickly into the other tube. Uniformity o f composition is ensured by transferring the result- ing solution quickly from one tube to the other twice more. Then place the reaction mixture back in the ice-bath over a white tile, and look vertically down through the solution. Record the time when the last tinge of red colour of the indicator disappears. Calibrate the method by using a solution of phenol of known concentration (0.01 M is suitable), and by varying the amount added in making the sample solution. To standardise the timing procedure, a blank experiment is made with the mixed solutions of sulphuric acid - methyl orange and bromate - bromide, but without phenol.This gives a bleaching time of about 3 seconds. The value for the blank is subtracted from the values obtained in the determination, and corrected times are used. The bleaching of the indicator occurs over the course of a few seconds. RESULTS AND DISCUSSION The results of 18 measurements on phenol, o-cresol and 9-cresol are shown in Fig. 1. The standard deviation The continuous line was calculated by the "least squares" method. from this line is 3.3 seconds. 0 Fig. 1 . Time of bleaching for various concentratioris: 0, phenol; A, o-crcsol; and ;< , p-cresol These results suggest that the method is capable of giving rapid analyses to an accuracy of 3 per cent. Equation (10) requires the slope of the calibration graph, shown in Fig. 1, to be 54ka4.The observed slope is 2-0 x mole per litre per second, whereas the calculated value under the quoted experimental conditions is 2.8 x mole per litre per second, assuming k to be 0-42 litre3 per mole3 per second. This agreement is acceptable, bearing in mind, (a), that this value of k has been extrapolated from a value7 at 25" C by using the Arrhenius equation, and, ( b ) , that the rate constant for this reaction varies considerably with ionic strength.8 In the examples quoted in Fig. 1, the time for the first precipitation of di-brominated or tri-brominated products was greater than that for the bleaching of the indicator. However, with m-cresol, precipitation occurred before the indicator was bleached. Examination of the structures of o-cresol, m-cresol and 9-cresol shows that it is only in the meta isomer that the346 BURGESS AND LATHAM [Analyst, Vol.91 methyl group is ortho or para to the site of substitution. As the methyl group activates strongly at the ortho and para positions and has little effect at the meta position, the rate constant for the bromination of m-cresol is much higher than that of o-cresol and 9-cresol. Hence, the mono-brominated m-cresol can be di-brominated at a rate comparable to that for mono-bromination of o-cresol and 9-cresol, and consequently the indicator method fails for m-cresol. REFERENCES 1. 2. 3. \.’ogel, A. I . , ‘ I A 4 Text-Book of Quantitative Inorganic Analysis,” Third Edition, Longmans, 4. 5. 6. 7 . 8. Koppeschaar, W. F., 2. analyt. Chem., 1876, 15, 233. Riemschneider, R., Chim. Ind., 1951, 66, 806. Green & Co. Ltd., London, 1961, p. 388. Kozak, G. S., and Fernando, Q., Analytica Chim. Acta, 1962, 26, 541. Bell, R. P., and Rawlinson, D. J.. J . Chern. SOC., 1961, 54, 63. Skrabal, A,, and Weberitsch, S. R., Mh. Chem., 1915, 36, 211. “Tables of Chemical Kinetics, Homogeneous Reactions,” National Bureau of Standards ( U . S . ) , Bray, W. C., and Liebhafsky, H. A4., J . Amer. Chem. SOC., 1935, 57, 51. Circular 510, 1951, 669. Received June 28th, 1966

ISSN:0003-2654    

DOI:10.1039/AN9669100343

出版商:RSC    

年代:1966

数据来源: RSC

摘要:

June, 19661 WALKER AND CAMPION 347 A Continuous Monitor for Hydrogen in Gases BY J. A. J. WALKER AND P. CAMPIOX ( U . K.A .E. A ., Reactor MateriaZs,Laboratory, Cztlcheth, Warrington, Lancashire) The construction and use of an instrument for the continuous monitoring of hydrogen in gas streams is described. The principles upor! which the instrument is based are the Catalytic oxidation of hydrogen to water, and the subsequent determination of the water with an electrolytic hygrometer. Factors relevant to the efficient operation of the instrument for laboratory and plant conditions are discussed. Results indicate a coefficient of variation of the order of $5 per cent. for the 80 to 1000-v.p.m. hydrogen range. IN a previous paper1 a technique was outlined for the continuous monitoring of hydrogen in carbon dioxide based gas mixtures.The work reported here is a description of the con- struction, operation and performance of an instrument used for monitoring hydrogen, both in the laboratory and under plant conditions. The principles upon which the instrument are based is the catalytic oxidation of hydrogen to water, and the determination of the latter with an electrolytic hygrometer. The instru- ment may be used for hydrogen monitoring of inert-gas streams other than carbon dioxide. EXPEKIMENTAL The apparatus is shown schematically in Fig. 1. APPARATUS- I t is housed in an 18 x 18 x 9-inch aluminium cabinet. The front panel contains a variable auto-transformer and pyrometer for controlling and indicating the temperature of the catalytic furnace, gas-input connections, flow controllers, rotameters and the hydrogen concentration meter.Two molecular-sieve 5 A driers, the electrolvtic cell and the catalvtic furnace are fixed on the rear Danel. The concent ration recorder. A = B = c = of hydr&en is recorded continuously by means of a 10-mV poientiometric L Recorart + To v e n t Molecular sieve, S A , I /16-inch D = Catalyst tube and furnace, 300' C Millaflow flow controllers F = Electrolytic cell Rotameters for sample and oxygen G = Electronics Schematic diagram of hydrogen monitor pel lets, ambient temperature E = Total-flow volameter Fig. 1 . The catalytic furnace consists of a stainless-steel vessel that is resistance heated and packed with small pieces of platinum ribbon. The heater is rated to give a maximum temperature of 800" C at the platinum surface.At the time of assembly, the stainless-steel pipework, desiccant containers and catalyst tube (together with its platinum) must be cleaned according to a standard procedure,l and all gas lines must be thoroughly dried. All couplings are of the de-mountable O-ring or compression-seal type.348 WALKER AND CAMPION: A CONTINUOUS [Analyst, 1701. 91 PROCEDURE- Connect the sample gas and oxygen supply to their respective pxitions on the front panel of the instrument, and adjust the gas pressure to 20 p.s.i. The optimum flow-rates are 100 ml per minute and 10 ml per minute for sample and oxygen, respectively, at a furnace temperature of 500" C. With a new instrument it has been found that clean-up periods of 3 days are required, with argon as the purge gas.A blank reading for the instrument is determined by passing oxygen or helium through the system. Under the above operating conditions, hydrogen values of less than 5 v.p.m. are observed, which are considered to be negligible. Replace the sample gas periodically with a standard hydrogen - carbon dioxide mixture to check the efficiency of the catalyst and electrolytic cell. Exposure of the electrolytic cell to an excess of moisture (most likely to occur during the commissioning of the instrument) leads to "flooding" and, possibly, to damage of the phosphorus pentoxide film. Rapid re- coating of the cell may be carried out according to the manufacturer's instructions. RESULTS AND DISCUSSION The following important factors were considered in the design and operation of the instrument.Cleanliness of s w faces- I t is of paramount importance to ensure that all internal stainless-steel surfaces in contact with the sample gas or gases are thoroughly clean with respect to grease and oxide. Surfaces not subjected to the cleaning technique previously describedl may act as a "sponge" to moisture and a high instrument blank may result. This stricture particularly applies to the catalytic unit and the tubing between the latter and the electrolytic cell. Minimzim delay time- The time required to register an alteration of hydrogen concentration in the sample gas is dependent on the delay volume and the flow-rate of sample gas. This delay volume of the pipework, valves and catalyst vessel must be as small as possible, consistent with efficient operation of the instrument.In the present design, the sample gas drier, immediately prior to the Millaflow flow controller, constitutes 95 per cent. of the delay volume. The equilibra- tion time required for the instrument described is of the order of 10 minutes. Minimum maintenance- The instrument has been used over plant operating periods of several weeks during which time no maintenance has been required. The equilibrium moisture capacity of molecular sieve 5 at 26" C and inlet moisture concentration* of 100 v.p.m. is approximately 30 per cent. w/w. Therefore, for a sample gas flow-rate of 100 ml per minute, containing 100 \'.p.m. of moisture, the life of the sample gas drier will be o f the order of 5 years.Frequent reduction in oxidation eficiency of the catalyst, for example, by the deposition of carbon from thcrmally unstable organic compounds or irreversible poisoning 1 ) j r sulphur compounds, must be avoided. In addition, potential catalyst poiwns, such as sulphur compoundy, are polar molecules and are therefore likeljv to be retained firmly on the molecular-+xe drier. Calibralinu -- Although the electrolytic hygrometer is a quantitative instrument and, therefore, frequent calibration is not necessary, the provision of a standard hydrogen gas supply facilitates rapid calibration of both the platinum catalyst and the electrolytic cell. A series of four hydrogen - carbon dioxide gas mixtures was prepared and analysed by means of helium gas cl-~romatography.~ The hydrogen concentration5 were 80, 160, 510 and 920 v.p.m.The effect of varying the catalyst temperature over the range of 150" to 500" C and halving the sample gas flow-rate to 50 ml per minute was investigated with the 920 v.p.m. hydrogen standard-gas mixture. The results (shown in Table I) indicate that complete oxidation of the hydrogen was achieved over the temperature range 300" to 500" C for a 100 ml per minute sample flow-rate and 200" to 500" C for a 50 ml per minute sample flow-rate. In addition, it was shown that varying the oxygen flow-rate from 2 to 30 ml per minute did not affect the indicated hydrogen concentration.June, 19661 MONITOR FOR HYDROGEN IN GASES 349 TABLE I VARIATION OF CATALYST TEMPERATURE AND SAMPLE FLOW-RATE Catalyst Standard gas Hydrogen monitor Indicated hydrogen temperature, flow-rate, reading, concentration, " C ml per minute v.p.m. v.p.m.500 50 470 940 500 100 910 910 450 50 480 960 450 100 910 910 400 50 470 940 400 100 930 930 350 50 470 940 350 100 890 890 300 50 470 940 300 100 900 900 250 50 460 920 250 100 359 359 200 50 465 930 200 100 138 138 150 50 83 166 150 100 83 83 A typical calibration of the instrument over the range 0 to 1000 v.p.m. of hydrogen, for a sample flow-rate of 100 ml per minute, oxygen flow-rate of 10 ml per minute and catalyst temperature of 300" C, is shown below- Hydrogen in standard gas, v.p.m. 920 513 160 80 Hydrogen monitor, v.p.m. . . 930 490 170 85 Calibration of this type, together with the determination of hydrogen in a gas mixture that had been analysed by helium gas chromatography, have indicated a maximum error of t-5 per cent.for the instrument. TNTEKFERENCE- Any hydrogenous compound that passes through the molecular sieve and is oxidised to water on the catalyst will create a positive error in the recorded hydrogen content. The lower alkanes and alkenes are examples. The four lowest alkanes were used to investigate the interference effect on the instrument. Methane, ethane, propane and butane were added to carbon dioxide to provide four sources of the basic gas with 500 v.p.m. of alkane as impurity. A mixture of alkane and carbon dioxide, flow-rate 100ml per minute, together with oxygen, flow-rate 10ml per minute, were passed through the instrument at a catalyst temperature of 300" C.The results are shown below- Methane Ethane Propane Butane Percentage of oxidation . . . . <0.001 0.5 1.0 2.0 On the basis of these results, the operating temperature of 300" C was selected in order to minimise interference by oxidation of higher hydrocarbons. 0 PE 1 ~ 4 T I 0 N l-1 N D E R PLAN T C 0 N D IT1 0 pi S- The hydrogen content o f the gas coolant in the Advanced Gas-Cooled Reactor at M'ind- scale was continuously monitored by gas chromatography and hydrogen monitor for a period of 6 weeks. The gas chromatograph and hydrogen monitor values were compared for a nominal 400-v.p.m. hydrogen concentration. A relative deviation of 7 per cent. was found for 137 paired determinations. 'The difference between this deviation and the deviation obtained with hydrogen standards (i.e., 5 per cent.) was attributed to a slight difference in coolant-sampling position and t o the deviation normally found in routine helium iunisation chromatography . C o N c LI! SI ON An instrument has been devised for the continuous monitoring of hydrogen in carbon Operation parameters have been determined for maximum efficiency An assessment for continuous monitoring under plant conditions has been dioxide and inert gases. and specificity. made and the instrument found to be satisfactory. REFERENCES 1. 2. Linde. Data Sheet No. 9890-E. 3. Ltd., London, 1962, p. 321. Walker, J . -4. J., and Campion, P., 14naZyst, 1965, 88, 280. Berry, R., in van Swaay, M., Editor, "Gas Chromatography 1962,'' Buttcrworths & Co. (Publishers) Received Oclohev 29th, 1965

ISSN:0003-2654    

DOI:10.1039/AN9669100347

出版商:RSC    

年代:1966

数据来源: RSC

摘要:

350 MELHUISH AND CHAPMAN: DETERMINATION OF [Analyst, VOl. 91 The Determination of Helium-3 in Argon at L’evels of BY K. R. MELHUISH AND H. CHAPMAN (Dounreay Experimeflntal Reactor Establishment, Thztrso, Caithness) The helium in the argon is first concentrated by removing the argon on an activated charcoal trap a t - 190” C. The helium is then transferred to a modified A.E.1. Ltd., MS2 mass spectrometer equipped with a Vibron amplifier, in which the volume of the helium-3 (and helium-4) is measured. With 1-litre samples the limit of detection is approximately 2 x p.p.m. by volume. Current experience on establishing the level of helium-3 in the argon blanket gas of the Dounreay Fast Reactor is outlined. THE measurement of the level of helium-3 and its day-to-day variation in the argon “blanket” gas of the Dounreay Fast Reactor has been carried out. Helium-3 is produced within the reactor from the radioactive decay of tritium, which is produced by two nuclear reactions: (i),.$Li (n, a ) :H on the lithium present as an impurity in the sodium - potassium metal (primary coolant), and (ii), ternary fission in the uranium fuel of the driver charge. The driver charge fuel elements are vented so that fission-product gases escape into the argon gas blanket. Calculations showed that the volume of helium-3 produced from tritium decay should be from about 0.3 to 0.6 ml per year. Therefore, as the argon blanket gas has a volume of about 108 ml, in order to detect the day-to-day change in the helium-3 level it was necessary to measure about 1 to 2 x lop5 p.p.m.by volume. By using refined techniques with a conventional mass spectrometer, the detection limit for the direct determination of helium in argon can be lowered to about 5 p.p.m. by volume and by a simple impurity-concentration technique1 to 0-05 p.p.m. by volume. Therefore, in order to achieve the required 1 to 2 x p.p.m. by volume, a superior concentration procedure was required and, as the sample size was limited to 1 litre, arising from the associated radioactive fission-product gases (radiation levels up to 1 Roentgen per hour per litre), improved spectrometric sensitivity was also required. The determination of 10-s-ml amounts of rare gas is usually only undertaken with special instruments and techniques, e.g., as used on rare gases from meteorites.2 EXPERIMENTAL MASS SPECTROMETRIC DEVELOPMENT- A standard A.E.I.Ltd., MS2 mass spectrometer and a Vibron Mass Spectrometer Amplifier Type 51A were made available for the work. By using the standard d.c. amplifier equipped with a 4 x 10lO-ohm resistor the limit of detection for helium was about 5 x lW5ml. The I‘ibron amplifier was then fitted (the Vibron head being fitted with a 1012-ohm resistor and 3-pF condenser). This lowered the detection limit to about 2 x 10P ml, but the noise level was appreciable and the time constant of the detection system long (about 3 seconds) for normal peak-scanning techniques to be used. Measurements had to be made with the instrument controls already “tuned” to the required mass number, and the stability of various voltage and current supplies was of paramount importance.For maximum sensitivity the mass-spectrometer ion-source voltages were adjusted (tuned) to the first ion repellor maximum on mass number 3 for the helium-3 determinations and on mass number 4 for early work with “natural” helium-4. To increase further the sensitivity of the mass spectrometer a 2-litre reservoir of the double-inlet system of the MS2 was replaced with a small reservoir of about 50-ml capacity. Corrections, however, have to be applied to the recorded results to allow for the pumping away of the gas through the mass spectrometer leak valve. (The leak has molecular flow characteristics, and hence the flow is proportional to M-, where M is the molecular weight of the gas concerned.) With these modifications, the detection limit should now be approxi- mately 5 x ml of helium.June, 19661 HELIUM-3 I N ARGON AT LEVELS OF 351 To establish the detection limit of the mass spectrometer, samples of air containing 5.4 p.p.m.by volume of helium were used. Aliquots (0-2 ml) were dispensed into the 50-ml reservoir, and the mass number 4 peak was measured. Five aliquots gave an average reading of 67.2 i 1.4 (2a) divisions on a 10-inch recorder chart (100 divisions = full-scale deflection) with a noise level of approximately 2 divisions. ml of helium-4 could be measured to within k2 per cent. ; 2 x 10 per cent. and the absolute detection limit, i.e., twice noise level, is about 2 x ml. Use of a calibrated helium leak also confirmed the accuracy of these absolute measurements.Taken in conjunc- tion with a 1-litre sample this gave a detection limit of 2 x The major drawback to the detection of helium-3 was that at mass number 3, the peaks from the hydrogen deuteride ion HD+ and, if sufficient hydrogen is present, the association ion H,+ also occur. To resolve the peaks in the mass doublet requires a resolving power of at least 500. Thus 1 x ml is measurable to within p.p.m. by volume. 3He : 3.01699 a.m.u. HD : 3.02289 a.m.u. The standard MS2 has a resolving power of about 200, with the collector slit a t 0.020 inch; this resolving power was insufficient to distinguish the two peaks at mass number 3 in a mixture of helium-3, hydrogen and deuterium (Fig. 1). Decreasing the collector slit width to 0.005 inch gave only sufficient resolving power to distinguish the two peaks (Fig.2). Decreasing of the collector slit reduced the sensitivity by a factor of four. In view of the poor resolution and the lowered sensitivity a hydrogen-removal stage was added to the concentration procedure, the collector slit being opened out again to 0.020 inch. It was found that, with volumes of air in excess of 0.2 ml, there was a distinct suppression effect on the mass-4 peak, probably arising from space-charge effects that caused de-focusing of the ion beam. This set the limit for the volume of concentrated gas from the concentration apparatus. Ion accelerating voltage Fig. 1. Peak shape of mass 3 Fig. 2. Peak shape of mass 3 with O.02-inch collecter slit: graph .A, with 0.005-inch collecter slit ; mixture hydrogen deutcridc only; graph B, a of helium-3 and hydrogen deuteride mixture of helium-3 and hydrogen showing partial resolution of mass-3 deu teride doublct CONCENTKATING THE HELIUM- Helium and argon are both members of the "inert"-gas family, therefore they cannot be separated by chemical procedures, and use must be made of variations in their physical properties.The boiling-point of argon is -1857" C and that of helium is -268.9" C, so the argon can be condensed at liquid nitrogen temperature (-195.So C) leaving helium as a gas.352 MELHUISH AND CHAPMAN DETERMINATION OF [Analyst, VOl. 91 A few experiments were made with a simple apparatus, the argon was condensed in a cold-trap containing an absorbent and the helium was recovered by pumping. It was established that activated charcoal held the argon better than a molecular sieve, and an apparatus was constructed (Fig.3). T U C1 T" 2 To rotary pump A = Sample attachment point F = Concentrate sample tube B = B.T.S. reagent container 0 = 3-way taps C,, C, = Cold traps (- 1 9 6 O C) T = Taps D = Mercury diffusion pump + = Ground-glass joint E = Simple manometer Fig. 3. Schematic diagram of the helium conccntration apparatus A series of experiments was carried out to determine the recovery of helium. The helium was added by using known volumes of air (5-4 p.p.m. by volume of natural helium)- the main constituents of air, i . e . , oxygen and nitrogen, are retained on the liquid-nitrogen cooled charcoal, the helium being pumped from trap 1 to trap 2 and finally into the sample tube.These are results on concentrates from 50-ml aliquots of air that should contain 2.7 x ml of helium. The analyses were carried out on a standard MS2 gas mass spectrometer. The results of the first series of experiments are summarised in Table I. TABLE I RECOVERIES OF HELIUM FROM 50ml OF AIR Total volume Volume of Aliquot of concentrated helium found, Helium added, Recovery, No. gas, ml ml ml per cent. 1 - 2-7 x 10-4 2.7 x 10-4 100 2 3.6 x 10-4 2.6 x 10-4 2.7 x 10-4 96 3 3.7 x 10-4 2.8 x 10-4 2.7 x 10-4 103 4 4.1 x 10-4 2.8 x 10-4 2.7 x 10-4 103 A second series of experiments was then undertaken, in which the samples consisted of Results are 1 litre of helium-free argon and 25 ml of air, i.e., 1-35 x 10-4ml of helium. summarised in Table 11.TABLE I1 RECOVERIES OF HELIUM FROM 25ml OF AIR I N 1 LITRE OF ARGON Sample No. 1 2 3 4 5 6 7 Total volume 3f concentrated gas, ml 2.9 x 10-4 2.3 x 10-4 2.8 x 10-4 4.0 x 10-4 2.5 x 10-4 3.8 x 10-4 2.5 x 10-4 Volume of helium found, ml 1.6 x 10-4 1.3 x 10-4 1.4 x 10-4 1.4 x 10-4 1.4 x 10-4 1.3 x 10-4 1.4 x 10-4 Helium added, ml 1-35 x 10-4 1.35 x 10-4 1-35 x 10-4 1-35 x 10-4 1-35 x 10-4 1-35 x 10-4 1.35 x 10-4 Recovery, per cent. 118 96 104 104 104 96 104June, 19661 HELIUM-3 IN ARGON AT LEVELS OF 353 As good recoveries of helium at this level from 1 litre of argon had been established, a third series of experiments was carried out with a very much lower level of helium. This time the helium-4 in the concentrated gas was measured with the modified mass spectro- meter.It should be noted that the concentration apparatus was made of Pyrex glass and may therefore be slightly porous to natural helium. Blank values on the apparatus were equivalent to 0.2 to 0.3 x Synthetic samples were prepared by using 1 litre of helium-free argon and 0.65 ml of air, i.e., 3.5 x 10-6 ml of helium-4. Results are given in Table 111. ml of helium. TABLE I11 RECOVERIES OF HELIUM FROM 0-65ml OF AIR IN 1 LITRE OF ARGON Volume of Sample helium, No. ml 1 3.8 x 2 3.5 x 10-6 3 3.9 x 10-6 4 7.0 x loF6 5 6.1 x 10-6 6 3-8 x Corrected for blank, ml 3.5 x 10-6 3.2 x low6 3.6 x 6-7 x 5.8 x 3-5 x 10-6 Volume of helium added, ml 3.5 x 10-6 3.5 x 10-6 3.5 x 10-6 3.5 x 10-6 3.5 x 10-6 3.5 x 10-6 Recovery, per cent. 100 92 103 191 166 100 The analysis showed that the hydrogen content of the concentrated gas was undesirably high, and in view of the resolution problem it was decided that it must be reduced to a minimal value.Experiments with helium-3 and varying amounts of hydrogen showed that a 10 per cent. increase in the mass-3 peak arising from H3+ (from the hydrogen) required more than 300 times as much hydrogen as helium. Hydrogen is not removed by activated charcoal a t -196” C ; however, several chemical methods are available and experiments were carried out with palladised asbestos and “B.T.S. reagent” (finely divided copper made up into pellets with an organic binder), as supplied by B.A.S.F. of Germany. Both methods appeared to be equally effective in reducing the hydrogen content, but the B.T.S. reagent was chosen because the addition of oxygen gas to effect the removal of hydrogen was not necessary. Blends, consisting of 1 litre of argon, 3.5 ml of air and 1 ml of hydrogen, were used for the experiments with the two reagents, and the results obtained are given in Table IV.TABLE IV RESIDUAL HYDROGEN FROM 1 ml OF HYDROGEN IN 1 LITRE OF ARGON Reagent and Sample No. Palladised asbestos 1 Palladised asbestos 2 Palladised asbestos 3 Palladised asbestos 4 B.T.S. 1 B.T.S. 2 Total volume 3f concentrated gas, ml 9.4 x 10-4 2.1 x 10-3 1.7 x 10-3 3.7 x 10-3 1.6 x 10-3 1.9 x 10-3 Volume of hydrogen, ml 8.1 x 10-4 1-94 x 10-3 1-62 x 10-3 3-58 x 10-3 1.8 x 10-3 1.51 x lop3 Hydrogen removal, per cent. 99-92 99.81 99.84 99.64 99.85 99-82 The hydrogen remaining was then 2 to 3 x ml, i.e., the hydrogen removal was 99.7 The hydrogen content of the concentrated gas has to be measured to 99.8 per cent.effective. to make sure that it does not exceed 300 times the helium-3 content. OUTLINE OF FINAL METHOD Samples of blanket gas are taken from a sample point in 500-ml stainless-steel, lead- shielded sample vessels at 35 p.s.i., and are allowed to stand for a day to reduce the radio- activity; they are then analysed. Between 500 and 1000 ml of gas are introduced into the helium-concentration apparatus (Fig. 3) and the volume is measured accurately. After passing the sample over the B.T.S. catalyst the argon is condensed in trap 1, the concentrated helium (and neon) is pumped into the interspace above the second Topler pump and then into trap 2, where any remaining traces of condensable gases are removed.The Topler pump is used to transfer the concen- trated gas into the sample tube where a pressure - volume measurement is made.354 MELHUISH AND CHAPMAN y l z a l y s t , Vol. 91 The sample tube is then transferred to the double-inlet system of the MS2 mass spectro- meter where the gas is expanded into the 50-ml reservoir. Measurements are made of the peaks at mass numbers 2, 3 and 4. To save the small amounts of gas from pumping steadily away via the leak valve to the mass spectrometer, the instrument controls are set to the desired mass number with monitor gas in the 2-litre reservoir. The leak valve to the 2-litre reservoir is closed and the leak valve to the 50-ml reservoir is opened for the sample peak height to be measured or recorded.The leak valve is then closed and the procedure repeated at the next mass number. A check is made on the time at which the leak valve was originally opened, To, the length of time for which it is open and the elapsed open-time at which each peak is measured; peak heights are then corrected for “pump-out” rates to give peak heights at To. The corrected peak height is then directly converted to the volume of helium-3 (or helium-4). The mass spectrometer is calibrated immediately before or after each sample by intro- ducing a known volume of an accurately prepared standard gas. RESULTS Sampling from the Dounreay Fast Reactor gas circuit was instituted in late January, 1965. Helium levels encountered were much higher than had been envisaged, being 1 to 2 x p.p.m. by volume of helium-3 and 20 to 50 p.p.m.by volume of helium-4. At these levels the size of sample taken was considerably reduced; 200 to 300 ml was usually quite sufficient. There was a considerable day-to-day variation of the helium-3 (and helium-4) content probably associated with sampling problems in a non-circulatory gas blanket system. Reproducibility has been checked by regular samples from a synthetic blend of helium-3 in argon at approximately 3 x p.p.m. by volume, and by taking several aliquots from one of the reactor gas samples. Results on the synthetic blend are as follows- helium-3 content: 0.0030 * 0.0001 p.p.m. by volume. Results on 3 aliquots from one reactor gas sample- helium-3: 0-8 x lod3 p.p.m. by volume, 0.8 x helium-4 : 31 p.p.m. by volume, 34 p.p.m. by volume, 34 p.p.m. by volume. p.p.m. by volume, 0-8 x p.p.m. by volume; DISCUSSION The limit of the method as used with the Dounreay Fast Reactor argon samples is 2 x 10-5 p.p.m. by volume, but for less radioactive samples the method should be capable of coping with at least 5 litres of argon, i.e., a limit of 4 x Other instrument modifications were considered, notably those by C ~ t h b e r t , ~ but were unnecessary in view of the helium levels encountered. Vse of the Cuthbert modifications should lower the limit to 1 x Recovery of helium from the argon is good and is certainly better than 40 per cent. Reproducibility is good, better than 10 per cent. The method has been proved in use over some 12 months. A method has been devised for measuring very low levels of helium-3 in argon. p.p.m. by volume. p.p.m. by volume. REFEREKCES 1. 2. 3. Parkinson, K. T., and Toft, L., AnaZyyst, 1965, 90, 220. Nier, A. O., in Waldron, J. D., Ediior, “Advances in Mass Spectrometry,” Pergamon Press, London, New York, Paris, Las Angeles, 1959, p. 507. Cuthbert, J., J . Scient. Instrum., 1964, 41, 431. Received December 221.ad, 1965

ISSN:0003-2654    

DOI:10.1039/AN9669100350

出版商:RSC    

年代:1966

数据来源: RSC

摘要:

June, 19661 QUAVLE AND COOPER 355 A Precise Coulometer BY J. C. QUAYLE AND F. A. COOPER (Inzperial Chemical Industries L t d . , Heavy Organic Chemacals Division, P.O. B o x No. 2, Billingham, Co. Durham) This paper is a contribution to the application of coulometry to accurate analysis, and describes an apparatus which can be used for the accurate assay of “pure” sodium carbonate. An instrument has been built which measures coulombs with a probable error of f 2 5 p.p.ni. as the product of a constant current and its time of flow. The current is maintained constant by an electric servo-system, and it is adjusted so that the voltage drop across a precise resistor is equal to the e.m.f. of a standard cell. Time is measured by a quartz-crystal clock. The resistor and cell are checked against local standards, which in turn have been calibrated against international standards by the National Physical Laboratory.The clock is checked against a broad- cast frequency and the General Post Office Speaking Clock. Thus the quantity of electricity for a titration is referred ultimately to the fundamental standards of mass, length and time; the titre is independent of knowledge of the purity of any chemical substance. COULOMETRIC titration as an analytical technique may be said to have originated with the work of Szebelledy and Somogyil in 1938. Since then the subject has developed rapidly, and many papers and reviews have appeared; Swift, Furman, Lingane and Tutundzic have been responsible for most of the early work. The accuracy reported has been adequate for routine analyses, but has seldom been better than kl000 p.p.m.* Despite this, Tutundzic2 suggested in 1958 that the coulomb should replace silver as the ultimate standard3 for acidimetry.Tutundzic’s proposal received considerable discussion, including some opposition. The following year Taylor and Smith4 reported the coulometric standardisation of acids and bases, including sodium carbonate, with the same order of precision as the best volumetric titrations. Their standard deviations ranged from 30 p.p.m. for potassium hydrogen phthalate to 100 p.p.m. for hydrochloric acid. They assayed sodium carbonate with a standard deviation of 70 p.p.m., but they controlled the current manually; the assay was not confirmed independently. The present work seeks to overcome these two objections recognising that, if accuracy as good as, or better than, that by volumetric titration can be attained, it may lead to the recognition of the coulomb as a universal standard in volumetric analysis.Such a standard is independent of absolute knowledge of the purity of the chemical substances used as standards (knowledge that is difficult to obtain), and takes advantage of the high degree of precision that can be reached by electrical measurements. The instrument must therefore be able to determine the number of coulombs taking part in a reaction with a coefficient of variation no greater than 100 p.p.m., this representing about the best that can be obtained in ordinary volumetric analysis. The efficiency of the electrode reaction must approach the theoretical very closely.Experimental work on the coulometric titration of sodium carbonate, to be described later, provides evidence that the neutralisation of bases by electrically generated hydrogen ions can be carried out quantitatively. The entire process, the generation of hydrogen ions and measurement of current and time, is simpler and no less precise than reference of the sodium carbonate to silver, which is probably the best chemical method of assaying sodium carbonate. APPAKATUS This paper describes the apparatus we have designed for the purpose. All the known types of coulometer, including the current - time integrating motor,5 were considered. Others, such as the silver perchlorate method, might give similar precision but none was so directly related to the fundamental physical standards.The instrument comprises (i) a precisely regulated constant-current source, whose output is monitored against a voltage standard, coupled with (ii) a means of measuring the time of flow of the current and (iii) a suitable cell wherein the reaction takes place. The cell is described elsewhere.6 The remaining two main items will now be described. quantities such as voltage and current, as well as chemical quantities. * All errors and deviations are expressed in p.p.rn. throughout the report. This applies to physical356 QUAYLE AND COOPER: A PRECISE COULOMETER [Analyst, Vol. 91 R I I A +YD b /-l f--W 1-7------- +w + D.C.source, R 4 + b constant voltage I I 9 1 Fii manually i I i I ------ ---I Power supply and control unit A, = Fast-acting current stabiliser D = Working reference cell A, = Slow-acting trim amplifier E = Ammeter A, = Amplifier G = Galvanometer B = Controlling reference cell R,, R,, R,, R, = Resistances C = Coulometer cell VR = Manual current adjuster Fig.1. Coulometer block diagram CURRENT SUPPLY (Fig. 1)- This is a rectified a.c. supply, the voltage of which can be adjusted manually to be approximately correct, and is thereafter not disturbed. The current flowing in the cell circuit is controlled at the desired value by comparing the voltage drop across a precise 4-terminal resistor with the reference voltage, so that in maintaining constant voltage drop it also main- tains constant current. This is achieved by varying the impedance of a group of control transistors influenced by two separate correcting amplifiers : (a) a fast-acting circuit which corrects for transient variations in the mains supply but does not have high long-term accuracy, ( b ) a slow-acting circuit which compensates circuit (a) for thermal and other drifts during the titration.Interaction between the two control amplifiers is avoided by the considerable difference between their time constants. Variations in the resistance of intermediate connecting leads and circuits are unimportant. In addition to these automatic corrections a manual adjustment is provided in the form of an adjustable shunt across R,. This enables the current to be brought to the exact value required. It is necessary not only to hold the current steady but also to measure it accurately.This is achieved by the use of a precise resistor connected in series with the titration cell. The value of the resistor is chosen so that, at the desired current, the voltage drop across it shall be equal to the e.m.f. of a Weston unsaturated cadmium - mercury cell. A galvano- meter is used to detect any errors, and manual correction to circuit ( b ) above can then be made. In practice these adjustments are small and are only occasionally required during a titration. A 1-mm deflection of the galvanometer represents a current change of about 7 p.p.m. The current variations can thus be held by automatic means, aided by occasional manual adjustment, to 10 to 20 p.p.m. during a titration, and its value is known to within +ZO p.p.m. For currents of the order of 1 amp a resistor of 1 ohm is used for measurement purposes.Provision is made for the use of lower currents by replacing the 1-ohm standards by 10 or 100-ohm standards. This means that R, and R, must also be replaced with appropriate values at the same time.June, 19661 QUAYLE AND COOPER: A PRECISE COULOMETER CIRCUIT DESCRIPTION 357 The principles of circuit operation are as follows. The current from the main d.c. source (Fig. 1) passes through the five regulator transistors connected in parallel, amplifier A,, resistors R,, R,, and R, and the coulometer cell. R, is a very precise and stable resistor which is accurate within +12 p.p.m. of its stated value (1 ohm), and is used for current measurement as described above. R, and R, are good quality wire-wound resistors, but have neither the stability nor the accuracy of R,.They are used to provide the voltage-drop signals for the two stabiliser circuits (a) and (b). FAST-ACTING STABILISER (a)- The voltage developed across R, is compared with the voltage from potentiometer R,. This is in turn supplied from a stabilised power unit. Any difference between the two voltages will appear as an error signal fed into amplifier A, and thence into the regulator transistors A,. The transistors change their impedance and cause the current to return to its correct value. The control action, which takes place in a fraction of a millisecond, can thus hold the current steady in the face of most cell-impedance changes, mains-voltage variations and so on. The stabilised power unit supplying R, is unlikely to produce transient variations in voltage but will certainly drift.This is corrected along with other slow drifts by the following circuit. SLOW-ACTING STABILISER (b)- The voltage developed across R, is compared with a Mallory RM42R cell, and any error signal is fed into amplifier A, which terminates in a servo motor geared to the potentiometer R,, and this re-sets the operating point of the fast-acting stabiliser. In practice two resistors, R, and R,, are connected in series across the 50-volt stabilised supply. The slow-acting servo motor corrects the value of R,, and R, is manually adjusted before the equipment is used so that R, starts from about the middle of its range. R, and its fine adjustment resistor, R,, are wound with precious-metal wire and use a precious-metal sliding contact, thus eliminating for all practical purposes the electrical noise from these components.The amplifier is a typical chopper type using vacuum tubes with an input sensitivity of about 12 pV. The response time for full-scale travel is determined by the servo-motor gearing and is about 10 seconds. The choice of the reference cell is governed by the requirements of low drift during a titration and the ability to deliver appreciable currents into the amplifier input. The control transistors perform their function by absorbing the difference between the voltage that the titration cell requires, and the output voltage of the power supply. The difference, which must be kept positive, is set manually to a value that is always within the maker’s rating.By means of a switch the titration cell can be replaced by a resistor of approximately equal resistance. This enables the current to be set to the desired value before a titration is started. After 15 minutes the rate of drift is low enough for the equipment to be used. The cell is then switched back into circuit and the current is controlled almost immediately at the correct value. PROTECTION- Protection of the power supply unit against accidental short-circuits is particularly necessary with transistor circuits, and is provided internally by a high-speed electronic trip circuit. Both this and the working of the fast-acting stabiliser are described more fully in Appendix I. TIME MEASUREMENT In the early stages of development a 100-kilocycle per second quartz-crystal oscillator was used, followed by several frequency divider stages using vacuum tubes.For a variety of reasons this proved unreliable and was replaced by the present equipment which uses a 10-kilocycle per second quartz-oscillator, followed by a number of solid-state binary frequency dividers to a final frequency of 10 cycles per second. This registers time in units358 QC'AYLE AND COOPER: A PRECISE COULOMETER [Analyst, Vol. 91 of 0.1 second on a pair of electro-magnetic counters. The counters are run simultaneously as a check on each other. One counter can be pre-set to switch off at a selected number of counts. The crystal is not temperature controlled as such effects are not significant over a range of 15" to 25" C.Checks on the time-keeping of this equipment against standard broadcast frequencies and the General Post Office Speaking Clock show the most significant error to be possibly one unit of 0.1 second at the end of the titration. In a normal period of 6000 seconds, this could be 15 p.p.m. STANDARDS- Current and time require to be measured accurately. Current is measured by observing the voltage developed across a resistor. Thus standards of voltage, resistance and time are required. KO standards of time are kept by the Agricultural Division as there is ready access to certain broadcast frequencies and the General Post Office Speaking Clock. RESISTANCE STANDARD- The lowest resistance (1 ohm) comprises a network of ten 10-ohm resistors, connected in parallel to make a 1-ohm, 4-terminal resistor.The wire is double fibre-glass covered which, besides acting as an insulator, provides a soft bedding layer against differential expansion stresses. The wire material is a grade of manganin with a resistance with temperature curve as Fig. 2. This curve was compensated by the addition of a single-series element of copper to give an improved form with a variation over the range 17" to 32" C of 15 p.p.m. The initial stresses are removed by prolonged annealing. -IZ0 t I I 30 Temperature, "C 10 - 20 0 Fig. 2. Comparison of compensated and uncompensated resistors The resistor, which is enclosed and oil filled, rises to an equilibrium temperature of 3" C above the oil temperature. This enables the final wire temperature to be compared with the resistance - temperature curve (Fig.3) to ensure that no significant resistance change has occurred. During the period of the tests (about 15 months) the resistance value changed from 1.00032 to 1.00035 ohms. VOLTAGE STANDARD- An unsaturated l!eston cadmium - mercury type cell is used (Muirhead D-942-C), which reduces the variations of e.m.f. due to temperature - electrolyte concentration changes found in the saturated type. The cell is enclosed and lagged to reduce temperature differentials between the limbs. The over-all variation of e.m.f. is about 2 pV per "C. Because of the supposed inferior long-term stability of the unsaturated type of cell, the working cell has been compared frequently with the I.C.I. Agricultural Division standards (see below).The results show that the working cell varied by *lo p.p.m. over a period of 3 months.June, 19661 QUAYLE AND COOPER: A PRECISE COULOMETER 359 REFERENCE STANDARDS- Both voltage and resistance standards are maintained by I.C.I. Agricultural Division Standards Laboratory along with a 5-decade standard potentiometer. All these items carry National Physical Laboratory certificates, so the possible errors can be assessed as follows- voltage: 5 p.p,m. * errors in the value of the absolute volt, resistance: within 1 or 2 p.p.m. errors in the value of the absolute ohm. The possible errors of measurement of coulombs are as follows: VOLTAGE ERRORS PARTS PER MILLION . . .. .. f 10 Uncertainty in the absolute volt . . . . National Physical Laboratory certification of standard .. .. f 5 Lagged unthermostatted . . .. .. .. .. .. f 10 Later lagged and thermostatted . . .. .. .. .. r t 4 Uncertainty in the absolute ohm . . .. .. .. .. f 10 Maker’s certification . . . . .. .. .. .. .. $ 1 .. .. * . .. f 5 15 Divisional standard cell temperature- RESISTANCE ERRORS Potentiometer discrimination . . .. TIMING ERRORS (including errors due to the timing of intervals) The probable over-all values for the initial and final arrangements are 25 and 24 p.p.m., respectively. Standard deviations of 50 p.p.m. have been obtained by using the coulometer in work that will be reported later. These results include the errors due to manipulation, weighing and any deviations from stoicheiometric reactions at the electrodes. .. DISCUSSION ACCURACY AND STABILITY- Within the limits of discrimination of the experimental measurements, no change in the working standards has been detected over a period of 3 months.Checking against the Divisional standards has become a precaution against random damage and unforeseen variations. Now that steady values are established, checking can be much less frequent. RELIABILITY- control. on the one occasion when the current drain from it exceeded the maker’s maximum. so that it gives no cause for anxiety. duration of the driving impulses. The working reference cell is protected by a variable high resistance as a sensitivity Its robust construction was shown by its return to its original e.m.f. within 24 hours, The working standard resistor is liberally rated, and its stability has been established, The timer counters gave some trouble at first but this was cured by an increase in the The remainder of the apparatus has functioned continuously without trouble.The original purpose of the work was to produce a coulometer of improved accuracy in comparison with the current - time, and motor coulometers previously r e p ~ r t e d . ~ , ~ A bench instrument was made to demonstrate the working principles, and the results of titration (see following paper6) confirm that our estimated probable errors of the order of 25 p.p.m. are realistic. Improved temperature control of the standards of resistance and voltage, and repeated calibrations at the National Physical Laboratory to establish their rates of drift, will reduce total possible electrical error to about 20 p.p.m.The error in time standardisation should always be negligible. With errors at this level, no further development is profitable until present work in various national laboratories to establish more accurately the values of the Faraday, the volt and the ohm has reduced their probable errors to below the present values. CONCLUSIONS The accuracy of the instrument, and its capability of being checked against internal and external standards, renders it suitable for the most precise work. It is reliable and versatile enough for general purpose work. We thank our colleagues, PuIessrs. J. J. E. Kess and J. Lindsley, who constructed and maintained the apparatus.360 QUAYLE AND COOPER: A PRECISE COC‘LOMETER [Analyst, Vol. 91 Appendix I DESCRIPTION OF REGULATED POWER SUPPLY^,^ (Fig.3) The main 52-volt transformer secondary winding, connected to a “Variac,” enables any voltage between 4 and 52 volts to be set manually. The a x . is rectified and fed to the regulating transistors, TR, to TR,,, which are connected in parallel. Voltage rise on open circuit is minimised by resistors R, and R,. The bases of TR, to TR13 are fed by the differential amplifier TR, and TR,, followed by TR,, TR, and TR,, giving an over-all control factor of 2000. The input signal to this amplifier is the difference between the e.m.f. developed across R, and the nominally equal e.m.f. from the stabilised supply. This supply is stabilised in two stages by gas-discharge tubes and is thus largely free from short-term variations. The current flowing through the cell is by this means held constant against short-term fluctuations arising from mains-voltage variations or changes in cell impedance.Protection against accidental overcurrent is provided by a trip circuit comprising TR,, which monitors the e.m.f. across resistor R,, and, if this is excessive, triggers the bi-stable TR,, and TR15 into its other state. This causes TR, to cut off, and with it TR, and TR,,, thus stopping the current flow before it can cause any damage. The change over of the bi-stable also releases relay RL, and opens the a.c. feed to the rectifier. Thesystemis re-set by applying a temporary earth potential to the base of TR,,, which causes the bi-stable to revert to the normal condition of TR,, conducting and TK15 cut off.The internal power supplies for the amplifiers and protection current are obtained, as shown, from additional secondary windings on the main transformer. These are conven- tional and do not merit comment. The output of the unit is up to 2 amps, 50 volts. Appendix I1 LIST OF COMPONENTS Rl, R2, R, = Precise 4-terminal resistors (see text) = 500-ohm potentiometer, palladium - silver, with J & M alloy 625 wiper = 0-076-ohm. 5-watt. wire-wound resistor = 1000-ohm, &watt, wire-wound resistors = 50,000-ohm potentiometer, molybdenum - palladium - gold, with J & M alloy = 33-ohm, +watt resistor = 15,000-ohm, &-watt resistor = 3900-ohm, &-watt resistor = 8200-ohm, &watt resistor = 12,000-ohm, +-watt resistor = 18,000-ohm, &-watt resistor = 1000-ohm, &watt resistor = 6800-ohm, +watt resistor = 3900-ohm, +-watt resistor = 12,000-ohm, +-watt resistor = 1000-ohm, Q-watt resistor = 10,000-ohm, &watt resistors = lfiOO-ohm, $-watt resistors = 100-ohm, &-watt resistor = 3000-ohm, 3-watt wire-wound Potentiometer = 100-ohm, $-watt resistor = 33,000-ohm, 78-watt resistor = 8200-ohm, 1-watt resistor = 5000-ohm, 3-watt wire-wound rheostat = 22,000-ohm, Q-watt resistor = 10,000-ohm, +watt resistor = 47.000-ohm, ,?-watt resistor 625 wiper 2-ohm, &watt resistors 250-puF capacitor, 50-volt working 8-pF capacitors, 450-volt working 47-pF capacitors, 150-volt working 2-pF capacitor, 12-volt working 250-pF capacitor, 50-volt working 5000-pF capacitor, 100-volt working Over-load trip relay GET 874 transistorsR3 22 Lv “t: TR - Control I ing reference cell R27 z‘oLJ R26 52 V 4 a v a.c. 7”””9 -Emitter --+ fol I o we r amplifier current amplifiers -Difference - Voltage Overload amplifier amplifier signal Fig.3. Power supply and control unit al Manual voltage setting[A7z,aZyst, Vol. 91 362 QUAYLE AND COOPER TR,, TR,, TR,,, TR,, = GET 102 transistors TR,,, TR,,, TRlS = OC 36 transistors TR,, TRs, TRg, TRIO, Vl v, = 150B2 valve = 85A2 valve = Main supply transformer, with earthed screen between primary and secondary V H = 1000-ohm, 3-watt rheostat Ti windings Illput, 10-0-200-220-240 Volts, 50 C/S. Output, 200 volts, 30 mA; 20 volts, 0.5 amp; 20 volts, 100 mA; 0-48-52 volts, 2 amps. = Rectifier, 5D23 = Rectifiers, OAZ 204 z, 2,s 2 3 2 4 = Rectifier, GJ4M z63 ' 8 = Rectifiers, 80 AS z,, 2 8 , 2 9 , 21, = Rectifiers, DDOOO Zll = Rectifier, lBlBlN538 REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. Szebelledy, L., and Somogyi, Z., 2. annlyt. Chem., 1938, 112, 313, 323, 332, 385, 391, 396, 400. Tutundzic, P. L., Analytica Chim. Acta, 1958, 18, 60. The Analytical Chemists' Committee of Imperial Chemical Industries Ltd., Analyst, 1950, 75, 577. Taylor, J . K., and Smith, S. W., J . Res. Natn. Bur. Stand., 1959, 63A, 153. Rett, N., Nock, W., and Morris, G., Ibid., 1954, 79, 607. Cooper, F. A., and Quayle, J. C., Amlyst, 1966, 91, 363. Brown, T. H., and Stephenson, W. L., Electron. Engng, 1957, 29, 425. Kemhadjian, H., and Newall, A. F., Mullard Tech. Commun., 1959, 4, 40. Received April 19th, 1963

ISSN:0003-2654    

DOI:10.1039/AN9669100355

出版商:RSC    

年代:1966

数据来源: RSC

摘要:

June, 19661 COOPER AND QUAYLE 363 Precise Coulometry: The Titration of Pure Sodium Carbonate BY F. A. COOPER AND J. C. QUAYLE (Imperial Chemical Industries Ltd., Agricultural Division, P.O. Box N o . 6, Billingham, Co. Durham) A method is given for the precise and accurate titration of sodium carbonate with hydrogen ions generated by the coulometer described in the preceding paper. This coulometer maintains automatically a constant current for a measured length of time. The results are in good agreement with those obtained by titration with standard acid referred to pure silver as the ultimate standard, and support proposals to establish the coulomb as a standard in volumetric analysis. Factors are discussed that affect the accuracy and precision of analysis by controlled current coulometry.TAYLOR and Smith1 have shown that acids and alkalis can be analysed coulometrically with high precision by using a manually controlled low current, but apparently there was no independent assay of the compounds titrated. No earlier work is known to the authors in which high currents, macro amounts and automatically controlled currents were used. The ifistrument used in our investigations2 measures time, and controls and accurately measures currents high enough to be used in macro-scale titrations. The construction of the coulometer and its mode of operation were described in the previous paper,2 but it was realised that other factors such as cell design, weighing and transfer of the sample, and even the value of the Faraday would affect the over-all accuracy and precision obtainable in coulometric titrations.In this paper these factors are discussed for the titration of sodium carbonate, a precise and accurate method for which is given. The sodium carbonate used was laboratory working standard material that had been analysed in recent inter-laboratory trials by the Society for Analytical Chemi~try,~ and was of the purity (100 0-2 per cent.) required by the Analytical Standards Sub-committee of the Analytical Methods Committee for primary standards. COULOMETER- The coulometer2 used in this work controls the current by maintaining a constant voltage across a precise standard resistor. The voltage drop across this resistor is compared with the e.m.f. of a standard cell, and the difference between them is shown on a sensitive galvanometer where a 1-mm deflection corresponds to a 7 p.p.m.error in current ; little manual correction is necessary. Three current levels of 1 amp, 100 mA and 10 mA are provided. Each level can be adjusted on a dummy load approximately equal to the cell resistance before the current is switched on to the cell. Time is measured by a quartz-crystal clock that is checked against standard radio transmission. The probable error2 in the quantity of electricity, i.e., the product of current and its time of flow, is t-25 p.p.m. Time is registered in units of 0.1 second by an electro-magnetic counter with cyclometer presentation of 5 digits. One counter can be pre-set to switch off at a given number of counts. Two independently driven counters are run in parallel; by comparing them, miscounts can be detected and erroneous experiments discarded .SUMMARY OF THE METHOD- The working electrode at which hydrogen ions or hydroxyl ions are generated is of platinum. Sodium sulphate is used as the supporting electrolyte, and is boiled in the cell before the titration to remove carbon dioxide. Copper and cupric sulphate are used as auxiliary electrode and electrolyte. The sodium sulphate solution is pre-titrated by generating, at 10 mA, hydrogen ions and hydroxyl ions to traverse the inflection, finishing at a pH just lower than that of the point of inflection. The sample is added and titrated a t 1 amp for a pre-set time that is approximately 0.2 per cent. longer than would be required if the sample were 100 per cent.pure. After boiling the solution again to expel carbon dioxide, it is cooled and back-titrated at 100 mA by generating hydroxyl ions until it is just alkaline. Hydrogen ions are then generated at 10 mA to give a known excess beyond the pH of inflection as in the pre-titration. The net anodic generation is used to calculate the purity of the sample. EXPERIMENTAL364 COOPER AND QUAYLE [Analyst, Vol. 91 In both the pre-titration and back-titration precautions are taken to ensure that no adsorption of hydrogen ions or hydroxyl ions has occurred, as shown by the reproducibility of repeated traverses of the point of inflection. TITRATION CELL- The cell used follows the design of Taylor and Smith,l and is modified to permit increased current, easier manipulation and easier replacement of the agar gel.Several designs in glass and Perspex were tried; the design eventually used, made of both borosilicate glass and Perspex, is illustrated in Fig. 1. Figures for weight loss, etc., are published in the Pyrex Bulletin No. 7, February, 1962. The rate of extraction of sodium was experimentally deter- mined as 0.5 mg on boiling 300 ml of de-mineralised water in the cell for 30 minutes, and then allowing it to stand for 20 hours. A = Titration compartment, B, C, D = Demountable sections, E =Auxiliary electrode compartment, Parts marked in black are made from PTFE. Fig. 1 . Cell for acid - alkali titration The glass titration compartment, A, containing M sodium sulphate is cylindrical with a diameter of 8.5 cm, and has a Fluon cap, the underside of which has a smooth domed shape to facilitate washing down.The lower edge of the rim is “feathered” (made thin and tapering) so that it produces a liquid-tight seal against the rim of the titration compartment. When the solution is boiled in the cell considerable differential expansion occurs, but the flexibility of the “feathered” edge accommodates this without overstressing the glass. To prevent the solid part of the cap from wedging itself into the taper aperture of the cell, because the feathered edge is flexible under load, the top edge of the cap is extended by a stainless-steel ring which rests on the rim of the glass (Fig. 2). The cap contains apertures to hold- (a) the generator electrode at which the titrant is produced. This is a piece of bright platinum foil, 7 cm by 4 cm, bent in an arc, approximately 4 cm in radius, outside the heater ( d ) ; (b) a combined pH-reference electrode (Ingold type 401-S) for end-point indication. Being coaxial in form it is less affected by potential gradients in the electrolyte than a separate glass electrode and reference half-cell ; a glass tube to inject nitrogen for removing carbon dioxide; (c) ( d ) a silica-sheathed heater ; (e) a resistance thermometer ; (f) a mercury-in-glass thermometer ; (g) a water condenser (when required), or a glass stopper (B34).Fig.2. Titration cell showing details of cell cap [To face page 364June, 19661 365 The heater is a multi-stranded helix of thin Nichrome wire, wound on a stout core wire insulated with Fibreglass braids.The core and heater are welded together at the inner end, and connected at the outer end to a metal-shrouded 2-pin plug with a bayonet safety catch. All the heating spiral is below liquid level; the rising portion of the sheath remains cool as it carries only connecting wires. Maximum heat dissipation is 13 watts per inch length, 150 watts total. A glass compartment, E, similar to A, contains the auxiliary generator electrode (copper sheet measuring 9 cm by 6 cm) and M cupric sulphate. The two compartments are connected by a tube 3-6cm in diameter, consisting of three de-mountable components B, C and D, held in position by screwed stainless-steel couplings. B and C are made of glass and are filled, by suction, with sodium sulphate from A. They contain medium-grade (No.3 porosity) sintered-glass discs to decrease the diffusion of sodium carbonate from the titration com- partmeqt A, and inlets, connected to a nitrogen supply, to blow back the solution into A. D is made of Perspex and contains a fine (No. 4 porosity) sinter cemented in with Tensol Cement No. 7 , and a gel of 3 per cent. of agar in 0-5 M sodium sulphate, to prevent liquid diffusion. PRECISE COULOMETRY: THE TITRATION OF PUKE SODIUM CARBONATE A No. 4 sinter is also held in a gasket between D and E. WEIGHING AND TRANSFER OF SAMPLE- The normal method of transfer from a glass weighing-bottle was attempted, but the capacity of the titration vessel did not allow a sufficient volume of wash water to be used. The finely powdered carbonate readily dispersed into the air when handled, probably aided by static electrification in the dry weighing-bottle.To overcome these difficulties a weighing capsule was designed in polythene, and was gold-coated all over to eliminate static effects (Fig. 3). The lid is a fairly tight fit so that it is possible to lift the capsule and contents by , 30 mm diameter Fig. 3. 12‘eighing capsule the knob on the lid. Samples are inserted by grasping the knob with ratchet forceps, lowering the capsule until it is completely immersed in the electrolyte, and pushing the bottom away from the lid by pressing on the platinum stud Lvith a spatula. The forceps and spatula are then rinsed with the minimum amount of water and removed. The capsule $Em sample is sufficiently heavy to settle to the bottom of the cell until most of the sample has dispersed; the capsule then rises clear of the electrodes and stirrer, but remains submerged, tilted on its side by the weight of the platinum stud so that the last traces of sample can dissolve, The capsule and top remain in the solution throughout the titration.pH MEASUREMC 4 NT- A Pye “Dynacap” pH meter is used to indicate the variation of pH with millicoulombs generated in the pre-titration and back-titration; the output of the pH meter is recorded on a Honeywell-Brown strip-chart recorder with a chart speed of 40 inches per hour. In order to check that the titration efficiency was 100 per cent. a t the beginning and end of366 COOPER AND QUAYLE [AnaZyyst, VOl. 91 the titration, the point of inflection was traversed three times, switching off the current at a chosen (but not critical) pH.The reproducibility of the number of coulombs required to titrate the solution from this pH to the point of inflection shows that one can ignore the possibility of the diffusion of impurities. To allow for the response time of the pH electrode only times of generation in the same direction are compared. REAGENTS- reagent grade) in de-mineralised water. grade). Sodium szd+hate--Prepare an approximately M solution of sodium sulphate (analytical- Cupric suZ$hate-Prepare an approximately M solution of cupric sulphate (general-purpose DETAILED PROCEDURE SAMPLE PREPARATION- dish, frequently stirring with a platinum rod. add a catch-weight of approximately 3 g to the capsule. AGAR GEL- Mix sufficient agar powder with de-mineralised water to give a 6 per cent.w/v solution, and boil. Add an equal volume of boiling neutral M sodium sulphate and mix thoroughly to give a final 3 per cent. w/v gel. Pour into a warm, dry gel compartment, and when the surface of the gel is firm (after approximately 20 minutes) cover with distilled water by using a tangential jet. Store in boiled-out 0.5 M sodium sulphate solution adjusted to pH 7 and protected from carbon dioxide. PREPARATION AND ASSEMBLY OF CELL- Remove any grease from the platinum electrode with chromic - sulphuric acid, rinse and finally immerse the electrode in aqua regia for a few seconds. Then rinse thoroughly. Clean the copper electrode with emery cloth. The deposition of copper is of no interest, but simply ensures that neither hydroxyl ions nor hydrogen ions are produced at this electrode.Clean the components of the glass cell thoroughly and finally boil in de-mineralised water. Grease the ground-glass faces with a silicone grease free from alkali-metal esters, and assemble the cell as indicated in Fig. 1, with A on a magnetic stirrer. Insert a plastic coated stirring rod. Partly fill A with M sodium sulphate solution, and fill compartments B and C completely with M sodium sulphate solution by suction from A. This expels air, containing carbon dioxide, from B and C, keeps the gel moist, and allows the solution level in ,4 to be adjusted so that the cell contains the minimum volume necessary for titration, thus reducing the risk of loss by splashing.E must also be filled above the level of the side-arm with sodium sulphate solution of at least 0.5 M concentration so that the gel is kept moist and its conductivity is not reduced by loss of ions due to diffusion. If cupric sulphate is in contact with the gel, some transfer of cupric ions may occur, so the sodium sulphate solution in E is not replaced by cupric sulphate solution until immediately before the titration. Insert a water-cooled condenser in the main aperture in the cap, switch the heater on full to boil the solution in A, and remove carbon dioxide. Blow back the solution from B and C, taking care to use a sufficiently low nitrogen pressure (corresponding to a flow of 2 litres per hour) to avoid rupturing the gel. Roil for 10 minutes, switch off the heater and cool the solution by bubbling through it a fast flow of "white spot" nitrogen, which is passed through a soda - asbestos column to remove carbon dioxide.The cooling rate can be increased by blowing air on to the side of A. PRE-TITRATION- In order to pre-titrate the solution, remove the condenser when the solution temperature has dropped below 75" C and insert the combined pH-reference electrode which can tolerate this temperature. Switch the pH meter to the 6 to 8 pH range, reduce the nitrogen flow, and fill B and C by suction to a depth of a few millimetres, which is sufficient to carry 100 mA but constitutes a negligible volume (3 per cent.) of untitrated solution. Titration of this volume at a pH between 6.5 and 7.5 to the equivalence point would require less than 1 second Dry the sodium carbonate sample to constant weight at 270" C, 2 10" C in a platinum Cool the sample in a desiccator and quickly Insert the lid firmly and re-weigh.June, 19661 367 at 10 mA, i.e., 2 p.p.m.of the total titre. Siphon the 0.5 M sodium sulphate solution from E, replace with M cupric sulphate solution and insert the copper electrode. Connect the electrodes to the output sockets of the coulometer. Adjust the current to 10mA on dummy load. Switch off, re-set the counter to zero, switch on the recorder chart drive and the current to the cell generating hydroxyl ions. Two hands are required. The recorder pen is not jolted on switching, but the slight effect of the generator field on the reading of the pH electrode gives a small marker (0.05 pH unit).Mark the start of the pre-titration on the recorder chart. Titrate at 10 mA to pH 7-4, or until sufficient of the curve is recorded to allow determination of the point of inflection (Fig. 4). Yote on the chart the point at which the current was switched off and the number of seconds counted. Ten p.p.m. of the total titre is equivalent to +5 seconds. Re-set the counters to zero, reverse the current and repeat the pre-titration until the number of milli- coulombs between the cut-off pH and the point of inflection in the direction of generation of acid is reproducible. PRECISE COULOMETRY: THE TITRATION OF PURE SODIUM CARBONATE Location to this precision permits an error of FO.1 pH units. Finish the pre-titration on the acid side.ADDITION OF SAMPLE- Fill B and C by suction from A before adding the sample, so that the sinters and intervening electrolyte minimise the risk of diffusion of carbonate into the gel. Switch off the stirrer to make manipu- lation of the sample capsule in the solution easier. Turn off the nitrogen supply and lower the capsule, held in ratchet forceps by the knob on the lid, under the liquid surface to prevent dispersion of the sample into the air when the capsule is opened. Push the capsule away from the lid by pressure on the platinum stud, using a glass rod or spatula. Release the lid, rinse the forceps and rod or spatula with de-mineralised water before removal so as to wash into the cell any traces of sample which may adhere to them. Insert the stopper in the cell cap and re-start the stirrer.Remove the combined pH electrode, rinse and store in distilled water. TITRATION- The resistance of the cell is approximately 30 ohms. The temperature reaches 40" to 50" C, but the gel will not suffer unless 70" C is exceeded. Re-set the counters to zero and pre-set the main counter for a time 10 seconds in excess of that required if the sample were 100 per cent. pure. Switch to hydrogen ion generation in the cell. Adjust the current if necessary every 2 minutes for the first 15 minutes, and then every 15 minutes until the end of the titration when the current will be switched off automatically. Adjust the current to 1 amp on the dummy load. Switch off the current. BACK-TITRATION- Switch the heater full on to boil the solution and expel carbon dioxide.Blow back the solution from B and C so that any carbonate which has diffused into these compartments will be neutralised and the carbon dioxide expelled. Wash down the underside of the cell top and stopper with de-mineralised water and replace the stopper with a water condenser. Boil for 10 minutes, switch off the heater, and cool by bubbling purified nitrogen through the solution. Fill B and C to a depth of a few millimetres. Adjust the current to 100mA on the dummy load. Switch off the current and re-set the counters to zero. Switch on the current to generate hydroxyl ions in the cell to a pH of approximately 7.4. Switch off and note the time. Fill and empty B and C until less than 0-03 pH change occurs (usually not more than three flushes are required) to ensure that no hydrogen ions or hydroxyl ions can be held in the sinters.Re-fill to a depth of a few millimetres. Adjust the current to 10 mA on the dummy load and repeat the procedure for pre-titration. CALCULATION- Calculate, from the chart speed and chart distance, the number of seconds of excess hydrogen ion generation after the last point of inflection in the pre-titration. In the same way, calculate the total number of seconds of hydrogen ion and hydroxyl ion generation at 10mA before the last point of inflection in the back-titration (Fig. 4). Multiply each current, in amps, by the time passed, in seconds, to give coulombs. Subtract the total number of coulombs of hydroxyl ions generated from the total number of coulombs of hydrogen368 COOPER AND QUAYLE [Aqcalyst, Vol.91 I I i I Point of inflection I , r . 1 - iL1 , pH 6.4 PH- Generation time (in seconds) at lOmA Traverse number H+ OH- I 76.0 (from starting pH) 2 - 103.0 3 102.0 - 4 - 102.0 5 52.0 (to point of inflection) - Total 230.0 205-0 Net time of H+ generation 25-0 seconds (Note: A 3 second error in back-titration is approximately 10 p.p.m. for a 3-g sample) Fig. 4. A typical record of back-titration. Chart speed 40 inches per hour ions generated; any lags in electrode response to changing pH in the pre-titration and back- titration will cancel out. Divide by the Faraday and the weight of sample, and multiply by the equivalent weight and by 100 to give the percentage purity, e.g., Standard cell voltage . . .. .. .. . . 1.01943 volts Resistor (nominal 1 ohm for 1 amp) .. .. . . 1.01806 ohms Current (nominal 1 amp) . . I . .. .. . . 1.00135 amps Weight of sodium carbonate in vacuo . . .. .. 2.99688 g Equivalent weight of sodium carbonate . . .. .. 52-9945 Gram equivalent of sodium carbonate in sample. . .. 0-0565508 Value of the Faraday . . .. . . .. . . 96,489 absolute coulombs per g cquivalent Number of coulombs required for 100 per cent. purity . . Switch off main current at . . .. .. .. . .-: + 10 = 5459 seconds 5456.52 Hydrogen ion generation- Current, in amps Time, in seconds Coulonibs 1-00135 5459-0 5466037 0.01 33.0 0.33 0.1 102.0 10.2 0.01 25-0 0.25 Total coulombs used = 5466.70 - 10.45 = 5456.25 couIombs Hydroxyl ion generation- 5456.25 Percentage purity = 5456.52 x 100 = 99.995 per cent.Atomic weights are based on lac = 12 scale. KESULTS An estimate of the diffusion of sodium carbonate into the agar gel was obtained by performing the pre-titration to pH 7 , then adding 3 g of sodium carbonate to the solution in A and leaving it there for 1.5 hours instead of titrating it. The pH of the solutions from compartments B and C was then measured, and the solutions titrated with 0.01 N sulphuric acid to pH 7 (see Table I). Diffusion into compartment C was negligible, and as C was separated from the gel by a KO. 4 porosity sinter, loss of carbonate into the gel was assumed to be nil. TABLE 1 DIFFUSION OF SODIUM CARBON-4TE INTO COMPARTMENTS B AND C Compartment PH required to bring p1-I to 7 Volume (in ml) of 0 . 0 1 ~ reagent B 10.0 6-0 C 6-5 0.1June, 19661 369 In order to assess the diffusion of cupric ions, compartments A, B and C were filled with sodium sulphate solution, compartment E was filled with cupric sulphate solution and they were left for 16 hours.No visible contamination was detected; the cupric-ion content of the solution from B and C was determined colorimetrically and polarographically and found to be 0.036 mg, thus showing that a transfer of cupric ion would give an error of only 20 p.p.m. if a titration had been performed. As the gel is never left in contact with the cupric sulphate during actual titrations for more than 7 hours, the error due to copper diffusion would be much less than 20 p.p.m. Diffusion of hydrogen ions from the cupric sulphate solution into compartment C was found to be nil.To test the reproducibility of the instrument, 25-ml aliquots of an approximately 120 g per litre solution of analytical reagent grade sodium carbonate were titrated. These were weighed into the titration cell from a glass weight b ~ r e t t e . ~ The balance was calibrated by using it to weigh Sational Physical Laboratory certified weights. The results are given in Table 11. The atomic weights used were those agreed by the International Union of Pure and Applied Chemistry5 in 1962, based on 12C = 12, giving 52.9945 for the equivalent weight of sodium carbonate. TABLE I1 DETERMINATIO?; OF REPRODUCIBILITY WITH A STANDARD SOLUTION OF SODIUM CARBONATE (SAMPLE 1) PRECISE COULOMETRY: THE TITRATION OF PURE SODIUM CARBONATE Weight of Weight of in grams in grams in grams sodium carbonate sodium carbonate Coulombs Sodium carbonate Percentage solution added, in vaczco, required found, purity 26-6718 3.142 15 5717.88 3.14042 25.9257 3.05425 5558-26 3.05275 25.47 13 3.00072 5460.1 9 2.99889 26.8610 3.16444 5758.59 3,16278 27.4556 3.23449 5885.55 3.23251 30.5787 3.60242 6555.32 3.60037 26.5084 3-12290 5683.09 3.1 21 31 Mean .. .. 99.945 99.951 99.939 99.948 99.939 99.943 99.949 99.945 Catch-weights of approximately 3 g of carefully dried sodium carbonate of a different make (sample 2) were weighed in gold-coated polythene capsules which were subsequently transferred to the titration cell. Results of the determination of purity are given in Table 111. TABLE I11 PURITY OF SODIUM CARBONATE (SAMPLE 2) sod Weight of in vacuo, in grams 3.0232 3.0067 3-0052 3.0077 3.0034 3.0099 3.0073 .ium carbonate Coulombs required 5504.16 5474.27 547 1.44 54 7 5.3 2 5468.16 5480-02 5475-15 Weight of sodium carbonate found, in grams 3-02304 3.00662 3.00502 3.00720 3-00327 3.00978 3.0071 1 Mean .. .. Percentage purity 99.995 99.997 99.994 99.983 99.996 99.996 99.994 99.994 Combined standard deviation (Tables I and 11) 0.005. Sample 2 is kept as a laboratory working standard and has been assayed independently by weight titration with hydrochloric acid standardised against silver. The mean value and the standard deviation of eighteen results obtained over 2 years were, respectively, 99.993 per cent. purity and 0-003, with extreme figures for purity of 100.000 and 99.990. The mean values obtained by the weight titration and coulometric assay differ by only 10 p.p.m.Eight samples of sodium carbonate from the second source were weighed into polythene capsules and titrated. The operator knew the weight only to k2 mg (660 p.p.m. in 3 g), and the excess acid was calculated from the upper limit of the weights. Table IV compares the sodium carbonate found with the sodium carbonate added, i.e., the weight in vacua corrected for the percentage purity found by the weight titration method. Sodium sulphate370 COOPER AND QUAYLE [Analyst, Val. 91 from a different manufacturer was used for the last four determinations. The solution was initially much more alkaline (pH >8 instead of between 6 and 7), but it was not realised until the titration was in progress that this solution had been used to make the gel.Diffusion of alkali from the gel made with this solution probably accounts for the high result of 190 p.p.m. ; the solution was neutralised before being used for gels for the subsequent titrations. TABLE IV TEST OF REPRODUCIBILITY ON UNKNOWN WEIGHTS Sodium carbonate added, in grams 3.00533 2.99783 3.00302 3.00131 3.00552 3.00508 3.00789 3.00167 Sodium carbonate found, in grams 3.00560 2.99773 3.00379 300143 3-00609 3.00488 3.00764 3-00137 Difference, in p.p.m. + 90 - 30 4- 60 + 40 + 190 - 70 - 80 - 100 In titrating sodium carbonate from the same source (sample 2), different quantities of excess hydrogen ions were generated in order to test the effect of varying the excess. The results are given in Table V, and also indicate the efficiency of generating hydrogen ions when no sodium carbonate is present to react with them, and of generating hydroxyl ions at one-tenth the current density used during the main part of the titration of sodium carbonate.TABLE V EFFECT OF VARYING THE EXCESS HYDROGEN IONS GENERATED Excess Percentage of I \ Percentage Weight (in grams) of sodium carbonate- h coulombs main titration added found purity 10 0.2 3.00051 3.00039 99.996 23 0.4 3.00704 3.00695 99.998 50 1.0 3,00172 3.00 145 99.991 As the mean of the results given in Table 111 is 99.994 and the standard deviation is 0.005 it can be seen that, unless errors compensate, within this range- (;) the quantity of excess hydrogen ions generated is not critical, (ii) the efficiency does not decrease when no titratable substance is present, (iii) the titration efficiency is at least 99 per cent.with hydroxyl ions generated at one-tenth the current density of that used in the main part of the titration of sodium carbonate, shown by the generation of 10,000 p.p.m. of hydroxyl ions which resulted in less than 100 p.p.m. error. SUMMARY OF POSSIBLE ERRORS- Errors in the weight of samples handled in capsules were initially kO.1 mg, but this value was later reduced to f- 0.05 mg by the use of a more sensitive balance. These correspond to 30 and 15 p.p.m., respectively, in 3 g. The equivalence point can be determined to + l o p.p.m. The errors associated with the determinations of atomic weight are those assigned by I.U.P.A.C. in 1962.5 A summary of the estimated possible errors is given below- Atomic weight of sodium.. .. . . .. . . &- 2-5 p.p.m. Atomic weight of carbon .. .. .. . . & 4 p.p.m. Atomic weight of oxygen.. . . .. . . . . + 6 p.p.m. Weight of samples in capsules . . .. . . . . 1 3 0 p.p.m. (initially) Weight of samples in capsules . . . . . . . . +15 p p m . (finally, see Tables IV and V) Determination of the equivalence point . . . . &lo p.p.m. Value of the Faraday . . . . .. .. . . 5 3 3 p.p.m. Timer . . . . . . .. .. .. . . & 3 p.p.m. Time (to nearest 0.1 second) a t 1 amp . . .. . . &lo p.p.m. Uncertainty in the absolute volt . . probably 5 1 0 p.p.ni. National Physical Laboratory certification . . . . & 5 p.p.m. Temperature . . ,. . . .. .. . . f 1 0 p.p.m. (initially) Temperature .. ,. .. .. .. . . f 4 p.p.m. (finally, see Tables IV and V) Standard cell voltage-June, 19661 PRECISE COULOMETRY: THE TITRATION OF PURE SODIUM CARBONATE Precise resistor- Uncertainty in the absolute ohm . . probably A10 p.p.m. Makers’ certification . . .. . . probably f 1 p.p.m. Instrument discrimination . . .. .. . . f 6 p.p.m. DISCUSSION 371 The determinations in Tables 111, IV and V were consecutive, though obtained over several weeks. The results in Tables I1 and I11 show that it is possible to achieve a standard deviation of 50 p.p.m. in the coulometric titration of sodium carbonate that is slightly less precise than that obtained by reference to silver. If there is any bias, values for the purity given by this method are likely to be high, due to contamination of the hydrochloric acid by other chlorides before analysis.Errors in the coulometric method are likely to be caused by low generation efficiency and will, therefore, also give high apparent purities. Xever- theless, the agreement to 10 p.p.m. of the means by the two methods suggests that the errors are comparable. The coulometric analogue of the normal volumetric practice of adding excess acid, boiling out carbon dioxide and back-titrating with dilute alkali was followed, because it was found tedious to remove the carbon dioxide by passing nitrogen through the solution.1 Because this method gave satisfactory results, no check was made of the effect of changing parameters other than the excess of hydrogen ions generated. It is thought unlikely that highly critical conditions were selected by chance.Nevertheless, further investigation to define more closely the optimum conditions might succeed in reducing the errors still further and simplifying the actual titration. CELL DESIGN- After the determinations in Table I11 were completed, considerable difficulty was experienced with leakage, particularly from the joint between A and B. The screwed stainless- steel coupling was finally replaced by “Tufnol” flanges (Fig. 5), so providing an exposed - 4 B A I 10.16 cm (b) Fig. 5. Tufnol flanges. Fig. 5 (a) : the flanges are held in position by silver steel rods with 2BA and 4RA threads. They are fitted by sliding into the holes and screwing the nuts tight. Fig. 5 ( b ) : method of fixing the flanges (centre section) glass-to-glass joint so that any leakage could be detected.Cell design is still a major problem involving a compromise between ease of cleaning and replacement, low electrical resistance and precautions to avoid loss of sample. The cell and electrode design were chosen purely for the practical reasons already mentioned, and since satisfactory results were obtained these conditions were not vaned, CHOICE OF AUXILIARY ELECTRODE SYSTEM- It is important to suppress the generation at the auxiliary electrode of ions which can migrate and take part in either the chemical or electrode reaction in the titration compart- ment. Taylor and Smith1 accomplished this in titrating acids by using the oxidation of silver372 COOPER AND QUAYLE [Amlyst, Yol. 91 to silver chloride as an auxiliary system.Insufficient details of their sodium carbonate analysis are given, but a silver cathode was apparently used, possibly after deposition of silver chloride so that the reduction involved only the liberation of chloride ions. A silver - silver chloride electrode with potassium chloride was therefore tried, but it was found difficult and tedious to deposit sufficient quantities of silver chloride in an even coating. During electrolysis hydroxyl ions were produced, and chloride ions migrated to the anode where they were oxidised, so giving incorrect results. No significant improvement resulted on replacing the potassium chloride solution with sodium sulphate. In deciding on an alternative auxiliary system the following points were considered- (a) electrode potential, (b) pH of the solution, (c) preference for a salt of sulphuric acid, (d) a simple method of detecting diffusion through the gel.Copper and cupric sulphate were chosen as fulfilling most of these conditions. The Cu2+/Cu potential is low under the given conditions, and the deposition of copper, preferen- tially to the reduction of water, is well-known. It gives an acid solution, and although the pH is as low as 3.2 for M cupric sulphate, hydrogen ions and cupric ions are attracted to the copper cathode during most of the titration. The strong blue colour of the hydrated cupric ion also gives immediate indication of penetration into the agar gel. SUPPORTING ELECTROLYTE- It is essential that the electrolysis of impurities in the sodium sulphate does not cause errors, particularly as the concentration of sodium sulphate is ten times that of sodium carbonate at the beginning of the titration.The maximum impurity which may react during electrolysis, according to the makers’ specification, is 0-002 per cent. as reducing substances; chloride, heavy metals and ammonium ions may be present up to 0.001 per cent., and iron up to 0.0005 per cent. In 300 ml of M sodium sulphate solution, in comparison with 3 g of sodium carbonate, these become 280 p.p.m., 140 p.p.m. and 70 p.p.m., respectively. In the authors’ experience chloride can be oxidised irreversibly, even in alkaline media. However, any irreversible reaction would take place during the pre-titration, Errors due to reversible oxidation or reduction should not appear, as the pre-titration and back-titration finish in oxidising conditions.Therefore, impurities in the sodium sulphate should not affect the accuracy of the titration, whether they are reversibly or irreversibly oxidised. For the last four determinations in Table IV, sodium sulphate from a different manu- facturer had to be used. Although this latter sodium sulphate was within specification, the initial pH was 8.1 instead of between 6.0 and 7.4, and the equivalence pH was 7.3 instead of approximately 7. Some precautions were taken, such as neutralising the solution before making up the gel. Care must obviously be taken to avoid absorption into the gel of any alkali (or acid), which may diffuse out and cause errors. THE VALUE OF THE FARADAY- For accurate coulometric titrations the value of the Faraday must be known accurately.Remy6 has summarised the determinations to date, re-calculated these on the basis of the isotope lZC = 12, and considered critically the reliability of the results. The mean of these, excluding the one which he considers seriously in error, is 96,489 absolute coulombs per g equivalent. A statistical examination of the errors which each observer gives for his deter- mination indicates that the 95 per cent. confidence limits for the mean are &3 absolute coulombs per g equivalent. THE COULOMB AS A STANDARD IN VOLUMETRIC AXALYSIS- This has already been suggested7 and would be useful because many of the present standard substances used in volumetric analysis continue to be the subject of much critical examination.The Society for Analytical Chemistry has, so far, recommended to the Inter- national Union of Pure and Applied Chemistry only sodium carbonate as being of sufficient purity (100 +_ 0-02 per cent.) for use as a primary standard, and this has not yet been accepted. The definition of the coulomb, on the other hand, is already agreed internationally and, as is well known, it can be measured without great difficulty against the fundamental standards of mass, length and time. Most inorganic reactions involve electrons, and if aJune, 19661 PRECISE COULOMETRY: THE TITRATION OF PURE SODIUM CARBOXATE 373 reaction can be made to proceed with 100 per cent. over-all efficiency by generation of ions at an electrode, and the completion of the reaction and the number of coulombs used can be accurately determined, then the coulomb may be used as a standard for volumetric analysis. These conditions have been satisfied for the titration of sodium carbonate with the technique described and electrical apparatus which is easily operated and stable. It is planned to investigate the titration of other primary standard compounds, including those that may be recommended for oxidation - reduction and precipitation reactions. CONCLUSION The results show that the coulometric titration of sodium carbonate can be almost as precise as titration with hydrochloric acid standardised against silver. The mean values of the coulometric and volumetric results for the purity of samples agree to within 10 p.p.m. Compared with the established method, which starts from silver and takes at least 5 days, the coulometric method is rapid, taking less than 1 day. It is therefore an attractive alternative to the present procedure. If investigation of coulometric titrations of other acids and alkalis, and of reactions involving oxidation and reduction shows the same accuracy and precision, this should establish the coulomb as the ultimate titrimetric standard. We thank Mr. R. M. Pearson for the initial suggestion on which this work was based, and for his continued help and advice. Mr. J. Lindsley and Mr. A. W. Remmer carried out most of the experimental work, and Mr. H. N. Redman prepared and weighed the samples. REFERENCES 1. 2 . 3. 4. Redman, H. N., I b i d . , 1963, 88, 654. 5. 6. 7. Taylor, J. I<., and Smith, S. W., J . Res. Natn. Bur. Stand., 1959, 63A, 153. Quayle, J. C., and Cooper, F. A., Analyst, 1966, 91, 355. Analytical Methods Committee, I b i d . , 1965, 90, 261. Commission on Atomic Weights, Pure and Applied Chemistry 1962, 1-2, 255. Remy, H., Chernikerzeitung, 1962, 86, 167. Tutundzic, P. S., Analytica Chim. Acta, 1958, 18, 60. Received April 19th, 1963

ISSN:0003-2654    

DOI:10.1039/AN9669100363

出版商:RSC    

年代:1966

数据来源: RSC

摘要:

374 CROMPTON : IODIMETRIC DETERMINATION OF [Analyst, 1701. 91 Iodimet ric Determination of Organo-aluminium Compounds BY T. R. CROMPTON (Carrington Plastics Laboratory, Shell Chemical Company Limited, Carringtow, Cheshire) The alkyl groups in various types of organo-aluminium compounds have been shown to react with iodine in hydrocarbon solution, and the stoicheio- metry has been determined of the reactions occurring between iodine and trialkylaluminium, dialkylaluminium chloride and dialkylaluminium alkoxide compounds. Based on these reactions a reasonably rapid and accurate iodimetric method has been devised for the determination of low- concentrations of organo-aluminium compounds in various hydrocarbon solvents. The method is applicable to the analysis of the hydrocarbon solutions of organo- aluminium catalysts used for the polymerisation of ethylene and propene.Good agreement is obtained between the iodimetric procedure and by a procedure based on conductiometric titration with a standard solution of isoquinoline for trialkylaluminium compounds and dialkylaluminium chlorides. The iodimetric procedure is also applicable to dialkylaluminium alkoxide compounds, which cannot be determined by isoquinoline titration. DILUTE hydrocarbon solutions of various types of organo-aluminium compounds are used as co-catalysts with Group IV and VI halides, in processes for the polymerisation of ethylene and propene to polyolefin polymers. Three of the principal types of organo- aluminium catalysts that are used in these processes are AlR,, AlR,(OR’) and A1R2C1 (where R and R’ are C, to C, alkyl groups).A rapid and simple method was required for determining these types of catalysts in the hydrocarbon solutions used for ethylene and propene poly- merisation, in amounts down to 10 millimoles per litre. Various methods, based on gasometric principles, have been described for determining methyl to butyl alkyl groups, and hydride groups in organo-aluminium compounds : Bonitzl and Ziegler, used the reaction between the organo-aluminium sample and 2-ethylhexanol, and determined the amounts of alkane and hydrogen gases produced by mass-spectrometric analysis- >AIC,H,n+, + ROH = >Al(OR) + CnH2n+2 >AlH + ROH = >Al(OR) 1- H2 Crompton and Reid3 and Dijkstra and Dahmen4 used the reaction between organo- aluminium compounds and hexanol, followed by water, and lauric acid, respectively, and analysed the resulting mixture of liberated alkane and hydrogen gases by gas chromatography.Neumann5 developed a gasometric method for determining aluminium-bound hydride groups, based on a reaction with N-methylaniline at low temperatures- Methods of analysis based on these principles, although capable of giving excellent information on the composition of the sample, were too lengthy and complex for routine control testing of plant streams. Bonitzl described a method based on a conductiometric or potentiometric titration with isoquinoline for determining organo-aluminium compounds. He described the colourless 1 to 1 complexes formed between isoquinoline and dialkylaluminium hydrides and trialkyl- aluminium compounds, and the strongly red coloured 2 to 1 complex formed between iso- quinoline and dialkylaluminium hydrides.These complexes were further studied by Neu- mann.5 Farina et ~ 1 . ~ and Sebbia and Pagani’ have reported modified potentiometric- titration procedures for determining organo-aluminium compounds.June, 19661 ORGANO-ALUMINIUM COMPOUNDS 375 Mitchens extended the studies of the red coloured 2 to 1 complex formed between iso- quinoline and dialkylaluminium hydrides and, based on his observations, devised a method for the simultaneous spectrophotometric determination of trialkylaluminium compounds and dialkylaluminium hydrides in mixtures. Wadeling utilised this colour-forming reaction to devise a photometric-titration method for determining the total isoquinoline-reactable organo- aluminium compounds.Razuvaev and GraevskiilO and Hagen and Leslief1 have devised methods involving the use of visual indicators for titrating organo-aluminium compounds with standard solutions of bases and ethers. Methods based on a conductiometricl q 5 or potentiometricl v 5 y 6 y 7 titration with organic bases, or the spectrophotometric method described, in which these reagents are useds v 9 are excellent for determining trialkyl-aluminium compounds and dialkyl aluminium chlorides (in both, only low concentrations of aluminium-bound hydride groups are assumed to be present). They cannot, however, be applied to the determination of dialkylaluminium alkoxide compounds, which do not co-ordinate with bases.In addition, these methods did not have the required sensitivity. Similarly, visual-indicator titration procedureslO could not be applied to the determination of dialkylaluminium alkoxide compounds as no reaction occurs between these compounds and organic bases or ethers. Bartkiewicz and Robinson1, have shown that a hexane solution of triethylaluminium consumes iodine according to the following equation, and have used this reaction for deter- mining the reducing capacity of triethyl aluminium- A1(C2H,), + 31, = AlI, + 3C2H5I It seemed that this reaction might offer the basis for a sensitive and rapid method for the analysis of organo-aluminium compounds. Based on this observation, the method discussed below has been developed for the determination of trialkylaluminium compounds and other types of organo-aluminium compounds in hydrocarbon solutions.EXPERIMENTAL The preliminary iodination experiments were carried out with a 5 per cent. solution of diethylaluminium chloride in anhydrous toluene. Several volumes of this solution were transferred by pipette into dry, nitrogen-purged reaction flasks (see Fig. 1) by using the tech- nique described by Crompton.13 A fixed volume (50 ml) of iodine reagent (0.4 N) was then added to each solution, and the mixture was left for 5 minutes to allow the reaction to proceed. C A = Burette with polyethylene stop- C = Glass stirrer B = B24-to-B24 adaptors with Gaco cock D = 250-ml, 3-neck (B24) flask seal Fig. 1. Apparatus for determining the iodine number Aqueous acetic acid was then added to each reaction mixture.The excess of iodine remaining was determined by titration with sodium thiosulphate solution and the amount of iodine consumed by the various sample volumes was calculated.376 CROMPTON IODIMETRIC DETERMINATION OF [Analyst, 1701. 91 A plot of iodine consumption against the volume of diethylaluminium chloride solution taken (Fig. 2, Procedure A) shows that the iodine consumption is not proportional to the volume of sample taken, and that the line drawn through the experimental points intersects the sample-volume axis at a positive value, indicating that low iodine consumptions are being obtained in these determinations. This suggested that the iodine reagent contained a small amount of an impurity that reacted rapidly with the alkyl groups in diethylaluminium chloride. Volume of sample solutioti, ml Fig.2. Iodine consumption of dilute diethylaluminium chloride solutions : graph A, 50 ml of iodine reagent added to sample; graph R, sample added to 50 ml of iodine reagent 6 taken, g Fig. 3. Influence of excess of iodine and reac- tion time on the iodine consumption: graph -1, triethylalumininm ; graph B, tripropylaluminium ; graph C, diethylaluminium chloride I t can be seen in Fig. 2 (Procedure B) that higher iodine consumptions are obtained when the order of mixing the diethylaluminium chloride solution and iodine is reversed, i.e., when the iodine is transferred by pipette into the reaction flask first. These conditions are, presumably, less favourable for the occurrence of the side reaction.However, the presence of the impurity still affects the determination of iodine consumption to some extent. The effect of the impurity was overcome by using a “double titration” procedure, described below. The iodine consumptions, I , g and I 2 g, of two different volumes, I/, ml and V , ml, of the sample solution are determined. The same volume of iodine reagent is used in each determination. The correct iodine consumption of Vl - I/, ml of sample solution is, therefore, equal to I , - 12.g of iodine. Several iodine-number determinations (grams of iodine consumed per 100 grams of sample) were carried out on a solution of diethylaluminium chloride by the “double titration” procedure, and these results are compared, in Table I, with the results obtained by the “single titration” procedure.The “double titration” results are consistently higher and do not vary appreciably with the amount of sample taken for analysis. TABLE I DIETHYLALUMINIUM CHLORIDE (Ti PER CENT. IN TOLUENE) : EFFECT OF DISSOLVED IMPUKITY IN THE IODINE REAGENT Iodine number, g of iodine consumed per 100 g of sample volume tincorrected Corrected A Sample - 7 taken, ml (“single titration”) (“double t i tration”) 2.5 3.0 5.0 7.5 22.7 26.2 26-7 - 28.6 28-5 28.1 28.4 The effect of excess of iodine on the iodine-number determination was examined. Varying volumes of dilute solutions of triethylaluminium, diethylaluminium chloride and tripropylaluminium were added to a fixed volume (50 ml) of the iodine reagent and theJune, 19661 ORGANO-ALUMINIUM COMPOUXDS 377 reaction was allowed to proceed for 10 minutes.It can be seen from Fig. 3 that when iodine consumption is plotted against sample size the iodine consumption is not affected, unless more than 80 per cent. of the iodine present is consumed in the reaction. These experiments were then repeated with dilute solutions of diethylaluminium ethoxide and dipropylaluminium isopropoxide. Fig. 4 shows that the iodine consumption of this less reactive type of organo- aluminium compound is more dependent upon the molar excess of iodine present. \Vith a 10-minute reaction period the iodine consumption is affected if more than 50 to 60 per cent. of the iodine is consumed. Extension of the reaction time to 20 minutes, however, circum- vents this effect. Complete iodination of all the types of organo-aluminium compounds examined is obtained in 20 minutes, even in the presence of only a 20 to 30 per cent.excess of iodine reagent. 5 Fig. 4. Influence of excess of iodine and reaction time on the iodine consumption : graph A, diethylaluminium ethoxide, 20 minutes’ reaction ; graph B, diethylaluminium ethoxide, 10 minutcs’ reaction ; graph C, dipropylaluminium propoxide, 5 minutes’ reaction In the experiments described so far, dilute aqueous acetic acid (2 N) has been added to the mixture of iodine reagent and sample before back-titrating the excess iodine with standard sodium thiosulphate solution. It was observed, however, that considerably higher iodine consumptions occurred for dialkylaluminium alkoxides if distilled water only was added at this stage.I t did not occur with trialkyl- aluminium and dialkylaluminium chloride compounds, and secmed to be connected in some way with the presence of alkoxide groups in the molecule. I t was decided to use aqueous acetic acid in the final analytical procedure. Xo explanation was found for this effect. STO I C HE I 0 $1 E TK Y 0 F THE I 0 D I SAT1 0 N 0 F 0 RG AN 0- AL U M I N I U 31 C OM PO V N L) S- The stoicheiometry of the reactions that occur during the iodination of iso-octane solu- tions of the purest available specimens of various organo-aluminium compounds was then examined. Ethyl, propyl, butyl, hydride and alkoxide groups in these samples were deter- mined by the procedure described by Crompton and Reid.3J3 Alkyl groups higher than butyl were determined by a procedure in which a cold toluene solution of the organo- aluminium compound was decomposed at -60” C by the addition of a dilute solution of glacial acetic acid in toluene.Aqueous sodium hydroxide was then added to the solution. Liquid paraffins, produced by the hydrolysis of the higher alkyl groups, were then determined in the separated toluene phase by gas chromatography. In Table I1 are shown the values of the determined iodine consumptions of iso-octane solutions of various organo-aluminium compounds. Each alkyl group in trialkylaluminium compounds consumes one mole of iodine. These results confirm the conclusions reached by Bartkiewicz and Robinson.lZ Similarly, each alkyl group in diethylaluminium chloride consumes one mole of iodine.The reaction between iodine and dialkylaluminium alkoxides follows a different course, however, as only 1.25 moles of iodine are consumed per mole of this type of compound, i.e., 0.625 moles of iodine are consumed per alkyl group.378 CROMPTON IODIMETRIC DETERMINATION OF [Analyst, VOl. 91 TABLE I1 STOICHEIOMETRY OF THE IODINATION OF THE ORGANO-ALUMINIUM COMPOUND Sample description Triethylaluminium .. . . . . Tripropylaluminium . . .. Diethylaluminium chloride .. .. Dipropylaluminium isopropoxide . . Composition of sample, w/w per cent. A1(C2H5)3 A1(C2H5)2H (C,H5)2(C,H,) A1(C2H5)2(0C2H5) 4.3 1.4 2-2 J 93.0 3.0 4.0 } Iodine consumption, moles of iodine consumed per mole of organo-aluminium compound 2.97 3.09 2.00 1.25 REPRODUCIBILITY OF THE PROCEDUKE- The reproducibility of the method for the determination of the iodine consumption was determined by statistical analysis.Iodine-consumption determinations were made with 6 different sample volumes of dilute solutions of 3 typical organo-aluminium compounds. The mean iodine number of samples, its standard error and its 95 per cent. confidence limits are shown in Table 111. I t can be seen that, for the 3 organo-aluminium compounds examined, the standard errors of the iodine-number determinations are acceptably low. TABLE I11 REPRODUCIBILITY OF IODINE-NUMBER DETERMIXATIONS Mean iodine number, g of iodine 95 per cent. Kumber of consumed per Standard confidence Sample description determinations 100 g of sample error limits Triethylaluminium . . . . 6 613.8 2.6 613.8 & 7.3 Diethylaluminium ethoxide .. 6 158.6 1.9 158.6 & 5.3 156.6 & 3.1 Dibu t ylaluminium 6 156.5 1.1 ethoxide . . . . , . 6 42.9 0.3 42.9 & 0.9 31 E TH o D APPARATUS- inlet sidearm and stopcock above the graduation mark. DiZutio7z jasks-These are 100-ml stoppered Pyrex-glass calibrated flasks with nitrogen Safety Pipettes-These are “Exe1o”-type plunger pipettes of 1, 2, 5, 10 and 25-ml capacity. T-Pieces ; glass, 3 iitches. Graduated cylinder, 50 ml. Burette, 50 mZ-This is preferably fitted with an E-MIL polythene stopcock (obtainable Reactio~zjask, 250 ml-This is a B24, three-neck flask with glass stirrer and nitrogen and from H. J. Elliott Limited, Treforest Industrial Estate, Pontypridd, Glamorgan). burette inlets (see Fig. 1).REAGENTS- able from British Drug Houses Ltd.) for 2 weeks. than 25 p.p.m. Iso-octane-Dry iso-octane by standing it over 50 g of molecular sieve, type 4A (obtain- Nitrogen-Dry by passing through a molecular-sieve packed tower, oxygen content less Swirl the bottle daily.June, 19661 ORGANO-ALUMINIUM COMPOUNDS 379 Iodine reagent (0-4 N)-To 2-5 litres of toluene in a dry bottle add 130 g of analytical- reagent grade iodine and shake the contents to dissolve the iodine. Add to the solution 50 g of freshly heated (at 120” C) 4A molecular sieves, stopper the flask, and leave it for several days, occasionally swirling it. Sodium thiosulphate (0-25 N) aqueous, standardised. Acetic acid (4 N) aqueous-Dilute 250 ml of glacial acetic acid to 1 litre with distilled water.SAMPLING- If the sample contains more than 20 per cent. of organo-aluminium compound it is necessary to dilute the solution with iso-octane in the following way. Transfer by pipette 20 ml of dry iso-octane into a dry 100-ml calibrated flask with a nitrogen inlet side-arm, and purge the solvent with nitrogen for 30 seconds. Connect a nitrogen line to the side-arm of the calibrated flask, open the stopcock and apply a gentle nitrogen purge. Transfer sufficient of the sample into the calibrated flask, by means of a safety pipette, to give a concentration of approximately 20 per cent. of the organo-aluminium compound in the diluted solution. Purge the exterior of the tip of the safety pipette with dry nitrogen during the transfer, as described by Crompton.13 Make up the volume to 100 ml with dry iso-octane, stopper the flask and mix the contents thoroughly .PROCEDURE- With a pipette fitted with a rubber suction bulb, transfer 50 ml of the same batch of iodine reagent into two dry 250-ml reaction flasks. Apply a gentle purge of nitrogen to displace the air from the flasks. Switch on the stirrers and adjust the speed to approximately 1 revolution per second. Transfer a different volume of the sample solution into each flask by means of a safety pipette. Observe the precautions described above to prevent decompo- sition of the sample during transfer. Stopper the reaction flasks immediately after the sample delivery. Suitable pairs of sample volumes required for the analysis of a 200 millimole per litre solution, of various types of organo-aluminium compounds, are shown in Table VIII.Corres- pondingly larger or smaller volumes should be taken if necessary. Maintain the gentle nitrogen purge during the subsequent reaction. TABLE VIII OPTIMUM PAIRS OF SAMPLE VOLUMES REQUIRED FOR THE ANALYSIS OF A 200 MILLIMOLES PER LITRE SOLUTION OF VARIOUS TYPES OF ORGANO-ALUMINIUM COMPOUNDS Sample volumes required* , 1 Type of organo-aluminium “A” “B” Trialkylaluminium compounds . . . . 6 12 compound analysed nil ml Dialkylaluminiuni chlorides . . . . 9 18 Diallq-laluminium allioxides . . . . 12-5 25 * The following relationships are used to calculate the sample volumes required :- 1 mole of trialkylaluminium compound = 6 x 126-9 g of iodine 1 mole of dialkylaluminium chloride = 4 x 126-9 g of iodine 1 mole of dialkylaluminium alkoxide = 2.5 x 126.9 g of iodine Let the reaction proceed for 20 minutes, then remove the nitrogen supply and introduce 40 ml of 2 N acetic acid into each reaction flask.Increase the stirrer speed until the aqueous and toluene phases are thoroughly mixed and then titrate the solution with 0.25 N sodium thiosulphate solution. Continue the titration until the solution becomes pale brown in colour. Commence drop-wise titration and stop the stirrer between each addition of titrant. Continue the titration until the pink colour completely disappears from the toluene phase and the toluene becomes pale yellow.380 CROMPTON IODIMETRIC DETERMINATION OF [,4?lnlyst, LT0l. 91 CALCULATIONS- Iodine consumption (g of iodine consumed per litre of sample) (TI - T,) x f x 126.9 x 1000 - - (V, - V,) x 1000 Trialkylaluminium compounds (millimoles of trialkylaluminium compound per litre of sample) Dialkylaluminium chlorides (millimoles of dialkylaluminium chloride per litre of sample) - (T, - T,) x f x 1000 Y 2 - Vl) x 4 Dialkylaluminium alkoxides (millimoles of dialkylaluminium alkoxide per litre of sample) ( T , - 7-2) x f x 1000 - - ( V , - V1) x 2.5 Where V , = Volume of sample solution taken (smaller volume), ml.V , = Volume of sample solution taken (larger volume), ml. T , = Back-titration of sodium thiosulphate obtained with smaller sample volume, T, = Back-titration of sodium thiosulphate obtained with larger sample volume, ml. f ml. = Normality of sodium thiosulphate solution. DISCUSSION OF RESULTS ANALYSIS OF DILUTE HYDROCARBON SOLUTIONS OF TRIALKYLALUMINIUM COMPOLXDS- The catalyst contents of dilute hydrocarbon solutions of various trialkylaluminium compounds was determined iodimetrically, by the conductiometric titration with isoquinoline and by the determination of aluminium.It can be seen in Table IV that reasonably good agreement is obtained between the iodimetric and the isoquinoline methods of analysis. The iodimetric method can be applied to solutions containing as little as 20 millimoles per litre of catalyst. TABLE IV DILUTE TKIALKYLALUMINIUM SAMPLES Sample description Triethylaluminium in iso-octane . . . . Triethylaluminium in iso-octane . . . . Tripropylaluminium in iso-octane . . . . Tripropylaluminium in iso-octanc . . . . Tripropylaluminium in iso-octanc .. . . Tripropylaluminium in iso-octane . . . . r Trialkylaluminium content, millimoles per litre Rased on Based on Based on determination consumption consumption 715 687 684, 673 1353 1314 1305 314 294 309 105 -* 104 A 7 aluminium isoquinoline iodine 62-8 -* 61.3 20.1 -* 19.6 * Isoquinoline method is not applicable because of the low electrical conductivity of the test solution. Commercial trialkylaluminium usually contains a small amount of dialkylaluminium alkoxide as an impurity, which is produced bv oxygen contamination during the manu- facture. An isoquinoline titration determines only the “active” trialkylaluminium content of the sample ; aluminium determinations include both “active” trialkylaluminium and “inactive” dialkylaluminium alkoxide.Higher results are therefore expected, and indeed found, in the latter method of analysis. The presence of small amounts of dialkylaluminium alkoxide in trialkylaluminium compounds causes little interference in the iodimetric method. Thus, the determined iodine number of a solution known to contain 180 millimoles of trialkylaluminium compound per litre and 20 millimoles of dialkylaluminium alkoxide per litre (i.e., total organo-aluminium content of sample contains 10 per cent. of the alkoxide derivative), indicates a trialkylaluminium content of 187 millimoles per litre, which is about 4 per cent. higher than the added amount.June, 19661 ORGANO-ALUMINIUM COMPOUNDS 381 ANALYSIS OF DILUTE HYDROCARBON SOLUTIONS OF DIALKYLALUMINIUM CHLORIDES- It can be seen from Table V that good agreement is obtained between the iodimetric and the isoquinoline methods of analysis. Diethylaluminium chloride usually contains a maximum total of 5 per cent. of ethylaluminium chloro-ethoxide and triethylaluminium or ethylaluminium dichloride as impurity.The presence of these contaminants at this level of concentration does not interfere appreciably in the iodimetric determination of diethyl- aluminium chloride. TABLE V DILUTE DIETHYLALUMINIUM CHLORIDE SAMPLES Diethylaluminium chloride content, millimoles per litre n I v Based on aluminium Based on isoquinoline Based on iodine determination consumption consumption 218 198 197 192 172 177. 179 ANALYSIS OF DILUTE HYDROCARBON SOLUTIONS OF DIALKYLALUMINIUM ALKOXIDES- Depending upon the method of manufacture used, dialkylaluminium alkoxide catalysts might contain small amounts of either trialkylaluminium or alkylaluminium dialkoxide as impurity.The dialkylaluminium alkoxide content of several dilute hydrocarbon solutions was determined iodimetrically . These values are compared in Table VI with results obtained by aluminium determinations. Good agreement was obtained between the two methods when the total organo-aluminium content of the test solution contained less than 5 per cent. of the previously mentioned impurities (sample A). As expected, poorer agreement was obtained for a solution of dipropylaluminium isopropoxide which contained an appreciable amount of propylaluminium di-isopropoxide as impurity (sample B) . Commercial preparations of dialkylaluminium alkoxides usually contain less than 5 per cent.of their total organo- aluminium content in the form of alkylaluminium dialkoxide or trialkylaluminium impurity and no serious interference from these impurities is therefore to be anticipated. TABLE VI DILUTE DIALKYLALUMINIUM ALKOXIDE SAMPLES Calculated as dialkylaluminium alkoxide, millimoles per litre 7- -- A t Based on Based on aluminium iodimetric Differences, Sample description determination determination per cent. Sample -4 : AlR,(OR) containing less than 5 per cent. of AlR, or -\lR(OR), impurity Diethylaluminium ethoxide in iso-octane . . 795 795 Nil Sample B: AlR,(OR) containing 25 per cent. Dipropylaluminium isopropoxide in iso- of =ZlR(OR), impurity octanc . . .. . . .. . . 272 273 238 - 12-5 The iodimetric method of analysis also presents a method for detecting whether a change in the composition of stocks of hydrocarbon solutions of organo-aluminium compounds has occurred during storage. This could arise from the contamination of the material with extraneous oxygen and/or water, thus leading to a reduction in the iodine number because of alkyl group decomposition as follows- 2 > A1R + 0, - - - 2 > Al(0R) > A1R + H,O - - - > Al(0H) + RH Regular iodimetric determinations present a method, therefore, of detecting whether contamination of the organo-aluminium compound occurs to any extent during storage.Such information could not be obtained from aluminium determinations alone, as these remain virtually unchanged even when the sample has become heavily contaminated.383 CROMPTON [Analyst, 1701. 91 APPLICATION OF THE IODIMETRIC METHOD TO HIGHER MOLECULAK WEIGHT ORGAXO-ALUMINIUM The iodimetric method was applied to a solution of impure trihexadecylaluminium in The usual stoicheiometry was assumed in the reaction of the components of this It is seen in Table VII that reasonable agreement is obtained between COMPOUNDS- toluene. sample with iodine. the expected and the found iodine consumptions of this substance. TABLE VII APPLICATION OF THE IODIMETRIC METHOD TO TRIHEXADECYLALUMINIUM Iodine consumption, g of iodine per 100 g of sample Composition of sample, r--7 w/w per cent. Expected Found 16H33) 47-8 4.0 4.5 9.2 1.6 32.0 55.7 54.0 - Total 99.1 - The author thanks the Ilirectors of Shell Chemical Company Limited for permission to publish this paper. 1. 2 . 3. 4. 5. 6. 7 . 8. 9. 10. 11. 12. 13. RE FE RE 5 c E s Bonitz, E., Chem. Ber., 1955, 88, 742. Ziegler, I<., Justus Liebags Annln Chem., 1954, 589, 91. Cronipton, T. R., and Reid, V. W., Analyst, 1963, 88, 713. Dijkstra, R., and Dahmen, E. ,4. M., 2. anal~rt. Chem., 1961, 181, 399. Neuniann, W, P., Justus Liebzgs Annln Chem., 1960, 629, 23. Farina, M., Donati, M., and liagazzini, RI., Aiznalz Chim., 1958, 48, 501. Nebbia, L., and Pagani, R., Chzmaca Ind., iikfzlano, 1962, 44, 383. Illitchen, J . H., Analyt. Chem., 1961, 33, 1331. \\'adelin, C. W., Talanta, 1963, 10, 917. Razuvaev, G. A., and Graevskii, X. I . , Dokl. Akad. iYauk SSSR, 1959, 128, 309. Hagen, D. F., and Leslie, W. D., AnalJit. Chem., 1963, 35, 814. Bartkiewicz, S. -4., and Robinson, J . W'., Analytzca Chiin. A d a , 1969, 20, 326. Crompton, T. R., Analyst, 1961, 86, 65%. Received Jirly 14th, 1965

ISSN:0003-2654    

DOI:10.1039/AN9669100374

出版商:RSC    

年代:1966

数据来源: RSC