ISSN:0003-2654    

DOI:10.1039/AN95176FX001

出版商:RSC    

年代:1951

数据来源: RSC

摘要:

2 ASCHAFFENBURG AND KING: A COLLABORATIVE STUDY OF THE [Vol. 76 A Collaborative Study of the Freezing-Point Depression (Hortvet) of Sucrose Solutions BY R. ASCHAFFENBURG AND J. KING DURING the last few years a committee of the British Standards Institution has been engaged in preparing a standard method of determining the freezing-point depression of milk. Some of the directions given by Hortvetl have in the past been interpreted in different ways. It was, therefore, necessary to formulate the Hortvet technique in more precise terms to ensure a higher degree of reproducibility, and this wi%S done by a sub-committee. The details of the technique arrived at, and recommended :for incorporation in a British Standard, are given below. To ascertain that satisfactory reproducibility was obtained when a number of laboratories followed the recommended procedure, a collaborative experiment was arranged by the following- Government Laboratory (Mr.J. King and Dr. H. Egan) Dr. Bernard Dyer and Partners (Dr. J. H. Hamence) National Institute for Research in Dairying (Dr. R. Aschaffenburg) Messrs. Cow & Gate Limited (Mr. H. C. Hillman) The Hannah Dairy Research Institute (Dr. R. Waite) Milk Marketing Board (Dr. A. L. Provan and Mr. F. J. Macdonald) Lancashire County Council (Dr. G. H. Walker). The collaborating laboratories were asked to determine the freezing-point depression of an 8.5 per cent. (w/v at 20" C) solution of sucrose with thermometers calibrated at the National Physical Laboratory, applying the corrections given on the certificates a's recom- mended by Aschaffenburg and These authors have confirmed the experience of many observers that it is difficult to obtain uniform results with sucrose solutions of 10 per cent.or even lower concentration. It did, however, seem reasonable from their findings that a solution of 8.5 per cent. could be expected to give uniform results that should be in accordance with those interpolated by Elsdon and Stubbs3y4 in this country from the work of Hortvetl in the U.S.A., since these observers used thlermometers calibrated on the international Temperature Scale at the National Physical Laboratory and the Bureau of Standards, respec- tively, as their standards of temperature. The seven laboratories were requested to carry out four determinations of the freezing- point of an 8.5 per cent.solution of sucrose and two determinations for water (one value before and one after the sucrose solutions) with either the Hortvet or the Temple cryoscope. When the latter was used, the motors driving the stirrer and the compressdr were to be switched off at the time of seeding. Any result considered unsatisfactory was to be recorded separately. Pure A.R. sucrose prepared from cane sugar was sent to all participating laboratories. 'Laboratory Apparatus A Hortvet B 9 9 B >I C 99 C 99 D Temple D 7) E 93 F 99 G Hortvet Thermo- meter No. 33988 99 99 62652 17424 4661 1 4839044 69279 4839004 3710288 TABLE I SUMMARY OF COLLABORATIVE RESULTS Corrected maximum temperature reading A I 7 8.5% sucrose solution Lowest, " C - 0.523 - 0.520 -0.517 - 0.530 - 0.520 - 0.520 -0.519 - 0.508 - 0.533 -0.513 Highest, " C a -0.520 -0.516 - 0.516 - 0.526 -0.519 -0.517 -0.517 -0.507 - 0.53 1 -0.512 Mean, " C - 0.52 1 -0.5175 -0.5165 - 0.528 - 0.520 -0.519 -0.5185 - 0,5075 - 0.532 -0.5126 Water & 1st test, " C - 0.00 1 + 0.006 + 0-004 - 0.008 - 0.003 + 0.002 +O-OOI + 0.010 -0.012 + 0.009 2nd test, " C - 0.001 -+ 0.004 + 0.004 - 0.005 - 0.002 + 0-002 0.000 $0.011 - 0.012 Freezing- point depression " C 0.520 0.522 (mean) 8 0.5205 0.5215 0.5175 0.52 1 0.519 0.518 0.520 General mean: 0.520 + 0-008 0.521 Remarks Thermometer on loan from Laboratory A different days Two sets of results on Standard deviation = 0.0015 Thermometer not N.P.L.standardised ; uncorrected thermometer readings4 ASCHAFFEXBURG AND KING EXPERIMENTAL PROCEDURE FREEZING-POINT OF 8.5 PER CENT.SUCROSE SOLUTION- Cool with prescribed rate of stirring5 (1 clomplete stroke per 1 to 2 seconds) until the temperature of the sucrose solution reaches - 1.65" C. Insert freezing starter tube holding ice fragment. Immediately remove freezing starter tube and cease stirring. After about 90 seconds the apparent highest temperature should be reached. Then stir three times at normal speed, tap the thermometer stem seven times at the level of the top of the mercury thread and read the thermometer. Thirty seconds after this reading, stir again three times followed by tapping seven times as before and again read. Repeat these operations a third time and record the final reading. The difference between the first and second reading may exceed 0.005" C but does not usually exceed 0.01" C; there should not be a greater difference than 0.003" C between the second and third redings.When the difference is greater, the result should be discarded and the test repeated. Stir at prescribed rate until the mercury thread rises. FREEZING-POINT OF WATER- Carry out the procedure as above, but if freezing is not spontaneous, insert the freezing starter tube at -1.1"C. RESULTS The results obtained by the seven laboratories are summarised in Table I. For each set of determinations, the range and mean value for 8.5 per cent. sucrose solution, and the two values for water, are listed together with thle mean value for the freezing-point depression of the sucrose solution. No abnormal results were reported other than those due to insufficient super-cooling or abnormal outside bath temperatures.Table I shows that it *is possible for different laboratories to obtain uniform resu.lts by following the recommended procedure. A wider range was obtained when each laboratory used its own technique. The mean for all results obtained by the technique given above is identical with the value of 0.520" C given by Stubbs and Elsdon.4 All but one of the mean values for the individual sets of results fall within &0.002" C of the general mean. This is well within the accuracy to which Hortvet thermometers are calibrateld at the National Physical Laboratory. It will also be seen that the results obtained with the Temple apparatus are indistinguishable from those obtained in the Hortvet cryoscope. These results from seven laboratories were obtained before the publication of the paper by Sutton and Markland.* They are not in accordance with the experience of these authors. Four of the laboratories have extended the work to more concentrated sucrose solutions of 9.0 and 8.75 per cent. The results with 9.0 per cent. solutions confirmed that values are less uniform at this level of concentration. With 8.75 per cent. solutions no difficulties were encountered, although they might have been expected from the data of Aschaffenburg and REFERE:NCES 1. 2. 3. 4. 5. 6. Hortvet, J . . J. Ass. OH. Agric. Chem., 1922, 5 , 470. Aschaffenburg, R., and Hall, J. A., Analyst, 1949, 74, 380. Elsdon, G. D., and Stubbs, J. R., Ibid., 1934, 59, 585. Stubbs, J. R., and Elsdon, G. D., Ibid., 1936, 61, 455. Hortvet, J., J. Ass. OH. Agric. Chem., 1921, 5, 175. Sutton, R. W., and Markland, J., Analyst, l!)fiO, 75, 251. NATIONAL INSTITUTE FOR RESEARCH IN DAIRYING GOVERNMENT LABORATORY SHINFIELD, NR. READING, BERKS. STRAND, LONDON, W.C.2 October, 1950

ISSN:0003-2654    

DOI:10.1039/AN9517600002

出版商:RSC    

年代:1951

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN95176BX003

出版商:RSC    

年代:1951

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN95176BP005

出版商:RSC    

年代:1951

数据来源: RSC

摘要:

12 BABB : A RADIO-FREQUENCY ELECTRONIC MOISTURE METER [Vol. 76 A Radio-Frequency Electronic Moisture Meter BY A. T. S. BABB (Presented at the meeting of the Society on Wednesday, February lst, 1950) A moisture meter based on change of permittivity with moisture content and utilising radio frequencies is described. Examples are given of its use for the examination of materials of widely differing texture and moisture content. The meter is shown to be accurate within 0.3 per cent. of moisture over its whole working range, which is from 1 or 2 per cent. to 60 per cent. of moisture, provided that measurements are made under the same conditions as are used for calibration. CONDUCTANCE and permittivity are properties of a substance which, as a rule, are greatly influenced by the moisture content, and many electronic meters have been designed to measure one or both of these properties and hence to indicate the moisture content of the material under examination.Meters based on the measurement of conductance are simpler than those designed to measure permittivity or capacitance, but the range of moisture content over which they are sensitive is usually limited. The conductance of the material is largely due to the solution of ionisable compounds. In many materials of low moisture content the water exists to a very great extent as “bound” water, and in these materials, for a small change of moisture content, the change of conductance is too small to be measured. When the moisture content is high, ionisation may be complete, and then a conductance meter may be insensitive to change of moisture content.A further objection is that for the accurate measurement of conductance the conditions of contact of the material with the electrodes must be reproducible, a requirement not easily satisfied if the texture of the material to be examined varies with moisture content or if it is not a homogeneous product. The conductance method could not, for example, be employed for the examination of lumpy or coarsely-chopped materials, particularly if the sample must be large in order to obtain a result that is representative of the bulk. Meters that depend on change of permittivity and that measure capacitance can be made t o indicate moisture content because the permittivity of water is 81 while that of most dry materials lies in the range 2 to 4.Provided, therefore, that the water is not combined, chemically or physically, with any other constituent of the material, permittivity should increase steadily with moisture content ; but the contribution to permittivity of “bound” water is very much less than that of “free” water, and for any material in which some of theJan., 19511 BABB : A RADIO-FREQUENCY ELECTRONIC MOISTURE METER 13 water is “bound” it would be expected that, as the moisture content is increased gradually from zero, the permittivity would increase slowly at first when the added water becomes nearly all “bound,” but more rapidly as the proportion of the added water that becomes “bound” progressively decreases. A meter for measuring capacitance should therefore be so designed that its sensitivity to change of moisture content increases as the moisture content decreases.Meters based on measurement of permittivity or capacitance are of two general types, those that employ low frequencies of the order of 60 to 5000c/s and those using radio frequencies of upwards of 100 Kc/s (the meter described in this paper utilises a frequency of 10 Mc/s). Low frequency meters are usually insensitive at low moisture contents because of the difficulty of measuring small changes of capacitance with the impedance bridge circuits they usually employ, and at high moisture contents the ohmic resistance of the sample may be affected by texture. A change of ohmic resistance that would have little or no effect on the response of a meter employing a high frequency could introduce serious errors in one operating at a low frequency.There are several radio-frequency moisture meters on the American market but, as far as is known, the only meter available in this country was designed specifically for the examination of textiles. The meter described in this paper was designed for the examination of a very wide range of materials with moisture contents as low as 1 or 2 per cent. or as high as 60 per cent., for powdered, granular or coarsely chopped products and for materials in the form of sheets, shaped pieces or wrapped packets. It was considered that a meter of such great versatility should find many applications, but it was also realised that, as distinct from an “all-purpose” meter, the need would arise in specific cases for a meter that would provide maximum sensitivity over a particular but limited moisture range.The instrument was designed with the following points in mind- (i) The provision of sample cells of varying dimensions to cater for materials of widely differing character. (ii) The ability to suit the dimensions of the sample cell to those of the articles to be tested in special cases. (iii) An electronic circuit that would be sufficiently sensitive to small changes of per- mittivity over a wide range of permittivity. (iv) An electronic circuit whose sensitivity to change of permittivity would be greatest when the permittivity was least. (v) An electronic circuit that, by the mere alteration of the values of some of its com- ponents, could, in a given instance, be made to operate with great sensitivity over any required, limited, moisture range.The “all-purpose” meter employs a frequency of about 10 Mc/s and operates with materials which have an effective permittivity between 1 and 20 and which increase the capacitance of the sample cell by 0 to 25pp.F. Only a brief description of its essential features need be given in this paper.* This has been found to be so in practice. THE ELECTRONIC CIRCUIT The circuit is shown in Fig. 1. Apart from the power pack it comprises three essential (i) T h e cell oscillator, which generates a signal whose frequency is governed solely by the capacitance of the tuned circuit. (ii) T h e frequency changer, which receives the signal from the cell oscillator and generates another signal whose frequency is governed solely by the frequency of the signal received.(iii) T h e indicator, which receives the signal from the frequency changer but which gives a visual response only if that signal has a single, pre-arranged frequency. It gives a visual indication when-and only when-the frequency of the original signal generated by the cell oscillator has a pre-arranged value, i.e., when the tuned circuit of the cell oscillator has a pre-arranged capacitance. sect ions- * It is intended to publish a more technical description of the circuit elsewhere. The meter has been named “The Kappa Moisture Meter” (Prov. Pat. No. 124752/47) and both the “all purpose” meter and meters for special applications are manufactured under licence by Messrs.Toplis Simpson and Co., Ltd.14 BABB A RADIO-FREQUENCY ELECTRONIC MOISTURE METER [Vol. 76 One component of this circuit is the sample cell, or condenser C2; when the capacitance of this condenser is increased, as when a sample is put into the cell, the capacitance of the tuned circuit is increased and the frequency of the generated signal is reduced. If the capacitance of the circuit be now gradually THE CELL OSCILLATOR- Only the method of control of capacitance need be described. E I - - - . . - - - $ .- reduced by the adjustment of a condenser, say the condenser C1A in parallel with the cell, the capacitance of the circuit will eventually reach its former value and the signal will have its former frequency. Thus, provided the meter had first been tuned to that frequency,Jan., 19511 BABB : A RADIO-FREQUENCY ELECTRONIC MOISTURE METER 15 the indicator would show that the capacitance added by way of the sample had been exactly nullified by the adjustment of ClA, and this adjustment would therefore be a measure of the capacitance of the sample. Such a method could not be adopted in practice because the added capacitance is small, and its variation with moisture content is so small that changes of the order of 0.01 ppF would have to be detected by the use of a very fragile and insufficiently stable component.Removal of capacitance by the operation of series condensers instead of parallel condensers has therefore been adopted and the design is such that the ratio of capacitance to be removed to capacitance added by way of the sample is at least 50 when the latter is 1 ppF and 12 when the latter has the maximum value of 25 ppF for which the meter provides. The removal is effected partly by the operation of the tuning condenser C8 which has a working range of 60 ppF and is ganged to a pointer which moves over a scale, and partly by a series of condensers, C4, C5 and C6, which by the operation of push-buttons remove capacitances in five increments each of 50 ppF. The push-button unit therefore functions as a range-selector switch so that, of the total capacitance to be removed, not more than 60 ppF is recorded on the scale, which is marked linearly with well-spaced graduations.The design of the circuit thus combines a very large magnification of the capacitance of the sample with an easily read scale.Two controls of this circuit are provided-the Standard control C3, operated by push- button, and the Set Standard control C9; in conjunction with the control C13, a component of the frequency changer, they are used to align the meter before use. By their adjustment the indicator is made to respond visually when the sample cell is empty and again when a standard capacity is introduced in parallel with the cell and a pre-arranged capacitance is removed by operation of the push-buttons and tuner. In this condition the meter operates correctly over its whole working range. THE FREQUENCY CHANGER- The frequency changer section contains two parts, a control oscillator of the same type as the cell oscillator and a signal mixer. The oscillator generates a signal of fixed frequency, about 10Mc/s (adjustable for alignment purposes by the Set Zero control C13).During the operation of the meter this signal, called the control frequency, is continuously fed to the mixer which also receives the signal from the cell oscillator. These two signals are electronically “mixed,” and the mixer is caused to generate a third signal whose frequency is equal to the difference between the frequencies of the two signals received. The third signal is passed to the last section of the meter, the indicator. The control frequency is made slightly greater than that generated by the cell oscillator when the cell is empty, so that a relatively small change of the cell oscillator frequency induces a relatively large change of the frequency of the third signal.For example, the “all-purpose” meter is designed to have a control frequency of 10.000 Mc/s and a cell oscillator frequency (cell empty) of 9-630 Mc/s; the third signal therefore has a frequency of 0.370 Mc/s, to which frequency the indicator is designed to respond. If the frequency of the cell oscillator decreases to 9.620 Mc/s, a change of 1 in 963, the frequency of the third signal increases to 0.380 Mc/s, a change of 1 in 37. In this way the sensitivity of the meter to change of frequency of the cell oscillator is magnified 24 times. THE INDICATOR- The indicator is designed to respond to a signal of fixed frequency (0.370 Mc/s in the “all-purpose” model) and only if the third signal has this frequency does the neon lamp respond.In practice the frequency band over which the indicator operates is about 0-02 Mc/s, from 0.360 to 0.380Mc/s, and the peak intensity of illumination can be judged to within 0.002 Mc/s. Thus the indicator shows that the frequency of the third signal lies between 0.368 and 0.372 Mc/s and therefore that the frequency of the cell oscillator signal lies between 9.632 and 9-628 Mc/s. It has not been found necessary to provide any stabilising circuits; this is because the cell and control oscillators are of the same type and, although mains fluctuation or temperature change might cause frequency drift, any drift would be common to each oscillator and the frequency difference (the third signal) would be unaffected. The meter is therefore extremely sensitive to change of frequency.16 BABB : A RADIO-FREQUENCY ELECTRONIC MOISTURE METER [Vol.76 SAMPLE CELLS To enable materials of widely different texture to be placed in the cell in a standardised manner, and for convenience and cleanliness, the sample is not put directly into the meter but into a container, made of Perspex, that slides smoothly into position between the plates of the condenser C2. According to the nature of the material, one of three sets of cells and containers should be chosen. The small container holds 100 to 200 g of material (depending on its bulk density) and is designed for use with powdered or finely granular material. The medium-sized container holds 400 to 800 g, and is for coarse granules, pellets,. tea, material in sheet form, biscuits, small tablets and similar articles. The large container is intended for larger irregularly shaped pieces, or for use when a very large sample must be taken to be representative of the bulk; this container holds 1000 to 2000 g.The small container is provided with a ‘compression block fitted with an adjustable stop so that, for materials of variable bulk density, the volume as well as the weight of the sample can, if necessary, be controlled. Within certain limits, cells and containers can be designed to suit the dimensions of shaped articles or packaged goods and to fit into the standard meter. OPERATION The meter having been aligned, the testing of a sample is performed as follows- A pre-arranged, convenient weight of sample is taken; weighing must be accurate to &O-5 per cent., or to &O-2 per cent. if the moisture content is low and the greatest accuracy is required.The sample is transferred to the container in a standardised manner and com- pressed if necessary. The container is placed in the meter and the push-button and tuner set to give a peak intensity of the indicating light. The position of the pointer on the scale card is recorded. The temperature of the sample is taken and the scale reading corrected for temperature if necessary. The moisture content is read from a calibration curve, or directly from the scale card if this has been graduated for the material under test. Tuning the meter to the peak intensity takes about one minute if all the moisture ranges have to be searched, but only a few seconds in routine work when the approximate moisture content is known.CALIBRATION- This change is governed by the permittivity of the dielectric, i.e., the sample under test, but whilst the permittivity of the sample generally increases with moisture content there is no simple relationship that enables one value to be expressed in terms of the other. Therefore, in common with all electronic moisture meters,, the meter here described must be calibrated for use with the particular material that it is required to examine. The meter is calibrated by taking three or more readings for each of a sufficient number of samples of known but different moisture contents so that a calibration curve to cover the required moisture range may be drawn. The sets of three or more readings show the degree of reproducibility of results and give an indication of the accuracy to be expected.The temperature correction is obtained by calibrating with the samples at two or three different temperatures. It will be appreciated that the routine followed in the subsequent determinations must be the same in all details as that adopted for the calibration. The scale card may be calibrated directly in terms of moisture content from the calibration curve. The meter measures indirectly the change of the capacitance of the condenser C2. PERMITTIVITY AND MOISTURE CONTENT The permittivity of the material depends mainly on its moisture content, but also on a (a) The proportion of the total moisture that is “bound” or chemically combined with the material. ( b ) The composition of the dry material.(c) The bulk density of the sample. (d) The disposition of the sample particles or pieces with respect to the condenser plates and to each other. (e) The temperature of the sample. number of other factors the most important of which are-Jan., 19511 BABB: A RADIO-FREQUENCY ELECTRONIC MOISTURE METER 17 It follows that unless these factors are constant, or change steadily with change of moisture content, the meter cannot give reliable results. It also follows that for any one material the method of filling the container must be standardised in such a way that “packing errors” are minimised. In this respect no difficulty is experienced with materials of uniform particle size unless texture varies largely with moisture content. With irregularly shaped pieces “packing errors” can be sufficiently reduced by taking large samples, and uniformly shaped articles should be packed in pre-arranged and regular formation.APPLICATIONS Without experience with similar materials it is impossible, in a particular example, to predict either the accuracy with which moisture content could be determined or the sensitivity of the meter to change of moisture content. This is because of the widely differing properties of materials-texture, composition, grain-structure, water-combining powers, etc.-and trial alone will show whether the meter can usefully be applied to the examination of a particular material. Some typical examples of the use to which the meter has so far been put are shown graphically in Fig. 2.The curves illustrate, for a great variety of materials and moisture MOISTURE CONTENT, PER CENT Fig. 2. Relationship between meter reading and moisture content. Curves for- (1) Biscuits (7) Tea (2) Spray-dried whole egg (several different (8) Fish meal brands) (9) Powdered gelatin (3) Dried ground chicory (4) Sulphamerazine granules ( 5 ) Cops of cotton (6) Cornflakes (ioj Rye grain - (11) (12) Dried chicory roots, sliced Flour of one type only - rn - rn - of several types, strong, weak, chlorinated, etc. __ + - + - ranges, that the meter is adequately sensitive to change of moisture content. As an example of a substance with a moisture content greater than 20 per cent., alumina granules gave a curve with an average slope of 7 scale divisions for 1 per cent. of moisture over the range 13 to 38 per cent.of moisture; determinations were made with an error not exceeding 0.3 per cent. of moisture over the whole range. The author wishes to thank the Directors of Messrs. J. Lyons & Co., Ltd., for permission to publish. J. LYONS & Co., LTD. THE LABORATORIES LONDON, W.1418 BABB : A RADIO-FREQUENCY ELECTRONIC MOISTURE METER [Vol. 76 DISCUSSION DR. J. D. DERMOTT HARDING asked what was the effect of the temperature of the samples on the determination of moisture by the instrument described. Could the method be applied to inorganic sub- stances, in particular t o stable inorganic salt hydrates, such as aluminium fluoride ? MR. BABB replied that a small temperature correction to the apparent moisture content had in some cases been found necessary.Apart from alumina, with which good results had been obtained, inorganic substances had not as yet been examined. DR. H. J . CALLOW asked whether the method could be applied to determine the average moisture content of a bulk sample such as a bale of jute which measured approximately 4 f t . x 2 f t . x 2 ft. and weighed about 400 lb. MR. BABB said he thought i t would be possible to design a cell for such a purpose provided there was no irregularity of shape. It might be found advisable to use a rather lower frequency with such a cell. MR. R. L. STEPHENS said that in view of the small capacitance involved, the frequency should be increased and not decreased. MR. BABB replied that the cell could be designed t o keep the capacitance about.the same as those met with in the normal cell.He had referred to the possibility of using a lower frequency, not to allow for a smaller capacitance, but to ensure that the wavelength would be many times greater than the dimensions of the cell. DR. D. W. KENT-JONES asked what the meter actually measured when used for testing cereals that contained both free and combined moisture. Would a meter which made an accurate measurement of the natural moisture in grain also measure the moisture in a freshly damped sample? DR. D. E. HAWKINS asked a similar question and stated that he had found electronic moisture meters unreliable for dried wheat, presumably because of uneven distribution of moisture in newly dried samples. MR. BABB replied that he thought he had made it clear that the meter reading obtained was governed solely by the permittivity of the sample, and any property of the sample which affected permittivity- one such property being the distribution of moisture--would affect the meter reading. If the material was not in equilibrium with its moisture or could not be brought to the same condition as obtained when the calibration curve was constructed, an error would be introduced. DR. PAGE asked whether any special packing precaxtions had to be taken for such materials as paper and biscuits. MR. BABB replied that the sample must be placed in the cell in a reproducible manner. The results on paper had been obtained with sheets cut to a suitable size, and biscuits had been stacked in regular formation in the cell. Biscuits could be ground if that were convenient. MR. H. T. SUTCLIFPE asked whether moisture could be accurately determined in a sample of soap chips taken directly from the drier and being dry on the outside and damp inside, and also in soap powder in which uncombined water is rapidly absorbed as water of crystallisation by sodium carbonate. MR. BABB referred to his previous reply on the question of uneven distribution of moisture and said that in such instances only a trial, using samples taken under working conditions, would show whether the meter reading could be accurately related to mois,ture content. Would the technique apply to biscuits of different shape and texture?

ISSN:0003-2654    

DOI:10.1039/AN9517600012

出版商:RSC    

年代:1951

数据来源: RSC

摘要:

Jan., 19511 MONEY AND CHRISTIAN 19 The '' Air Current " Method of Moisture Determination With Particular Reference to Moisture in White Sugars BY R. W. MONEY AND W. A. CHRISTIAN (Presented at the meeting of the Society o n Wednesday, February lst, 1950) For the routine determination of moisture in fine-grain white sugar, vacuum drying at 100" C was found to be inaccurate, the A.O.A.C. method, although valuable as the reference method, was too slow, and the Karl Fischcr method was unsatisfactory owing to the need to maintain a supply of reagent, which deteriorates rapidly, for only three or four determinations per week. Experiment showed that a method in which a preheated current of air was drawn through the sample in a specially designed apparatus gave satis- factory results. The apparatus and the method of operation are described and some typical results are tabulated to show their reproducibility.A comparison of results by the air-flow method with those by the -4.O.A.C. and Karl Fischer methods showed satisfactory agreement. THIS investigation arose from the necessity to determine moisture contents of about 0.05 to 0.3 per cent. in fine-grain white sugar (crystal caster) for use in a manufacturing process requiring freely flowing sugar particles. It was essential that the determination should be made rapidly, i.e., in less than an hour, and should be accurate to within 5 per cent. The rapid method of drying a 1 to 2-g sample in a vacuum oven at 100°C was insufficiently accurate owing to the great significance of errors introduced by slight charring and by the normal experimental errors when such a small moisture content was being determined, while the A.O.A.C.method of drying to constant weight in vacNo at 70" C was too slow for routine purposes, although valuable as the reference method. Consideration was given to the Karl Fischer method and good results were obtained, but the method was not entirely satisfactory as it entailed the maintenance of a supply of reagent, which deteriorates rapidly, for the sake of three or four determinations a week. Various distillation methods were also considered but were unsuitable owing to the very small amount of moisture involved. It was thought that the rapid removal of moisture could best be effected by a current of warm dry air passed through the sugar in such a manner that all the particles were exposed to the air current and kept in motion by the air; the apparatus evolved and the results obtained by employing this principle are described below.A preheated current of air has been used in many methods of moisture determination, but in only a few has the air been passed through the sample. In an oven method described by Spencer,l air is sucked over an electric heating element and through a metal capsule fitted with a metal filter cloth on which the sample is placed; drying periods of up to 20 minutes at 105" C were used for raw sugars. Meade2 used the Spencer oven to determine the moisture content of molasses, syrups and honey by absorbing a dilution on asbestos, which was subsequently dried for a specified time; drying to constant weight was not recommended.Whittaker and Ross3 described an apparatus, the "Moisture Teller,"4 that was similar to the Spencer oven and consisted of a sample dish fitted with a 500-mesh screen as the bottom; the air stream was provided by a blower. The use of this apparatus for "roughages" was described by Monrose and perk in^.^ Since the method described in this paper was devised, Hardesty, Whittaker and Ross,6 and Ross and Love,' have described an apparatus embodying crucibles fitted with sintered glass plates; they used this apparatus to determine the moisture content of fertilisers by heating at 60" C for two hours. In all these methods, the preheated current of air is drawn or blown through the sample in a downward direction so that loss of the finer particles is avoided, but tight packing may be aggravated and this would tend to lengthen the drying time.It was considered that the use of an upward current of air combined with precautions against the loss of fine particles would ensure that packing is minimised, that the particles are agitated and continually mixed, and that an even temperature would be attained.20 MONEY AND CHRISTIAN: THE AIR-CURRENT METHOD [Vol. 76 THE APPARATUS The general principle of the apparatus is shown in Fig. 1, in which the various parts are shown diagrammatically and unassembled. Standard ground glass joints were used. The sample container, S, is formed of two B29 cones fused together, and a sintered glass plate, P, of grade 3 porosity is fused into one cone.The sample container fits into the B29 socket of the heating unit, H, and is closed by the vacuum head, V, placed over the top cone. By the application of a vacuum at the outlet, 0, air previously dried (by passage through silica 0 H V Fig. 1. Apparatus for air current moisture determination gel, calcium chloride, sulphuric acid or other desiccant and filtered through cotton wool) is drawn in at I, round the heater, through the porous plate and the sample. Temperatures may be taken by the insertion of thermometers fitted with B14 cones into pockets T, and T,; the temperature reached by the sample is referred to again below. Some care is necessary in the design of the heating unit to avoid the risk of volatilisation of metal or metallic oxide from the heating wire on to the porous plate, so causing an increase in weight.It is for this reason that the wire is enclosed in a sealed Pyrex tube, which serves to heat the incoming air. The heater current is obtained from a mains transformer tapped for 12 volts; the heater consumes 24 watts. The rate of air flow is controlled by the vacuum applied and is, of course, dependent on the porosity of the plate and tightness of packing of the drying towers, filter plugs, and so on. The flow should be just sufficient to keep the particles of the sample in motion without violent disturbance. In practice, the vacuum is gradually increased until the sample begins to “dance”; for caster :sugar this occurs when the pressure in S (measured by a manometer connected to T,) is reduced to about 36cm of mercury.AJan., 19511 OF MOISTURE DETERMINATION 21 dry vacuum pump was used to produce the air current in this investigation; if a water pump must be used, suitable traps might be necessary to prevent water vapour passing back to the sample. In order to guard against the possibility of fine particles of the sample being carried away by the air current, a plug of cotton wool is inserted in the upper cone of S, and, as is shown below, provides an effective safeguard against the loss of even extremely fine particles. METHOD OF OPERATION A number of interchangeable containers, S, are fitted with cotton wool plugs, dried in the apparatus, cooled in a desiccator for 20 minutes and weighed, the procedure being repeated until constant weight is reached.The plug is removed, the sample of sugar, 6 to 8 g, introduced, the plug replaced and the weight found by difference. The sample container is then placed in the apparatus previously warmed by means of the heater, and air is passed for 30 minutes. The container is then removed, cooled in a desiccator for 20 minutes and reweighed. In general, for caster sugars, 30 minutes is sufficient time, but as a check, the container is replaced and air passed for a further 30 minutes. During the preliminary work difficulty was experienced in that the moisture content of the sugar used as reference sample was neither constant nor uniform but changed with surprising rapidity with changing temperature, despite storage in a jar fitted with a screw cap and rubber washer.This was undoubtedly due to changes in the distribution of moisture over the surface of the sugar particles and could not be avoided by apparently thorough shaking. The effects were avoided by "conditioning" the sugar in a moist atmosphere (over 85 per cent. relative humidity) at 25" C for various periods of time, the sugar being spread out in very shallow layers and being well mixed at frequent intervals. The well mixed sample was then placed in small specimen tubes, each holding 7 or 8 g, which were completely filled with the sugar before being corked and waxed. The entire contents of a tube were then used for a determination. The method of operation is as follows. TEMPERATURE OF THE SAMPLE The temperature to which the sample was heated in relation to the temperature of the heated air before its passage through the porous plate was measured by modifying the apparatus to allow a thermometer bulb to be immersed in the 7-g sample of sugar in the container.The results are shown in Table I. The temperature of the air was measured by a thermometer placed in T,. TABLE I Duration of air flow, minutes 1 2 3 6 9 12 15 18 21 24 27 30 Temperature of sugar, " C 24.5 29 36 50 57.5 61 64 65.2 66.2 67-0 67.3 67.7 Temperature of air at TI, " C 80 92 101 110 111 112 112 112 112.5 112.5 112.5 112.5 These data showed that during the 30 minutes' heating period the temperature of the sugar did not exceed 70" C. As in practice it would be highly inconvenient to have a thermometer immersed in the sample, a further set of temperatures was taken employing similar conditions of air flow and heating, but taking the temperature of the air above the sample, i.e., by means of a thermometer inserted at T,.Hence, to reach a sample temperature approaching but not exceeding 70" C, it is necessary to attain a temperature of 112" to 114" C below the porous plate (i.e., at TI), and one of 28" C above the sample (i.e., a t TJ. These results are shown in Table 11.22 MONEY AND CHRISTIAN: THE AIR-CURRENT METHOD [Vol. 76 POSSIBLE LOSS OF SAMPLE In order to test whether or not fine particles of sugar might be carried away by the air current despite the cotton wool filter, some experiments were made with icing sugar, which contained about 10 per cent. of particles smaller than 270 mesh (0.053 mm) and so provided a much more severe test than the caster sugar, the particles of which were all greater than TABLE I1 Duration of air flow, minutes 1 2 3 6 9 12 13 18 21 24 27 30 Temperature: above sugar a t T,, "C 20 21 21 23 24 25 26 26.5 27 37.5 28 28 Temperature of air a t T,, "C 93 102 106 111 112 113 114 114 114 114 114 114 100 mesh (0.147 mm).The apparatus was prepared as described above wih the exception that the air after leaving the sample container was bubbled through a U-tube containing water. When the sugar had reached constant weight, no sucrose could be detected in the water by colour reactions sensitive to 1 mg. A second experiment was carried out in which five discs of Whatman No. 5 filter-paper were cut to fit the container above the cotton wool. These were dried and weighed and then placed in position over the container.When the icing sugar had reached constant weight, the discs were tested for the presence of sucrose by the method of Partridge and We~tall.~ No sugar was found on any of the discs, although a similar disc on which 1 mg of sugar had been deposited gave a positive reaction. It was therefore concluded that the cotton wool plug was an effective safeguard against the loss of sugar. TYPICAL RESULTS The reproducibility of the results can be judged from the triplicate determinations of the moisture in three samples of sugar shown :in Table 111. The loss of relative precision when the moisture content is very small is due to the increased effects of experimental, sampling and distributional errors. TABLE I11 RESULTS OF TRIPLICATE DETERMINATIONS OF MOISTURE I N SUGAR Moisture I Sample (4 (b) (4 % % % A 0.446 0.437 0-440 B 0.197 0.180 0-187 C 0.055 0.059 0.062 Mean Spread, % 0-441 f l 0.188 &4 0.059 f 7 The moisture contents of ten samples of sugar are shown in Table IV, together with those obtained by drying to constant weight in a vacuum oven at 70°C.The moisture in four of the samples was also determined by the Karl Fischer method. The agreement in general is satisfactory. Whether the observed differences are due to experimental error (in which must be included errors introduced by the uneven distribution of moisture in the samples taken), or to causes fundamental to the methods used, cannot be stated, but the good agreement between the results by the air current method and the chemically determined water of the Karl Fischer method would indicate that the vacuum oven method used was open to doubt for this type of determination.Although the A.O.A.C. method specifies an air vent to the oven, the flow of air is small and, as has been shown by Iles and Sharman,8 it is difficult to ensure a uniform air movement; moreover the sample of sugar on the bottom of a dish is virtually shielded from any air current.Jan., 19511 OF MOISTURE DETERMINATION 23 TABLE IV MOISTURE DETERMINATIONS BY DIFFERENT METHODS Moisture by air current method, 0.415 0.171 0.166 0.164 0.1 21 0.103 0.065 0.064 0.061 0.060 76 Moisture by drying at 70" C, /O 0.399 0-165 0.161 0.164 0.120 0.110 0.075 0.081 0.052 0.080 01 Moisture by Karl Fischer method, 0.420 0.174 % - - 0.067 0.058 - Our thanks are due t o the Directors of J.Lyons & Co., Ltd., in whose laboratories the work was carried out, for permission to publish, and to Dr. E. B. Hughes for his advice and interest. REFERENCES 1. 2. Meade, G. P., Ibid., 1921, 13, 924. 3. 4. 5. 6. 7. 8. 9. THE LABORATORIES Spencer, G. L., I n d . Eng. Chenz., 1921, 13, 70; U.S. Pat. 1,348,767. Whittaker, Colin W., and Ross, William H., J . Ass. 08. Agric. Chem., 1942, 25, 132. Manufactured by Harry W. Dietert Co., 9300, Roselawn Ave., Detroit, Michigan, U.S.A. Monroe, C. E., and Perkins, A. E., Dairy Sci., 1939, 22, 37. Hardesty, John O., Whittaker, Colin W., and Ross, William H., J . Ass. Off. Agric. Chem., 1947, Ross, William H., and Love, Katharine S., Ibid., 1947, 30, 617.Iles, W. G., and Sharman, C. F., J . SOC. Chem. Ind., 1949, 68, 174. Partridge, S. M., and Westall, R. G., Biochem. J . , 1948, 42, 238. 30, 640. J. LYONS & Co., LTD. T nwnnw 1x7 1 A DISCUSSION MR. H. POWERS said that there was a possible loss of dust from pulverised sugar, a small proportion of which may be as small as Some of the coarser conglomerates and grouping of crystals might not dry as soon as the separate particles. MR. MONEY replied that the method was developed for the routine testing of caster sugar of specified particle size that excluded fine dust, but, as shown in the paper, no loss occurred even with icing sugar, all particles of which were less than 0.147 mm. No trouble has been experienced with conglomerates, which were normally absent from the sugar examined, but which have been produced artificially by damping the sugar and breaking up the lumps.MR. ALFRED WRIGHT said that in a very similar method for resins i t had been found that variations in particle size made very large differences to the results, even though resins were amorphous and not crystalline like sugar. Errors also arose from the tendency of the finely powdered portion of the sample t o be blown away. As absolute uniformity of particle size was impossible without sieve grading and its consequent losses, and also since the method required the particle size to be very small for consistency of results and the small size of particle led to losses by blowing away, the method was considered to have too many inherent disadvantages and was discarded.MR. A. L. BACHARACH suggested that figures for the reproducibility of the A.O.A.C. method, together with those reported by the author for his own, should make it possible to assess the significance of the occasional small differences that he found between them. DR. I<. 4. WILLIAMS said that the principle described by the authors had been applied to other materials. In particular, he mentioned that in 1918 the Ministry of Food adopted as standard a method put forward by their Committee of Analysts for determining moisture in oils, and this was very similar to that now put forward. It consisted in spreading the oil over a wad of asbestos contained in a tube and driving off moisture contained in the oil by means of a stream of dry inert gas a t about 50" C.In the early form of the method the moisture removed from the oil was caught on calcium chloride or sulphuric acid and weighed. Later, however, it proved simpler to ascertain the loss of weight of the tube containing the oil on asbestos. Some care was necessary in choosing asbestos suitable for the test as some specimens had proved to lose moisture slowly and contirfuously when dry gas was aspirated over them, and these were clearly useless. inches.24 MONEY AND CHRISTIAX [Vol. 76 MR. R. A. FINCH said that Mr. Money had referred to the determination of moisture in dehydrated vegetables by his air flow method. It was well known thak there was, as yet, no accurate means of measuring moisture other than by drying in vacuum for many hours. This was of no practical value where a large number of samples was being examined daily, and a method of drying dehydrated vegetables to constant weight in a short time, preferably in their unground form (dice or strip) would be very useful.He would be interested t o know how long the author’s method took, and what vegetable he had used. The amount of work done was very small, but drying did take some hours, and the indications were that the method was not as satisfactory as with sugar particles where the moisture was all on the surface. MISS E. IRENE BEECHING asked the author whether he had used the method he described for the determination of moisture in high-boiled sweets, milk powders and chocolate crumb. MR. MONEY replied that the method gave good results with chocolate crumb, but had not been tried for milk powders.Boiled sweets were effectively a glass, and the method could not be used unless the sample were pulverised, during which procedure it would probably absorb a certain amount of water, and even if it were possible to devise a method of pulverising the substance in absence of water, the result would probably depend on the particle size of the powdmer. Much work was needed on the determination of moisture in high-boiled sweets, before a satisfactory method could be devised. MR. L. G. BECKETT drew attention to a statement in the paper that difficulty had been experienced in maintaining known moisture conditions in sugar retained in a Kilner jar closed b y , a rubber ring and screw-on cap. He said that it was not generally realised that rubber was permeable to water vapour.Considerable difficulty in this respect was experienced in the packaging of dried pharmaceutical preparations. Greavesf and Flosdorf2 both referred to this permeation. It was possible that the variation in sugar moisture was due to water vapour flow through the diaphragm. Mr. Beckett asked whether the author had had any experience of back diffusion of water vapour from a water jet pump when used for aerating a sugar sample as described in the paper. He would not have expected back diffusion with the through-put of air necessary to maintain a pressure of 34 cm of mercury. In view of the long time necessary for com- pletely drying sugar under vacua by the method described, had the author any experience of tumbling sugar under high vacuum conditions and the consequent effect upon the time required ? Had Mr. Money considered equating his shorter drying time by air current methods to fundamental considerations of the kinetic theory of gases ? MR. MONEY replied that it was only a rubber washer, not a rubber diaphragm, that was used and therefore the joint was practically glass to glass. He agreed that back diffusion of water was extremely unlikely, but he preferred to take that precaution. The sugar was virtually tumbled but not a t a very high vacuum and he had had no experience of the effects of tumbling under such conditions. MR. MONEY replied that he had tried tomato powder and potato powder. REFERENCES TO DISCUSSION 1. Greaves, R. I. N., in Keynes, G. L., “Blood Transfusion,,’ Simpkin Marshall, Ltd., London, 2. 1949, p. 686. Flosdorf, E. W., ‘‘ Freeze-Drying,” Reinhold Publishing Corp., New York, 1949, pp. 150-164.

ISSN:0003-2654    

DOI:10.1039/AN9517600019

出版商:RSC    

年代:1951

数据来源: RSC

摘要:

Jan., 19511 FRYD AND KIFF: THE DETERMINATION OF MOISTURE IN TOBACCO 25 The Determination of Moisture in Tobacco BY C. F. M. FRYD AND P. R. KIFF (Presented at the meeting of the Society op2 Wednesdgy, February lst, 1950) Moisture in tobacco is commonly determined by the statutory method as 'loss in weight on drying at a defined temperature. During recent years attention has been directed to the effect of variations in the time of drying and in the degree of ventilation in ovens; such variations, it has been shown, may seriously affect the apparent moisture content. This sensitivity of the apparent moisture content to drying conditions is demonstrated to arise at least in part in a component which does not exist as moisture in the undried tobacco, b u t is produced by reactions of the Maillard type or by other reactions involving reducing sugar, during the process of drying.Apparatus is described by the use of which experimental variations in the apparent moisture content of tobacco can be minimised. FOR revenue purposes it is necessary to determine the moisture content of tobacco; and the relevant Acts of Parliament prescribe that this shall be taken as the decrease in weight when the tobacco is dried at a temperature of 212" by Fahrenheit's thermometer. As temperature is the only parameter mentioned, it is not surprising that until recently, interest was concentrated almost entirely upon it. Tatel drew attention to difficulties in the equalisa- tion of temperature within ovens, and Fig. 1, reproduced from his paper, illustrates the extent of the variation that may take place in readings of the temperature at various points within an approximately cubical electrically heated oven.An internal fan rotating at sufficient speed was shown to obviate these variations, and a marked improvement in the repeatability of moisture determinations made in ovens so fitted was the result. Iles and Sharman2 have recently described the effect of the rate of ventilation within ovens on the apparent moisture content of Bright Virginia tobacco. This type of tobacco has long been known to yield moisture results that vary with the apparatus in which the determination is made. Variations of this kind suggest that similarly discordant results may be expected in the moisture figures for other tobaccos and indeed for biological material in general.As a sequel to the work of Iles and Sharman, which had been made available t o us before publication, an oven was constructed in the Government Laboratory in which up t o six 10-g samples of tobacco could be submitted for any desired time and at any pressure between fairly high vacuum and atmospheric to a metered ventilation. It was arranged that the apparatus could be totally and deeply immersed in a well-stirred glycerine - water bath, the temperature of which could be held to within 0.2" C at any point between 60" and 105" C (Fig. 2). Preliminary experiments with this apparatus made it clear that the repeatability of moisture determinations (or more exactly loss-in-weight determinations) on cut tobacco was limited by the homogeneity of the samples. For certain reasons interest in the Government Laboratory was directed especially to cigarette tobacco, and so the raw material for the experiments was obtained from a large number of cigarettes that had been submitted as samples. Cigarette tobacco consists physically of finely shredded lamina of the leaf proper together with portions of the veins and midribs (known as stalk) contaminated with a small proportion of "sand," the residue of those particles of blown dust and soil which have not been detached during the process of manufacture. It is not difficult to mix this tobacco thoroughly; but in samples of 10 g (an amount frequently used for moisture determinations) variations in the proportion of stalk or sand from sample to sample of the mixed material still produces detectable variations in the apparent moisture.After several experiments, a technique to minimise these effects was devised. In a blender (Fig. 3) consisting of a closed drum some 56 cm in diameter and provided with some sixty inward-pointing 4-cm spikes attached to the inner periphery, the tobacco, in lots of about 1 kg, is tossed by rotation of the drum26 FRYD AND KIFF: THE D-ETERMINATION OF [Vol. 76 for 24 hours at such a speed that the tobacco is just not carried round by centrifugal force but falls continuously from the highest point. (One end of the drum is made wholly of quarter-inch Perspex t o facilitate the choice of the appropriate speed.) During this process the sand particles are shaken free and the shreds of tobacco are reduced in length more than are the portions of stalk and vein.The tobacco at this stage is removed and sieved, only the portion which passes 12 meshes to the inch and fails to pass 40 meshes to the inch being retained and returned to the blending drum for a further 30 minutes. After immediate removal at the expiry of this time, the tobacco is packed tightly into glass jars provided with # d 3 P TEMPERATURE. OF Fig. 1. Variation of temperature in electrically-heated oven rubber sealing rings and holding some 200g per jar. Experiments indicate that in these containers no noticeable alteration in apparent moisture takes place during storage of several months, as long as the containers themselves are stored in conditions of equable temperature. For large-scale replicate work, the weighing out of samples requires the utmost care if loss or absorption of moisture by the sample is to be avoided.Two operators are needed, one of whom places in the previously tared and numbered drying-pan an approximate 10 g of the sample; the other weighs it immediately to the required (1 mg) accuracy on an aperiodic optical balance, and returns it to his companion, continuing with the next pan which has been prepared in the interim. The weighed pans a:re stored immediately in airtight containers, in order that the whole series of perhaps one hundred samples may enter the ovens in the same condition. Precautions must be taken to ensure that neither sunlight nor currents of air affect the moisture content of the bulk from which portions are being drawn; and allJan., 19511 MOISTURE IN TOBACCO 27 portions removed from an overfull pan, or remaining in almost empty storage bottles, must be immediately discarded.With these precautions repeatability can be attained which for organic natural materials is of high standard (Table I). m Inner oven Outer Container- I I Fig. 2. Experimental oven Table I1 shows the extent to which variations in the apparent moisture content can occur in homogenised material if the conditions of drying are artificially varied. Results of this nature called not only for the development of apparatus in which moisture determinations could be made under standard conditions, but also for some rational explanation of the cause of the discrepancies. Table I11 shows some results obtained in the Government Laboratory which illustrate the chemical effect of heating for various times and under varied Fig. 3.Blender conditions. It is immediately apparent that the major quantitative changes have taken place in the sugars present. Iles and Sharman2 suggest that reactions of the MaillardS type may be responsible and point to the blackening of the heated tobacco in support of this suggestion; but in addition it may be mentioned that the production of dark and humic substances by the direct action of organic acids on reducing sugars is well Table IV gives some results obtained in the Government Laboratory for certain of the acids and sugars28 FRYD AND KIFF: THE IIETERMINATION OF [Vol. 76 known to be present in tobacco. Some preliminary work, still mainly of a qualitative nature, has been carried out on the volatile end-products of these reactions associated with the heating of tobacco.It is certain that the loss in weight, or apparent moisture, is for the main part accounted for by the liberation of water itself. There is little, if any, carbon dioxide, but TABLE I APPARENT PERCENTAGE OF MOISTURE IN HOMOGENISED BRIGHT VIRGINIA TOBACCO A series of 10-g samples dried at 100" C for 17 hours % f \ 11-63 11.62 11.59 11.56 11.61 11.56 11-56 11.63 11-59 11-66 11-52 11.58 11.55 11.56 11.58 11.59 11.66 11-63 11-61 11.54 11.63 11-53 11.53 11.55 11.59 11-61 11.58 11.55 11.48 11-52 11.55 11.56 11.59 11-58 11.58 11.55 11.53 11.49 11.55 11.57 11.60 11.63 11-61 11-59 11.55 11-57 11-59 11-61 some organic matter of an aldehydic nature is evolved which is not formaldehyde. I t is trapped by concentrated sulphuric acid, with which it reacts to form an insoluble carbonaceous residue.TABLE I1 Further work is in progress on this aspect of the problem. APPARENT PERCENTAGE OF MOISTURE IN HOMOGENISED BRIGHT VIRGINIA TOBACCO 10-g samples dried under varied conditions Treatment Loss in weight % 1. 17 hours drying a t 60" C with 4 litres/min. air flow . . .. . . 8-62 3. 17 hours drying a t 100" C with 0-05 litres/min. air flow . . 13.37 4. 30 hours heating at 100" C followed by 17 hours drying at 100; C with 4 litres/min. air flow . . .. .. .. .. .. . . . . 16.62 2. 17 hours drying a t 100" C with 4 litres/min. air flow . . .. . . 11-91 Sufficient evidence is available therefore to show that the operation of heating tobacco for any appreciable period at a temperature of about 100" C results not only in the removal of all or part of the moisture originally present, but also in a chemical reaction in which moisture is produced.As may be expected, the component of the total apparent moisture accounted for by the results of chemical decomposition varies not only, as has been shown,2 TABLE I11 EFFECT OF OVEN CONDITIONS ON THE COMPOSITION* OF HOMOGENISED BRIGHT CIGARETTE TOBACCO Dried at 100" C for 17 hours -----7 After pretreatment of 17 hours at 100" C Dried at 60" C, full ventilation, without ventilation, With immediate :4 % % Total soluble matter .. .. .. 48.8 45.6 44.0 Total insoluble matter . . .. .. 42.7 43.1 40.8 Apparent moisture .. .. .. .. 8.4 11-3 15.1 Sum of above . . .. .. .. 99.9 100.0 99.9 Total sugars . . . . . . .. .. 16.3 11.9 4.6 Reducing sugars . . .. .. .. 13.2 9.5 4.3 Free acid (ml of 0-1 N NaOH per log tobacco) 72 64 53 Light petroleum ext. .. . . . . , 2.75 2.77 2.66 Total nitrogen . . .. . . . . 1.47 1.39 1-32 Nicotine . . .. . . . . . . 1.51 1.23 1.20 Free ammonia . . .. .. . . 0.12 0.09 0.02 Nitrogen, other than ammonia and Resins .. .. .. .. .. 1.88 1-80 1.76 nicotine nitrogen . . .. . . . . 1.11 1-10 1-10 * For the analytical methods used, see appendix, p. 31.Jan., 19511 MOISTURE IN TOBACCO 29 with the degree of ventilation, but also with the temperature attained (Fig. 4) and with the time during which the tobacco is held at that temperature (Fig. 5). Other considerations, which have effects fortunately of a minor order, are the rate of initial heating of the sample and the barometric pressure within the oven.Thus it is apparent that the use of ovens to determine the moisture in tobacco does not result in the determination of anything that can TABLE IV EFFECT OF HEATING MIXTURES OF 1 G OF VARIOUS ORGANIC ACIDS WITH 5 G OF EITHER LAEVULOSE OR GLUCOSE FOR 36 HOURS IN A DRYING OVEN AT ABOUT 96°C Sugar Acid Loss, Colour Effervescence Residue Glucose 33 33 > 3 Laevulose >? >) 33 Malic Citric Oxalic Protein Malic Citric Oxalic Protein hydro1 y sate h y drolysate % 3.7 Straw 4-5 Straw 16.4 Black 16.6 Black 8.5 Dark brown 9.4 Dark brown 27.4 Black 12.1 Black nil nil Much Much nil nil Little Much Soluble Soluble Soluble Soluble Soluble Soluble Insoluble Partly in soluble Sufficient water was added to ensure solution of the reagents, but the loss-in-weight percentages quoted below are based on the weight of the residue compared with the sum of the weights of the sugar and acid ingredients only.be called the “true moisture” of the sample. The most to be expected from oven determina- tions is a series of consistent and repeatable figures in which the effect of oven decomposition is minimised and standardised. But, as for revenue purposes the loss-in-weight on drying at 212” C remains the legal moisture content of tobacco, the Government Laboratory and presumably the trade in general will continue to use oven drying; hence it may be of interest to describe briefly the type of apparatus that is in use for moisture determination to-day and the requirements that have led to the present design.It is clear that the major con- siderations must be- (1) Ventilation sufficient to ensure that at no time is evaporation from the surface of the tobacco itself slowed down, or the unavoidable chemical reactions allowed to occur at a high rate of velocity in the liquid phase. (2) Constancy of final temperature within the oven. (3) Identity of heat history for all the samples during the process of drying. (4) Speed in dealing with a large number of samples. (5) Economy of the time of skilled operatives, which in effect means drying overnight so that the working period may be devoted to weighing. This requirement itself necessitates that the oven shall be fully automatic.If the heating of an individual sample pan placed in a hot oven is considered, it is clear that heat may reach it by one of three paths-contact with the ambient air, conduction through metal, or radiation from the surrounding surfaces. Some early experiments carried out in the Government Laboratory indicated that the heat flow through the air, though not negligible, contributes only slowly to the rise in temperature of the sample. Conduction through metal is important, but is likely to become uncertaiqas the pristine surface of the metal suffers from routine use. Radiation from surrounding surfaces is probably the largest and most consistent contributor to the considerable amount of heat required to raise the temperature of the sample and supply energy for the evaporation of moisture. To ensure identity of heat history for each sample, the Government Laboratory has found it necessary to construct ovens in which each pan is equidistant from the radiating surfaces and in which the whole of the internal surface is a t the same temperature.In such an oven there is no balancing out of hot and cold spots as is necessary with, for instance, electric ovens of the normal type, and assurance can be felt that not even for short periods is any sample exposed to radiation from surfaces at a higher temperature than that finally attained by the whole oven. Acknowledgment must be made of the great help of the Imperial Tobacco Company, Limited, in this connection. In 1936, Dr. Jollyman of that company showed us an apparatus embodying an oven consisting of a cylindrical shell heated by an outer steam jacket; and30 FRYD AND KIFF: THE DETERMINATION OF [Vol.76 although his successors and ourselves have introduced modifications, his apparatus is very little different in essence from that shown in Fig. 6, which is a diagram of one of the ovens now in use in the Government Laboratory. LOSS OF WEIGHT 'x Fig. 4. Variation of decomposition with temperature; drying time 17 hours In this device the oven proper is the central chamber, circular in plan, into which an internal movable rack (on which a number of sample pans are arranged symmetrically in tiers) is lowered. Up the central axis of the oven rises a perforated tube, distributing radially a flow of preheated air forced in by a centrifugal, pump (not shown).Round the bottom, sides, and top of the chamber circulates a flow of steam generated from a container of distilled water below, into which the condensate returns. With ovens of this type, replicate determina- Fig. 5. Variation of decomposition with time at 100" CJan., 19511 MOISTURE I N TOBACCO 31 tions having a degree of repeatability of the order of those shown in Table I may be confidently expected as a routine operation. Steam 7j=-i+ heated lid from lid Steam inlet to lid +* Air preheating coil 7- Air distribution tube An apparatus including some of the features of such an oven is the subject of a patent application made at the instance of the Imperial Tobacco Company, Limited, and quoted by Iles and Sharman.2 APPENDIX NOTES ON THE METHODS USED IN OBTAINING THE DATA IN TABLE 111 Nicotine-Five grams of tobacco is steam distilled with 2 g of magnesium oxide and 50g of sodium chloride, and 250ml collected.A further 5ml is collected and tested for absence of nicotine. A 100-ml aliquot is acidified and the nicotine is precipitated with silicotungstic acid. The precipitate is filtered through a Gooch crucible, ignited, and weighed. Light petroleum extract-Tobacco, 13-33 g, is in contact with 100 ml light petroleum for 18 hours. Final residue is heated at 100" C for 14 hours. Total nitrogen-One gram of tobacco, 10 g of potassium sulphate, 0-5 g of copper sulphate and 20 ml of concentrated sulphuric acid are boiled in a Kjeldahl flask for 4 hours or until clear. The solution is then diluted, neutralised with sodium hydroxide, distilled into 25 ml of 0.1 N sulphuric acid and titrated back.75 ml of supernatant liquor is removed and evaporated on steam.32 FRYD AND KIFF [Vol. 76 The mass is made up to 200 ml with silicotungstic acid, 12 per cent. solution; 100 ml of filtrate is then treated with 10ml of neutral, 30 per cent. lead acetate solution. After filtration through a Buchner funnel, 100 ml is collected, 5 In1 of saturated potassium oxalate is added, the solution made up to 110 ml and again filtered (Solution A). This solution is used to titrate Fehling's solution, using methylene blue ;is indicator. Total sztgars-As above, but 50 ml of Solution A is boiled for 30 seconds with 3 or 4 drops of concentrated hydrochloric acid and made up to 100ml after cooling.Total soluble-Five grams of tobacco is digested with 150 ml of water overnight. The mass is filtered through a Buchner funnel and washing continued until 1OOOml of extract is obtained. Total insoluble-Fibrous matter from the above is carefully removed and dried at 100" C to constant weight. Ammonia-Five grams of tobacco, 20 ml of 1 : 4 hydrochloric acid and 20 ml of silico- tungstic acid, 12 per cent. is made up to 100 ml with water, 50 ml of filtrate collected, distilled with sodium hydroxide into 15 ml of 0.1 N sulphuric acid and titrated back. Resins-Five grams of tobacco is digested with 100 ml of ethanol overnight, 50 ml of filtrate evaporated to dryness and digested with water. After re-filtration, the insoluble resins are dried at 100" C and weighed.Reducing sugars-Tobacco, 12.1 g, is digested with 150 ml of water overnight. 100 ml of extract is dried for 18 hours on a steam-bath. We wish to thank our colleagues in the Government Laboratory for help in the preparation of this paper, and the Government Chemist for permission to publish. REFERENCES 1. 2. 3. 4. CLEMENTS INN PASSAGE Tate, F. G. H., Analyst, 1934, 59, 168. Iles, W. G., and Sharman, C. F., J . Soc. Chem. Ind., 1949, 68, 174. Maillard, L. C., Compt. rend., 1912, 154, 66. Evdokimov, A. G., Sci. Mem. St. Univ. Leningrad, Chem. Ser., 1938, 3, 38. GOVERNMENT LABORATORY STRAND, LONDON, W.C.2 D I sc u SSION MR. L. G. BECKETT pointed out that there was a considerable amount of oxygen at a pressure of 1 cm of mercury, and this could account for the blackening of the tobacco leaf in spite of the authors' contention to the contrary. He Considered it essential that any remark concerning pressure should be qualified by the words "gauge" or "absolute" to avoid difficulties that may be experienced by other workers in interpreting results. He drew attention to the necessity for care when referring to degrees of vacuum. DR. J. H. HAMENCE asked if the new oven really reached a tsmperature of 100" C. MR. FRYD replied that it did. If the oven was empty it would reach 100" C in a very short time When the oven was fully loaded with about 100 samples, it would take from 2& to 4 hours- DR. HAMENCE said that the meeting had served a valuable purpose because it had brought the air The latest conclusions for the estimation of water suggested indeed. depending naturally upon the water content of the sample material-to reach 100" C. flow methods to the notice of the Society. that the air flow method was superior to other available methods.

ISSN:0003-2654    

DOI:10.1039/AN9517600025

出版商:RSC    

年代:1951

数据来源: RSC

摘要:

Jan., 19511 HASLAM AND CLASPER 33 Some Further Observations on the Analysis of Nylon Type Polymers BY J. HASLAM AND M. CLASPER Methods have been devised for the analysis of mixtures of hexamethylene diamine dihydrochloride and 6-amino caproic acid hydrochloride that may be obtained on the application to certain types of nylon polymer of the methods of hydrolysis previously reported by the authors. The method for the determination of the 5-amino caproic acid hydrochloride is based on titration with standard alkali, phenolphthalein being used as indicator; the method of determination of hexamethylene diamine dihydrochloride depends on the passage of a solution of the mixed hydrochlorides through a column of Amberlite IRA 400 resin and titration of the hexamethylene diamine passing through the column with standard alkali, methyl orange being used as indicator.Samples of polymer produced ( a ) by the interaction of hexamethylene diisocyanate and 1 : 4-butanediol and ( b ) from w-amino undecanoic acid have been submitted to hydrolysis by hydrochloric acid solution and the hydrolytic products examined along the lines laid down for nylon type polymers in the previous paper. As a result of our work on the examination of nylon and related polymers,l it seemed to us that methods for the examination of mixtures of hexamethylene diamine dihydrochloride and 5-amino caproic acid hydrochloride would be extremely useful. THE RESOLUTION OF MIXTURES OF HEXAMETHYLENE DIAMINE DIHYDROCHLORIDE W'e have been able to show that 5-amino caproic acid hydrochloride may be readily determined in mixtures of 5-amino caproic acid hydrochloride and hexamethylene diamine dihydrochloride by titration of the aqueous solution of the hydrochlorides with 0-1 N sodium hydroxide solution, phenolphthalein being used as indicator.Known weights of the two hydrochlorides were dissolved in 20ml of water and the solution was titrated with 0.1 N sodium hydroxide, phenolphthalein being used as indicator. One millilitre of 0.1 N sodium hydroxide is equal to 0.01675 g of 5-amino caproic acid hydrochloride. The results are shown in Table I. AND &AMINO CAPROIC ACID HYDROCHLORIDE TABLE I RECOVERY OF &AMINO CAPROIC ACID HYDROCHCORIDE FROM SYNTHETIC MIXTURES WITH HEXAMETHYLENE DJAMINE DIHYDROCHLORIDE Composition of mixture A I -l Hexamethylene diamine 5-Amino caproic dih ydrochloride, acid hydrochloride, g g nil 0.5014 0.1512 0.3494 0.2512 0.2496 0-3496 0- 1489 0.5021 nil 5-Amino caproic acid hydrochloride recovered, g 0-4989 0.3469 0.2479 0.1503 0.0017 Further, a method has been devised for the determination of hexamethylene diamine dihydrochloride in mixtures of hexamethylene diamine dihydrochloride and 5-amino caproic acid hydrochloride.The method is based on the passage of an aqueous solution of the hydrochlorides through a column of Amberlite IRA 400 ion-exchange resin. We are indebted to Dr. K. W. Pepper of the Chemical Research Laboratory, Teddington, for the suggestion that this resin might prove to be useful for our purpose. The hexamethylene diamine passing through the column is finally titrated with 0.1 N hydrochloric acid solution, methyl orange being used as indicator.34 [Vol.76 ION-EXCHANGE METHOD- The method is as follows: 10 g of Amberlite IRA 400 dried at 100" C are mixed into a slurry with water and transferred to an ion-exchange column, Fig. 1. The excess of water is then run off until the resin in the column is just covered with water. At all times care must be taken to ensure that the liquid in the column is not drained beneath the head of the ion-exchange resin: the passage of air into the resin impairs the absorption capacity. The Amberlite IRA 400 resin is activated by passing about 800 ml of N sodium hydroxide through the column at a rate of about 2 to 3 ml per minute. The apparatus and the resin are then washed with water until 50 ml of effluent give a titration of less than 0.1 ml of 0.1 N hydrochloric acid, methyl orange being used as indicator.Normally, about 250 ml of wash water are sufficient. The ion-exchange column. is now ready for use; it must be used immediately after the water washing has been completed. About 0.5 g of the mixed hydrochlorides is, weighed, dissolved in 50 ml of water and the solution transferred to the separating funnel, A. This solution is allowed to pass through the column at a rate of 2 to 3 ml per minute. The separating funnel and sides of the column are carefully washed down with water, the washings being allowed to pass through the column. When this washing has been completed the separating funnel is filled with water, which is allowed to pass through the column at a steady rate of 2 to 3 ml per minute.The first 150 ml of effluent are collected and titrated with 0.1 N hydrochloric acid, methyl orange being used as indicator, and each succeeding 50 ml of effluent are collected and similarly titrated until the titre is less than 0.1 ml of 0.1 N hydrochloric acid. The combined titres are then calculated tcl a percentage of hexamethylene diamine dihydrochloride in the mixture by means of the following factor- HASLAM AKD CLASPER: SOME FURTHER OBSERVATIONS ON THE 1 ml of 0.1 N hydrochloric acid = 0.009457 g of hexamethylene diamine dihydrochloride Application of the above method to five mixtures of hexamethylene diamine dihydro- chloride and &amino caproic acid hydrochloride gave the results shown in Table 11.The titres of the successive volumes of liquid passing through the columns are also shown. TABLE I1 RECOVERY OF HEXAMETHYLENE DIAMINE DIHYDROCHLORIDE FROM SYNTHETIC MIXTURES WITH &AMINO CAPROIC ACID HYDROCHLORIDE MIXTURES TAKEN- Col. ;1 Col. 2 Col. 3 Col. 4 Col. 5 Hexamethylene diamine dihydrochloride, g 0.5013 0.3510 0.2520 0-1502 nil 5-Amino caproic acid hydrochloride, g. .. nil 0.1480 0.2471 0.3490 0.4997 TITRE, ML OF 0.1 N HYDROCHLORIC ACID- Effluent, ml: 150 .. .. .. .. . . 51.86 35.70 25.52 15.12 0.23 50 . . .. .. .. . . 0.29 0.28 0.18 0.08 0.06 50 . . .. .. .. . . 0.23 0-18 0.10 50 . . .. .. .. . . 0.06 0.08 0.06 Tptal titre, ml . . .. I . . . 52.44 36.24 25.86 15.20 0.29 Hexamethylene diamine added, g 0,5013 0.3510 0.2520 0.1502 nil dihydrochloride { found, g 0.4958 0.3427 0.2445 0.1437 0.0027 Although the results for the recovery of hexamethylene diamine dihydrochloride are slightly low, we have found that the above method when used in conjunction with the direct titration method for the determination of 5-amino caproic acid hydrochloride, p.33, is extremely valuable in the examination of the hydrolysis products of interpolymers of nylon 66, 610 and 6. NOTES ON THE ION-EXCHANGE METHOD- (i) The volume of sodium hydroxide solution used to generate the active base from the Amberlite IRA 400 is extremely large, but in our experience the use of smaller volumes of sodium hydroxide solution or faster rate of passage of the sodium hydroxide solution through the column is accompanied by a reduction in efficiency.(ii) Other proprietary resins have been tried in place of Amberlite IRA 400, but without success. When De-Acidite F was used, the removal of excess alkali by washing with water after the initial activation was much more difficult than when Amberlite IRA 400 was used. When an aqueous solution of hexamethylene diamine dihydrochloride was passed through anJan., 19511 ANALYSIS OF NYLON TYPE POLYMERS 35 activated Amberlite IRA 400 column, the recovery of hexamethylene diamine in the effluent was almost theoretical. When the corresponding experiment was carried out with De-Acidite F, although the recovery of hexamethylene diamine in the effluent was reasonably in agree- ment with theory, it was extremely difficult to reach finality in the process. When experiments were carried out by passage of aqueous solutions of 5-amino caproic acid hydrochloride through activated columns of Amberlite IRA400, the proportion of base in the effluent titratable to methyl orange indicator was negligible.With the De-Acidite F, however, successive volumes of effluent gave distinct positive titrations to methyl orange indicator. These titrations were of the order of 1.5 ml of 0.1 N hydrochloric acid per 50 ml of effluent. EXAMINATION OF POLYMER PRODUCED BY THE INTERACTION OF HEXAMETHYLENE The polymer produced by the interaction of hexamethylene diisocyanate and 1 : 4- butanediol appears to be known to the trade as Igamide “U” or Perlon.2 The sample melted at 179” C and had a nitrogen content (macro-Kjeldahl) of 10.58 per cent.The infra-red spectrum was as shown in Fig. 2a. DIiSOCYANATE AND 1 : 4-BUTANEDIOL Fig. 2. Infra-red spectra of polymers (a) produced by the interaction of hexamethylene diisocyanate and 1 : 4-butanediol, and ( b ) produced from w-amino undecanoic acid Considerable attention has been paid to the identification of the products of hydrolysis of this type of polymer, and the following experiments were carried out. Experiment 1-The sample, 5 g, was hydrolysed with 200 ml of 50 per cent. v/v hydro- chloric acid for 60 hours. The hydrolysis product had the characteristic odour of a chlorinated hydrocarbon. A Dean and Stark apparatus was then fitted to the flask and the solution was boiled for 2 hours. At the end of this time a small amount of immiscible liquid was collected at the bottom of the graduated side arm.This part of the apparatus was dis- connected when cool, and the upper liquid in the graduated arm was removed by a suction pipette. The recovered liquid remaining in the Dean and Stark apparatus was washed36 HASLAM AND CLASPER: SOME FURTHER OBSERVATIONS ON THE [Vol. 76 successively with about 2 ml of water until the wash1 water was neutral to litmus; five washings were required. The washed liquid was transferred to a 2-ml centrifuge tube, a small amount of sodium sulphate (anhydrous) was added and the whole allowed to stand overnight. The dried liquid had a boiling-point of 154.5" C, a refractive index of 1.4556 and a chlorine content of 55.25 per cent. These figures were consistent with the material being 1 :4-dichlorobutane, characteristic figures for which are : boiling-point, 155.0" C, refractive index at 20" C, 1.4566, and chlorine eontent, 55-84 per cent.Experiment 2-In a second experiment 15 g of the sample were hydrolysed with 600 ml of 50 per cent. v/v hydrochloric acid in the apparatus shown in Fig. 3. As the hydrolysis proceeded, a layer of immiscible liquid began to accumulate in the side arm. The liquid in this side arm was run off from time to time arid separated in a separating funnel. The E 0 u1 T Sintered Glass i / i 1 n Fig. 1. Ion-exchange column Fig. 3. First apparatus for hydrolysis upper layer was reserved and the lower layer returned to the hydrolysis flask. After about 24 hours, approximately 26ml of upper layer had been obtained, and this liquid was dried overnight with anhydrous potassium carbonate.The dry liquid was then distilled in a small side-arm distillation flask. Three fractions were collected of the following approximate boiling ranges and amounts- Boiling range, Volume, " c ml Fraction 1 . . .. .. 63 to 65 0.6 Fraction 2 . . .. .. 65 to 70 1.3 Fraction 3 . . .. . . Above 70 (boiled 0.6 mainly at 160" C)Jan., 19511 ANALYSIS OF NYLON TYPE POLYMERS 37 A micro-analytical examination of these fractions gave the following carbon, hydrogen and chlorine figures- Carbon, Hydrogen, Chlorine, % % Fraction 1 . . .. .. 64.0 9-3 1.4 Fraction 2 . . .. .. 65.4 9-8 0.3 Fraction 3 . . .. .. 49.8 7-2 24.4 0' /O The figures for carbon, hydrogen and chlorine contents of the fractions pointed strongly to the presence of a large proportion of tetrahydrofuran in fraction 2 and a rather smaller proportion in fraction 1.Tetrahydrofuran was known to oxidise with nitric acid to give succinic acid, and fractions 1 and 2 were therefore oxidised with nitric acid, authentic tetra- hydrofuran being oxidised similarly as a control, in the following manner. Half a millilitre of water and 0.2 ml of the fraction were taken in a test tube and cooled in ice-cold water. The cold liquor was added to 2 ml of ice-cold concentrated nitric acid and the mixture left standing overnight in a beaker of ice and allowed to come slowly to room temperature. The nitric acid solution was evaporated to dryness on the water-bath in a tared evaporating basin and the residue moistened once or twice with water previous to re-evaporation. After drying at 100" C for 1 hour, a white crystalline residue was obtained which had the following properties- Fraction 1 Fraction 2 Tetrahydrofuran Melting-point, O C .. . . 182 to 187 185 to 188 185 to 187 Equivalent weight . . .. 59.2 59.5 59.6 The figures for succinic acid are- Melting-point, 185" C Equivalent weight, 59.05 As a result of the above experiment we concluded that the hydrolysis of this type of polymer with 50 per cent. v/v hydrochloric acid under the above conditions produced an appreciable proportion of tetrahydrofuran. At the same time we concluded from Experiment 1 that when the action of hydrochloric acid was prolonged, 1 :4-dichlorobutane could also be isolated from the hydrolysis products. Experiment 3-A further 1 g of the sample was hydrolysed for 60 hours with 50 per cent.v/v hydrochloric acid, and the aqueous solution was filtered and the filtrate was taken to dryness on a water-bath. The residue was then purified by dissolving in alcohol, boiling with animal charcoal, filtering and re-precipitating by adding acetone to the cold alcohol solution. This procedure was repeated three times and the precipitate finally obtained was dried under vacuum. The material so obtained melted at 252" C, and micro-analytical examination gave the following ultimate analyses: carbon, 38.15 per cent. ; hydrogen, 9.5 per cent. ; nitrogen, 14-6 per cent.; chlorine, 37.60 per cent. Corresponding figures for hexamethylene diamine dihydrochloride are as follows : melting- point, 253" C; carbon, 38-10 per cent.; hydrogen, 9.6 per cent.; nitrogen, 14.81 per cent.; chlorine, 37.49 per cent.We now sought to use the information we had obtained as a result of the three experiments to develop a test which could be applied in a simple way to samples suspected to be of this type of polymer. Such a test is described below as applied to 1 g of a sample. One gram of the polymer was hydrolysed by boiling with 20ml of 50 per cent. v/v hydrochloric acid in the apparatus shown in Fig. 4. There was little visible change in the appearance of the dispersion during the first 24 hours of boiling. It was necessary to shake the flask and contents from time to time so that the whole of the sample was removed from the sides of the flask and subjected to hydrolysis.After 24 hours the particles in suspension appeared to become much finer and this process continued. After about 48 hours the mixture appeared to clear to a certain extent and the hydrolysis was then continued for a further period of 12 hours, that is, a total of 60 hours. At the end of this period the solution was a pale brownish colour and contained a small amount of material in suspension. After hydrolysis for about one hour the liquid in the condenser and siphon began to assume an oily appearance, and after three hours a distinct layer of oily liquid was visible at point A, Fig. 4, above the aqueous layer in the siphon. At this stage, i.e., on the appearance of an oily upper layer in the siphon, the liquid in this tube, including the aqueous layer,38 HASLAM AND CLASPER: SOME FURTHER OBSERVATIONS ON THE [Vol.76 was withdrawn by means of a capillary pipette. After hydrolysis for a further 3 hours a second oily upper layer was formed in the siphon and this also was withdrawn and added to the original distillate. The combined distillates were cooled in ice-cold water and added to 2 ml of ice-cold concentrated nitric acid contained in a test tube. The test tube was then n 85 mrn. I - 7 rnm. I 160 mm. -A - - - Fig. 4. Second apparatus for hydrolysis Fig. 5. Modified apparatus for hydrolysis All dimensions in millimetres placed in a beaker containing iced water and the whole left overnight. As the ice melted there was a gradual increase of temperature until. room temperature was reached and also a gradual increase in colour of the solution from colourless to yellowish-green. On the following morning the nitric acid solution was evaporated to dryness on the water-bath and the test was completed as described under Experiment 2, p.37. The white crystalline residue so obtained weighed 0.0763 g, had a melting-point of 185" to 187" C and an equivalent weight of 59.4: as determined by semi-micro titration in alcoholic solution with 0.02 N sodium hydroxide and phenolphthalein indicator. The pale brownish coloured hydrochloric acid solution from the hydrolysis was filtered through a tared sintered glass crucible No. 1, G:3, and the residue well washed with water prior to drying at 100" C. The hydrochloric acid filtrate was evaporated to dryness in a tared evaporating basin and dried t o constant weight at 100" C.This residue weighed 0.7258 g and had a melting-point of 248" to 254" C and a chlorine content of 36.6 per cent. We have shown that the principle of the above test can he applied to as little as 0.5g of sample by use of the modified apparatus shown in Fig. 5. The dry residue weighed 0.0170 g.Jan., 19511 ANALYSIS OF NYLON TYPE POLYMERS 39 EXAMINATION OF POLYMER PRODUCED FROM w-AMINO UNDECANOIC ACID Within recent months we have been called upon to examine several samples of a nylon- type material that has been proved to consist of the so-called polymer "R".3 This polymer has also been described as type 11 nylon, i.e., the polymer made from w-amino undecanoic acid by the removal of the elements of water. The infra-red spectrum of the polymer is shown in Fig.2b. We have found that this polymer is attacked by 50 per cent. v/v hydrochloric acid, and when 0.5 g of an authentic sample of the polymer was boiled under reflux for 40 hours with 20ml of 50 per cent. v/v hydrochloric acid and the solution subsequently cooled, a crystalline deposit was formed. The hydrolysis product possessed a characteristic odour. The hydrolysis product was transferred with the aid of about 20 ml of water to the apparatus for continuous ether extraction1 and the solution extracted with ether for approximately 6 hours. The ether extract was evaporated to dryness and the residue dried at 100" C and weighed. The solution of the hydrochlorides was evaporated to dryness, dried to constant weight at 100" C and weighed. The melting-point and chlorine content of the residual hydrochlorides were determined, and the equivalent weight was calculated from the results of the titration of the hydrochlorides with standard alkali, phenolphthalein being used as indicator.determined, as well as the melting-point. In addition, carbon, hydrogen and nitrogen in From this examination the results shown in Table I11 TABLE I11 EXAMINATION OF 0 . 5 ~ SAMPLE OF POLYMER w-AMINO UNDECANOIC ACID Polymev- Nitrogen, 7.01 % : carbon, 70.957; ; hydrogen, 11.76% Melting-point, 185" to 186" C Hydrolysis products of polymer- Ether extract of hydrolysed product . . .. . . products . . .. ,. .. . . . . . . Residue after evaporation of ether extracted hydrolysis , Base hydvochlovide- the original polymer were also were obtained.PRODUCED FROM 1 2 4.1 yo 5.7 yo 125.20/:, 122.5% Melting-point of base hydrochloride . . . . . . 145" to 147" C 146" C Chlorine in base hydrochloride . . . . . . . . 14.5276 14.65 Yo Equivalent weight of base hydrochloride (calculated from phenolphthalein titre) . . .. .. . . 245.8 242.0 OBSERVATIONS ON THE DATA IN TABLE III- (i) The nitrogen, carbon and hydrogen figures are not in strict agreement with those we should expect to obtain for the pure polymer obtained from w-amino undecanoic acid by removal of one molecule of water. The theoretical figures for this pure polymer would be: nitrogen, 7.64 per cent. ; carbon, 72.06 per cent. ; hydrogen, 11.54 per cent. (ii) The melting-point of the sample examined was 185" to 186" C. We have encountered samples of this polymer of rather lower melting-point, i.e., about 180" C.(iii) The proportion of ether extract and hence of residual base hydrochloride do not agree. Infra-red examination has indicated that this ether extract consists essentially of the base hydrochloride, i.e., the hydrochloride of m-amino undecanoic acid. This hydro- chloride is slightly soluble in ether and the varying results are due to the fact that the through- put of ether is not the same in the duplicate extractions. (iv) There is a certain amount of evidence to support the view that the hydrolysis product of this polymer does not consist completely of the hydrochloride of w-amino undecanoic acid. In our experience there is present in the hydrolysis a rather flocculent material that is to a certain extent insoluble in water and which is almost certainly present as an impurity in the base hydrochloride. This would account for the fact that the figures obtained are not in strict agreement with theory. The theoretical figures for the chlorine content of w-amino undecanoic acid hydrochloride and its equivalent weight are : chlorine content, 14-91 per cent. ; equivalent weight, 237.8.40 RILEY: THE DETERMINATION OF ACETYL VALUES FOR USE I N [Vol. 76 We are indebted to Mr. H. Willis for the infra-red spectra of the two polymers and to Mr. N. Payne for his assistance in the design of the apparatus used in the examination of polymers produced by the interaction of hexamethylene diisocyanate and 1 : 4-butanediol. REFERENCES 1. Clasper, M., and Haslam, J., Analyst, 1949, 74, 224. 2. First Review of German Science, 1939-46, “Preparative Organic Chemistry, Part 111,” Office of the Military Goverment for Germany, Field Information Agencies, Technical ; British, French, 1J.S.; p. 310. Salk and Rayon, December, 1948. 3. IMPERIAL CHEMICAL INDUSTRIES LIMITED PLASTICS DIVISION BLACK FAN ROAD WELWYN GARDEN CITY, HERTS. July. 1950

ISSN:0003-2654    

DOI:10.1039/AN9517600033

出版商:RSC    

年代:1951

数据来源: RSC

摘要:

40 RILEY: THE DETERMINATION OF ACETYL VALUES FOR USE IN [Vol. 76 The Determination of Acetyl Values for Use in Component Fatty Acid Analyses of Castor Oils BY J. P. RILEY A comparison has been made between the British Standards Institution (second method) and the Association of Official Agricultural Chemists' methods for the determination of acetyl values, and it has been concluded that both procedures give very similar results. Where results of high accuracy are required several determinations should be made and the mean calculated. A procedure has been developed for the complete analyses of castor oils and tested by application to mixtures of fatty acids of known composition. NUMEROUS methods have been proposed for the determination of the acetyl value of fats, most of which are based either on direct acetylation of the oil in the cold with a mixture of pyridine and acetic anhydride and subsequent determination of the amount of acetic anhydride consumed,l y 2 or on acetylation with boiling acetic anhydride and determination of the acetic acid obtained by hydrolysis of the resultant acetyl comp0und.39~ The former method suffers from the defect that the end-point of-the titration is rather vague and many modifications have been suggested for increasing the accura~y,~9~3~ but for the ultimate purpose of component acid determination direct acetylation with acetic anhydride alone has been found preferable.In the present study a comparison has been made between the British Standards Institution (second m e t h ~ d ) ~ and the Association of Official Agricultural Chemists4 methods for the determination of acetyl values, both of which depend on the determination of the saponification values of the oil and the acetylated oil.It has been found that both methods give very similar results, but it is suggested that where results of high accuracy are required the saponification values both of the oil and its acetyl compound should be the mean of at least five determinations. The accuracy of the B.S.I. method has been tested by using pure specimens of methyl ricinoleate and methyl 12-hydroxystearate ; and it is concluded that with care results having an accuracy of &I per cent. can be obtained. The acetyl value method, when applied to castor oils, does not give a true estimate of the amount of ricinoleo-glycerides present, since small amounts (about 0.6 per cent.) of 9 : 10-dihydroxystearo-glycerides that are present6 are also estimated.The dihydroxystearic acid may be determined with sufficient accuracy by crystallisation of the mixed fatty acids of the oil (after removal of unsaponifiable matter by the S.P.A. methods) from ethyl acetate at 0" C. The precipitated dihydroxystearic acid, which has only a low solubility under these conditions, is weighed after filtration and washing, and to this weight is added a correction for its slight solubility in the ethyl acetate used as solvent and wash liquor. Knowledge of the acetyl value and percentage of dihydroxystearic acid enables the percentage of ricinoleic acid in the oil to be calculated. Besides ricinoleic acid, the other principal constituent of castor oils is linoleic acid, which occurs to the extent of about 4 to 6 per cent, The linoleicJan., 19511 COMPONENT FATTY ACID ANALYSES OF CASTOR OILS 41 acid occurring in castor oil has been identified as the normal 9 : 12-linoleic acid of seed fats (J.P. Riley, unpublished observation). It may be determined by spectrophotometric examination of the mixed acids (after removal of unsaponifiable matter), after alkali isomerisa- tion at 180" C for 60 minutes under the conditions of Hilditch et al.1° I t is not necessary to make a correction for the absorption due to ricinoleic acid since it has been shown that ricinoleic acid treated under these conditions shows negligible absorption. Oleic and saturated acids, which occur in small amounts in the oil, are determined by iodine value and by difference respectively .As a test of the proposed method for the analysis of castor oils, mixtures have been made up from pure methyl ricinoleate, oleate and linoleate and analysed by the recommended procedure with results which agree with the calculated values. EXPERIMENTAL PREPARATION OF PURE ACIDS AND ESTERS- Methyl ricinoleate-Brown and Green11 first prepared methyl ricinoleate by low tempera- ture crystallisation, but attempts to repeat their work were unsuccessful. After a study of a number of solvents it was concluded that acetone was the most suitable, but that the solubility of methyl ricinoleate in it varied greatly with temperature. Crystallisations were therefore carried out by slowly cooling the solution to a temperature at which the desired amount of solid had separated and then maintaining it a t this temperature for 3 hours.A commercial cold drawn castor oil (600 g, I.V. 82-9) was hydrolysed and the un- saponifiable matter extracted. The recovered acids were methylated at room temperature. The neutral esters (566 g) were crystallised from acetone first in 11 per cent. solution a t -50" C and then- in 9 per cent. solution at -40" C. The soluble product (341 g) from the latter crystallisation was then recrystallised three times in 5 per cent. solution in acetone a t -65" C, -50" C and finally at -70" C, by which means there resulted 190.7 g of methyl ricinoleate S.E. 311.2 (S.E. (theory) 312.0), I.V. 80.0 (I.V. (theory) 81.2), n? 1.4657, nz 1.4603.Ricinoleic acid-Methyl ricinoleate (1.88 g) was hydrolysed by boiling for 10 minutes with 10 ml of 10 per cent. alcoholic potash. The soaps were acidified with dilute sulphuric acid, and the liberated acids extracted with ether. Most of the ether was removed by distilla- tion, and the last trace in vacuo at room temperature. Yield of ricinoleic acid 1.73 g, I.V. 83.8 (I.V. (theory) 85.2), EtZi at 234 mp after alkali isomerisation at 180" for 60 minutes. 1.1. MethyZ 12-hydroxystearate-Commercial cold drawn castor oil was alkali refined by washing with 10 per cent. potassium hydroxide solution and then hydrogenated with Raney nickel catalyst at 100" C. The product (I.V. 2.4) was saponified and the resultant acids (90 g) were ground up and extracted in a Soxhlet extractor with light petroleum (b.p.40" to 60" C) for 25 minutes. The residue (86 g) was recrystallised three times in 1 per cent. solution in petroleum (b.p. 60" to 80" C) yielding 69 g of 12-hydroxystearic acid, m.p. 82" C. The acid (20 g) was esterified at room temperature with methyl alcohol containing 0-5 per cent. of hydrogen chloride, yielding 19.7 g of methyl 12-hydroxystearate which after crystallisation from acetone a t -20" C had m.p. 57.5" to 58" C (Grummitt and Siedschlag12 record 56" to 9 : 10-Dihydroxystearic acid-The mixed acids (296 g) from 304 g of castor oil were dissolved in 1500 ml of ethyl acetate and cooled to 0" C. After standing overnight a t 0" C, the precipitated dihydroxystearic acid (1.36 g) was filtered off and twice recrystallised from 50 ml of ethyl acetate at 0" C, yielding 1.01 g of 9 : 10-dihydroxystearic acid, m.p.141" to 142" C. \ Determination of the solubility of dihydroxystearic acid-Dihydroxystearic acid (0.5 g) was stirred a t 0" C for 3 hours with 550 ml of redistilled ethyl acetate, and the solution was filtered through a cooled sintered glass filter. 500ml of the filtered solution was evaporated to dryness in a platinum basin, leaving 0.0925 g of dihydroxystearic acid; whence the solubility of dihydroxystearic acid is 0.0185 g per 100 ml of ethyl acetate at 0" C. Il.lethyZ oleate-Methyl oleate prepared as described by Hilditch and Pathak13 had the following characteristics-I.V. 84.8, E:?i at 234 mp after alkali isomerisation at 180" C for 60 minutes 4-3, whence its component acid composition was-saturated 1.6 per cent., oleic 97-9 per cent.and linoleic 0.5 per cent. by weight. Methyl Zinoleate-A concentrate of linoleic acid obtained by low temperature crystallisa- tion of the mixed fatty acids of a sunflower seed oil was methylated and fractionated. The 57" C).42 RILEY: THE DETERMINATION OF ACETYL VALUES FOR USE I N [Vol. 76 fraction having I.V. 165.3, Ei:& at 234 mp after allkali isomerisation at 180" C for 60 minutes 796, was used in the investigation. The component acids of the sample, calculated from the above data were: oleic 12.1 per cent., linoleic 8'7-9 per cent. by weight. DETERMINATION OF ACETYL VALUE BY B.S.I. AND A.O.A.C. METHODS- SaponiJication equivaled-Weigh out 3 g of tlne oil (or 1-5 g of the acetylated oil) into a 300-ml flat-bottomed flask, and add 25 ml of neutral alcohol, followed by 50 ml of approxi- mately 0.5 N alcoholic potash.Heat the mixture under reflux for 2 hours and then titrate with 0-5 N sulphuric acid using phenolphthalein as indicator. Run a blank determination under the same conditions but omitting the sample. Carry out saponification value determinations on both the oil and its acetyl compound in quintuplicate and calculate the acetyl value according to the formula: Acetyl value-Acetylate 15 g of the oil as described in B.S.S. 684 (1950). S' - s 1 - 0*001^)75s Ac.V. = where S is the mean sap. value of the oil S' is the mean sap. value of the acetylated oil. All the saponification tests in the text and tables of this paper are recorded as saponifica- tion equivalents (S.E.), and not as in the B.S.I. and A.O.A.C.specifications as saponification values. The formula for converting saponification equivalents to saponification values is- 56,100 S.E. COMPARISON OF THE B.S.I. AND A.O.A.C. MEiTHODS FOR THE DETERMINATION OF ACETYL VALUES A neutralised commercial castor oil and the rnethyl ,esters prepared from another castor oil were examined by both the B.S.I. and A.O.A.C. methods with the results shown in Table I. COMPARISON OF B.S.I. AND A.O.A.C. ACETYL VALUE METHODS Oil S.E. 309.3 310.8 309.7 310.2 310.0 310.6 Acetylated oil I--- B.S.I. A.O.A.C. S.E. S.E. 180.8 179.3 180.4 179.7 181.3 179.6 181.6 180.5 181.1 180.2 Methyl esters S.E. 314-3 312.9 312.0 313.4 312-7 3 12.5 Acetylated methyl esters -7 B.S.I.A.O.A.C. S.E. S.E. 189.1 189.3 188.0 189.1 188.7 188.7 187.5 189.4 187.8 188-3 188.7 189.3 Mean . . . . .. . . 310.2 Mean deviation . . .. . . 0.14y0 Standard deviation . . . . 0.566 Acetyl value . . .. .. - 181.0 179.9 0.17% 0.22% 0.469 0.707 149.3 152.4 313.0 0.20y0 0.787 - 188.3 189.0 0.27% 0.19% 0.624 0.436 137.1 135.8 From the above results it will be seen that both methods give very similar results and that the determined acetyl values do not differ from the mean by more than 1 per cent. For consistency in the later parts of the work, the B.S.I. method has been employed. EXAMINATION OF METHYL RICINOLEATE AN11 METHYL 12-HYDROXYSTEARATE BY THE B.S.I. METHOD The accuracy of the adopted method has been further tested by the examination of pure samples of methyl ricinoleate and methyl 12-hydroxystearate with the results shown in Table 11.The figures shown indicate that there is good agreement between the determined and calculated saponification equivalents and acetyl values. The determined acetyl values areJan., 19511 COMPONENT FATTY ACID ANALYSES OF CASTOR OILS 43 about 0.6 per cent. low, but it is impossible to say whether this is due to an error in the method, or to the presence of small amounts of impurities in the esters used as standards. COMPLETE ANALYSIS OF CASTOR OILS Dissolve about 60 to 80 g of the castor oil in 300 ml of ether and neutralise it by washing with 10 per cent. aqueous potassium hydroxide and then with water till free from alkali. Divide the neutral oil into two portions.(i) Use 30g for the determination of acetyl value. (ii) Saponify 30 to 40g of the oil by boiling for 1 hour with excess of 10 per cent. alcoholic potash, extract the unsaponifiable matter with ether and determine it according to the S.P.A. methodg (with volumes increased proportionately). Liberate the fatty acids from the soaps with dilute sulphuric acid and extract them with ether, remove the ether by distillation. Use the first portion (2 g) for Divide the resultant acids into two portions. TABLE I1 EXAMINATION OF PURE ESTERS BY B.S.I. PROCEDURE Methyl ricinoleate Methyl 1 2-hydroxystearate Acetylated Acetylated 7- Ester ester Ester ester Number of determinations . . .. 5 5 2 2 S.E. mean .. .. .. . . 311.2 177.3 313.8 178-4 S.E. theory . . .. .. . . 312.0 177.0 314.0 178-0 Mean deviation .. .. .. 0.12% 0.13% 0.06 yo 0.06% Standard deviation . . .. . . 0.510 0.374 - - Acetyl value (found). . .. .. 157.4 156-7 (theory) . . .. .. 158.5 157.6 the determination of linoleic acid by alkali isomerisation at 180" C for 60 minutes followed by spectrophotometric examination at 234 mp by the method of Hilditch et aZ.1° Dissolve the remaining portion of the acids (about 25 to 35 g) in 10 ml of ethyl acetate for each gram of acids and cool to 0" C overnight. Filter the precipitated dihydroxystearic acid through a cooled, weighed sintered glass filter (porosity 3), wash it with a known volume (about 30 ml) of cold ethyl acetate in small portions. Dry the washed precipitate in a vacuum desiccator under high vacuum and weigh. The product consists of substantially pure dihydroxystearic acid and should have a melting-point of not less than 139" C (pure 9 : 10- dihydroxystearic acid has m.p.141" to 142" C). In calculating the percentage of dihydroxy- stearic acid in the mixed fatty acids, a correction must be made for its solubility (0.0185 g per 100 ml at 0" C) in the known volume of ethyl acetate used as solvent and wash liquor. CaZcztZations-Percentage of triacetyltriricinolein in the acetylated oil- = P. Ac.V. x 100 159.1 The percentage of ricinoleic acid in the mixed fatty acids of the oil is different from the above because of the change in molecular weight on acetylation. It can be calculated with sufficient accuracy by means of the following approximation Let 298p + 0.96 (100 - P) = y 352-7 Then, if the proportion of ricinoleic acid in the mixed fatty acids is R per cent.298P x 100 352.7~ This figure for ricinoleic acid is too high, since it includes the contribution of the two hydroxyl groups in the dihydroxystearic acid also present. Let the percentage of the latter = D. Then the actual percentage of ricinoleic acid = R - 2D. Oleic acid is determined by its iodine value after making allowance for the ricinoleic and linoleic acids. R = Saturated acids are determined by difference.44 RILEY [vol. 76 The proposed method for the analysis of castor oils has been tested by the examination of two mixtures of methyl ricinoleate, oleate and linoleate in known amounts, with the results shown in Table 111. TABLE I11 EXAMINATION OF MIXTURES OF KNOWN COMPOSITION Esters .. .. .. .. .. S.E. .. .. .. .. .. S.E. mean deviation .. .. S.E. acetyl compound . . .. S.E. mean deviation .. .. Acetyl value . . .. .. .. Iodine value . . .. .. .. Mixed acids: Iodine value . . .. .. Et zi a t 234 mp (180°/60 min.) . . Mixture 1 Mixture 2 found 308.7 183.1 144.4 86.0 0.16% 0.23 %, 89.5 38.4 Whence the component esters of the mixtures are- found % w . 1 Methyl ricinoleate . . .. .. 90.1 Methyl oleate .. .. .. 5.7 Methyl linoleate . . .. .. 4.2 calc . 310.2 183.5 144.3 85.5 - - 89.5 39.9 calc. 90.0 5.6 4.4 % (wt.)* found 306.6 189.4 131.3 90.0 0.08 % 0.07 % 93.9 82.9 found 80.0 11.2 8.8 % ( W e ) calc. 308-7 190.9 129.6 90.1 - - 94.3 83.3 calc. % d(g)* 9.9 9.2 * Calculated on the assumption that the composition of methyl oleate and linoleate concentrates are: saturated ester 1-6%, methyl oleate 97.9% and meth,yl linoleate 0.5%; and methyl oleate 12.1% and methyl linoleate 87-9y0 (wt.) respectively.An example of the analysis of a commercial cold drawn castor oil of B.P. quality by the proposed procedure is given below- Characteristics of alkali re$ned oil-S.E. 309.7; I.V. 82.9; Ac.17. 150.2; n:6 1.4778; n;O 1-4741; [cc]zO + 4-42’. The refined oil (50-57 g) yielded 0.244 g of unsaponifiable matter and 46.32 g of mixed fatty acids, which gave 0-1681 g of dihydroxystearic acid, m.p. 139” C (corrected weight 0.261 g). Mixed acids after removal of unsafionijable matter-1.V. 87-0; Ei& at 234 mp (after alkali isomerisation at 180” for 60 minutes) 48.5. The component fatty acids of the oil calculated from these results are-saturated 0.9; oleic nil ; linoleic 5-4 ; ricinoleic 92-6 ; dihydroxystearic 0.6 ; unsaponifiable 0.5 per cent.by weight. Further application of the method to castor oils from various sources will be reported See page 41. Acetylated oil, S.E. 180.5. elsewhere. The author thanks Professor T. P. Hilditch, F.R.S., for his criticism and guidance during this work. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. REFERENCES British Standard Specification 684 (1950) (first method, p. 42). Verley, A,, and Bolsing, F., Ber., 1901, 34, 3354. Britis, Standard Specification 684 (1950) (second method, p. 44). “Methods of Analysis of the Association of Official Agricultural Chemists,” 6th Edition, 1945, West, E. S., Hoagland, C. L., and Curtis, G. H., J . Biol. Chem., 1934, 104, 627. Wilson, H. N., and Hughes, W. C., J . SOC. Chem. Ind., 1939, 58, 74. Ogg, C . L., Porter, W. L., and Willets, G. O., I n d . Eng. Chetn., Anal. Ed., 1945, 17, 394. Eibner, A., and Munzing, E., Chem. Umschau, 1925, 32, 159. “Report of Sub-committee on the Determination of Unsaponifiable Matter in Oils and Fats,” Hilditch, T. P., Morton, R. A., and Riley, J. F’., Ibid., 1945, 70, 67. Brown, J. B., and Green, N. D., J . Amer. Chem. SOC., 1940, 62, 738. Grummitt, O., and Siedschlag, K. G., J . Amer.. Oil Chem. SOL, 1949, 26, 690. Hilditch, T. P., and Pathak, S. P., Proc. Roy. :TOG., A , 1949, 198, 323. Washington, D.C., p. 501. Analyst, 1933, 58, 203. THE UNIVERSITY LIVERPOOL May, 1950

ISSN:0003-2654    

DOI:10.1039/AN9517600040

出版商:RSC    

年代:1951

数据来源: RSC

摘要:

Jan., 19513 BELCHER AND STEPHEN 45 The Determination of Small Amounts of Hydroquinone BY R. BELCHER AND W. I. STEPHEN A method is described for the determination of small amounts of hydroquinone in the presence of methacrylic acid. The solution con- taining hydroquinone is allowed to react with an excess of ferric ammonium sulphate and the ferrous iron produced is treated with o-phenanthroline and determined absorptiometrically. The interference of the ferric iron un- consumed in the reaction is avoided by conversion to the ferrifluoride ion. HYDROQUINONE is used as an inhibitor in the polymerisation of methacrylic acid, and it is often necessary to determine the amounts present at various stages of the reaction. None of the methods available for the determination of hydroquinone was sufficiently sensitive to determine accurately the small amounts present under our conditions (1 to 200 pg), and it became necessary to find an entirely new method.Several reactions are known whereby various substances are determined colorimetrically after treatment with hydroquinone ; hence it seemed possible that by suitable modification, the same reactions might be exploited for the determination of hydroquinone itself. Hydroquinone has been used to determine the following substances colorimetrically : silver,l copper2 and nitritea3 It has also been used to reduce ferric iron to the ferrous state before determining iron colorimetrically by its reaction with o-phenanthr~line.~ All these reactions were examined in turn, but only the last proved to be suitable for the determination of hydroquinone.Excess of a ferric salt is added to the solution containing hydroquinone. The ferric salt is reduced,with the production of an amount of ferrous iron equivalent to the amount of hydroquinone originally present. o-Phenanthroline is added, and the amount of ferrous iron is determined colorimetrically after suppressing the colour due to the excess ferric iron by addition of a suitable complexing agent. A visual comparison method based on this principle was first developed, and a brief note on its use has appeared elsewhere.6 More recently another method has been described for the determina- tion of similar amounts of hydroquinone based on the formation of molybdenum blue by reduction of a solution containing phosphotungstate.s The present method uses a Spekker absorptiometer for the measurement of the colour intensity and is, accordingly, much more accurate than the visual comparison method.THE DETERMINATION OF FERROUS IRON- As the first step in the development of the method, solutions containing ferrous iron were used to find the optimum conditions for maximum colour development. Graphs were prepared, by using 0.5-cmJ 1-cm and 2-cm cuvettes, relating the amount of ferrous iron present to the drum reading. An Ilford spectrum blue filter No. 602 was used, with the instrument set at 1-0 against water. It was found that a period of 30 minutes was required a t a pH between 3 and 4 for the maximum colour development. The correct pH range was obtained by adding suitable amounts of sodium acetate solution.A straight line relationship between the amount of ferrous iron and the drum reading was found under these conditions (Figs. 1, 2 and 3). THE EFFECT OF FERRIC IRON- Since an excess of ferric iron would necessarily be present in an actual determination, it would be expected to interfere unless its effect could be eliminated in some way. Suitable amounts were added to solutions containing known amounts of ferrous iron, and the latter was then determined. Although the interference was not marked, it was sufficient to affect the accuracy of the determination. However, it was found that the interference could be eliminated by complexing the ferric iron by the addition of ammonium fluoride. THE DETERMINATION OF HYDROQUINONE- Known amounts of hydroquinone were allowed to react with ferric iron for various times, the solution then being treated with o-phenanthroline and the intensity of colour The basis of the reaction is as follows.46 BELCHER AND STEPHEN : THE DETERMINATION OF [vol.76 50 30 HY DUOQUINONE.Pg Fig. 1. Graph obtained with 2-cm cuvette HYDROQUINONE,pg Fig. 2. Graph obtained with 1-cm cuvetteJan., 19511 SMALL AMOUNTS OF HYDROQUISONE 47 measured after complexing the excess ferric iron with ammonium fluoride. A reaction time of 30 minutes was found necessary for complete oxidation of the hydroquinone by the ferric iron. Under these conditions the reaction was found to proceed quantitatively, and hence it was possible to prepare the standard graphs by using a solution of ferrous ammonium HY DROQUlNONE~g Fig.3. Graph obtained with 0-5-cm cuvette sulphate instead of a solution of hydroquinone. The equivalent amount of hydroquinone is calculated on the basis that 2 molecules of ferrous ammonium sulphate are equivalent to 1 molecule of hydroquinone. Some results are shown in Table I and indicate that the method is sufficiently accurate for a determination of this type. TABLE I THE DETERMINATION OF HYDROQUINONE IN THE ABSENCE OF METHACRYLIC ACID Hydroquinone present, CLg 275 2 20 165 110 55 44 33 22 11 6 3 2 1 Hydroquinone found, Pg 272, 273 222, 221 164 112, 110 56 45 33 23 11 5 3 2 1 THE DETERMINATION OF HYDROQUINONE IN THE PRESENCE OF METHACRYLIC ACID- The amount of methacrylic acid likely to be present in the solutions to be analysed was of the order of 2 to 3 ml of a 1 per cent. solution.Three-millilitre quantities of a 1 per cent. solution were added to various known amounts of hydroquinone and the latter was then determined. In every case a control determination was carried out in which the same48 BELCHER AND STEPHEN : ‘THE DETERMINATION OF [Vol. 76 amount of hydroquinone was determined withlout the addition of methacrylic acid. From the results included in Table I1 it can be seen that the methacrylic acid has no effect on the determination. TABLE I1 THE DETERMINATION OF HYDROQUINONE IN THE PRESENCE OF 3 M L OF 1 PER CENT. METHACRYLIC ACID SOLUTION Hydroquinone found Hydroquinone present, CLg 220 165 110 55 33 17 6 3 1 r Methacrylic acid absent, Pg 220 164 110 545 33 16 (i 3 1 3 Methacrylic acid present, Pg 221 167 110 55 32 17 7 4 2 Some further tests were also carried out .with much stronger solutions of methacrylic acid, 3 ml of an 8-6 per cent.solution being added. The results are included in Table I11 and show that the accuracy of the determination is not impaired by the presence of fairly large amounts of methacrylic acid. TABLE I11 THE DETERMINATION OF HYDROQUINONE I N THE PRESENCE OF 3 M L OF 8.6 PER CENT. METHACRYLIC ACID SOLUTION Hydroquinone found A 7 \ Hydroquinone Methacrylic acid Methacrylic acid present, absent, present, Pg Pg tLg 220 220 220 55 5 8 57 6 6 7 4 4 5 PROCEDURE FOR THE DETERMINATION OF HYDROQUINONE REAGENTS- sulphuric acid per litre. Ferric ammonium sulphate-A 0.001 M solution containing 4 ml of 50 per cent. v/v Ammonium JEuoride-A 0-1 M solution.Sodium acetate-A 1 M solution. o-Phenanthroline-A 0.1 per cent. aqueous, solution. PROCEDURE- Transfer 5 ml of the solution to be analysed to a 25-ml graduated flask and add 5 ml of 0.001 M ferric ammonium sulphate. Add sufficient of the 1 M sodium acetate solution to bring the pH to 3.5 to 4. The appropriate amount should be determined beforehand by adding bromophenol blue indicator to separate quantities of ferric ammonium sulphate and hydroquinone solution, and noting the minimum amount required to give a blue colour to the solution. Shake the flask to mix the solutions thoroughly and allow to stand for 30 minutes. Add 5 ml of 0.1 per cent. o-phenanthroline and after 5 minutes add 1 ml of 0.1 M ammonium fluoride solution and dilute the solution with water to the mark.It is essential to add the reagents in this order. Stand the solutions for 30 minutes and read the absorption on the Spekker absorptiometer with Ilford spectrum No. 602 filters and a setting of 1.0 to water. PREPARATION OF THE GRAPH- Prepare solutions of ferrous ammonium sulphate of the concentration required by appropriate dilution of a 0.2 M solution containing 4 ml of 50 per cent. v/v sulphuric acidJan., 19511 SMALL AMOUNTS OF HYDROQUINONE 49 per litre. Maintain the same acid concentration in the diluted solutions. Transfer suitable amounts to a 25-ml graduated flask. Adjust the pH as described above, add 5 ml of 0.1 per cent. o-phenanthroline and dilute the solution to the mark. Allow the solutions to stand for 30 minutes and then take the absorptiometer reading as before. Convert the amounts of ferrous iron present to the equivalent amounts of hydroquinone and prepare graphs relating concentration to drum reading. Prepare graphs for the 0.5-cm, the l*O-cm and the 2.0-cm cuvet tes. Thanks are due to Mr. T. S. West for independently carrying out a large number of check determinations. REFERENCES 1. 2. 3. 4. 5. 6. Costeanu, R. N., Mikrochemze, 1939, 26, 170. Muller, R. H., and Burtsell, A. T., Ibid, 1940, 28, 209. Vagi, S., 2. anal. Chem., 1925, 66, 101. Willard, H. H., and Hummel, F. C . , I n d . Eng. Chem., Anal. Ed., 1938, 10, 13. Stephen, T?'. I., Metallurgia, 1948, 37, 333. Whettem, S. M. A., Analyst, 1949, 74, 485. DEPARTMENT OF CHEMISTRY THE UNIVERSITY BIRMINGHAM May, 1950

ISSN:0003-2654    

DOI:10.1039/AN9517600045

出版商:RSC    

年代:1951

数据来源: RSC