DECEMBER, 1968 THE ANALYST Vol. 93, No. I I13 The Effect of Filtration and Centrifugation on Raw Sugar Polarisation Analysis BY R. A. M. WILSON, C. G. SMITH, R. H. JAMES AND R. R. WALLACE (The Colonial Sugar Refining Co. Ltd., Central Laboratory, Sydney, Australia) In raw sugar analysis, the apparent sucrose content is determined as its polarisation value, a measure of the rotation of plane-polarised light passing through a solution of the sugar, which is directly proportional to sucrose concentration when the impurity level is low. Before taking a reading with a polarimeter, a “normal” raw sugar solution (26.000 g per 100 ml) is clarified by addition of basic lead acetate solution to precipitate impurities, which are removed by simple filtration ; about 50 ml of filtrate are needed for analysis.When the solution is filtered, the sucrose concentration increases because of preferential absorption of water by the filter-paper. Initially in the current study, the magnitude of the absorption has been examined, and it has been determined that the first 10 ml of filtrate should be discarded to minimise this effect. Differences of 1 part in 40,000 in the polarisation value of sugar solutions, however, can now be detected by using modem photo-electric polarimeters and the refined analytical techniques described in this paper. This compares with 1 part in 2000 when using older visual instruments and standard tech- niques. Therefore, an insight into evaporation and humidity effects has also been gained, and a comparison of filtration and centrifugation, an alternative method of clarification, made.Centrifugation has practical advantages over filtration in laboratories in which large numbers of analyses are performed. In addition, it is not subject to preferential absorption and evaporation errors, and currently appears to give the best estimate of polarisation value, as defined and specified by the International Commission for Uniform Methods of Sugar Analysis (ICUMSA) . THE polarisation value of a sugar is a measure, in units called International Sugar Degrees (” ISS), of the rotation of plane-polarised light passing through a solution of the sugar. The basis of the 100” point on the International Sugar Scale is defined by the International Commission for Uniform Methods of Sugar Analysis (1CUMSA)l as the optical rotation of the ‘normal” (26.000 g per lo0 ml weighed in air) solution of pure sucrose in a 200*000-mm tube at 20*00° C at the wavelength of the green line of the mercury-198 isotope (A = 546.227 nm in vacuo).The “normal” solution of a raw sugar is usually not clear enough to enable a reading to be taken with a polarimeter, and is, therefore, first clarified by the addition of basic lead acetate solution. This forms a flocculent precipitate that settles at the bottom of the flask, carrying with it suspended particles. The solution is separated from the precipitate by filtration, for which a 3-inch diameter stemless filter funnel (covered to minimise evaporation) resting in a 200-ml filter glass is used. The polarisation value of the filtrate is then determined as the optical rotation in O ISS measured in a sugar polarimeter.Hardin and Zerban2 have reported that the first portions of sugar solution passing through a filter-paper undergo an increase in polarisation, which they attributed to preferential absorption of water from the solution by the filter-paper. To minimise this effect, they recommended that at least 25 ml of initial filtrate be discarded. The latest official ICUMSA method for raw sugar polari~ation,~ however, specifies that 1 O m l be discarded. 0 SAC and the authors, 773774 WILSON et al. : EFFECT OF FILTRATION AND [Analyst, Vol. 93 At the time of the 14th Session of ICUMSA, in 1966, it was not known whether an air-dried filter-paper would reach equilibrium with the solution after only 10ml had been filtered, or whether the concentration of the subsequent filtrate, and hence the polarisation value, would still be affected.It was, therefore, recommended by this Session4 that "the optimum amount of filtrate discarded be investigated." This amount needs to be sufficient to ensure that the filtrate is unaffected by preferential absorption, but as small as possible to reduce the sample volume requirements, the time needed to complete the analysis and the consequent risk of concentration changes from solution evaporation. The primary aim of the work presented in this paper, therefore, was to determine the optimum amount of filtrate to be discarded. During the course of the work, however, the development of improved analytical techniques, in conjunction with the use of a high-precision photo-electric polarimeter, enabled a detailed study of evaporation and humidity effects, and a comparison of filtration and centrifugation methods, to be undertaken.In the usual method of centrifuging, the leaded raw sugar solution is sealed in a 60-ml capacity stainless-steel tube of about 2.7-cm diameter and centrifuged with either an MSE High Speed 17 or a Sorvall SS-3 Automatic Superspeed centrifuge. The speed is increased to 14,000 r.p.m. and then decreased to zero, under automatic braking. The cycle time is about 9 minutes for the MSE centrifuge and 5 minutes for the Sorvall machine. Under these conditions, the clarity of the final solution, measured as optical density at both 646 and 589 nm, is as good as, or better than, that of filtered solutions. If a raw sugar solution is clarified by centrifugation there is a significant difference in its polarisation value, but centrifugation has practical advantages over filtration in a labora- tory in which large numbers of samples are being analysed.In addition, centrifuged samples should not suffer from the effects of evaporation, preferential absorption or other possible sources of error occurring with filtration; therefore, centrifugation should give a better estimate of the polarisation value, as defined and specified by ICUMSA, than filtration. The 14th Session of ICUMSA,* in 1966, also recommended "that the optimum moisture content in filter-paper . . . be investigated.'' The current study has been extended to include a determination of the equilibrium moisture content of filter-paper and a limited attempt to gauge the effect of filter-paper moisture content on the polarisation result.EXPERIMENTAL TECHNIQUE USE OF HIGH-PRECISION POLARIMETER- Before 1960, the polarisation value of a clarified raw sugar solution was determined with a visual polarimeter, calibrated in O ISS, by measuring the rotation of plane-polarised light passing through the solution contained in a cell of 200-mm length. For a single, clear solution, two consecutive readings may differ by up to O*lOo ISS (1 part in 10oO). The usual practice is to determine the average of five readings to the nearest 0.01" ISS, but the standard error of the mean is about 0-02" to 0-04" ISS, depending on the type and condition of the instrument, and the experience of the analyst. In the early 1960's, the Bendix-NPL, Model 143A, automatic polarimeter was introduced into this Laboratory for raw sugar analysis.This instrument combines such features as short cell length (10 mm) and, therefore, small angular rotation, with the Faraday magnetic - optic effect for modulation of the plane of polarisation and compensation for the angle of rotation. These features made possible a considerable increase in the precision of results, so that consecutive readings seldom differ by more than 0.01" ISS. The use of a Solartron 1420.2 digital voltmeter to monitor the output enables each reading to be made to the nearest 0.0025" ISS (1 part in 40,000). The Bendix polarimeter can, therefore, be used to examine effects producing much smaller polarisation errors than could previously be detected with visual p olarimet ers .COLLECTION OF SMALL ALIQUOTS OF FILTRATE- To take full advantage of the high instrument precision, special analytical technique5 required to be developed to minimise experimental errors in the preliminary solution prepara- tion steps. In particular, special apparatus was designed to facilitate collection of filtrate in small aliquots without introducing sources of error, while at the same time preserving procedural and environmental consistency with normal filtration.December, 19681 CENTRIFUGATION ON RAW SUGAR POLARISATION ANALYSIS 775 A thistle funnel of 20-ml capacity was attached with wax to the underside of a 3-inch diameter, stemless, glass filter funnel.To keep evaporation at an absolute minimum, 10 ml of filtrate were first collected in the thistle funnel and then, by loosening a spring-clip, released quickly through a short piece of small-bore plastic tubing into a 10-ml calibrated Quickfit test-tube (Fig. 1). The test-tubes were stoppered immediately after filling; the filter funnel was not kept full during filtration, and was, of course, always kept covered. Cover Fig. 1. Filtration apparatus A “normal” solution (250 ml) of raw sugar (sucrose content between 96 and 100 per cent.) was prepared, defecated with basic lead acetate solution and filtered. For each such solution, 80 ml of filtrate were collected in 10-ml portions, as described above. Simultaneously, the usual filtration procedure was followed as a control, the first 10 ml of filtrate being discarded and about 70ml collected.Carlson Ford, BIC quality, U4512 filter-papers were used throughout this work. Other papers of similar grade may be used, provided that they are quick filtering and yield a sparkling clear filtrate ; preferential absorption characteristics may then be different. POLARISATION OF SMALL ALIQUOTS ( ‘‘MINI-POLS”)- At least 50ml of solution are required to flush and fill a 200-mm visual polarimeter tube, and 30ml to flush and fill the 10-mm long, 1-ml capacity, Bendix polarirneter flow- through cell by the usual gravity siphoning method. By using a vacuum, however, it was found that a minimum volume of 8 ml of solution was required with a Bendix polarimeter cell when two consecutive solutions differed by no more than 4” ISS; 8 ml of solution, with a small air bubble between samples to reduce sample mixing, was found to flush and fill the cell with no detectable contamination, that is, to within 1 part in 40,000.Without the small capacity of the Bendix cell, a detailed study of the effect of filtrate volume on polarisation would not have been possible. 1” C. In addition, the polarimeter cell is provided with a water jacket, and strict temperature control was maintained at 20.0” 4 0.1” C . RESULTS TRIAL 1- Polarisations of twenty-eight raw sugars were examined by the “mini-pol” procedure described above. By using the average polarisation value of the control over all twenty-eight samples as a base-line, the graph of average polarisation value against the progress of the altration was plotted (Fig.2). Results show a decrease in polarisation as filtration progresses, until about 40ml have been filtered. This presumably results from preferential absorption of water from the solution during the early stages of filtration. At this point the curve is about 0~001” ISS lower than All of the trials were carried out in a temperature-controlled laboratory at 20”776 WILSON et a2. : EFFECT OF FILTRATION AND the control. The polarisation then increases steadily until, after 80 ml, it is about 0@09" ISS higher than the control. The increase is, presumably, caused by evaporation of water from the solution. Filter-papers used in the trial were in equilibrium with ambient conditions and contained about 7 per cent. of moisture, determined by both Karl Fischer and oven-drylng methods.The ambient relative humidity during the trial was 70 per cent. TRIAL 2- The alternative procedure of centrifugation was compared with filtration. Trials with thirty-eight raw sugar samples showed a highly significant (more than 99.9 per cent. signifi- cance) difference of 0-013" ISS between the polarisation values of solutions clarified by filtration and the same solutions clarified by centrifugation, the results for the former being higher than those for the latter. A horizontal line representing the result by centrifugation as 0*013" ISS lower than the filtered control is drawn in Fig. 2. [Afinalyst, Vol. 93 s - 0.005 - 0.0 13" ISS Fig. 2. Effect of filtrate volume on polarisa- tion at 70 per cent. relative humidity (air-dry filter-papers used with about 7 per cent.of moisture). A, Experimental curve; and B, curve corrected for evaporation 0-85 x evapora- tion at 60 per cent. relative humidity subtracted (see Fig. 5) 80 0-10 10-20 20-30 30-40 40-50 50-60 60-70 71 Progress of filtration - (10-ml portions of filtrate) Fig. 3. Effect of filtrate volume on polarisation at 60 per cent. relative humidity (air-dry filter-papers used with about 7 per cent. of moisture). A, Experimental curve; and B, curve corrected for evaporation (see Fig. 6) Earlier trials with 151 samples had shown a difference of 0.025" ISS, but 0.013O ISS of this was later attributed to insufficient rinsing of the centrifuge tubes and lids with the test solution. Tubes and lids are normally washed with water and, in the early trials, were rinsed only once with the test solution; three rinsings were subsequently found to be necessary, and this practice was adopted for the thirty-eight sugars mentioned above, and for later trials TRIAL 3- The polarisations of seven raw sugars were examined by the "mini-pol" procedure as in Trial 1, except that the control on each sample was centrifuged. Filter-papers again con tained about 7 per cent.of moisture, but the atmospheric humidity was lower (60 per cent.) The average polarisation value compared with the control is plotted against the progress of filtration in Fig. 3. The form of the curve is similar to that for Trial 1, except that the right-hand portion (50 to 80 ml) is about 20 per cent.steeper than for Trial 1 ; this reflects a higher rate of evaporation at the lower prevailing ambient humidity.December, 19681 CENTRIFUGATION ON RAW SUGAR POLARISATION ANALYSIS 777 The lowest value of the curve at about 30 ml is higher by about 0*015O ISS, with respect to the centrifuged control, than that in Trial 1, with respect to the filtered control. This agrees well with the result of Trial 2 for the difference in polarisation between filtered and centrifuged samples. TRIAL A To examine further the filter-paper preferential absorption effect, Trial 3 was repeated on eight raw sugars, except that the filter-papers were pre-conditioned to contain about 12 per cent. of moisture. The average polarisation value compared with the centrifuged control is plotted against the progress of filtration in Fig.4. Fig. 4. Effect of filtrate volume on polarisa- tion at 60 per cent. relative humidity (moisture- conditioned filter-papers used with about 12 per cent. of moisture). A, Experimental curve; and B, curve corrected for evaporation (see Fig. 6) In the early stages, the polarisation increased as filtration progressed until the value of the curve at 40 ml was about the same as the control. The moisture content of the paper was presumably greater than the equilibrium value for the paper in contact with the “normal” sugar solution and, therefore, sucrose instead of water was preferentially absorbed from the solution during the early stages of filtration. From 50 to 80 ml, the curve is similar to that for Trial 3, and suggests that evaporation has become the dominant effect.TRIAL 5- To obtain further insight into the evaporation effect, the “mini-pol” procedure was carried out with six previously clarified raw sugar solutions. However, no filter-papers were used. To be consistent with the other trials, about 90 ml of clarified solution were added to the filter funnel (without a filter-paper) and 80 ml collected in 10-ml portions at the same rate as in Trials 1 and 3. The rate of flow was controlled by replacing the thistle funnel with a short length of small-bore plastic tubing and a screw-clip. In this way the preferential absorption effect by the filter-paper was eliminated. Average polarisation results are plotted against the progress of filtration in Fig. 5. The polarisation of the 70 to 80-ml portion is 0.027” ISS higher than that of the 0 to 10-ml portion, indicating that evaporation causes a significant increase in polarisation during filtra- tion, even when elaborate precautions are taken to prevent it.The prevailing relative humidity during Trial 6 was 60 per cent. DISCUSSION In earlier polarisation work, an accuracy of better than 0-01” ISS could not be expected. However, the use of modern photo-electric polarimeters and refined analytical techniques have enabled smooth filtration graphs to be drawn, in which the points are calculated and778 WILSON et al. : EFFECT OF FILTRATION AND plotted with a precision as high as 0~001" ISS, polarisation differences consequently detected with a range of precision far beyond that previously possible and factors causing small but consistent errors studied in detail for the first time.The factor most likely to affect the polarisation of a sugar solution during filtration is preferential absorption of water or sugar by the filter-paper. The true polarisation value of the solution would then be approached as the volume of filtrate increases. This effect for the first 40 ml of filtrate is shown in Fig. 2. However, the sharp rise in polarisation over the last 30ml of filtrate cannot be explained by preferential absorption, and is almost certainly caused by evaporation. An alternative method for the removal of precipitate from a "leaded" raw sugar solution is centrifugation. Trial 2 shows that filtration gives a higher polarisation value than centri- fugation, presumably as a result of preferential absorption and evaporation, and a comparison of Trials 1 and 3 (Figs.2 and 3) confirms this finding. When the lead precipitate in a raw sugar solution is removed by centrifugation, the effects of preferential absorption and evaporation are eliminated. [Analyst, VOl. 93 Two questions remain to be answered- (i) how much initial filtrate should be discarded to minimise the effect of preferential absorption and to obtain a filtered polarisation result as close as possible to the correct polarisation value ; and (ii) what is the correct value, and whether a centrifuged solution gives, on average, a more accurate estimate than the corresponding filter solution? It is best first to consider the two effects, preferential absorption and evaporation, separately .PREFEREXTIAL ABSORPTION- Karl Fischer moisture determinations have shown that filter-papers used for polarisation analysis have a moisture content of 6 to 8 per cent. when in equilibrium with ambient condi- tions of 20" C and 60 to 70 per cent. relative humidity. Oven-drying methods, although not precise, have confirmed a value of 7 per cent. for moisture. Papers with this moisture content preferentially absorb water from sugar solutions, as shown in Figs. 2 and 3. If the moisture content were increased, we would expect less preferential absorption in the early stages of filtration and, if sufficiently high, a preferential desorption, that is, the equivalent of an absorption of sucrose from the solution and a consequent underestimate of polarisation from the initial filtrate runnings.However, we would not expect the increase in polarisation from 40 to 80ml of filtrate, presumably from evaporation effects, to be more than slightly affected by different initial filter-paper moisture contents. These conclusions are confirmed by the results of Trial 4, shown in Fig. 4, in which papers pre-conditioned to 12 per cent. of moisture were used. Fig. 4 shows a reversed preferential absorption effect, followed by the characteristic increase in polarisation over the last 40 ml of filtrate collected. Therefore, to minimise the effect of preferential absorption, filter-papers should apparently have a moisture content of 9 to 10 per cent., when it should not be necessary, in theory, to discard any of the first runnings of filtrate.This is contrary to some beliefs that an optimum moisture content of 20 per cent. is required. AMOUNT OF FILTRATE TO BE DISCARDED- It is difficult to condition filter-papers to a pre-determined moisture content, and the first filtrate runnings need to be discarded, in any event, as the first few millilitres are some- times slightly cloudy. It is, therefore, most convenient to use papers with the moisture content in equilibrium with ambient conditions (about 7 per cent.). Under these conditions, sufficient of the first runnings must be discarded to ensure a subsequent sparkling, clear filtrate and that the effect of preferential absorption has become small enough to be insignificant. Figs. 1 and 3 indicate that at least 10 ml need to be dis- carded.However, the amount discarded should be kept to a minimum so that the time required for the analysis will be reduced to a minimum; there will be no need to replenish the solution in the filter funnel; and risk of evaporation will be minimised. For example, we see (Fig. 2) that if a 20 to 80-ml portion is collected, rather than a 10 to 70-ml portion, preferential absorption errors will be reduced, but evaporation errors will be increased with respect to the control.December, 19681 CENTRIFUGATION ON RAW SUGAR POLARISATION ANALYSIS 779 At least 50ml of filtrate need to be collected for flushing and filling a 200-mm visual polarimeter tube. As both the preferential absorption and evaporation effects result in an overestimate of polarisation, the 50, 60 or 70-ml portion that gives the lowest polarisation should be collected.In Table I, the average results for individual 10-ml portions over spans of 50, 60,70 and 80 ml are compared for Trial 1 with the filtered control, and for Trial 3 with the centrifuged control $Zus 0.013" ISS. (0*013" ISS is the difference between filtered and centrifuged polarisations determined in Trial 2.) TABLE I COMPARISON OF AVERAGE POLARISATION RESULTS OF SEVERAL 10-ml Polarisation difference (" ISS) ; solution average minus control FRACTIONS WITH CONTROL SAMPLE RESULTS Trial 1 Trial 3 (Fig. 2) (Fig. 3) Filtered control (10 to 80 ml) . . . . o*ooo - Centrifuged control +0.013" ISS . . - 0-000 0 to 60 ml 0.027 0.028 0 to 60 ml 0.023 0,025 0 to 70 ml 0.021 0.023 0 to 80 ml 0.019 0.023 10 to 60 ml 0.002 0.004 10 to 70 ml 0-003 0.006 10 to 80 ml 0.003 0.008 20 to 70 ml 0.001 0.007 20 to 80 ml 0-003 0.009 30 to 80 ml 0-003 0.01 1 The agreement between the 10 to 80-ml portion in Trial 1 and the filtered control is excellent and, in Trial 3 with the centrifuged control, satisfactory.Clearly, if the first 10 ml are not discarded, the polarisation result will be seriously in error (by about 0.02" ISS). However, if additional filtrate is discarded, results will not differ significantly for most applications. We therefore recommend that the first 10 ml be discarded and the filtration continued until 50 to 60 ml have been collected. I I I I I I 0-10 10-20 20-30 30-40 40-50 W-60 60-70 70- 0.0ooI: 1 Progress of filtration - (10-ml portions of filtrate) Fig.5. Evaporation during filtration at 60 per cent. relative humidity (no filter- paper used) EVAPORATION- To study the evaporation effect independently of the preferential absorption effect, rrial5 was conducted with solutions that were "filtered" under specially controlled conditions without filter-papers. There is a steady rise in polarisation throughout which, in the late780 WILSON et al. : EFFECT OF FILTRATION AND stages from 50 to 80 ml, is approximately linear in form, with a slope (Fig. 5) of 0.0060" ISS per ml. The slope of the 50 to 80-ml portion of the curve in Fig. 3 is 0*0045" ISS per ml; this is less steep than the evaporation curve of Fig. 5, presumably because of a small, gradually diminishing effect of preferential absorption.The slope of the 50 to 80-ml portion of the curve in Fig. 4 is, however, 0.0060" ISS per ml, the same as for the evaporation curve in Fig. 5. If the evaporation curve is now subtracted from the experimental curves in Figs. 3 and 4, the resultant curve, corrected for evaporation, represents a plot of preferential absorption alone. The best estimate of polarisation should now be that value which is approached asymptotically as preferential absorption diminishes. For Trial 3 (Fig. 3), the corrected curve is relatively flat and approaches closer to the centrifuged control result in the final stages of the filtration. After 80 ml have been filtered, the corrected curve is only 0.005" ISS higher than the centrifuged control. For Trial 4 (Fig. a), there is a similar effect but, because the preferential absorption effect is reversed, the approach to the centrifuged control is from the low polarisation side; after 80 ml have been filtered, the corrected curve is 0.005" ISS lower than the centrifuged control.This indicates that clarification by centrifugation causes no significant errors in the determination of raw sugar solution polarisations. Centrifugation is, therefore, a more accurate method than filtration and gives a better estimate of the polarisation, as defined and specified by ICUMSA. As mentioned earlier, centrifugation is preferred for large numbers of raw sugar analyses because it is less susceptible to the effects inherent in the filtration procedure; it is also rapid in comparison with filtration and requires less bench space.[Analyst, VOl. 93 HUMIDITY EFFECTS- Apart from the different controls, the curves of Figs. 2 and 3 represent similar trials differing only in humidity conditions; Trial 1 was carried out at 70 per cent. and Trial 3 at 60 per cent. relative humidity. Examination of the curves over the last 30ml of filtrate shows that the experimental curve in Fig. 2 has a slope of 0.0038" ISS per ml compared with 0*0025" ISS per ml for that with 60 per cent. humidity in Fig. 3. Therefore, the evapora- tion effect seems to be only 0.85 times as large at 70 per cent. as at 60 per cent. relative humidity. If the evaporation curve in Fig. 5, with all values reduced by a factor of 0-85, is now subtracted from the experimental curve in Fig. 2, again the preferential absorption effect alone is illustrated.Again the corrected curve approaches the centrifuged result obtained in Trial 2, and plotted in Fig. 2, but here it seems that the effect of evaporation based on the above assumptions has been slightly overestimated. Evaporation at 70 per cent. relative humidity is probably only about 0-8 times that at 60 per cent. The amount of water required to saturate air of 70 per cent. humidity at 20" C is calculated to be about 0.8 times the amount required to saturate air of 60 per cent. humidity. These trials were conducted in a laboratory at 20" C, considerable precautions being taken to prevent evaporation. The over-all effect in Trial 5 for 60 per cent. relative humidity was a polarisation increase of 0.027" ISS from the first 10-ml portion to the last.Under routine conditions, for lower ambient humidities and higher ambient temperatures, the effect is likely to be several times greater. The average effect over a 50 or 60-ml portion in a laboratory in the tropics could easily be 0.05" ISS for a raw sugar, or 0.05 per cent. of the polarisation value for cane juices and sugar products. CONCLUSIONS When a raw sugar solution is defecated with basic lead acetate and filtered through a single filter-paper that has a moisture content in equilibrium with the atmosphere, polarisation is significantly increased as a result of preferential absorption of water by the filter-paper. Although this effect continues until at least 50 ml of filtrate are collected, the effect is negligible for most practical purposes after the first 10 ml are filtered.In accordance, therefore, with the official method of the International Commission for Uniform Methods of Sugar Analysis for raw sugar polarisation, the initial 10ml should be discarded. The optimum moisture content of filter-papers to minimise the preferential absorption effect is 9 to 10 per cent. However, for convenience, papers in equilibrium with the atmosphere (containing 6 to 8 per cent. of moisture) should be used.December, 19681 CENTRIFUGATION ON RAW SUGAR POLARISATION ANALYSIS 781 Even when great care is taken to prevent evaporation, there is a steady increase in the polarisation of small consecutive aliquots of filtrate, apart from the preferential absorption effect. The effect is about 20 per cent. higher for 60 per cent. relative humidity ambient conditions than for 70 per cent., with both at 20” C. For routine analysis under tropical conditions, errors as large as 0.05” ISS could be expected. To minimise this effect, not more than 10 ml of initial filtrate should be discarded, and only as much as is required for the polarisation measurement, usually about 50 to 60 ml, should be collected thereafter. A raw sugar defecated with basic lead acetate solution and clarified by centrifugation, rather than by filtration, is not subject to preferential absorption and evaporation effects. Centrifuged solutions give the better estimate of polarisation value (as defined and specified by ICUMSA) , whereas filtered solutions have polarisation values 0.013” ISS higher. The authors thank the Colonial Sugar Refining Company Limited for permission to publish this paper. REFERENCES 1. 2. 3. 4. Saunier, R., Editor, “Proceedings of the XIVth Session of the International Commission for Uni- Hardin, G. H., and Zerban, F. W., La Plr Sug. Mfv, 1924,73,388; Ind. Engng Chem., 1924,16, 1175. Gross, D., Editor, “Proceedings of the XIth Session of the International Commission for Uniform Saunier, R., Editor, “Proceedings of the XIVth Session of the International Commission for Uni- Received July llth, 1968 form Methods of Sugar Analysis, 1966,” ICUMSA, Subject 6, p. 16. Methods of Sugar Analysis, 1958,” ICUMSA, Subject 21, p. 86. form Methods of Sugar Analysis, 1966,” ICUMSA, Subject 11, p. 61.