Conductometric Measurement of Ash in White Sugars T. R. GILLETT California and Hawaiian Sugar Resning Corporation, Ltd., Crockett, Calif. in 100 ml. of solution. Only 43740 of total conductivity of water should be subtracted in making the determination. Sucrose purity of white sugars may be calculated by conductivity ash measurements, moisture determinations, and known ratios of ash to total dry nonsucrose in sugar products.
Conductometric methods provide a simple, rapid, and accurate means of determining ash content of white sugars. The relation for white sugars derived from Hawaiian raws is: 740 ash (resulfated less 10%) = 0.00047 X specific conductivity (micromhos). The most desirable concentration was 25 grams of sugar
T
HE value of conductometric measurements for determining the ash content of sugar products has been generally recog-
nized in the sugar industry for a number of years. Numerous papers have been published on the subject, and the methods are described in varying detail in a number of handbooks (1,2,6). Because of the speed, simplicity, and precision offered by conductometric measurements, these methods were investigated in this laboratory several years ago. This investigation led to the replacement of the time-consuming gravimetric ash procedure with conductometric methods for routine ash determinations on white sugars, raw sugar, char waste water, refinery sirups, and certain other refinery products. The present paper describes the investigation of conductometric methods for use in determining the ash content of white sugars. ANALYTICAL METHODS USED
Conductometric Method. The conductometric method of determining ash in sugar products is based on the principle that in a solution of the sugar, the mineral matter that constitutes the ash dissociates, whereas the sucrose, a nonelectrolyte, does not dissociate. A measurement of the conductance of the solution will therefore give a measure of the concentration of the ions present, and thus provide a direct indication of the total mineral or ash content of the product.
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16
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z
12
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u v)
41 0
I
10
I
20
I 30
I
40
I 50
J 60
SUGAR CONCENTRATION % BY WEIGHT
Figure 1. Typical Relation between Conductivity and Concentration of White Sugar Solutions
As conductance, measured in mhos, is the reciprocal of resistance in ohms, a Wheatstone bridge can be used to measure the conductivit,y of a solution. The instrument employed in recent years is the portable Leeds & Northrup sugar ash bridge (No. 4961). The conductivity cell used with this equipment is a dipping type cell with platinized gold electrodes. The instrument has a range in conductance from 1 to 120,000 micromhosLe., mhos X 10-e-and is equipped with means for compensating for the temperature of the solution and for the cell constant of the conductivity cell. This permits obtaining the specific conductivity directly. In making a determination, the sample of sugar is first dissolved in a low conductance water such as a distilled water of good quality. The concentration of solution selected for white sugars was 25 grams of sugar dissolved in water to give 100 ml. of solution (dry substance = 22.8% solids). This concentration was used because it provided the most accurat>ebasis for conductivit,y measurements on white sugars.
Figure 1 was obtained by preparing a series of solutions of different concentrat,ions from the same white sugar. The conductivity increases up to about 20% rds, levels off between 20 and 30% rds, and then decreases from 30% rds up to saturation. By using a concentration equivalent to 22.8% rds, approximately the maximum conductivity reading is obtained and the flat part of the curve, where analytical error has minimum effect, is utilized. Numerous tests of this kind have been made with varioua types of sugar products of different ash content and the same characteristic type of curve has been obtained in all cases. Sees ( 4 ) reported similar data and also came to the conclusion that a concentration of 25 grams in 100 ml. was optimum. Because of the preserice of small amounts of mineral matter in the dissolving water, it is necessary to apply a correction for the conductivity of the water. However, because of the depressing effect' of the sucrose on conductance, only a portion of the water conductivity should be subtracted from the measured conductivity of the solution. In order to determine the amount of correction; a series of solutions was prepared, using the same sugar sample but water samples of increasing conductivity, starting with a specially prepared, triple distilled conductivity water of practically zero conductance. The results of these tests are shown in Figure 2. A straight-line relationship was obtained, and calculation of the water correction from the slope of the line indicates that only 43% of the specific conductivity of the water should be subtracted for sugar solutions of this concentration. [Similar tests have been made in this laboratory with solutions of other concentrations and the water correction factors established. Calton, Weitz, and Calendar ( 3 ) have used this method to determine the water factor for a concentration of 5 grams in 100 ml.] The conductometric method used in this investigation on white sugars involved weighing out 25 grams of the white sugar dissolving it in a distilled water of good quality to give a tota! of 100 ml. of solution, measuring the conductivity at, 20" C. on
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ANALYTICAL CHEMISTRY
1082 the sugar ash bridge, and subtracting 43% of the distilled water conductivity, to give the net specific conductivity in micromhos. Filtration was not found necessary, and no adjustment or correction was made for the alkalinity or acidity of the solution, as tests showed that pH had little effect between the ranges of about pH 5 and 8, which would cover the pH range of practically any normal sugar product.
Gravimetric Method. The resulfated ash method with a 10% deduction was used for the ignition ash determinations in this investigation. Although other methods such as carbonated ash, and sulfated ash without deduction are also widely 0 used, the sulfated ash with 10% a w deduction is the method officially recognized by the International Commission for Uniform Methods Figure 2. of Sugar Analysis and is the one most generally used in the sugar industry. Obviously, a relationship with conductivity can be easily established for any gravimetric method desired.
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Effect of Conductivity of Water on Conductivity of White Sugar Solution Concentration, 25 grams of mugar per 100 ml. of malution
The procedure for determining sulfated ash involves weighing a small amount of sample into a dish, adding a few drops of sulfuric acid, igniting the sample in a muffle a t about 550” C., and cooling the sample. In this investigation, the additional ste of resulfation was employed to msure complete conversion o r a l l ash to sulfate. This requires a second addition of acid, and a second igniting and cooling before the ash can be weighed. The ash is then calculated to a percentage basis and a deduction of 10% of the total ash is applied. This gives the ash content in t e r m of “per cent sulfated ash less lo%.” DESCRIPTION OF INVESTIGATION
The relationship between the conductance of a solution and the gravimetric or ignition ash of the same product is usually established by a direct comparison of the two determinations. A conversion factor is then developed by taking the average of a large number of such comparative determinations. This method appears satisfactory in many cases where an appreciable amount of ash is present. However, in products such as white sugars which contain extremely small amounts of ash, the comparative method is subject to considerable error, chiefly due to the inaccuracies in the gravimetric procedure. In the investigation described, a procedure, which is believed to be considerably more accurate, was used in establishing this relationship for white sugars. This procedure involved adding known amounts of sugar ash to white sugar solutions of extremely high purity and then comparing the incremental ash added and the corresponding incremental increase in conductivity. Experimental data are based on products derived from Hawaiian raw sugars. The addition of ash was accomplished by preparing several stock solutions, each containing a different sugar product of relatively high ash content. Stock solutions of raw sugar, brown sugar, No. 4 liquor, and molasses were thus prepared and added in increasing quantities (5, 10, 15, 20, 30 ml., etc.) to a series of solutions of a given high purity white sugar. The amount of added ash, based on the ash content of the stock solution, could then be easily and accurately calculated. By determining the conductivity of each solution in the series, a ratio was established between the incremental amount of ash added and the incremental increase in conductivity and this formed a basis for establishing an accurate conversion factor, in accordance with the following formula:
Factor =
% ’ ash increment av. conductivity increment
Table I indicates the results of one of a series of such tests. The stock solution added was prepared from a light brown sugar containing 2.168% ash (resulfated less 10%). This stock solution was made up to a concentration of 10 grams of sugar per liter, so that each milliliter contained 0.01 gram of sugar or 0.0002168 gram of ash. A series of white sugar solutions was then prepared to which increasing quantities of the stock solution were added. The solutions were adjusted in each case for the amount of solids added in the form of stock solution, so that a standard concentration of 25 grams of solids in 100 ml. of solution was maintained. Table I shows the amount of stock solution added, the specific conductivities of the final solutions, and the corresponding conductivity and ash increments for a c h solution. The ratio of the averages of these two increments indicates a factor of 0.000468 for this particular series of samples. In the course of this investigation, 28 series of tests were made, involving 224 different samples and determinations. White sugars of high purity were used; the confectioners’ sugar had an ash content of about 0.001% and the granulated sugar about 0.01%. Details of each series of tests are not shown. However, a summary of the entire group of tests is presented in Table 11. The final average factor obtained was 0.000472. Expressing this as a formula:
Table I. Typical Example of ,Method of Calculating Conductivity-% Ash Conversion Factor for White Sugar (Series 2, test 16; stock ash solution = 10 g r a m of light brown sugar ,per liter; granulated sugar solution = 25 g r a m of sugar per 100 ml. of solution)
MI. 5 10 15 20 30 40 50 60
Micromhos 32.4 37.2
41.6 46.2 55.2
64.8 74.2 83.4
Av. yo ash increment
%
Micromhos
.,....
i
0.002168
4’. 4.4 4.6 4.5 4.8 4.7 4.6
4.63
Factor = av. conductivity increment
-0.002168 4.63
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0.002168 0.002168 0,002168 0.002168 0,002168 0.002168 0.002168
0.000468
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V O L U M E 21, NO. 9, S E P T E M B E R 1 9 4 9 Table 11. Tabulation of Conductivity-Ash Factors folr W h i t e Sugars Ash in Source
Grade of Sugar Confec tioners'
Test So. 1 2 3 4 5 6
7 8 9 10 11
(% Resulfated
Source of Added Ash Ash Less 10%) 0.4328 Raw sugar 0.415 Raw m g a r 1.08 4 liquor 1.02 4 liquor 2.03 Light brown sugar 2,105 Light broirn sugar 1.92 Light b r o u n sugar 2.52 l f e d i u m hroirii sugar 2.595 JIediuiri brown sugar Nedium brown siigar 2 ,643 3.315 Dark brown sugar Molasses 12.775 12.915 Molasses 13.035 hlolasses
D, Ash Increment,
70
0.002229 0.002087 0.002225 0.002101 0.002091 0.002168 0.001978 0.002596 O.OO2673 0.002724 0.003414 0.002555 0,002583 0.002607
E, Average Conductivity Increment,
Factor
2:
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5.27 4.58
0.000~10 0,0004.59 0 .0 ~ O . j o ~ 0.000466 0,000421 0.000477
4.18 4.66 4.69 5.43 5.97 6.04 7.49 4.70 5.40 5.33
0.000431 0.000456 0,000543 0.000478 0.000490
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method of determining ash in white sugars is used for ash determinations on all routine control samples of all grades of white sugar. It has also found wide use for refinery investigations of centrifugal washing, quality comparisons of white sugars, and calculation of white sugar purities. CONDUCTIVITY-PURITY DETERMINATIONS
O.OOOJ48
The calculation of purity from conductivity has been found of extreme 4.21 0.0004g6 value because of the difficulty and in0.4130 0 002087 Raw sugar 15 Gran d a t e d 0 4328 0 002229 Raw w e a r 16 accuracies involved in making polarimetric 1.02 0.002101 17 0.002225 1.08 5.27 0.000422 purity determinations on such high purity 18 0 002168 4.63 0.000468 2.105 Light brown sugar 19 4.60 0.000~29 products as white sugars. The method 1.92 0 001978 Light b r o a n sugar 20 2.03 0.002091 Light brown sugar 21 of conductometric purity determination 2,595 0.00?673 hIediuin hrown sugar 22 2.645 hIedium brown sugar 0 002724 23 6.01 0.000453 is simple, if the moisture is known and 2.52 llediiim brown sugar 0.002596 5.43 0 . 000477 2.4 3.315 7.69 o.000~4 the ratio of ash to total dry nonsucrose D a r k brown sugar 0.003414 23 0.002583 12.915 Molasses 26 constituents is established. 0.002607 13.035 Molasses 27 0,002555 12.775 4.70 0.000543 Molasses 28 For a given type of raw sugar, the Average factor 0.000472 Probable error ;to.ooooo~ ratio of ash to dry nonsucrose is generally about constant. This ratio furthermore is approximately the same for products, such as soft sugars, derived from % resulfated ash less 10% = 0.000472 X specific conductivity in this raw sugar. I t is assumed that this same ratio holds for white micromhos sugars also. It has been found by determining the ratio for these products over a long period of years that under the particular = 472 x specific conductivity in mhos conditions in this refinery, the ash content represents approsiIn practice, the last figure is dropped, and a factor of 0.00047 mately 29% of the total dry nonsugars. If this ratio is known is used, as the instrument reads directly in micromhos. and the conductometric ash and moisture are determined, the conductometric purity or sucrose content can be determined a s DISCUSSION OF RESULTS follows: The stock solutions were prepared from sugar products varying % conductometric ash 7 0 total dry nonsucrose = from about 0.5 to 13% ash. The factors shown in Table I1 0.29 represent the average for each series of samples. The average Then of all factors was 0.000472 and a calculated probable error was about *0.000004. The results of this study indicate that the % sucrose = 100 - ( % total dry nonsucrose Yo moisture) type of sugar ash has no appreciable effect on the relationship and that a value of 0.00047 can therefore be used to give a reliable Assume that a white sugar has a conductivity of 20 microniho.,. This is converted t o per cent ash by multi lying by 0.00047, indication of the per cent ash in white sugars derived from giving an ash content of 0.0094%. The totaf dry nonsucrose is Hawaiian raw sugars. then calculated as 0.032%. If the sugar has a moisture content An inspection of the literature shows a record of similar factors of 0.026%, by other investigators. A number of years ago Nees ( 4 ) re yosucrose = 100 - (0.032 0.026) or 99.94% ported the establishment of a similar relationship for beet sugars, based on a direct comparison between sulfated ash less 10% and Comparative polarimetric punties of high purity products such conductivity. as white sugars are very difficult to ascertain with any degree of yo sulfated ash in beet sugars = accuracy. However, comparisons on products of slightly lower specific conductance X 105 a t 25' C. purity have shown good agreement. This method of calculating 231.5 purity has proved of considerable value for rapidly determining the sucrose content of white sugars of various types and grades, Converting this to the basis employed in the present investiga.particularly in special investigations on refinery processes. One tion-i.e., 20" C. and specific conductivity in micromhos-this is large sugar refinery in England uses this method of purity equivalent to a factor of 0.00048. control altogether-Le., refiniug operations are controlled on a Another relationship for white sugar, including unwashed and conductometric purity basis rather than the usual polarimetric washed granulated and also remelt sugars, was established by purity basis. Zerban and Sattler (6),both of whom have done a large amount of investigative work on conductivity measurements on a variety SUiMMARY of sugar products. The relationship between conductivity and Conductometric methods provide a simple, rapid, and accurate sulfated ash less 10% reported in this second case, when expressed means of determining the ash content of white sugars. The on the same basis, indicates a factor equivalent to 0.00053. This relationship for white sugars derived from Hawaiian raws is: slightly higher factor may be due to the different type of sugars tested, their generally higher range of ash content, and the slightly yoash (resulfated less 10%) 0.00047 X specific conductivity different method of conductivity determination employed. (in micromhos) In view of the results of this investigation, the factor of 0.00047 The method used in determining this relationship is believed was officially adopted in this laboratory about 10 years ago and to be much more reliable than the usual direct comparison has been in regular use since that time. The conductometric 12 13 14
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ANALYTICAL CHEMISTRY
method. This factor compares very favorably with those developed by other investigators. The most desirable concentration for conductivities of white sugars was found to be 25 grams of white sugar in 100 ml. of solution. Only 43% of the total conductivity of the water should be subtracted in making such a determination. The sucrose purity of white sugars may be calculated by means of conductivity ash measurements, moisture determinations, and known ratios of ash to total dry nonsucrose sugars in sugar products. Conductometric ash determinatioris made on the basis described are much more accurate than gravimetric methods, have provided a precise index of white sugar quality, and have proved valuable for control and investigation of refining operations.
LITERATURE CITED (1)
Bates, F. J., and associates, Bur. Standards Circ. C-440, 265 (1942).
Browne, C. A., and Zerban, F. W., “Sugar Analysis,” 3rd ed.. p. 1021,New York, John Wiley & Sons, 1941. (3) Calton, F. R., Weitz, F. W., and Calendar, Elmer, Proc. Am. SOC.
(2)
Sugar Beet Technol., 4, 542 (1946). (4) Nees, A . R., Ind. Eng. Chem., 19, 225 (1927). ( 5 ) Spencer and Meade, ”Cane Sugar Handbook,” 8th ed., pp. 450-5. New York, John Wiley 8: Sons, 1945.
(6) Zerban, F. W., and Sattler, L., IND. ENG.CHEM.,ANAL. ED.. 3, 41 (1931). R E r E I V E D October 4, 1948. Presented before the Division of Sugar Chemist r y and Technology of the . ~ M E R I C A S CHEMICALSOCIETYa t the 114th Meeting, Portland, Ore.
Conductometric Determination of Ash in Raw Sugars T. R. GILLETT California and Hawaiian Sugar Refining Corporation, Ltd., Crockett, Caiif.
This paper describes a conductometric method for determining the ash content of Hawaiian raw sugars. A uniform relationship was found to exist between the specific conductance of a solution of raw sugar and the ignition ash content of the sugar. This is a direct relationship, and avoids the special methods used by other investigators, which involve acid addition, multiple readings, etc. The relationship established for a solution of 5 grams of raw sugar per 100 ml. of solution (4.9% solids) is: % resulfated ash = 0.00160 X specific conductance (micromhos). The procedure described has been in very satisfactory use in this laboratory for over 10 years.
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OKSIDERABLE investigative work has beeii done in this laboratory on conductometric methods of determining the ash content of sugar products. The application of these methods for determining the ash content of white sugars has been described ( 2 ) . The present paper discusses another application of conductivity methods-the measurement of the ash content of raw sugars. The samplrs involved in this study were raw sugars from plantations in the Hawaiian Islands. The relationchip described is therefore primarily applicable to raws from this fiource. METHODS OF ANALYSIS E!MPLOYED
Conductometric Method. The method used for determining the conductivities of raw sugars was similar to that employed for white sugars ( 2 ) . The Leeds & R-orthrup sugar ash bridge with dipping type conductivity cell was utilized and specific conductivity readings were thus obtained directly. The solutions wpre made up to a concentration of 5 g r a m of raw sugar dissolved in a distilled water of good quality to give 100 ml. of solution (4.9yo solids). This is the generally accepted concentration for conductivity measurements on raw sugar. In accordance nith the usual practice, a correction for the conductivity of the distilled water was applied. However, tests showed that the extent of correction was dependent upon the concentration of sugars present and that the correction of 4370 of the specific coiiductance of the distilled water that was determined for a 2.5-gram-per-100-ml. solution did not apply to a 5-gram-per-lOO-ml. solution. I n order to determine the extent of correction, a series of solutions was prepared from the same sample of raw sugar but with various water samples of different conductivities. The results plotted in Figure 1 indicate that 73% of the distilled water specific conductivity should be subtracted from the specific conductivity of the solution to give the specific conductivity of the raw sugar. This factor is apparently independent of the amount of ash normally present in such sugar products.
A similar correction factor was determined by Calton, Weitz, and Calendar ( 1 ) in their work on beet sugars. They employed a concentration of 5 grams of granulated sugar in 100 ml. of solution and determined a water correction factor of 77% of the distilled water conductivity, although they reported variations in their correction factor from 74.6 to 79.9%. Although this factor of 77% is somewhat higher than the factor determined in the present case, i t is in reasonably good agreement, particularly when the difference in the two types of sugars involved is considered. Furthermore, the small difference in factors would have little effect on the conductometric ash of the raw sugar, inasmuch as the total water correction will usually amount to only about 1% of the solution conductivity. I n summary, the conductometric method involves weighing 5 grams of the raw sugar, dissolving i t in distilled water to give a total of 100 ml. of solution, and measuring the specific conductivity a t 20” C. on the sugar ash bridge. From this reading is subtracted 73% of the specific conductance of the dissolving water to give the net specific conductivity of the raw sugar in micromhos. The sample is not filtered, as there is small difference between the conductometric ash of a filtered and an unfiltered sample. No correction is made for pH, as tests indicate that between about pH 5 and 8 there is little change in conductivity. The pH of normal raw sugars falls well within this range. Gravimetric Method. In determining the ash chemically, the resulfated ash method, described in standard reference books, was employed, with a 10% deduction. The determinations were made directly on the raw sugars to give the total ash rather than the soluble ash, d i c h would be obtained if the samples had been dissolved and filtered prior to ashing. The former procedure was followed because the total ash is considered to be of primary significance. DESCRIPTION O F INVESTIGATION
I n developing the relationship between the conductivity of a raw sugar solution and its ash content, direct comparisons were made between the specific conductivity and the resulfated ash,
