Determination of Moisture in Sugar Products' Maltose and Dextrose J . E. CLELAND
AND
W. R. FETZER, Union Starch and Refining Company, Granite City, 111.
C
Using this value, a final compaiison was made by Brown, ,\Io and Millar between their value and that of Ost: ( o ) of~ maltose at 15.5' C. (2 to 20 per cent concentrations), Ost corrected 138.12, Bran n, RIoriis, and llillar 137.93. Lintner and Dull (6) examined starch hydrolytic products and obtained the amount of solids in solution from the specific gravity, the weight constant being determined by drying 2 to 3 grams of a 10 per cent solution a t 100" C. or sometimes a t only 60" C. until the n-eight of the residue was constant. Brown, Morris, and hlillar ( 2 ) showed that it was impossible to remove the last traces of water from starch hydrolytic products in this manner and that nothing short of heating in vacuum over phosphorus anhydride at a temperature above 100" C. was effective. In a more recent publication, Walker (IO), co-author of the Munson-Kalker tables, reports on the determination of the copper number of maltose. The data obtained on maltose are important because the values for reducing power obtained in this LI ork are those appearing in the hlunson-Kalker tables ( 1 ) and more recently adopted by the International Sugar Commission. The maltose used was prepared by recrystallizing RIerck's c. P. maltose three times, and drying below SO" in vacuum. This gave a specific rotation of 130.48",corresponding to 137.2' for the anhydrous maltose, which was stated to bhow a very pure prepnration. In viea of the criticism of this method of preparation by Brown, Morris, and hlillar in an earlier paper, it i y unlikely that a pure hydrated product was obtained.
ORK sirup contains maltose and dextrose, both of which form hydrates. A literature survey reveals little information in regard t o the determination of d r y substance of dextrose and dextrose hydrate, leading one to believe t h a t they offer no serious analytical difficulties. On the contrary, there is considerable controversy over the dry substance of maltose and its hydrate. Apparently the difficulty in the determination of d r y substance is the reason why no generally accepted values for specific rotation and reducing power exist for this sugar. Since these constants are indispensable in sugar analysis, and both sugars are constituents of corn sirup, i t mas deemed advisable t o apply the new drying technique to these products. Important early aork done on maltose and its anhydride was reported in 1894 by Lobry de Bruyn and Van Laent ( 7 ) . These authors called attention to the observations of Soxhlet, who stated that maltose failed to lose its water of crystallization a t 100" C. unless heated in a vacuum. Thev checked this observation by drying maltose several hours a t 100' to 105' C. in an air oven and followed this with a specific rotation determination on the dried material which was found to be 130' (temperature not specified, concentration 1 gram in 10 cc., D line used). For vacuum drying, they made use of a device consisting of two flasks connected by a curved tube, one containing the maltose and the other phosphorus pentoxide. The flark containing the maltose was heated at 105" C. for 8 hours. The specific rofation of the maltose was found to be 137.7'. These authors called attention to the stability of maltose by stating that the anhydride could be produced by placing a solution of maltose in a platinum dish and heating in a calcium chloride bath held a t 130-135" C. They stated that the sirup must be stirred during evaporation and the heating continued 1 to 2 hours after the last traces of water have been removed. Again the evidence that the anhydride had been obtained was the specific rotation found. Brown, RIorris, and U l l a r (2) in their classic 1%ork on products formed by diastatic conversion of starch, experienced great difficulty in driving off the last traces of moisture from these carbohydrates. After many attempts, they finally adopted a form of apparatus similar to the one recommended by Lobry de Bruyn and Van Laent ( 7 ) and were able to render crystallized hydrated maltose completely anhydrous in a few hours at 105-106" C. 1%ithout any signs of fusing or discoloration. Hydrated dextrose was still more easily made anhydrous and crystallized levulose completely lost all traces of adherent moisture below the fuqing point. The same workers determined the specific rotation of maltose, drying hydrated maltose in the vacuum apparatus to constant weight, the temperature being gradually raised from SO" to 108" C. The specific rotation value was based upon actually weighed quantities of dried anhydrous maltose and an average value at 15.5' C. for 2 to 20 per cent solutions was 137.93'. This value was compared to that of Ost (9),which was 137.04' for a temperature of 20" C., by applying hleissl's (8) correction for temperature to Oat's number. The following comparison was obtained: ( a )of~ maltose at 15.5" (2 to 20 per cent concentrations), Ost 137.96, Brown, AIorris, and hlillar 137.93. The determinations of Obt n-ere all made on weighed quantities of hydrated maltose which had been brought to a constant meight in a desiccator over sulfuric acid. The anhydrous maltose \\as then calculated on the basis that the compobition of the sub. H ? O , it contained exstance being used TWS C I ~ H ~ ~ O ~ ~?nd.that actly 5.00 per cent water of crystallization. Brown, Morriy and llillar (2) found that hydrated maltose, although readilv coming to a constant \!eight when dried in this manner, invariably contained an amount of water greater than that corresponding to one molecule, even after several neeks of drying. This was shown by the loss in weight when the hydrate was dried carefully in a vacuum over phosphorus anhydride in the Lobry de Bruyn apparatus in such a manner as to prevent any decomposition. The average amount of water retained by the hydrate was 0.46 per cent in excess of that corresponding to one molecule.
The purpose of this investigation n a s not to determine the specific rotation of maltose, as such a study would er,tail the preparation of pure maltose, which is the subject of controversy, but to examine the drying technique employed in t h e past n i t h particular reference t o the newer drying methods of the present authors. KOclaims other than those of the manufacturer are made for the maltose used, nor is the dried mateiial defined ab to its cy- or /?-maltose content. It is believed that the maltose was substantially pure material as i t is commonly known. The methods and technique are presented m-ith the belief that they constitute more efficient and reliable means of drying difficult hydrates than have hitherto been used
Materials Commercial dextrose hydrate (Cerelose). Pfanstiehl c. P. maltose. Several lots of maltose hydrate varying in moisture content from 4.9 to 6.1 per cent have been obtained from this firm, The specific rotation from dry substance values obtained have always checked to the first decimal place.
Apparatus Weighing bottles, Pyrex glass, 40 X 65 nim., standard-taper stoppers. General vacuum oven, an older type fitted n ith rubber gasket closure and used vith individual pump, but not suitable for highvacuum work. The pressure maintained \\ as approximately 60 to 70 mm. Tests were made on this equipment, as it is of the type more often found in laboratories and usually deemed adequate for vacuum oven determinations. JJ'eber lead gasket, a new oven fitted with lead gasket and employed mith a Mega-Vac pump whereby pressures of 1 mm. or less Ivere readily obtainable. Air oven, Elconap, double wall with temperature regulations + l o c. I obry de Rruln apparatus, as described (4).
Procedure Approximately 5 gianis of the maltose hydrate or dextrose hydrate were weighed into the neighing bottle. Tests were made at 60", 70°, SO", go", loo", and 110' C. in the vacuum oven, and at 100' C. in the air oven. Vacuum was released by introducing air through an extended drying train. Weight loss was determined at definite intervals over an extended period of time, depending upon the temperature of the test.
1 This is the third article in a series on this subject. The first t w o appeared in IND. EWQ.CHEM, ANAL E D ,13, 855 and 858 (1941).
27
28
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 14, No. 1
The maltose data have been regraphed, taking the moisture obtained a t 5 , 10,20,30, etc., hours a t different temperatures, as shown in Figure 2. Under the operating conditions of the general vacuum oven (60 to 70 mm. pressure), there was very little loss of water of crystallization below 80" C. Above 80" C., the rate of loss of hydrate water became increasingly rapid. The data obtained with dextrose and maltose hydrates employing the Weber oven are shown graphically in Figure 3. As before, &gram samples were used and the pressure in the oven was maintained a t 0.5 mm. or less, by means of a MegaVac pump. Dextrose proved stable at all temperatures and values obtained could be expressed best by a straight line. The actual data are given in Table I. 70'C.
TABLEI. DATAON DEXTROSE
/
Elapsed Time Hours
60°C.
60' C.
%
10 17
The data obtained are best shown by Figure 1. With maltose hydrate, temperatures of 90" C. and above produced a n active loss of water. Temperatures of 100" and 110" C. yielded identical values for moisture within the experimental difficulties of weighing such a hygroscopic material. Below 90" C. water was lost with difficulty, and at 60" C. the water loss gave indication of reaching the theoretical hydrate with 5.00 per cent only after extended drying time. The slowness of removing the adsorbed water is attributed t o the leaking oven and the hygroscopic nature of the material. &he specific rotation was determined on the material obtained a t go", loo", and 110" using a 10 per cent solution (10.000 grams per 100.00 ml.) with the following results:
Temperature O
c.
90 100
110
Time Necessary to Reach Constant Weight Hours
Jloisture Lost %
84 79 80
6.04 6.10 0.10
(OOD
200
c.
137.5 137.42 137.38
6
5.
35 44 57 66 103
TFMPFRATllRF ' C 80 85 90
FIGURE 2
Atmosphere, 1000 c.
%
7:+8
7: 7.79
is
.. ..
..
7176 7:77
..
These data are for the crystalline material and do not necessarily follow for solutions where the initial p H of the solution is a function. A series of dextrose solutions ranging from pH 2.0 to 6.0 was dried. Those in the pH range of 4.2 to 5.4 were stable in so far as drying data are a criterion. Solutions above and below this range gave indications of instability based on reduction in reducing power. The authors believe that dextrose solutions should be dried on a dispersing medium (4,5). The data on maltose are not comparable in moisture values t o Figure 1, as the material was a new lot of maltose hydrate. The specific rotation, however, was the same, as judged by identical drying tests carried out a t 100" C. The rate of moisture removal a t 60" C. with the higher vacuum shows a marked increase and probably would have reached the value obtained a t 100" C. if the drying time had been extended sufficiently. The data for the atmospheric oven a t 100" C. were erratic and did not give a smooth curve. For thys reason, the approximate drying curve is shown by the dotted line. The isolated point a t 120" C., vacuum oven, gave discolored material, indicative of decomposition. This difficulty in drying hydrates has been noted before.
1.C
75
7:i7
7:77
2.1
70
c.
%
7:77
7.78
3.
65
1000
7.76
4
0
Vacuum
95
100
I65
Browne (3) lists dextrose, maltose, lactose, and raffinose as requiring special precautions. He warns against raising the temperature during the initial stages of dryin above the melting point of the hydrate, as otherwise the sugar will liquefy to a thick viscous mass, from which it is difficult to expel the last traces of moisture without decomposition. For dextrose hydrate, he advises a thin layer of material for several hours at 50' to 60" C. followed by a gradual increase in temperature to 105' C. On the other hand, maltose hydrate loses
ANALYTICAL EDITION
January 15, 1942
29
water incompletely a t 100' C. under atmospheric pressure, so that vacuum dehydration IS necessary, and he recommends careful heating at 90" to 95' C. under high vacuum with the completion of dehydration at 100' to 105' C. The recourse to higher drying temperatures of 105' C. in order to remove the last traces of water seems unnecessary in t h a t it raises the moot question of decomposition. The method of dispersion described for corn sirup (6), when applied to a maltose hydrate, gave unexpected results and a new drying technique for such materials. The Lobry de Bruyn technique at 38" C. (4) was used. I n one instance, the crystalline material was used, and i n the other the nia1tor;e \vas dissolved and dispersed in Filter-Cel. As a further indication of the effectiveness of dispersion, a sample was dispersed in FilterCel and dried in a vacuum oven a t 60" C., employing a pressure of 0.5 mm. or less (Figure 4). The maltose was the same material as used for Figure 1. A comparison of the two Lobry de Bruyn tests is very striking. Even though the pressure on the apparatus containing the crystalline material was reduced to 0.01 mni., the
FIGURE3
loss of Ratel \\as exceedlngly slow. The test was continued for 850 hours with approximately 25 per cent of the water unremoved. The loss of water from the Filter-Cel sample was more rapid and a t the end of 250 hours equaled the value obtained at 100" C. in the vacuum oven, thus establishing identical dry substance values for 38" and 100" C. The data obtained for the vacuum oven a t 60' C., using dispersion, were equally striking. I n marked contrast to the behavior of the crystalline material, as shown in Figures 1 -
'
DEXTROSE
{
IOO°C. PRESSURE A T M O S . 60°C. * 0.5mm.
I0O0C.
MOISTURE
.
0.5
'
.. ..
O h
a
120°C.PRES 0.5m m .
6
I I
/
I
II J
0
IO
20
30
TIMF: 40
50
FIGURE4
60
70
80
90
100
30
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
and 3, the loss of water was extremely rapid and at the end of 50 hours, the value equaled that at 100’ C.
Conclusions Identical values have been obtained by drying maltose hydrate under low pressures a t temperatures of 38” and 100’ C. A new drying technique has been developed for the removal of water of crystallization from sugar.
Literature Cited (1) Assoc. Official Agr. Chem., Official and .4nalysis, 1935.
Tentative Methods of
Vol. 14, No. 1
(2) Brown, H. T., Morris, G. H. and Millar, J. H., J . Chem. SOC. Trans., 71, 73-123 (1897). (3) Browne, C. A., “Handbook of Sugar Analysis”, p. 25, New York, John Wiley & Sons, 1912. (41 Cleland, J. E., and Feteer, IT, R., ISD.ENG.CHEM.,d s - \ r,. ED., 13, 858 (1941). ( 5 ) Evans, J. W., and Feteer, W.R . , Ibid., 13, 855 (1941). (6) Lintner, C. J., and Dull, G., Ber., 26, 2533 (1893). (7) Lobry de Bruyn, C. A . , and Van Laent, F. H., Rec. tvau. cliim. Paus-Bas., 13, 218 (1894). ( 8 ) Rfeissl, J . prakt. C‘hem.. 21, 274 (1880). (9) Ost, H., Chem.-ZtO., 19, 1501 (1895). (10) Walker, P. H., J . Am. Chrm. Soc., 29, 541 (1907).
Ruthenium Dipyridyl-A
New
Oxidimetric Indicator JOSEPH STEIGRIAN, NATHAK BIRNBAUM, AND SYLVAN M. EDMONDS College of the City of New York, New York, N. Y.
I
T IS well known that various iron complex compounds of
the ferrous-phenanthroline type act as high potential redox indicators. These indicators are reversible and reach equilibrium rapidly. Inasmuch as the chemistry of ruthenium parallels that of iron in many respects, a n investigation of possible indicator properties of analogous ruthenium complexes has been undertaken. The behavior of ruthenium tridipyridyl dichloride, first prepared by Burstall ( I ) , has been examined from this point of view. The results demonstrate that this complex enters into a mobile reversible oxidation in which the reduced divalent form is orange-red and the oxidized trivalent form is green in concentrated solution. At dilutions comparable with those of indicator solutions, the corresponding color change is from yellow to colorless on oxidation. I n marked contrast to the behavior of the corresponding iron complexes, both forms of the new indicator exhibit stability towards acid and do not dissociate appreciably even a t the boiling temperature.
Materials Tridipyridyl ruthenium dichloride was synthesized in the manner described by Burstall (1). The complex was precipitated as the thiocyanate, which was metathesized with a slight excess of silver nitrate. The resulting solution, which was adjusted t o a concentration of approximately 0.02 molar, was used for the determination of the oxidation potential of the complex and as an indicator in the titrations described below. Solutions of ceric ammonium nitrate in molar nitric acid and ceric ammonium sulfate in molar sulfuric acid were prepared from reagent chemicals. They were standardized against Iiational Bureau of Standards sodium oxalate with phenanthrolineferrous ion as indicator ( 2 ) .
Oxidation Potential The oxidation potential of the indicator was determined by titrating 0.002 molar solutions of the nitrate with 0.02 molar ceric sulfate in molar sulfuric acid and with 0.02 molar ceric nitrate in molar nitric acid. Corresponding concentrations of ferrous sulfate and ferrous nitrate were employed in the back-titration. The reference electrode was the quinhydrone in either molar sulfuric acid or molar nitric acid.
indicator is close to that of the ceric ion in sulfate solution. Thus the indicator cannot be used successfully with this particular oxidant. However, ceric nitrate in nitric acid solution possesses a higher oxidation potential than the corresponding sulfate solution. The end points are sharp with this reagent and the Eacan be read directly from the curve with ease. The molar oxidation potential found in this manner is 0.58 volt higher than that of the ferric-ferrous system, measured against the quinhydrone reference electrode. The corresponding potential on the hydrogen scale is 1.33 volts, compared with 1.44volts for ceric sulfate in molar sulfuric acid and 1.61 volts for ceric nitrate in molar nitric acid solution. The indicator solutions reached stable potentials a few seconds after addition of each increment of reagent, indicating a highly mobile, easily reversible redox system.
Application to Titration of Oxalate The direct titration of sodium oxalate in 2 molar perchloric acid solution a t room temperature with ceric nitrate is rapid precise, and accurate. Two drops of 0.02 molar indicator solution were added to 100 ml. of 2 molar perchloric acid in which from 0.12 to 0.15 gram of sodium oxalate was dissolved. The titrations were then carried out in the cold with 0.1 molar ceric nitrate. At the end point the color changes from yellow to colorless. Successive standardizations of the oxidant solution were in agreement to less than one part per thousand and checked to the same precision standardizations through ferrous sulfate and ceric sulfate, using ferrous phenanthroline as the indicator.
Acknowledgment The authors wish to acknowledge the aid of George H. Walden, Jr., of Columbia ‘C‘niversity, who first suggested this problem. One of the authors (J. S.) wishes to thank the American Platinum Co., Sewark, N. J., for the ruthenium chloride.
Literature Cited The titration curves for the sulfuric acid soIutions exhibit a point of inflection at the equivalence point. However, the end point is not sharp, indicating that the potential of the
(1) Burstall, J . Chem. SOC.,1936, 173. (2) Walden, Hammett, and Chapman, J . Am. Chem. SOC.,55, 3649 (19.73.
