ANALYTICAL EDITION
136 Table I SUGAR
COLOR
% a
b
2 a
COLOROA CANDY^
%
SUGAR
VOl. 3; No. 2 I
coLoR %
85 41 50 i h 200 166 55 50 42 j 198 40 k 60 45 78 120 1 92 39 m 71 42 85 n 86 100 Stronger standard and more dilute solution used.
COLOROF CANDYa
% 47 60 24 25 83 36
From these values of the color in the sugar and in the candy
it is evident that the color of the sugar has no definite relation to the color of the candy produced from the sugar, and consequently there is no need of using the same standard color solutions in grading the sugar and the candy. The only comparisons that may be drawn are those between the sugars themselves and those between the candies themselves. Literature Cited (1) Ambler, Manufacluring Confectionery, 1, No. 1, 17 (1927). (2) Balch, IND. ENG. CHEM., 22, 255 (1930).
Impurities in White Sugars I-Determination of Phosphorus’sz S. Byall and J. A. Ambler BUREAUOF CHEMISTRY AND SOILS,WASHINGTON, D. C.
The importance of traces of impurities in sugar is T IS common knowledge very often possible to debeing recognized more and more, necessitating the among sugar c h e m i s t s termine the total quantity of development of methods of analysis of greater sensiand technologists that in a group of compounds havtivity. Inorganic and organic phosphorus compounds evaluating white sugars for ing in common some characwhich are present in the plant find their way into the certain uses the o r d j n a r y teristic r e a c t i o n or atomic juices and eventually into the white sugars crystallized gravimetric, volumetric, and grouping. The n i n h y d r i n from these juices. The Briggs modification of the physical chemical methods test for a m i n o acid comBell-Doisey coerulo-molybdate method of determining of analysis are i n a d e q u a t e . pounds (1) is a group test phosphates is sufficiently delicate to show the amounts For example, two samples of this character, since it is of inorganic phosphorus and, by difference, will show of white sugar may be pracspecific for no i n d i v i d u a l the amounts of organic phosphorus present in sugar. tically i n di s t i ngu i s h a b 1e amino acid, polypeptide, or The test is capable of showing as little as 0.3 p.p.m. when examined for ash, inother hydrolytic product of Pz06 when applied to a 10-gram sample. Analyses vert sugar, sucrose, pH, color, albumin, but is given by the of a large number of white sugars of all kinds have turbidity, and so forth, and class of compounds which revealed the fact that organic phosphorus compounds yet one sample may show a contain an amino group in are not entirely eliminated by modern methods of greater t e n d e n c y t o w a r d the alpha position to a carsugar production. caramelization on heating or boxyl group. With the remay be more susceptible to f i n e m e n t of a n a l v t i c a l fermentation than the other. Such differences in behavior methods and technic it may eventually be possible t’o submust be caused by impurities which are present in quantities divide such groups of related compounds into smaller ones, so minute that either they have b‘een formerly considered un-’ or in some cases to determine chemical individuals. important and therefore not determined, or they are capable Phosphorus Compounds in White Sugars of estimation only by special methods of a sensitivity greater than that of the methods in general use. It has long been known that phosphates are present in the In studying the causes of such differences in behavior, juices of the sugar cane and of the sugar beet. I n the procthe first point of attack is from the analytical end. Only esses of manufacture of sugar, the lime which is added in the when the amounts of the impurities are known is it possible, clarification of the juice combines with the phosphates present by adding appropriate quantities of each to pure sucrose, to to form the relatively insoluble calcium phosphates But determine from any desired viewpoint the effect of each foreign because the latter are appreciably soluble, a small quantity substance and of combinations of them. The determination of inorganic phosphates always remains in the liquors and, of the quantities of contaminants which are present only as will be shown, m6ay often be determined in the white sugar. in traces is often difficult, not only because of the lack of Organic phosphorus compounds such as lecithin and the methods of sufficient sensitivity but also because of the nucleic acids also occur in the tissues of the sugar cane (6) extreme reactivity of sucrose itself toward chemical reagents. and of the sugar beet (4). They will therefore pass into Therefore, it is necessary to devise methods or to modify the mill and diffusion juices and will be subjected to the existing methods so as to increase their sensitivity and over- action of lime in the clarification. The lecithins on treatcome interference by the sugar. Several such methods ment with alkalies yield among their hydrolytic products have been worked out and will be the subjects of the first glycerophosphoric acid which, on account of the solubility part of a series of publications on the “Impurities in White of its calcium salt (6), will be removed from the liquors only Sugars.” The effects of these impurities will be discussed to the extent that it is adsorbed by the compounds precipilater in the series. tated in the sludge. The nucleic acids are more-stable toAlthough in dealing with traces of organic substances ward lime than the lecithins and probably are hydrolyzed to a it is rarely possible in the present state of knowledge even to less extent during the processes of sugar making. Neither detect the presence of an individual organic compound, it is type of these phosphorus compounds responds to the ordinary tests for phosphates until the organic portion of their mole1 Received November 28, 1930. cules has been destroyed or removed by hydrolysis. The 2 Contribution No. 105, Carbohydrate Division, Bureau of Chemistry occun-ence of organic phosphorus in white sugars has not and Soils.
I
April 15, 1931
INDUSTRIAL AND ENGINEERING CHEMISTRY
been reported in the literature, so far as the authors are aware. By using the method to be described, the quantity of organic phosphorus is found generally to exceed that of the inorganic phosphorus. Because of the extremely small quantity of phosphorus in the sugar, it is not feasible to use any of the ordinary gravimetric or volumetric methods of determining it. This was clearly indicated by preliminary experiments in which it was necessary to use samples of sugar weighing several hundred grams in order to obtain sufficient phosphate for gravimetric determination. These attempts showed, however, that the proportion of total phosphorus (obtained by wet oxidation with nitric acid whereby all organic matter was destroyed) was best expressed in terms of parts per million of sugar. The Briggs (5) modification of the Bell-Doisey ( 2 ) colorimetric method for phosphates was found to be rapid and economical. Sucrose does not interfere with the development of the color. The test is specific for inorganic phosphates and therefore lends itself to the differentiation of organic and inorganic phosphorus compounds. It is sufficiently sensitive to allow of easy determination of 0.003 mg. of PzOs, or, in a 10-gram sample of sugar, of 0.3 p. p. m. The reagents used are as follows: STANDARD PHOSPHATE SOLUTION-A sample of potassium dihydrogen phosphate weighing 0.4394 gram is dissolved in water and diluted to one liter. Twenty-five cc. of this solution are then diluted to 200 cc. This gives a solution which contains 0.02866 mg. of P206per cubic centimeter. MOLYBDATE SoLuTIoN-Twenty-five grams of ammonium molybdate are dissolved in 300 cc. of water and to this are added 200 cc. of water containing 75 cc. of concentrated sulfuric acid. HYDROQUINONE SOLUTION-Ha1f a gram of hydroquinone iS dissolved in 100 cc. of water to which a drop of sulfuric acid has been added to retard oxidation. SULFITESOLUTION-Twenty grams of sodium sulfite are dissolved in 100 cc. of water. The solution should be freshly prepared. NITRIC ACID-One volume of concentrated nitric acid is diluted with an equal volume of distilled water. PREPARATION O F COLORIMETRIC STANDARDS-The standards for the colorimetric comparison are developed simultaneously with the unknowns. Measured volumes ranging from 0.1 to 10.0 cc. of the standard phosphate solution are placed in 100-cc. Nessler tubes. To each are added, in the order given, 5 cc. of the molybdate solution, 1 cc. of the sulfite solution, 1 cc. of the hydroquinone solution, and enough distilled water to make a final volume of 100 cc. The solutions are mixed and allowed to stand for 30 minutes. Standard solutions made up with and without the addition of pure sucrose showed no difference in hue or depth of the colors developed. Procedure
TOTAL P20b-Five grams of the sugar were mixed in a platinum dish with 0.2 gram of anhydrous sodium carbonate. The mixture was carefully charred over a free flame and finally incinerated to white ash in a muffle furnace heated to a temperature below that a t which the ash will fuse. The ash was cooled, dissolved in exactly 1 cc. of nitric acid reagent, and taken up in a little hot water. The solution was filtered, and the filtrate and washings were collected in a 100-cc. Nessler tube and allowed to cool to room temperature. The reagents were then added in the same order and the same quantities as described for the standard tubes. After 30 minutes the solutions were compared colorimetrically with standards which had been developed at the same time. INORQANIC PpOs-Ten-gram portions of the sugars were dissolved in a little water in the Nessler tubes. To these solutions the reagents were added exactly as already described, and the colors which developed in 30 minutes were compared with those in the standard tubes. ORGANICP20s-The value found for inorganic P20s was mbtracted from that found for total P206.
137
Table I gives the results found in representative white sugars. Table I-Phosphorus a s PaOr in White Sugars INORGANIC ORGANIC INORGANIC ORGANIC
SAMPLE
PZOS P. p. m.
P2On P. p. m.
DIRECT CONSUMPTION B E E T SUGARS
0.9 0.9 0.9 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.9 0.9 0.6
n
0.9 1 4
0
10.6 4.9 8.3 6.0 9.7 8.9 45.6 5.4 11.3 8.9 4.9 8.3 2.9 7 2 8 9
SAMPLE
P206 P. P. m.
P206 P. p. m.
DIRECT CONSUMPTION CANE SUGARS
P
3.7 2.6 0.9 0.9 20
?,
r S
t
18.7 7.7 2.6 0.3 6.0
R E F I N E D CANE SUGARS
u
v
W X
Y 2
0.0 0.0 0.0 0.6 0.0 0.0
4.9 9.2 6.3 6.9 0.9 0.0
I n the determination of the total phosphorus it is essential to add sodium carbonate to the sugar before the charring in order to prevent the volatilization of some of the phosphorus pentoxide which will occur if there is not sufficient alkali or alkaline earth in the sugar to fix the phosphorus as phosphates in the ash. Table I1 shows the results obtained when the carbonate was added before incineration in comparison with those in which the carbonate was added after the incineration but before the treatment with nitric acid. of T i m e of Adding Carbonate on Total Phosphorus PHOSPHORUS AS P20, NazCOs added before NatCOs added after SAMPLE incineration incin er ation P. p . m. P. p . m. 9.2 8.6 I 45.9 45.9 5.7 5.2 22.3 18.6 P
Table 11-Effect
a
I n dissolving the incinerated ash, the presence of some nitric acid is desirable to hasten the transformation of pyrophosphate to orthophosphate, since the latter is the only form of phosphorus which will react with the molybdate reagent. If an excess of nitric acid is used, however, it will interfere with the development of the color so that the results obtained will be too low. Thus, a determination of total phosphate in sugar u was made in which the ash was dissolved in 2 cc. of the dilute nitric acid and compared with one in which only 1 cc. was used. The former showed only 0.57 p. p. m. P2OSwhile the latter showed 4.87 p. p, m. A few of the colors developed from incinerated sugars had, in addition to the blue of the reduced molybdate, a slight yellowish cast caused probably by a small amount of iron in the ash of the sugar. It was found that the interference of this yellow color could be eliminated by viewing the unknown and the standard solutions with which it was being compared through a yellow Wratten light filter (K-3 No. 9), which equalized the colors and made possible a perfect match. As in all colorimetric work, blank determinations should be made on the reagents without sugar and, if phosphates are found, suitable corrections must be made on the readings obtained with the unknowns. The occurrence of such large proportions of organic phosphorus in refined sugars is unexpected. The explanatioii must be sought in further studies on the elimination of phosphorus during the refining processes. Literature Cited (1) Ambler, Intern. Sugar J., 29, 382, 437 (1927). (2) Bell and Doisey, J . Bzol Chem., 44, 55 (1920). (3) Briggs, Ibid., BS, 13 (1922). (4) Rtimpler, "Die Nichtzuckerstoffe der Ruben," pp. 32, 242,302-21, and 352-5, Vieweg, Braunschweig, 1898 (5) Shorey, J . Am. Chem. S O L , 20, 113 (1898), 21, 609 (1899). (0) Willstatter and Lkdecke, Ber., 87, 3755 (1904).
