Carbohydrate Dehydration Demonstrations David A. ~olson'and Rubin Battino Department of Chemistry, Wright State University, Dayton, OH 45435 Trevor M. Letcher, K. H. Pegel, and N. Revaprasadu Department of Chemistry and Applied Chemistry, University of Natal, Durban, South Africa The "charring reaction" of a carbohydrate with concentrated sulfuric acid is a demonstration of the dehydrating power of sulfuric acid. I t is also a standard test for a carbohydrate ( I ) . This is a relatively simple reaction (which requires eood safetv urocedures! see below) and is oRen used & chemical dem%trations. Perhaps, because of the simplicitv . - of the reaction, most authors do not define the type of sugar, or the degree of fineness of the sugar crystals-@7). We will show that these factors can alter the rate of reaction by an order of magnitude. Furthermore, some texts suggest the addition of water to the sugar before reaction with sulfuric acid. We will show that the amount of water added is important and can either speed up the reaction sienificantlv. or. if too much water is added. cause the reaczon to fa$ ~o&~letely. We became curiou; about the reaction of sulfuric acid with other carbohydrate materials, such as those available in supermarkets or a lumber yard. These materials were tested with interesting results. Finally, we offer some specific safety precautions. We trust that the results of our systematic studv will make this demonstration more interesting and easier and safer to perform. Materials The various materials used were purchased locally or were commonly available from standard suppliers. The concentrated sulfuric acid was 18 molar. Procedure In order to obtain meaningful results, we standardized our experiments by using 40 g of each sample and 40 mL of sulfuric acid. In almost all cases the sulfuric acid was added carefully to a test material in a 400-mL beaker, and stirred with a elass rod to obtain a uniform mixture. For safety purposeswe recommend placing the beaker on several layers of newspaper or, preferably, in a square borosilicate glass baking dish. Caution. Depending on the anticipated result, you will want to have available a I-, 2-, or 4-Lbeaker to cover the reaction vessel ta minimize the spreading of the noxious fumes. The general course of the reaction with, say, sucrose is to first turn vellow. then black. and then this is followed bv a rapid e~&vescentexpansion. We desi~matcdthc time,tl, as the lirst sim of formation of the black matcnal, and. tr. as the time irtook for the rising mass to reach the300-&L mark on the beaker. (Some of the materials tested never showed the rising charring effect.) Results and Discussion Results for a variety of materials are shown in Table 1. The times are ones we obtained for several reoetitions. Because there are many variables like the len&h of stirring and the s h a ~ of e the 400-mL beaker and the source and state of the materials, these times should be taken as rela'Author to whom corresoondence should be addressed
tive guidelines for doing this demonstration. The results with sucrose are a case in point. Supermarket sucrose comes i n manv Dhvsical forms from a laree coarse brownish cryst2 tb &e finely ground confectioners sugar. For exam~le.one U.S.A. coarse eranular s a m ~ l eturned black, bucththe mass that formed never reached [he 300-mL mark. The powdered confectioners sugar (U.S.A.) was the fastest to respond of the sucroses tested. The effect on the rate of reaction of adding varying amounts ofwater is particularly interesting. Over the wide range of 15 to 50 mL the rate of reaction for sucrose increased markedly over the dry material. But, note that adding 100 mL of water resulted in no reaction a t all. (There is always a warming effect due to the enthalpy change on dilution of sulfuric acid.) The addition of water to the other sugars significantly increases the rate of reaction, but not as dramatically as for sucrose. Also, it only took the addition of 50 mL of water to stop the reaction for the other sugars. It is interesting to note that sucrose is a non-reducing sugar, while fructose is a reducing ketose, and that maltose, lactose, and glucose are reducing aldoses. The order of solubility in water (sugar - g1100 g HzO: fructose - 404, sucrose - 203. elucose - 100. maltose - 83. lactose - 22) does not relatk to the order of reaction. ~or'exarn~le, con+ 40 e suear + 40 mL sidering the mixture of 25 mL Hz0 HzS04, the order of the rate of reaction is: sucrose = fructose > maltose > lactose > elucose. In view of the ease of solubility of the sugars in water, it would follow that the water of crvstallization (either 2 or 4 e ner suear) - . does not make a major contribution to the reaction rates. If you calculate the amount of "water" in 40 g of the five sugars it turns out to be 23.14 g for the disaccharides and 23.98 g for the monosaccharides. The amount of water extracted and which is available to dilute the concentrated sulfuric acid is approximately the same in all cases. Thus, this cannot be the determining factor in the differing rates of reaction. Out of curiosity we determined the temperature rise in the 400-mL beaker to be about 125 "C for the addition of 40 mI, HzS04 to 23 mL of water. One test of the effect of the heat of dilution was to start with solutions madc! up of 40 mL sulfuric acid plus either 7.5 or 25 ml. H 2 0 cooled to room temoerature. These were added to 40 g of the U.S.A. granulateh sugar. The reaction of the solution containine 7.5 mL Hz0 was slowed to ti = 65 s (all black by 130 s), G t h "steam"observed a t 350 s &d tz = 410 s. There was no reaction with the more dilute solition within 40 min, although the mixture gradually darkened. The reaction of concentrated sulfuric acid on an aqueous suear solution (40 e suear + 50 mL water) could be used as a g s t for disting&hiig between sucrosk and fructose on the one hand, and glucose, lactose, and maltose on the other hand. The reaction with the sugars in the dry h e > crystal size > 53 pm) could be crystalline form (63 used to distinguish fructose from the other sugars because it reacts so much more rapidly.
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Table 1. The Reactions Times, t~and O, for the Reaction between Concentrated Sulfuric Acid and Various Materials
Table 2. Effect of Varying Volumes of HzS04 with 40 g USA Granulated Sucrose mL H2S04
Material sucrose
fructose
maltose
lactose
glucose (dextrose)
Karo light corn syrup glycerin popcorn, rice, rolled oats, Grape-Nuts baby food (bananas) dextrin sawdust cornstarch d-mannitol wheat flour honey light molasses brown sugaf . spagheai apple . . .ielly . white breada 'South Africa 'U.S.A. 'large
~~ystals. 2-3mm
'53-63 mm
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Physical Form
Vol. Water addedlml
granulara granularb granularC fined powderede solutiona solutiona solutione solutiona solutione solutionb solutionb fined solution solution solution Solution Solution fined solution solution solution solution solution fined solution solution solution solution solution fined small ciyst. solution solution solution solution solution liquid liquid solid
h/s
h1s
hls
hls
160 180
20 12 40 10 60 12-1 5 80 7WO 'Estimated amount that fell over
39 35 70 148
Column HeighVcm 29 25+ Isa 17+10a 18
Table 2 shows the time delays that accompany the addition of various volumes of sulfuric acid to 40 g of sucrose (granular, U.S.A.). There were differences in the times and in the nature of the carbon column. Over a wide range, the results are quite good for demonstration purposes. The addition of 80 mL HzS04 resulted in a dark maroon (not black) color a t 15-20 s. The results of tests with various common substances are given in Table 1and will be further commented upon here. The solid materials of popcorn, rice, rolled oats, GrapeNuts (cereal), and pasta (spaghetti et al.) show a gradual darkening within 3-10 min, but no formation of a carbon column even after several hours. However, grinding up pasta gave a standard reaction but with a h a 1 height of the carbon column of only 225 mL aRer 7 min. Cornstarch was perhaps the best of all of the materials tested. A t l of 10 s permitted a safe stirring time, a tz of 34 s was a reasonable dramatic pause, and the carbon column was the most dramatic produced with an estimated volume of 1200 mL or more. Solid d-mannitol slowly turned a light caramel color with no further reaction. The light Karo corn syrup needed continuous stirring to attain a ti of 66 s followed by a tz of 92 s. Glycerin (or glycerol) darkened with stirring a t about the 2-min mark, but never reacted. Our sample of banana dessert baby food blackened and "steamed" within 3 s, but there was no formation of a carbon column. Dextrin (which is a form of starch) gave a t l of 56 s with stirring, and a long reaction time of about tz = 400 s. The reaction with 40 g of sawdust obtained from a carpentry shop was interesting. First, due to the low density, we had to use a 1-L beaker for safety. Stirring was initially difficult, with an almost immediate blackening. Although a vigorous reaction was completed within 60 s, there was only a small increase in volume. The report& results for white bread were for removing crust and breaking up the bread into smaller pieces. The sample mass was onfy 16.4 g or one slice of biead. Using more bread would have required a larger beaker as in the case of sawdust. The mixture was stirred for about 30 s. The carbon column was very frothy! The apple jelly column collapsed a t the 300-mL mark. Brown sugar (U.S.A.) gave standard results. A thermometer was inserted approximately into the midpoint of the formed carbon column at 150 s (tz = 65 s). At t = 360 s we measured a peak temperature of about 120 'C in the carbon column. Both honey and light molasses cave fast reactions.. . ~robablvdue to the amount of wateFin these materials.
son powder coarse powder powder powder liquid liquid
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Caution. The very short interval between t , and t2 means that these reactions must be done with great caution. The foaming, lava-like, carbon column almost leaps out of the beaker! The viscous nature of the materials means they need stirring, but this should be done with great caution.
powder jelly. . -
'U.S.A. confectionerssugar
'NR = no reaction g16.4 g sample broken into -1
cm pieces, i.e.,one slice
Journal of Chemical Education
We tried the reaction between cornstarch and &So4 in a 1-L graduated cylinder, thinking this might make bandling easier and safer. The mixture was stirred with a long 6-m& glass rod with some difficulty. The carbon column did rise out of the cylinder. Although the reaction proceeded in a normal way, it was not as easy to observe (or
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dramatic!). We do not recommend the use of eraduated cvlinders uniess kinetic measurements of column volume versus time are kev to the demonstration. The carbon column may be floatedhut of the cylinder with caution by the addition of water. Smith (6) has briefly summarized the extensive experiments of Meek (7)who analvzed the uroducts of the reaction of iurrl,>c. with sulfur~cacid. An immediate rwnpwarure rise to 60-80 C occurwd ijn mixirle. Durlne the foaming Meek found the temperature to rGe to 12t-160 "C. This temuerature i s hieh enoueh to boil off water. and Meek attribited almost thk entiremass loss to be dke to escaoine steam. The eases evolved during the reaction of dry Hucrose with theacid were about 67-volume percent CO, 17 COz, and 17 SOz. The gases were analyzed using infrared absorptions and checked by a chemical method. Adding different amounts of water to sugar varied the proportions of the three gases, but the predominant one was always CO. The sucrose was not completely dehydrated because Meek detected carbonyl groups i n the carbon mass, a s well a s positive sugar tests. He concluded that the foaming was caused by the evolution of steam. We found that the carbon columns varied significantly in density, heing more dense a t the bottom. Uses of These Demonstrations First, this is an engaging demonstration that illustrates a property of carbohydrates and the powerful dehydrating nature of concentrated sulfuric acid. Because we tried this demonstration under a wide variety of conditions and with many substances, i t i s obvious that this i s a general reaction for a class of compounds. Discussion of the varying results can be a fruitful exercise. Although the test must be done with caution, the reaction can be used to distinguish between various sugars. Finally, the following safety procedures based on our extensive experience should prove to be useful. Safety Procedures Concentrated sulfuric acid is a very corrosive substance and a powerful dehydrating agent. Experimenters need to wear proper safety goggles, rubber gloves (latex work well), and a rubber apron. Acontainer of sodium bicarbona t e should be nearby, a s well a s access t o sufficient
amounts of water for flushine. The beaker should be placed in a large borosilicate baking dish. An exrra beaker or two for the srlrrinrr rods should be available Cse measured quantities of tlhe chemicals. A heaker sufficiently large to cover the expected mass of carbon should be available to minimize the escape of noxious fumes after the reaction occurs. We generally cover the reaction beaker before t h e reaction compieted. This serves a secondary purpose of seeing the escaping steam condense on the inner surface of the cover. We found that the safest way to dispose of the spent reactants was to immerse them using tongs in a very large beaker (4L) or other suitable bucket in a large sink. Neutralization of the acid may be accomplished by the addition of NaHC03 i n small amounts until no evolution of COz is observed. This must be done with some care so that the beaker does not overflow. We have found i t to be faster to neutralize some of the acid with dilute NaOH and to finish with NaHCOn. Crushine the carbon column into smaller pieces facilitates the neutralization. Aonroximatelv 58 e of NaOH or 121 e of NaHCO? are requLed to react with 40 mL of H2SO4. G t e r neutral: zation the carbon slurry was filtered and rinsed using a paper-and-net filter (the type used for paints) in a large funnel. A wire screen, kitchen sifter, or coarse filter paper also would be suitable for collecting the carbon. I t may then he safely disposed of by wrapping in several layers of newspaper. You can check for residual acidity with litmus paper.
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Acknowledgment The authors wish to thank the FRD (South Africa) for financial assistance. Literature Cited
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1. Mann. F. G.: Saunders. B. C. Ploclicol Omonic Chemislm 4th ed.: h n m " a n s : London.19601 p367. ~D ~ ~~ ~ ~ chemistly; ~~ ~chemistry ht ~ nopartment. ~~ t i M ~~ ~~ uni. g~ M versity: Hamilton. ON. 1983: X6. 3. Shakhaahiri 8. Z. Chemical Demondmfions. Vol. I: The University of Wisconsin Press: Madison, 1983: p 77. 4. Summerlin, L. R.: Esly, 3. L. Chemkol Demondmtrons. A Sourcebook for 7%zchrrs; Amer Chem. Soc.: Washington. DC. 1985: p 122. 5. Sse,A.Chem,cnlMapicfiom theGmery Stam Easiem New MedcoUniversity: Portales.NM. 1991:oo .. 61-62. 6 . Smith. D. 0. J,ChemEdue. 1980.57.805. 7 . Meeks. E. G.Schon1 Sci. Re". 1919.61.281 2. ~
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