ANALYTICAL EDITION
January, 1946
and when the oscillating solution moves away from the opening, a stream of mercury and liquid pours back into the flask. The whirling solution creates a suction on the siphon tube, E , and keeps the liquid in the electrode chamber a t a constant level. The stream of mercury mixes the contents of the chamber, so that the pH readings are representative of the whole sample. The pH control is effected as follows: One hand is used to operate the buret which contains the standard acid or base while the other hand taps the electrometer key. Thus, if base is liberated during the experiment and the desired range is 3.5 to 4.5, the dial of the pH meter is set at pH 4 and standard acid is added at a rate which causes minimum deflection of the needle. With a little experience the operator can maintain the pH of the reaction mixture within 0.5 t o 1.0 pH units in a continuously changing system. EXALIPLE. The precision of the method can be demonstrated by a study made on the rate of decomposition of 25-gram samples of 2.5% sodium amalgams of different particle size and at various temperatures in contact with an aqueous phase maintained in the pH range of 3.5 to 4.5. The deviation of the points from the smooth curve in Figure 2 may be taken as a measure of the
7s
degree of control attained. The entire reaction vessel may be submerged in a constant-temperature bath when it is desirable to control the temperature a t which the reaction is allowed to proceed. ACKNOWLEDGMENTS
The authors wish to thank Harold E. Zaugg of the Abbott Laboratories, Korth Chicago, Ill., for furnishing the stainlesssteel stirrer and for helpful advice during the course of the work. LITERATURE CITED
(1) Dole, M., “Glass Electrode”, New York, John Wiley &- Sons, 1941. ( 2 ) Forrest, R.E., IND.EXG.CHEM.,AXAL.ED., 14, 56 (1942). PAPER 2248.
Scientific Journal Series, ZIinnesota Agricultural Experiment Station. Abstracted from part of a thesis presented t o t h e Graduate School ‘of t h e university in partial fulfillment of t h e requirements for t h e degree of doctor of philosophy, March, 1945.
Estimation of Iodine Color of Starches and Starch Fractions STANLEY A. WATSON AND ROY L. WHISTLER Starch and Dextrose Division, Northern Regional Research Laboratory, Peoria, 111.
S
menclature as simple as possible. I n general, comparisons have been made with standard colors or with common color names. It is recognized that the designation of color by name is inexact to the extent that most names may properly describe a number of colors within a given small range. Hence, for more exact designation of color, the more precise not,ation devised by RIunsell ( 7 ) has also been used. The results obtained in applying the test method to several starches and starch fractions are shown in Table I. The method is particularly useful in determining the purity of amylopectin fractions, The presence in amylopectin of amylose in quantities so low as to be scarcely evident on potentiometric iodine titration ( 2 ) is readily shown by the blue color produced on addition of 0.1 ml. of iodine to thc starch dispersion, while more iodine up to 0.5 ml. brings out the typical amylopectin color. The persistence of the blue color and its influence on the final color may serve as a rough indication of the amount of amylose present. With appreciable amounts of amylose in the amylopectin, the blue color of the amylose-iodine complex masks the color of the amylopectin with iodine and prevents its visual detection. This method, therefore, does not serve to detect amylopectin in mixtures where Table I. Colors Produced b y Iodine on 0.03% Dispersions of Starches, Starch Fractions, and amylose constitutes more than Starch Derivatives 6 to SY0 of the total carbohyColors Observed with Various Amounts of 0.01 N Iodine on 10-b11. Starch Dispersions drate present. I n the latter soluColor Notation from hlunsell tions the amylose-iodine complex Color Comparison M a d e with Ridgeway Color Charts ( 7 ) usually precipitates. Color Charts (8) 0.10 ml. 0.50 ml. Starch Sample 0.10 ml. (2 drops) 0.50 ml. (10 drops) ( 2 drops) (10 drops) Under the controlled condiAmylose Spectrum blue Deep blue precipitate 5 P B 4/12 5 P B 3/12 tions of test employed here, the Whole cornstarch (disintegrated) Spectrum blue Deep blue precipitate 5 P B 4/12 5 P B 3/12 Corn amylopectin (not purifiedla Light spectrum blue Spectrum violet 6PB 6/10 3 P 4/10 color produced by iodine on (49b) b Corn amylopectin (cotton treated, Light blue-violet Amethyst-violet to 9PB 5 / 8 4 P 3/10 waxy cornstarch is true purple 1)C violet-purple and not red as has often been Potato amylopectinc Light purple (6Sb) Magenta 1OP 5/10 5 R P 4/12 Waxy cornstarch (hand-polliLight amethyst-violet True purple t o magenta 5 P 5/8 4 R P 4/10 reported. The red (sometimes nated) (61b) Waxy rice Light pink (671) c a l l e d r e d d i s h - b r o w n ) color Reddish magenta (69’) 5 R P 7/8 7 R P 4/12 Waxy barley starch Light spectrum violet Violet-purple 1OP 5/10 1OP 4/10 f r e q u e n t 1y a s so c i a t e d with (59b) @-Amylase limit dextrin (corn- Spectrum blue Spectrum violet 5 P B 4/12 3 P 3/10 waxy c o r n s t a r c h is observed starch) only in the presence of an exWheat dextrin (roasted 6 hours Light amethyst-violet True purple 5P 5/8 4 R P 4/10 a t 180’ C.) (61b) cess of iodine and is in part due Light jasper red t o 5 R P 7/5 4R 6/10 Wheat dextrin (roasted 6 hours Pale pink (3’f) a t 190’ C.) light coral red to the color of the free iodine a Iodine absorption ( 8 ) 16 mg. per gram: approximate amylose content 5.0%. in the solution. h number of b Xumbers refer t o plates whose color nomenclature has been simplified. ilpproximate amylose content 0%. i n t e r m e d i a t e stages of color can be observed (6) in waxy T..IRCHES and starch fractions are usually characterized in part through the color produced by addition of iodine to their solutions. Exact comparison of these solutions can be made only by means of the spectrophot’ometer(1, 3, 4,5 , 9 ) , but in many instances, and for routine testinp, visual estimation of color is useful. Heretofore, however, no standard method for comparing starch-iodine colors has been employed, and, in most cases, authors have failed to record the procedure used in making the color test. Since the color with iodine of some starches and starch fractions changes with the concentration of the starch solutions and with the amount of iodine added, many starch-iodine colors recorded in the literature without accompanying description of the test method are not suitable for comparative work. I n order to facilitate and standardize the designation of color of starch-iodine solutions, the following method has been devised for visual comparison of these solutions. The method gives reproducible results and is sensitive to small differences in color. All colors are referred to standard color charts (7, 8) and hence are capable of direct comparison in different laboratories Color names have been chosen with a view to keeping the no-
.
.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
76
starch dispersions LLS the amount of iodine is increased toward an excess. Under identical conditions of test the waxy starches seem to differ in the color produced by iodine as one proceeds from type t o type. Listed in order of decreasing red and increasing violet contents the starch-iodine dispersions may be arranged as fdllows: waxy rice, waxy corn, and waxy barley. To this series may be added in order potato amylopectin and corn amylopectin. This order coincides with the arrangement of Baldwin, Bear, and Rundle (1) who have suggested that i t is also the order of increasing length of chain ends for the respective starch molecules. METHOD
'
Thirty milligrams of starch are dispersed in 10 ml. of 1 N potassium hydroxide by allowing the mixture to stand with ooeasional shaking a t 0 " C. for 1to 2 hours. For starch in the whole granule state, 20 ml. of 0.5 N potassium hydroxide are uBed for the dispersion. The dispersion is neutralized with 1 N hydrochloric acid to a phenolphthalein end paint and 1 drop of acid added in excess. The solution (pH 4 to 6) is then made up to 100-ml. volume, giving a starch concentration of 0.03%. By proceeding in this fashion, a constant amount of salt is introduced into each sample. Large amounts of salt, which produce changes in the iodine color, are to be avoided. To 10 ml. of the solution in a 16- by 150-mm. test tube is added 0.5 ml. (10 drops) of 0.01 N iodine solution (KI = 0.014 M ) dropwise, with shaking. After 0.1 ml. (2 drops) has been added, the color of the solution is observed. At this point, the presence of s m d l amounts of amylose in amylopectin will be evidenced by a blue color. The remainder of the 0.5 ml. of iodine is then slowly added and the color again observed. Further addition of iodine is unnecessary and in some cases may vitiate the calor t.est by superimposing the color of
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Vol. 18, No. 1
free iodine on the starch-iodine color. All observations of color are made by transmitted daylight or a daylight fluorescent lamD and
St .I_... _I._""1"1, . "_. "..~~..""-l~.."..~".~..~l_ starches or starch fractions and to be comparable among different laboratories, must be perfomed under uniform conditions in the absence of a large excess of iodine. A procedure is outlined for the rapid determination of starch-iodine colors. I t is especially satisfactory for rapid estimation of the purity of amylaveetin when contaminated with less than 6 to 8% amylose. I._.. I
LITERATURE CITED
\., ,I\
.....,_". Soc.. 66. 111 (1944).
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D A".,
-., ""2
D &*. Q
D..-AL
-., *r, *-
D 77 A*_
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(2) Bates, F. L.. Frenoh, D., and Rundle, R. E., Zbid., 65, 142 (1943). (3) Hanes, C. E., and Cattle, M.. Pmc. Roy. SOC.(London), B125,387 , 101m \L.,yy,.
(4) Kerr, R. W., and Trubell. 0. R.. Paper Trade J., 117, No. 15, 25 (1943). (5) McCready, R. M., and Hassid, W. 2.. J . Am. C h m . Soc., 65. 1154 r\'a1*1. rnnm
(6) MacMasters. M. M., and Hilbert, G. E., IND.ENG.CHEM.,36,
958-63 (1944). (7) Munsell, A. E. O., "Munsell Book of Color", Baltimore, Md.,
Munsell Color Co.. 1929. (8) Ridgeway, Robert, "Color Standards and Color Nomenolaturd',
(9)
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Washington, D. C.. published by the author. 1912.
Versatile I4rc-SI park Stand
JOS. W. MACECIO U. S. Naval Ordnance Plant, Ttle Amorhwp Corp., Forest Park, Ill.
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N arc-spark stand combining the adjustments of the pin-
type stand and the versatility of the Petrey stand (1) has been developed which is capable of supporting heavy weights . .. ana does not place any Strain on.the Ovtlcal bar or the svectrograph. The &e, shape, and vertical adjustments of thk stand simplify nondestructive spectrographic analyses on relatively massive parts. In many laboratories this stand will broaden
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Th, Da.np.~p'Laye 11 IIIauLLIucu, , U I L I ~ i ~L l~ rU L ~ L L L U ~ UJ. Y L L ~D Y ~ U U hy two 1.8-cm. (0.5-inch) thick sections of micarta (Figure 1). All screws which fasten the micarta to the sample plate eto., are offset from each other a t least 2.5 cm. (1 inch). By ths means micart,%insnlst,e ~ t,he mmnle nlat,n~ a t least 2.5 em. (1 inchi of air or ~ A~-from the remainder of ihe stand. The micarta is fastened to two ~
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the edge at the sample plate towards the spectrograph slit 8 notch is cut, 2.5 cm. (1 inch) wide and 2.5 em. (1 inch) deep, similar to the cut-out portion of the Petrey stand. The sample lying on the sample plate can easily be e x i t ed from the bottom, using the sample as one electrode and a gra phite rod as the other ($3, 3). The notch must be large enough s30 that the excitation will be lite rod and not between the
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by trial. I n th;; case, extra in the tap of the plate, in order thrit the plate may be turned to bring the proper notch toward the 5;pectrograph slit.
YlMPLE PLATE
2 S€CT/ONS
of
STEEL
h7CARTA ,Figure
1. Method of lniidating Sample Plate
Figure 9.
Arc-Spark Stand
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