Reaction Vessel for Maintaining Constant p H by Continuous Electrometric Titration during Sodium Amalgam Reductions NATHAN SPERBER'
AND
D.R.
BRIGGS, Division of
Agricultural Biochemistry, University of Minnesota, St. Paul, M i n n .
A
forated glass disk, C, 3 cm. in diameter, containing a large number of 1- to 2-mm. holes. The perforations serve t o diminish the impact of the solution against the electrodes, especially when mercury is used in the reaction mixture. The neck of the electrode chamber is large enough to accommodate a No. 6 rubber stopper. The main portion of the electrode chamber is a flat cylinder 2.5 em. deep on its horizontal axis, one base of which is the perforated disk. A conical taper, 2 cm. deep, connect's the bottom side of this cylinder to the lower side of the flask at a point vertically about 1 em. from the bottom of the flask by means of an 8mm. glass tube, D, which has a slight U-bend. This tube serves as the main path for the return of the mercury from the electrode chamber to the flask. Another glass tube, E, connects the center of the electrode chamber to the lower third of the flask. This tube is joined to the flask by an inner seal just above the point where the stirrer blade moves along the !Tall of the flask and opens within the flask as a spout which points in the direction of movement of the blade along the wall. The tube acts as a siphon and keeps constant the level of the aqueous solution in the chamber; it also provides for a rapid change of the liquid in the electrode chamber. The pH changes occurring in the electrode chamber can be most accurately followed when a direct-reading pH meter is employed, but the ballistic type of instrument may be used almost as readily. The electrodes may be constructed t o fit the dimensions of the cell, or any of the long electrodes furnished commercially can be adapted to fit the cell. An agar-potassium chloride bridge was found to be superior to the capillary glass type, but a commercial calomel cell with a flexible rubber tip containing a pinhole was satisfactory. The elect.rodes are inserted in a two-holed rubber stopper and are adjusted t o fit into the lower portion of the electrode chamber TTithout touching the sides. A 50-ml. calibrated buret is, placed in one side neck and the other neck is available for measuring the temperahre and adding reagents.
S U M B E R of chemical reactions occur at definite optimum pH but liberate substances which alter the pH. I n order to maintain the desired p H it is therefore necessary to add controlled amounts of acid or base to the system during the course of the reaction. Such control has been accomplished by use of indicators, buffers, or electrometric instruments. The first method is not practical where the reaction medium is turbid or colored, or the pH of the solution is changing rapidly; the second is limited by questions of buffer capacity and the isolation of the product.
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66
OPERATION
A 10- to 15-ml. pool of clean mercury is placed in the flask and the stainless-steel stirrer is fitted as close to the bottom as possible. The bearing for the stirrer may be simply a glass tube with a rubber-mineral oil seal. The speed of the stirrer is considered properly adjusted when all the mercury is thrown against the side of the flask and no large globules remain at the bottom when the stirrer is operated. The reagents are added together with 200 ml. of water and the speed of the stirrer is regulated to force the solution and the stream of mercury into the electrode chamber a t a level sufficient to keep the electrodes covered. The electrode assembly is then inserted into the neck of the electrode chamber. The stirrer and the whirling mercury produce an oscillating, pumping action whereby the liquid and the mercury are forced into the chamber and back through the return tubes. Mercury collects in the U-portion of tube D during one phase of the action,
Electrometric control can be maintained in large-scale reactions by the use of automatic potent,iometric equipment,. On a smaller scale it is possible to use an external centrifugal pump and electrode chamber with manual control of the addition of acid or base required to hold the p H constant, but this system requires fairly large volumes of liquid and may result in corrosion in the pump chamber. A number of micro and semimicro control techniques have been devised, but, these methods are not readily adapted to the usual laboratory preparat,ion ( 1 ) . -1simple, all-glass vessel is needed to accomplish this purpose without the use of external circulating pumps and expensive automatic control instruments-i.e., one suited for laboratory use. The authors have devised such a vessel for use in studies of the sodium amalgam reduction of aldonolactones t o aldoses where the reduction was studied a t several pH ranges. With slight modification this vessel may be employed for many other types of reactions where pH cont,rol is necessary. APPARATUS
The reaction vessel (Figure 1) consists of a 1-liter, three-necked Pyrex flask, A , with a small electrode chamber, B, sealed on just above the middle of the flask. Larger or smaller flasks may be used just as readily. A stainless-steel stirrer, S,having a hinged blade, n-hich is a circular segment of the cross section of the flask and fitting to within 1 to L mm. of the bottom, furnishes the pumping action (2). The stirring motor should be both fast and powerful. A motor controlled by a centrifugal governor is preferable to one with a rheostat control. The flask and the electrode chamber are separated by a perI
tLLL-.-,
0"
Figure 2.
Present address, Schering Corporation, Bloomfield, N. J .
74
EGO
400
TIME
1
-
,200 '
'
"
' 1600
"
' 2000
SECONDS
Decomposition of Sodium Amalgam in 0.104 Sulfuric A c i d
N
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 p H 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 p H 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 drate present. I n the latter soluDispersions Color 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-
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