V O L U M E 23, NO. 10, O C T O B E R 1 9 5 1
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Amylodertr
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dissolved out is to be found at the front of the solvent, extending downward in streaks. The weak concentric rings in the amylose spots (IA, 1B)appear only when retrograded material is present. In a series of eight amylopectins tested, the intensity of the iodine color complex after formamide treatment appeared to vary with the molecular weight of the polysaccharide. This observation may serve BS a basis for approximation of molecular weights of amylopectins. Depending on the molecular weight, as little as 25 t u 75 micrograms of amylopectin can be detected by this test. A number of other solvents, such ae water, $cohol, acetone, and pyridine, were tried. Pyridine was found to be the only solvent that can be substituted for forinamide with satisfaotqy results.
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Amylopeotin ~ ~ e r e . gmoleaular e weight SOU 000 Amylopectin: average molecular weight 4.OdU.OW
ACKNOWLEDGEMENT
The author is grateful to the Corn Industries Research Foundation for the support of this work, t o W. 2.Hnssid for his interest and advice, and to Dexter French for SuDDl~ines a m ~ l eof s smvlodextrins. LITERATURE CITED
(1) Barker, S. A,, Bourne, E. J., and Peat. S.. J . (2)
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190, 673
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IIYED
chern. SOC..1949,
May 28, 1951.
High-Precision Semimi w
EDWARD N. ISE' University of Calgoornia, Los Alamos S eientlfie Laboratory, Los Alamos, N . M . An automatic titrator has heen constructed for assaying sernimioro samples of substances by redox Titration. Operation of the titrator is demonstrated hy assaying samples of from 2.4 to 4.8 mg. of iron, which are converted to the sulfate and are reduced t o the +2 valenoe state with an excess of titanous sulfate before being transferred to the titrator. The titrator automatically adds either sulfatoceric acid e r potassium permanganate solution from a motordriven syringe, both continuously and incrementally, to determine accurately first the oxidant-
L F hav! b!en published on p method for stopping a potentiometric titrahon a t its equivalence point (e) and an :& mechanically operated buret (S), but an article by Lingane (I), .concerning the use of a motor-driven hypoderrnic.syringe for the addition of titrant, gave the key to the solution of the problem of achieving automatic titration with a high degree of accurrtcy. A :study of this method of titrant addition, and an attempt t o scale down the method to the titration of 3- to 4mg. samples of iron in a total solution volume of 5 ml., are responsible for the develop ,merit of the apparatus described here. P
tita nous equivalence point and then the oxidantimrI equivalence point. From the number of revogear train between the lutions of the syrinse-drive . . two equivalence points, the operator o m calculate the quantity of iron in the sample, based on previous Stalidardization runs of samples of sodium oxalate. The automauc aamuon OI approximately I-NI. a n erements of titrant in the regions of the equivalence points permits determination of these points with ease and precision. A n average precision of assay of 0.9 part per thousand was obtained.
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Present address. University of Kmrar, Lnrrrenoe. Kan.
point of a titration by changes in the potential of a platinumcdomel electrode system. Titration to a definite electrode-system potential was rejected because of the effect of temperature and of the condition of the platinum electrode on the potential produced. Iteverting t.o the definition of the potentiometric equivalence point, it was apparent that a method for determining the maximum A E / A V m'a6 to be desired. This wa8 achieved by adding the titrant in small increments, in the neighborhood of the equivalence point, evenly spaced with respect t o time. The true equivalence point was denoted by that increment which produced the maximum change in the electrode-system potential. Thie
1480
ANALYTICAL CHEMISTRY
incremental addition was easily obtained hy permitting a suitahle shaft of the gear train driving the titrant syringe to make only one rapid revolution a t We beginning of each Wsecond period, and the change in electrode-system potential was recorded on a Brown Electronik recording potentiometer whose chart speed was constant during the entire titration. The chemistry of the proposed assay method included the initial complete reduction of iron t o the +2 valence state, by the addition of an excem of a solution of titanous sulfate, followed by titration first to the oxidant-titanous equivalence point and then to the oxidant-iron equivalence point. As a measure of only the quantity of oxidant added between these equivalence points was desired, a method was devised for counting only the revolutiom of the gear train driving the titrant svringe between these points
face of the s?-ringe I ~ W T . Two snisll tshs were soldered onto the barrel of t h i micrometer and a longitudinally split collar was slipped over the barrel to provide the drive from the gear train. A mechanical differential which w a s connected to the gear train permitted the counter to be engaged or disengaged from the gear train by the operation of a solenoid. A small 3 r.p.m. motor was used to actuate a microswitch momentarily every 20 seconds. Two relays were included in the titration zssemhly; the lower one wits controlled by the refill-delivery swit.ch and permitted rapid refill of the syringe, while the latching relay supported on a 4-inch (IO-Cm.) pillar limited the motion of the gear train to a single revolution when desired.
CONSTRUCTION O F APPARATUS
The titration assembly was constructed from the “mechmical breadboard’’ parts manufactured by Servomechanisms, Inc., and is shown in Figures 1 and 2.
Right Rear View of Titration Assembly Because the method is R volumetric one, the entire apparatus hack of the Bakelite support panel should he enclosed and provided with a thermostatically controlled heater to maintain a constant temperature. The inclusion of a small intermediate titrant reservoir would Dermit, the titrant to attain temnernture eauilihnium before enterkg the syringe. METHOD OF OPERATION
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hv _-_I_I. referer The e.-lortrirrl ___ rirniit m9.r hr heat nnrlerstonrl I____.I .___ to the complete electrical circuit shown in Figure 3. l_l
Figure 1.
Left Front \ i e n of‘ritration Assembly
A Bakelite panel on the front of the titrator supported a magnetic stirrer, a platform far the titration vessel, a delivery-refill stopcock, and an interlocked delivery-refill switch. The Lucite platform was provided with four holes through which the platinum and calomel electrodes, the inert atmosphere lead, and the titrant addition tip Were admitted to the titration v e s d A phosphor-bronze spring with a cuplike depression a t its lower end held the titration vessel up into a shallow circular groove in the bottom of the Lueite platform. The Bakelite panel was spaced from the vertical breadboard plates to provide for the barrel of
I n the controller unit a 6AG5 miniature tube was emuloved a pentode-connected cathode follower to match the very high impedance requirements of the electrode system to the low input impedance of the recorder. A variable compensating voltage was incorporated in the output to balance the no-signal current in the cathode circuit to obtain a recorder reading of aero. or a desired value above zero, when no voltage was deceloped by the electrode system. By proper adjustment of R M ,RM, and Rlr (Figure 3) the recorder range was selected to read from -200 to +I000 mv. electrode-wstem uotential. Ah OB2 voltage r e d a t o r wab used to obtain a constant volt~
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was ground t o a concave spherical shape, in which was placed a 3/10 inch bearing hall, which in turn pressed against the flat SUI-
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the s%n& vlunger.’ Excellent vrecision of translation of the
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980 mv. were availaGle fo
