Amperometric Titration of Nornicotine C. 0. WILLITS AND CONSTANTINE RICCILTI Eastern Regional Research Laboratory, Philadelphia 18, Pa. L4sshoan in Figure 1, nornicotine had a half-wave potential of -0.78 volt referred to a standard calomel electrode, and the flat portion of its wave was a t -0.90 volt. Silicotungstic acid in the same electrolytic medium also has the flat portion of its reduction wave a t -0.90 volt, as shown in Figure 2. Figure 3 shows a typical amperometric titration plot obtained for 50 ml. of 1.440 X 10-3 M nornicotine in 0.1 .TI hydrochloric .lf acid and 0.1 M lithium chloride titrated with 6.197 X silicotungstic acid in increments at 3-minute intervals. The points were obtained by plotting the wave height in millimeters against volume of titrant added. Repeated trials Rith the nornicotine-silicostungstate titration, in which a relatively short period ( 3 minutes) elapsed betveen additions of the titrant always showed a similar but erratic pattern of points 1%hich indicated the formation of an unstable precipitate.
can be determined by quantitative precipiN ORSICOTINE tation with silicotungstic acid by a method similar to that used for nicotine ( 1 ). Bowen and Barthel(2) determined nicotine and nornicotine in the steam distillate of tobacco from the difference in the weights of the total alkaloid-silicotungstates before and after the destruction of nornicotine with nitrous acid. Larson and Haag ( 8 ) determined nornicotine by forming a colored complex with cyanogen bromide and then measuring its absorption intensity in a spectrophotometer. 6.0
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POLAROGRAM O F S l L l C O T U N G S T l C ACID
APPLIED E.M.F., VOLTS
Figure 1. Polarogram of Nornicotine For some time amperometric titration methods have been successfully used for inorganic analyses (5-7',11-13), and recently the method has been applied to organic compounds (3,4,IO). TKO prerequisites of the amperometric titration method are: that the substance being analyzed or the titrant used should be polarographically reducible, and that the substance being measured be removed from solution a s either a precipitate or a nonreducible complex. The titration of nornicotine with silicotungstic acid fulfills these conditions. Both nornicotine and silicotungstic acid are reduced polarographically and the nornicotine is quantitatively precipitated by silicotungstic acid. The amperometric method should therefore be applicable to the determination of nornicotine (in the absence of nicotine) and should also provide quantitative information on the mechanism of formation of the nornicotine-silicotungstate precipitate.
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APPLIED
Figure 2.
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E Y F , VOLTS
Polarogram of Silicotungstic 4cid
Figure 4 shows a t \ pica1 alnperometric titration using 50 nil. of 2.231 X .If nornicotine in the same electrolyte solutioii to which portions of 4.892 X 31' silicotungstic acid were added a t 15-minute intervals. The intersection point of the projected straight lines was 11.35 ml. of added silicotungstic acid. In this case the molar ratio was 2 t o 1, as the calculated volume of silicotungstic acid required for this ration was 11.40 nil. The anonia-
EXPERIMENTAL
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A4continuous reading Sargent Model XX polarograph and a
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Lingane H-cell (9) with a saturated calomel electrode as reference electrode were used. The cell was covered with a plastic cap with openings for introducing the buret tip and capillary. With the capillary in place, oxygen-free nitrogen was bubbled through the solution for 2 minutes after each addition of titrant t o remove any dissolved oxygen which would produce an interfering polarographic wave. I n addition to the nornicotine, the electrolyte solution contained 0.1 mole of lithium chloride and 0.1 mole of hydrochloric acid per liter as an indifferent or supporting electrolyte. The applied potential of -0.90 volt used in the amperometric titration of nornicotine x a s established from a polarogram of a 2.879 X M nornicotine with 0.1 Af hydrochloric acid and 0.1 M lithium chloride as supporting electrolytes. The nornicotine polarogram was recorded in a cell held a t 5 O C., as a t higher temperatures the wave was not w l l defined.
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Figure 3. 1712
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ACID (0.006197 M), ML.
Typical Amperometric Titration Plot of Nornicotine
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V O L U M E 23, NO. 11, N O V E M B E R 1 9 5 1 300 - 1
1713 tion was conducted rapidly by adding the silicotungstic acid a t 3-minute intervals, a n unstable precipitate was formed. However, when the same titration was conducted a t a slower rateLe., adding the acid a t 15-minute intervals-the stable precipitate with a 2 to 1 molar ratio of nornicotine to silicotungstic acid was obtained. The points for the slower titration were less scattered than those for the rapid one. The points were even less erratic when the titration was conducted at 5' C. than at higher temperatures. The close agreement between the values obtained from eight separate amperometric titrations and the calculated values indicates t h a t this method can be used to determine concentrations of 2 to 3 mg. of nornicotine per nil. with a probable error of +2y0of the amount of nornicotine present.
2:l MOLAR RATIO STOICHIOMETRIC POINT FOUND 11.35 ml. CALCULATED 11.40ml.
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E 200
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> ACKNOWLEDGMENT
100
The authors wish to acknowledge the assistance of -4bner Eisner, who prepared the samples of nornicotine used in these studies. LITERATURE CITED
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2 4 6 6 1 0 1 2 14 SILICOTUNGSTIC ACID (0.004892 M),
Figure 4.
(1) hssociation of Official Agricultural Chemists, "Official and Tentative Methods of Analysis," 6th ed., p. 74, par. 6.108-9,
ml.
.4mperometric Titration Plot of
Nornicotine
IOU$ nonreducibility of the nornicotine solution when titrated under the "slow" conditions is not readily esplained. It is not due to 3 pH effect, as the p H remains constant throughout the titration. The horizontal, linear segment of Figure 4 is above the zero ordinate because no correction has been made for residual current, 40 111111. or 0.8 microampere. Such a correction would lower this linr to the zero ordinate. DISCUSSION
The aniperometric curves for nornicotine titrated with silicoturigdic acid showed t h a t the composition of the precipitate depentlrd upon the time allowed for equilibrium. When the titra-
1945. (2) Boaen, C. V.,and Barthel, IT. F., IXD.ESG. CHEM.,AN.iI.. ED., 15, 740 (1943). (3) Conn, J. B., h a L . CHEM.,20, 585 (1948). Ibid., 21, 565 (1949). (4) Elofson. R. h l . , and Mecherly, P. 8 , (5) Kolthoff, I. M., and Langer, A , , J . A m . C R e m Soc., 62, 211 ( 1940). (6) Ibid., p. 3172. (7) Kolthoff, I. hl., and Lingane, J . J., "Polarography," Kew York, Interscience Publishers, 1941. (8) Larson, P. S.,and Haag, H. I3., IXD.EX. CHERf., ha^. ED., 16, 86 (1944). (9) Lingane, J. J., and Laitinen, H. .I.,Ibid., 11, 504 (1939). (10) Smith, L. I., Kolthoff, I. &I., and Spillane, L. J., J . Am. Chem. Soc., 64, 640 (1942). (11) Spalenka, J., Collection Czechosloc. Chew. Communs.. 11, 146 (1939). (12) Stock, J. T., A n a l y s t , 72, 291 (1947). (13) Thanheiser, J., and Willems, J.. A i d i . Eisc,ihiittenu~., 13, 73 (1939). R E C E I V EJanuary D 26. 1931
Determination of Radioniobium in the Presence of Soluble Orthophosphates R. B. HA"' AND C. L. BURROS Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tenn. HE accurate determination of radioniobium in solutions conTtaining soluble ort,hophosphates is difficult ( 1 ) . The phosphntr ion is coprecipitat,ed with niobium pentoside and repeated washings remove only a small portion of this impurit,y. The smorint of phosphoric anhidride carried down by the niobium pentoside precipit,ate may be as much as one t,hird of the total weight. This is due t o the formation of a n insoluble niobium phosphate ( 4 ) . Thr conventional procedure (3)for t8hedetermination of radioniolium in fission product mixtures gives erroneous results if the sonip1c.s contain much orthophosphate. The phosphate ion accompanies the niobium throughout the entire analytical procedure and frequently the final precipit,at,e of niobium pentoxide ustd for counting weighs more t,han the added carrier. ('onvrntional methods for removal of phosphate prior t o analysis are unsatisfactory, because all or a large portion of t,he niohiuni is removed along with the phosphate ion. The method of 13urgus and Davies ( 2 ) is not sa (,tory in solutions contain-
' On loan froill
R'ayne Universitl-.
ing large amounts of soluble orthophosphates because of a considerable loss of the niobium in t'he silver phosphate precipitate. I n studying the precipitation of niobium pentoside from various acid media it was found that the iodate ion is coprecipitat,ed with niobium in the same manner as the phosphate ion. When niobium pentoxide is precipitated from solutions containing both iodate and phosphate ions, bot'h ions are coprecipit'ated. The ratio of these ions in the final precipitate is dependent upon their relative concentrations in the original solution; hence precipitation of niobium pentoside from solutions containing a high concentrat,ion of iodate ion will eliminate most of the coprecipitate of the phosphate ion. Unlike the phosphate ion, the iodate ion can be removed from the niobium pentoside by washing with dilute ammonium hydroxide and by ignition. Using these properties, the following method was devised to eliminate the interference of the phosphate ion in the procedure of Glendenin ( 3 ) . PROCEDURE
The following step should be substituted for Step 1 of Glendenin's procedure (3):
