INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 13, No. 11
graphically in Figure 3 and indicate that the new method is reliable over a considerable range of silver concentrations. The slight deviation from the theoretical line may be due to traces of dissolved oxygen in the water used for dilution.
The end point, on the other hand, is different in the two cases, as is shown in Figure 5. Titrations a t any potential are reproducible but the potential should be carefully adjusted for the most precise results.
Influence of Potassium Iodide Concentration
Accuracy of Estimation of Metallic Silver In the titrations described above, 0.0001 N solutions were used. When 0.00001 N solutions were tried, the electrode
In order to determine photolytic silver in silver halide sols free from gelatin it is necessary to dissolve the sol in nearly 30 per cent potassium iodide. This factor was studied from 0 to nearly 50 per cent of iodide. The result is shown in Figure 4. The data a t low iodide concentration are somewhat erratic and this appears to be caused by nonuniform sampling of the solid iodide. Use of a stock solution eliminates the difficulty. Above 5 per cent, however, samples are uniform and the solid salt may be used. Effect of Voltage across the Electrodes The voltage across the electrodes affects both the end point and the slope of the straight-line portion of the titration curve. The slopes for titration from either direction increase as the voltage increases UD to about 100 millivolts, after which they remain relatively constant.
action became somewhat sluggish but the end point was definite and reproducible. From Figure 2 it would appear that titrations may be reproduced to *0.002 cc. but in actual silver determinations the authors were not able to check better than dO.01 cc. of 0.0001 N iodine, which is equivalent to * 1 X 10-e mole of silver. Five micrograms of silver can be determined to * 1 per cent or 0.5 microgram of silver to * 10 per cent.
Literature Cited (1) Foulk, C. W,* and Bawden, A. T., J. Am. Chem. SOC.,48, 2046 (1926). (2) Wernirnont, G . , and Hopkinson, F. J., IND.ENG.CHEM., ANAL. ED., 12, 308 (1940).
v
COMYUNICATION No. 801 from the Kodak Research Laboratories.
Iodosulfate Microchemical Identification Tests for Cinchona Alkaloids CHARLES C. FULTON Alcohol Tax Unit Laboratory, U. S. Treasury Department, Saint Paul, Minn.
T
HE identification of alkaloids and other amines by microscopical observation of their crystalline precipitates has been developed to a high degree. I n the past these tests have been made almost invariably on aqueous solutions, although usually the dry substance is available, either submitted as powder, pill, or capsule in the first place, or isolated by extraction. Sometimes the best identifying crystals are obtainable from strong acids without admixture of water; and often it is easier to hold the acidity, concentration of the precipitating compound, and other factors within the limits giving the best crystals if the reagent embodies these conditions and can be applied directly to the solid alkaloid or its salt. Accordingly reagents for direct application are being developed for various alkaloids. Among the advantages of microscopic crystal tests an important one is the definite discrimination of isomers or closely related compounds. Not all such tests will accomplish this, for often there is a family resemblance even in the crystal forms. Usually, however, some reagent can be found or devised which will readily and clearly distinguish between a particular pair of isomeric alkaloids. The tests of this article yield entirely different crystals with quinine and quinidine, cinchonidine and cinchonine. The microscopic tests in present use or recommended for these four alkaloids are made on their aqueous solutions, except for some variations of the herapathite test. The best known may be found in the books by Stephenson (14) and Amelink (1). A number of the tests they give were due originally to Behrens (3)or Grutterink (8). These may be supplemented by certain suggestions of the writer: chloromercuric acid with 15 volumes per cent of hydrochloric acid for quinine (6),bromoplatinic acid for cinchonine (6),dilute
(0.2 per cent) picric acid for cinchonine (7), and ferrocyanide with phosphoric acid for cinchonine and cinchonidine (7). Recent studies, with new recommendations, have been made by Whitmore and Wood (16)and Martini (11). There does not appear to be as yet any general agreement as to the best test for any one of the four alkaloids. The crystals of quinine iodosulfate, or herapathite, so notable and well known for their extreme dichroism, provide certain identification of quinine when they can be obtained. Even without a microscope they are characterized by a dark beetle-green metallic luster or iridescence. Numerous variations of this quinine test have been given since its discovery by Herapath (1,2, 4,9, 10, 12, 16). Mulliken (IS) and Fuller (6) state that cinchonidine in this test yields microscopic needles without metallic luster, and that quinidine also yields an iodosulfate precipitate, which according to Fuller is reddish brown and much more soluble than that of quinine, so that it often takes a considerable time to form. Quinidine may not give the test, depending on the variation used. When quinidine iodosulfate crystals are obtained they do not resemble herapathite in optical properties. In the writer's experience none of the previous versions of the herapathite test has been very satisfactory, even for quinine alone, on the small scale used for other microcrystal tests for alkaloids. His purpose a t first was to perfect this quinine test for microchemical use. It was found that excellent iodosulfate crystals could also be obtained with quinidine, cinchonine, and cinchonidine. Quinidine, the stereoisomer of quinine, gives canaryyellow, nondichroic crystals. Cinchonine gives plate crystals varying in depth of color but also nondichroic, as observed with polarized light. Of the isomers cinchonine and cin-
November 15, 1941 d ,
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ANALYTICAL E D I T I O N .
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solve the iodine and potassium iodide in 2 to 3~cc. of the water, then dilute with the rest of the water. IODINE IN (2 1) ACETICACID. Dissolve 0.5 gram of iodine in 50 cc. of glacial acetic acid (this may take several days, with occasional shaking); then dilute with 25 cc. of water. Leave in contact with excem iodine crystals. Glacial acetic acid, water, and diluted sulfuric acid (1 3): concentrated sulfuric oz3$usby3v:% $ : volume of water.
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Reakents and Crystals IODINE REAGENT Q. Wagner's No. 2, 3.0 glacial acetic acid, 1.5 cc.; sulfuric acid + 3), 1.5 cc. Quinine C v s t a k . Plates, mostly of a pale
cc.;
(1 FIGURE
1.
L)lXNINE GRYSTALS
(HERAPATHITE), WITH YOLARUED IiIGHT
olivaceous green tint by ordin By polarizy where they overla are variously cogred: practically colorless, ycllosish, gkenish, oliic-green, .@ray-gmm; pink, red, pnd blork. ltotition 01 tllc stage or of the nieol diicloscs the olcoehroirm. c d w less or meenisb to dark re'd or black.' The photom6rograph (Figure 1, left) is by polarized light. The test is sensitive and only a little quinine need be used; but the characteristic crvsbak arc? seen at the soot of meatest concehtration. Out in the solution,lwhere there is excess of iodine, brown colored crystals may form. Quinidine CTystals. Canary-yellow plates, not dichroic. of various irregular shaDes. Figure 1 (right) shows o n e type. -Crystah vary somewhat in shape with this and the c 4 fo!lowing reagents, hut are nearly uniform in color. I This reagent mxy yield crystals with cinchonine and cinchonidine, but does not give FIGURE 2. CINCHONINE CRYSTALS, RECTANGULAR AND PENTAGONAL TYPES dependable tests for them.IODINE ILEAGENTCI. Wagner's No. 1.4.4 ce.; (left); CINCHONINE CRYSTALS, TRIANGULAR TYPE(right) 1) acetic acid, 3.1 eo.; glacial iodine in (2 acetic acid, 2.1 cc.; water, 1.8cc.; andsulfm-ic acid (1 3), 0.6 cc. Quinine Clyslals. As with the preceding reagent, but usually chonidine, cinchonidine corresponds the more closely to smaller and more densely matted together. quinine, cinchonine to quinidine. Cinchonidine gives two Quinidine Crg8tals. As with the preceding reagent, but gendistinct kinds of crystals, under different conditions, needles erally thinner and more branched and irregular in shape. and plairu, t l i Iatcer ~ el.owing dichroism a s extrcnie ns tlint Cinchonine CnJstals. Plates, commonly red-brown in color, varying to red, brown, black, and yellow; scarcely if at all diof hernpathite. This dichroism is an almost complete abchroic. Thes are of several different but definite shapes. the sorption of light vibrnting in one direction in the crystal, nitti transpnrsncy tolight vibritingnt rightangles. All tl.ecrgst& are highly birefringent, and oppcur briellt nitti crossed nicols.
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(left); QUINIDINE CRYSTALS (right)
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Method Put a small amount of the dry, powdi:red alkaloid or its salt in a little heap on the microscope slide, acId a drop of reagent, and apply a cover glass. The tests are sensitive, but the characteristic crystals are formled at a fairly high ratio of alkaloid to reagent, for quinine and cinchonidine; hence the
test-is sensitive and usually ives nufierous crystals in various are= under the cover glass; $ut occasional difficulty in readily obtaining crystalhation has been experienced. Submicroscopic seeding, even just by the reuse of slides, washed and then wiped
G t h c h e others. E&&iue
