Alpha-Picoline Methiodide as a Reagent for Bismuth KENNETH WHELAN and FRANK J. WELCHER Indiana University Extension Division, Indianapolis, Indiana
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UMEROUS organic con~poundscontaining trivalent nitrogen, usually with the nitrogen in heterocyclic form, have been used for the detection and the determination of metals. One of the first of the nitrogen bases to be used for this purpose was quinine. A solution prepared by dissolving quinine sulfate and potassium iodide in water has been used by Aubry (1) to detect bismuth in urine. Antipyrine (Z), qninoline (3), cinchonine (4), and urotropine (5) have been employed by different investigators for the detection of bismuth and antimony in various materials. Reactions employing these reagents take place in an acid solution and in the presence of potassium iodide to give highly colored precipitates. Krumholz and Watsek (6) assign the composition HBi11,Base to these precipitates. The relationship between the molecular weight of the base and its sensitivity as a reagent has been studied by Krumholz and Watsek (6) and they conclude that as the molecular weight of the organic base increases the sensitivity increases proportionally. One of the methods tried by them of increasing the molecular weight of the base was to form the quaternary ammonium salt, usually the salt formed by the base with ethyl iodide. The increase in sensitivity exhibited by the quaternary ammonium salts was found to be more than could be justified by the increase in molecular weight alone. Alpha-picoline methiodide was one of a number of compounds used by them in their studies, but no details of its use as an analytical reagent were given. A search of the literature does not reveal that or-picoline methiodide has been used as an analytical reagent for bismuth. The authors of this paper have found that or-picoline methiodide gives a perceptible reaction with solutions of bismuth salts a t concentrations as low as 0.002 mg. Bi+++ ion per ml. of solution, or when used as a drop reaction, with 0.4 7 Bi+++ ion per drop of solution. The reagent has been found to be more sensitive when used alone than with KI, which has been an essential part of the reagents containing the nitrogen bases. The iodide ion which is necessary for the formation of the bismuth precipitates is derived from the a-picolime methiodide. The reagent, while not specific for bismuth, is much better as a confirmatory test in the regular analytical procedure than the usual sodium stannite reduction. REAGENTS
Preparation of or-Picoline Methiodide. Alpha-picoliie
methiodide is prepared by mixing two moles of methyl iodide (reagent grade, Baker's) with one mole of a-picoliue (two-degree boiling range which includes 129.3"C., obtainable from Reilly Tar and Chemical Corporation, Indianapolis, Indiana) in a large beaker. A spontaneous reaction takes place and may become rather violent. The mixture solidifies and a great deal of heat is evolved. The solid is allowed to dry in an acid-free atmosphere. No further purification of the compound is necessary, and it will keep indefinitely in closed containers. The use of less methyl iodide than the ratio specified above will cause a decrease in the yield of a-picoline methiodide. Reagent. A solution of 25 g. of the or-picoline methiodide in 100 ml. of water is used in the following tests. An acid solution of the reagent is used in place of t h ~ above aqueous solution in certain of the tests. This acid solution can be prepared by dissolving 25 g. of the a-picoline methiodide in 100 ml. of 2 N HCl. In the following procedures, the aqueous solution of the reagent is used unless the acid solution is specifically designated. METHODS
Test-tube Method. If interfering substances are absent, the solution to be tested is made just acid to litmus with dilute HCl. To 1 ml. of this solution add 0.5 to 1 ml. of the reagent. An orange-red precipitate or orange opalescence forms depending upon the concentration of the Bi+++ ion. The color obtained with the more dilute solutions of bismuth is best viewed by looking vertically through the tube as in colorimetric work. Under optimum conditions of acidity it was found possible to detect 0.002 mg. of Bi+++ ion per ml. of solution by this method. The color a t this dilution is slow in forming and may require 5 to 10 seconds. Best results are obtained by using 0.5 ml. of the reagent for the test, and if the color does not develop in 5 to 10 seconds, by adding an additional 0.5 ml. In concentrations of Bi+++ ion of greater than 0.1 mg. Bi+++ ion per ml., the orange-red precipitate is formed. Drop Readimz. In the absence of interfering substances the reagent is adaptable to a drop technic. Two to three drops of the solution to be tested are placed on a white porcelain spot-plate and acidified slightly with dilute HCl. The addition of one or two drops of
the reagent causes an orange-red precipitate or an orange color to form if bismuth is present. Under optimum conditions of acidity 0.4 y Bi'++ ion in one drop of solution is detected. Care should be exercised in the control of the acidity. Too much acid will cause the test to fail, a pale yellow solution appearing. The test also fails if there is not sufficient acid to prevent the precipitation of BiOCl with the addition of the aqueous solution of the reagent. INTERFERENCE
Ag+ and Hg+ ions interfere with the bismuth reactions by precipitating the insoluble iodides of these metals, but this interference may be eliminated by complete precipitation of the metallic ions with HCl. The resulting insoluble chlorides are removed by filtration and the Htrate neutralized, if necessary, with NaOH until alkaline, and then acidified with dilute HC1 until just acid to litmus before applying the test. The excess HC1 does not interfere if the concentration of the Bi+++ ion is high (0.1 mg. per ml. or greater) and need not be neutralized. Copper, ferric iron, and other oxidizing substances interfere by oxidizing the iodide of the reagent to free iodine. This interference can be removed by adding an excess of KCN to the sample and filtering off the precipitated bismuth hydroxide. The bismuth hydroxide is then washed with hot KCN solution and finally with cold water to remove the KCN. The precipitate is then dissolved in 0.5 to 1 ml. of 2 N HCl and the reagent added. The characteristic orange-red precipitate or orange-red opalescence is observed if bismuth is present. The bismuth may be detected on the paper by pouring 0.5 to 1.0 ml. of the acid solution of the reagent (see reagents) over the washed precipitate of bismuth hydroxide. The usual orange-red color is formed immediately with bismuth. In the presence of antimony, stannous tin, or lead, the regular separation for Group IV should be used. The precipitate of bismuth hydroxide is obtained as usual, and 0.5 to 1.0 ml. of the acid solution of the reagent poured over the precipitate. An orange-red coloration appears on the paper if bismuth is present. The precipitate of bismuth hydroxide, obtained from the Group IV separation procedure, may also be dissolved in 2 N HCl by pouring 1 to 2 ml. of the dilute acid over the precipitate and then testing in the usual manner with the aqueous reagent. Either method seems to be equally satisfactory. In either case the bismuth hydroxide must be washed several times with water in order to remove the NH4+ ion, a large concentration of which interferes with the formation of the orange-red precipitate or orange opalescence. A n t i m & is ~ ~the mbst troublesome of ;he interfering substances, ~t precipitates with the reagentas light yellow which quickly to a brick-red mass. The dierence between the precipitate form5d with antimony and that formed with bismuth is not difficultto distinguish, but in mixtures of the two ions
the orange-red crystals of the bismuth complex will be obscured and it is impossible to obtain a positive test without removing the antimony. Lead, mercuric mercury, silver, and stannous tin form white to light yellow precipitates with the reagent. If the concentration of these ions does not exceed one-half of the concentration of the bismuth ion, and the concentration of the bismuth ion is 0,1 mg. per ml. or greater, no trouble should be experienced in getting a positive test for bismuth. The white or light yellow precipitates formed with lead, mercuric mercnry, silver, or stannous tin are colored a definite shade of pink if bismuth is present in the mixture, and this color is a strong indication of the presence of bismuth. In all cases of a doubtful test, the Group IV separation must be applied. In order to compare the effectiveness of the reagent with the usual sodium stannite reduction test for bismuth, students of qualitative analysis in this laboratory were advised to use this test as well as the sodium stannite test on their unknowns. The filter paper containmg the bismuth hydroxide obtained by the usual separation procedure was tom in half and to one half was applied the sodium stannite reduction while the other half was tested with the reagent by the method outlmed above. These students then reported the presence or absence of bismuth as shown by both of these methods. The results based upon the sodium stannite reduction showed a relatively high percentage of error while the results based upon the a-picoline methiodide were without error. SUMMARY
The authors submit that the reagent or-picoline methiodide is a sensitive reagent for the detection of bismuth salts, and while not specific for the Bi+++ ion, the interfering substances can be eliminated without considerable amount of trouble. No work has been done at present on the reason for the increase in the sensitivity of the reagent upon eliminating potassium iodide. The reagent seems to show most promise as a confirmatory test in the regular procedure in place of the sodium stannite reduction. The authors are indebted to the Reilly Tar and Chemical Corporation of this city for furnishing the highly purified (now commercially available) a-picolme used in these experiments; also for the many other picolines and substituted pyridines which they have so kindly given for use in further experiments. Additional work on the use of the quaternary ammonium salts in place of the added potassium iodide is now in progress with several of the common organic bases available. LITERATURE CITED
(1) AUBRY, "Detection of bismuth in urine," I. pharm. chim., 25, 1 5 4 (1922); C k n . Abstrads, 16,2343 (1922). (2) CAILLEAND VIEL."A new reaction of the alkaloids and the preparation of the iadostibinates of these substances in the crystalline state," Compt. rend., 176, 1156-9 (1923); C k m . Abstracts, 17, 2289 (1923). (3) GILPCHENKO AND SHE~NTZIS, "New (calar) test for bismuth."
Zauodskaya Id.,4 , 835 (1935); Chew. Abdractr, 30, 984 (1936). (4) FEEL AND NemEa. "The detection of the elements of the hydrogen sulfide group with particular regard to spot reactions," Z . anal. &m., 62, 369-84 (1923). (5) COLE."Hexamethylenetetramine as a reagent in qualitative
analysis," Philippine I. Sci., 22, 631-7 (1923); Chen. Abslracts, 17,2686 (1923). (6) K R ~ H O LAND Z WATSEK,"Studies concerning the relation between sensitivity and molecular weight (weighting effect)," Mikrochenie, 19, 55-9 (1935): Chem. Abdrads. 30, 51 (1936).
