November 15, 1941
TBLE I.
845
F ~ u o ~ o a d e T 8 1DETERMINATION c OF GALLIUM
Addition
MU.
ao AI
30 AI 30 A I 3U AI
KO AI 10 Fell1
Gallium present
MioruRT.Wl 0.0
0.1
0.5 1.0 0.5 1.0
Gallium Found MiW*l7PC-l
0.0 0.1 0.55 1.0 0.4 0.9
ful fluorescence after shaking with 1 ml. of chloroform, a blank being used for comparison: the fluorescence is less than that of 0.1 microgram of gallium. The detection of 0.5 microgram of gallium offers no difficulty in the presence of the following metals (25 to 50 mg. present except as indicated) : alkalies, alkaline earths, silver, beryllium (10 mg.), maguesium, zinc, cadmium, mercury (Hg' and Hd', 10 mg.), scandium (2 mg.), yttrium (3 mg.), lanthanum (2 mg.), aluminum, indium (1 mg.), thallium (TI'), zirconium (1 mg.), cerium (Ce"', 10 mg.), thorium (2 mg.), lead, arsenic (As"), chromium (Cr"'), tungsten (W", 5 mg.), uranium, manganese, cobalt (5 mg.), nickel (5 mg.), platinum (0.2 mg.), and palladium (0.2 mg.). It is believed that less than 0.5 microgram of gallium can be detected in the presence of most of these metals even when they are present in amounts greater than those specified. In the case of tin, antimony, and bismuth, which yield heavy precipitates by hydrolysis, 1 to 2 micrograms of gallium are required for a deiinite reaction when 10 mg. of one of these metals are present. The precipitate in the chloroform makes it difficult to recognize a fluorescence, and these elements are best separated before applying the test. Titanium, columbium, tantalum, and tellurium also should be sep% rated unless present in very small amounts. The metals interfering seriously with the detection of gallium are those already mentioned as forming hydroxyquinolate8 soluble in chloroform under the conditions of the testnamely, ferric iron, cupric copper, vanadate, and molybdate. By reduction or precipitation of these metals as described above, i t is possible to detect 0.5 microgram of gallium in the presence of 30 mg. of iron, 5 mg. or more of vanadium, and
10 mg. of copper. One microgram of gallium can be detected in the presence of 5 mg. of molybdenum. Fluoride reduces the sensitivity of the gallium reaction enormously. In 5 ml. of solution of pH 3.0 containing 10 mg. of sodium fluoride, 2 micrograms of gallium are barely detectable. However, if aluminum is present in sufficient amount, the sensitivity is not reduced. Citrate also inhibits the reaction. Phosphate reduces the sensitivity only slightly. Onehalf microgram of gallium in 5-ml. solntion of pH 3 containing 100 mg. of ammonium monohydrogen phosphate still gives a fluorescence, but a weaker one than in the absence of phosphate. With 10 mg. of ammonium monohydrogen phosphate in 5 ml., 1.0 microgram of gallium gives approximately the same intensity of fluorescence as 0.9 microgram in the absence of phosphate. Quantitative Application Preliminary experiments indicate that the reaction described may be applied to the determination of small quantities of gallium in the presence of relatively large amounts of such elements as aluminum and iron. The results given in Table I were obtained by fluorometric titration. A dilute standard solution of gallium was added to a comparison solution having the same volume and pH as the unknown solution, and containing the same amount of 8-hydroxyquinoline and chloroiom, with shking nfrcr p.wh addition until < l i b rhloroiorm I n w r v in boil, 5olutiont showed the imiw inremit! or fluorrseenec. T h solutions \vcrc nrliirstcd to 011 3.0 u r d ierric iron WIL. reduced with hydroxylamin< hydrochioride. The comparison solution did not contain aluminum or iron. There appears to be but little diminution in the fluorescence intensity of gallium in the presence of moderate amounts of aluminurn or iron. Zinc, however, reduces the intensity of the gallium fluorescence markedly. Thus, 1.0 microgram of gallium in the presence of 20 mg. of zinc (5-ml. volume, pH 3.0) gave approximately as much fluorescence as 0.5 microgram of gallium in a zinc-free solution. Therefore, if gallium is to be determined in the presence of zinc without making a separation, approximately an equal amount of zinc must be present in the comparison solution
Micromethod of Chromatographic Analysis M. O'L. CROWE Division of Laboratories and Research
New York State Department of Health, Albany, N. Y.
T
HE chromatographic method of analysis originated by Tswett (9)and revived by Knbn and associates (6, 6)
has proved a valuable physical method for the isolation and purification of substances from mixtures. Considerable difficulty is sometimes encountered and large quantities of material are required in the selection of suitable solvents, adsorbents, and elutriants for the substance to he investigated. A microchromatographic method that requires only a few drops of material has been developed, and has been used in this laboratory for the past two years for rapid preliminary survey before filtering solutions through columns in preparation for spectroscopic examinations. A number of adsorbents ace placed in the cups of a "spot plate" or cupped porcelain dish such a8 water colorists use. A v e v small
FIGURE 1. TOPVIEW OF PETRI DISH, SHOWINGZONE FORMATION IN ADSORBENT MATERIAL
846
INDUSTRIAL AND ENGINEERING CHEMISTRY
quantity (0.25 teaspoon) of the adsorbent in the powdered form i s placed in the cup and moistened with various solvents; 2 or 3 drops of a prepared creamy mixture of adsorbent and solvent may be used. A drop or two of the solution to be investigated is then laced at the rim of the cup and allowed to flow into the adsorgent; the combination that is most suitable for the particular substance to be studied is determined. Then a glass Petri dish about one quarter full of the chosen adsorbent is gently shaken in a tilted position, so that the adsorbent settles in the form of a wedge that is very thin at the upper edge and a few millimeters thick at the lower edge. The solution that is t o be analyzed is dropped from a 1-ml. pipet into the center of the Petri dish, which is held in a tilted position (Figure 1)so that the solution may flow gently towards the thin edge of the wedge and then filter slowly downwards through the adsorbent. The solvent is added drop by drop to form broad zones of separated material. Experience will determine in each case whether it is better t o allow the solution to flow into the adsorbent in the dry state or when moistened slightly with the solvent.
If material that contains fluorescing substances is used, these spread out into broad semicircular fluorescing zones in the light of a Hanovia analytic quartz lamp. This method is very useful in the analysis of biologic materials when only
Vol. 13, No. 11
very small quantities are available, and has been found more satisfactory than other micromethods (1-4, 7, 8, 10) in the analysis of certain biologic substances.
Literature Cited (1) Brown, W. G.,Nature, 143,377-8 (1939). (2) Flood, H.,Tids. Kjeimi Benpesen, 17,178-9 (1937). (3) GoppelsrBder, F.,Verh. Naturf. Ges. Basel, 19,81 (1908). (4) Iamanov, N.A., and SchraIber, M. S., F a r m t s i y a , 1938,No.3, 1-7; Khim. Referat. Zhur., No.2, 90 (1939). (5) Kuhn, Richard, and Lederer, Edgar, 2. phusiol. Chem., 200, 108-14 (1931). (6) Kuhn, Richard, Winterstein, Alfred, and Lederer, Edgar, Ibid., 197,141-60 (1931). (7) Lou, C. H., Stain Technol., 12,119-24 (1937). (8) Schwab, G.M.,and Jockers, Kurt, Angew. Chem., 50, 546-53 (1937). (9) Tswett, M.,Ber. deut. botan. Ges., 24,238,316,384 (1906);Ber., 41,1352 (1908);43,3199 (1910);44,1124 (1911). (10) Zechmeister, L., and Cholnoky, L. von, “Die chromatographisohe Adsorptionsmethode”, 1st ed., pp. 56, 57, Vienna, Julius Springer, 1937.
Modified Electrometric Determination of Metallic Silver By a Dead-Stop End-Point Procedure R. H. LAMBERT
N
AND
R. D. WALKER, Eastman Kodak Company, Rochester, N. Y.
0 CHEMICAL method is known for determination of out. The authors studied the titration of iodine and arsenite metallic silver in suspensions, such as one would have solutions from both directions and found both end points rein exposed photographic emulsions, if silver is present to the peatable. order of 1 to 10 x 10-9 mole per liter. A volumetric method Experimental Procedure can now be reported for estimating such small amounts of solutions All chemicals to be titrated used werewas of reagent adjusted quality. to pH 8.3 The with alkalinity o.02 of silver which makes use of the polarized-electrode method of Foulk and Bawden (I), This method measures sodium bicarbonate. Solutions were made with freshly distilled silver only and is not affected by the presence of silver ions. conductivity water and stored in Pyrex bottles. The iodine The reactions involved in the- determination are: 2Ag0 11+ 2AgI (1) Is + NasAsOs HzO + NasAs04 2HI (2) The silver iodide formed is dissolved by the excess potassium iodide and any hydrogen iodide is neutralized by the sodium bicarbonate. At such low concentrations of silver, special precautions to reduce errors are necessary. A method of following the titration was developed which allows greater precision to the end point. This paper is an account of these factors. Titration of iodine using the deadstop end point may be divided into four categories-addition of an iodine solution to a reducing agent in an acid or in an alkaline medium; and addition of a reducing agent to an iodine solution which is acid or alka4.0 4.1 42 4.3 4.4 4.5 line. Wernimont and Hopkinson (2) C C . I X10-4N IODINE report that titration of iodine with c c I x IO-^ N ARSENITE FIGURE 2. TITRATION OF ARSENITE sodium thiosulfate in an acid FIGURE:^ TITRATION OF IODINEWITH ARSENITE WITH IODINE medium is not so reproducible as 30 per cent KI, 0.02 M NaHCOs 30 per oent KI 0.02 M NaHCOa when the reverse titration is carried E. m. i., 100 d i l l w o l t e E. m. f., 100 milljvolta
+ +
+
