The MICRO-DETECTION of GASES and VAPORS EUGENE W. BLANK The Colgate-Palmolive-PeetCo., Jersey City, New Jersey
HE correct identification of a gas, provided a macro sample is available, offers little difficulty However, a search to the experienced analyst. of the miaochemical literature reveals that little emphasis has been placed upon the more difficult identification of micro volumes of gases. Emich (1, 2) describes a so-called "gas chamber" in which may be suspended drops of reagent to be acted upon by a gas evolved within the chamber. Charnot (3) goes into greater detail giving a description of an apparatus and mode of procedure in determining the identity of a gas or vapor by its use. Essentially it consists of a small gas generator connected to a long, capillary tube. Fragments of solid reagents are placed in the latter and observed under the microscope as the gas passes over them, or the gas can be made to pass into a drop of reagent in which a reaction may be produced. The writer has devised an apparatus of great applicability to a wide number of tests calling for various experirnental conditions. Figure 1 shows a small gas generator connected to a gas chamber in which may be suspended a drop of the absorbing cornpound which is being used to determine the nature of the gas issuing
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A
- 100 FIGURE
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1.-APPARATUS BOR THE DETECTION OF EVOLVED GAS
III AN
from the generator. The drop may be suspended from a cover glass or a platinum loop. In the former case, after a short interval, the cover glass is removed, inverted, and the drop of liquid examined under the
microscope either directly or after the addition of a suitable reagent. The formation of a characteristic precipitate may often be observed under the microscope without uncovering the gas chamber. A loose wad of cotton is placed a t ( B )to prevent the mechanical carrying over of material from the generator (D). If the generator is to be subjected to a high temperature, as is the case in the amalgamation test for Hg, the cotton is replaced by asbestos. In arsine tests the cotton is saturated with lead acetate solution and dried before use. The generator may be heated with a miaoburner to effect the decomposition of the material being tested. The rubber bulb is surmounted by a saew clamp so that the tip of the capillary (G) may always be kept full of liquid, thus preventing escape of gas from (D) into (G). A small glass tube (F) may be used to hold strips of test paper, or discs of test paper, if moistened, may be made to adhere to the under surface of the cover glass closing ( A ) . If the gas to be tested is lighter than air the tip of the capillaty is turned upward; if heavier than air a modified tube shown in Figure 2 can be used. In cases where the gas chamRGuRB 2.-MoDIaIBD H,,,T,~, ber is closed with a rubber T,,,.~ stopper as in t h e u s e o f test P A P E R SW H E N T H E papers the capillary ( H ) is EVOLVED GASISHEAVIER THAN AIR loosely inserted a t (E) to (H) is capillary, (T) is Prevent excessive Pressures test paper, (F) is glass from developing in the ap- tube. paratus. A drop of water may be placed in the bottom of ( A ) to prevent the test papers from drying out during the test, or drops of reagents serving to intensify the test may be added. Figure 3 shows a gas chamber which may be used in pressure or vacuum work. I t can also be used as a vacuum micro-desiccator by closing (C) and exhausting through ( A ) . In such use a suitable desiccant is placed on the lower slide and the material to be dried is suspended from the top slide. For pressure work the slides (B) must be held together by rubber bands. The apparatus may be warmed by placing on a steam bath or aluminum block heated by means of a microburner. A stopcock barrel serves admirably as the cell of this form of gas chamber. The two ends of the
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barrel are ground down with coarse emery and finished off with 6ne emery paper. The cell may be attached to the slides forming top and bottom by a heavy stopcock lubricant, or the lower slide may be permanently attached to the chamber by means of Canada balsam.
Many of the m a a o tests involving gases can be readily adapted to mino treatment. Such tests are given in various of the qualitative analysis manuals (4, 5, 6, 7) to which reference may be made for a detailed description. Carbon dioxide may be detected by hanging a drop of Ca(OH)2, Ba(OH)%,or ammoniacal solution of lead acetate in the gas chamber. The latter reagent is very sensitive. In performing this test it should be borne in mind that carbonates of the alkaline-earth metals are converted to soluble acid carbonates by an excess of COz and it is possible to miss the test due to this cause. Warming the hauging drop will cause it to effervesce with escape of COz and reprecipitation of the normal carbonate. Warming may be accomplished by removing the gas chamber from the miaoscope stage and flaming with a micro-burner. HCN can be detected by test paper moistened with a 110 Dm. solution of benzidine acetate and cupric acetate. FIGURK VACUUM A P ~ R A T U POR S THE DETECTTON On exposure to HCN the paper turns blue. Howoa AN EVOLVED GAS ever, this is a very misleading test and apparently the reaction is not specific (8). Pertusi and Gastaldi (9) In use this cell is exhausted through (A) which is claim that the test is specific in the presence of N a then closed with a clamp. The gas is generated in a HP04. It is preferable to absorb the HCN in a drop of generator similar to that shown in Figure 1 with the AgN03 solution with subsequent examination for exception that a cut-off buret is used in place of the crystals of AgCN. The material to be tested is placed capillary (G) and bulb (C). A drop of the liquid re- in the generator, several drops of dilute HzS04(1 :1) are agent or a moist test paper is placed on the top slide, added, and the generator is warmed gently. The AgCN the cell is exhausted through (A), and the clamp is forms as tiny, highly refractive, short, stout prisms or tightened, after which acid is added to the generator rods, or sheaves of very slender needles. A magnificaby opening the buret tap. If the gas to be evolved is tion of 150 to 200X is satisfactory for the examination. liberated by the interaction of solid reactants the buret Chamot (10) mentions the use of (NH4)zS as an absorbmay be dispensed with and the generator heated with a ent for the HCN. After the absorption has continued miao-burner to produce the gas. Or, if the test so a short time remove the hanging drop and evaporate to requires, drops of BIZ, NH40H, etc., may be placed on dryness with gentle heat. Add a drop of FeCla soluthe bottom of the cell and vaporized by exhaustion. tion which will result in the formation of blood red By using an evacuated test chamber the sensitivity of Fe(SCN)3 if HCN has combined with the (NH4)nSto the various tests is considerably increased. give NH4SCN. The advantages of this form of a gas chamber and Fluorine is detected by the turbidity produced in a generator are numerous and worth mentioning. The drop of water by silicon tetrafluoride that has been small volume of the generator permits a very small formed by the action of Ha04 on a dry mixture of a volume of gas to reach quickly the outlet of the capil- fluoride and powdered silica or silicate. The turbidity lary (H) and this, combined with exhaustion as in the is caused by silicic acid (11). Fluorides can also be use of the apparatus shown in Figure 3, enormously tested for by causing Sip4 to react with (NH4)zMo04, increases the sensitivity of the test. The position of whereby a yellow color is produced. the side arm of the generator permits heavy gases to To detect NHs the sample under test is warmed with flow readily into the capillary. Manipulation of the solid KOH and a drop of water and any liberated amapparatus is rapid and easy. Breakage is limited to monia is detected with Nessler's reagent (12). If the the capillary (H)which is a small item. Connection amount of NH3 in the sample is a t all appreciable the of the capillary and generator a t by means of rubber use of the generator test is needless. A blank should be tubing allows of easy insertion of solid reagents if it is run on the KOH and HzO. desired to observe the capillary under the microscope The detection of small amounts of mercury in certain while a t the same time cotton, glass wool, or asbestos solutions can be effected by reduction with powdered is readily inserted in (23). For extended tests involving copper and subsequent amalgamation on gold foil the use of test papers the latter may be kept moist by a (13, 14). To detect small amounts of mercury add a drop of water placed in the gas chamber, or drops of small quantity of powdered copper to the solution of reagents that intensify a test may replace the water. the sample acidified with HCl. After it is judged reThe drop of test reagent may be placed on the lower duction is complete, filter the solution through a miaoslide or bottom of the cell and the capillary tip inserted filter and transfer filter and copper to the generator. into the liquid if desired. In short, this apparatus A thin strip of gold leaf is placed in the capillary tube allows of an exceptional number of experimental condi- and the generator is heated strongly with a small flame. tions to be realized quickly and readily. The generator is closed with a stopper protected with
(a
asbestos paper or preferably by means of a tightly rolled is bright red (18). Other organic compounds have been strip of asbestos. Minute quantities of mercury pro- suggested, such as rosaniline bisulfite (19), strychnine duce a bright silver amalgamation of characteristic ap- (ZO), and Hofmann's violet (21). The writer has only pearance on the gold foil. utilized fluorescein as the basis of a Brz test. The HzS is most readily and commonly detected by the conversion to eosin is rapid and the chmge quite disuse of test papers moistened with lead acetate solution. tinctive. A drop of NHIOH on the paper or the use of NazPbOz Van Eck (22) describes 51 test-paper reactions inin place of lead acetate apparently increases the delicacy cluding tests for the gases ozone, NH3, HCN, Clz, of the test (15). The sample under test is warmed with HCl, Brz, 12, CO, Sop, and HzS. dilute HzS04(1:I). Arsenic compounds are detected by their reduction, SOz turns a mixture of iodic acid (HI03) and starch, in acid solution, by means of metallic zinc to arsine violet to blue. According to Eegriwe (16), if zinc (23, 24). Chamot (23) gives full details for performing nitroprusside is exposed to ammonia vapors, then to this test. HgBn paper can be employed as a test paper SO%,a rose-red color will appear. The same writer for arsine. gives tests for NOz using sulfanilic-acid-cr-naphthyl- Stibine and phosphine are detected by methods analom i n e or a less sensitive reagent based upon the f a d gous to the Marsh test (25). Both blacken moist that benzidine after exposure to NO2 couples with 8- AgN03 paper. naphthol to give a red dyestuff. Conversely, zincnitroFree iodine colors starch solution blue in the presence prusside can be used as a test for NH3. A drop of of a soluble iodide (26). dilute KMn04 is decolorized by SOz. The brief recapitulation of these few tests by no Dimethyl-p-phenylenediaminehydrochloride gives a means limits the applicability of the apparatus but bright red color in the presence of free chlorine (17) and should, on the contrary, serve as a stimulus in developthis may be used as the basis of a test. ing technic and adaptmg this f o m of apparatus to the A very delicate test for BIZ is based upon the fact detection and identification of numerous and more that the latter converts fluorescein into eosin which varied materials. LITERATIJRE CITED
(1) E m m , "Lehrbuch der Mikrochemie," Munich. 1926, p. 41. (2) EMICE, "Mikrochemisches Praktikum," Munich, 1931, p. 23. AND MASON, "Handbook of chemical microscopy," (3) CAAMOT John Wiley & Sons, Inc., New York City, 1931, Vol. 2, p p 15,40. (4) MCALPINEAND SOULE,r'Qualitative chemical analysis." AND JOHNSON, D. Van based upon the text by PRESCOTT Nostrand Co., Inc., New York City, 1933. (5) BASKERVILLE AND CURTMAN, "Qualitative chemical analysis," The Macmillan Co.. New York City, 1917. (6) Goo- AND BROWNING, "Outlines of qualitative chemical analysis," John Wiky & Sons, Inc., New York City, 1925. (7) TREADWELL AND HALL,"Analytical chemistry," John Wiley & Sons,Inc., New Ymk City, 1921. (8) STEVERTS AND H E R M S D O 2.~angew. , Chem., 34,3 (1921). (9) PERTUSIAND GASTALDI, Chem.-Ztg.,37,609 (1913). bc. it., p. 306. (10) C H A M AND O ~ MASON,
AND LUNDELL, "Applied inorganic analysis," (11) HILLEBRAND John Wiley & Sons, Inc., New York City, 1929, p. 596. Ibid.. p. 642. 13) I M . , p. 176. J.Am. Med. Assoc., 68,1693 (1917). 14) ELLIOTT, (15) TRUESDALE, Ind. Eng. Chem., Anal. Ed., 2,299 (1930). Z. anal. Chem., 65, 182 (1924). (16) EEGRIWE, Fimka Kemistsamfundets Medd.., 36, 109 (1927); (17) ALFTAAN, Chem. Ahstr., 23,53 (1929); 25,2071 (1931). Bull. Acod. Roy. Belg., [3], 17,359 (1889) ; LOREN=, (18) SWARTS, et al., Z . nnorg. allgem. Chem., 136,90 (1924). Comfit. rend., 155, 721 (1912). (19) DENIGES. Bull. soc. chim., 9,542 (1911). (20) DENIGES, (21) GUARESCHI, Z. a w l . Chem., 52,545 (1913). (22) VANECK,P. N., Phnrm. Weekblod, 62,365 (1925). LOG. cit.. p. 199. (23) Cnmor AND MASON, AND HALL,lot. tit., Vo1. 1, p. 247. (24) TREADWELL (25) Ibid., pp. 257, 408. (26) Ibid., p. 325.
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