V O L U M E 19, NO. 2, F E B R U A R Y 1 9 4 7 one can obtain a n exact positional measurement. Manually, this is given by the mean of two values, t., and h,. Tracking circuits are available for positioning the 1inear.delay trigger exactly halfway between these markers automatically. The exact details of each of these operations and the circuits for their achievement are omitted here because they are adequately described in several sources (3, 6). These and still other circuit resources are available for the assimilation and measurement of the patterns without recourse to their visual presentation on an oscillograph screen. Figure 8 illustrates a case in which a series of lines was scanned, differentiated, and used to trigger a microsecond pulse generator The lines in the original pattern were spaced in a square law sequence and were not of uniform intensity. (For purposes of reproduction it mas necessary t o retouch this figure in order to reveal the very narrow lines.) This difficulty is ordinarily overcome by beam intensification. CONCLUSIONS
Brief mention has been made of double scanning, but in general, an elaborate series of patterns can be scanned from top to bottom, if after each sweep the scanning beam is depressed by an appropriate sweep increment along the vertical axis. bionlinear traces have important uses. For example, if a sine wave is phase-shifted and used to produce a circular trace on the
77 scanning oscillograph, a pattern may then be prepared in which a series of radial lines is drawn. As the circular sweep passes over these lines, a series of pulfies will be produced which are integral multiples of the original sine wave frequency. The use of polarizing film also presents the possibility of scanning anisotropic samples. These and other applications are currently under investigation with an improved high-precision instrument. ACKNOWLEDGMENT
These experiments were conducted in the Central Research Laboratory of the General Aniline and Film Corporation. For the facilities and technical assistance the author is greatly indebted to L. T. Hallett, F. A. Hamm, F. C. Snowden, and L. V. M eyers. LITERATURE CITED
(1) Garman, R. L., “M.I.T. Radiation Laboratory Series”, M.I.T. Radiation Laboratory Report 105-3, New York, McGrawHill Book Co. (to be published in 1947). (2) Hexem, J., in ( 1 ) . (3) Lewis, W. B., “Electrical Counting”, New York, Cambridge University Press, 1943. (4) Muller, R. H., in (1). (6) Miiller, R. H., and Garman, R. L., Mikrochemie, 21,302 (1936). (6) Puckle, 0. S., “Time Bases”,’New York, John Wiley I% Sons Co., 1943.
Rapid Determination of Small Amounts of Carbon Monoxide Preliminary Report on the NBS Colorimetric Indicating Gel MARTIN SHEPHERD National Bureau of Standards, U. S . Department of Commerce, Washington 25, D. C.
This condensed report furnishes the minimum preliminary information necessary to make and use the NBS indicating gel for the rapid colorimetric determination of small or physiologically significant amounts of carbon monoxide in air in the field or laboratory. The gel will detect and estimate less than 1 part of carbon monoxide in 500,000,000 parts of air. It will detect 0.001% by volume in less than 1 minute, and determine physiologically significant amounts in approximately 1 minute. Field use requires a small, inexpensive apparatus without maintenance problems, and involves procedures so simple that untrained personnel will be able to obtain reliable results.
T
HE object of this condensed report is to furnish the minimum information necessary to make and use the KBS indicating gel for the rapid colorimetric determination of small amounts of carbon monoxide in air. This preliminary report will serve those who may have immediate use for the indicating gel. A complete report on the bureau’s work on this project is being prepared in collaboration with others associated with the various phases of the investigation: Initial development, Shuford Schuhmann and Mary Ann Somervell Palladium solutions, W. Stanley Clabaugh and Edward Wichers Purification of gel, Richard L. Thomas Production of NBS indicating tubes, Marthada Vaughn Kilday and John H. Eieeman Glass sealing machines, E. R. Weaver
Indicating instruments, Shuford Schuhmann, Harry Bailey, and Robert Thiebeau Applications of the indicating tube, Shuford Schuhmann, E. R. Weaver, and Marthada Vaughn Kilday Color standards, Marthada Vaughn Kilday and Shuford Schuhmann Laboratory methods
Of
analysisi Shuford
The formulation of the gel and equilibrium in the indicating tube are also discussed. DEVELOPMENT, NATURE, AND USE OF INDICATOR
During 1941 the Royal Aircraft Establishment, Farnborough, England, developed an indicator for the detection and estimation
ANALYTICAL CHEMISTRY
78 of small amounts of carbon monoxide on board aircraft ( 3 ) . This indicator was a silica gel impregnated with ammonium molybdate, sulfuric acid, and palladium chloride. A yellox silicomolybdate complex, the heteropoly acid Ht[Si(Pvl0~0~)~], was 'thus formed, and the palladium served to catalyze its reduction by carbon monoxide. The reduction product appeared to be a mixture of oxides, predominantly the blue tritoctoxide, hfo308. Extremely small amounts of carbon monoxide caused the reaction, but the colors developed were too muddy to permit good colorimetric differentiation, and accordingly the length of discoloration in a column of gel was used to measure the concentration of carbon monoxide. This measurement was not deemed sufficiently accurate for acceptance tests of aircraft in this country. In addition, the reduction occurred only when the gel contained an optimum amount of water. Since the gel lost its sensitivity wTith passage of air samples containing more or less water vapor than corresponded to this equilibrium, its general field use was not always practicable. In November 1941, the Bureau of Aeronautics of the Navy Department (and later the U. s. Army Air Forces) requested this bureau to study the RAE (Royal Aircraft Establishment) gel, with the object of improving i t or developing a suitable substitute. The relatively high sensitivity of the RAE gel suggested further investigation in this direction. Since it was entirely practicable to dry small air samples in the field, the first hope was to make an indicating gel sufficiently sensitive in the dry state. When dry air was passed through the yellow R.4E gel, it turned a deep brownish buff and became insensitive to carbon monoxide. The buff color suggested loss of the catalyst by deposition of un-ionized, insoluble palladium chloride according to the dissociation: PdCl, = PdC4 Clp. In starting with the chloride, this would be the inevitable pattern, since no amount of excess sulfuric acid could be expected to substitute the sulfate radical for the stable chloride ion. The indicated course was completely to eliminate the chloride from the start and hope that enough palladous sulfate to furnish the necessary palladium would be retained in the sulfuric acid solution on a nearly dry gel. Accordingly, palladium metal, or the oxide, PdO, was digested in hot sulfuric acid. When this solution, together with ammonium molybdate, was added to a highly purified silica gel, the product was an indicating gel which was highly sensitive to small amounts of carbon monoxide when nearly dry. Dried air passed for hours through this gel did not materially alter its sensitivity. It was about four times more sensitive than the RAE gel. I t yielded a series of clear green to bluish green colors which were easily differentiated, and made possible an analysis by actual colorimetric procedure rather than the measurement of the length of discoloration. The gel was capable of detecting and determining less than 1 part of carbon monoxide in 500,000,000 parts of air in 20 minutes; of detecting as little as O . O O l ~ oby volume in less than 1 minute; of determining physiologically significant amounts of carbon monoxide (0.01 to 0.4%) in about 1 minute a t ground level; and of detecting and estimating concentrations of 0.0025 to 0.0570 on board aircraft in 1 to 5 minutes, depending on altitude and concentration. The color response was a function of the product of time and the concentration of carbon monoxide-a direct parallel to the physiological response to this gas. This NBS indicating gel was first produced in December 1941 and adapted for field use in the form of NBS indicating tubes early in 1942. These sealed glass tubes are 7 mm. in outside diameter, 120 mm. long, and contain, in addition to the indicating gel, guard gels which remove interfering gases and vapors. During 1942 and 1943 the optimum formula of the indicating gel and the optimum equilibrium in the indicating tube T T C ~ Cexperimentally determined. Experimental production of both gel and tuhes was expanded to the scale of a production laboratory, with necessary revision of experimental procedures. Methods for using the gel and tuhes in the field and laboratory were developed.
+
Field and laboratory instruments for using the tubes were designed and models were constructed and put into use. A machine for making and sealing the tubes was developed. Altogether, more than half a million tubes were made for the military services of the United States, Canada, and Great Britain. Of these, about 200,000 were used by these services, and 300,000 were ready for delivery to fill an Army Air Forces request a t the close of the war. The tubes were used in the field to determine carbon monoxide in and around aircraft, hangars, carriers, tanks, gun turrets, flame throwers, garages, and other military equipment and installations. They were used in many laboratories for the accurate determination of small amounts of carbon monoxide in connection with various military investigations. One of the novel uses was the very rapid determination of the degree of saturation of the blood with respect to carbon monoxide. This was done without taking a blood sample from the patient, who had only to exhale alveolar air into a 20-cc. syringe, whence it was drawn through an NBS indicating tube for analysis. Of the instruments developed, a very simple field kit proved the most popular. The sample was drawn through the indicating tube by an ordinary 2-ounce rubber aspirator bulb equipped with a special rate-controlling valve. A spot test could be made in 1 minute; and such tests could be entrusted to untrained personnel. The NBS indicating gel has a wide prospective peacetime application, since it provides a simple, rapid, and inexpensive means for the determination of carbon monoxide in the air and the diagnosis of carbon monoxide poisoning. In either application it can detect and estimate any quantity of physiological significance. The tubes can be modified to serve as detectors of other reducing gases and vapors, including many organic vapors. PREPARATION OF INDICATING AND GUARD GELS
Traces of chlorides or organic matter will spoil the indicating gel. All glassware must be carefully cleaned, preferably with fuming sulfuric acid. Purification ot Silica Gel. The Davison Chemical Corporation's (Baltimore, hld.) KO.697539-600-0 silica gel was used for the preparation of the indicating gel, and its So. 695204550 for the guard gel. Gels should be free from bits of wood, paper, or other foreign matter. Either gel must be purified by oxidation with nitric acid. This has been done satisfactorily as follows: Put approximately 1.45 kg. (3.2 pounds) of gel into a S o . 690 4-liter Pyrex serum bottle, cover 5 cm. (2 inches) with nitric acid (sp. gr. 1.42), and immerse the bottle to its upper shoulder in a steam bath. Digest for 1 week, during which time the level of the acid should be maintained, and free flow of steam kept stirring through the bath. [A 45/50 TS male joint (Trade Standard interchangeable ground-glass joint) should be drawn into a retortshaped delivery tube and inserted in the corresponding grinding of the bottle neck to discharge condensate into a beaker; or a reflux condenser can be used. The average hooded steam bath will accommodate six such bottles, and a special top can be made t o fit them. I Remove the acid by means of an immersion filter attached to a trap and suction. [-4satisfactory filter is made from a 25-mm. porcelain plate with 1-mm. holes, sealed loosely into an 18 X 22 mm. Pyrex tube. Smaller holes tend to plug. In removing acid (or water) from gel, avoid producing a water hammer with the suction. ] Wash the gel with distilled water according to the following schedule, replacing in the steam bath after each complete washing:
01
Complete Changes of Water, with Removal of Water after Each Change 12 7
2 3 4
2
Time after Removal of Acid, Days
On even-numbered days to 20
5 3
2
79
V O L U M E 19, NO. 2, F E B R U A R Y 1 9 4 7 The pH of the wash water after a final 2 days’ leaching in the steam bath should be .5.0 or greater for the KO.697535-600C gel, and 4.9 or greater for the KO.69520-650 gel. Continue washing if necessary to achieve this condition. When proper p H is reached, remove mater thoroughly with the immersion filter, place the bottle in,electric oven a t 100’ C. until gel appears dry, then heat 3 days at 320” C. In the same oven, place a tube containing guard gel. (This tube is made from a TS 45/50 male joint extended 5 cm. a t the small end and then closed except for twenty 1-mm. holes. The large end is rounded and drawn down to a 1-cm. ringed neck which may be grasped with crucible tongs. The whole tube is filled with guard gel previously prepared.) After 3 days, remove bottle of “activated” gel and immediately insert guard tube. When gel is cool, i t is ready for use. Preparation of Guard Gel. The No. 69520-650 gel needs no treatment other than the purification given above. I t is the guard gel used to protect the indicating gel in the final indicating tube. It must be kept dry and free from organic vapors, and must therefore be stored in TS 45 bottles equipped with guard tubes, themselves filled with guard gel. Preparation of Indicating Gel. After purification and activation, the No. 697535-600C gel is converted into indicating gel by impregnating with a palladium and a molybdenum solution, followed by drying in two successive stages. Preparation of the Palladium Sulfate Solution. Heat about 14 grams of finely divided palladium metal a t 600’ C. overnight in a furnace to convert as much as possible to PdO. Transfer this to a 500-ml. Pyrex boiling flask having a thermometer well and TS joint, add 175 ml. of concentrated sulfuric acid, connect a reflux condenser, and heat to boiling (about 300’ C.) until all the palladium and its oxide appear to have been converted to sulfate. At this point, a dark brown, shiny, crystalline precipitate mill be formed. Continue heating, and add water slowly and cautiously (never more than 1 ml. a t a time) through the top of the condenser. If sulfur dioxide I S evolved (as evidenced by its odor), stop adding water and heat a t the then existing temperature until no more sulfur dioxide is evolved. Then slon-ly continue the addition of water, stopping the refluxing if necessary, until all the crystalline precipitate has dissolved and the boiling temperature is approximately 200” C. Cool the solution, filter through a porous porcelain or glass filtering crucible of fine porosity, and dilute to 500 ml. in a volumetric flask. Dilute a 5-ml. aliquot of this solution to 250 ml. and determine the palladium content by precipitation with dimethylglyoxime (2) and the sulfuric acid content by titrating the filtrate and washings from the palladium precipitation with standard alkali, using methyl red as the indicator. Calculate the amount of additional sulfuric acid needed and the volume to which the solution must be diluted to give a final solution containing 0.013 gram of palladium and 0.33 gram of sulfuric acid per ml. Molybdenum Solution. Dissolve 50 grams of reagent quality ammonium molybdate in 1 liter of distilled water. Impregnating the Gel. Mix 750 ml. of distilled water, 450 ml. of molybdenum solution, and 100 ml. of palladium solution in a clean KO.690 Pyrex 4-liter serum bottle. Slowly add to this 2 liters of the purified, freshly activated silica gel, rotating the bottle so that the gel will always be submerged. The bright yellow silicomolybdate complex forms immediately. Allow to stand 2 hours or overnight. Initial Drying. Heat the bottle of impregnated gel to 60” C. in a water bath reduce the pressure in the bottle to about 4 cm. of mercury, and evaporate the excess water During this process, remove the bottle from the water bath every minute or two, and tap i t firmly on a sponge rubber pad in such a way that the solid which tends to coat out on the walls of the bottle is continually moistened and taken up by the gel. When the gel is surfacedry, continue to heat and evacuate, rotating the bottle vigorously i n the water bath every 2 or 3 minutes. At first, when a fresh portion of the surface-dry gel comes in contact with the hot wall of the bottle, the evolution of water vapor is evidenced by the quivering and dancing of the gel. When this finally ceases the initial drying is completed. The gel has now turned from pale to bright yellow. (The time required for this process is about 6 hours.) To reduce the pressure in the bottle for the above process, first connect to it an adapter made from a TS 45 150 male joint draTvn out to a 10-mm. tube pointing downward. (This grinding should fit the corresponding female one in the neck of the bottle well
enough so that when distilled water is used as a “lubricant” no excessive leakage occurs a t a pressure of 4 cm. absolute. About two thirds of the bottles and grindings taken from stock should meet this requirement Sever lubricate this grinding with anything but distilled water. It will operate satisfactorily dry during the last stages of drying.) Connect the adapter to a 2-liter filter flask, and this in turn to an efficient water aspirator. (Arthur H. Thomas, Philadelphia, Pa., KO.8842 nitrometer tubing is satisfactory for the connections.) A mercury manometer may be teed into the line to indicate proper operation of the aspirator. A 25-pound lard can will serve as the water bath, and a rubber tube tied between its handles keeps the bottle submerged t o the proper level betm-een the periodic shakings. Final Drying. Transfer equal portions of the partially dried indicating gel to fourteen 500-ml. round-bottomed boiling flasks having necks 35 cm. long and 2.5-cm. bore which terminate in T S 24/40 joints. (This transfer is facilitated by an adapter made from a TS 45/50 male joint, drawn down and curved like a spout.) Connect these flasks to a manifold having a series of 24/40 female joints sealed as tees to a horizontal tube, which in turn connects through a trap immersed in liquid air to a McLeod gage, a mercury vapor pump, and a mechanical backing pump. (The cold trap should accommodate about 50 grams of ice without plugging.) iipiezon L may be used to lubricate TS joints and stopcocks in this vacuum system, but the pumps must be started immediately after the flasks are connected and some positive flow from the gel to the trap and pump must be maintained thereafter. Evacuate the system until the pressure has dropped to approximately 0.028 mm. of mercury. (This will probably vary somewhat with each ap aratus. The time required is about 20 hours.) Ehut off the pump, gage, and trap, and slowly open the flasks to the atmosphere through a large dryer containing alternate layers of silica gel, Hopcdite, and silica gel. When atmospheric pressure is reached remove the flasks, very carefully remove grease from the maie joints, using only a dry, clean cloth, and transfer the dried gel back into the serum bottle, closing this with a dry TS 45 stopper. Determine the water content of the gel as directed below. Determination of Water Content, Optimum Water Content, and Blending. An empirical test is made as follows: Weigh out 1 to 2 grams of the gel in a conical glass-stoppered weighing bottle, place this in a small Pyrex vacuum desiccator containing purified and activated silica gel, and put this in turn in a vacuum electric oven a t 105” C. for 16 hours while continuously evacuating. Admit air to the cooling desiccator through a drying tube, and weigh the cooled bottle to determine the loss in weight. This test is not a true determination of water, since all the water is not driven off, and some of the acid probably is; however, i t is sufficient for controlling the preparation of the gel. The sensitivity of the gel is a function of ~ t residual s water content, and while gels are sensitive over a fairly wide range-about 2 to 20 mg. per gram-the optimum lies between 8 and 12 mg. per gram. For immediate use in laboratory-type tubes against dry guard gel, reduce the water to 10 or 12 mg. per gram. (See following section for description of laboratory tubes.) For field use after storage for 1 month or longer in sealed tubes against dry gel, reduce the water to 15 or 16 mg. per gram. (Laboratory and field types of tubes are described below.) If the drying process yields gels with less water than corresponds to the optimum, blend these with gels containing more water than the optimum. Blend by mixing thoroughly in 4- or 9-liter serum bottles in the proper proportion by weight (or volume) to produce the desired total water content per gram. The water content of the wet gel should not exceed 100 mg. per gram; and the dry gel should contain no less than 2 mg. per gram. 41low one month’s storage to secure equilibrium. Blending is not necessary if a product of uniform calibration is not required. Thus in the laboratory, determinations may be made with color standards prepared from indicating tubes, and the same gel used for the unknowns. But matching to a definitely set series of colors in the field requires a gel of uniform sensitivity. PREPARATION OF INDICATING TUBES
Laboratory Tubes. Laboratory tubes are for immediate use for accurate determinations. They are capable of yielding within 0.00027, by volume in the range 0 to 0.01% carbon monoxide.
80 These tubes cannot be stored longer than 8 hours. The indicating gel need not be adjusted to a standard sensitivity as long as one lot of gel is used to make the color standards and tests of the unknowns. Use 7-mm. clear glass tubing without noticeable color (Pyrex is suitable and most easily worked). Cut into 15-cm. lengths and fire-polish. Clean with fuming sulfuric acid, or by heating a t 550” C. for 1 hour, or select freshly drawn tubing which has been carefully protected from dirt. Plug a 000 cork in one end, insert loosely a small pad of absorbent cotton, and then fill alternately and in order with 4 cm. of guard gel, 15 mm. of indicating gel, and 3 cm. of guard gel. Insert a second cotton pad and tamp loosely to hold the gel in place, then close with a second 000 cork. Glass-stoppered pear-shaped separatory funnels (Pyrex No. 6400) with dry TS 4 stopcocks make convenient gel venders. A system of dosing cocks may be made from T S 19/22 male joints sealed to TS 4 stopcocks. The 19/22 joint plugs into a T S 19 reagent bottle (Pyrex No. 1500) which holds the gel. Two cocks can be s aced to form a pocket of the proper size for a dose of guard gef A solid T S 4 stopper can be drilled with a pocket of proper size to hold a dose of the indicating gel. The day’s supply of laboratory tubes may be stored in a desiccator over guard gel.
NBS Indicating Tubes for Field Use. Tubes for field use are for the rapid determination of any physiologically significant amount of carbon monoxide. The colors developed are matched against painted chips, and the indicating gel must accordingly be adjusted to a standard sensitivity. Use the same size of tube, but for convenience cut 23-cm. lengths, and clean and fire-polish as before. (A special arrangement with the glass works and the use of the third method is recommended.) Draw down one end of the tube to a tip about 1cm. long and 1 to 1.5 mm. i n outside diameter. The length from the rounded shoulder to the end of this tip should be nearly 15 mm. (If many tubes are to be made, a glass-sealing machine will be necessary.) Heat in an electric oven to expel any condensed water. Insert a rectangular piece of KO.ECClB 1 X 0.007 inch Fiberglas tape, cut 1 X ’/16 inch, which has previously been cleaned by heating i n an electric furnace to 550” C. for 2 hours. Tamp this firmly into the sealed end with a clean 4.5-mm. steel drill rod. (A large number can be prepared and stored in clean, tin-plated 25- or 50-pound lard cans.) Fill the tubes with alternate layers of guard and indicating gel as follows: 4.5 cm. of guard gel, 14 mm. of indicating gel, and 2.5 cm. of guard gel. Insert a second Fiberglas plug and tamp firmly against the gel. Tap down the gel thoroughly by vibrating the tube held vertically. (A standard shaking machine can be used for large batches.) Tamp down the glass plug a second time. Seal off the tube 2 om. or more away from the gel, drawing into a second tip like the first. I n handling large batches of tubes, large desiccators containing a layer of guard gel offer satisfactory temporary storage. However, the tube should be sealed within an hour after filling. The room where this work is done should be free from carbon monoxide, organic vapors, reducing or oxidizing gases such as ethylene, acetylene, and hydrogen sulfide, or ozone and nitrogen peroxide. The humidity should not be excessive, and air-conditioning is helpful. If a glass-sealing machine is used, proper mechanical adjustment will always keep the tip of the flame away from an open end of the tube. Unless care is taken in sealing off by hand, the carbon monoxide generated in the partial combustion caused by contact of flame with cold glass will usually ruin the indicating tube. If the indicating tube is properly prepared, i t should remain useful for severd years. Just how long is not determined, but tubes made 4 years ago still retain their sensitivity, and probably will continue to do so. CO WRIMETRIC DETERMINATION OF CARBON MONOXIDE WITH NBS INDICATING TUBES
The color response of the indicating gel is a function of the product of time and t.he concentration a t any constant rate of
ANALYTICAL CHEMISTRY
flow,and most of the analpical methods used or proposed have taken advantage of this fact. A fairly close estimation of the concentration of carbon monoxide in an unknown may be made by simply measuring the time (at constant flow) necessary to produce a standard color. A more accurate determination can be made by matching an unknown with a set of standard colors produced by exposing the tubes themselves to knova concentrations. This method is described below. Still another possibility is to make all the carbon monoxide contained in a measured sample react completely B-ith the gel. Many forms of apparatus, some of them remarkably simple, have been used or suggested. Only two are described here. Interfering substances can be strong reducing or oxidizing gases or vapors which are present i n amounts sufficiently large to escape the inlet guard gel. The Geld tubes were made to remove any interfering substances ordinarily present in any of the atmospheres associated with military equipment or installations, and are probably good for any ordinary peacetime use. However, this should be determined in special cases. Curiously enough, the indicating gel has so little sensitivity toward hydrogen that its presence in large amounts does not interfere with determinations of carbon monoxide. It is obviously possible to modify the indicating tube so that it will serve to detect and/or estimate the presence of other reducing or oxidizing gases. Hopcalite will serve to remove any carbon monoxide present, and an anhydrous calcium sulfate can be substituted for the guarding silica gel. This combination will detect many organic vapors or unsaturated gases. The oxidation of a previously reduced tube is also possible, A Laboratory Method. Air to be tested is passed through an KBS indicating tube (laboratory type) a t 90 ml. per minute for a definite period. The color obtained is compared with freshly prepared standards, which are themselves indicating tubes exposed to known amounts of carbon monoxide. In the range 0 to O.Olyoby volume, carbon monoxide can be determined with a reproducibility of 0.0002% and a probable accuracy of 0.001% or better. Above O.Ol%, the sample may be diluted with a known proportion of carbon monoxide-free air if sufficient differentiation of the darker colors proves difficult. Apparatus. One calibrated flowmeter registering 90 ml. per minute, two or three additional flowmeters for diluting samples in known proportions with carbon monoxide-free air, one stop watch, standard mixtures of carbon monoxide in air under pressure (see below), and laboratory-type indiaating tubes. Procedure. Pass the sample to be tested through a freshly prepared indicating tube a t a measured rate of 90 ml. per minute. Note time (in seconds) required for approximately matching the color developed by exposure to o . o o 5 ~ 0carbon monoxide for 50 seconds (an easy color to match). The composition of the unknown can now be approximately computed. Example: Unknown developed the “0.005 %-50-second color” in 95 seconds. Concentration of unknown was therefore 50/95 X 0.005 = 0.0026 (approximately). Now prepare a set of color standards from freshly filled indicating tubes and the 0.005% standard mixture, using the knowledge derived above in order to bracket the unknown with lighter and darker colors, and taking the nearest 10-second interval as the exposure time for the unknown. To continue the example given above: Exposure time for unknown is conveniently selected as 100 seconds. Since i t is approximately 0.0025%, to bracket i t would require ex osure of 0.005% for 50 seconds aa a base time, with greater andyesser exposures. With 0.005% for 50 seconds as a base, each 1-second difference would correspond to 0.0001% carbon monoxide. If tubes were then exposed to this concentration for 46, 48, 50, 52, and 54 seconds, this standard set would represent the percentages 0.0046, 0.0048, 0.0050, 0.0052, and 0.0054, respectively, on the 50-second base, but only one half these amounts on the 100second base-0.0023 0.0024, 0.0025,0.0026, and 0.0027. The unknown, ex osed for 100 seconds, is now compared with this standard set. guppose its color lies between the 52- and 54-second tube, or between 0.0026 and 0.0027%. It would be reported as 0.00265 * 0.00005 During the determination of the order of the color intensity of these tubes, the identifying labels should be obscured. If the
’
V O L U M E 19, NO. 2, F E B R U A R Y 1 9 4 7
81
observer then ananpes the color standards in their prouer order,
To prrl,arc a mixture, fill a .mnll ryliudcr u i t h enrhon monoxide of known ~niriiy,let stand ovrrntghi, cvxuatL'. refill to slightly ovpr xtmoipl.eric pwssurc, opvn in atrno&pl.en.lhrough O,:$-?m. t ~ rm. (1 ynnl) long, close (0.125-incli, OutsiJr diameter t u h i ~ 00 $:vlindcr. rind read banmetcr and cvlinder iemucrarura (with
~~~~~~~~~~~~
~
~~~~
~
~
~
~~
Ig the 1 per cent stbck'solution for thi 'Lpure"~&rbon monoxide, and again compress t o 99 atmospheres gage with carbon monoxidcfree dried air. Other desired dilutions may be similarly prepared. Compute composition according to the relation:
P A 14.696 Per cent co = co - 760 PG 15
+
f
2 TI
P A = atmospheric pressure (barometer when clhsing cylinder PG fs
f,
before adding nitrogen or air) final gage pressure in pounds per square inch = valve of PVjnRT for Ns (or air) a t 100 atmospheres and T, = value of PVjnRT for CO (or NJ a t 1 atmosphere and =
T,
T, = final cylinder temperature (" C. absolute) TI = initial cylinder temperature (" C. absolute) Figure 1. Coloi Chart and Instruotion Card Mix contents of cylinders by inclining at 45" and heating lower end to ahout 60' C.while cooling upper end to about 15" C. Mixing is usually complete in 16 hours. I n actual practice, if the accuracy indioated above is t o be realized, the standard color tubes should be prepared from a mixture containing 0.0025% carbon monoxide in air. The color developed is practically, but not exactly, determined by concentration of carbon monoxide times duration of exposure. If an exposed tube is immediately flushed with carbon monoxidefree air, thus removing unreacted residual carbon monoxide, the expressed relation is more nearly exact. If a 0.005% standard is used to make color tubes for determinations in the range 0 to O.Ol%, a reproducibility of 0.0002% and an accuracy of 0.001% or better are easily achieved, however. As the concentration of carbon monoxide increases, the colors developed become darker and are more difficult to match. Reduction of the exposure time with some loss in reproducibility and accuracy will serve for awhile; hut above O.OZ%, dilutionis comforting and, with increasing concentrations, is &ally new=
Field Method. A %ounce aspirator bulb equipped with a ratecontrolling valve was the most popular field instrument during the war. This device poweased three noteworthy advantages: It was cheap and readily available, it worked, and apparently anyone could use i t successfnlly. Figure 1 shows the color chart and instruction card furnished with this bulb-type indicator. The instructions are self-explanatory, and serve t o show the simplicity and speed of operation. The black and white reproduction of the colors does no more than suggest the ease with which colors can be differentiated and mrttched. The change from bright yellow to bright greens, and W y bluish greens, is indeed decisive; and the colored chart which is actually used has enabled even untrained observers t o reach the correct colorimetric conclusions.
S W .
Prewation of Standard Mixtures. PreDare and test carbon monoxide according to the methods p r e ~ o u s l ydescribed ( I ) . [Option: For most work, carbon monoxide containing hydrogen as its only significantimpurity, and whose purity is 98% or better when determined according to (Z), is satisfactory.] Prepare the mixture in steel cylinders (ICCSA) whose inner walls are clean, and free from rust, scale, or pits. Valves must be tight, and preferably of the diapbragm packless type. Dry cylinders thoroughly by alternate evacuation and admission of dried air. Make mixtures in a room whose temperature varies no more than 1" C. during the preparation. M o w cylinders t o stand 2 hours before significant pressure readings are taken and check pressure thereafter t o assure constancy within the deiired significance. Use laboratory test gage calibrated to 1400 kg. per sq. meter (2 pounds per square inch) and good to 4 pounds per square inch
ACKNOWLEDGMENT
Nearly all members of the Gas Chemistry Section of the National Bureau of Standards were engaged in this project at one time or another during the past. Those who were mainly concerned and who will he the authors of various sections of the forthcoming complete report are listed on page 77. LITERATURE CITED
..
(1) Branham. J. R.. Sheoherd. Martin. and Schuhmann, Schuford. J . Rm&h Nhtl. BUT.Standards. 26. 671 (1941). (2) Gilchriat. Raleigh, and Wichers, Edward, J . Am. Chem. Sac., 57. 2565 (1936). A procedure for separl~tionof the sin platinum metals from one another and far their grsvimetric 6etermha-
tion.
(3) MainSmith. J. D.. Royal Aircraft Establishment, Reppmt 324 (August 1941).
CE
