Acidimetric Method for Determination of Carbon Monoxide in Air CH4RLES H. LIYDSLEY'
.JOHN H. YOE, University of Virginia, Charlottesville, V u .
An acidimetric method for the determination of low concentrations of carbon monoxide in air is described. The monoxide is oxidized, resulting dioxide absorbed in an excess of barium hydroxide solution, and excess alkali measured by titration with a standard solution of oxalic acid. Appreciable error may be introduced by the use of phenolphthalein as indicator (color change at pH 7.8 to 8.0), owing to action of acid on the finely divided barium carbonate suspension. The magnitude of this error was determined by using thymolphthalein (color change Comparison with at pH 9.0); allowance is made by a correction of +O.OOlc/o. values of the iodine pentoxide method and with known values for samples supplied by Sational Bureau of Standards showed results accurate within 0.001%.
UMEROUS methods and iristrunients have been developed for the rapid and accurate deterniination of small amounts of carbon monoxide in air. Thermometric methods using the heat of oxidation of the gas (4,6 ) and colorimetric procedures involving formation of a blue compound with palladous silicomolybdate (15)require calibration with gas nlixtures containing known amounts of carbon monoxide. One of the best chemical methods uses iodine pentoxide to liberate iodine quantitatively in contact with carbon monoxide a t elevated temperatures (3, 16). In other methods carbon monoxide, first bound to hemoglobin, is liberated and measured gasometrically (11, 14). In one niicromethod the monoxide is catalytically oxidized to dioxide, which is determined gravimetrically (12); another is based on the loss in weight of red mercuric oxide upon reaction with carbon monoxide (1). The mercury vapor liberated in the latter reaction has recently been employed in an instrument for the niicrodetermination of carbon monoxide in air (8); the mercury-containing air reacts with a selenium sulfide test strip, producing a black coloration proportional to the quantity of monoxide in the original sample. A method requiring only readily available equipment is offered here for determining concentrations of carbon monoxide t ( J 0.005% with 0.001% accuracy.
Oxalic Acid, 0.00447 Molar. T w o solutions of about equal concentration were used. The "bulk acid'' was used only in 100-ml. portions, and was added to the barium hydroxide after absorption of carbon dioxide in order to neutralize most of the alkali. Its concentration need be known only approximately, because the same volume was used in both the blank and the actual determination. The concentration of the "standard acid," however, must be accurately known, as it waj used for the final titration. For each liter of solution, 0.563 gram of oxalic acid dihydrate (reagent grade) was dis-jolvedin water. If the crystalline material is of sufficient purity, this weight per liter may be assumed to give 0.00447 molar acid. The concentration was checked occasionally by comparison with a standard hydrochloric acid. One milliliter of this acid was equivalent to 0.1 ml. of carbon monoside a t N.T.P.
c
The carbon monoxide is oxidized by passage over the catalyst hopcalite, a specially prepared mixture of copper and manganese oxides (5,7, IO). The resulting carbon dioxide is determined by a modification of the method described by Dennis ( 2 ) : it is absorbed in an excess of barium hydroxide solution, and excess alkali is measured by titration with a standard solut,ion of oxalic acid. MATERIALS AND SOLUTIOlYS
Hopcalite. The instrument grade catalyst, 14- to 18-mesh, prepared by the Mine Safety Appliances Company was used. Mixtures of Carbon Monoxide in Air. Carbon monoxide was prepared in the usual way by reaction of formic and sulfuric acids, and the diluted mivtures in air wcrP ma+ up using suitable calibrated volumetric apparatus. Several samples of gas mixtures in steel cylinders were obtained from the Leeds 8: Northrup Co., Philadelphia, and the Kational Bureau of Standards. Barium Hydroxide, 0.005 Molar. For each liter of solution, 1.58grams of barium hydroxide octahydrate and 0.25 gram of barium chloride dihydrate were dissolved in water. The barium chloride was added to reduce the solubility of precipitated barium carbonate. The solution was kept in a stock bottle guarded by a soda-lime tube, and delivered t o a buret with a side-entering tube t o prevent contact with air, and a soda-lime tube attached a t the top. The solution was allowed to settle 24 hours in the stock bottle before use. 1
Present address, Institute of Textile Technology, Charlottesville. V a .
Figure 1.
J
Analytical Train for Determination of Carbon Monoxide in Air
A , s o d a lime; B, calcium chloride; C, Ascarite; D Dehydritei hopcalite (2 9.) heated i n water bath; F , G, sampling fiasks. Flowmeter m a y b e connected to exit tube
Indicators. Both phenolphthalein and thymolphthalein solutions were used. The former was prepared by dissolving 1 gram of the solid in 60 ml. of ethanol and adding 40 ml. of water; the latter was obtained as a 0.57, solution from the Lahlotte Chemical Products Company, Baltimore. APPARATUS AND PROCEDURE
Water vapor lowers or destroys the activity of hopcalite and hence must be removed from the air before the air comes in contact with the the catalyst. Carbon dioxide present in the air must also be removed. To accomplish these ends the adsorption train is used (Figure 1). Soda lime removes most of the carbon dioxide, and the calcium chloride holds back most of the moisture taken from the soda lime. Ascarite completes the removal of carbon dioxide, while magnesium perchlorate (Dehydrite) is used to dry the air thoroughly. A band of indicating desiccant, such as Drierite or anhydrous silica gel impregnated with cobalt chloride, may be placed in the last drying tube to give warning when the desiccant is nearly spent. After drying, the air is led through the catalyst tube, made of 7-mm. Pyrex, containing 2 grams of hopcalite. iilthough hopca5 13
ANALYTICAL CHEMISTRY
514 lite oxidizes carbon monoxide rapidly a t room tcmperature, the life is greatly prolonged by immersing it in a bath of boiling water. The air then passes through two or more 2- or 3-liter Erlenmeyer sampling flasks arranged in series. Suction applied a t the outlet of the last flask draws 1 to 1.5 liters per minute of air through the system (a water aspirator is convenient). A flowmeter placed after the last flask may be used in regulating the air flow. As air is drawn through the train, the air initially present is swept out, and the sampling flasks become filled with dry air containing only carbon dioxide, resulting from oxidation of the monoxide. Experience has shown that when 10 liters of air have been drawn through the train for each 3-liter flask, each flask contains a sample of gas suitable for analysis. The flasks me then removed from the train and the ends of the inlet and outlet tubes are immediately closed. The rubber stopper in each flask has a third hole in it (not shown in Figure 1) closed by a tightly fitting glass plug, through which reagents are added. This hole should be large enough to give a close but not a tight fit about the pipet and burets used. When sampling is complete, 100 ml. of barium hydroxide and 4 drops of indicator are added, and the plug is immediately replaced. Complete absorption of carbon dioxide is effected by vigorous though intermittent agitation with a swirling motion for about half an hour. [If the caibon monoxide content is known to be low (
