Determination of Nitrogen Oxides in Air Permanent Color Standardsfor Use with Method of Patty and Petty P. H. IVERELL, W. F. K t R T , N. T. WOODBERRY, AND W. R. BR-IDLEY Stamford Kesearch Laboratories, American Cyanamid Company, Stamford, Conn. CALIBRATION OF CELLOPHANE STANDARDS
COLORIMETRIC field method for estimating the content
A of nitrogen oxides in industrial atmospheres has been de-
The cellophane standards were calibrated by comparing with nitrite standards prepared as described by Patty and Petty.
scribed by Patty and Petty (1). The method is based on the hydrolysis of nitrogen dioxide or tetroxide t o nearly equimolecular amounts of nitric and nitrous acids in a reagent containing anaphthylamine, sulfanilic acid, and acetic acid, with which the nitrite ion reacts t o give a red color; nitrous oxide and nitrogen pentoxide are not acted upon by this reagent, but are not commonly encountered in appreciable quantities in industrial atmospheres. A 50-ml. ground-glass syringe (B-D, Yale) serves as container for the reagent, sampling device, reaction vessel, and color comparator tube. A field kit was designed to contain six loaded syringes, six standards (made from a standard sodium nitrite solution), and bottles of solutions of the two reagents. Since the color produced by the reaction fades considerably upon standing several hours, it is necessary to carry a standard nitrite solption in the kit to make fresh standards daily. This, in turn, necessitates carrying a larger stock of the reagent solutions for making up the color standards and also adds to the time of preparation for each test. It was suggested by Patty and Petty that the use of permanent glass color standards of suitable form nould cut down the size of the field kit as well as the time for preparation.
Approximately 10 ml. of the colored solution were placed in the syringe, the solution was agitated so as to wet the ground-glass surface above the solution, and the mounted strip was held in front of the syringe, so that the clear half was over the colored solution, the dyed cellophane over the empty space above the solution, and the boundary just a t the surface of the solution. In this way, the blank due to the plastic and the syringe walls was automatically canceled out. Different solution standards, progressively differing by 5 p.p.m., r e r e compared until a match was made, and the strip was then properly labeled. The comparison was made against a background of diffused daylight-Le., looking through a window at the sky, with a thin sheet of white paper directly behind the syringe and touching it. An artificial source of diffused white light would probably Serve equally well. TEST PROCEDURE
The procedure for testing air for nitrogen oxides is exactly the same as described by Patty and Petty up to the actual evaluation. .4fter 10 ml. of the mixed reagent hsve been placed in the syringe, 50 ml. of air drawn in and the mixture shaken and allowed to stand about 10 minutes, the dyed cellophane standards are compared until the closest match is found.
PREPARATION OF PERMANENT STAhDARDS
STABILITY WITH TIhlE
K i t h the aid of a General Electric recording spectrophotometer, the authors founda suitable blend of dyes with which to dye a series of cellophane strips to different depths of color, so as to match a series cf color standards corresponding to 5, 10,20, 30,40, and 50 p.p.m. of nitrogen dioxide in 50 ml. of air a t 26" C.
Two identical sets of cellophane standards were made, one of which was kept in a sealed envelope, while the other was used in the field as occasion demanded for evaluating the nitrogen dioxide content of industrial atmospheres. The latter set was thus given a limited exposure t o indoor diffused daylight or artificial light over a period of 2 years. At the end of this time, both sets of cellophane standards were compared with freshly prepared nitrite standards. Any differences in depth of color between the cellophane standards and the fresh sloution standards were imperceptible, and the two sets were likewise practically indistinguishable. It seems safe to conclude, therefore, that these color standards are sufficiently stable for practical purposes. Frequent or prolonged exposure to direct sunlight Rill cause some degree of fading in practically any dye, and is not recommended. The blight darkening of the cellulose nitrate in the mount, which occurs over a period of time, has no cffect on the results obtained, because of the self-contained blank.
Small squares (7.5 X 7.5 cm., 3 X 3 inches) of non-moistureproof cellophane (du Pont 600) were treated for various lengths of time in the following dye bath: Calcomine Fast Red 8 B n Calcomine Sky Blue F F , Ex Conc.a Sodium chloride Total volume of bath Temperature
25 nig. per liter 0 . 2 mg. per liter 5 25 grams per liter 2 liters 800 C .
a Made by Calco Chemical Division, American Cyananlid Co., 30 Rockefeller Plaza, S e w York, !i Y..
In order to ensure uniform dyeing, the strips of cellophane were held on special glass frames and the dye bath was kept in motion, so that the strips would not stick t o one another. As the strips were removed, they were rinsed in distilled water and stretched over thc mouth of a wide-mouthed bottle to dry as a smooth sheet, free from wrinkles. The following results were obtained after the diffusion rates of thc two dyestuffs were ascertained by preliminary experiments. Dyeing Time, M i n . 2 3 7 11.5 17.5 25
EFFECT OF TEVPERATURE
As stated by Patty and Petty, the speed of the color reaction varies inversely with the temperature, the time for maximum development of color being 30 minutes a t 4" C., 10 minutes a t 25' C., and 5 minutes a t 50" C. While the value of the nitrite solution standards in p.p.m. of nitrogen dioxide by volume is calculated on the basis of tests taken a t 25" C., actually a variation of 20" on either side of 25" C. would cause a difference of less than 4 p.p.m. in the 50 p.p.m. standard.
Equivalent I'.P.hZ. o! XOz per 50 1\11. of Air 5 10 20 30 38 45
The dyed cellophane strips were laminated with 3 X 5 cm. pieces of cellulose nitrate sheet in such a way that oGe lengthwise half (1.5 X 5 cm.) of the piece contained the dyed cellophane, while the other half was the clear plastic. The lamination was done by wetting the inner edges of the two sheets of plastic &
