Abstract:
An improved detection reaction for benzene in gas analysis with a gas detector tube formed of a carrier material impregnated with gold(III) oxide in a display layer.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of priority under 35 U.S.C. §119 of German Patent Application DE 10 2010 012 468.0 filed Mar. 24, 2010, the entire contents of which are incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention pertains to a gas detector tube and to a process for determining benzene. 
       BACKGROUND OF THE INVENTION 
       [0003]    The gas analysis of benzene gas mixtures by means of gas detector tubes has hitherto been carried out colorimetrically on the basis of p-quinoid compounds with paraformaldehyde/sulfuric acid or with iodine pentoxide. The determination with I 2 O 5  yields full, sharp display, which can be read by the user with certainty. The sensitivity of 2 ppm of benzene and the selectivity against, e.g., gasoline hydrocarbons are, however, insufficient. 
         [0004]    A process for detecting benzene in gases, which comprises the reacting of the gas to be tested with a mixture of paraformaldehyde and anhydrous sulfuric acid, which said mixture is preferably applied to silica gel, is known from DE 865 384 B. The sensitivity of the detection reaction can be increased by adding dimethyl sulfate. The detection on the basis of paraformaldehyde/sulfuric acid is selective and sensitive enough in its application with a detection limit of about 0.5 ppm of benzene. However, the display accuracy is insufficient because of a diffuse change in color, so that the user can read the concentration displayed in an only poorly defined manner. In addition, the measurement times with 20 minutes and 20 strokes are not user-friendly and not accepted in the container fumigation market. Said tubes cannot be used for release measurements, e.g., in the container fumigation market because of poor sensitivity, reading accuracy, measurement time and selectivity. A qualitative measurement, which provides information on whether a container can be safely accessed or not, is required for this application. 
       SUMMARY OF THE INVENTION 
       [0005]    The basic object of the present invention is to provide an improved detection reaction for benzene in gas analysis with a gas detector tube as well as a corresponding process. 
         [0006]    According to the invention, a gas detector tube is provided for detecting benzene. The detector tube comprises a display layer which consists essentially of a carrier material impregnated with gold(III) oxide. 
         [0007]    According to a further aspect of the invention, a process is provided for detecting benzene in a gas sample. The process comprises the steps of providing a gas detector tube with a display layer which consists essentially of a carrier material impregnated with gold(III) oxide and passing the gas sample into the gas detector tube such that the gas sample is reacted with the carrier material impregnated with gold(III) oxide. 
         [0008]    According to still another aspect of the invention the process further comprises using the gas detector tube to detect benzene in the gas sample by observing a change in color in the display layer. 
         [0009]    Gas analyses with gas detector tubes, which have a display layer based on gold(III) oxide, yield a sharp, easy-to-read, selective blackish-purple display. Gasoline hydrocarbons, e.g., 800 ppm of n-octane, do not interfere with the display of benzene, and aromatics, e.g., toluene or xylene, are retained in a benzene-permeable absorption layer acting as a prelayer and displayed to the user qualitatively with a brownish-purple change in color. 
         [0010]    The gas detector tube also has, in addition to the benzene-permeable absorption layer, a dry layer, which is arranged upstream of the absorption layer. 
         [0011]    The dry layer consists in the known manner of silica gel impregnated with 100% sulfuric acid. It binds the humidity of the air and protects the moisture-sensitive absorption and display layer. 
         [0012]    The absorption layer consists of silica gel, which is impregnated with 100% sulfuric acid and selenium dioxide. It oxidizes toluene and xylene, so that these cannot enter the display layer and react with gold(III) oxide there. The presence of toluene and xylene is qualitatively displayed in this layer with a brownish-purple change in color. The absorption layer is already used in prior-art gas detector tubes with another detection reaction. 
         [0013]    The display layer consists of quartz glass grit impregnated with 100% sulfuric acid and gold(III) oxide. The gold(III) oxide dissolved in sulfuric acid reacts with benzene into colloidal gold and is selectively displayed with a blackish-purple change in color. Based on the high price of gold oxide and the especially complicated preparation of the carrier by means of cleaning with aqua regia, the display layer is filled only to the extent, approximately 13 mm, that only a concentration of, e.g., 1 ppm can be displayed with 6 strokes with a stroke volume of 100 mL after 4 minutes. Concentrations of, e.g., 0.5 ppm and 2 ppm are correspondingly displayed with 12 strokes and 3 strokes. 
         [0014]    Since the quantity of display layer needed is limited by the short filling, the tube can be manufactured cost-effectively despite high manufacturing costs. Alternative materials for the carrier materials are gelatinous silica and silica gel. 
         [0015]    The display layer advantageously has a length between 10 mm and 15 mm at an internal diameter of the glass flask of between 1 mm and 3 mm. The length of the display layer is selected to be such that a change in color takes place in the display layer at a sample gas volume of approximately 1.2 L at a benzene concentration of 0.5 ppm. 
         [0016]    The process according to the present invention for detecting benzene in a gas sample is characterized in that the gas sample is reacted with a carrier material impregnated with gold(III) oxide. Gold(III) oxide dissolved in sulfuric acid is used for the impregnation. 
         [0017]    An advantageous use of a carrier material impregnated with gold(III) oxide in the display layer of a gas detector tube is the detection of benzene. 
         [0018]    An exemplary embodiment of the gas detector tube according to the present invention is shown in the figure and will be explained in more detail below. The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0019]    In the drawing: 
           [0020]      FIG. 1  is a schematic view showing a gas detector tube for detecting benzene. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]    Referring to the drawing in particular, the only figure schematically shows a gas detector tube for detecting benzene. The gas detector tube includes a tube structure that allows flow therethrough, in a flow direction indicated by the arrow in  FIG. 1 . The gas detector tube has, in the direction of gas flow indicated by the arrow, a dry layer  1 , an absorption layer  2  and a display layer  3  impregnated with gold(III) oxide. The sorption layers—the dry layer  1 , the absorption layer  2  and the display layer  3 —are held by a ceramic holding element  4  and by a wire hat  5 . 
         [0022]    The display layer  3  consists of a carrier material impregnated with the gold(III) oxide. The carrier material consists essentially of one or more of quartz glass, gelatinous silica or silica gel. The impregnation is gold(III) oxide dissolved in sulfuric acid. 
         [0023]    The absorption layer  2  is for oxidizing toluene and xylene. The absorption layer  2  is arranged directly upstream of the display layer  3  in the embodiment of  FIG. 1 . The absorption layer  2  may advantageously consist essentially of carrier material impregnated with sulfuric acid and selenium oxide. 
         [0024]    The dry layer  1  is arranged upstream of the absorption layer  2 . The dry layer may advantageously consist essentially of a carrier material impregnated with sulfuric acid. 
         [0025]    The display layer  3  may advantageously have a length between 10 mm and 15 mm at an internal diameter of between 1 mm and 3 mm. The length of the display layer  3  is selected to be such that a change in color takes place in the display layer  3  at a benzene concentration of 0.5 ppm at a sample gas volume of 1.2 L. 
         [0026]    According to the process of the invention, the gas sample is reacted with a carrier material impregnated with gold(III) oxide. The gold(III) oxide dissolved in the sulfuric acid is used as the impregnation. 
         [0027]    While specific embodiments of the invention have been described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.