Diffusion tube

This paper describes the construction and use of a diffusion tube for sampling NO2 from the atmosphere. Examples of its use include the determination of NO2 concentrations at various heights above ground level in an urban environment and through a tree s leaf canopy. 

T07 Stabili d Diffusion Tubes, Heraeus-Amersil, SayreviUe, N. J., 1980. 

WELL-DEFINED DIFFUSION TUBE Fig. 13-4. Static sampler based on the diffusion principle. 

Table 10.18(c) Direct-indicating Drager diffusion tubes - no pump required... 

Gas burners are of various low- to high-pressure (2-25 psi) designs, including gas-ring burners, center-diffusion tube gas burners, and turbulent gas burners. 

Whitaker, S, Role of the Species Momentum Equation in the Analysis of the Stefan Diffusion Tube, Industrial and Engineering Chemistry Research 29, 978, 1991. 

Figure 8.31 l q ratus for preparing standard mixtures of organic vapors. A, exponential dilution flask B, diffusion tube system. 

Within limits, broad concentration ranges can be prepared by varying the tube dimensions and/or the flow rate of the diluent gas. Diffusion tube systems are preferable to permeation tubes when the latter are no fl Mercially available. 

Solution-diffusion tubing, flow through, 15 722, 723 Solution dumps, 9 797 Solution/electrode interface, 9 574-581 Solution-enhanced dispersion by... 

B. 2-Naphthalenethiol. In a 250-ml. flask, fitted with a diffusion tube2 and swept with nitrogen, is placed 23.1 g. (0.10 mole) of O-2-naphthyldimethylthiocarbamate (Note 4). The flask is heated at 270-275° for 45 minutes in a salt bath (Note 5). After cooling, a solution of 8.4 g. (0.15 mole) of potassium hydroxide in 10 ml. of water and 75 ml. of ethylene glycol is added to the flask. The diffusion tube is replaced by a condenser, and the mixture is heated at reflux for 1 hour (Note 6). The cooled reaction mixture is poured onto 150 g. of ice. After the ice has melted, the mixture is shaken two times with 150-ml. portions of chloroform. The chloroform layers are discarded, and the aqueous layer is cautiously acidified with concentrated hydrochloric acid (Note 7) and shaken three times with 75-ml. portions of chloroform. The organic layers are combined and dried by filtration through anhydrous magnesium sulfate. The solvent is removed by distillation to yield 13-15 g. of crude product. Distillation yields 10.3-12.8 g. (71-80%) of pure 2-naphthalenethiol, b.p. 92-94° (0.4 mm.), m.p. 80-81° (Note 8). 

The need for an air sampling pump can be eliminated by use of a diffusion tube having a set length to diameter (L/d) ratio in its geometry for introduction of a gas sample. Proper selection of the geometry and L/d ratio of the diffusion tube results in an electrochemical cell with a response which is independent of external gas flow rate. A schematic of a solid polymer electrolyte diffusion head sensor cell is shown in Figure 13. 

The effect of air flow past the diffusion tube inlet was studied to simulate ambient convection. The effect of external air flows perpendicular to the diffusion tube for two different CO concentrations is shown in Figure Ik. Little variation was seen in the response over the range 200 - 600 ft/min. Response times to... 

Recent work has extended the use of a diffusion based instrument to one in which the diffusion tube has been rotated 90° with respect to that of Figure 13. In this configuration, there is a possibility of flow directly into the diffusion tube. This tube has dimensions L = 1.6 cm and d = O.87 cm. Substituting these values into Equation (2) gives a predicted diffusion cell response of 1.5 jua/ppm CO. Observed values of 1.1 -1.2 jua/ppm CO were again in good agreement with theoretical predictions. 

The last boundary condition states that the flux of A out the end of the diffusion tube at z = —L results in an increase in the concentration of A in the attached reservoir. This problem may be solved by the method of separation of variables to give (C2, p. 107) ... 

A clever means of dynamic generation of standards at the part-per-million level involves permeation through a polymer. In 1966 O Keeffe and Ortman (34) described this technique primarily for air pollution standards. A condensable gas or vapor is sealed as a liquid in a Teflon tube under its saturation vapor pressure as shown in Figure 4.14. After an initial equilibration period the vapor permeates through the tube wall at a constant rate. This rate is determined by weight loss over a period of time. Temperature must be controlled to within .0.1°C to maintain 1% accuracy. In use the tube is thermostatted in a chamber that permits a diluent gas to fully flush the chamber. The concentration is then determined by the same equation used for diffusion tubes. However, since the rate is generally much less in permeation tubes it is usually reported in ng/min. 

Figure 4.13. Cross-sectional diagram of a diffusion tube (courtesy Analytical Instrument Development).

Gibson, L.T., Cooksey, B.G., Littlejohn, D. and Tennent, N.H. (1997b) A diffusion tube sampler for the determination of acetic acid and formic add vapours in ambient air. Analytica Chimica Acta, 341, 11-19. 

In these experiments, and those of Raabe (1968), the levels of airborne activity, and hence of ionisation in the chamber, were sufficient to ensure that decay products were neutralised, by collision with negative air ions, before entering the diffusion tube. 

Porstendorfer Mercer (1979) did similar experiments with 220Rn-laden air at 106 to 109 Bq m-3. Their diffusion tube had a central electrode and deposition was measured with and without an electrical field. Collection on the charged electrode was more efficient in moist than in very dry air (relative humidity, RH < 2%). In moist air, D was 6.8 x 10-6 m2 s 1 irrespective of whether the 212Pb was partially or wholly neutralised before deposition. In very dry air and low 220Rn concentrations, D was 4.7 x 10-6, and it was concluded that the charged component had D equal to 2.4 x 10-6 m2 s 1. Paradoxically, this would correspond to k = 1 x 10 4 m2 V-1 s 1, the mobility found by Jonassen Hayes (1972) for 222Rn decay products in moist air. In all Porstendorfer Mercer s experiments, the ageing time was very short. 

Busigin et al. (1981) considered that values of D for 218Po reported in the literature depend on the ratio of flow rate to length of the diffusion tubes, or on the age of the ion. They measured the efficiency of... 

One of the problems with cold wall systems is the difficulty in maintaining a very uniform temperature on the wafers. Such concern can be eliminated if the entire reactor chamber is placed within a furnace maintained at a very uniform temperature. An ideal candidate for such a furnace is the standard diffusion tube furnace already in wide use for integrated circuit fabrication. If in addition, wafers could be loaded vertically as in a diffusion furnace, the reactor throughput could be substantial. 

The diffusion tube was designed and constructed such that the length L of the upper section is equal to the length of the lower section plusihe length of the bore of stopcock A. Hence the stopcock bore and the lower section of the tube are both filled with CO2. 

The CO2 and He (or Ar) gas tanks are connected to the diffusion tube as shown in Fig. 3. Stopcocks A through Fshould be opened and the line should be pumped on for about 20 min. Close stopcock F and check for an increase in pressure. The rate of increase should be no greater than 5 Torr h . Then evacuate the entire apparatus again for a few minutes. 

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