APPENDIX: USE OF PORTABLE ORSAT APPARATUS

DESCRIPTION OF APPARATUS

The portable Orsat apparatus shown in figure 18 consists essentially of a burette, e, four pipettes, a, b, c, and d, and manifold j. The burette has a total capacity of 100 cc. and is divided into 100 large divisions, each of which is further subdivided into five small or 0.2 cc. divisions. The burette is surrounded by a large-diameter glass tube or water jacket. The annular space, i, between the burette and the jacket is filled with water to prevent sudden changes of temperature in the burette during an analysis.

100

Wiring diagram

FIGURE 18.-Details of portable Orsat apparatus for analyzing mine gases.

The pipettes contain the following solutions: Pipette a, potassium hydroxide or sodium hydroxide solution for the absorption and removal of carbon dioxide from the sample; b, alkaline pyrogallote solution for the removal of oxygen; and c, acid cuprous chloride (or other reagent containing cuprous salts) for the removal of carbon monoxide. Pipette d is the slow-combustion pipette in which methane (or other combustible gas) is burned. Pipettes a and b contain vertical glass

To sample

o-container

n

tubes, which retain a film of the absorbing reagent on their surfaces when gas is introduced into the pipette and thus present a large area wetted with the reagent, which accelerates absorption of the gas that is to be determined. Pipette c is designed so that the gas sample may be bubbled through the reagent. The cuprous chloride solution ordinarily used in pipette c is not viscous enough to form an adherent film on glass tubes as used in pipettes a and b, and therefore intimate contact between gas and reagent is obtained by the bubbling action. Pipette c is provided with a two-way, parallel-bore stopcock, one branch of which communicates with a capillary glass tube extending almost to the bottom of the pipette, and the other branch with the top of the body of the pipette. This cock is turned first so that the gas sample may be forced down through the central tube, from which it emerges and bubbles upward through the reagent, collecting at the top of the pipette. The gas is withdrawn from the top of the pipette by turning the stopcock 180°.

The combustion element inside the slow-combusion pipette d is made from 212 to 3 inches of No. 28 or 30 B. & S. gage platinum wire formed into a 16-inch-diameter coil supported by two glass tubes. The ends of the platinum wire are hermetically sealed through the upper ends of the supporting tubes and are welded to copper wires that extend down through the tubes to the exterior electrical connections. A power source capable of furnishing 3 to 5 amperes at 6 to 8 volts usually is necessary to heat the wire to a temperature that will cause complete combustion of methane. This power source may be a number of cap lamp batteries or No. 6 dry cells connected in series or, if alternating current is available, a transformer of at least 150 watts capacity with taps for 6 or 8 volts on the secondary. In using a transformer, it should be noted that the applied primary voltage and frequency are those for which the transformer is designed. The temperature of the platinum coil in the combustion pipette is regulated by connecting a variable resistance in series with the coil and the lowvoltage power source. If a rheostat is not available, a suitable resistance may be made from a 12-inch length of No. 22 nichrome wire. One end of this wire is attached to one lead from the combustion pipette, and the other is made variable by slipping it through the eye of an ordinary screw binding post to which the corresponding lead from the power source is attached. The wiring diagram is shown in figure 18.

The solution reservoirs of pipettes b and c are closed with 1-hole rubber stoppers to which are attached rubber expansion bags. These bags protect the solutions in the pipettes from the oxygen of the air, which would cause rapid deterioration of the solutions if the reservoirs were left open. Pipette a does not need protection while in use and may be closed with a rubber or cork stopper when not in use.

The marks on the capillary stems or inlet tubes of the pipettes, which indicate the height to which the solutions are to be drawn, usually are etched into the glass. If not, a suitable mark can be made by pasting a narrow strip (about 1/16 inch wide) of paper around the stem about one-fourth to one-half inch below the lower end of the rubber connection to the manifold. This distance gives space for observing and adjusting the level of the liquid in the pipette stem and eliminates dead space as much as possible. "Dead space" is the term applied to

the space in the manifold and capillary glass tubing connections that is not filled with displacing liquid during an analysis.

The manifold j, with its attached stopcocks, permits admission of the original sample to the burette through stopcock p and passage of the sample into the various pipettes through the stopcocks communicating with them. Connections between manifold and pipettes and to stopcock p are made with snug-fitting rubber tubing.

To stopcock on manifold

Automatic valves

FIGURE 19. Alternate types of bubbling pipettes.

Leveling bottle ƒ contains the confining liquid for the burette, and leveling bottle g contains the confining liquid for the combustion pipette.

The portable Orsat apparatus shown in figure 18 illustrates a simple design that contains the basic parts necessary to conduct an analysis. Apparatus obtained from commercial laboratory supply houses may embody other features of design that differ somewhat from the apparatus shown in figure 18, although the fundamental purpose of these modified parts is the same.

The bubbling-type pipette may be modified as shown in figure 19, so that an automatic ball check valve directs the gas through the bubbling tube as the sample is forced into the pipette and permits exit of the gas from the upper part of the pipette when the sample is withdrawn, thus eliminating the necessity of turning a stopcock each time the sample is passed into and withdrawn from the pipette.

To manifold stopcock

Tube containing catalyst

Catalyst tube heater

Heater
switch

Temperature-control rheostat

500° C. thermometer

To gas reservoir

Reservoir for gas that has
been passed through
catalyst tube

Confining liquid

FIGURE 20.-Apparatus for the catalytic combustion of methane.

In some types of commercial apparatus the slow combustion pipette shown in figure 18 has been replaced by a small-diameter tube containing a catalyst, which is heated to cause the combustion of methane. The essential parts of this device are shown in figure 20. Operation of this type of combustion unit is described in a following section.

The design of stopcocks and manifold may be modified as shown in figure 21. The manifold shown at A in this figure is similar to that illustrated in figure 18, in that the stopcock through which gas is admitted to or discharged from the burette is situated at the same end of the manifold as the burette. This design is to be preferred to that shown at B in figure 21, in which the admission stopcock is situated at the opposite end of the manifold from the burette. In design B the horizontal portion of the manifold is filled with the original sample when the sample is initially drawn into the burette, and unless the

FIGURE 21. Types of manifolds that may be used in the portable Orsat gas apparatus.

graduation of the burette is designed to include the volume of the manifold, slight errors in gas-volume measurements may be introduced. Also, in design B a small volume of each constituent of the sample may remain unabsorbed in a section of the manifold beyond the pipette in which the major portion of that respective constituent is absorbed. The presence of these small, unabsorbed residuals in the manifold will cause slight errors throughout the successive steps of an analysis. However, in the analysis of mine gases, the errors introduced by the use of design B usually are small, and this type of manifold has been