an application of the method used in water analysis for the determination of nitrates. The procedure is as follows: Shake samples of the gas to be tested with or bubble them through 10 per cent caustic soda or potash solution, then treat this solution for oxides of nitrogen as described later. In the Bureau of Mines gas laboratory the samples are received in vacuum-bulb containers. The neck of the bottle is scratched with a file and broken off, and the opening immediately closed with a cork stopper. Put 5 c. c. of 10 per cent caustic soda and 5 c. c. of hydrogen peroxide in the bottle and stopper it again. Rotate the bottle to coat the inside with the solution and then leave it for 30 minutes. Open the tube, wash the contents through a filter paper into a 150-c. c. beaker, and evaporate to dryness on a hot plate. During the evaporation cover the beaker with a watch glass to prevent contamination of nitrates from outside sources and to prevent loss of precipitate from spattering during the final drying period. Do this drying at a lower temperature to prevent the precipitate from caking. Moisten the residue with 2 c. c. of a 50-50 mixture of phenol-disulphonic acid and cold, concentrated sulphuric acid, dilute with 10 c. c. of distilled water, and run through a filter paper into a Nessler tube; rinse and wash as usual. Then add 15 c. c. of 11 to 1 strong ammonia and water; make up the whole to 100 c. c. and compare with color standards.

REAGENTS FOR DETERMINATION

PHENOL-DISULPHONIC ACID

Dissolve 25 grams of pure white phenol in 150 c. c. of concentrated sulphuric acid; add 75 c. c. of fuming sulphuric acid (15 per cent SO,); stir well; and heat for 2 hours at 100° C.

STANDARD NITRATE SOLUTION

Make a solution of potassium nitrate containing 0.72 gram of KNO, per liter. Evaporate 10 c. c. of this solution on the water bath until about 1 drop is left, and remove the remaining water with a current of dry air. Quickly and thoroughly moisten the residue with 2 c. c. of the phenol-disulphonic acid solution, and make it up to 1 liter; 1 c. c. of this solution equals 0.001 mg. N2, or 0.00175 c. c. of NO.

AMMONIA SOLUTION

Use equal parts of strong ammonia and distilled water.

CAUSTIC SODA SOLUTION

Prepare a 10 per cent, by weight, caustic soda solution, being sure that the caustic soda does not contain nitrates.

49530°-26- -6

HYDROGEN PEROXIDE

Use standard commercial 3 per cent hydrogen peroxide free from acetanilide and nitrates.

Standards used for determining oxides of nitrogen are usually

The parts per million given in the table are based on a 250-c. c. sample, but by increasing the volume of sample taken the accuracy of the method can be increased accordingly, so that one or two parts per million can be obtained easily. Should the oxides of nitrogen be present in amounts which exceed the set of standards given, the solution should be diluted enough to come within the set of prepared standards and the results corrected for the amount of dilution. The hydrogen peroxide is added for the purpose of oxidizing any nitric oxide (NO) to nitrogen peroxide (NO2).

USE OF THE GAS INTERFEROMETER

In its investigation of mine gases the Bureau of Mines has tested the laboratory-type Rayleigh interferometer as adapted to gas analysis by Haber of Berlin and Löwe of Jena, and made by Zeiss of Jena. Mohr, Haber,38 Löwe,39 and Küppers 40 have published studies concerning its use.

37

The instrument is made in two different models. The laboratory type (Pl. VII and fig. 25) is of rigid construction, has gas chambers 1 meter long, and is employed for exact gas analysis. It corresponds to apparatus that use mercury as the confining fluid, and with which results accurate to 0.02 to 0.03 per cent can be obtained. Some experience is required before one becomes sure of the readings. The laboratory type only is discussed here.

37 Mohr, O., Die Verwändung des zeisschen Interferometers zur technischen Rauchgasanalyse: Ztschr. angew. Chem., Jahrg. 25, June 28, 1912, pp. 1313, 1317.

38 Haber, F., Optische Analyse der Industriegase: Ztschr. angew. Chem., Jahrg. 19, Aug. 17, 1906, p. 1418-1422.

39 Löwe, F., Ein neues Interferometer für Gase und Flüssigkeiten: Physikal. Ztschr., Jahrg. 11, No. 23, 1910, pp. 1047-1051.

40 Küppers, E., Die Bestimmung des Methangehaltes der Wetterproben mit Hilfe des tragbaren Interferometers: Glückauf, Jahrg. 49, Jan. 11, 1913, pp. 47-50.

The other model, the short or portable type, is employed for the analysis of gas mixtures when extreme accuracy is not required. Its accuracy is about 0.2 to 0.3 per cent and is comparable with that usually obtained in technical gas analysis when water is employed as the confining fluid. Both instruments depend upon the same principle. The short model is constructed with a view to portability. When in position it is upright, about 10 cm. in diameter and 50 cm. high, and weighs about 11 pounds.

DESCRIPTION OF LABORATORY TYPE

Figure 25 illustrates the principle of operation of the laboratory type. The pencil of parallel rays which comes from the light of a Nernst lamp passes through the object glass of the collimator c and travels in three parts to the observation telescope d. The upper

FIGURE 25.-Diagram of parts of laboratory interferometer, vertical and horizontal sections. For explanation, see text

half of the pencil, in shape of a half cylinder, passes directly toward a double diaphragm placed at the end of the object glass of the telescope and covering all of the object glass. This pencil of rays passes through the diaphragm into the upper half of the object glass of the telescope. In its focal plane this part of the pencil of rays produces an image of the slit, together with Fraunhofer's diffraction phenomena, which consist of two straight parallel dark bands in a white field, with colored bands on each side. The lower half of the pencil of rays is divided into two parts by the metallic partition ab. One part passes through the gas chamber e and the other through chamber f, which contains the gas for comparison. The pencil of rays also passes through compensator plates h and g. The part h is movable by means of a micrometer screw, k, which measures the deflection of the plate. The part g is fixed. By this arrangement the optical path of the gas in chamber e can be varied. From the compensator plates the rays pass through the diaphragm into the telescope.