Spectrographic Detection and Determination of the Halogens W. W. A. JOHNSON AND DANIEL P. hTORMAhNew England Spectrochemical Laboratories, West Medway, Mass.
T
through the center of two rubber stoppers (the interelectrode gap was still maintained a t 2 mm.). Off-center holes in each stopper held 3-mm. diameter glass tubes. Carbon dioxide was introduced through the lower tube and escaped through the upper tube. A few ounces of carbon dioxide pressure sufficed to sweep the tube clear of nitrogen in less than a minute. It was, of course, necessary to use pure carbon dioxide. Carbon dioxide generated from dry ice was found t o be contaminated with chlorine, and ordinary cpmmercial carbon dioxide contained traces of nitrogen and chlorine. Through the courtesy of E. N. Ellis of the S. S. White Dental Mfg. Co. of Philadelphia, the authors obtained U. S. P. carbon dioxide customarily used in anesthesia. No nitrogen or chlorine could be detected in this gas by spectrographic methods.
HE spectrochemical analysis of the halogens has been
confined, with one exception, to the detection and determination of fluorine as calcium fluoride ( 1 , 5 , 7) or silicon fluoride (6). Pfeilsticker (8) has analyzed salts for bromine, chlorine, and iodine at reduced pressures b y the use of a special, very sparklike spark (2000-microfarad condenser, 0.05-0.1 millihenry inductance, at 220 volts). Nevertheless, the intensity of the spark lines of the halogens tabulated b y Exner and Haschek (2) suggested strongly t h a t it would be possible t o detect all the halogens spectrographically in the usual spark source, if i t were not for the presence of air lines, principally of nitrogen. T h e air lines could not be quenched b y the usual expedient of introducing self-inductance into the spark circuit because the sparklike qualities of the source would thereby be degraded toward the arc, and the spark lines of the halogens would become exceedingly faint. Despite this difficulty, i t seemed feasible to determine the halogens spectrographically without the use of special auxiliary equipment and this possibility mas investigated.
The spectrographic sensitivity of the halogens varied with the matrix in which they were found and the cations to which they were attached. In general, amounts of the following orders of magnitude could be detected with certainty: Element
Wave Length
Intensity
Element
A ...
F
6856.02 6902.46
60 40
Br .
4678.69 4704.86 4785.50 4816.71
10 30 15 10
Wave
Length
A.
4794.54 4810.06 4819.46
20 10 8
I
5161.19 5464.61
50 30
70
in C O ~
500 900
0.3 0.6
in air
200 100
0.1 0.07
Summary Materials containing the halogens were sparked without prior chemical treatment in the usual high-voltage spectrographic spark. B y this method the halogens could be determined with an accuracy comparable t o t h a t obtained in the spectrochemical analysis of metals. In the case of chlorine and bromine the most sensitive lines were masked b y air lines and observations were carried out in a carbon dioxide atmosphere in a simple cylindrical tube. T h e limits of detection were 0.5 per cent for chlorine and 0.3 per cent for bromine, in a carbon dioxide atmosphere; 0.1 per cent for fluorine and 0.07 per cent for iodine, in air.
Inten. sity
c1
Sensitivity
where the limit of identification refers to the quantity sensitivity, and the limit of sensitivity refers to the concentration sensitivity, in accordance with the definitions given b y Feigl ( 3 ) . One half these limiting amounts could be detected in many materials, one fourth these amounts in some materials. Satisfactory quantitative determinations of chlorine were carried out in the carbon dioxide atmosphere in the range from 1to 15 per cent of chlorine when 150 mg. of sample, introduced in five separate loads, were sparked. The accuracy of the determinations varied from *10 per cent of the amount of chlorine found when the chlorine content was less than 5 per cent to =t5 per cent when the chlorine content was 15 per cent. The precision of the determinations was uniformly of the order of *5 per cent.
T h e following spectral lines have proved to be the most satisfactory for the detection and determination of the halogens : Element
Limit
Microprams
z~
All spectrograms were taken in the first order of a 3-meter grating spectrograph, dispersion 5.6 A. per mm. Eastman 33 plates were used for determining chlorine and bromine; Eastman spectroscopic I-F plates for fluorine and iodine. Overlapping higher order spectra were eliminated by suitable Corning glass filters placed before the spectrograph slit. The finely divided samples were introduced into the cored crater of 0.6-cm. (0.25inch) graphite electrodes (National Carbon Co., Acheson grade) and flattened off level with the rim of the crater. The loaded electrode was placed in the lower clamp of a spark stand. A sharply pointed electrode was placed in the upper clamp of the spark stand and so adjusted that its point was exactly 2 mm. above the surface of the sample. The electrodes were connected to the terminals of a conventional spectrographic, alternating current, condensed-spark circuit with the following constants: input, 130 volts; output, 25,000 volts, 17 to 21 milliamperes; capacity 0.004 microfarad. The large inductance customarily used in such circuits to eliminate the air lines was shorted out, so that the only inductance was the natural self-inductance of the circuit. All leads were made as short as practicable. The spark was focused on the slit of the spectrograph by a simple quartz lens.
Identification
The wave lengths are taken from the M. I. T. Wavelength Tables (4). T h e intensity estimates are the authors’ and are based on the intensity scale of 1 t o 1000 used b y Exner and Haschek in their measures of the air lines (9). Air lines did not interfere with any of the strongest lines of fluorine and iodine, b u t all the chlorine lines except 4794.54 A. were masked b y nitrogen lines, as was the strongest line of bromine. T h e masking of these sensitive lines reduced the sensitivity of the spectrographic test for chlorine and bromine very considerably.
Literature Cited (1) Churchill, H.V.,J . Am. Water Works Assoc., 23 1399 (1931). (2) Exner and Haschek, “Die Spektren der Elemente bei Normalen Druck”, Vol. 111, Leiprig, Franz Deuticke, 1912. (3) Feigl, F., Mikrochem., 1,4(1923). (4) “M.I. T. Wavelength Tables”, New York, John Wiley & Sons, 1939. (6) Papish, J., Hoag, L. E., and Snee, W. E., IND. ENQ. CHEM., ANAL.ED., 2,263 (1930). (6) Paul, W., Angew. Chem., 49,901 (1936). (7) Petry, A. W., IND.ENQ.CHEM.,ANAL.ED.,6, 343 (1934). (8) Pfeilsticker, K.,Spectrochirn. Acta, 1, 424 (1940).
To overcome the interference of the air spectrum, the electrodes were introduced inside a 5-cm. diameter Pyrex tube 119
