DETERMINATION OF HYDROGEN Universal Gasometric Micromethod LEONARD P. PEPKOWITZ AND EVERETT R. PROUD Knolls Atomic Power Laboratory, General Electric Company, Schenectady, N. Y .4 new universal gasometric micromethod is presented for the determination of
hydrogen in organic, inorganic, and metal-organic compounds and low melting metals. The basis of the method is the evolution of the hydrogen in the sample within a sealed iron capsule and the complete diffusion of the liberated hydrogen through the walls of the capsule into a simple vacuum system. The procedure is rapid; a complete determination can be made within an hour. The average error (0.27~) is comparable with that of the standard combustion procedure (0.3%). There are no interferences from other elements.
T
HE many difficulties of the combustion procedure for the determination of hydrogen (carbon and hydrogen deterniination) are well known. Many attempts (4)have been made to simplify it and to adapt it to a universal method independent of composition of the sample. This has resulted in numerous modifications ( 4 ) to eliminate interferences from elements such as nitrogen by use of lead peroxide with all its attendant difficulties, the halogens and sulfur by use of the “universal” filling, etc. The high vacuum fusion procedures (1, 3, 5-7) used in nietallurgical analyses involve extensive and intricate equipment, and are not adaptable to organic and inorganic compounds or low melting metals. The method described in the present paper obviates these difficulties, and is generally applicable for the determination of total hydrogen in organic, inorganic, and metalorganic compounds and the low melting metals. The basis of the present method is the evolution of the hydrogen in the sample within a sealed iron capsule and complete diffusion of the liberated hydrogen through the walls of the capsule into a simple vacuum system. The hydrogen is then determined by measuring the reduction in pressure which occurs in a static system when the hydrogen is converted into water over hot copper oxide. Because hydrogen is the only gas that diffuses through the walls of the capsule a t the operating temperature (700” C.), interferences by other elements are eliminated. To ensure the complete reduction of the sample and the products of the pyrolysis such as water, an excess of hydrogen-free magnesium turnings or sodium is sealed into the capsule. This procedure has proved satisfactory for all samples, organic and inorganic, to which it has been applied thus far. In the cnse of sodium hydroxide, which was selected as a representative inorganic compound whose hydrogen content could not be determined by available methods because of its stability toward thermal decomposition, oxidation, or reduction, an excess of distilled sodium was used as the reducing agent. The following reaction takes place quantitatively under the conditions of the determination: 2Xa 2SaOH + 2Na20 H2.
+
450” C.
+
APPARATUS
Vacuum System. The simple vacuum system employed is diagramed in Figure 1. A photograph of the equipment is given in Figure 2.
The furnace tube, C , can be of Pyrex and is cooled by a blast of compressed air. The iron capsule, B, is supported on a molybdenun1 wire rack, A . The thermocouple gage, D,is used for the qualitative indication of the pressure in the system and as a leak detector. The cold trap, E, is cooled with liquid nitrogen, which has been found more convenient to use than dry ice mixtures. The double McLeod gage, F , provides for two ranges of pressure measurements, 2 and 10 mm. For sume applications where
IRON CAPSULE
B A
.060”0IA.MOLYBDENUM W I R E CAPSULE
d
SUPPORT
Figure 1. Vacuum Apparatus
larger amounts of hydrogen are to be measured, a suitable manometer may be used. The blcLeod gage is connected to the system by a spiral to reduce the risk of breakage. The mercury cut-off, G, is preferred to a stopcock for a static system. The copper oxide tube, H , is made of quartz in order to withstand the rapid heating and cooling which it undergoes. Because the adjacent graded seals fatigue after a short time, the copper oxide tube is supported by a glass helix, as shown. With such a device, the copper oxide tube has been used continuously for over a year. The vacuum is produced by a mechanical pump only, because a vacuum of greater than 1 to 2 microns is unnecessary. The original apparatus included a Toepler pump, but this was e!iniiiiated, as the thermal cycling is sufficient to convert the hydrogen in the system to water in 35 minutes. The Toepler pump decreased this time by only 5 minutes. Iron Capsules. The iron capsules (Figures 1 and 3), are the shells of 6C5 metal radio tubes, with the flange machined off. The covers are a press-fit cap stamped from 10-mil sheet iron. I t is estimated that t.he capsules have withstood pressures as high as 4 atmospheres. In ninny hundreds of analyses, only one rupture has been encountered. In this instance, too large a sample of very ]OK hydrogen content was used and the capsule heated too rapidly, so that an estimated internal pressure of 10 atmospheres mas reached. The capsule ruptured on the side and riot at the weld, and the apparat,uswas not broken. Welding. The closure is made with a simple shielded argon arc welding technique, utilizing a tungsten electrode. Direct current is preferable, although 110-volt alternating current can be used. For rapid heat conduction, the capsules are held in a copper block
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V O L U M E 21. NO. 8, A U G U S T 1949
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prepared (8 to 10) for the day's analyses. samplhg. STANDARD SODIUM HYDROXIDE SAMPLES.
Standard samples were made up by weighing out sodium hydroxide of known composition into the iron capsules, adding a small bulb contrtining hydrogen-free, triple-distilled sodium (ca. 1 gram), which had its drawn tip broken off just previous to the addition and then welding the capsule shut. The pr5cedure proved to he simple and reproducible. The standard sodium hvdroxide was prepared by allowing C.P. flakes to come to equilibrium with the atmosphere of alarge desiccator containing silica gel. Thetiter, carbonatecontent, and moisture oontent (by difference) were determined an large samples. From these data it was calcuIated that there was 2.48% of hydrogen in the sodium hydroxide. The theoretical Apparatus for Microdetermination of IIydrogen value for pure sodium hydroxide is 2.520/0. There has been no significant change in the hydrogen value of this mat e d for more thsn G months. Itaw SuniuM S~MFLES. Sodium is used as an example of a low melting metal with a high vapor pressure at moderate temperatures: ,Spryle? ai rpw p i c k so3um wefetaken from !he interim ~~
Figure 3.
Iroj
(Fiaure 41 which is rotated bv a 1
tratian from wei luires less than 5 ng. The capsule ture (700" C.). d and a variatic 750" C. has no effe,at upon the react PROCED
Capsule Prepar;Ition. The caps washing with acetc>neand thorough then degassed in the apparatus by pumping out the system t,o 2 to 3 microns, an
