Determination of Water Vapor in Nitrogen Thermal Conductivity Measurement of Hydrogen Liberated from Calcium Hydride HARRY W. LINDE' and L. 8. ROGERS Department of Chemistry and laboratory for Nuclear Science, Massachusetts Institute of Technology, Cambridge ,Though the reaction of water vapor with calcium hydride can be used for determining water, the reaction is not clean-cut, as evidenced by the variability of the results. As expected, the lower alcohols and ammonia interfere seriously, although, if present alone, they can b e determined in the same way as water.
H
and gash (2) reported a simple and extremely sensitive method for determining water vapor of an inert gas stream by measurement of the temperature rise due to the heat of reaction. Later, others discovered that a new calibration curve was required each time the hydride was changed ( 3 ) . Some variation would be expected as a result of differences in the reactivity of the hydride, particularly if one batch reacted more slowly and less completely than another. If so, the use of long columns of hydride would eliminate the problem. Then, instead of temperature rise, an extremely sensitive method for hydrogen such as thermal conductance (1) would have t o be used. Although variations from batch to batch were somewhat smaller than when temperature rise had been used, they were larger than those obtained with different inert calibrating gases. Attempts to improve reproducibility by substituting other hydrides or using solutions of a hydride were fruitless. The reaction of calcium hydride with water vapor must be very complex and is apparently unsuitable for highly precise routine measurements. The sensitivity of the hydride toward lower aliphatic alcohols indicates that they constitute serious interferences or, if present alone, can be determined with about the same precision as water. ARRIS
APPARATUS
A modified Veco 139-2 thermal con-
ductivity cell (Victory Engineering Corp., Springfield Road, Union, N. J.) was used to measure conductivities. Present address, Department of Anesthesiology, Hospital of the University of Pennsylvania, Philadelphia 4, Pa. 1250
ANALYTICAL CHEMISTRY
Only one thermistor was used; it formed one arm of a Wheatstone bridge whose other arms were precision, wirewound resistors. The thermistor was kept a t a constant resistance of 500 ohms by controlling the current flow to the bridge. At 500 ohms, the thermistor temperature was constant a t about 71' C. The brass cell-block assembly was kept a t 30" f 0.02' C. in an oilbath thermostat. This setup was considerably less sensitive than the original unmodified bridge. However, it had been used for an unpublished study in which it had been calibrated using a number of pure gases of known conductivities (6'). The relationship for this apparatus was k = 2.61 I2 - 0.0000168, where IC is in cal./cm.-see.-' C. and I is in amperes. The increase in current required to balance the bridge in the presence of hydrogen was the parameter used in preparing calibration curves. The bridge current was measured as an I R drop across a 100-ohm resistor, using a Rubicon potentiometer sensitive to 0.1 mv. The balance of the bridge was checked daily and drift found to be barely perceptible. The calibration, checked periodically on pure inert gases, proved constant. The hydride was packed loosely into a 12 X 180 mm. glass tube and heated by a microchemical mortar whose temperature was controlled manually by an autotransformer. To obtain nitrogen of a known water content, a gas train was built along the lines of that of Walker and Ernst (6). The train consisted of a dry nitrogen stream and a vapor-containing nitrogen stream, each passing through a separate flowmeter into a mixing bottle. The saturator consisted of two flasks containing water and chipped ice and fitted with gas dispersion tubes through which dry nitrogen was passed. The effluent from the first flask passed through the second ice flask, through a Fischer and Porter rotameter, and into a 500-ml. mixing bottle. At the same time, a stream of dry nitrogen from another cylinder passed through a similar rotameter and into the mixing bottle. As both rotameters were graduated between 10 and 690 ml. per minute, and the vapor pressure of water over ice is 4.6 mm., the effluent from the mixing bottle could be adjusted to
39, Mass.
contain between 100 and 6000 p.p.m. of water vapor in nitrogen. (The gas stream from the ice water flasks was assumed to be saturated, because addition of more flasks of ice water to the saturation train produced no measurable difference in the water content of the effluent gas.) Part of the gas stream from the mixing bottle was fed directly into the bottom of the vertically mounted hydride tube and then through the thermal conductivity cell to the atmosphere. The flow rate through the hydride was limited to 20 ml. per minute t o conserve the hydride; a t lower rates, readings became erratic. The flow rate over the hydride was regulated by venting the excess of wet nitrogen t o the atmosphere through a controlled leak placed between the mixing bottle and the hydride tube. When methanol and butanol were to be passed over the hydride, they were vaporized from baths a t ice temperature and 25' C., respectively, and the alcohol-nitrogen mixtures were treated in the same manner as the water-nitrogen mixture. Ammonia was vaporized similarly frem an aqueous solution, the water removed by Ascarite, and the dry stream then diluted as the other vapors had been. Care was taken to make glass-to-glass contact when Tygon was used for connections. Minimum lengths of tubing were used in the apparatus to minimize the time required to saturate the walls with a sample, in order to obtain a reading that did not drift during 15 minutes. REAGENTS
IThe hydrides were the commercial grade supplied by Metal Hydrides, Inc., Beverly, Mass. (4). The calcium hydride was gray and ranged from a coarse powder to lumps about 3 to 5 mm. in diameter. The sodium hydride grains were about 0.5 to 1 mm. in diameter and brownish in color. The lithium aluminum hydride was a porous gray solid in various sizes between a fine powder and 30- to 40-mm. lumps. The methanol was Mallinckrodt analytical reagent grade. The 1-butanol was distilled and the portion boiling in the range of 116-17' C. was collected. The ammonia vapor was prepared by vaporizing a 3-44 aqueous solution of reagent grade ammonia and removing
the water by passage over Ascarite. The ammonia content of the vapor from the ammonia solution was determined by absorbing measured gas samples in standard acid, and back-titrating with standard base. The nitrogen was Air Reduction Co.'s prepurified grade. PROCEDURE
The water vapor-nitrogen mixtures were passed over calcium hydride in a glass tube held a t the desired temperature, and the conductivity of the resulting hydrogen-nitrogen mixture was measured. A blank reading was necessary, as a current greater by 0.001 to 0.003 ma. was required when the dry nitrogen was passed through the hydride directly than when it was fed through the cell without contact with the hydride. This increase in conductivity was probably due to traces of water in the nitrogen and to the liberation of adsorbed or chemically bound hydrogen from the hydride. Sodium hydride and lithium aluminum hydride were used in the same manner as calcium hydride. Moist nitrogen was bubbled through solutions of lithium aluminum hydride in several ethers in attempts t o obtain rapid, complete conversion of water vapor to hydrogen. RESULTS
Water Determination. When moist nitrogen was passed over the calcium hydride a t room temperature, somewhat more than the calculated amount of hydrogen (6) was released, but the results were erratic. When the temperature of the hydride was raised to 100' C., a slightly larger amount of hydrogen was evolved, but the system still gave erratic results. Increasing the hydride temperature to 150" C. did not significantly improve reproducibility. When the temperature of the hydride was raised to 235' C., however, there was less variability in the results, although the slope and intercept of the curve remained about the same. A steady reading was usually reached within 10 minutes. These data for calcium hydride a t 235' C. are plotted in Figure 1. Brief trials with the hydride heated to 290' C. indicated no improvement and, because nitrogen begins to react with calcium hydride a t higher temperatures and the hydride itself begins to decompose with the liberation of hydrogen (4), the temperature was not raised higher. Xhen sodium hydride was used instead of calcium hydride, roughly similar results were obtained, but they were extremely erratic. The sodium hydride was used only a t room temperature, because it decomposes when heated to about 300' C. The erratic results were attributed, at least in part,
6751' ~ 0
'
' 1 500
"
'
' 1 loo0
Water Vapor,
Figure 1.
'
'
' ' 1500
1
'
' ' 2000
'
~
'
~
RPM.
Data for calcium hydride at
235' C.
0 Water
m Methanol A 1-Butanol
to the formation of a sodium hydroxide layer on the surface of the particles with which the gas first came into contact. The sodium hydride in the entrance end of the tube appeared moist after use and had changed from its original brownish color to white. Solid lithium aluminum hydride, when exposed to moist nitrogen, did not give off hydrogen until about 6000 p.p.m. of water was present. It was not heated, because it begins to decompose a t 125' C. (4). When it was brought into contact with liquid water, a violent reaction ensued. If surface coatings were interfering with the release of hydrogen, it was thought that a solution of this hydride in a suitable ether might solve the problem. Solutions of lithium aluminum hydride in n-butyl ether, di-nbutyl Cellosolve, and diethyl Cellosolve were examined The results were more encouraging than those obtained with the solid aluminum hydride but were still erratic and did not appear useful for analytical purposes. Furthermore, formation of a gelatinous precipitate of lithium aluminate in the bubbling tubes was extremely troublesome. When ether solutions were used, the blank value was considerably different, because of the conductivity of the ether vapor which entered the nitrogen stream. Interferences. The hydrides react not only with water, but also with other active hydrogen compounds which could be present in the vapor phase, such as alcohols and ammonia. To study these reactions, ammonia gas and methyl and n-butyl alcohols were passed over calcium hydride a t 235' and 100' C. and room temperature. The concentration of the alcohols in the vapor phase was calculated from their known partial pressures, assuming saturation of the
original nitrogen stream. The ammonia concentration was determined by titration after absorption of the gas in standard acid. The results of these studies a t 235' C. are also shown in Figure 1. With the calcium hydride, the alcohols gave about the same amount of hydrogen as did water-as expected for complete reaction with the hydride. KO hydrogen was liberated when ammonia was passed over the hydride, until the ammonia concentration was about 2000 p.p.m. The hydrogen liberated a t this point was equivalent to 200 p.p.m. of water and may have been due to incomplete removal of the water by the Ascarite rather than to reaction of ammonia with the hydride. When the temperature of the hydride was reduced to loo', interference due to the butanol was lessened (1200 p.p.m. of liberated hydrogen equivalent to 200 p.p.m. of water); methanol gave the same results as a t the higher temperature. Ammonia was not tried a t this temperature. With calcium hydride a t room temperature, ammonia did not react, the butanol reaction was about the same (2000 p.p.m. of liberated hydrogen equivalent to 500 p.p.m. of water), and the amount of reaction of the methanol mas reduced slightly (1000 p.p.m. of liberated hydrogen equivalent to 700 p.p.m. of water). A second lot of a similar grade of calcium hydride tested under these conditions gave the same results. A third lot of a different grade (5-mm. lumps) was tested briefly and appeared to give somewhat larger amounts of hydrogen. Conclusions. Water vapor in nitrogen in the range of 200 t o 2000 p.p.m. was determined by reaction with calcium hydride a t 235' C. and measuring the liberated hydrogen b y thermal conductivity. The results VOL. 30, NO. 7, JULY 1958
1251
were erratic when the hydride was at a lower temperature. A single sample could be analyzed in about 10 minutes. The method could easily be applied continuously to a process gas stream by diverting a small side stream through the calcium hydride t o the thermal conductivity cell. Attempts to determine moisture using sodium hydride or lithium aluminum hydride in place of calcium hydride were unsuccessful. Akohol vapors interfered with the determination of water vapors, but, in the absence of water vapor, the alcohols themselves could be determined. At room temperature, and possibly a t 235’ C., ammonia gas did not react.
ACKNOWLEDGMENT
The authors gratefully acknowledge partial support of this work by the Atomic Energy Commission and the Office of Naval Research. I n addition, one (HWL) is grateful to the Allied Chemical and Dye Corp. for generous support in the form of a research fellowship. LITERATURE CITED
(1) Daynes, H. A,, “Gas Analysis by
Measurement of Thermal Conductivity,” Cambridge University Press, 1933. (2) Harris, F. E., Nash, L. K., ASAL. CHEX 23, 737 (1951).
(3) Lux, A. R., E. I. du Pont de Kernours & Co., Inc., Penns Grove, N. J., private communication, 1952. (4) Metal Hydrides, Inc., Beverly, Mass., bulletins on calcium hydride, sodium
hydride, and lithium aluminum hydride. (5) Walker, A. C., Ernst, E. J., IND.EKG. CHERI., A S A L . ED. 2, 134, 139 (1930): ( 6 ) Weber, S., Ann. Physik. 54, 481 (1917). RECEIVEDfor review May 18, 1957. Bccepted February 12, 1958. Taken from a thesis presented by Harry W. Linde to the Department of Chemistry, Massachusetts Institute of Technology, in partial fulfullment of the requirements for the degree of doctor of philosophy in September 1953.
PoIa rogra phic Determination of Hy droge n Peroxide, Formaldehyde, and Acetaldehyde in Mixtures SAMUEL SANDLER and YU-HO CHUNG‘ Department o f Mechanical Engineering, University o f Toronto, Toronto, Ontario, Canada
b A three-step polarographic method i s presented for the quantitative determination of hydrogen peroxide, formaldehyde, and acetaldehyde in solutions. Previous attempts to analyze such solutions b y one-step polarographic methods have been successful in determining formaldehyde in the presence of hydrogen peroxide only after calibration using standard solutions containing concentrations o f the constituents similar to those in the unknown samples. The present method uses known reactions of the components to improve the analytical determination of each. The diffusion current-concentration relationship for each pure substance i s the only calibration requirement. Hydrogen peroxide is determined in an acid buffer solution in which the aldehydes d o not interfere. Formaldehyde i s determined in an alkaline solution to which titanium tetrachloride i s added to eliminate the interference of hydrogen peroxide. Acetaldehyde is estimated in a similar solution in the presence of dimedon, which reacts rapidly with formaldehyde.
T
products of the low-temperature oxidation of hydrocarbons consist chiefly of hydrogen peroxide, organic peroxides, aldehydes, acids, and sometimes alcohols. Some of these products, like the peroxides and aldehydes, can be determined conveniently, HE
1 Present address, 19 Bonham Road, Hong Kong.
1252
ANALYTICAL CHEMISTRY
qualitatively and quantitatively, by the use of polarographic methods. Various papers on this subject have been published. Dobrinskaya and Seiman (1) found it preferable to carry out the electroreduction of hydrogen peroxide and organic peroxides in acid solution, as they decomposed rather rapidly in neutral or alkaline media. RIacSevin and Urone (6)added a solution of titanium tetrachloride in LV hydrochloric acid t o separate organic peroxides from hydrogen peroxide in a mixture. Titanium tetrachloride reacts with hydrogen peroxide rapidly, leaving the organic peroxides unchanged. Skoog and Lauwzecha ( 7 ) reported values of the half-wave potentials and diffusion-current constants for 17 alkyl hydroperoxides. The half-wave potentials for all compounds in the series fell within a range that was too narrow for the determination of any single member of a mixture. Whitnack and Moshier (9) determined formaldehyde in the presence of acrolein and other aldehydes. They obtained the most satisfactory results in an alkaline medium, a solution 0 . W in lithium hydroxide and 0.01N in lithium chloride. Karshowsky and Elving (8) applied this method for the simultaneous determination of formaldehyde and acetaldehyde, but observed rapid condensation of acetaldehyde in the presence of strong alkalies. The waves of the reduction of formaldehyde and some unsaturated aldehydes like acrolein were superimposed; consequently, it was impossible to determine the
amount of each aldehyde directly. Holyever, Neiman and Gerber (4) analyzed solutions containing both formaldehyde and acrolein by using dimedon to remoTe the wave due to formaldehyde. Shtern and Pollyak (6) studied various mixtures of aldehydes and peroxides in six different electrolytes. To detect peroxides and aldehydes in one polarogram, they recommended the use of 0.1S lithium chloride as supporting electrolyte with preliminary expulsion of oxygen from the water used in preparing the solutions. The error in their peroxide determination was usually less than s%,but that for formaldehyde was as much as 40%. MacNevin and associates (3) reduced these errors by running polarograms on standard mixtures with compositions similar to those of the samples. As this method cannot be applied easily when samples of widely varying concentrations are encountered, another method of analysis for such mixtures seemed t o be required. APPARATUS
The apparatus used was a Type PO% ink recording polarograph made by Radiometer, Copenhagen. The electrolysis cell consisted of a standard calomel half-cell, an electrolysis vessel with a dropping mercury electrode, and an agar-salt bridge to connect the calomel half-cell with the test solution. The capillary characteristics in distilled n-ater and a t zero applied voltage mere m = 3.91 mg. of mercury per second and t = 3.34 seconds. The electrolysis cell
