Water : Combining Volumes Of Hydrogen And Oxygen

Early experiments on the composition of water by volume are those of Cavendish (1781), who obtained the ratio H/O = 201:100; Gay-Lussac and Humboldt (1805) who found 199.89: 100; and Bunsen, whose numerous determinations indicated an almost exact ratio of 2:1.

An accurate determination of the combining volumes was attempted by Alexander Scott, whose experiments, made in 1887-9 and 1893, at first yielded slightly varying ratios, from 199.4:100 to 200:100. The later experiments showed that this variation was due to a very thin film of grease carried over from the lubrication of the stopcocks into the eudiometer, which took up a little oxygen during the explosion, burning to carbon dioxide and steam. By using pure hydrogen, prepared by passing steam over sodium, and pure oxygen from silver oxide, and by lubricating the stopcocks with syrupy phosphoric acid, the combining ratio at S.T.P. was found to be a little greater than 2:1. viz., 200.285:100. Morley found 200.269:100.

A more recent determination of the ratio is that of Burt and Edgar (1915). The final result, the average of 59 determinations, was 200.288:100, agreeing with Scott's to within 3 parts in 200,000. I he special points of the research were: (1) very carefully purified gases were used; (2) the actual measurements were carried out at 0°, and under 1 atm. pressure, so that the temperature and pressure corrections were eliminated.

The hydrogen was prepared by the electrolysis of recrystalhsed barium hydroxide; it was dried by phosphorus pentoxide, and further purified: (i) by passing over charcoal cooled in liquid air, which readily absorbs oxygen and nitrogen, but hydrogen only to a slight extent; (ii) by passing through a tube containing palladium black to convert oxygen to water, and then pumping the gas through the walls of a closed palladium tube heated electrically. The palladium tube was welded to a short platinum tube, and the latter sealed into a glass tube. This was sealed inside a wider tube, and the palladium heated by a platinum spiral wound on a quartz cylinder slipped over it. The palladium was protected from mercury vapour from the pumps by plugs of gold wire sponge. The palladium was charged with hydrogen at 100°, 300 c.c. of gas were then pumped off at 180°, and the metal was recharged with hydrogen at 100°.

The oxygen was prepared: (1) by the electrolysis of baryta, liquefaction in fresh liquid air, and fractionation; (a) by heating pure potassium permanganate in glass tubes, and washing the gas (a) with strong caustic potash solution, (b) with saturated baryta solution, (c) with very strong potash solution. The gas was then dried by sticks of potash, and phosphorus pentoxide, liquefied, and fractionated.

The apparatus

Fig: Water composition

Volumetric composition of water: apparatus of Burt and Edgar.

consisted of a 300 c.c. glass pipette, A, sealed to capillary tubes at each end. The lower capillary was expanded to a dead-space, B, of about 1 c.c. capacity, with a glass levelling-point. The upper capillary led to a 3-way tap, C. The pressure of the gas in the bulb was equal to the vertical distance between the mercury surface in B and that in the upper chamber, D, also provided with a levelling-point; these two vessels were kept at a constant distance apart by a stout glass rod sealed between them. The manometer head passed to a mercury pump. The T-piece, H, and the tap, J, formed a volume adjuster; the capacity of the pipette could be varied within narrow limits by withdrawing mercury from J; this mercury could be weighed, and its volume thus accurately determined. The bulb and upper part of the apparatus were enclosed in an ice-bath; the lower dead-space was surrounded by a small brine bath, M. The mercury for displacing the gas was contained in O; the air-catch, F, protected the pipette from air leaks through the rubber. The volume of the apparatus, from C to the level of the glass point in the dead-space, D, was determined by weighing the contained mercury. The exit tubes from the oxygen and hydrogen apparatus joined beyond the taps, X and Y, in a T-piece, Q, which divided again, one branch leading to the pump through R and the other to the measuring pipette, A.

The gas was allowed to enter the pipette, displacing mercury into O, until the mercury surfaces in the dead-space and manometer stood at the glass-points. Since there was a vacuum above the mercury in the manometer, the gas was measured under the pressure of this mercury column, which was very approximately 1 atm. The tap X, or Y, was then closed, and the fine adjustment made by the pressure adjuster, J, by which small amounts of gas could be added to the pipette.

The gas had previously been allowed to attain the temperature of the ice-bath, which took about three hours, and was then passed to the explosion bulb, Z, by opening C and raising O, mercury being displaced from Z through an air-trap, α, to the reservoir, β. Z had a capacity of about 1 litre. Two pipettes of hydrogen with a little excess, measured by the pressure adjuster, were thus passed into Z. A pipette of oxygen was then added in portions, firing after each addition.. The small residual volume of wet hydrogen was sparked for a few minutes. The explosion vessel was then cooled by a mixture of solid carbon dioxide and acetone to freeze the water, the pressure reduced, and the residual gas sucked off through a phosphorus pent-oxide tube into a small pump, δ, a spiral, η, cooled in liquid air, being also interposed. The gas collected in the small vessel, E, and its volume was measured as follows. The pipette, A, was filled with hydrogen from the generator, and carefully levelled. The small volume of residual gas was then added from E, and the pressure adjustment made by running a little mercury from the adjuster, J. From the weight of this mercury the volume of the residual gas was calculated.

From the results of these experiments we can calculate the ratio of the hydrogen and oxygen by weight from a knowledge of the densities of the gases. The weights of 1 litre of hydrogen and oxygen at S.T.P. are, according to Morley, 0.089873 gm. and 1.42900 gm. respectively. With these figures, the values of Burt and Edgar give, for the weight of oxygen combining with 1 part by weight of hydrogen:

The atomic weight of hydrogen is therefore:

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