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Atomic Weight of Nitrogen


Prior to 1860, the names of Berzelius, Dulong, Gmelin, Turner, Penny, and Marignac were associated with atomic-weight determinations of nitrogen. An exhaustive series of investigations by Penny resulted in the mean value of 13,967 from the ratio Ag:AgNO3, and Marignac's value for the same ratio was 13,982. This latter experimenter mentions the difficulty of obtaining pure silver nitrate, owing to the tendency of the crystals to retain nitric acid.

The elaborate researches of Stas between the years 1860 and 1881 resulted in the mean value of 14,040 as the atomic weight for nitrogen, which was taken as standard for many years. Reinvestigation by Rayleigh, Leduc, D. Berthelot, Guye, Gray, Clarke, and others, caused a revision to be made in the atomic-weight values of a number of elements, and in 1907 the International Atomic Weight Commission adopted 14,010 as the value for nitrogen. A critical examination of the values obtained by Stas was made by Gray, who adduced considerable evidence to show that the experimental results of Stas for the atomic weight of nitrogen are erroneous. For example, the redetermination of the ratios Ag:NH4Cl and Ag:NH4Br by Scott, the ratio Ag:NaCl by Richards and Wells, and the ratio Ag:AgNO3 by Richards and Forbes, resulted in the lowering of the Stas value in each case. Although the precautions taken by Stas were, as usual, most elaborate, yet later experimenters had many refinements not available to the great Belgian chemist. Thus, the ratio Ag:AgNO3, according to Richards and Forbes, results in the value of 14,008 when corrections are applied for the traces of water and ammonium nitrate in the synthesised silver nitrate.

The researches on the atomic weight of nitrogen, carried out at Geneva since 1904 by P. A. Guye and his collaborators, include the analysis of nitrous oxide, both gravimetrically and volumetrically, the gravimetric analyses of nitrosyl chloride and nitrogen peroxide, the indirect volumetric analysis of ammonia, the determination of the densities of nitrous oxide and ammonia by the volumetric method, and the density of nitric oxide by the ordinary globe method, and, finally, the measurement of various gaseous compressibilities and critical constants.

It may be stated here that all pieces of apparatus were weighed by the method of vibrations against counterpoises of similar material and shape, and of nearly equal weight, and all weighings corrected to the vacuum standard.

Preparation and Purification of Nitrogen Compounds Used

Guyes and Pintzas apparatus
Guye's and Pintza's apparatus.
Nitrous Oxide. - Since the gas, prepared by any of the usual methods, is always slightly contaminated with nitrogen, recourse was had to the method devised by V. Meyer. Concentrated sodium nitrite solution was dropped from the vessel D into a neutral solution of hydroxylamine sulphate contained in the flask E. The gas evolved was passed through potassium hydroxide solution and concentrated sulphuric acid in the bottles N and P respectively, and finally dried with phosphoric anhydride in S. The solutions were prepared in air-free distilled water, and at the commencement of each experiment the apparatus was evacuated through the tube F; a small quantity of gas was then disengaged, and the apparatus again exhausted. A repetition of this procedure once or twice served to eliminate completely the last traces of air, when the tube F was sealed.

Nitric Oxide. - This gas was produced by three distinct methods, namely: (a) Nitric acid (25 per cent.) was allowed to flow, drop by drop, into a boiling concentrated solution of ferrous sulphate in dilute sulphuric acid, or a concentrated solution of sodium nitrite was slowly added to one of ferrous chloride in hydrochloric acid. The evolved gas (20 litres) was collected over air-free water in a copper gas-holder, from which it was afterwards driven through concentrated sulphuric acid and over phosphoric anhydride, and the dry gas condensed in a receiver cooled in liquid air. (b) A 2 per cent, solution of sodium nitrite in concentrated sulphuric acid was run into an Erlenmeyer flask, the bottom of which was covered with a layer of mercury. The nitric oxide produced was led through concentrated sulphuric acid, and then condensed at the temperature of liquid air. Air was initially eliminated from the apparatus by a process similar to that described under Nitrous Oxide. (c) A 10 per cent, solution of sulphuric acid was added, drop by drop, to aqueous sodium nitrite (6 per cent.). Nitric oxide is produced in this reaction by auto-oxidation of the nitrous acid initially formed:

3HNO2 = HNO3+2NO+H2O.

The gas was thoroughly dried by conducting it through three vessels containing sulphuric acid, and finally liquefied.

It will be noted that higher oxides of nitrogen were always absorbed by concentrated sulphuric acid, and not by potassium hydroxide, since the latter reagent slowly decomposes nitric oxide, producing potassium nitrite and nitrous oxide.

The impurities still present in nitric oxide, prepared by the above methods, include small quantities of nitrous oxide and nitrogen, and traces of higher oxides of nitrogen, and such substances as hydrochloric acid, sulphur dioxide, and chlorine derived from the reagents used. A consideration of the boiling-points of the impurities suggested that it should be possible to prepare nitric oxide free from all these impurities, with the possible exception of nitrogen, by liquefaction and fractional distillation. The presence of even 0.15 per cent, of nitrogen, however, would only cause an error of 0.01 per cent, in the value for the density of the gas. Accordingly, the nitric oxide was liquefied, and boiled under reduced pressure, whereby the more volatile portions escaped, and the residue solidified. It was again liquefied, the more volatile portion again boiled off, and the procedure repeated five or six times, until the volume of the liquid had been reduced by one-half. The solidified residue was then slowly sublimed, under reduced pressure, and the final third rejected. This fractional sublimation was repeated in various experiments from two to five times.

Nitrogen Peroxide. - This was prepared by mixing, at -20° C., purified and carefully dried nitric oxide with excess of pure oxygen, prepared from potassium chlorate. The nitrogen peroxide was repeatedly distilled under reduced pressure in the presence of oxygen, to ensure the absence of trioxide, which would have been difficult to remove by distillation. Special precautions were taken to free the final product from dissolved oxygen.

Although nitrogen peroxide decomposes into nitric oxide and oxygen at moderately high temperatures, at ordinary temperatures the extent to which this decomposition proceeds is quite insignificant, and the peroxide may therefore be regarded as a stable substance.

Ammonia. - Ammonia prepared from commercial ammonium salts is always contaminated with traces of organic bases such as pyridine, which materially raise its density. Accordingly, the gas obtained from a cylinder containing 20 kilograms of liquid ammonia (the first half collected in the distillation of a supply of commercial liquid ammonia) was slowly passed through a long, hard glass tube, packed with little pieces of lime and heated to redness. The organic compounds were thereby decomposed, the nitrogen contained in them being converted into ammonia. The issuing ammonia was collected in pure hydrochloric-acid solution, and on crystallisation pure ammonium chloride was obtained.

The apparatus for obtaining pure, dry ammonia for density measurements, consisted of a large cylindrical tube, filled with a mixture of pure ammonium chloride and quicklime, and connected with a purifying and drying system of six tubes. The first three of these were filled with solid potassium hydroxide; the others contained anhydrous barium oxide. All the connections were of glass. The apparatus was initially evacuated, and then swept out once or twice with dry ammonia, liberated at a convenient rate by suitably warming the mixture of ammonium chloride and lime.

Nitrosyl Chloride. - This substance was prepared by Tilden's method, a mixture of nitrosyl hydrogen sulphate and sodium chloride in equivalent quantities being warmed in vacuo in a small flask to 85° C. The distillate was collected in a small receiver cooled in a bath of solid carbon dioxide and ether. The materials were previously dried in a vacuum over phosphoric anhydride, and the apparatus thoroughly desiccated, since nitrosyl chloride is immediately decomposed by water. For the purification, the nitrosyl chloride was several times fractionally crystallised, the final crystals melted, and the liquid submitted to fractional distillation, with the rejection each time of the first and last fractions. The impurities thus eliminated included traces of hydrogen chloride, chlorine, sulphur dioxide, and higher oxides of nitrogen.

Analytical Methods

(a) Gravimetric Analysis. - In the cases of nitrous oxide and nitrogen peroxide, weighed quantities of the compounds were decomposed by red-hot iron, which quantitatively absorbed the oxygen. Iron spirals were employed, wound on thin porcelain rods, and electrically heated. After an experiment, a spiral was prepared for further use by being heated in a current of pure hydrogen. Since each spiral used was oxidised and reduced a number of times in succession before the final experiments were carried out, traces of impurities in the iron, capable of forming volatile oxygen compounds, must have been eliminated.

Apparatus for analysis of nitrous oxide
Apparatus for analysis of nitrous oxide.
The experimental arrangement adopted for nitrous oxide is indicated in fig. The decomposition vessel A contained the iron spiral, the ends of which were silver-soldered to platinum leads fused into the vessel. Connection with a mercury pump was initially made through B, which was sealed off after A had been completely evacuated. Nitrous oxide was absorbed in C, containing wood charcoal which had previously been carefully purified by igniting in chlorine, boiling with concentrated hydrochloric acid, washing with water and drying in vacuo. The tube C was alternately saturated with nitrous oxide and evacuated several times before being finally charged with the gas. In carrying out an analysis, A was first evacuated and weighed. The apparatus being then fitted up, B was exhausted and sealed off, the spiral heated to bright redness, and by manipulating taps D and E, nitrous oxide was passed, little by little, into A, tap F remaining closed. After the nitrous oxide in A was judged to be completely decomposed, a very slow current of the gas was allowed to pass over the incandescent spiral, the nitrogen escaping by F through the sulphuric acid in G. The current was stopped while a considerable portion of the surface of the spiral still remained unoxidised, D closed and A slowly evacuated by connecting the pump G; throughout the evacuation the spiral was maintained at a red heat to ensure the absence of any iron nitride from its surface. Finally, taps E and F were closed, and, on cooling, the increase in weight of the vessel determined. The amount of nitrous oxide used was given by the loss in weight of C.

Apparatus for analysis of nitrogen peroxide
Apparatus for analysis of nitrogen peroxide.
The decomposition vessel used in the experiments with nitrogen peroxide is shown in fig. It was first evacuated and weighed; pure nitrogen peroxide was distilled into it, and frozen in the tubulure A. The vessel was again exhausted, and the increase in weight, due to the peroxide, was determined. The spiral was then heated to bright redness, and the nitrogen peroxide in the tubulure allowed to evaporate slowly, its vapour being completely decomposed. The residual nitrogen was then pumped out, the spiral being still at a red heat, and finally the vacuous apparatus weighed again, after cooling, in order to determine the weight of the oxygen fixed by the iron.

For the complete analysis of nitrosyl chloride, the pure, dry vapour was very slowly led through a U-tube containing finely divided silver, heated to 400°-500° C.; special experiments showed that the chlorine was thereby quantitatively retained. The residual nitric oxide was conducted through a second U-tube containing finely divided copper at the same temperature in order to absorb the oxygen; and the nitrogen was finally absorbed by metallic calcium, contained in a straight glass tube heated to redness. Each absorption tube was provided with stopcocks, all connections being made by ground-glass joints according to Morley's method; and a manometer was introduced between the second and third absorption tubes. The entire apparatus was evacuated at the beginning of an experiment, and if the experiment was successful, the apparatus remained vacuous at its termination. Perfect desiccation of the interior of the apparatus was, of course, absolutely necessary; the silver and copper were prepared for use by washing with ether and drying in carbon dioxide and hydrogen respectively, while the calcium was heated to redness in vacuo, to eliminate any trace of volatile impurity.

(b) Volumetric Analyses. - A cylindrical bulb containing an iron spiral and connected by a capillary tube to a mercury manometer formed the apparatus used in the volumetric analysis of nitrous oxide. The manometer was provided with the usual opaque-glass point, to which the mercury was always adjusted whenever a measurement of pressure was required. The bulb was calibrated, and also the "dead space" along the capillary connection as far as the zero-point; and the volumes of gas were corrected for the difference in temperature between that in the bulb and the small amount in the dead space.

The bulb was initially filled with pure, dry nitrous oxide and packed with melting ice; the mercury was then adjusted to the zero-point, the pressure of the gas read (to within 0.02 mm.) and reduced to the value at 0° c. The globe was next dried and the nitrous oxide decomposed by heating the iron spiral to whiteness: to prevent permanent deformation of the bulb, the spiral was repeatedly heated to an exceedingly high temperature, but only for a few seconds at a time, the apparatus being allowed to cool considerably between successive heatings. Each experiment was continued till further heating had no influence on the final volume. Finally, the bulb was allowed to cool, packed around with melting ice, and the pressure of the residual gas observed after adjusting the mercury to zero once more.

The volumetric analysis of ammonia was indirect; the relative volumes of nitrogen and hydrogen which combine to form the gas being deduced from the densities and compressibilities of hydrogen, nitrogen, and the mixture of these gases that results from the decomposition of ammonia. The mixed gases were prepared by generating pure ammonia from ammonium chloride and quicklime, and passing it slowly through a cylindrical tube containing a platinum spiral heated to bright redness. The greater part of the ammonia decomposed; the products passed through a U-tube containing glass beads moistened with sulphuric acid to absorb unchanged ammonia, and were dried over phosphoric anhydride. The mixed gases were led into a volumeter, and the density measured as described later.

Density Measurements

Modern determinations of gas densities have been effected by two methods: (i) the "globe" method, and (ii) the "volumeter" method. Each of these was employed by Guye, the former with nitric oxide, and the latter with ammonia, nitrous oxide, and the mixture of nitrogen and hydrogen obtained from ammonia.

In each method the weight of pure, dry gas, which, at an observed temperature (always 0° c. for Guye's measurements) and pressure (approximately atmospheric), occupies a known volume, is determined. The "globe" method has been already outlined; the "volumeter" method differs from it in that the measurements of pressure, volume, and temperature are effected in one apparatus, while the gas is weighed in another.

(i) The "globe" method, first adapted to accurate work by Regnault, is comparatively simple to carry out, permits of several experiments being conducted simultaneously, and furnishes results little, if at all, inferior to those obtained by the volumeter method. It is not necessary to employ very large globes, the results obtained with quite small globes being at least as concordant among themselves as those obtained with 8 to 10 litre globes. Lord Rayleigh's accurate experiments were carried out with a globe the volume of which was about 1.8 litres, while the density of nitric oxide was measured by Gray, using a globe of only 0.267 litre capacity. In their work on the density of nitric oxide Guye and Davila employed three globes of capacities (at 0° C.) 379.80, 385.01, and 817.65 c.c. respectively, calibrated by weighing them empty and then filled with water at 0° C.

A small globe possesses the advantage that the correction necessary on account of its contraction when evacuated is proportionally less than that for a large globe. In fact, the only error that is augmented by employing a small globe is that due to "adsorption" of gas on the surface of the glass.

To obtain accurate results, it is absolutely necessary to have the interior of the globe perfectly dry, and to ensure this, after first repeatedly filling the globe with pure air dried by phosphoric anhydride, it is necessary to fill the globe repeatedly with the pure, dry gas and evacuate. Successive density determinations then give concordant results, provided care is taken to maintain the globe vacuous between the experiments.

The contraction undergone by each globe when evacuated was directly measured by the method suggested by Travers; 1 for this purpose the globe was supported in the interior of a large desiccator, its stem passing through one of the holes in a stopper in the desiccator lid. Through the other hole a calibrated vertical capillary tube was passed. The desiccator was filled with water and immersed in a constant temperature bath; the contraction of the globe was obtained by observing how far the level of the water fell in the capillary tube when the globe was evacuated.

The globes were always filled with nitric oxide at the temperature of melting ice and under a pressure slightly in excess of atmospheric; the tap was opened to allow the pressure to fall to that of the atmosphere, and then closed again. The globe was then dried and weighed against its counterpoise.

The calculation of the results, with all necessary corrections, is explained on p. 130, Vol. I. No correction was applied for "adsorption," but allowance was made for the deviation of the gas from Boyle's Law.

(ii) The volumeter method renders it possible to deal with large quantities of gas, since the apparatus for measuring the volume need not be portable. The weight of the gas may be determined in two ways, either by disengaging the gas from an apparatus which only allows pure, dry gas to escape, and determining the loss in weight of the apparatus, or by removing the gas from the volumeter after its volume, temperature, and pressure have been determined, absorbing it by suitable means, and determining the increase in weight of the absorption apparatus and contents. The former method was used for the nitrogen-hydrogen mixture; the latter for ammonia and nitrous oxide.

Guyes and Pintzas apparatus
Guye's and Pintza's apparatus.
The arrangement adopted by Guye and Pintza is shown in fig. The globes A and B were calibrated by determining the weight of water at 0° C. that filled them to the marks a and b. Their combined volumes amounted to 3502.63 c.c. The "dead space" extending from these marks to the tap L in one direction, and the zero-point n of the mercury manometer in the other, was separately determined, as also was the space between the taps H, I, and L.

In conducting an experiment, the absorption tube c, containing cocoanut charcoal, was evacuated, weighed, and attached to the apparatus as shown. The apparatus, which had previously been rinsed out several times with pure, dry gas, was evacuated, and then slowly filled with gas until the pressure was about one atmosphere. The globes were surrounded by melting ice, and when the temperature of the gas had reached 0° C., the taps I and L were closed, the mercury adjusted to the mark n, and the initial pressure of the gas accurately observed.

The space H I L was next evacuated, the tap I closed, and then, by suitably manipulating the taps H, L, and R, the gas was absorbed in the charcoal contained in the tube c; this tube was cooled in a bath of ether and solid carbon dioxide. After most of the gas had been absorbed, the taps H and L were closed, and the pressure of the residual gas in the globes determined. The gas contained between H, I, and L was pumped out and measured, and the absorption tube removed and re weighed.

In calculating the results, the difference between the initial and final pressures was taken, and the densities deduced; the correction for elasticity of the glass is, however, unnecessary. Due allowance was made for the fact that the temperature of the gas in the dead space was not 0° c., and the results were corrected for the known deviations of the gases from Boyle's Law.

Guye and Pintza considered that by still leaving an appreciable amount of gas in the volumeter at the end of an experiment, any "adsorption" effect was eliminated from their results; but this conclusion is erroneous.

Reference only can be made to the measurements of compressibilities and critical constants (vide supra).

Statement of Results

Analytical Methods.
    Gravimetric analysis of nitrous oxide. In five experiments, 5.6269 grams nitrous oxide yielded 2.0454 grams oxygen.

    N2O:O::44.015:16, and N = 14.008.

    The presence of a trace of air or oxides of carbon in the gas would lead to a slightly high value for the atomic weight of nitrogen.
  1. Gravimetric analysis of nitrogen peroxide. In seven experiments,
    10.3522 grams nitrogen peroxide yielded 7.1999 grams oxygen. Hence
    N2O:O2::46.010:32, and N = 14.010.

    A trace of oxygen dissolved in the nitrogen peroxide would cause the result to be rather low.
  2. Gravimetric analysis of nitrosyl chloride. In five experiments,
    0.6067 gram nitrogen was obtained and 0-6931 gram oxygen. Hence
    N:O::14.006:16, and N = 14.006.

    (ii) Volumetric analysis of nitrous oxide. As the mean of four experiments,
    1 litre of nitrous oxide at N.T.P. yielded 1.00717 litres of nitrogen.
    But increase in volume of iron spiral, due to oxidation, amounts to 0.30 c.c.; hence corrected volume of nitrogen is 1.00687 litres. Also

    1 litre of nitrous oxide at N.T.P. weighs 1.9777 grams (vide infra);
    1 litre of nitrogen at N.T.P. weighs 1.2507 grams (Rayleigh).
    (N2O-N2):N2::16:28.042, and N = 14.021.
  3. Volumetric analysis of ammonia. As the mean of three experiments,
    Weight of "normal" litre of mixed hydrogen and nitrogen = 0.3799 gram.

    Assuming the data of other observers for the densities and compressibilities of hydrogen and nitrogen, it follows from this result that the molecular volumes of these gases are in the ratio of 1.00057:1 at N.T.P., and

    H2:N2::2.0152:28.030, whence N = 14.015 (H=1.0076).

    Guye and Pintza only attach a confirmatory significance to this result, and therefore the details of the calculation are omitted.

Physical Methods. -
    Data. - The values obtained for the weight in grams of a normal litre of gas are as follows:

    NH3 - 0.77079 (mean of 5)
    N2O - 1.9777 (mean of 3)
    NO - 1.3402 (mean of 14)
    The following are the values of A01 at 0° C.:


    The critical constants are as follows:

    Critical temp. (abs.)405.3°311.8°179.5°
    Critical press. (atm.)109.677.871.2

    The values for nitrous oxide and nitric oxide are due to Villard and Olszewski respectively.
  1. Calculation of Molecular Weights; Atomic Weight of Nitrogen. -
    1. (i) By Limiting Densities. Assuming the values 1.4290 and +0.00097 respectively for the normal density and the coefficient A01 for oxygen, we have:

      L.1-A'01Molecular Weight.Atomic Weight of Nitrogen.
      N2O1.97770.9925844.00114 001

    2. From Critical Constants. Applying the formulae of p. 134, Vol. I., to the data already given, the following results are obtained:

      L.10-5a10-5b10-5a010-5b0Molecular Weight.Atomic Weight of Nitrogen.

Summary of Results

The various results obtained by Guye and his collaborators for the atomic weight of nitrogen are given in the following table:

Gravimetric Analysis.Volumetric Analysis.Density Limits.Critical Constants.
NO2:O214.010N:3H (indirect)14.015N2O14.001N2O14.004
N:O (in NOCl)14.006NO14.006NO14.009

The mean of the three gravimetric values is N = 14.008, much the best series being undoubtedly that relating to the analysis of nitrogen peroxide; in each case the determination was a direct one.

The results of the volumetric analysis confirm the gravimetric value; the value obtained from the analysis of nitrous oxide depends, however, on the densities of nitrous oxide and nitrogen as well as on the volumetric ratio measured, while the uncertainty concerning the value deduced from the analysis of ammonia has been already mentioned.

Turning to the physical results, it is seen that the mean value deduced by the method of critical constants is N = 14.009, in close agreement with the gravimetric value. It should be remembered, however, that this method of calculation is empirical, although it yields good results for a number of other atomic weights.

The results obtained by the method of limiting densities are, in the cases of ammonia and nitrous oxide, distinctly lower than the gravimetric value. These two gases are readily liquefied, and with such gases the molecular weights obtained by this method are usually rather low, probably owing to A01 being overestimated. The method of extrapolating for A01, adopted by Jaquerod and Scheuer for ammonia, is open to criticism; while Rayleigh's value for A01 for nitrous oxide is almost certainly too large, since it is obtained by a linear extrapolation. The value N = 14.006 furnished by nitric oxide is, however, in good agreement with the gravimetric value, as also is the value deduced from the density and compressibility of nitrogen.

The experimental work carried out at Geneva therefore leads to the rounded-off value N = 14.01.

A description of the work of R. W. Gray is now given as confirming, in a remarkable manner, the values obtained by Guye. Gray preferred nitric to nitrous oxide on account of the close approximation with which the former gas obeys the gas laws at ordinary temperatures, and also because nitric oxide contains nitrogen and oxygen in approximately equal weights.

Preparation and Purification of Nitric Oxide. - Great precautions were taken to obtain nitric oxide free from nitrogen and nitrous oxide. The gas was prepared by Deventer's method from potassium nitrite, potassium ferrocyanide, and acetic acid. Nitrogen tetroxide was removed by means of strong aqueous caustic potash, and the gas was then passed over solid caustic potash, and finally over phosphorus pentoxide to remove water.

An extensive series of fractionations was then performed in order to remove completely nitrous oxide and nitrogen. The crude, dried nitric oxide was liquefied in a vessel surrounded by a vacuum vessel, and a fresh supply of gas bubbled through the liquid and condensed in a second vessel, which communicated with a similar third vessel, so that the gas which solidified in this last container had again passed through its own liquid in the second vessel. The nitric-oxide gas from the third vessel was now largely freed from nitrous oxide, and was stored over water in a gas-holder. After passing the gas over solid caustic potash and phosphorus pentoxide, the gas was subjected to another series of fractionations, after which it was possible to demonstrate complete absence of nitrous oxide.

The removal of the last traces of nitrogen was a matter of greater difficulty owing to the occlusion of nitrogen by the solid nitric oxide. Although most of the nitrogen could be removed by means of the Topler pump, removal of traces was only effected by subliming the solid gas under 170 mm. pressure, at which the boiling-point and melting- point of nitric oxide coincide. Finally, the solid nitric oxide was sublimed under a pressure of 50 mm., and the last traces of nitrogen removed through the Topler pump.

Determination of the Density of Nitric Oxide. - The density of nitric oxide was determined relatively to that of oxygen by filling a glass bulb of about 300 c.c. capacity with the purified gas, after first exhausting the bulb by means of the Topler pump. The greatest precautions in weighing were taken. Thus, the sealed counterpoise bulb and that containing the gas were treated in an identical manner. The weights of nitric oxide and oxygen were then compared by filling the bulb with pure oxygen under similar conditions.

The weight of a litre of nitric oxide was found to be 1.3402 grams, when Rayleigh's figure was used for the weight of a litre of oxygen at N.T.P., lat. 45°, namely, 1.42905 grams.

The exact molecular weight of nitric oxide was calculated (a) by using the results of Jaquerod and Scheuer for the relative compressibilities of nitric oxide and oxygen, and found to be 30.004; (b) by reduction to 0° C. of the critical constants, and found to be 30.008.

The physical value for the atomic weight of nitrogen was thus found:

(i) Method of limiting densities - 14.004
(ii) Reduction of critical constants - 14.008
Mean value - 14.006

determining the composition of nitric oxide
Gray's apparatus for determining the composition of nitric oxide.
The Gravimetric Analysis of Nitric Oxide. - For the purpose of very exact analysis, the nitric oxide was subjected to a third fractionation and sublimation. The apparatus employed is illustrated in fig. Into the neck of the combustion bulb A was fitted a carefully ground glass stopper B carrying two thick (2 mm. diam.) platinum electrodes EE. Fused to the stopper were a capillary tube and a stop-cock joined to a capillary ground glass joint D. Attached to the electrodes were leads of thick nickel wire supporting a small porcelain boat H. About 75 cm. of fine platinum wire were evenly wound around the whole length of the boat and connected with the nickel leads. By this arrangement the boat and its contents could be raised to any desired temperature by passing an electric current through the electrodes EE.

By means of a short length of capillary glass tubing, the bulb A was connected with the nitrogen absorption bulb M. The latter, which was filled with powdered cocoanut charcoal, was connected by means of a second ground capillary joint K with the rest of the apparatus. A stop-cock D led to a tube connected on the one side with the Topler pump and on the other with the storage bulbs containing the nitric oxide. The capacity of the bulb A was about 300 c.c., whilst that of M was about 100 c.c.

In carrying out an experiment the gas was decomposed in the combustion bulb A. The nitrogen was completely absorbed in the bulb M, which contained cocoa-nut charcoal, by putting A and M into communication after having exhausted the air from the capillary between the ground glass joints K and D. The bulb M was cooled in liquid air, and the weight of nitrogen obtained from a given weight of nitric oxide in A calculated. Similarly, the weight of oxygen taken up by the nickel from the nitric oxide could be obtained by weighing A for the third time. The atomic weight of nitrogen can now be calculated from any of the three ratios, NO:O2, N2:O2, NO:N2, and as a mean of twenty-two experiments Gray obtained the value 14.010.

The density of the nitrogen resulting from the decomposition of nitric oxide was obtained by substituting a density bulb for M, and the mean of two experiments gave the value of 14.008 for the atomic weight of nitrogen.

The final conclusion of Gray, obtained by giving each individual result an equal value, is that the atomic weight of nitrogen is 14.0085, which agrees excellently with the value of 14.0090 as the result of the contemporary work of Guye and his collaborators.

Wourtzel in 1912 calculated the ratio N2:O2, determined from the weight of oxygen required to convert nitric oxide into nitrogen peroxide, and obtained 14.0068 as the average atomic weight of nitrogen from five concordant results.

More recent determinations are in good agreement with the results of Guye and Gray, and the accepted value of 14.008 may be considered as correct to 2 or 3 in the fifth significant figure. Moles and Clavera give 14.008 from the density of nitrogen prepared by ignition of pure sodium azide, NaN3. Batuecas, from the densities and compressibilities of nitrous and nitric oxides, obtains the values 14.003 and 14.006.

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