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The Isotopes of Nitrogen, Mass 15, and Oxygen, Mass 18 and 17, and Their Abundances

1930; American Institute of Physics; Volume: 36; Issue: 2 Linguagem: Inglês

10.1103/physrev.36.333

1536-6065

S. M. Naudé,

Atmospheric chemistry and aerosols

I. The isotopes of nitrogen and oxygen.---The absorption spectrum of the NO $\ensuremath{\gamma}$ bands, in particular that of the (0, 0) band at $\ensuremath{\lambda}2269$, the (1, 0) band at $\ensuremath{\lambda}2154$ and of the (2, 0) band at $\ensuremath{\lambda}2052$ was studied in search of isotopes of nitrogen and in order to verify the recently discovered isotopes of oxygen of mass 18 and 17.A hydrogen continuous source was constructed which could be operated by means of a 5 K.W. transformer which gave 0.5-0.75 amps. through the secondary. It was shown that the silvering of the capillary tube joining the electrodes acts as a catalyzing agent which accelerates the recombination of the hydrogen atoms to molecules th increasing the intensity of the continuous spectrum. Owing to a chemical change in the fused quartz window on the hydrogen source which leaves the quartz coloured to a violet tint if light of shorter wave-length than $\ensuremath{\lambda}1850$ falls on it, the intensity of the continuous spectrum below $\ensuremath{\lambda}2300$ is reduced considerably. This was overcome by using a crystal quartz window which did not show this effect. The photographic plates were sensitized with vaseline.A glass tube 92 cm long and 5 cm in diameter with quartz windows sealed onto either end was used as an absorption tube. NO was prepared by dropping a solution of NaN${\mathrm{O}}_{2}$ into FeS${\mathrm{O}}_{4}$ and ${\mathrm{H}}_{2}$S${\mathrm{O}}_{4}$.Band heads were observed in all three bands investigated corresponding to the calculated heads for the four kinds of molecules ${\mathrm{N}}^{14}$${\mathrm{O}}^{16}$, ${\mathrm{N}}^{15}$${\mathrm{O}}^{16}$, ${\mathrm{N}}^{14}$${\mathrm{O}}^{18}$ and ${\mathrm{N}}^{14}$${\mathrm{O}}^{17}$, the maximum deviation of the observed wave-lengths from the calculated values being 0.035A. The results obtained therefore provide new evidence for the existence of a nitrogen isotope of mass 15 and verifies the existence of the oxygen isotopes of mass 18 and 17.II. The relative abundance of ${\mathrm{O}}^{16}$ and ${\mathrm{O}}^{18}$.---From the atmospheric bands of oxygen Babcock obtained the relative abundance of ${\mathrm{O}}^{16}$ and ${\mathrm{O}}^{18}$ to be 1250 with a probable error of 25 percent. ${({\mathrm{O}}^{16})}_{2}$ is symmetric whereas ${\mathrm{O}}^{16}$${\mathrm{O}}^{18}$ is not. This may give different absorption coefficients for the two molecules. This difficulty disappears in NO. By comparing the pressures of NO in the absorption tube at which the (1, 0) ${P}_{1}{N}^{14}{O}^{16}$ head could be made to have the same intensity as the ${P}_{1}{N}^{14}{O}^{18}$ head the relative abundance of ${\mathrm{O}}^{16}$ and ${\mathrm{O}}^{18}$ was found to be 1075 \ifmmode\pm\else\textpm\fi{} 110.The relative abundance of ${\mathrm{N}}^{14}$ and ${\mathrm{N}}^{15}$.---Because in the (1, 0) band the ${P}_{1}{N}^{15}{O}^{16}$ head has the same intensity as the head ${Q}_{1}{N}^{14}{O}^{18}$ head, the relative abundance of the ${\mathrm{N}}^{15}$ and ${\mathrm{O}}^{18}$ isotopes is inversely proportional to the relative intensity of the ${P}_{1}$ and ${Q}_{1}{N}^{14}{O}^{16}$ heads. This relative intensity was found to be 0.65 \ifmmode\pm\else\textpm\fi{} 0.1. The relative abundance of ${\mathrm{N}}^{14}$ and ${\mathrm{N}}^{15}$ is therefore 1075 \ifmmode\times\else\texttimes\fi{} 0.65 or 700 \ifmmode\pm\else\textpm\fi{} 140.From the relative abundance of ${\mathrm{O}}^{16}$ and ${\mathrm{O}}^{18}$, and ${\mathrm{O}}^{16}$ and ${\mathrm{O}}^{17}$ the mass of the ${\mathrm{O}}^{16}$ isotope was calculated to be 15.9980 \ifmmode\pm\else\textpm\fi{} 0.0002 if the atomic weight of the mixture of isotopes was taken to be 16.0000. Aston defines the mass of the ${\mathrm{O}}^{16}$ isotope as 16.000 and consequently his atomic weights are higher than the corresponding chemical atomic weights. The chemical atomic weights and Aston's should agree if we correct the ${\mathrm{O}}^{16}$ isotope to an atomic weight of 15.9980 \ifmmode\pm\else\textpm\fi{} 0.0002, that is by 1.25 parts in 10000.The mass of the ${\mathrm{N}}^{14}$ isotope was calculated to be 14.0069 \ifmmode\pm\else\textpm\fi{} 0.0012 which is in close agreement with Aston's corrected value of 14.0063 \ifmmode\pm\else\textpm\fi{} 0.0029.