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The use of a high-frequency tesla discharge tube for the determination of nitrogen and oxygen in helium
THE
USE OF A HIGH-FREQUENCY TESLA DISCHARGE TUBE FOR THE DETERMINATION OF NITROGEN AND OXYGEN IN HELIUM P. EMMOTTand R. E. WILSON Chemical Inspectorate,
The War Office, Woolwich, London S.E.18, England
Smtmary_-A study has been made of the variation in light intensity, within the wavelength range 300-700 mp, of a high-frequency (B&a> discharge in helium and in mixtures of this gas with nitrogen and oxygen. The increase in intensity in the presence of nitrogen and the decrease in intensity in the presence of oxygen have been related ta the amounts present, and applied to the determination of up to 1% of nitrogen and up to 0.7% of oxygen (v/v). The interfering effect of nitrogen on the latter determination has been studied. The precision of the determinations and the recovery on synthetic mixtures are satisfactory. INTRODUCTION
THE theory and practice of the high-f~quency excitation of gaseous mixtures, used as an analytical tool, have been reviewed by M&rath, Magee, Pickering and Wilson1 The possible applications of a Tesla discharge tube for the analysis of gaseous mixtures have been mentioned by Stemberg and Poulson,a and preliminary experimental work confirmed that the light intensity of such a discharge varies considerably with the impurity content. An analytical technique based on this phenomenon would have the advantages of not requiring expensive equipment and of being simple and potentially versatile. It was decided, therefore, to study the changes in the light intensity of a discharge tube containing helium on the introduction of nitrogen or oxygen, using a selenium photocell mounted on the side of the tube as a light detector. It was realised, however, that an analytical method based an such a study must be largely empirical, and that the effect of possible interfering factors, particularly pressure variatiuns, would require careful investigation. The important fundamental can be expressed as follows:
R+C*-+R++2e R+C*+R*+C R* -+R+hv where R R* R+ and C*
processes involved in a high-f~quency ionisation by electron impact excitation by electron impact light emission
discharge (11 (2) (3)
is a gas molecule or atom, is an excited molecule or atom, is an ion, is an electron with a high kinetic energy.
Processes (1) and (2) are often referred to as collisional processes of the first kind.3 1003
1004
P. EMMOTTand R. E. WILSON
Excitation and ionisation can also be effected by the impact of fast moving atoms or molecules, and by light quanta. The excitation or ionisation of a molecule or atom by an excited atom or ion is referred to as a collisional process of the second kind. The ionisation of a gas by metastable helium atoms is particularly important and is referred to as the Penning effect: He*+R+He+R++e
(4)
The energy of a metastable helium atom is about 20 ev, sufficient to ionise the permanent gases, most of which have an ionisation potential of about 15 ev. In absolutely pure helium the effective collisional process is of the first kind [processes (1) and (2)], but in the presence of traces of impurity, process (4) becomes effective, and is made use of in the present paper for the determination of nitrogen. The principal loss of ions in a discharge is through the recombination of positive and negative ions. This is 105-108 times as probable as the recombination of a positive ion with an electron. Oxygen, the halogens and compounds of either are able to capture free electrons to form negative ions, which then combine with positive ions to decrease the number of ions present. R++O,--+R+O,
(5)
This recombination effect means that the presence of oxygen, in particular, can seriously interfere with the spectral examination of a gas, but it has been made use of in the present paper as a method for the determination of oxygen and by Lovelock and Lipsky in the Electron-Capture Ionisation Detector. A further process involves non-elastic collision between fast electrons and gas molecules, whereby electrons lose energy and the processes (1) to (4) are partially suppressed : C* + R + C + energised R (6) Collisions between electrons and an inert gas molecule are elastic, and the suppressing effect of the introduction of a foreign gas into an inert gas discharge is made use of by Ellis and Forrest5 as the basis of a gas chromatographic ionisation detector. Reaction (6) is very important in high-frequency spectra work, and may outweigh the Penning Effect (4) at high impurity concentrations. EXPERIMENTAL Apparatus High-frequency generator A mains-operated Ferranti Tesvac was used, having an output frequency of four megacycles, a maximum output voltage of 30 kv and a power consumption of 30 W. Discharge tube High-frequency discharge tubes with internal electrodes are subject to difficulties associated with contamination from the electrode materials and adsorption on the electrode surface. Preliminary work indicated that discharge tubes without internal electrodes gave more reproducible results, and accordingly this type of tube was used in the present work. The discharge tube (Fig. 1) consisted of a Pyrex capillary tube 1 mm id. and 100 mm long, fitted with external electrodes and sealed at one end. The other end was sealed to a bulb of 150-ml capacity fitted with a tap, to permit evacuation and filling. This reservoir was necessary to minimise the effect of “clean up” in the electrical discharge. Before use, the tube was evacuated, baked at about 400” and conditioned to the sample. The gaseous mixtures were blended and introduced into the discharge tube by the use of conventional high-vacuum gas-transfer techniques.
Determination
of nitrogen and oxygen in helium
loo.5
/--4Omm-j
I
Alumlnlumfoil electrodes ----/--I
1r
’
Eo;th
FIG. I.-Apparatus. Di&arge tube holder and detector assembly The holder (Fig. 1) was fabricated from Tufnol and kept the discharge tube fron+he light detector whilst excluding extraneous light. A fixed aperture, with light from the tube to fall upon the detector, a selenium photocell (EIiIger No. taken from the photocell to a Pye Scalamp galvanometer (Cat. No. 7901(S) which of measurement of the light intensity in arbitrary units.
I
I
1
I
I
50
100
150
200
250
Pressure,
FIG. 2.-Variation
mmHg
of light intensity from helium with pressure.
at a fixed distance a shutter, allowed 633). Leads were provided a means
P. EMMOTTand R. E. WILSON
1006
Sampling pressure Helium was introduced into the discharge tube, and the light intensity of the discharge was measured at different pressures. A curve was drawn (Fig. 2) relating the light intensity and the gas pressure. This indicated a maximum light intensity at a pressure of 3 mm of mercury. This condition was difficult to reproduce exactly, and it was decided to sample the helium at the more convenient pressure of 200 mm, where the pressure/light intensity curve is not steep and the intensity is still adequate for accurate measurement. Effect of prolonged discharge The discharge tube was filled with helium at 200 mm pressure, and was subjected to a continuous discharge. Readings of the light intensity were taken at intervals. These indicated that the light intensity did not vary with time except during the first minute of the discharge, when the readings were somewhat unsteady. In all succeeding experiments 1 min was allowed to elapse between striking the discharge and measuring the light intensity.
r
20
0
I
I
I
I
0.5
I.0
I.5
2.0
Nitrogen,
FIG. 3.-Variation
%
of light intensity with nitrogen content, oxygen being absent.
Variation of light intensity with impurities Mixtures containing up to 2 % of nitrogen in helium were made up in a simple gas-blending rig, and were introduced at 200 mm pressure into the discharge tube. The light intensity of each mixture was measured, and a graph was drawn of nitrogen content against light intensity (Fig. 3). This curve showed an increase in light intensity with an increase in nitrogen content up to 2%. In the same way, mixtures containing up to 0.7% of oxygen in helium were prepared, and a graph was obtained (Fig. 4) showing the reduction in light intensity with increase in oxygen content up to 0.7 %. It is apparent that the presence of either oxygen or nitrogen will interfere with the determination of the other. N.B. In these and succeeding experiments the helium used in making up synthetic mixtures was purified by passing through a 5-8, molecular sieve at 77°K before use. RESULTS Determination
of nitrogen
AND
DISCUSSION
in helium
The light intensities of the discharges in mixtures of helium containing up to 1 y0 of nitrogen were measured at a sampling pressure of 200 mm, and Fig. 3 was used as a calibration curve to calculate the nitrogen content. Table I compares the known nitrogen content with that found using Fig. 3, and indicates that up to 1 o/o the error is about O-O2o/oof nitrogen.
Determination
1007
of nitrogen and oxygen in helium
I -
I 0.1
0
I 0.2
I 0.3
Oxygen,
FIG. 4.-Variation
I 0.6
I 0.7
%
of light intensity with oxygen content, nitrogen being absent.
TABLE I.--TIIE
DETERMINATION
Nitrogen added, %, u/v
Light intensity (arbitrary units)
0 0.04
5.1 6.1 8.4 8.5 10.7 13.3 15.2 15.0
0.10 0.15 0.27 0.52 0.96 0.98
I 0.5
I 0.4
OF NITROGEN
IN
HELIUM
Nitrogen recovered,
(Fig. 3) %, 40 0.04 0.14 0.13 0.27 0.52 1.00 0.94
Error 0.00 +o+M -0*02 0.00 0.00 +oGI -0M
The use of Fig. 3 as a calibration curve for the determination of nitrogen in helium required the prior removal of oxygen to eliminate interference. Commercial helium was therefore passed through a manganous oxide trap to remove any oxygen present and was then introduced into the discharge tube at 200 mm pressure. The light intensity of the discharge was measured under the same conditions as before and Fig. 3 was used as a calibration curve to calculate the nitrogen content of the helium sample. Replicate results were obtained for a sample of helium containing O-10?; of nitrogen (determined mass spectrometrically), and are recorded in Table II. Determination of oxygen in helium The interfering effect of nitrogen on the determination of oxygen in helium cannot be easily eliminated, because the removal of the nitrogen from the sample is diflicult. It was decided to investigate this interference in more detail. Mixtures of helium and nitrogen were prepared containing up to 40% of nitrogen, and the light intensity of each was measured. Initially, the discharge colour was pink and the light intensity rose sharply with increasing nitrogen content; it remained sensibly constant between 2% and 5% of nitrogen; and it then fell rapidly throughout the rest of the range considered, where the colour of the discharge was deep purple. 3
P. EMMO~ and R. E.
1008
WILSON
TABLE 11.--I&~DERATION IN
OF
NITROGEN
HELIUM
Light intensity (arbitrary units)
Nitrogen found, %, V/U (Fig. 3)
8.8 8.4 6.5 6.3 7.2 8.0 7.8 7.7
0.15 0.14 0.06 0.05 O-09 0.12 0.11 0.10
Mean result Standard deviation
0.10% 0‘03
A series of mixtures containing up to 05% oxygen in helium were made up and various amounts of nitrogen were added to each. The light intensity of each mixture was measured, and Table III and Fig. 5 show the relationship between oxygen content and light intensity despite the variation in nitrogen content from 1-6’$4 to 53%. Two results, for concentrations of O-1y0 and 150% nitrogen respectively, are included in the Table to show the necessity for control of the nitrogen content. TABLE III.-THE DE~~A~ON
OF OXYCBN
IN
HEW
CONWNING
NlTRWEN
Nitrogen added, %. eio
;:I: 3.1 2-3 3.9 2.1 l-6 5.3 1.9 1.7 0*1* 1s=o*
oxygen added, %, o/v
0.02 040 0.11 0.20 0.24 0.30 0.34 0.42 OS1 066 O-02 0.12
Light Intensity (arbitrary units)
14.8 160 13.8 12.6 12.5 11.6 11.4 10.6 ;:; ;:;
Oxygen recovered, %, v/v (Fig. 5)
0.04 0.11 0.21 0.22 0.30 0.32 0.41 o-57 O-60 >0*7 >0*7
Error, %
+0*02 003 -t-o*01 -0*02 0.00 -@02 -0.01 $0.06 -0@6
* Effect of nitrogen content outside the set Iimits.
Fig. 5 can be used as a calibration curve for the determination of up to 0.7% of oxygen in helium. An analytical method based on this curve would first require the determination of the nitrogen and then its adjustment to between 2% and 5% by the addition of the calculated amount of pure nitrogen. Table III gives the oxygen content as determined from Fig. 5 using the observed light intensity values, and compares them with the actual oxygen figures. It can be seen that the average error is about 0.02% of oxygen.
Deiermination of nitrogen and qgen
0
I
I
I
I
04
0.e
0.3
o-4 Oxygen,
E%tk
S.-Variation
I 0.5
1009
in helium
1 0.6
I 0.7
%
of light intensity with oxygen content, nitroger being present, CONCLUSIONS
A selenium photocell connected directly to a galvanometer is sufkiently sensitive for the satisfactory measurement of the light intensity from a Tesla discharge tube containing helium. The light intensity from such a discharge increases on the addition of small amounts of nitrogen, and if oxygen is removed, e.g., by a manganous oxide trap, it is possible to use this property to measure the nitrogen content of helium over the range O-l yOby volume. The light intensity decreases on the addition of oxygen, and this property can be used to measure the oxygen content of helium over the range U-O*?% by volume, provided that no nitrogen is present or that the nitrogen content can be adjusted to between 2 and 5%, The probable error involved in the determination of nitrogen or oxygen by these techniques is about -&0-03~&, The method is a rather restricted one, and liable to extreme error ~ind~scriminately applied to the analysis of gas mixtures in general. ft is also very sensitive to physical parameters such as pressure. Zusammenfassuna-Die &derung der Lichtintensit~t zurischen300 und 7OOmp einer H~~r~uenz-(Tesla-)entladung in Helium und Gemischen von Helium knit Stickstoff und Sauerstoff wurde untorsucht. Der IntensZitszuwachs in Gegenwart von Stickstoff und die Abnahme mit Sauerstoff wurde zu den anwesenden Meneen in BeziehunE gesetzt und fiir die Bestimmung von his N 1% S&c&off und 47% Sauerstoff 0 verwendet. Die St&untz van Sti&stoiY bei der ~ue~~~~~g wurde ebenfails unt&sucht. I% Genatigkeit der bang und die Ergebrdsse an seIbst hergestellten Miischungen sind ~~te~i~d~
1010
P. EWMOIT and R. E. WILSON Resume-Dans Ie domaine de longueurs d’onde compris entre 300 et 700 m,u, on a Ctudie la variation de l’intensite de lumiere dune decharge haute frequence (Tesla) dans l’hblium et dans des melanges de ce gaz avec I’azote et l’oxygene. L’accroissement de l’intensite en presence d’azote, et son decroissement en presence d’oxygene ont et6 relies aux quantites presentes, et appliques aux dosages de l’azote jusqu’a 1% et de I’oxygene jusqu’a 0,7x (v/v). On a Btudie l’effet perturbateur de l’azote sur le demier dosage. La precision du dosage et la r&cup&ation, pour des melanges synthetiques, sont satisfaisantes. REFERENCES
1 W. D. McGrath, R. J. Magee, W. F. Pickering and C. L. Wilson, Talanta, 1961, 8, 892. * J. L. Stemburg and R. E. Paulson, J. Chromatop.. 1960. 3.406. s L. B. Loeb, B&c Processes of Ga&ous Electron&. University of California, 1955. b J. E. Lovelock and S. R. Lipsky, J. Amer. Chem. Sot., 1960,82,431. 5 J. F. Ellis and C. W. Forrest, Analyt. Chim. Acta, 1961, 24, 329.
USE OF A HIGH-FREQUENCY TESLA DISCHARGE TUBE FOR THE DETERMINATION OF NITROGEN AND OXYGEN IN HELIUM P. EMMOTTand R. E. WILSON Chemical Inspectorate,
The War Office, Woolwich, London S.E.18, England
Smtmary_-A study has been made of the variation in light intensity, within the wavelength range 300-700 mp, of a high-frequency (B&a> discharge in helium and in mixtures of this gas with nitrogen and oxygen. The increase in intensity in the presence of nitrogen and the decrease in intensity in the presence of oxygen have been related ta the amounts present, and applied to the determination of up to 1% of nitrogen and up to 0.7% of oxygen (v/v). The interfering effect of nitrogen on the latter determination has been studied. The precision of the determinations and the recovery on synthetic mixtures are satisfactory. INTRODUCTION
THE theory and practice of the high-f~quency excitation of gaseous mixtures, used as an analytical tool, have been reviewed by M&rath, Magee, Pickering and Wilson1 The possible applications of a Tesla discharge tube for the analysis of gaseous mixtures have been mentioned by Stemberg and Poulson,a and preliminary experimental work confirmed that the light intensity of such a discharge varies considerably with the impurity content. An analytical technique based on this phenomenon would have the advantages of not requiring expensive equipment and of being simple and potentially versatile. It was decided, therefore, to study the changes in the light intensity of a discharge tube containing helium on the introduction of nitrogen or oxygen, using a selenium photocell mounted on the side of the tube as a light detector. It was realised, however, that an analytical method based an such a study must be largely empirical, and that the effect of possible interfering factors, particularly pressure variatiuns, would require careful investigation. The important fundamental can be expressed as follows:
R+C*-+R++2e R+C*+R*+C R* -+R+hv where R R* R+ and C*
processes involved in a high-f~quency ionisation by electron impact excitation by electron impact light emission
discharge (11 (2) (3)
is a gas molecule or atom, is an excited molecule or atom, is an ion, is an electron with a high kinetic energy.
Processes (1) and (2) are often referred to as collisional processes of the first kind.3 1003
1004
P. EMMOTTand R. E. WILSON
Excitation and ionisation can also be effected by the impact of fast moving atoms or molecules, and by light quanta. The excitation or ionisation of a molecule or atom by an excited atom or ion is referred to as a collisional process of the second kind. The ionisation of a gas by metastable helium atoms is particularly important and is referred to as the Penning effect: He*+R+He+R++e
(4)
The energy of a metastable helium atom is about 20 ev, sufficient to ionise the permanent gases, most of which have an ionisation potential of about 15 ev. In absolutely pure helium the effective collisional process is of the first kind [processes (1) and (2)], but in the presence of traces of impurity, process (4) becomes effective, and is made use of in the present paper for the determination of nitrogen. The principal loss of ions in a discharge is through the recombination of positive and negative ions. This is 105-108 times as probable as the recombination of a positive ion with an electron. Oxygen, the halogens and compounds of either are able to capture free electrons to form negative ions, which then combine with positive ions to decrease the number of ions present. R++O,--+R+O,
(5)
This recombination effect means that the presence of oxygen, in particular, can seriously interfere with the spectral examination of a gas, but it has been made use of in the present paper as a method for the determination of oxygen and by Lovelock and Lipsky in the Electron-Capture Ionisation Detector. A further process involves non-elastic collision between fast electrons and gas molecules, whereby electrons lose energy and the processes (1) to (4) are partially suppressed : C* + R + C + energised R (6) Collisions between electrons and an inert gas molecule are elastic, and the suppressing effect of the introduction of a foreign gas into an inert gas discharge is made use of by Ellis and Forrest5 as the basis of a gas chromatographic ionisation detector. Reaction (6) is very important in high-frequency spectra work, and may outweigh the Penning Effect (4) at high impurity concentrations. EXPERIMENTAL Apparatus High-frequency generator A mains-operated Ferranti Tesvac was used, having an output frequency of four megacycles, a maximum output voltage of 30 kv and a power consumption of 30 W. Discharge tube High-frequency discharge tubes with internal electrodes are subject to difficulties associated with contamination from the electrode materials and adsorption on the electrode surface. Preliminary work indicated that discharge tubes without internal electrodes gave more reproducible results, and accordingly this type of tube was used in the present work. The discharge tube (Fig. 1) consisted of a Pyrex capillary tube 1 mm id. and 100 mm long, fitted with external electrodes and sealed at one end. The other end was sealed to a bulb of 150-ml capacity fitted with a tap, to permit evacuation and filling. This reservoir was necessary to minimise the effect of “clean up” in the electrical discharge. Before use, the tube was evacuated, baked at about 400” and conditioned to the sample. The gaseous mixtures were blended and introduced into the discharge tube by the use of conventional high-vacuum gas-transfer techniques.
Determination
of nitrogen and oxygen in helium
loo.5
/--4Omm-j
I
Alumlnlumfoil electrodes ----/--I
1r
’
Eo;th
FIG. I.-Apparatus. Di&arge tube holder and detector assembly The holder (Fig. 1) was fabricated from Tufnol and kept the discharge tube fron+he light detector whilst excluding extraneous light. A fixed aperture, with light from the tube to fall upon the detector, a selenium photocell (EIiIger No. taken from the photocell to a Pye Scalamp galvanometer (Cat. No. 7901(S) which of measurement of the light intensity in arbitrary units.
I
I
1
I
I
50
100
150
200
250
Pressure,
FIG. 2.-Variation
mmHg
of light intensity from helium with pressure.
at a fixed distance a shutter, allowed 633). Leads were provided a means
P. EMMOTTand R. E. WILSON
1006
Sampling pressure Helium was introduced into the discharge tube, and the light intensity of the discharge was measured at different pressures. A curve was drawn (Fig. 2) relating the light intensity and the gas pressure. This indicated a maximum light intensity at a pressure of 3 mm of mercury. This condition was difficult to reproduce exactly, and it was decided to sample the helium at the more convenient pressure of 200 mm, where the pressure/light intensity curve is not steep and the intensity is still adequate for accurate measurement. Effect of prolonged discharge The discharge tube was filled with helium at 200 mm pressure, and was subjected to a continuous discharge. Readings of the light intensity were taken at intervals. These indicated that the light intensity did not vary with time except during the first minute of the discharge, when the readings were somewhat unsteady. In all succeeding experiments 1 min was allowed to elapse between striking the discharge and measuring the light intensity.
r
20
0
I
I
I
I
0.5
I.0
I.5
2.0
Nitrogen,
FIG. 3.-Variation
%
of light intensity with nitrogen content, oxygen being absent.
Variation of light intensity with impurities Mixtures containing up to 2 % of nitrogen in helium were made up in a simple gas-blending rig, and were introduced at 200 mm pressure into the discharge tube. The light intensity of each mixture was measured, and a graph was drawn of nitrogen content against light intensity (Fig. 3). This curve showed an increase in light intensity with an increase in nitrogen content up to 2%. In the same way, mixtures containing up to 0.7% of oxygen in helium were prepared, and a graph was obtained (Fig. 4) showing the reduction in light intensity with increase in oxygen content up to 0.7 %. It is apparent that the presence of either oxygen or nitrogen will interfere with the determination of the other. N.B. In these and succeeding experiments the helium used in making up synthetic mixtures was purified by passing through a 5-8, molecular sieve at 77°K before use. RESULTS Determination
of nitrogen
AND
DISCUSSION
in helium
The light intensities of the discharges in mixtures of helium containing up to 1 y0 of nitrogen were measured at a sampling pressure of 200 mm, and Fig. 3 was used as a calibration curve to calculate the nitrogen content. Table I compares the known nitrogen content with that found using Fig. 3, and indicates that up to 1 o/o the error is about O-O2o/oof nitrogen.
Determination
1007
of nitrogen and oxygen in helium
I -
I 0.1
0
I 0.2
I 0.3
Oxygen,
FIG. 4.-Variation
I 0.6
I 0.7
%
of light intensity with oxygen content, nitrogen being absent.
TABLE I.--TIIE
DETERMINATION
Nitrogen added, %, u/v
Light intensity (arbitrary units)
0 0.04
5.1 6.1 8.4 8.5 10.7 13.3 15.2 15.0
0.10 0.15 0.27 0.52 0.96 0.98
I 0.5
I 0.4
OF NITROGEN
IN
HELIUM
Nitrogen recovered,
(Fig. 3) %, 40 0.04 0.14 0.13 0.27 0.52 1.00 0.94
Error 0.00 +o+M -0*02 0.00 0.00 +oGI -0M
The use of Fig. 3 as a calibration curve for the determination of nitrogen in helium required the prior removal of oxygen to eliminate interference. Commercial helium was therefore passed through a manganous oxide trap to remove any oxygen present and was then introduced into the discharge tube at 200 mm pressure. The light intensity of the discharge was measured under the same conditions as before and Fig. 3 was used as a calibration curve to calculate the nitrogen content of the helium sample. Replicate results were obtained for a sample of helium containing O-10?; of nitrogen (determined mass spectrometrically), and are recorded in Table II. Determination of oxygen in helium The interfering effect of nitrogen on the determination of oxygen in helium cannot be easily eliminated, because the removal of the nitrogen from the sample is diflicult. It was decided to investigate this interference in more detail. Mixtures of helium and nitrogen were prepared containing up to 40% of nitrogen, and the light intensity of each was measured. Initially, the discharge colour was pink and the light intensity rose sharply with increasing nitrogen content; it remained sensibly constant between 2% and 5% of nitrogen; and it then fell rapidly throughout the rest of the range considered, where the colour of the discharge was deep purple. 3
P. EMMO~ and R. E.
1008
WILSON
TABLE 11.--I&~DERATION IN
OF
NITROGEN
HELIUM
Light intensity (arbitrary units)
Nitrogen found, %, V/U (Fig. 3)
8.8 8.4 6.5 6.3 7.2 8.0 7.8 7.7
0.15 0.14 0.06 0.05 O-09 0.12 0.11 0.10
Mean result Standard deviation
0.10% 0‘03
A series of mixtures containing up to 05% oxygen in helium were made up and various amounts of nitrogen were added to each. The light intensity of each mixture was measured, and Table III and Fig. 5 show the relationship between oxygen content and light intensity despite the variation in nitrogen content from 1-6’$4 to 53%. Two results, for concentrations of O-1y0 and 150% nitrogen respectively, are included in the Table to show the necessity for control of the nitrogen content. TABLE III.-THE DE~~A~ON
OF OXYCBN
IN
HEW
CONWNING
NlTRWEN
Nitrogen added, %. eio
;:I: 3.1 2-3 3.9 2.1 l-6 5.3 1.9 1.7 0*1* 1s=o*
oxygen added, %, o/v
0.02 040 0.11 0.20 0.24 0.30 0.34 0.42 OS1 066 O-02 0.12
Light Intensity (arbitrary units)
14.8 160 13.8 12.6 12.5 11.6 11.4 10.6 ;:; ;:;
Oxygen recovered, %, v/v (Fig. 5)
0.04 0.11 0.21 0.22 0.30 0.32 0.41 o-57 O-60 >0*7 >0*7
Error, %
+0*02 003 -t-o*01 -0*02 0.00 -@02 -0.01 $0.06 -0@6
* Effect of nitrogen content outside the set Iimits.
Fig. 5 can be used as a calibration curve for the determination of up to 0.7% of oxygen in helium. An analytical method based on this curve would first require the determination of the nitrogen and then its adjustment to between 2% and 5% by the addition of the calculated amount of pure nitrogen. Table III gives the oxygen content as determined from Fig. 5 using the observed light intensity values, and compares them with the actual oxygen figures. It can be seen that the average error is about 0.02% of oxygen.
Deiermination of nitrogen and qgen
0
I
I
I
I
04
0.e
0.3
o-4 Oxygen,
E%tk
S.-Variation
I 0.5
1009
in helium
1 0.6
I 0.7
%
of light intensity with oxygen content, nitroger being present, CONCLUSIONS
A selenium photocell connected directly to a galvanometer is sufkiently sensitive for the satisfactory measurement of the light intensity from a Tesla discharge tube containing helium. The light intensity from such a discharge increases on the addition of small amounts of nitrogen, and if oxygen is removed, e.g., by a manganous oxide trap, it is possible to use this property to measure the nitrogen content of helium over the range O-l yOby volume. The light intensity decreases on the addition of oxygen, and this property can be used to measure the oxygen content of helium over the range U-O*?% by volume, provided that no nitrogen is present or that the nitrogen content can be adjusted to between 2 and 5%, The probable error involved in the determination of nitrogen or oxygen by these techniques is about -&0-03~&, The method is a rather restricted one, and liable to extreme error ~ind~scriminately applied to the analysis of gas mixtures in general. ft is also very sensitive to physical parameters such as pressure. Zusammenfassuna-Die &derung der Lichtintensit~t zurischen300 und 7OOmp einer H~~r~uenz-(Tesla-)entladung in Helium und Gemischen von Helium knit Stickstoff und Sauerstoff wurde untorsucht. Der IntensZitszuwachs in Gegenwart von Stickstoff und die Abnahme mit Sauerstoff wurde zu den anwesenden Meneen in BeziehunE gesetzt und fiir die Bestimmung von his N 1% S&c&off und 47% Sauerstoff 0 verwendet. Die St&untz van Sti&stoiY bei der ~ue~~~~~g wurde ebenfails unt&sucht. I% Genatigkeit der bang und die Ergebrdsse an seIbst hergestellten Miischungen sind ~~te~i~d~
1010
P. EWMOIT and R. E. WILSON Resume-Dans Ie domaine de longueurs d’onde compris entre 300 et 700 m,u, on a Ctudie la variation de l’intensite de lumiere dune decharge haute frequence (Tesla) dans l’hblium et dans des melanges de ce gaz avec I’azote et l’oxygene. L’accroissement de l’intensite en presence d’azote, et son decroissement en presence d’oxygene ont et6 relies aux quantites presentes, et appliques aux dosages de l’azote jusqu’a 1% et de I’oxygene jusqu’a 0,7x (v/v). On a Btudie l’effet perturbateur de l’azote sur le demier dosage. La precision du dosage et la r&cup&ation, pour des melanges synthetiques, sont satisfaisantes. REFERENCES
1 W. D. McGrath, R. J. Magee, W. F. Pickering and C. L. Wilson, Talanta, 1961, 8, 892. * J. L. Stemburg and R. E. Paulson, J. Chromatop.. 1960. 3.406. s L. B. Loeb, B&c Processes of Ga&ous Electron&. University of California, 1955. b J. E. Lovelock and S. R. Lipsky, J. Amer. Chem. Sot., 1960,82,431. 5 J. F. Ellis and C. W. Forrest, Analyt. Chim. Acta, 1961, 24, 329.