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A RAPID METHOD FOR THE DETERMINATION OF NITROGEN OXIDES IN AIR
John F. Flagg
University of Rochester
This document consists of 3 pages.
Date of Manuscript: September 5, 1944
Date Declassified: May 15, 1947
This document is issued for official use.
Its issuance does not constitute authority
to declassify copies or versions of the
same or similar content and title
and by the same authorss.
Technical Information Division, Oak Ridge Directed Operations
Oak Ridge, Tennessee
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A RAPID METHOD FOR THE DETERMINATION OF NITROGEN OXIDE
By Johnl F Flagg and Ralph.Lobenp
There are available at present two distinctly different types of method for determining nitrogen
oxides (NO, NO,, N20O
The first of these' involves oxidation of the oxides to niThsajatdAising
hydrogen peroxide, afterr first absorbing the gas from a definite volume of air in a suitable solution.
' The nitric acid is then'determiined bmconveneiohal meas, such as the diphe'nyIniine, chloranil, pie-
- noldisulfonic acid, or polarogaphic methods. -
The second method2 involves absorption of the oxides in a suitable qobi a"d tei in4awnof
the nitrous acid produced. This is done by use of the alpha napthyl amine-sulfanilic acid erS.
The first method gave good results in the hands of theoriginators and would appear to suffer no
defect save the fact that a two-hour oxidation period is necessary before the final analysis can be
The methods of final analysis ,(diphenylamin; etce):are standard and accurate. The second
method,i w e more rapid, is empirical.
'heq gojlrimetrie determination of the nitrous, aid is made
in the syringe with which the air sample wa taken mainng for greate peed.
is satisfactory of the order of 20 per cent.
The accuracy claimed
The method presented he're'was devised for rapid, semquati e id work, using a minimum
f o equipment and reagents.
PRINCIPLE OF THE METHOD
The adsorptive power of silica ge1 is well known a d t see8dd imke9 hsm amoumtas m nitro-
gen oxides could be concentrated upon silica for furtl tesg.Thi iroveA
air samples of suitable size containing nitrogen dioxidewere dp~rnover small amounts of silica in
glass tubes, complete removal of the gas was noted. Ai~pmaeAten4l "ly to find a reagent. with
which the silica might be treated in order to.develop a col withth adsorbed gas. Diphenyazamine
in sulfuric acid was found a satisfactory reantfg i,18psej elpLU eo rl nsi
, ,of whichh is proportional to the amount of NOn iem*A fmIW a t,$olQwmye shall.assume that we are
dealing primarily with the oxide NO2.) '
The colors developed by treating the silica with diphenylamine reagenit could te compare4wfith
known standards and thus the concentration of NO2 in the air-gas mixture determined. Another method
was also tried in which :the.dipheiylamine was added.directly to the silica without removing'it from
the tubes .The length of the colored zone was observed after the manner of chromatographic adsorp-
.The first method gave more
sat isfactory results.
:r EXPERIMENTALL PART.
. l .a' nt .
obtained from the Davidson Chemical Co
., Baltimore, Maryland, was
cleaned by heating about 50 cc with 100 cc of 6N HCL.
Heating proceeded lor 30 minutes; the acid was
decanted, and the silica was washed hot with several hundred cc of distilled water.
Iron is the
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principal impurity removed by this process.
The silica was then dried, ground, and sieved.
tions saved were 40-60*
, 60-80, 80-100, 100-150.
These portions were dried at 150 to 180fC
until no more steam was evolved (about 1 hour)
then stored in tightly capped bottles.
should be almost perfectly white after this treatment.
2. The nitrate standard was made by dissolving 72.16 mg of K N in4 0Qpl water. This s
contained 328 pnm of nitrogen as NQ.. This solution was diluted to secure standards of lower
3. Diphenylamine reagent was prepared by dissolving 50 mg of diphenylamine in 25 ml of concen-
4. Nitrogen dioxide was prepared before each series of experiments by treatinogo~per; con-
centrated nitric acid.
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The only apparatus used consisted of (1) z 50-cc Yale B-D syringe fgr talking s ples| and
5-liter flasks for storing the air-gas mixtures, and (3) numerous glass tubes, -m insid gneter,
about 40 mm long, in which the silica was placed.
1. Synthetic samples: The method of preparing atmospheres co~t$iknih knowth bif d of Ncq
was as follows. Nitrogen dioxide was 'generated By the action f nitric ac n cpir
flask. When the pressure of the gas was slightly greater thanxattmspherid
in an evacuated pipet.
a ample as c&k ted
Several pipets were used with capacities of 2 to S>b. "Aft er ~fi liii&e
pressure of the sample with the atmosphere, the NQ waswashed with *aiint0o 4ithe e -orS-lter
The mixture was thoroughly mixed by pumping with ile syrrige.
Using the texdpi rr d
barometric pressure, the volume of NO2 at
diluted again in the manner described.
2. Taking the samples:
A glass tube filled with 20 mm of silia (100-150) and plugged at each end
with glass wool was attached by means oat rubber tubing to the syringe. A 50-cc sample was drawn in
slowly, then slowly expelled,. The silica wasthen ready for treatment with di eniylmin p
3. Development of color: One plug of glass Wool was removed from the tubmid 'e sfaic aoured
into a depression on a spot plate. In adjacent spots similar amounts of fr&dh siita wei plai Two
drops of the various nitrate standards *ere placed or the fresh silica ThenWo both stafdr d
unknown were added 12 drops of diphenylamine reageht. After 4 to 5 minittds, StibgnMiegkhe, the
colorswere compared. Determninations were usually~ tiade 'in tiplieate amiltThtIrbn aibkilica
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* The conditions finally choMsn for Iakingte~iwtlysn tsd~ed ktiaingh f50-J cciiUdEnple
and comparing it with the nitrate stadaddsdas teseribd it F1igure 1*sfiOWthresiloieese
The relationship between ppni in the ta"htl iethf~ dlittib is Rltn eaf '4he isfitul
When the gas concentration is over 50 to 60 ppm, the color produced from a 50-cc sample is
too intense for accurate comparison with the standards. Presumably a smia~ier sample (25 cc) could
be taken, although this was not tested carefully. W
with a 50-ec sample is faint, though easy to match.
might be taken.,
A" Vi" d, E
ith concentrations of 1 to 5 ppm, the color produced
With smaller concentrations, a 100- ITfie
* Passed the 40 mesh sieve, not the 60, etc.
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Inasmuch as the nitrate standards can be read to only 3 ppm, it is seen from the curve that the
concentration of the gas can Ie read only to +5 ppm. Thus the accuracy of the method is limited, al-
though at lower concentrations the error is probably somewhat less.
A reasonably good check may be obtained, upon calculation, between the amount of NO2 on the
silica and in the nitrate standard. Consider the gas concentration at 20 ppm. From Figure I it is seen
to match the nitrate standard having 10 ppm. In the 50-cc gas sample there was present 50/1000
x .04 mg = 0.002 mg of NO2. (20 ppm of NO2 is .04 mg/liter.) The nitrate standard contained grams
of N, or about 10 grams of NOa, per million grams of water. Since, however, only 2 drops of this solu-
tion were used for the test, the total amount of NO2 on the spot plate was .1l106 x 10 g =10-" gram, or
0.001 mg of NQ. Thus, the nitrate solution is about twice as efficient in producing a color as the
adsorbed gas. The explanation for this probably lies in the findings of Patty and Patty? who found
that NO2, when treated with water, is converted only to the extent of 57o into HNQO,.
CHARACTERISTICS OF THE ABSORPTION PROCESS
Mesh Size of the Silica: The larger mesh sizes (40-60, 60-80, 80-100) work well, and offer little
resistance to drawing the sample. The color produced upon development is difficult to compare with
the standards, since it is the particles themselves tha are colored. The color is more uniform with
small mesh sizes (100-150). Particles smaller than these offer considerable resistance to drawing
and are not satisfactory.
Depth of Silica Layer: Using a 4-mm (inner diameter) tube, 10 mm of silica will remove 10 or 20
ppm of NO, from a 50-cc sample; 20 mm of silica will remove up to 100 ppm from a 50-cc sample.
Use of deeper beds of silica offers no advantage and only makes comparison difficult later.
Retention of the Gas: Tubes containing adsorbed NQ, from a 20-ppm sample, showed no loss of
gas after one week.
Interfering Substances: Presumably other oxidizing gases such as ozone would interfere. No
tests have been made.
Sample Sizes: There is a direct proportionality between sample size and color developed. For
example, if a 10-cc sample of gas gives a color matching 1 ppm nitrate standard, then a 20-cc sample
will match 2 ppm nitrate standard, etc.
"Chromatographic" Adsorption: In a number of cases, diphenylamine was added directly to the
silica tube after adsorption of the gas. The depth of the c lored zone was observed and an attempt
made to correlate zone depth with concentration. Zone depth was influenced too much by washing and
time of standing to be a reliable indicator of concentration.
1. A method has been devised for adsorbing NO2 on silica gel, treating with diphenylamine, and
comparing the color with standards.
2. The method is applicable to air-gas mixtures containing from 1 to 50 ppm of NOq.
3. Advantages claimed are sensitivity and speed; only simple apparatus and reagents bring
required. Disadvantage lies in only moderate accuracy.
1. Cholak and McNary, J. Ind. Hygiene and Toxicology, 25, 35-; (1943)
2. Patty and Patty, ibid, 25, 361 (1943).
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