Jnited States
nvironmental Protection
600777017
Office of
Research and
Development
Environmental Monitoring and Support
Laboratory
Cincinnati, Ohio 45268
NITRATE INTERFERENCE IN
TOTAL KJELDAHL NITROGEN
DETERMINATIONS AND ITS
REMOVAL BY ANION EXCHANGE
RESINS
Interagency
Energy-Environment
Research and Development
Program Report
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EPA-600/7-77-017
February 1977
NITRATE INTERFERENCE
IN TOTAL KJELDAHL NITROGEN DETERMINATIONS
AND ITS REMOVAL -BY ANION EXCHANGE RESINS
by
Albert Schlueter
Department of Chemistry
Central State University
Wilberforce, Ohio 45384
Grant No. R-802755-02
Project Officer
Morris Gales
Environmental Monitoring and Support Laboratory
Cincinnati, Ohio 45268
ENVIRONMENTAL MONITORING AND SUPPORT LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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DISCLAIMER
This report has been reviewed by the Environmental Monitoring and
Support Laboratory, U.S. Environmental Protection Agency, and approved for
publication. Approval does not signify that the contents necessarily re-
flect the view and policies of the U.S. Environmental Protection Agency, nor
does mention of trade names or commercial products constitute endorsement
or recommendation for use.
11
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FOREWORD
Environmental measurements are required to determine the quality of
ambient waters and the character of waste effluents. The Environmental
Monitoring and Support Laboratory-Cincinnati conducts research to:
°Develop and evaluate techniques to measure the presence and
concentration of physical, chemical, and radiological
pollutants in water, wastewater, bottom sediments, and
solid waste.
°Investigate methods for the concentration, recovery, and
identification of viruses, bacteria, and other microbio-
logical organisms in water. Conduct studies to determine
the responses of aquatic organisms to water quality.
°Conduct an Agency-wide quality assurance program to assure
standardization and quality control of systems for monitor-
ing water and wastewater.
The standard methods for analysis of water and waste samples are
under continual review to assure that the most accurate results possible
are obtained. If a chemical interference in an important analytical
procedure is discovered it must be evaluated and if necessary a procedure
modification made to circumvent the interference. This report investi-
gates the interference of nitrate in the analysis for total Kjeldahl nitro-
gen and suggests a method for removing this interference.
Dwight G. Ballinger
Director
Environmental Monitoring and Support Laboratory
111
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ABSTRACT
The interference of nitrate with the determination of total Kjeldahl
nitrogen (TKN) was investigated. TKN losses of greater than 90% were
observed in solutions containing a nitrate-nitrogen concentration ten times
greater than the TKN level. The loss was found by infrared spectroscopy to
be occurring due to nitrate and ammonia decomposition to nitrous oxide at
the elevated TKN digestion temperatures. Prevention of the nitrate inter-
ference in TKN analyses was found to be possible only by removal of nitrate
prior to TKN analysis. Several anion-exchange resins in the chloride form
were found effective in this removal. Samples containing known TKN levels
and high nitrate concentration when treated by these resins prior to TKN
analysis gave nearly 100% TKN recovery on TKN analysis.
IV
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CONTENTS
Foreword iii
Abstract iv
Tables vi
Acknowledgment vii
I. Introduction 1
II. Summary 2
III. Conclusions 3
IV. Recommendations 4
V. Experimental Procedures 5
VI. Data 7
References 16
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TABLES
Number Page
1 Comparison of the response of the ammonia electrode to
standard solutions with and without TKN reagents for
ionic strength adjustment 8
2 TKN determinations of standard solutions 9
3 The effect of nitrate on TKN recovery 9
4 Loss of TKN as a function of reflux temperature and time .... 10
5 Addition of successive nitrate aliquots to distilled water
and Dowex - 1 X 8 resin 13
6 The use of anion-exchange resins to remove nitrate for TKN
analyses 15
VI
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ACKNOWLEDGMENT
The efforts of Marcia Lewis and Francisca Ogene, two chemistry majors
at Central State University, in collecting most of the data contained in
this report are gratefully acknowledged. The assistance of Michael Taylor
in the initial stages of this work is also noted.
The initial work of Morris Gales, Jr., Project Officer of this grant,
laid the groundwork for this project. His preliminary work and correct
suggestion of the mechanism of the nitrate interference pointed the
direction for the work in this report.
Finally, the support of the U.S. Environmental Protection Agency in
this grant is acknowledged. With this support Central State University
faculty and students have had the invaluable opportunity of participating
in important and relevant research work.
vn
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SECTION I
INTRODUCTION
Amounts of ammonia and organically bound nitrogen, called total Kjeldahl
nitrogen (TKN), are determined by the Kjeldahl method,* a long-accepted
standard method of analysis used for example, to evaluate the protein con-
tent of food. Recent work, however, has demonstrated that high nitrate con-
centrations result in low TKN analysis results. In some cases (nitrate
levels 20-times greater than ammonia), greater than 90% of the Kjeldahl
nitrogen present is lost in analysis. This plus the common natural occur-
rence of nitrate makes the study of its interference a significant one. The
purpose of this study was to: (1) confirm this interference of nitrate in
the Kjeldahl nitrogen determination, (2) understand the mechanism of the
loss of nitrogen, and (3) find a simple, yet effective way to prevent the
nitrate interference and enable accurate TKN determination of any sample.
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SECTION II
SUMMARY
The work presented here demonstrates the interference of the nitrate
ion in TKN determinations, a proposed mechanism of the interference, and one
effective, simple way of eliminating the interference. The results show that
nitrate-nitrogen significantly interferes with TKN determinations whenever
the nitrate-nitrogen levels are ten-times or greater the Kjeldahl nitrogen
levels. By collection of the gases released during a TKN determination and
infrared analysis, the predominant product is found to be nitrous oxide.
This gas unquestionably is formed through combination of ammonia and nitrate
at the high temperatures (greater than 300°C) of the Kjeldahl digestion reac-
tion. Subsequent loss through the digestion reaction causes lowered TKN
results.
As the interference in the TKN determinations is a result of the pre-
sence of nitrate and the use of high temperatures during reflux, elimination
of one or the other is necessary. Because the high temperature is necessary
for decomposition of organic compounds, nitrate removal is the only feasible
approach. The high solubility and low complexing ability of nitrate com-
pounds led to the selection of ion exchange resins as a simple, effective
way to remove the nitrate interference in TKN determinations. Several anion
exchange resins in the chloride form were found effective in reducing nitrate-
nitrogen levels of 25 ppm to essentially zero. Prior treatment of samples
containing greater than 50 fold excess of interfering nitrate with these
resins, followed by TKN analysis, resulted in quantitative recovery of the
original TKN.
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SECTION III
CONCLUSIONS
This work has demonstrated the following:
1. Accurate determination of TKN levels using the ammonia
electrode requires use of standard ammonia solutions containing
electrolytes to adjust the ionic strength to the level present
in the sample.
2. Nitrate does indeed interfere and reduce the TKN in a sample
when it is present in concentrations ten times or greater
than the TKN level.
3. The loss of TKN when nitrate is present occurs by formation of
nitrous oxide, due to the high temperature used in the Kjeldahl
digestion.
4. The interference can only be prevented by removal of nitrate
prior to TKN analysis. Anion-exchange resins in chloride form
were found effective in this removal.
5. Nitrite seems also to interfere in TKN analysis when present
in high concentration. This interference probably occurs due
to the combining of ammonia and nitrite to release nitrogen
gas. As nitrite is generally present in very low levels in
natural water samples, this interference is probably not
significant in most cases.
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SECTION IV
RECOMMENDATIONS
As a result of this work the following need attention:
1. The need for ionic strength adjustment in standards when the ammonia
electrode is used in TKN analysis should be noted.
2. The interference of nitrate in TKN analyses should be noted in the
Methods for Chemical Analysis of Water and Waste manual, as well as
the suggested (or another) procedure for elimination of this
interference.
3. The evaluation of TKN analysis procedures different from the one used
in this study needs to be made to determine the extent of nitrate
interference. The effect of using block digesters with higher digestion
temperatures needs to be investigated. The effect of a catalyst other
than mercury (II) needs to be considered.
4. A re-evaluation of past TKN analyses should be made. Seawater, for
example, can have nitrate levels as high as 0.7 ppm with TKN levels
of 0.03 ppm, and interference would be anticipated. TKN determinations
of run-off water in rural areas might be low due to high nitrate levels
resulting from fertilizers. In some specific TKN applications, invalid
low levels of Kjeldahl nitrogen could be reported due to high nitrate
levels.
5. The interference of nitrite needs to be substantiated more thoroughly.
The blocking of nitrite interference by anion-exchange resins should
be confirmed.
6. Frequently, time-tested analytical procedures stifle efforts to develop
new and better methods. Development of a faster, yet reliable way of
releasing organic nitrogen than the Kjeldahl procedure needs to be
considered. If such a procedure could be developed, nitrate inter-
ference might be avoided.
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SECTION V
EXPERIMENTAL PROCEDURES
MEASUREMENTS
All electromotive potential readings in this report were taken using
an Orion Model 801A digital pH/mv meter, an Orion Model 95-10 ammonia elec-
trode, an Orion Model 90-02 double-junction reference electrode, and an
Orion Model 94-17A chloride electrode. The infrared spectra were obtained
using an IR-8 Beckman spectrophotometer and a Perkin-Elmer long path gas
cell.
SOLUTIONS
All solutions in this work were prepared using distilled water deionized
by a Corning Model LD-2a demineralizer.
Standard solutions of ammonia-nitrogen ranging from 100 ppm to 0.01 ppm
were prepared using reagent grade ammonium chloride. To each solution were
added a few drops of dilute hydrochloric acid to ensure against loss of
ammonia. A second series of ammonia standards was prepared, containing ionic
concentrations identical to those present in samples after distillation
and digestion using TKN reagents. One liter of each of these standard solu-
tions contained 26.7 g of K2S04, 0.4 g of HgO dissolved in 5 ml of 1:5
sulfuric acid, and 40 ml of concentrated ^SO^-all of which were neutralized
ed using 10 M NaOH before addition to the standard plus the required quantity
of ammonium chloride.
The solutions needed for the Kjeldahl analysis were prepared as des-
cribed in References 4 and 6. The 10 M NaOH (alkaline reagent 1) was pre-
pared using reagent grade 50% NaOH solution purchased commercially. The
alkaline reagent 2 (400 g NaOH, 300 g Nal and 2 g EDTA per liter) was
likewise prepared using this 50% NaOH solution.
For most of this work, standard solutions of alanine were analyzed
for TKN. A stock solution of alanine containing 50 ppm nitrogen was prepared
using reagent grade alanine. This solution was prepared fresh every two
weeks because it appeared to hydrolize somewhat within this interval.
Nitrate solutions containing 200 ppm nitrogen, 500 ppm nitrogen, and
100 ppm nitrogen were prepared using NaNO,.
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OPERATING PROCEDURE
The TKN determinations in this study were made by the modified proce-
dure using an ammonia ion electrode (see References 4 and 5). In this
procedure, 50.0 ml of sample are placed in a 100 ml round bottomed flask,
and 10 ml of the sulfuric acid-mercury (II) sulfate-potassium sulfate
solution (called TKN reagent) are added. The mixture is evaporated until
SO? fumes are emitted, then is allowed to digest for 30 minutes. It is
allowed to cool,and the residue'is dissolved in distilled water, rinsed
from the flask into a graduated cylinder, and diluted to 44 ml. An addition
of 6 ml of 10 M NaOH neutralizes most of the sulfuric acid, resulting com-
monly in a pH between 3 and 5. The resulting solution is cooled in an ice
bath to 25°C, a stirring bar added, stirring begun, and the ammonia elec-
trode inserted. After adding 4 ml of alkaline reagent 2, a millivoltage
measurement is made as soon as the reading stabilizes. Generally a 1-minute
wait is sufficient although longer times are sometimes necessary for more
dilute solutions.
Calibration curves of electrode potential versus the negative logarithm
of parts per million ammonia-nitrogen were prepared periodically. These
plots were linear and Nernstian over the concentration range of 0.1 ppm to
100 ppm. General operating procedure included measuring a standard solution
both before and after each sequence of important measurements.
Apparatus for six simultaneous runs was available. For many important
measurements an average of six determinations was made. In all cases deter-
minations were made at least in triplicate.
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SECTION VI
DATA
The results of this study will be reported in the following five sections:
1. The use of the ammonia electrode in TKN analyses,
2. Confirmation that the presence of nitrate causes low TKN results,
3. Analysis of the mechanism of nitrate interference in TKN analyses,
4. The use of anion-exchange resins to remove nitrate interferences
in TKN analyses, and
5. Evidence for nitrite interference.
1. The use of the ammonia electrode in TKN analysis
The use of the ammonia electrode in TKN determinations is becoming
increasingly widespread because it obviates distilling the ammonia from the
sample and carrying out a titration.6>?>8 In this approach the residue
after Kjeldahl digestion is almost neutralized and diluted to 50 ml.
Addition of akaline reagent 2 raises the solution pH to release the ammonia,
simultaneously preventing precipitation of mercury or other metal hydroxides
on the ammonia electrode membrane. Shortly after addition of alkaline re-
agent 2, the electrode millivoltage stabilizes and can be read.
Initially a series of standards was prepared using ammonium chloride
ranging from 0.01 ppm to 100 ppm ammonia-nitrogen. A few drops of dilute
HC1 were added to each to ensure against ammonia loss. These standards,
when made alkaline with 10 M NaOH, gave a linear plot of millivoltage
versus log (ppm ammonia-nitrogen between 0.1 ppm and 100 ppm.) The plots were
Nernstian commonly having a slope close to -59 millivolt/decade. However,
a curve obtained from these standards consistently showed greater than 100%
recovery of ammonia in TKN analyses of standard solutions. The cause of
this error was the disparity between the standard solutions and the TKN
solutions being analyzed. Although the ionic strength of the standard
solutions is nearly zero, the ionic strength of the TKN solutions, containing
H2S04, K2S04, and H SO*, is greater than 2.5 M. This very large value could
have a considerable influence on the polar ammonia molecule. To test this,
the standards were prepared including identical to those of concentrations
of HgSC>4, K2SO^, and Na2SC>4 (formed by neutralization of ^804) solutions
resulting from TKN analysis. In Table 1 a comparison of the response of the
ammonia electrode to standard solutions with and without the TKN reagent is
shown.
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TABLE 1. Comparison of the response of the ammonia electrode to standard
solutions with and without TKN reagents for ionic strength
adjustment
Millivoltage
Standard Solution with Standard Solution
TKN Concentration TKN reagents with TKN reagents
1 . 0 ppm
2.5
30.3 mvolt
8.7
21.9 mvolt
0.0
10 -25.9 -35.7
The above data demonstrate the presence of the TKN reagents causes a negative
shift of the standard curve by 9.0 millivolts. The fact that the shift is
constant for all readings implies an ionic strength effect rather than
ammonia contamination. This result led to the use of standard solutions
containing TKN reagents for all succeeding measurements to minimize experi-
mental error.
2. Confirmation that the presence of nitrate causes low TKN results
The typical standard curve of the plot millivoltage versus the logarithm
of the ppm of ammonia-nitrogen was linear and Nemstian over the concentration
range 0.1 to 100 ppm ammonia-nitrogen. During this study, this standard
curve shifted by ± 15 millivolts but always retained a slope very close to
59. To correct for these small shifts standard solutions containing TKN
reagents were run at least twice daily and both before and after any readings
deemed important.
Periodic measurements of the distilled, deionized water used in this
study typically gave readings of 120 millivolts, implying less than 0.01 ppm
ammonia-nitrogen. TKN analysis of 50 ml of this deionized water, 50 ml of
a standard 2.0 ppm alanine solution, and 50 ml of 10.0 ppm NH4C1 gave the
results given in Table 2.
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TABLE 2. TKN determinations of standard solutions
Sample
# of runs
Average
Millivoltage
(Deviation)
mg/1
NHyN (Recovered)
Deionized water
(blank)
2.0 ppm N (alanine)
10 ppm N (NH4C1)
5
3
3
75.0 (3.0)
-7.6 (.1)
-25.8 (1.2)
0.2
2.5
9.6
The rather large result for the blank apparently is the result of ammonia
contamination of the reagents used and could not be lowered below the 0.2 ppm
value given above. Difficulty in obtaining accuracy and precision with
samples containing 1.0 ppm TKN or less, possibly due to the large blank, led
to work with solutions containing 2.0 ppm TKN in many of the succeeding
determinations.
The effect of large concentrations of nitrate-nitrogen on the results of
TKN determinations of 50 ml of 2.0 ppm alanine are shown in Table 3.
TABLE 3. The effect of nitrate on TKN recovery
mg/1
N03-N
0
5
20
50
# of runs
3
3
3
6
Millivolts
(Deviation)
-7.6 ( .1)
13.8 (1.6)
72.4(20 )
69.7 (7.2)
mg/1 TKN
Recovered
2.5
2.3
0.28
0.18
% Recovery
100
92
11
7
Great difficulty was experienced in obtaining good precision in TKN deter-
minations made where high nitrate levels were present. The extent of ammonia
loss seemed effected by distillation speed, temperature of the heating mantle,
and, particularly, the amount of water left in each flask for digestion.
Only by boiling all water from each solution and digesting the sulfuric acid-
electrolyte solution remaining was satisfactory precision achieved. This
aspect of the analysis will be discussed later in this report.
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The data in Table 3 confirm other workers' observations (References 2
and 3) and illustrate the great magnitude of nitrate interference in TKN
determinations.
3. Analysis of the mechanism of nitrate interference in TKN analyses
In order to determine a way of carrying out accurate TKN measurements
without nitrate interference, a study of the mechanism of interference was
undertaken. A literature search revealed that solid NffyNC^ decomposed slowly
at 240°C 9 and rapidly at 290°C 10 into primarily nitrous oxide, N20, and
water. Furthermore, chloride was thought to catalyze the reaction. "
Measurement of the temperature during Kjeldahl digestion in several different
determinations gave 312°C as an average reflux temperature. This temperature
exceeds the literature values for decomposition of solid ammonium nitrate,
and although a solution phase is present during the Kjeldahl digestion, this
fact strongly supports decomposition as the mechanism of interference. To
verify experimentally nitrous oxide loss during Kjeldahl nitrogen digestion,
solutions were prepared containing various concentrations of ammonium and
nitrate in concentrated sulfuric acid. Concentrated sulfuric acid was used
as a solvent in these studies because its boiling point of about 315°C
demonstrates that essentially concentrated sulfuric acid is the liquid phase
in the Kjeldahl digestion.
The heating of two solutions of concentrated sulfuric acid, one contain-
ing ammonium nitrate to 10 ppm ammonia-nitrogen and the other 100 ppm NH3-N
(typical of the concentrations present during TKN digestion), gave the read-
ings given in Table 4.
TABLE 4. Loss of TKN as a function of reflux temperature and time
Time of Sampling
Millivolts
10 mg/1 NH3-N 100 mg/1 NHg-N
Initial reading
180°C pot temperature
250°C " "
300°C " "
315°C " "
After one hour of reflux
+22.8
19.6
46.8
76.6
73.7
78.2
-3.2
20.7
76.4
76.1
71.0
72.2
10
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These data indicate that solutions heated to over 300°C containing equal
concentrations of ammonia and nitrate-nitrogen undergo significant decomposi-
tion and loss of Kjeldahl nitrogen. Furthermore, they demonstrate that the
loss occurs quickly, before a temperature of 250°C, and that the percentage
losses are very great, 95% for 100 ppm NH3-N and 88% for 10 ppm NH3-N. The
surprising conclusion, nitrate reacts with ammonium in concentrated H-SO^
when both are present in concentrations as low as 10 mg/1, led to a determin-
ation of whether the Kjeldahl reagents inhibit this reaction. A solution of
40 ml concentrated H2S04, 26.7 g K2S04 and 1.0 g HpS04 and 100 ppm NH4N03
was heated, as above. Although it appeared that tne decomposition required
slightly higher temperatures, the same final effect was observed at about
the same magnitude. With these data it appears the reason no decomposition
is observed in TKN determinations when low levels of nitrate-nitrogen are
present is the presence of greater quantities of water than those in the
concentrated H2S04 used in the above studies. As the proposed reaction
shown causing TKN interference:
NH4+ + NO" > N20 + 2H20,
yields water as well as nitrous oxide, it would be expected that the pre-
sence of water in the reflux mixture would inhibit the decomposition. Thus
the decomposition only becomes important when the nitrate-nitrogen levels
are large relative to ammonia-nitrogen levels. If this is true, it would
be expected that the nitrate interference would be of even greater signifi-
cance where block digesters are used and higher, more water-free digestion
temperatures achieved.
Confirmation of the presence of nitrous oxide in the gaseous effluent
from the heating of ammonia nitrate in concentrated sulfuric acid was made
by using infrared spectroscopy. The peaks at 1160 cm", 1275 cm"1,
2200 cm'1, and 2560 cm"1 were used to confirm the presence of N20 and semi-
quantitative ly evaluate its concentration. Infrared spectra revealed some
nitrous oxide in the gaseous effluent of a solution of NH4NC>3 in sulfuric
acid containing 100 ppm ammonia-nitrogen heated to 230°C. Large quantities
of nitrous oxide were formed when this solution was heated to 260 C and to
295°C. After heating to 315°C and refluxing for one hour, only traces of
nitrous oxide were observed. Although the brown color of nitrogen dioxide
was apparent from about 200°C upward, none was present in the infrared
cell (perhaps because it was absorbed in the drying tube). Its presence
presumably was due to decomposition of nitric acid formed during digestion.
In one final infrared study, loss of TKN from 50 ml of a solution con-
taining 10 ppm ammonia-nitrogen and 100 ppm nitrate-nitrogen was found to
occur within the first 5 minutes of digestion. Collection of the effluent
gases during this period and subsequent infrared analysis showed the pre-
sence of nitrous oxide.
11
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4. The use of anion-exchange resins to remove nitrate interferences in TKN
analyses
From earlier work, the interference of nitrate in TKN determinations
results from combination of nitrate and ammonium at elevated digestion temp-
eratures with the subsequent loss of the Kjeldahl nitrogen as nitrous oxide.
Prevention of the interference would only be possible by lowering the diges-
tion temperature or prior removal of the nitrate. A few digestions carried
out with only 90% of the sample water distilled and thus at a lower tempera-
ture resulted in incomplete recovery of the Kjeldahl nitrogen. This demon-
strated that the water-free, high temperature is necessary to release all
organic nitrogen in the Kjeldahl analysis. This conclusion mandated the
physical removal of the nitrate prior to digestion as the only way to block
the nitrate interference. The high solubility of all nitrate salts and the
poor chelating ability of nitrate ruled out removal by precipitating or com-
plexing. Anion exchange resins were studied for their potential in removing
nitrate.
Dowex- 1 X 8 - 200 (100 to 200 mesh) anion exchange resin in the chloride
form was used first in attempts to remove nitrate in the milligram -per-liter
concentration range. A 5.00 g sample of the Dowex resin was washed several
times with ammonia-free water. A chloride ion-selective electrode was
inserted into 100 ml of stirred deionized water containing the resin. After a
stable reading was achieved a few milliliters of 500 ppm nitrate-nitrogen
solution were added. Instantly the millivoltage decreased, indicating the
nitrate was being removed from solution replaced by the chloride from the
resin. Within about ten seconds the millivoltage reading had stabilized again.
To determine quantitatively the extent to which the resin removed nitrate,
the previous experiment was repeated, adding 1.00 ml of 500 ppm nitrate-
nitrogen to 100 ml of deionized water and 5.0 g of washed Dowex resin. The
chloride-electrode responses after successive additions are given in Table 5.
12
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TABLE 5. Addition of successive nitrate aliquots to distilled water and
dowex -1X8 resin
Sample
Millivoltage
Cl-electrode
mg/1 Cl'
Increased in Cl"
in mg/1 Cl"
Washed Dowex and
100 ml water
Above + 5 mg/1
212.5
2.8
N03-N
+10ml/l N03-N
+15ml/l N03-N
+20mg/l N03-N
+120mg/l N03 -N
+220mg/l NO" -N
J
176
160
152
146
121
110.5
15.5
31.6
44.7
57.6
182
295
12.7
16.1
13.1
12.9
124.4
113
For the first four additions up to 20 mg/1 N03~ -N, an average of 13.7 ppm
Cl" are released by each 5 ppm NO?" -N addition. By calculation 12.7 ppm Cl"
should be released for each ppm N0~ -N addition. This implies that nitrate
is being quantitatively removed ana replaced by chloride at these low concen-
trations. In increasing from 20 ppm to 120 ppm N03~ -N addition, only 124
ppm chloride is recovered, implying an only 50 percent exchange of resin
chloride and solution nitrate. Treatment with a second portion of resin or
treatment with a larger portion of resin would presumably remove all nitrate
in these higher concentration.
With confirmation that nitrate could be removed by Dowex - 1 X 8 anion
exchange resin, it was next necessary to confirm that the resin did not
significantly increase the TKN blank. As this resin, like most anion exchange
resins, is a quarternary ammonium salt, this concern was very real. A 5.0 g
portion of Dowex resin was treated with 1.0 M NaOH to re-convert to the
chloride form, and finally rinsed with several portions of distilled water.
The rinsed resin was stirred with 50 ml of deionized water, the resin filter-
ed from the water, and the filtrate analyzed by the usual Kjeldahl procedure.
Analysis of three samples gave readings of 52.0 - 0.5 millivolts. This cor-
responds to readings at the 0.5 ppm TKN level, an increase in TKN over the
13
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deionized water blank level of 0.2 ppm. While some increase in TKN results
from use of the resin, a run was made using the resin to remove nitrate prior
to TKN determination. To a solution containing 2 ppm alanine-nitrogen and
50 ppm nitrate-nitrogen was added 5 g of rinsed Dowex resin. The solution
was stirred, the resin filtered from the solution, and the filtrate analyzed
in the usual manner. Analysis of three solutions gave an average reading of
12.8 i 2.8 millivolts corresponding to that of 2.0 ppm TKN. Earlier deter-
minations (see Table 3) without prior resin removal of the nitrate resulted
in only 0.2 ppm TKN, or 7%, recovery in this analysis. An analysis using
the same procedure with a solution containing 2 ppm alanine-nitrogen and
20 ppm NO^'-N gave a millivoltage reading of 8.2 ±1.7 for six runs,
corresponding to readings for 3.2 ppm TKN. This increase, while difficult
to explain, gives further evidence that all interfering nitrate had been
removed.
Considering that the increase in TKN recovered might be due to contamina-
tion from the resin and being concerned with the larger blank (0.5 mg/1)
resulting with use of the Dowex- 1X8 resin, other anion resins were in-
vestigated. Of the others studied, Permutit S-l gave the best results.
Blanks run on deionized water, after being stirred with this resin, filtered,
and analyzed, gave an average for three determinations of 68.5 t 2.2
millivolts. This compares very well with a blank reading run on deionized
water, 75.0 millivolts. Thus, with Permutit S-l very little TKN contamina-
tion results. Analysis of a solution containing 2 ppm alanine-nitrogen and 20
ppm nitrate-nitrogen after prior treatment with Permutit S-l gave a reading
of 17.2 * 6.7 millivolts corresponding to readings of 2.2 mg/1 TKN recovery.
In a final determination analyses of a solution containing 5ppm alanine-
nitrogen and 50 ppm nitrate-nitrogen gave results of 99.6 ±6.1 millivolts
without treatment and -.8 1 1.4 millivolt with prior treatment. These
values correspond to those of 0.1 and 4.5 ppm, implying a 90% recovery
with treatment as compared to 2% recovery without. The data for these
analyses are summarized in Table 6.
14
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TABLE 6. The use of anion-exchange resins to remove nitrate for TKN analyses
mg/1
TKN
2
0
2
2
2
0
2
5
5
mg/1
N03-N
50
0
50
20
20
0
20
50
50
Anion
Exchange Resin
Treatment
None
Dowex-1 X 8
Dowex-1 X 8
None
Dowex-1 X 8
Permutit S-l
Permutit S-l
None
Permutit S-l
# of
runs
6
3
3
3
6
3
3
3
3
recovered
TKN— mv
69.7
52.0
12.8
72
8.2
68.5
17.2
99.6
-.8
± 7.2
! .5
! 2.8
i20
± 1.7
i 2.2
i 6.7
± 6.1
± 1.4
Recovered
TKN
mg/1 % recovery
.2
.5
2.0
.3
3.2
.3
2.2
0.1
4.5
7
100
11
160
110
2
90
5. Evidence for nitrate interference
Although the nitrate concentration in natural water samples is commonly
very low and well below TKN concentrations, its interference is possible in
special TKN applications. The decomposition of ammonium nitrite into
nitrogen and water occurs readily at even lower temperatures than ammonium
nitrate decomposition, suggesting serious interference. Early in this
project reduction of nitrate to nitrite was considered as a means to block
the nitrate interference in TKN analysis. With this in mind the effect of
nitrite in TKN determinations was investigated. Twelve runs were made
determining the TKN concentration of a solution containing 2 mg/1 of alanine
and 50 mg/1 of sodium nitrite. Although the precision was quite poor for
these determinations, an average of only 20% recovery of TKN was found. The
TKN analysis procedure had not been developed to a high precision at the
time of these measurements, but interference of nitrite is clearly implied by
the results.
15
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REFERENCES
1. Kjeldahl, _Z. anal. Chem., 2^2, 366(1883)
2. Parkin, G.F. Nitrate Interference in the Kjeldahl Method For Determining
Organic Nitrogen. Communication to Charles Mashni, Nov. 21, 1974.
3. Gales, M.E., Nitrate Interference in the Kjeldahl Method for Determining
Organic Nitrogen. Memo to Record, USEPA, ORD, EMSL, March 18, 1975.
4. Methods for Chemical Analysis of Water and Wastes, EPA-625-16-74-003,
Methods Development and Quality Assurance Research Laboratory, National
Environmental Research Center, U.S. Environmental Protection Agency,
Office of Technology Transfer, Washington, D.C. 20460
5. Evans, W.H. and Partidge, B.F., Determination of Ammonia Levels in Water
and Wastewater with the Ammonia Probe. Analyst, 99, 367(1974)
6. Gales, M.E., Determination of TKN with the Ammonia Ion Electrode Following
Kjeldahl Digestion. EPA, Environmental Monitoring and Support Laboratory,
Cincinnati, Ohio, 1975
7. Thomas, R.F. and Booth, R.L. "Selective Electrode Measurement of Ammonia
in Water and Wastes", Environmental Science and Technology, ]_, 523(1973)
8. Bremmer, J. M. and Tabotaboi, M. A., "Use of An Ammonia Electrode for
Determination of Ammonium in Kjeldahl Analysis of Soils1.', Comm. in
Soil Science and Plant Analysis, 3^ 159 (1972)
9. Saunders, H. L., "The Decomposition of NH4N03 by Heat.", .J. Chem. Soc.,
121, 698 (1922)
10. Shah, M.S. and Oza, T.M., "Decomposition of NH4NO^.", J. Chem. Soc., 1932
725
11. Tramm, Heinrich and Vehde, Hermann, "The Spontaneous Decomposition of
NH4N03 Melts", Angew. Chem., £7, 782 (1934)
16
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/7-77-017
2.
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
Nitrate Interference in Total Kjeldahl Nitrogen
Determinations and Its Removal by Anion Exchange
Resins
5. REPORT DATE
February 1977 issuing date
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Albert Schlueter
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Central State University
Wilberforce, Ohio 45384
10. PROGRAM ELEMENT NO.
EHE625
11. CONTRACT/GRANT NO.
R-802755-02
12. SPONSORING AGENCY NAME AND ADDRESS
Environmental Monitoring and Support Laboratory -
Office of Research and Development Cin., OH
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
13. TYPE OF REPORT AND PERIOD COVERED
14. SPONSORING AGENCY CODE
EPA/600/06
15. SUPPLEMENTARY NOTES
16. ABSTRACT
The interference of nitrate with the determination of total Kjeldahl
nitrogen (TKN) was investigated. TKN losses of greater than 90% were observed
in solutions containing a nitrate-nitrogen concentration ten times the TKN
level. The loss was found by infrared spectroscopy to be occurring due to
nitrate and ammonia decomposition to nitrous oxide at the elevated TKN digestion
temperatures. Prevention of the nitrate interference in TKN analyses was found
to be possible only by removal of nitrate prior to TKN analysis. Several
anion-exchange resins in the chloride form were found effective in this removal.
Samples containing known TKN levels and high nitrate concentration when treated
by these resins prior to TKN analysis gave nearly 100% TKN recovery on TKN
analysis.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Ammonia
Nitrogen organic compounds
Water analysis
Monitors
Nutrients
Ammonia electrodes
Kjeldahl nitrogen
Interference
Nitrate
07
14
18. DISTRIBUTION STATEMENT
Release to public
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
25
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
17
• U. S. GOVERNMENT PRINTING OFFICE: 1977-757-056/5595 Region No. 5-11
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