2414
ANALYTICAL CHEMISTRY, VOL. 51, NO. 14, DECEMBER 1979
Semiautomated Determination of Nitrogen in Nitrate-Containing Fertilizers
Sandra L. Larson" and Harvey P. Peterson Wisconsin Department of Agriculture, Trade and Consumer Protection, Bureau of Laboratory Services, Madison, Wisconsin 53705
Rapid and accurate measurement of the nitrogen content of fertilizers is important to quality control in the fertilizer industry. As t h e number of samples increases, automation becomes increasingly necessary. T h e time, space and labor requirements of t h e Kjeldahl method ( I ) have led t o the development of automated alternatives t o the Kjeldahl method (2-9). Characteristically, regulatory laboratories receive a wide range of samples for nitrogen analysis. Therefore, any useful, automated system must be both rapid and accurate, and also flexible enough to handle the wide variety of substances submitted for analysis. The Kjel-Foss Automatic (KFA), designed for automated protein determinations, follows the conventional Kjeldahl analysis: acid digestion, neutralization, distillation, and titration ( I O ) . Sample analysis rates are increased over the conventional Kjeldahl analysis, resulting in a 15-min, rather than a 3-h delay for results of the first sample. Results for subsequent samples are obtained at fixed 3-min intervals, allowing 20 samples to be analyzed per hour. T h e KFA can be used to analyze any substance that can be analyzed by the standard Kjeldahl and has been accepted as an official method for feeds (11, 12). One drawback of t h e KFA is t h a t it does not reduce the nitrates often found in commercial fertilizers. T h e Comprehensive Nitrogen Method (CNM) (13),however, is applicable to all fertilizer samples and is a n official method (14). This study concerns itself with the development of a semiautomated method for total nitrogen in nitrate-containing fertilizers using t h e C N M predigestion step coupled with t h e KFA nitrogen determination.
EXPERIMENTAL Apparatus. Analyses were performed using the Kjel-Foss Automatic, Model 16210, Kjeldahl (nitrogen/protein) analyzer (Foss America, Inc., P.O. Box 504, Rt. 82, Fishkill, N.Y. 12524). Reagents. Reagents were prepared according to the Kjel-Foss Automatic operating manual (15) except that dried, reagent grade KNOBwas used for the low (1.1482 g/L) and high (45.9300 g/L) standard solutions instead of (NH412S04.The KN03 was determined to contain 13.60% nitrogen by the CNM. The calibration of the KFA was based on this value. Instead of the recommended Kjel tabs, 15.7 g of Pope Kjeldahl mixture was used for the analyses. Procedure. Weigh sample containing about 60 mg of nitrogen into a 250-mL beaker. Add 35 mL of water and 1.2 g of chromium powder and let stand 10 min with occasional swirling. Add 7 mL of HC1, cover, and wait until a visible reaction occurs. Digest on hotplate until 20-25 mL remain. Quantitatively transfer to a 50-mL volumetric flask, leaving residue behind, and dilute to volume. Prepare a series of standards using 10-mL aliquots of the high standard solution and 10-mL aliquots of the low standard solution that are reduced and diluted as described above. The KFA is calibrated using 25-mL aliquots of these prepared solutions. Analyze 25-mL aliquots of the sample solutions according to the procedure for liquid samples given in the operating manual with the following modifications: no H202need be dispensed with the H2S04in position 1, and at least 130 mL must be distilled over in the 3-min distillation step in position 5 instead of the recommended 100 mL. For accurate results, the aliquot size of the sample analyzed should be the same as that of the standards used to calibrate the instrument.
Table I. Five-Trial Comparison between t h e CNM and KFA Results for Total Nitrogen in Fertilizers Group A Ma gr u d er check fert. 7610 7611A 761 2 7702 7703 7704-B 7705 7707A 7708 7606 7605
grand average
grade 5-10-15 5-15-25 8-32-16 9-29-19 18-46-0 12-24-6 13-13-13 8-0-24 3-23-23 15-15-15 10-10-10
5.42 t 0.07 4.90 I0.07 7.82 i 0.10 8.89 i 0.08 17.98 i- 0.11 12.12 i 0.11 13.17 i 0.15 7.92 t 0.11 3.20 i 0.03 14.18 t 0.12 9.14 i 0.10
CNM
iange 0.07 0.06 0.23 0.15 0.22 0.32 0.13 0.14 0.14 0.18 0.11
-
x
S
5.40 4.89 7.79 8.91 18.14 11.98 13.13 7.82 3.16 14.22 9.01
0.0277 0.0239 0.0852 0.0628 0.09127 0.1322 0.04722 0.0581 0.0632 0.07259 0.0497
range 0.07 0.06
0.07 0.1 5 0.2 0.3 0.4
0.14 0.10
0.3 0.18
KFA X
5.40 4.90 7.80 8.93 18.0 12.1 13.2 7.87 3.21 14.2 9.12
S
t-value
0.0292 0.0255 0.0292 0.0596 0.0837 0.130 0.167 0.0555 0.0361 0.130 0.0756
-0.640 -0.248 -0.517 2.53' -1.45 -0.902 -1.39 -1.54 0.300 -2.72'
S
t-value
0.114
0.741 2.80' --7.81'
0
Group B
cNR4
sample 1
2 3 4 5
6 a
grade
range
urea (undried) 5-1-1 organic liquid 28-0-0 liquid 9-18-9 liquid 5-10-5 liquid urea
0.45 0.09 0.47 0.22 0.12 0.42
-
KFA
X
S
range
45.88 5.09 26.03 9.19 5.21 45.56
0.2129 0.0354 0.2216 0.0851 0.0462 0.1725
0.3 0.28 0.3 0.20
0.36 0.4
Significant a t P = 0.05 level. 0003-2700/79/0351-2414$01.OO/O
C 1979 American Chemical Society
X
45.8 4.95 26.9 9.19 5.20 45.8
0.106
0.114 0.0721 0.161
0.158
0
0.133 - 2.29
ANALYTICAL CHEMISTRY, VOL. 51, NO. 14, DECEMBER 1979
RESULTS AND DISCUSSION
2415
LITERATURE CITED
The nitrogen contents of a diverse set of fertilizer samples were determined according to the above procedure. These results are compared to results from the official CNM in Table I. Kitrogen values obtained by the KFA compare favorably with the target values obtained by the CNM. Of the 17 fertilizers involved in the study, four means were shown to be significantly different by the Student's t-test (16). Three of these four, however, have means within 0.15% nitrogen. The average difference between the means in Group A (Magruder check fertilizers) is -0.025% nitrogen and that of Group B (routine samples) is -0.15% nitrogen. The difference for Group B can be reduced to -0.002% nitrogen if the result from sample 3 is ignored. Sampling errors are suspected as contributing to the high deviation because of the small sample weight required for analysis. T h e KFA demonstrated its ability to analyze fertilizers using the predigestion step of the CNM. This is necessary only, however, when the sample contains nitrates. Samples can be rapidly screened for nitrates by a method described in the Official Methods of Analysis (1 7). Those samples containing nitrates can be analyzed by the method described above. Samples not containing nitrates can be analyzed by the KFA strictly according to the operating manual. I t has been this laboratory's experience that the majority of the samples are nitrate-free. Since the predigestion step is not required for this fraction, results from these samples can be obtained very rapidly.
(1) "Official Methods of Analysis", 12th ed.,AOAC, Washington, D.C., 1975, sec. 2.049-2.050. (2) D. L. Park and R. L. King, J . Assoc. Off. Anal. Chem., 5 7 , 42-47 (1974). (3) D. A. Biggs, J . Assoc. Off. Anal. Chem., 5 5 , 488-497 (1972). (4) W. H. Doty, D. E. Wood, and E. L. Schneider, J . Assoc. Off. Anal. Chem., 52, 953-956 (1969). (5) M. E. Ebeling, J , Assoc. Off. Anal. Chem., 51, 766-770 (1968). (6) H. W. Schafer and N. F. Olson, Anal. Chem., 47, 505-509 (1975). (7) C. W. Gehrke, L. L. Wall, Sr., J. S. Killingley, and C:. Inada, J . Assoc. Off. Anal. Chem., 54, 651-657 (1971). (8) c. W. Gehrke, J. S.Killingley, and L. L Wall, Sr., J . Assoc. Off. Anal. Chem., 5 5 , 467-480 (1972). (9) L. L. Wall, Sr. and C. W. Gehrke, J . Assoc. Off. Anal. Chem., 6 0 , 881-889 (1977). (10) R. Obberreith and H. M. Neil, Food Techno/., 28, 40-43 (1974). (11) R. J. Noel, J . Assoc. Off. Anal. Chem., 59, 141-147 (1976). (12) "Official Methods of Analysis", 2nd Supplement to 12th ed., AOAC, Washington, D.C., 1975, sec. 7.801-7.804. (13) C. W. Gehrke, J. P. Ussary, C. H. Perrin. P. R. Rexroad, and W. L. Spangler, J . Assoc. Off. Anal. Chem., 5 0 , 965-975 (1967). (14) "Official Methods of Analysis", 12th ed.,AOAC, Washington, D.C., 1975, sec. 2.05 1-2.052. (15) Kjei-Foss Automatic 16200 Operating Manual, PIN 117242 (May 1976) Issue 4. Foss-America. Inc.. Fishkili. N.Y. 12524. (16) W. J. Youden, "Statisticil Methods for Chemists", John Wiley and Sons, New York, 1951, pp 24-29. (17) "Official Methods of Analysis", 12th &., AOAC, Washington, D.C., 1975, sec. 2.046.
RECEIVED for review June 7 , 1979. Accepted September 24, 1979.
CORRECTIONS Coulometric Determination of Aromatic Nitro Compounds with Electrogenerated Chromium(l1) In this article by I. M. Al-Daher and B. G. Kratochvil, Anal. Chem. 1979,51, 1480, on page 1482, the first line of the last paragraph before the Acknowledgment should read "Constant current potentiometric end-point detection often". The second sentence of the Abstract on page 1480 should read "Samples of the order of 0.2 to 0.5 umol could be determined. . ."
Room Temperature Phosphorescence of Selected Pteridines In this article, appearing in the October issue, Anal. Chem. 1979,51, 1921-1926, the second author's name is misspelled. The correct name is Richard S.Freedlander.
