Quantitative Determination of Organic Nitrogen in Water, Sewage, and Industrial Wastes GEORGE B. MORGAN, JAMES B. LACKEY, and F. WELLINGTON GILCREAS Sunitury Engineering Reseurch iaborafory, Universify of Florida, Gainesville, Fla.
F A modified Kjeldahl method gives
98,470 recovery
of organic nitrogen with a standard deviation from the average of o.23570. Such recovery is 770 higher than that obtained with the copper oxide-Kjeldahl method. The method is simpler to perform, requires less time, is more precise and accurate than other methods; it can handle the usual interfering substances, carbohydrates and lipides. Ammonia can be separated and recovered from organic nitrogen. N o appreciable loss of organic nitrogen results from the presence of nitrates and nitrites.
S
ULFURIC acid
with various catalysts will digest organic material and convert nitrogen to ammonium acid sulfate After ammonia is made alkaline and distilled, it is collected in either boric or sulfuric acid and titrated. The selection of a proper catalyst for the Kjeldahl method has been the subject of many investigations and reports. Willits, Ogg, and coworkers (3, 6, 6) have shown that mercury is much superior to copper, and they have also condemned the use of selenium as a catalyst (3, 4). These authors, as well a. RfcKenzie and Wallace ( 2 ) , have conclusively shown t h a t the mercury catalyst, concentration of potassium sulfate, temperature, and time of digestion are the important factors McKenzie and Wallace have proposed a method for the microdetermination of organic nitrogen t h a t gives excellent results with a much shorter digestion time than is required by present methods. The following data are presented to deterniine the feasibility of a new method for the determination of organic nitrogen in routine analysis of water, sewage, and industrial wastes.
of 0.2% methyl red in 95% ethyl alcohol with one volume of 0.27, methylene blue in 95% ethyl alcohol. Indicating boric acid. Dilute 20 grams of boric acid and 10 ml. of mixed indicator to 1000 ml. with ammoniafree distilled water. Potassium acid iodate solution, 0.02N, 0.7998 gram of potassium acid iodate (analytical reagent grade) per 1000 ml. of solution. Standard amino acid solution. Dissolve 0.5365 gram of glycine, 0.8368 gram of m-valine, 0.5365 gram of DL alanine, and 0.7295 gram of l-tryptophan in 500 ml. of ammonia-free distilled water. Dissolve 0.8583 gram of 1-cystine in 50 nil. of 0.1N sodium hydroxide. Mix the two solutions and dilute to 1 liter. This standard nitrogen solution contains 0.5 mg. of nitrogen per ml. Sulfuric acid solution, O . O 2 S , 1 ml. = 0.2802 mg. of nitrogen. Copper sulfate solution, 10% .by weight of copper sulfate, analytical reagent grade. Sodium hydroxide-sodium thiosulfate solution. Dissolve 500 grams of sodium hydroxide and 25 grams of sodium thiosulfate pentahydrate in distilled mater and dilute to 1 liter.
Table 1.
Sulfuric acid-mercuric sulfate-potassium sulfate solution. Dissolve 125 grams of potassium sulfate in 800 ml. of distilled water and 400 ml. of 36N sulfuric acid. Add 50 ml. of mercuric sulfate solution and dilute to 2000 ml. Sodium hydroxide solution. Dilute 500 grams of sodium hydroxide, analytical reagent grade, to l liter. Phosphate buffer solution, 0.5111. Dilute 14.3 grams of monobasic potassium phosphate, (analytical reagent grade) and 68.8 grams of dibasic potassium phosphate (analytical reagent grade) to 1 liter with distilled water. Standard ammonium chloride solution. Dilute 1.9107 grams of chemically pure ammonium chloride to 1 liter. This solution contains 0.5 mg. nitrogen per ml. A six-unit Precision macro-Kjeldahl apparatus was used for the Kjeldahl determinations. Titrations were performed using two 50-ml. Normax burets certified by the National Bureau of Standards. EXPERIMENTAL PROCEDURES
McKenzie and Wallace Kjeldahl Method. Samples of the standard
Recovery Tests on Standard Amino Acid Solution," McKenzie-Wallace Method
Sulfuric Acid TitrantC Nitrogen recovered, Dev. % from av. 96.96 1.17 97.20 0.93 98.40 0.27 99 I84 1.71 99.28 1.15 100.16 1.03 95,04 3.09 Av. 98.13 1.33
Potassium Acid Iodate Titrant* Kitrogen recovered, Dev. % from av. 98.40 0.07 98.80 0.33 0.89 99.36 98.00 0.47 99.52 1.05 99.36 0.89 98.16 0.31 98,96 1.51 98.80 0.33 98,96 0.49 98.08 0.39 97.28 1.19 Av. 98.47 0.66
REAGENTS AND EQUIPMENT a
Sulfuric acid, analytical reagent grade, 36N. Potassium sulfate, analytical reagent grade. llercuric sulfate solution. Dissolve 8 grams of red mercuric oxide and dilure to 100 ml. with 6N sulfuric acid. Mixed indicator. A h two volumes
12.50 mg. nitrogen applied.
Accuracy
=
98.47 f 0.815%.
Std. dev. of av. Accuracy
=
= u.t =
2-'98.47 f
42
0.815 a
= 98.47 i 0.23570,.
98.13 i 1.68%; std. dev. of av. = 98.13 f 0.63%.
VOL. 2 9 , NO. 5, MAY 1957
833
*
Table II. Recovery Tests on Standard Amino Acid Solution" Copper Oxide-Kjeldahl Method* ModSed Kjeldahl Methodc Nitrogen Nitrogen recovered, Dev. recovered, Dev. % from av. % from av. 91.84 0.71 98.16 0.57 93.44 2.31 97.76 0.97 90. 72 0.41 99.28 0.55 88.48 2.65 99.28 0.55 93.04 1.91 98.88 0.15 91.20 0.07 96.48 2.25 91.68 0.55 98.64 0.09 90.08 1.05 98.64 0.09 89.44 1.69 99.36 0.63 99.84 0.11 91.36 0.23 99.20 0.47 Av. 91.13 1.16 99.28 0.55 Av. 98.73 0.582 12.50 mg. nitrogen added. Accuracy = 91.13 i 1.53y0,;std. dev. of av. = 91.13 i 0.4847& Accuracy = 98.73 i 0.719%; std. dev. of av. = 98.73 A 0.265ojC.
Table 111.
Comparison of Methods by Nitrogen Determination of Primary Effluent Copper Oxide-Kjeldahl Modified Kjeldahl Dev. Dev. hlg. nit.rogen from av. Mg. nitrogen from av. 6.674 0.034 6.596 0.044 0.002 6.599 0.041 6.862 0.017 6.874 6.637 0.003 0.005 6.550 0.010 6.893 0.014 6.781 0.141 6.885 0.006 6.879 Av. 6.640 0.0455 0,0078 7.358 0.240 7.565 0.047 7.468 6.672 0.046 0.050 7.146 0.028 7.497 0.021 7.295 0.177 0,025 7.543 Av. 7.118 0.123 7.518 0.036
amino acid solution were diluted to 250 ml. with ammonia-free distilled water in 650-ml. Kjeldahl flasks. One milliliter of mercuric sulfate reagent, 5 grams of potassium sulfate, and 10 ml. of 36N sulfuric acid were added to each flask. These samples were digested for 20 to 30 minutes beyond the fuming point, along with a reagent blank for each run. After digestion, 250 ml. of ammonia-free distilled water and 50 ml. of sodium hydroxide-sodium thiosulfate reagent were added to the samples, which were then distilled into 50.0 ml. of indicating boric acid solution. Two hundred-milliliter aliquots were collected and titrated with 0.02N potassium iodate solution to a lilac end point. Samples of the standard amino acid solution were treated as above, except that the distillates were titrated with 0.02N sulfuric acid. Results are shown in Table I. Copper Oxide-Kjeldahl Method ( I ) . Samples of the standard amino acid solution were again diluted to 250 ml. with ammonia-free distilled water; 10 ml. of 36N sulfuric acid and 1 ml. of copper sulfate reagent mere added. These samples were then digested for 30 minutes beyond the fuming point. After the addition of 25 ml. of sodium hydroxide solution and boiling saddles, they were distilled into indicating boric
834
ANALYTICAL CHEMISTRY
acid solution. The distillates were titrated with 0.02N sulfuric acid. Results are shown in Table 11. ModifiedKjeldahl-Mercuric Sulfate Method. Samdes of the standard amino acid so1;tion were diluted t o 200 ml. and to each was added 50 ml. of the sulfuric acid-mercuric sulfatepotassium sulfate solution. These samples were digested for 20 minutes beyond the fuming point. After the addition of 50 ml. of the sodium hydroxide-sodium thiosulfate solution, the samples were distilled into 50.0 ml. of the indicating boric acid solution. The distillate was titrated with 0.02,V sulfuric acid. These results are also shown in Table 11. Nitroeen determinations for samdes of sewage primary effluent were run' by the modified method and on aliquots of the same Drimarv effluent samoles using the copier oxidkKjeldah1 met'hod (Table 111). ' To determine whether carbohydrates and lipides normally present in small quantities in sewage will interfere with the modified Kjeldahl method, nitrogen determinations were made on 20 samples of primary effluent. Nothing was added to 10 of the samples. To the other 10 samples were added 5 ml. of the standard amino acid solution, 0.5 gram of sucrose, 0.5 gram of Swift shortening, and 50 ml. of the sulfuric
Table IV. Nitrogen Recovery in Presence of Interfering Substances"
8.070 7.997 8.010 8.114 8.095 8.135 7.983 7.999 8.029 8.046
nitrogenb 10.474 10.493 10.516 10.508 10.501 10.576 10.503 10.462 10.493 10.515
% 96.16 99.84 100.24 95.76 96.21 97.64 100.80 98.52 98.56 98.76 Av. 98.25
'
AV. 2.09 1.59 1.99 2.49 2.01 0.61
2.55 0.27 0.31 0.51 1.44
a 0.5 gram each of sucrose and fat added. 2.50 mg. nitrogen added. Accuracy = 98.25 i 1.776%; std. dev. of av. = 98.25 rt 0.571%.
Table V. Recovery of Ammonia in Presence of Organic Nitrogen" Organic Nitrogen Ammonia Recoveredb Recoveredc Dev. Dev. from from 70 av. % av. 99.20 0.11 98.80 0.04 0.29 0.36 98.80 98.40 98.30 0.46 99.40 0.31 ._ _. 99.10 0.34 99.30 0.21 99.00 0.09 99.20 0.44 98.50 0.59 99.00 0.24 99.80 0.71 98.20 0.56 98.90 0.19 98.40 0.36 99.50 0.41 97.60 1.16 99.50 0.41 99.60 0.84 99.09 0.33 .4v. 98.76 0.48
10 mg. each of organic nitrogen and ammonia added. b Accuracy = 98.76 i 0.59670; std. dev. of av. = 98.76 rt 0.179%. c Accuracy = 99.09 rt 0.40476; std. dev. of av. = 99.09 i 0.121%.
acid-mercuric sulfatepotassium sulfate reagent along with 10 ml. of 36N sulfuric acid. All samples were examined in the usual manner. Results are shown in Table IT'. Free ammonia KVRS determined by adding 20 ml. of standard ammonium chloride solution to 200 ml. of animoniafree distilled water. Tn-enty milliliters of standard amino acid solution, previously neutralized with IN sulfuric acid, were also added. The resulting solution was adjusted to pH T.4 with 0 . W sodium hydroxide, and 25 ml. of phosphate buffer were added. The ammonia was distilled, collected, and titrated with 0.02N sulfuric acid. After removal of the free ammonia. organic nitrogen was determined (Table V). The interference of nitrates with the Kjeldahl method was investigated by adding 5 p.p.m. of nitrate to samples of the standard amino acid. Ten milliliters of ethyl alcohol were added to five samples, and the rest were used as
Table VI. Recovery of Organic Nitrogen in Presence of Nitrates
Ethyl Alcohol Added, M1.
Organic Nitrogen Recovered,
%
98.35 98.02 99.34 97.80
0
98,02
5
controls.
99.04 97.95 99.36 98.74 98.94
Results are shown in Table
VI. RESULTS AND CONCLUSIONS
An average of 98.47% recovery of nitrogen was obtained with the modified method. Recovery tests from amino acids are not entirely satisfactory because samples of 100% purity are difficult to obtain and the hygroscopic properties of these compounds are marked. Although the expanded McKenzieWallace micromethod gives slightly more precise results than other methods tested, the ease and simplicity of a procedure in routine work must be
considered. The potassium acid iodate solution can be made more accurately than sulfuric acid but the high purity is expensive. Sulfuric acid can be standardized against reagent grade sodium carbonate and stored for several years without appreciable change in normality. Although high purity of reagents is desirable. it is not mandatory if reagent blanks are examined along with the samples. The mixed indicator usually gives a much sharper end point than methyl red alone. The boric acid solution used to absorb the ammonia is about 0.05X so that the pH a t the end point is well within the range of the mixed indicator. The modified Kjeldahl method, using sulfuric acid in place of potassium acid iodate, is preferred by the authors over other methods. The usual interferences in this method cause a pyrolytic loss of nitrogen due to the high acid-salt ratio which results. To determine to what extent this occurred, samples of primary effluent were examined in comparison with duplicate samples to which had been added sucrose, fat, and amino acid nitrogen. Results given in Table IV indicate that with excess sulfuric acid added there is not enough interference to be significant. There is, however, a greater spread in the per cent recovery, partially because it was cal-
culated on the relatively small quantity of amino acid nitrogen that was added to the primary effluent. The modified Kjeldahl method has further merit in that there is no bumping during distillation. This permits the solution to be distilled a t a faster rate. The effect of nitrates in concentrations usually found in water and sewage on the determination of organic nitrogen was insignificant. Less than 0.5% loss resulted. The modified Kjeldahl-mercuric sulfate method has been successfully used in the Sanitary Engineering Research Laboratory of the University of Florida as a routine procedure for 11 months. LITERATURE CITED
( I ) American Public Health Assoc., New
York, N. Y., “Standard Methods for the Examination of Water. Sewage, and Industrial Wastes,” 10th ed., 1955. (2) McKenzie, H. A., Wallace, N. S., Australian J . Chem. 7, 55-70 (1954). (3) Ogg, C. L., Brand, R. W., Willits, C. 0.. J . Assoc. Offic. Aar. Chemists 31, 661, 663 (194g). (4) Willits, C. O., Coe, 31. R., Ogg, C. L., Ibid., 32, 118 (1949). (5) Willits, C. O., Ogg, C. L., Ibid., 31, 565 (1948). ( 6 ) Ibid., 33, 100, 179 (1950). RECEIVEDfor review October 3, 1956. Accepted January 28, 1957.
Automatic Recording Thermo balance CORNELIUS GROOT and V.
H. TROUTNER
Corrosion and Coating Operation, Reactor and Fuels Research and Development Operation, Hanford Afornic Products Operation, Richland, Wash.
b An automatic recording thermobalance was constructed from standard laboratory equipment a t a minimum of expense, to plot pyrolysis curves. The unbalance is detected by a photoelectric null-point detector and is restored by an electromagnetic restoring device. The balance recorded weight changes over the ranges of 50, 100, and 200 mg. with an accuracy within 3t0.3, k0.4, and h0.8 mg., respectively. The sensitivity was 0.3 mg. The balance required 10 minutes to register a full range weight change. Construction details and a discussion of the thermobalance are given.
A
measures the changes in weight of a substance as it is heated. Several systems for THERMOBALANCE
automatically recording changes in weight have been described (2). Duval has shown (1) how the curves of weight us. temperature (pyrolysis curves) can be used to study the composition of many substances. Because the recording thermobalance used by Duval was not commercially available a t the time of this work, an automatic recording thermobalance was constructed from standard laboratory equipment a t a minimum expense. Weight changes were detected by a photoelectric cell and measured with an electromagnetic restoring device. The temperature of the furnace, surrounding the material being studied, was increased linearly with time and the weight mas plotted as a function of time. The resulting curves of weight us. temperature were used to study the composition of hydrated aluminum oxide samples. This
report presents the construction details and a discussion of this thermobalance. The automatic recording thermobalance consists of four main systems: 1. The thermo system for holding and
heating the sample.
2. The balance system for weighing the
sample. 3. The automatic or control system to compensate the balance fo; changes of sample weight. 4. The recording system for recording the force needed to compensate the balance, and thus the weight change of the sample. The complete apparatus is shown in Figures 1, 2 , and 3. THERM0 SYSTEM
The thermo system (Figure 4) consists of a vertically mounted electric tube furnace, a quartz rod which supports the sample within the furnace, VOL. 29, NO. 5 , MAY 1957
835
