ANALYTICAL CHEMISTRY
878 (20) Hogness, T. R., Zscheile, F. P., Jr., and Sidwell, A. E., Jr., J. Phys. Chem., 41, 379 (1937). (21) Holiday, E. R.. and Beavan, G. H., Photoelectric Spectrometry Group, Bull. 3, 53 (October 1950). (22) Kemmerer, A. R., J. Assoc. Ofic.Agr. Chemists, 2 9 , 1 8 (1946). (23) Kortum, G., “Kolorimetrie und Spektralphotometrie,” Berlin, Springer-Verlag, 1948. (24) Lowry, 0. H.. and Bessey. D. A., J . Biol. Chem., 163,633 (1946). (2F) Matossi, F., Phys. Rev., 76,1845 (1949). (26) Miller, W. C., Hare, G., Strain, D. C., George, K. P., Stickney, M. E., and Beckman, A. O., J . Opt. SOC.Amer., 39,377 (1949). (27) Optical Society of America, Committee on Colorimetry, Ibid., 34, 183 (1944). (28) Tunnicliff, D. D., ~ I L CHEM., . 20, 828 (1948).
Twyman, F., and Allsopp, C. B., “The Practise of Absorption Spectrophotometry,” P. 57, London, Jarrell-Ash Co., 1934. Vandenbelt, J. hl., Forsyth, J., and Garrett, A., IND.ESG. CREM.,ANAL.ED., 1 7 , 2 3 5 (1946). Vitamin Oil Producer’sInstitute, San Francisco, Calif., Collaborative Assay No. 1, 1943. RECEIVED for review April 11. 1951. Accepted February 6, 1953. Presented before the Pittsburgh Conference o n Analytical Chemistry and Applied Spectroscopy, February 1950. Supported i n part by the joint program of the O 5 c e of S a v a l Research and the Atomic Energy Commission. This paper is a joint contribution of Beckman Instrumente, Ino.: and the Laboratory of Suclear Science and Engineering and Department of Chemistry of the Massachusetts Institute of Technology.
Polarographic Determination of Nitrates in Sanitary Analysis M. C. RAND AND HOVUNESS HEUKELEKIAR .Yew Jersey .4gricultural Experiment Station, R u t g w s L‘nirersity, .Yew Brunswick, N. J .
Research dealing with biological oxidation under the conditions prevailing in natural bodies of water requires accurate routine determination of nitrates to ascertain what portion of the total oxygen demand is due to nitrification. Chemical methods for nitrate assay tend to be complicated, subject to interferences, or inaccurate at the low concentrations involved. In the polarograph, nitrates produce a characteristic current wave in supporting solutions of zirconyl chloride. The wave is eliminated by addition of excess ferrous sulfate. Analysis by direct observation of the wave height is somewhat subject to interferences. A preferable technique is to ob-
I
T HAS become almost customary, in studying the biological
deoxygenation of water, to avoid nitrification rather than try to evaluate it. Several more or less elaborate procedures for preventing nitrification have appeared in the literature, such as pasteurization of the sample or selective poisoning of the nitrifying organisms. The usual practice is to determine nitrites rather than nitrates, and when the analyses show that nitrification is occurring in spite of the precautions taken, the nitrifying samples are discarded, because of the limitations of the available chemical methods for nitrate determination. The literature indicates that under certain conditions nitrates produce a polarographic wave. It was considered possible, therefore, that this approach might provide an assay method. Generally speaking, polarographic analyses are simple in technique, and can be made specific by proper selection of conditions. The accuracy usually compares favorably with chemical methods. The objectives of this study were to define conditions and evolve a technique which would permit applying a polarographic determination of nitrates to sewage, water, and particularly to biologically treated sewage effluents, and to compare the method critically with existing chemical methods of nitrate determination. SELECTION OF BASE ELECTROLYTE
A variety of electrolytes have been employed as base solutions in the polarography of nitrates, including lanthanum chloride and cerous chloride (3, 6, 7-9) and alkaline earth chlorides (8, 9). Lanthanum, cerous, alkaline earth, and zirconyl chlorides were among those tested in this laboratory. Figure l , A shows the type of polarogram obtained in a supporting solution of lan-
serve the change of limiting diffusioncurrent which is produced by addition of ferrous sulfate. The latter method is capable of determining nitrate nitrogen from 0.02 to at least 25 p.p.m., with a maximum variance of 3.470. Recoveries were from 98 to 103q0 in well water and treated sewage. In raw sewage, the standard addition technique had to be used, which gave recoveries ranging from 91 to 108q0,with a standard error of 6%. The technique is particularly useful in studying biological nitrification, but it should find application in other branches of sanitary analysis, as its accuracy and sensitivity are superior to those of chemical nitrate determinations.
thanum chloride. The disadvantages of this electrolyte have been explained by Kolthoff et al. ( 4 ) ; the height of the wave is not proportional to nitrate concentration, and the nTaveJsposition depends upon the concentration of nitrate. When 0.1 N solutions of magnesium, calcium, or barium chloride are used as the base electrolyte, a wave occurred on addition of nitrate, but was in each case so close to the decomposition potential of the base electrolyte that its true height could not be determirird (Figure 1,B). With 0.1 N cerous chloride, the base line, \Tith or without added nitrate, rose rapidly from the origin (Figure 1,C) and no nitrate wave was observed. This effect may have been due to an impurity in the reagent grade cerous chloride, such as small quantities of tetravalent cerium, but the matter was not investigated further. In zirconyl chloride solutions, nitrates produce a clear-cut wave (Figure I$). The wave is not ideal for analytical purposes; it is somewhat extended (almost 0.5 volt wide) and it barely reaches its limiting diffusion current before electrolysis of the base solution begins. However, this electrolyte was selected for further work because it was the only one of those tested which produced a wave suitable for analytical work. The concentration of zirconyl chloride is not critical in the range around 0.1 N . When the concentration is reduced ta 0.01 N , the nitrate wave is shifted about 0.1 volt in the negative direction, while in 0.55 N zirconyl chloride the base line becomes overly steep, and the nitrate wave is somewhat suppressed. When 0.1 N zirconyl chloride is used as the base electrolyte in water and sewage, no special precautions for pH control are required. The 0.1 N salt solution is a strong buffer, and it
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that none of the mineral constituents known to be present in the water supply were responsible. DESTRUCTION OF NITRATES AND DETERMINATION BY DIFFERENCE
Because attempts to eliminate and even to identify the source of the observed interference were not successful, a different approach was tried. If nitrates can be destroyed in the sample, the resulting change of diffusion current will serve as an indication of the original nitrate conD - N/IOZr3Cl2 C N / IO CeC13 centration. Even in the presence of materials which cause a rise in the base line, the currpnt difference should be proportional to the nitrate concentration alone. It was found experimentally that 0.5 ml. of 1 S ferrous sulfate eliminates the nitrate wave in zirconyl chloride base solutions. In the absence of nitrate, the ferrous sulfate, if free of ferric compounds, produces no change in the base line in tap water, and very little in sewage. 0 04 08 12 t6 0 04 00 12 16 20 Ferrous sulfate is knonm to reduce nitrates to VOLTS VOLTS nitric oxide ( S O ) in acid solutions. This is preFigure 1. Nitrate Polarogams in Various Base Electrolytes sumably the reaction involved, but it has not been experimentally proved. The relationship hetween nitrate concentration and current change on addition of ferrous sulfate was found to be linear from 0.1 to 25 p.p.m. of nitrate nitrogen. Figure 3 shows the relationship observed by the technique described, using standard solutions of sodium nitrate, n-ith 0.1 N zirconyl chloride as the base electrolyte.
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'CORRECTION FOR FERRIC ION
As the ferrous sulfate solution ages, addition of the reagent to a nitrate-free sample sometimes produces a slight increase in diffusion current. This effect was proved to result from the formation of ferric salts in the stock solution, by atmospheric oxidation. The effect may be corrected by observing its magnitude
0
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2
3
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N 0 3 - N IN P P M
Figure 2. Relationship of Wave Height to Nitrate Concentration in 0.1 N Zirconyl Chloride
was found experimentally that the pH after addition of the base electrolyte is uniformly 1.7, as indicated by the glass electrode, whether the sample consists of distilled water, tap water, or sewage. When dealing with strongly alkaline samples of industrial wastes, it may prove necessary to neutralize, as the height of the nitrate wave is affected by pH (5). ANALYSIS BY OBSERVATION OF WAVE HEIGHT
Figure 2 shows that the relationship between nitrate concentration and wave height is linear in distilled water solutions of sodium nitrate, containing 0.1 N zirconyl chloride. It was presumed that observation of the wave height might provide a suitable analytical technique, but it developed that stale sewage, and sometimes even tap water, produced a distortion of the base line such that the presence of nitrates was indicated when none was added. In some cases the interference exceeded 2 p.p.m. Treatment of the sample to remove cations, by ion exchange or chelation, did not improve the results. It was established also
0
5
10
15
20
25
30
NO3- N, P P M Figure 3. Relationship between Nitrate Concentration and Current Change on Addition of Ferrous Sulfate in 0.1 N Zirconyl Chloride Base Solution
at the beginning of each day's work and adding the necessary correction to all current changes subsequently observed. It has been established that no systematic error in the determinations results from such a procedure. However, when the correction becomes more than about 10% of the current change observed in the sample, the volume of ferrous sulfate added becomes critical, and a new solution should be prepared. This became necessary at intervals of from 2 weeks to 1 month.
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ANALYTICAL CHEMISTRY
The “ferric ion correction” may be maintained a t a low value almost indefinitely if the solution contains about 0.5% of free sulfuric acid and is stored in contact with iron wire.
Table 11. Recovery of Nitrates Added to Sewage Original 0 0 0 0.210
PROCEDURE FOR ANALYSIS OF UNKNOWN SAMPLES
The necessary reagents are: Zirconyl chloride, 1.1 N. One hundred millimeters of solution containing 17.7 grams of zirconyl chloride octahydrate. Ferrous sulfate, 1 A-. Place 27.8 grams of ferrous sulfate heptahydrate in a 200-ml. volumetric flask, and add 1 ml. of concentrated sulfuric acid. Add water to dissolve and dilute to the mark. Xitrogen gas. Place a 10-ml. sample of the solution to be analyzed in the cell of the polarograph, and add 1 ml. of 1.1 S zirconyl chloride. Free the solution of dissolved oxwen bv bubbling nitrogen through it for 5 minutes. Raise th
