Nitrite Nitrogen Standards W. F. RELNDOLLAR State of Maryland Department of Health, Baltimore, 3rd.
P
ROCEDUFSS for the determination of nitrite nitrogen
in water and sewage are given by the American Public Health Association (f), and in cereals, and maters, brines, and salt by the Association of Official Agricultural Chemists ( 2 ) . Although each of these methods is based upon the diazotization reaction originally suggested by Griess (4)and is employed to estimate quantities of nitrite nitrogen in approximately the same range of concentration, the reagents differ both in composition and in mode of preparation. This sulfanilic acid is variously described as an 0.8 per cent weight by volume solution in 5 N acetic acid, a solution of 0.5 gram of the compound in 150 cc. of 20 per cent acetic acid, and a 1 per cent aqueous solution. The a-naphthylamine reagent is prepared in one case by dissolving 0.5 gram of the base in 100 cc. of 5 B acetic acid; in the other two cases the hydrochloride is employed, making solutions of 0.13 and 0.5 per cent concentration in dilute acetic acid and water, respectively. In every case sodium nitrite is prepared from silver nitrite by interaction with sodium chloride. In one procedure (that for cereals) the quantity of the sulfanilic acid and a-naphthylamine reagents is increased from 1 to 2 cc. and the time period alloaed for the color to develop is changed from 30 to 60 minutes. Investigations made in connection with the development of this test point out the conditions necessary to obtain optimum results. Griess (S), in 1879, called attention to the fact that an acidified mixture of sulfanilic acid and cu-naphthylamine gave a pink coloration in the presence of minute amounts of nitrous acid. Ilosva (5, 6) studied this reaction and suggested the substitution of acetic acid for the hydrochloric acid used, claiming that the former induced a more rapid development of color and a more uniform gradation of tint. Weston (9) in a comprehensive review of the subject concluded, in part, that a n excess of hydrochloric acid interferes with the delicacy of the reaction whereas a n excess of acetic acid does not, and that the procedure as modified by Ilosva is more rapid than the original method. He recommended also that the reagents be made more concentrated to avoid inconvenient dilution of the sample. Although numerous articles have since appeared suggesting further modifications, the procedure of Weston is essentially the standard today. Mason (7) explains the use of silver nitrite rather than sodium nitrite as primary standard on the basis of the questionable purity and deliquescent nature of the latter salt. Appropriate inquiry has shown that the first objection no longer obtains; sodium nitrite of high quality may be readily secured. This is fortunate, as the employment of the silver salt to prepare nitrite standards entails certain difficulties which are likely to impair accuracy. Silver nitrite is somewhat photosensitive, deteriorates slightly with age, and may become unreliable as a standard. It dissolves slowly and with difficulty. When sodium chloride is added to effect decomposition, the silver chloride, owing to the necessary absence of acid, does not precipitate in its characteristic clotted form but occurs as a colloidal dispersion. The standard method ( 1 ) specifies that a definite quantity of solution from this mixture be used in preparing the more dilute reagents, leaving the choice of methods of clarification to the operator. If filtration is employed for this purpose, i t is necessary to return the liquid to the filter repeatedly and to wash thoroughly to ensure complete removal of sodium nitrite. If a clear liquid is to be obtained by settling and decantation, as suggested by Mason ( 7 ) , 2 to 3 days must elapse before the
diluted reagents can be prepared. Centrifuging for a reasonable period does not yield a clear liquid. I n order t o learn whether pure sodium nitrite may serve as a primary reagent in the preparation of nitrite nitrogen standards, this investigation was undertaken.
Reagents S o m m NITRITE. Solution A . A small quantity of Merck’s reagent grade granular sodium nitrite was dried overnight in a desiccator, assayed by the U. S. 1’. X I (8) procedure, and found to contain 99.4 per cent of sodium nitrite. A stock solution was prepared making due allowance for the slight amount of impurity present, and from this liquid a standard solution containing 0.0005 mg. of nitrogen per cc. was prepared by successive dilution, Solution B. This solution was prepared by the interaction of silver nitrite and sodium chloride as directed (1) and reduced by successive dilution, as specified, to the same concentration as A . Clarification of the stock solution was effected by repeated filtration and washing of the filter and its contents. Solution C. Preparation of this solution was similar to that of B except for the method of clarification. In this case the stock reagent was allowed to stand for 52 hours, and although still somewhat. turbid was withdrawn and diluted as specified. SULFANILIC ACID SOLUTION. Prepared as directed (1). NAPHTHYLAMINE -4CETATE SOLUTION. A fresh SUP ly O f the base was obtained and dissolved in 5 N acetic acid folrowing the procedure of (1). The finished reagent was perfectly clear and had a weak purple tint. Color standards were prepared from the three sodium nitrite solutions and a comparison of their respective values was made at the concentrations suggested ( I ) . I n every case slightly higher values were obtained for A than for B, indicating that with the latter solution some nit#ritehad been lost, probably in the process of clarification. Values obtained when A and C were compared were in good agreement. I n view of this fact i t appears that sodium nitrite solution may be substituted for the silver nitrite-sodium chloride solution and that where the latter is employed, decantation should be specified as the method of clarification.
Summary Sodium nitrite of a sufficiently high degree of purity to serve as a primary standard in the nitrite nitrogen determination is readily obtainable. I t s use obviates the delay attendant upon the use of silver nitrite and permits the preparation of all three dilutions of the reagent in a short period. As sodium nitrite is relatively inexpensive and the solution is readily prepared, it may be made fresh when needed, thus eliminating any question of deterioration arising from the use of an old solution.
Literature Cited (1) Am. Pub. Health Assoc., “Standard Methods of Water Analysis”, 5th ed., pp. 46, 133, New York, 1936. (2) Assor. Official Agr. Chem., Official and Tentative Methods of Analysis, 4th ed., pp. 216, 506, 1935. (3) Griess, Peter, Ber., 12, 426 (1879). (4) Griess, Peter, Proc. Roy. Soc., 9, 494 (18583, ( 5 ) Ilosva, M. L. I. de N., Bull. SOC. chim., (3) 2, 347 (1889). (6) Zbid., p. 388. (7) Mason, W. P., and Buswell, A. hl., “Examination of Water”, 6th ed., p . 51, New York, John Wiley Bi Sons, 1931. (8) United States Pharmacopeia XI, p. 344, 1936. (9) Weston, R. S., J . Am. Chem. Soc., 27, 281 (1905).
325
