Determination of Nitrogen Dioxide by Cerate Oxidimetry F. M. STUBBLEFIELD Davis and Elkinr College, Elkinr, W. Va.
I
to assure maximum accuracy in the permanganate determination. The potassium permanganate solution was floated upon the surface of the acid water, and not stirred until very close to the end point, when very slow stirring was initiated. The prescribed 3-minute period was taken to assure permanency of the pink potassium permanganate color. Table I presents data comparing percentages of nitrogen dioxide as determined by the potassium permanganate and cerate procedures. The determinations by the cerate method were completed in much less time than those by the permanganate procedure. The results compare favorably. I n the higher percentages of nitrogen dioxide it is believed that results by the cerate procedure are more accurate than by the permanganate, because of sharper end point and absence of opportunity for possible loss of nitrogen dioxide, since the gas is always in contact with an excess of oxidizing agent. hTo attempt was made to extend the method directly to the nitrosyl sulfuric acid determination. Since the potassium permanganate titration for this is identical with that for nitrogen dioxide, the writer can see no reason why the cerate titration should not be entirely satisfactory for this estimation.
N T H E manufacture of nitric acid by the Ostwald process and its modifications, the amount of nitrogen dioxide in the final acid must be determined. I n addition to the effect of nitrogen dioxide in industrial uses of the acid, the amount present is a measure of the efficiency of the air bleach, and represents a waste of the gas. The amount of nitrosyl sulfuric acid in a mixed acid must be determined in order to establish the percentage of the nitric and sulfuric acids present. The standard method (8) of nitrogen dioxide determination, used in most plants where such work is needed, is titration of a 10-ml. sample with 0.2 N potassium permanganate until a pink color persists for 3 minutes. The method is accurate only with careful manipulation of the sample. In actual control practice loss of nitrogen dioxide from the solution under even the best of conditions, speed of addition of otassium permanganate, amount of stirring of the solution wtile titrating, and individual definition of what constitutes a permanent pink color, as well as the error caused when a rushed analyst shortens the 3minute period to 2 minutes or even 1-all contribute to inaccuracy. Nitrosyl sulfuric acid is also determined by permanganate kitration, using either a 10-ml. sample or an aliquot thereof. The many published works, and especially those of Smith (1, S), amply prove the advantages of cerimetry in regard to stability and oxidizing power of the solution, reversibility of the reaction, and lack of side reactions and interferences. These advantages have been retained in developing a method for determination of nitrogen dioxide that is accurate and rapid, even in the hands of an inexperienced analyst.
Table 1. Sample NO.
REAGENTS
1 2 3 4 5
The solutions required are a 0.1 N solution of sodium oxalate, an approximately 0.1 N solution of ammonium nitrato cerate containing 170 ml. of 72% perchloric acid per liter and the indicator, nitro-o-phenanthroline ferrous sulfate. The cerate solution is prepared by adding 55 to 56 grams of ammonium nitratoaerate to the perchloric acid, stirring for half a minute, and adding 100 ml. of water. The solution is stirred for another half minute, a second 100-ml. ortion of water is added, and the process is repeated until a voyume of 1 liter is reached. If the salt is not dissolved in this way, an insoluble salt may precipititate in a few days’ standing. The cerate solution is checked periodically in terms of exactly 0.1 N sodium oxalate.
6 7 8 9 10
11 12 13 14 15 16 17 18 19 20
Determination of Nitrogen Dioxide (In approximately 60% nitric acid) Nitrogen Dioxide Found Permanganate method Cerate method
470
%
0.15 0.28 0.36 0.48 0.59 0.80 0.88 1.02 1.24 1.46 1.58 1.69 1.84 2.09 2.26 2.63 9.97 3.49 -1.16 5.34
0.15 0.28 0.37 0.48 0.60 0.80 0.88 1.02 1.24 1.47 L59 1.70 1.85 2.10 2.27 2.64 2.98 3.51 4.17 5.36
PROCEDURE
To 100 ml. of water in a 250-ml. beaker, 5 ml. of 72% perchloric acid are added. To this acidified solution in most cases 25 to 50 ml. of 0.1 N cerate are added, the amount varying with the quantity of nitrogen dioxide expected to be present. A 10-ml. sample of the acid is pipetted accurately into the beaker. A t the start of delivery, the pipet tip should be near the bottom of the beaker; near the end of dehvery, the tip is raised until it ba+y touches the surface of the solution. It is let stand 1 or 2 minutes, then stirred slowly. Two to 3 drops of nitroferroin are added, and the excess cerate is titrated with sodium oxalate. The end point is sharp. Since the exact cerate equivalence in terms of a 0.1 N solution is known by the earlier oxalate titration of the cerate, the milliliters of cerate actually used in oxidation of the nitrogen dioxide will be the equivalence figure minus the milliliters of 0.1 N oxalate used in titration of the residual cerate. The percentage of nitrogen dioxide may be calculated by the usual equations recognizing that 0.1 N solutions have been used in place of 0.2 d potassium permanganate.
S U M M A R Y AND C O N C L U S I O N S
A procedure utilizing a cerate solution as the oxidizing agent has been outlined for the determination of nitrogen dioxide, and its application to the estimation of nitrosyl sulfuric acidauggested. The method has none of the disadvantages of the potassium permanganate procedure and may be carried out rapidly. Experimental work has proved it to be fully as accurate as the permanganate method in lower concentrations of nitrogen dioxide and slightly more accurate at higher concentration, under conditions in which extreme care was taken with the permanganate procedure. Under rushed control laboratory conditions, the cerate procedure is much more accurate than the permanganate. LITERATURE CITED
Kolthoff. I. M., and Furman, N. R.,“Volumetric Analysis”’ Vol. 11, pp. 491-5, New York, John Wiley & Sons, 1929. (2) Scott, W. W., “Standard Methods of Chemical Analysis”, 5th ed., p. 653, New York, D. Van Nostrand Co., 1939. (3) Smith, G. F., “Cerate Oxidimetry”, Columbus, Ohio, G . Frederick Smith Chemical Co., 1942. (1)
EXPERIMENTAL
The method was carefully checked against the potassium permanganate procedure, using a series of acid samples containing varying amounts of nitrogen dioxide. Extreme care wits taken
366
