1630
ANALYTICAL CHEMISTRY
with suction through filter paper on a Buchner funnel until the cake is firm and can be broken up into small particles. The damp articles are air dried with continued suction a t low relative Rumidities t o give a product which has little or no odor of namyl alcohol, and which dissolves completely in boiling water. If high relative humidities prevail, the air should be dried. A vacuum desiccator evacuated with a water aspirator might be used where the relative humidity is high or where faster drying is desired. The dry product should be stored in a tightly stoppered bottle. The yield of purified linear fraction varies with the degree to Fhich the precipitates settle out (especially the undissolved starch granules in the first step). An average yield is about 2 grams of dry linear fraction from 50 grams of raw potato starch, where only 50% of the total volume of the initial extraction was supernatant solution. The absorption spectrum of the blue complex of this linear fraction with triiodide ion, I*-, is similar to that found previously with the same fraction recrystallized twice with n-butyl alcohol from the crude Pentasol-precipitated fraction (2). Maximum absorbance occurs a t approximately 610 mp.
ACKNOWLEDGMENT
The research of which this work was a part was facilitated by a grant from Research Corp. LITERATURE CITED (1) Krishnaswamy, K . G., Sreenivasan, d.,J . Biol. Chem. 176, 1253 (1948). (2) Lambert, J. L . , ASAL. C H E W23, 1247 (1951). (3) Ibid., p. 1251. (4) I b i d . , 25, 271 (1953). (5) I b i d . , p, 984. (6) Lambert, J. L., Arthur, P., X o o r e , T. E., I b i d . , 2 3 , 1101 (1951). (7) L a m b e r t , J. L., Yawda, S.K., Ibid., 27, 444 (1955). (8) Lansky, S.,Kooi, II.,Schoch, T. J., J . Am. Chem. SOC.71, 4066 (1949). (9) Schoch, T. J., “ d d v a n c e s in C a r b o h y d r a t e C h e m i s t r y , ” vol. I, pp. 247-77, Pigman, W. W., Wolfrom, &I. L., eds., Academic Press. New York, 1915. (10) Schoch, T. J., J . Am. Chem. SOC.64, 2957 (1942). RECEIVED for review February 27, 1956. Accepted July 3,1956. Condensed from material t o be presented by Stanley C. Rhoads to the Graduate School of Kansas State College in partial fulfillment of the requirements for the degree of master of science.
Rapid Colorimetric Method for M. H. SWANN
and
M. L. ADAMS
f a i n t and Chemical Laboratory, Aberdeen Proving Grounds,
A quantitative method for small amounts of nitrates applicable to samples in high acid concentrations was considered useful for the analysis of certain coating materials. The pale red color that frequently forms when the famous “brown ring” test is applied tosamples containing small amounts of nitrates was tested for quantitative analytical possibilities. The method is applicable in the presence of many other anions. The reagent used and color formed are stable.
Md.
Table I.
Comparison of Theoretical and Experimental Value
Solution Concentration (Potassium Nitrate), % 0.20 1.04 5.01 10.00
T
HE red-purple color that develops when ferrous sulfate and small amounts of nitrates react in sulfuric acid has not been reported for quantitative analytical purposes. A rapid colorimetric method, based on this reaction, uses simple apparatus and stable reagents, the color is stable for several hours, and the method is applicable to dried samples and to some organic materials such as nitrocellulose. Sitrites and thiosulfates interfere, but chromates, dichromates, sulfates, phosphates, chlorates, sulfites, acetates, and the halogens do not. This allon-s broad application of the method, which has been used with success in the analysis of phosphate coating compositions and nitrocellulose. Although the nitrate content of the latter can be readily determined, oils and some plasticizers interfere and prevent application of the method to the analysis of lacquer coatings. The color formed is due to an addition compound of nitric oxide and ferrous sulfate of the formula (FeS0)SOa (1). Some analytical results are shown in Tables I and 11. ANALYTICAL PROCEDURE
Reagent. Dissolve 0.5 gram of ferrous sulfate heptahydrate in 25 ml. of water and add slowly, with cooling, 75 ml. of concentrated sulfuric acid. Cool to room temperature before using. Potassium nitrate is used for standardization, Dissolve a weighed quantity of sample or standard in water and dilute to volume, so that small aliquots containing 0.5 to 2.5 mg. of nitrate can be withdrawn into 25-m1. Erlenmeyer flasks having ground-glass stoppers. Make these samples slightly alkahne with a few drops of 0.1-Vsodium hydroxide and dry in an oven
Nitrate Present, Mg. 1.24 2.48 1.91 1.28 1.84 1.23 1.85 1.23
Nitrate Found, Ma. 1.22 2.49 1.85 1.23 1.80 1.18 1.76 1.16
Table 11. Analysis of Some Phosphate Coating Materials Nitrate Present (APP~ox.),% 15 16 15
Nitrate Found, % Nitron method Colorimetric method 14.8 14.8 14.7 14.8 15.3 15.4 14.9 15.2 14.7 15.2
a t 105” C. If the material to be analyzed is insoluble in water, acetone or similar solvent may be used. Dissolve nitrocellulose samples in acetone and dry a t 60” C. If rapid analysis is needed, use a current of air t o speed the evaporation of solvent. T o the dried sample, add exactly 20.0 ml. of the ferrous sulfate reagent preferably from a buret. Stopper the flask and al!ow it to stand for 30 minutes (90 minutes for organic materials like nitrocellulose) with frequent agitation. Determine the absorbance on a colorimeter or spectrophotometer a t 525 mp, using some of the reagent as a blank. From a graph prepared with known samples, the nitrate content of unknowns may be determined. LITERATURE CITED
(1) Feigl, F., “Qualitative Analysis b y S p o t Tests,” 2nd ed., p. 210, Nordemann Publishing Co., New York, 1939. RECEIVED for review May 29, 1956. Accepted August 1, 1956.
