ANALYTICAL EDITION
198
unreliable in small sizes and gives one a false sense of security. By running preliminary experiments in small autoclaves that can be readily dropped into a pail of water should the
Vol. 3, No. 2
pressure become too high, and by avoiding the use of straight oxygen or of acetylene under combined pressure and temperature, accidents should not occur.
Microanalysis with an Ordinary Balance I-Determination
of Nitrogen by Micro-Dumas Method’ Wm. J. Saschek2
LABORATORY OB MICROCHEMISTRY, WASHINGTON SQUARECOLLEGE,NEWYORU UNIVERSITY,NEW YORK,N . Y.
M
ANY laboratories engaged in industrial analysis or research find it necessary or convenient to carry out numerous quantitative nitrogen determinations. The macro-Dumas method is tedious and time-consuming. The many advantages of the micro methods of elementary analysis devised by Pregl are so apparent that quite a number of adaptations have been made to utilize these methods in ordinary industrial work. An obstacle to a general adoption of the micro methods has been the cost and elaborate operating conditions of the micro balance required (W,9, 6, 7). The usual method of attacking this problem is to increase the weight of the sample from the 2 to 5 mg. used by Pregl to 20 to 30 mg., and use an analytical balance of very high sensitivity such as 0.05 mg. (4,8). These methods, however, do not retain all the advantages of Pregl’s micro methods. I n the first place, a balance of such high sensitivity costs almost as much as a micro balance. Second, the time of analysis, on account of the larger sample, is not very much shorter than the ordinary macro method. Furthermore, the apparatus used is considerably more complicated than that employed by Pregl. One method retaining Pregl’s apparatus in its entirety, but still avoiding the use of a microbalance, was worked out in this laboratory (5). Briefly, this method consists in dissolving the sample in a suitable solvent, spreading an aliquot portion of this solution on copper oxide in the mixing tube, and evaporating the solvent. While this method gives fairly good results, it was believed that there was room for further improvement. I n the first place, the choice of a suitable solvent presents some difficulty where the solubility of the unknown substance in various organic solvents is not known. Then, in transferring the aliquot portion from the bottle to the mixing tube, some solvent is lost both from the sample and from the solution in the bottle, thereby introducing an error. Difficulty may also be experienced in removing the solvent after weighing. I n order to overcome these difficulties, a solid mixture was employed in place of the solution. Since the sample must be thoroughly mixed with copper oxide before introducing into the combustion tube, fine precipitated copper oxide was used as the base of the mixture. A comparatively large sample (0.1 gram) was ground up with a large amount of copper oxide (5 grams), and after thorough mixing, an aliquot portion of about 0.2 gram was taken for analysis. If uniform distribution of the substance in the copper oxide is assumed and the aliquot sample is weighed out to within 0.1 mg. it can be seen that the sample of the substance has been weighed to 0.001 mg., which is the accuracy of the microbalance. If the mixture is ground fine so that no particle is larger than 0.05 mm. in diameter, the error due to sampling can be calculated by the fobmula of Baule and BenedettiPichler (1): 1 Received
February 13. 1931. Present address, Department of Biological Chemistry, College of Physicians and Surgeons, Columbia University, New York, N. Y. 1
M =
p d dv.6.+1(1 - pl) 4 E
where M = “absolute average deviation” from theoretical percentage of element PI and Pa = percentage of element to be determined ( N 2 ) in two components of the mixture (organic nitrogenous substance and copper oxide) E = weight of entire sample v = volume of single particle 6 = density of mixture 91 = ratio of number of particles of substance to be analyzed to total number of particles in mixture
To illustrate: If we take a substance containing 50 per cent nitrogen and mix it with fifty times as much copper oxide, the mixture should have practically the same specific gravity as pure copper oxide, since the small amount of organic substance will not affect it to an appreciable extent. If ground in an agate mortar, the particles will have a maximum diameter of 0.05 mm. Substituting in the formula we have 1M =
50 - 0 4 1 2 5 . lo-“ z/%O
6 . 0 . 0 2 .(1
- 0.02)
=
0.01%
Thus it can be seen that using a substance containing 50 per cent nitrogen, the absolute average deviation from the correct percentage due to sampling is 0.01 per cent; in other words, the per cent nitrogen obtained for the above substance will not be over 50.01 per cent nor under 49.99 per cent. These calculations are based upon the most unfavorable conditions. I n actual practice the particles are much smaller, and the nitrogen content very rarely is as high as 50 per cent. Table I-Results of Andyses of Samples (Substance used, 0.1000 gram) MIX-
SUBSTANCE o-Toluamide
CuO
CON-
TURE
TENT OB
OUT
STANCE
WEIGHEDSUB-
Grams
Gram
Ms.
4.9000
0.3285 0.3168 0.3306 0.3034 0.3318
6.570 6.336 6.612
6.636
0,2528 0.2288 0.2802 0.2506
5.056 4.576 5.604 5.012
0.068
Acetanilide
4.9000
Acetanilide
4,9000 0.2643 5 , 2 8 6 0.2630 5.260
Azobenzene
5,0000 0.2256 0.1402 0 1716
Beuzamide
5 0000
4 424 2 749 3 365
0.1914 3.752 0 . j 1 0 2 6 082
NITROGEN OBTAINED Volume Temp. Pressure Cc. C. Mm. 70 0.586 22 762 10.35 0 . 5 5 0 17 769 10.34 0.581 20 769 10.36 0.544 24 756 10.25 0.502 22 764 10.37 Av. 10.33 Theory 10.36 0.458 2 0 . 5 761 10.55 0 . 4 0 9 27 768 10.27 0.ciOl 27 768 10.28 0.446 25 765 10.26 Av. 10 34 Theory 10 37 0.481 26 761 10.40 0.484 29 767 10 49 Av. 10 44 Theory 10.37 0 580 20 767 15 41 0 360 22 773 16 41 0 444 21 767 15 46 Av. 15 43 Theory 15 39 0.364 23 767 11 30 0.682 21 773 11 30 Av. 11 30 Theory 1 1 . 5 8
,
INDUSTRIAL AND ENGINEERING CHEMISTRY
April 15, 1931
A mixture containing about 2 per cent of the substance to be analyzed was found to be suitable. It was prepared as follows: 0,1000 gram of the finely powdered substance was first thoroughly mixed on high-gloss paper with 4.9000 grams of pulverized copper oxide which had been previously ignited and ground in the mortar. It is not necessary to have the sample and copper oxide in exactly this proportion; i t was only used to simplify final calculations. The mixture was then transferred quantitatively to a glass or, preferably, an agate mortar, where it was ground for several minutes. The particles of this mixture had a maximum diameter of 0.02 mm. which is much less than that assumed in the theoretical discussion. AIiquot samples of about 0.2 gram were taken for analysis. The course of the analysis from this point on was the same as described by Pregl. The results are given in Table I. To sum up, the economy and simplicity of Pregl’s micro-
199
Dumas determination of nitrogen have been retained in a method which eliminates the use of a microbalance when 0.1 gram of substance is available. It has the further advantage that as many analyses as desired can be made and the unused sample recovered by the use of a suitable solvent. If a more sensitive balance is available, or if so great an accuracy is not necessary, the amount of substance can be reduced still further. Literature Cited Baule and Benedetti-Pichler, 2. anal. Chem., 74, 442 (1928). Emich, “Lehrbuch der Mikrochemie,” Muenchen, 1926. Flaschentraeger, 2.angtw. Chem., 39, 717 (1920). Lauer and Sunde, Mikrochemic, Pregl-Festschrift, 236 (1929). Niederl, Trautz, and Saschek, Ibid., Emich-Festschrift, 219 (1930). Pregl, “Die Quantitative organische Mikroanalyse,” Springer, 1930. Translated by Ernest Fyleman, Blakiston, 1930. (7) Schwarz-Bergkampf, 2. anal. Chem., 69, 321 (1926). (8) Wise, J. A m . Chcm. Soc., 39, 2055 (1917).
(1) (2) (3) (4) (5) (6)
The Hehner Test for Formaldehyde’ Charles C. Fulton U. S. INDUSTRIAL ALCOHOL BUREAU,OMAHA,NEB.
The Hehner test can be improved by using bromine description ( 1 ) sets the senHEN a solution of for the oxidizing agent and by diluting the sulfuric sitivity of the Hehner test f o r m a l d e h y d e in acid somewhat before adding the milk to be tested. ataboutl:10,000. Thisisan milk is underlaid The color can be developed in a zone, or uniformly obvious error, as the test is with concentrated sulfuric throughout the solution. The improved test is sensieasily more sensitive than the acid containing a little ferric tive to 1 part of formaldehyde in 1,000,000 of milk. Rimini, or phenylhydrazine salt or other oxidizing agent, hydrochloride and s o d i u m a violet color develops at the junction. This is the well-known Hehner test, introduced nitroprusside test, for which the sensitivity is set a t 1:70,000 in 1895 and still in use. The reaction with formaldehyde to 1:80,000. Leach sets the sensitivity of his test, which is depends on the tryptophan component of the proteins. similar to Hehner’s, a t 1:250,000 ( 3 ) . Tryptophan is one of the amino acids. Proteins other than The improved test is sensitive to 1 part of formaldehyde in those of milk can be used for the test so long as they con- 1,000,000 of milk and it has certain other advantages also. tain a moderately large proportion of tryptophan. Egg al- Two methods of making the test are given in the following bumin is sometimes used. Peptone has been highly recom- paragraphs. The first gives a zone reaction, as in the ordimended for this test or for the Leach test, which substitutes nary Hehner test, and the second gives a uniform color concentrated hydrochloric acid and heating for the concen- throughout the solution. trated sulfuric acid. However, some samples or kinds of First Method-Dilute 5 cc. of concentrated sulfuric acid with 1 peptone give only a feeble reaction. The test is an aldehyde-oxidation reaction of an aromatic cc. of water, cool, and put 3 cc. of this diluted acid in a test tube. Drop in a small crystal of potassium bromide. Shake, then overamine, analogous to the aldehyde-oxidation reactions of lay at once with 1 cc., or a little more, of the milk to be tested, phenols, which have been previously discussed ( 2 ) . Experi- without mixing. Bromine, set free by the action of the strong ments with formaldehyde and tryptophan itself show that bro- acid, effects the oxidation. If formaldehyde is present, even as mine is the best oxidizing agent, and that the proper strength little as 1 part in 1,000,000 parts of milk, a violent zone quickly If no violet appears by the time the acid has become of sulfuric acid for the strongest color is secured by diluting develops. orange-yellow, the test is negative. Owing t o the prior dilution the concentrated acid with from half its volume up to an of the acid, the color forms in a fairly broad zone rather than in a equal volume of water. Tryptophan alone, in aqueous solu- narrow ring. The escape of bromine and hydrobromic acid also tion, gives a purplish pink color with bromine but this bro- tends t o mix the two solutions. The reaction is strongest with 1 part formaldehyde in 50,000 of milk. The color then mine test for tryptophan cannot be obtained in strong sul- about spreads through practically the whole of the liquid in the test furic acid solution and does not interfere at all with the form- tube, and is strong violet a t the bottom, ranging to deep red a t the center, and back to purple a t the top. For good results the aldehyde reaction. If the conditions found most suitable for the tryptophan formaldehyde should not exceed about 1 part in 1000; this is of the usual Hehner test also reaction are applied to the detection of formaldehyde with true Second Method-Dilute 8 cc. of concentrated sulfuric acid with milk, the test is considerably improved. The usual test, 5 cc. of water, cool, and put 4 cc. of this diluted acid in a test if carefully performed, is sensitive to about 1 part of formal- tube. Add 1 cc. of the milk t o be tested, and mix with running dehyde in 300,000 of milk. On account of the color ob- water while cooling. A clear and practically colorless solution result, unless a large proportion of formaldehyde is present tained with a blank (yellowish green changing to brown), should (see Table I). The curd at first formed is redissolved. Prepare and the charring produced if the concentrated sulfuric acid a bromine oxidizing solution by mixing equal volumes of concenis accidentally partly mixed with the milk suddenly, the trated sulfuric acid and saturated bromine water, and cooling test is none too reliable for very small proportions of form- Add about 0.5 cc. of the oxidizing solution t o the sulfuric acidsolution, and shake. I n the presence of formaldehyde a aldehyde-that is, less than 1part in 100,000. The A. 0. A. C. milk violet color develops a t once, a color ranging t o light purplish 1 Received
January 21,1931.
pink for very small proportions of formaldehyde.
The blank is
