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-
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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 to 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 at 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 to 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 to the sulfuric acidmilk solution, and shake. I n the presence of formaldehyde a aldehyde-that is, less than 1part in 100,000. The A. 0. A. C. violet color develops at once, a color ranging to light purplish 1 Received
January 21,1931.
pink for very small proportions of formaldehyde. The blank is
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ANALYTICAL EDITION
practically colorless, but has a faipt pink tinge. The color with as little as 1 part in 1,000,000 of formaldehyde is easily distinguishable Table I gives the results with various proportions of formaldehyde. The above proportions of acid, water, and milk can be varied a little. Two cubic centimeters of concentrated sulfuric acid can be diluted with 1 cc. of water,and this diluted acid mixed with 1 cc. of the milk to be tested. These proportions will give a stronger purple with very little formaldehyde (1 part in 500,000 or less), but on the other hand the blank then shows a distinct though slight pink color. The faint pinkish color obtained in the blank in this method is probably due to a trace of formaldehyde formed from the carbohydrate of the milk by the action of the strong acid.
The second method is particularly well suited to the detection of formaldehyde in a distillate. Instead of diluting the concentrated sulfuric acid with pure water, dilute it with the distillate to be tested, and then mix with pure milk. All varieties of the Hehner test are spoiled by a large proportion of formaldehyde. This is not a defect of the Hehner test alone, for a number of other formaldehyde tests have the same flaw. However, when enough formaldehyde
Vol. 3, No. 2
is present to interfere seriously with the test, its odor can be distinguished. T a b l e I-Reactions ,FORMALDEHYDE I N
MILK
with B r o m i n e Oxidation a n d F o r m a l d e h y d e in Hehner Test BEFOREADDING BROMINE SOLN.
AFTERADDING BROMINE SOLN.
Pam None 1 in 1,000,000 1 in 500,000 1 in 100,000 1 in 50,000 1 in 10,000 1 in 1000 1 in 100
None None None None None Light greenish yellow Greenish yellow, some undissolved material Yellowish white, undissolved material
Whitish faint pink tinge Slight phrple-pink Light pink-purple Deep violet Strong deep violet Intense violet Becomes purplish pink undissolved matter bro&nish Becomes somewhat brown ish; test negative
Literature Cited (1) Assocn. Official Agr. Chem., Methods, p. 132 (1925). (2) Fulton, J . Assocn. Ofinal Agr. Chem., 12, 434 (1929). (3) Leach and Winton, “Food Inspection and Analysis,” p. 165, Wiley, 1920.
The Opium Alkaloids as Reagents for Formaldehyde’
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Charles C. Fulton U. S. INDUSTRIAL ALCOHOL BUREAU, OMAHA, NEB. Y.,
Phenolic compounds (phenols and ethers of phenols) HE opium alkaloids, strength sulfuric acid, the rein general, a n d certain of t h e opium alkaloids in parparticularly morphine action is not very sensitive ticular, are useful as reagents for formaldehyde. The and a p o m o r p h i n e , to f o r m a l d e h y d e , and the tests are best made by mixing some of t h e aqueous have had considerable use in color with small amounts of solution t o be tested (preferably a distillate) with condetecting f o r m a l d e h y d e . the aldehyde is &t the brilcentrated sulfuric acid in appropriate proportion, and The r e a c t i o n is that of a liant crimson to:’purple obapplying this solution t o t h e alkaloid. A n oxidizing phenol with an aldehyde in tained with more formaldeagent is also added for t h e more sensitive tests. Morthe presence of strong acid, hyde or more dilute acid, but phine a n d apomorphine give good phenol-aldehyde is a dark blue that develops producing a color. tests. These tests are sensitive t o about 1 part formTests much more sensitive gradually. Hence the test aldehyde in 5000 t o 6000 parts of water. With the made by dissolving morphine both to the phenolic comaddition of t h e oxidizing agent apomorphine, codeine, pound and to the aldehyde in concentrated sulfuric acid pseudomorphine, a n d papaverine give t h e best tests. on a watch glass, and floating can be obtained by the addiThese reactions are n o t specific for formaldehyde, a s tion of an appropriate oxidizthis on the liquid to be exother aldehydes m a y give similar colors, b u t under t h e ing agent to the mixture (4). amined (1, 3) by way of exconditions a n d with t h e precautions given they provide posing the acid solution to the The alkaloids most suitable fairly certain identification, as well as detection of very vapor of formaldehyde, is not for such tests are apomorsmall quantities. They are sensitive t o as little as 1 a good one. It is better to phine, codeine, pseudomorpart formaldehyde in 500,000 or even 1,000,000 parts phine, and p a p a v e r i n e . p r o v e that the reaction is i f aqueous solition. (Phenolic tests in strong acid due to a volatile substance succeed with ethers of phenols by using a distillate from the as well as with true phenols.) The tests with the addition of suspected solution. Mix 1 cc. of the distillate with 2 to 5 an oxidizing agent can be so made as to be sensitive to 1 part cc. of concentrated sulfuric acid, while cooling; then apply to morphine on a spot plate. A purple-red to purple color of formaldehyde in 1,000,000 parts of aqueous solution. denotes formaldehyde. The color appears practically at; Phenol-Aldehyde Reactions once down to about 1 part of formaldehyde in 6000 of water. MORPHINE-The test with morphine is usually made by The test can be carried out as a zone reaction if desired. APOMORPHINE-The above remarks in regard to mordissolving the alkaloid in concentrated sulfuric acid and adding a drop or two of the solution to be tested. It is better phine apply in large part to apomorphine also. With the to dilute the sulfuric acid with the aqueous formaldehyde latter, the color in concentrated acid is purple, rapidly darksolution and then apply the mixture to the morphine, but the ening to blue-black and then changing to dark green; with texts do not state what proportion of acid and aqueous solu- only a little formaldehyde the color develops as dark green. tion should be used for such a test. There is a too-prevalent With acid of 75 to 85 per cent sulfuric acid, the purple to violet assumption that phenol-aldehyde reactions take place best color is better and more persistent. Gettler recommended in full-strength, concentrated sulfuric acid, but this assump- apomorphine over morphine. He mixed 5 cc. concentrated tion is often erroneous. The morphine reaction is best in sulfuric acid with 2 cc. of distillate from the suspected solusulfuric acid of 75 to 85 per cent sulfuric acid. With full- tion, then added the alkaloid (5). “Allen’s” misrepresents him as recommending apomorphine for Bonnet’s test (3). I Received January 21, 1931.
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