A S A L Y T I C A L EDI TIOiY
244
cated by the electrical measurement, still the deviation for this extreme case is only about 2.0 per cent absolute moisture content. Effect of Surface Finish
Tests were made upon white-pine specimens that had a surface finish on one face, to see if it would affect the results. Two coats of each of the following finishes were tried: white lead paint, a typical prepared paint, spar varnish, white shellac, lacquer, and aluminum powder in spar varnish. T o eliminate as far as possible all unknown factors other than the effect of the finish, the specimens were conditioned in the 60 per cent relative humidity room prior to finishing and were left there until the finish was entirely dry. Moisturecontent readings were made on both sides of the specimens with the electrical apparatus, and then the moisture content was determined from loss in weight upon oven drying. I n no case were the deviations among these three determinations more than 1 per cent absolute moisture content. This fact shows that it is possible to determine the moisture content of material having any of the common finishes on the surface. Even with aluminum powder the particles are evidently sufficiently enveloped by the electrically resistant varnish to prevent appreciable surface leakage of the current. I t must not be assumed, however, that a n equal accuracy can be obtained for any specimen coated on one face, since abnormal moisture gradients, which might readily be set up, could affect the resulting electrical readings considerably.
Vol. 2,
90.
3
Conclusions
The present form of apparatus using point contact electrodes that are inserted into the wood so that the current flow is across the moisture gradient rather than along the moisture gradient is far more generally applicable than the old forin using surface electrodes. It is possible to determine the average moisture content of specimens with a n accuracy of perhaps better than 2.0 per cent absolute moisture content, not only when the specimen has a uniform distribution of moisture, but also when the moisture is distributed according to any normal drying gradient. Unfortunately, abnormal moisture gradients and absorbing moisture gradients may cause considerably larger deviations of the observed moisture content from the true average value. Experimental cases are given, however, in which the deviation is not excessive. Though no appreciable variation among the various woods tested was observed, and no consistent variation with changes of density, more accurate results may be expected if the instrument is calibrated for the type of stock with which it is to be used. If the electrical conductivity instrument is used with an understanding of its limitations, and proper precautions are taken not to make measurements under the most unfavorable conditions, it may prove a valuable tool for commercial practice. Literature Cited (1) Stamm, I N D Ehic CHEM, 19, 1021 (1927) ( 2 ) Stamm, I b i d , Anal Ed , 1, 94 (1929)
Determination of Small Amounts of Acid in Ether' Lawrence P. Hall 3fALLINCKRODT CHEUICAI. \vORKS,
ST LOUIS,X I 0
Methods are described and discussed for the titration UCH importance has s e r i e s of c h a n g e s w h i c h of acid in ether. When phenolphthalein is used as an usually been placed ether undergoes upon long indicator and alcohol used to keep the solution homostanding under various condiupon tests for acid in geneous, fair results are obtained by a skilled operator. tions. ether in the belief that it was A two-layer titration using phenolphthalein yields Older methods of testing difficult to prepare ether envery large errors in determining small amounts of f o r acidity, discussed b y tirely free from acid. This acid. The substitution of a sulfonphthalein indicator Baskerville and Hamor ( I ) , was unquestionably true in such as bromothymol blue gives much more dependincluded testing with litmus the earlier days of the manuable results. This method is also simpler, easier, paper-a method inadequate facture of ether when purimore rapid, and can be extended to a wider range of to distinguish between a fication processes were not concentrations without special arrangements. really fine grade of ether thoroughly understood, but Pure ether has been found to be neutral. Ether with and an imperfectly prepared under modern methods it alcohol, as ether for anesthesia, has been found to product-and titrating with is commercially possible to exert an influence on the color of phthalein indicators. standard solution using prepare ether in large quanBy careful neutralization of decomposed ether, it has phenolphthalein as an inditities free from acid. Apprebeen possible to separate the sodium salts of acetic cator. Various modifications ciable acidity is, therefore, acid and formic acid and prove thereby the presence of these methods are in curindicative of contamination of these acids as products of decomposition. rent use. I n the modificafrom i m p r o p e r l y c l e a n e d tion in which ether is shaken containers, soldering flux, or decomposition of thk ether itself resulting from exposure to with pure water and titrated, errors of 0.15 to 0.50 cc. of alkali are usual, owing to almost complete extracair and light. As is well known, upon long standing in the 0.01 presence of oxygen, ether may acquire small amounts of tion of phenolphthalein from the aqueous layer and extracorganic peroxides and other substances and gradually develop tion of alcohol from the ether layer by the water. The an acid reaction. Hence it is important to have available an first effect requires the addition of sufficient free alkali to accurate method for determining very small amounts of cause the phenolphthalein to redissolve, and the second acid in ether, particularly anesthetic ether. The correct influences both the color and the end point. determination of acid is also useful in studying the complex I t is customary to allow 0.3 to 0.4 cc. of 0.01 S alkali for 20 cc. of ether. If this represented the true acid content, 1 Received February 28, 1930 Presented before the Dirision of the amount of acid would be much more than should be Physical and Inorganic Chemistry a t t h e 79th Meeting of the American present in any properly purified ether, even when allowance Chemical Society, Atlanta, G a , April 7 to 11 1930
M
*
July 15, 1930
I.YDUSTRIAL AND ESGI.YEERING CHEMISTRY
is made for slight unavoidable contamination due to soldering flux and to a reasonable amount of deterioration. I n other words, the error inherent in the method of analysis is greater than the total amount of impurity which should be permitted in anesthetic ether. Another method using phenolphthalein was devised by H. V. Farr, of this laboratory,2 in which phenolphthalein is added to 80 per cent alcohol, followed by just sufficient alkali to produce a distinct pink tint. Upon the addition of 25 cc. of ether to this solution, the two liquids mix completely, forming a homogeneous solution which in the case of pure ether does not show any diminution of the color of the phenolphthalein. If acid is present, it can be titrated with 0.01 N alkali. This method gives more nearly correct results than can be obtained with that of Baskerville and Hamor. It is, however, open to some objections. I n the first place, the phenolphthalein end point in 80 per cent alcohol has a reddish or pink tint, whereas after the addition of ether a violet end point is obtained, thus making it difficult to compare accurately with a color standard. I n the second place, if the amount of acid present requires more than about 4 cc. of 0.01 N alkali, the solution will again separate into two layers. There is also the objection that phenolphthalein, particularly in this menstruum, is extremely sensitive to carbon dioxide, so that the utmost care must be used to avoid absorption of even the slightest traces of this impurity. 0
Recommended Method
By using a sulfonphthalein as an indicator, which is not extracted from the aqueous layer. the employment of a simpler technic is permitted. Dibromothymol sulfonphthalein, commonly known as bromothymol blue, has been found very suitable for t,his purpose. Sole-Cresol red may also be used. T h e color change for this indicator is from yellow t o red over t h e pH range of 7 . 2 t o 8.8. T h e color change for bromothymol blue is from yellow t o green t o blue over t h e p H range of 6.0 t o 7.6. T h e intermediate color of green is very useful in giving a warning of the approach of the end point and preventing over-titration. An obvious saving of sample and time results.
The method of titration has been to take 10 cc. of distilled water in a well-stoppered cylinder, introduce 2 drops of broinothymol blue solution, add a fern drops of 0.01 N sodium hydroxide solution until the first blue color is permanently developed, add from a pipet' 25 cc. of the ether to be tested, shake in order to mix the two layers together, and add 0.01 S alkali until the first blue color remains for several minutes. Care should be taken to avoid introduction of carbon dioxide during the pipetting or titrating. Well-fitting stoppers in the flasks are necessary to prevent loss in shaking. The final color should not be judged until the layers have separat'ed. When ether which contains alcohol is t'itrated, the brilliant blue seen in water is not quite so bright, but there is no difficult'y in observing the change from green to blue, which is the proper end point'. I n t'he titrations discussed below, carbonate-free sodium hydroxide solution was used which had been standardized against 0.01 S oxalic acid solution with phenolphthalein and also bromothymol blue indicators. When the first blue of the latter indicator was taken as the end point, the two sets of standardizations agreed within 0.1 per cent. By these titrations the alkali was 0.0100 normal. Comparisons were also made, using Farr's method and the bromothymol blue methods, for the titration of 10-cc. samples of 0.01 S acetic acid in ether. The two methods agreed within 0.02 cc. of 0.01 S alkali. 2 A description of this method appears in the U. S.Pharmacopeia X but so fhr a s the author is aware has not been published elsewhere.
24 5
Ether solutions of various concentrations made by careful quantitative dilutions of 0.010 N acetic acid in ether, were titrated as "unknowns" by three men. Their results are given in Table I (columns 4, 5, and 6). The averages of these titrations agree well with the equivalent amounts of 0.01 N NaOH which were theoretically required according to the dilution of 0.01 N acetic acid. For comparison, figures are also given for averages of titrations using phenolphthalein and alcohol (column 3). Table I-Titration
of Acetic Acid i n Ether by Various Operators
EQUIV.0.01 S NaOH
SOLN. REQUIRED PHTH.
(1)
A
Cc. 1.32
A v . cc. 1.46
B
0.46
0.51
0.42 0.43
C
0.02
0.04
0.03 0.02
D
3.98
3.67
3.98
0.01 S NaOH USISG B. (2) (3)
T. B.
Av.
Cc.
cc.
cc.
cc.
1.27 1.26
1.32 1.32 1.33 0.45 0.45 0.44 0.02 0.03 0.02 4.06 4.01
1.40 1.38
1.32
0.49 0.4s
0.45
0.04 0.04
0.03
3.92 3.94
3.98
0.03 0.04
0.04
0 14 O,l5
0.14
0 00 0 00
0 00
4
E
0.04
0.00
0.04
F
0.13
0.02
0.15
G
0 00
0 09
0 00 0 00
nn
3.98 3.98 3.99 0.04 0.04 0 04 0 . 0.5 .. 0.14 0.14 0.14 0.14 0 00 0 00 0 00
Table I1 gives a set of titrations of very old ether, containing considerable amounts of decomposition products, which was carefully analyzed for acid and diluted successively with fresh ether to give solutions of various concentrations. I n the last two columns are recorded the differences between the extremes in the titrations for each concentration by both methods. Such differences are smaller for the bromothymol blue method. The absolute error of titration becomes relatively very large by the phenolphthalein method when titrating the minute amounts of acid which may be present in good ether. It should be noted here that when less than about 0.4 cc. of alkali is required titration by the plienolphthalein method is difficult. Table 11-Titrations
EQKIV. NaOH
0 01 S ALKALI .4v. N O . OF V-ARIATIOUS B. T. B. TITRATIONS Phth. B. T. B.
SOLN. REQUIRED P h t h .
cc.
.4
6.15
B
3.08 1.54 0.77 0.38
C
D
E
of Old Ether
cc.
cc.
6.15 3.02 1.54 0.72 0.39
6.10 3.09 1.55 0.76 0.39
cc.
6 3 3 3 3
0.09 0.12 0.07 0.07 0.06
cc
0.10 0 00 0.00 0.03 0.00
Effect of Alcohol
Since. when ether containing ethyl alcohol is tested by this method, a slight shift in color from blue to green is observed, and a drop or two of 0.01 alkali restores the color, the question arose whether the ether could have been acid. This effect was investigated in several ways. Many different samples of freshly made anesthetic ether were tested and none was found to require more than a drop of 0.01 12' alkali to restore the original intensity of the color, although the shade might not be identical with that in water alone. Freshly distilled ether containing no alcohol did not change the color or shade and appeared inert. When bromothymol blue was added to alcohol, a similar change in shade was obtained. Quantitative dilutions of the 0.01 h' acetic acid-ether
246
AAVALY TZCAL EDI TIO-V
solutions were made with fresh ether containing about 2.5 per cent alcohol to give solution7 which should have required 2.50, 1.00, 0.50, 0.25, 0.10, 0.05, and 0.026 cc. of 0.01 A' alkali. I n titrating these solutioiis very closely agreeing values were found in every case. the maximuin error being 0.01 cc. Hence, this ether could not have been acid by itself, and yet it altered slightly the blue color of the indicator. About 10 cc. of 0.01 N sodium acetate solution were placed in each of several cylinders of appropriate size. A drop of bromothymol blue indicator was added to each and a bluish green color produced. Amounts of anesthetic ether ranging from 10 to 100 cc. were added to the different cylinders. The color changes were very nearly alike in all cases whether 100 or 10 cc. of ether were present. Hence, the ether could not have been acid. The shifting of the color must hare been due to the influence upon the indicator of the alcohol and ether taken up by the water. The effect of solvents upon phthaleins has received comment before ( 2 , 5). Small amounts of impurities which might be found in poorly made ether or decomposed ether, such a? acetone, acetaldehyde, or peroxide, were added to ether and no change in acid values was found. Acids in Very Old Ether
I n studying the spoilage of ether, acidity has been found to develop a t a late stage. Relatively large amounts of peroxide, representing the first stage of decomposition, can be formed without the appearance of acid. Usually when aldehyde. or at least substances giving the aldehyde reaction. form, acidity begins to develop, but there has not been found a fixed ratio of aldehyde to acid, or of peroxides to aldehydes such as reported b y King ( 3 ) . I n some cases no peroxide and very little aldehyde hare been found, but considerable acid was present. There was on hand some very old ether which had been exposed to air to favor decomposition. Part of this ether was carefully distilled off in a still similar to that of Peters ( 4 )
Vol. 2, S o . 3
to concentrate the acids. The residue, which still contained alcohol and et'her, was extracted with 5 per cent sodium hydroxide solution, using bromothymol blue as indicator. The aqueous layer was then separated and evaporated. I n this way the sodium salts of the acids present, were obtained. There was a small amount' of an oil which was soluble in alcohol and thereby separated. Crystals then fornied which corresponded to those of sodium acetate. The mother liquor was treated with sulfuric acid and ethyl alcohol. K h e n the mixture was poured upon water, odors were present of acetic acid, ethyl acetate, and other substances, including ethyl formate. Tests with sodium carbonate and potassium permanganate solutions showed a decolorizing of the latter. Silver salts were reduced. Bromine water was not clecolorized. S o precipitate could be obtained by the addition of calcium chloride or acetate mith the subsequent addition of aiinnonia. These tests eliminated the unsaturated acids and those vhich form insoluble calcium salts. They confirmed the presence of formic acid as one of the decomposition products. King (.?) detected formic acid in old ethers by odor alone. Acetic acid has generally been accepted as the final decomposition product, based upon the work of Richardson and Fortey (6). The presence of both these deconiposition products is confirmed, and thus the work of these various investigators is substantiated. Acknowledgment
The writer wishes to acknowledge the assistance and cooperation received from members of the staff of this laboratory in connection with this investigation. Literature Cited (1) Baskerville and Hamor, J. IND.ENG. C H E X . ,3, 306 (1911). ( 2 ) Cohn, Z . angetu?. Chem., 19, 1389 (1906). (3) King, J . Chem. .Coc., 1929, 738. (4) Peters, J. IND. EXG.CHEY.,14, 476 (1922). ( 5 ) P r a t t and Coleman, J . A m . Chem. Soc., 40, 238 (1918). (6) Richardson and Fortey, J. Chem. Soc., 69, 1352 (1896).
Determination of Centralite in Double-Base Smokeless Powders' H. Levenson PICATINNY ARSENAL,DOVER, K. J.
HE dialkyldiphenylureas, known as centralites, are
T
used as stabilizers (anti-acid compounds) in the manufacture of some double-base powders made from nitrocellulose and nitroglycerin. The analysis of such powders has hitherto presented considerable difficulty inasmuch as there was available no direct method of determining the centralite present with known limits of accuracy. Such a method has now been developed and the procedure used is recorded in this paper. Previous Methods
The initial step in the analysis of a double-base powder containing centralite consists of extracting with absolute ether, the nitroglycerin and centralite being dissolved and the nitrocellulose left as a residue. The ether is evaporated from t h e solution and the relative concentrations of nitroglycerin and centralite in this residue are determined. As but small quantities of centralite are used, previous Published by permission of the Chief of 1 Received February 18, 1930. Ordnance, U. S War Department.
workers have attempted to determine the nitroglycerin and find the centralite by difference. Cope and Rarab ( I ) have shown that the nitron method is not applicable to the accurate determination of nitroglycerin. The use of the nitrometer for this purpose is open to two serious objections: The nitrogen content of the nitroglycerin must be assumed to be within an arbitrary range, and the nitric acid liberated reacts with the centralite and a low result for nitroglycerin is obtained. Giua and Gastalla (2') state that, on treatment with fuming nitric, diethyldiphenylurea is nitrated to 2, 4, 2", 4"-tetranitrodiethyldiphenylurea, m. p. 176-177" C., and note that the nitrometer gives low results because of the action on the centralite of nitrous vapors resulting from the decomposition of the nitroglycerin by sulfuric acid. They recommend the determination of the total nitrogen in the mixture by the Dumas method and the calculation of the relative proportion of the components from this and the nitrogen content of each, but here too an assumption of a nitrogen content of 18.5 per cent for the nitroglycerin is made. I n practice, the
