January, 1946
ANALYTICAL EDITION
Make the test solution ammoniacal, warm, acidify with acetic acid, and then add the reagent solution. This procedure is not 50 sensitive as those already outlined. The nonspecific phenol reaction of tannic acid with dilute sodium acetate-ferric chloride solution is not so sensitive as the test described here. As a test-tube reaction the identification limit is 25 micrograms; the limiting concentration is 1 to 40,000. The corresponding figures for the drop reaction on a spot plate are 1 microgram and 1 to 50,000. Hence the new test is three times as sensitive as the ferric-phenol reaction when carried out in a test tube, and just as sensitive as the latter when spot test techniques are used. DETECTION O F N A T U R A L T A N N I N S AND DIFFERENTIATION F R O M SYNTHETIC T A N S
The test solution (0.5%) is prepared from the solid specimen. One milliliter of the filtered liquid is carried through one of the procedures as outlined. When testing extracts of tanning agents, test portions are prepared by diluting the specimen 1 to 10 and 1 to 100, and 1-ml, portions are used. The behavior of all solutions toward ammonia and acetic acid should be determined before adding the reagent. If a precipitate appears on adding acetic acid to the warm ammoniacal solution, it should be removed, and the test tube procedure carried out on the filtrate after again making the solution ammoniacal. The test with Fe(a,a’-dip)aSO4 was tried on a variety of tanning materials. The following natural tannins gave a positive reaction: gallotannin; tara powder; gambier; myrobalan; quebracho; and extracts of wattle, mangle, sumac, and fustic. I n marked contrast the following synthetic tanning agents gave a negative reaction: orotan N, syntan A, syntan S,mertanol 7 L, maxyntan, and tanasol. These findings indicate that this test is suitable for distinguishing commercial natural from synthetic tanning agents. Similar differentiating tests were also made on’finished, mostly colored, leathers. About 0.5 gram of the leather was cut into tiny bits and boiled in 2 ml. of ammonia water for 2 minutes. The clear filtrate was carried through the test procedure. Preliminary tests with ammonia and acetic acid were made in all
Gas
Bubble Releaser
63
cases. This step is indispensable when testing leathers colored with coal-tar dyes, because the addition of the reagent to the ammoniacal solutions of many acid dyes results in a precipitate that does not dissolve in acetic acid. If the preliminary test produces a precipitate, it is filtered off, and the tannin test is made on the clear filtrate after i t is again made ammoniacal. I n 19 out of 20 cases, the test revealed the nature (vegetable or synthetic) of the tanning agent. The single exception was a leather that had been tanned with a sulfited quebracho extract; it gave a negative response. Further studies will be necessary to determine whether the test fails with leathers tanned with strongly sulfited tannin extracts. A trial with a technical sulfited quebracho extract showed that it still gives a distinct tannin reaction a t a dilution of 1 to 1000. DETECTION OF T A N N I N S IN BEVERAGES
Three milliliters of the samples were taken for the tests. Positive reactions were given by 4 varieties of red wine (tart, sweet): 3 varieties of white wine (tart, sweet); and water extracts of tea, mat6, and guarana. Negative responses were obtained with Cinzano (Italian origin); beer (light, dark); and water extracts of coffee (raw, roasted). ACKNOWLEDGMENT
The samples of leather, and of the natural and synthetic tanning agents, were donated by Cortume Carioca, Rio de Janeiro. The authors express their gratitude to this firm for its cooperation. LITERATURE CITED
(1) Feigl, F., and Miranda, L. I., IND.ESG. CHEM.,ANAL.ED., 16, 141 (1944). (2) Nierenstein, M., Section on “Tannins”, “Allen’s Commercial Organic .4nalysis”, Vol. 5 , p. 5, Philadelphia, P. Blakiston’s Son & Co., 1927. (3) Powell, A. R., and Schoeller, W. R., Analyst, 50, 485 (1925); Z. anorg. Chem., 151, 221 (1926). (4) Yoe, J. H., and Sarver, L. A., “Organic Analytical Reagents”. pp. 130,256, New York, John Wiley & Sons, 1941.
for Use in Dumas Nitrogen Determination Asotometers RENATO POMATTI, The Texas Co., Beacon, N. Y.
A
RATHER common and annoying occurrence in running a Dumas nitrogen determination is the sticking of gas bubbles to the surface of the mercury in t,he azotometer. This has been attributed to too narrow an opening of the gas inlet of the nitrometer, too short a distance between the gas inlet and the level of the mercury, excessive greasing of the gas inlet stopcock (S), and the use of perfectly pure clean mercury when the azotometer is firjt filled ( 4 ) . FlaschentriiLer 1,f) and Keygand ( 5 ) say that this difficulty can be overcome by the addition of powdered copper oxide to the surface of the mc’rcury. Sichols ( 2 ) suggests the use of mercurous oxide for the same purpose. S o n e of these measures, which are prevent,ivc in character, has been found completely reliable. A simple direct method for releasing gas bubbles already sticking to the mercury has been used effectively in this laboratory for over a year.
A piece of steel or iron wire about 1 em. long, slightly curved, is placed in the azotometer so that i t rests on the surface of the mercury. Khenever gas bubbles are to be released, the piece of wire is swept over the mercury by means of a small permanenttype magnet held outside the azotometer near the level of the mercury and opposite the wire. The wire is attracted to the mag-
net and, in passing over the surface of the mercury, releases the bubbles. The wire can thus be moved back and forth simply by placing the magnet in the proper position opposite it. To facilitate this opbration two magnets can be used. These are held on opposite sides of the azotometer and brought near the azotometer alternately. I n addition, this device can be used to break up small bubbles a t the potassium hydroxide solution-gas interface. This is done by attracting the mire to the magnet and moving it up slowly to the interface. By moving the wire up and down through the interface, the gas bubbles are broken. Large gas bubbles stuck or moving up very slowly in the graduated portion of the azot,ometer can be released also by this means. LITERATURE CITED
(1) Flaschefiriiger, B., Miikrdchemie, 8, 1 (1930). (2) Nichols, &I. L., I N DENG. . CHEM.,A w a ~ED., . 5, 149 (1933). (3) Niederl, J. B., and Kiederl, V.,“Micromethods of Quantitative
Organic Analysis”, 2nd ed., p. 98, Sew York, John Wiley & Sons, 1942. (4) Pregl-Fyleman, “Quantitative Organic Microanalysis”, 2nd English ed., p. 97, Philadelphia, P. Blakiston’s Son & Co., 1930. (5) Weygand, C., “Quantitative analytische Mikromethoden der organischen Chemie in vergleichender Darstellung”, pp. 24-8, Leipaig, Akademische Verlagsgesellschaft, 1931.
NOTES ON ANALYTICAL PROCEDURES Determination of Reducing Sugars Mathematical Expression of Reducing Action in the Lane and Eynon and Volumetric Ferricyanide Methods
F. W. ZERBAN, W. J. HUGHES, AND C. A. NYGREN N e w York Sugar Trade Laboratory, N e w York,
IN
T H E determination of reducing sugars by direct titration against Fehling solution according to Soxhlet, Violette, or Pavy, it has been generally assumed that the concentration, x, of sugar solution, multiplied by the volume, y, required for complete reduction of a fixed quantity of copper solution, is constant (xy = C). But Soxhlet (6) showed as early as 1878 that this simple relationship holds only if the concentration of the unknown sugar solution is approximately the same as that used for standardization of the Fehling solution. Later, Lane and Eynon ( 4 ) found that the "factor" (C/lOO) may vary as much as 5% for a range of 15 t o 50 ml. of sugar solution used. When it became apparent about two years ago that copper salts might be difficult to obtain, it was decided to devise a substitute for the Lane and Eynon method, and alkaline ferricyanide solution was tried, as first proposed by Ionescu and Vargolici (f), with methylene blue as indicator of complete reduction. When the results were plotted by Louis Sattler, of this laboratory, it was discovered that there is a straight-line relationship not between x and y, but between their logarithms, and t h a t the results can be expressed by the equation log y = log b or
- m log x
y = bx-m
Table
=
log b
m log b
Deviation Average Maximum
M1.
M1.
0.023 o,013 0.012
0.08 o,04 0.04
10 ml. of Fehling solution
l n ~ ~ ~ ~ ~ $ ~ m 1.0341 1,0166 o f 3.7886 3,7405 0.2730 o,2718 Plus 5 grams of sucrose 1.0030 3.6848 0.2722
"-
erose lo grams Of 0.9830 3.6240 Plus 25 grams of sucrose 0.9552 3.5308 Dextrose 1.0354 3.7789 1.0315 3.7966 &;&hydrate 0.9759 3.8444 Lactose hydrate 1.0010 3.8348
0.2713
0.067
0.21
0.2698 0.2740 0.2717 0.2512 0.2610
0.109 0.017 0.023 0.008 0.057
0.28 0.04 0.06 0.03 0.14
0.018
0.05 0.04 0.05 0.04 0.05 0.04
25 ml. of Fehling solution Invert sugar 1.0124 4.1270 0.2453 D ~ ~ o ~ ~ r a m o f s u c r 1.0086 ose 4.1130 0.2452 1.0121 4.1138 0.2458 Levulose 1,0108 4.1362 0.2444 Maltose hydrate 0.9597 4.1932 0.2289 Lactose hydrate 0.9760 4.1593 0.2344
O.Oo9 0.022 0.016 0.024 0.016
(1) the method of averages from 18 pairs of values for milligrams of sugar in 100 ml. of solution and the corresponding titers given in the Lane and Eynon tables ( 3 ) . A detailed comparison between
(2)
the figures in the table for invert sugar alone, with 10 mi. of Fehling solution, and those calculated from the equation is shown in Table I. I n this particular case m was found to be 1.0341, and log b = 3.7886. In Table I1 the values of nz and log b, calculated as explained, are shown for all the sugars and sugar mixtures studied by Lane and Eynon, together with the maximum and average deviations of the calculated titer from t,hat given in their tables. I n two of the equations the value of m is so close to unity that the formula y = C / x could be used without serious error. The ratio of m to log b for invert sugar is about midway between the ratios for dext,rose and levulose. W t > hincreasing quantities of sucrose added to invert sugar both m and log b decrease, m more rapidly than log b, as shown by the ratio between the two. The agreement between the titers given in the Lane and Eynon tables and those calculated is remarkably close, the average deviations being in most instances around 0.02 ml. or less and the maximum deviations well within 0.1 mi. Larger discrepancies are found in the case of invert sugar in the presence of 10 or more grams of sucrose, titrated against, 10 ml. of Fehling solution. Lane and Eynon have pointed out that the total time of boiling has a pronounced effect on the reducing pow-er of invert sugar mixed \vith large amounts of sucrose. The maximum discrepancies occur usually when the titer is very high, close t o 50 ml.
1.
EQUATIONS FOR LANE AND EYNON METHOD
T o test the validity of this 1aTv of reducing action for the Lane and Eynon method, the values of m and log b were calculated by
I.
Comparison between Lane and Eynon Titers and Those Calculated from Equation I for Invert Sugar A l o n e (10 ml. of Fehling solution) Sugar Sugar Sugar in Solution, Solution, 100 hll. L. & E. Equation 1 Difference Mo. M1. hll. Ml. 336 15 13.00 0.00 298 17 16.98 -0.02 267 19 19.03 4-0.03 242.9 21 20.98 -0.02 0.00 222.2 23 23.00 204.8 25 25.02 4-0.02 190.4 27 26.98 -0.02 177.6 29 29.00 0.00 166.3 31 31.04 4-0.04 156.6 33 33.03 4-0.03 147.9 35 33.04 4-0.04 140.2 37 37.03 4-0.03 133.3 39 39.01 +0.01 127.1 41 40.98 -0.02 121.4 43 42.98 -0.02 116.1 45 45.01 +0.01 111.4 47 46.97 -0.03 107.1 49 48.92 -0.08 Average difference *0.023 Maximum difference -0.08
Table
Constants in Equation 1 for Reducing Effect of Various Sugars, as Determined b y Lane and Eynon m
where b and m are constants. The original equation, = C, or y = C / x , is a special case of the general equat'ion Y = C/xm, with m
II.
N. Y.
An interesting case is presented by lactose, titrated against 10 ml. of Fehling solution. Here the factor, C/lOC, found experimentally by Lane and Eynon decreases between 15- and 3064
