INDUSTRIAL A N D ENGINEERING CHEMISTRY
472
TABLEI. RECOVERY OF KNOWN ANOUNTSOF POTASSIUM AS POTASSIUM CHLORIDE ADDEDTO SOLUTIONS OF PLANT ASH
ME.
M1.
Mo.
0
10 8 6 4 2 0
3.2s 3.5s 3.90 4.25 4.55 5.00
2
4 6 S
10
...
3.62 3.97 4.31 4.65 5.00
%
...
-1.1 -1.7 -1.4 -2.2
...
0.001 Gram of K per M1.
a
Made to 10-ml. volume before precipitation.
digestions either; thus no silica will separate as silica gel when the solutions are acidified. The light absorption of the various partially reduced dichromate solutions remains the same over the temperature range 15' to 30" C. The color of the solution remains constant for as long as one week after the precipitate is oxidized and the solution is made to volume, if the solution is protected from dust and strong light. It should not, however, be expected that these solutions will remain so indefinitely, because the chromium complex may change from green to violet, especially if considerable reduction has taken place. Absorption readings made on solutions one
Vol. 14, No. 6
week old did not vary more than 0.5 absorption unit from those made immediately after the solution was prepared. This variation represents 0.03 to 0.05 mg. of potassium, depending upon the amount of potassium actually present, and this error, which might be attributed to the photoelectric colorimeter, is of no greater magnitude than other errors in the method. Table I presents results obtained on the recovery of known amounts of potassium added in increasing increments to a constant or known volume of unknown solution obtained by wetdigesting plant tissue. These results and the curves in Figure 1 would indicate that 1 to 7 mg. of potassium can be determined within 2 to 3 per cent error. By using a few simply made solutions and short-cutting the usual methods of color development used in colorimetry, determinations can be made much more rapidly without sacrificing the accuracy of reproducibility expected in this type of determination. Only one easily made and stable standard solution is necessary, whereas two standard solutions are required for the volumetric estimation of the precipitate. The use of a working curve eliminates a factor which presupposes a straight-line function in the development of the precipitate.
Literature Cited (1) Adie, R. H., and Wood, T. B., J. Chem. SOC.,77, 1076-80 (1900). (2) Brown, D.S., Robinson, R. R., and Browning, G . M., IND. ENG. CBEM.,ANAL.ED.,10,652 (1938). (3) Kramer, B., J . Biol. Chem., 41,263 (1920). (4) Morris, V. H.,and Gerdel, R. W., Plant Phydd., 8, 315-19
(1933). (5) Piper, C.S.,J.SOC.Chem. Ind.,53,392T(1934). (6) Wilcox, L. V., IND.ENG.CHEM.,ANAL.ED.,9,136 (1937).
A Fiber Identification Stain H. L. DAVIS AND H. J. RYNKIEWICZ, Johnson &Johnson, New Brunswick,N. J.
T
HE identification of unknown fibers is frequently requested of the laboratory. The rapid procession of new materials in fiber or sheet form makes it impossible to keep up with all of them, but certain dye mixtures give significant colors which aid in the detection of the more common materials. The National Bureau of Standards (1) gives several dye systems for the identification of the rayons. Chief among these are two Hahn stains : A.
1% picric acid and 0.2% Soluble Blue 2B Extra (presumably Colour Index No. 707)
B. 1% tannic acid and 0.2% Soluble Blue 2B Extra and 0.1% eosin (presumably Colour Index No. 768)
These stains produce the following rayon colors: A
B
Acetate Yellow Colorless
Cuprammonium Deep blue Deep blue
Nitrocellulose Colorless Lavender
Viscose Pale blue Lavender
A study of these formulas and the color produced suggested that it might be possible to combine them so as to get a more versatile and selective stain. Because of the low solubility of the eosin, it was replaced by acid fuchsin, and the hydrochloric acid was omitted. After trials of other proportions, the following mixture was found to be most useful: Acid fuchsin (Colour Index No. 692) Picric acid Tannic acid National Soluble Blue 2B Extra (Colour Index No. 707)
Grams 6 10 10
6
The dyes may be ground together and dissolved together, or dissolved separately in any order and diluted to 1 liter.
While the over-all concentration may not be very critical, these ratios of components appear to give the best differentiation. The dye mixture can be dissolved readily only in hot water, but the solution may be used hot or cold. Momentary immersion in the hot solution is sufficient, but commonly something over 2 minutes is allowed for cold dyeing. A thorough rinsing in water completes the test. Some dyed textiles may be identified without previous bleaching. The fibers are treated as usual, and then rinsed. When pressed wet (after rinsing) between white absorbent papers, a dye mixture characteristic of the color which would have been shown by the undyed fibers is transferred to the papers. The colors shown by common fibers are: Vegetable fibers Synthetic fibers
Animal fibers
Cotton or linen Acetate or nylon Cuprammonium Viscose Vinyon Wool Silk (raw) Silk (degummed)
Light blue Pale greenish yellow Drirk blue Lavender Very pale blue Yellow Black Brown
The stain is also useful in the identification of films of cellulose acetate or viscose (cellophane), giving the above colors. The colors realized with any such dye mixture will depend somewhat on the history of the sample tested, and increased coddence follows a check by other dye mixtures. A solution of this dye mixture is being made available by Eimer and Amend, Xes, York, 5 .Y.
Literature Cited (1) Natl. Bur. Standards, Circ. C423 (1939).
