Photometric Determination of Potassium I. W. WANDER, Agricultural Experiment Station, Wooster, Ohio
Potassium is determined photometrically by oxidation of the dipotassium sodium cobaltinitrite precipitate with standard potassium dichromate i n the presence of sulfuric acid, and estimation of the resulting colored solution in a photoelectric colorimeter.
1
THE estimation of potassium in solutions of plant ash and soil extracts b y methods based on precipitation as cobaltinitrite has been reported by many workers, using modifications of the original work of Adie and Wood ( 1 ) . Work with some of these proposed modifications (2, 4, 6) suggested t o the writer that further modifications would result in a simpler and more rapid method of determination without sacrifice of accuracy and reproducibility. The method described takes advantage of the fact that when a solution of potassium dichromate is reduced it changes from bright yellow to green, and that the amount of change depends upon the amount of reduction. The dipotassium sodium cobaltinitrite precipitate formed in the potassium determination is used to reduce a definite volume of standard potassium dichromate, forming various shades of yellow to green solutions. The absorption of these solutions is obtained with a photoelectric colorimeter. In this manner, a standard or working curve is obtained, from which the potassium content of unknown solutions can be determined. Reagents Precipitating reagent, an aqueous solution of trisodium cobaltinitrite containing 1 gram of reagent quality salt in 5 ml. of distilled water. The solution is filtered before use. Nitric acid, approximately 1 N (64 ml. of concentrated nitric acid per liter) and 0.01 N . Concentrated sulfuric acid. Potassium dichromate, exactly decinormal solution prepared by dissolving exactly 4.9035 grams of oven-dry potassium dichromate crystals and diluting t o exactly 1 liter with distilled water.
made up and being immediately ready for use. Only enough should be made up to run the determinations that can be handled in one day. The temperature a t which the precipitation proceeds should be kept nearly constant, about 20" C., since it has been noted ( 5 , 6) that the temperature affects the composition of the precipitate. This can be accomplished by using a water bath. By carrying several known concentrations of potassium through the procedure simultaneously with a set of determinations and referring to the curve to which these known concentrations correspond, the temperature factor can be eliminated. The effect of length of time of precipitation can be compensated for in a similar manner. The curves shown in Figure 1 confirm the generally recognized observation that the amount of precipitate is influenced by the time permitted for precipitation. It is very important to keep the time of precipitation constant once a working curve is made, and to check by running a few standards with each set of determinations. Xeither the accuracy nor the reproducibility of the method was affected appreciably by washing the precipitate only once with 15 ml. of 0.01 N nitric acid. This shortened procedure saves considerable time, a conclusion also reached by others ( 2 ) . Other ions, such as those of sodium, calcium barium, magnesium, strontium, iron, zinc, chloride, nitrate, sulfate, and phosphate, in the concentrations usually found in plant tissues and soil extracts, d q n o t interfere with the precipitation of potassium by trisodium cobaltinitrite (3, 6). The ammonium ion is the only one which will interfere seriously, and its presence should be guarded against throughout the procedure. Wet digestions in which both nitric and hydrochloric acid have been used with sulfuric acid will not contain ammonia. There Kill be no silica in solution with these wet
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Procedure Place a 10-ml. aliquot of approximately neutral aqueous solution containing from 1 to 7 mg. of potassium in a 25 X 150-mm. Pyrex test tube. Add 1 ml. of N nitric acid, mix, add 5 ml. of trisodium cobaltinitrite reagent, and again mix. Allow the mixture t o stand 2 hours at a temperature near 20" C. Wash down the walls of the test tubes and balance them with 0.01 N nitric acid. Centrifuge at about 2200 r. p. m. for 10 minutes, pour off the supernatant liquid, and allow the tubes to drain several minutes. Wash the precipitate with 15 ml. of 0.01 A; nitric acid, being sure to mix the precipitate thoroughly with the \\.ash solution, centrifuge, and drain as before. Add exactly 10 ml. of decinormal potassium dichromate solution, then 5 ml. of (eoncentrated sulfuric. acid, and mix thoroughly. Heat will be produced, and the prec'ipitate will be oxidized; a clear solution \vi11 resclt. Place tubes in a cool water bath. When cooled t o room temperature, pour the solution and rinse the tube into a 100-ml. volumetric flask and make to volume xith distilled water. Mix the solution and read absorption in a photoelectric colorimeter using a filter of about 425 millimicrons transmission. Plot the absorption against milligrams of potassium carried through the procedure as outlined. Unknown concentrations of potassium ('an then he obtained from this curve.
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M I L L I G R A M S OF
FIGURE1.
Discussion The advantages of the trisodium cobaltinitrite precipitating reagent used in this method are discussed by Uyilcox (6). This reagent also has the advantage of being easily
EFFECT O F UPON
5 K
S
AMOUXTO F POTASSIUM LIGHTABSORPTION
7
DICHROMATE
Potassium dichromate reduced by dipotassium cobaltinitrite in different lengths of time for precipitation. This working curve wa8 obtained by using for the ordinate the scale A readings (absorption) obtained with a Fischer AC model electrophotometer.
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
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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 & J o h n s o n , New Brunswick,N. J.
T
HE identification of unknown fibers is frequently re-
quested 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).
