INDUSTRIAL AND ENGINEERING CHEMISTRY
98
potassium, and the crucible is placed in ice water for at least 15 minutes. Without removing the crucible from the ice water the precipitate is collected on a filter stick (cooled in ice water) as shown in Figure 3. This stick consists of a 4-mm. glass tubing containing two constrictions between which asbestos fibers are placed. The mother liquor is removed by suction, and the precipitate is washed with 2 drops of ice water (washing solution 1, gravimetric procedure), then with 7 to 10 drops of washing solution 2, and finally with one drop of ice water. The filter stick is removed from the suction and the longer section of the stick is filled with acetone by means of an eye dropper. The end opposite the asbestos is placed in the mouth and the acetone is blown through the asbestos, collecting the FIGURE 3 solution in the crucible in which the precipitation has been made. This is repeated until all the precipitate has dissolved and the acetone has become colorless. The acetone solution is then diluted t o 100 ml, (01 any other desired volume) with water containing 1 ml. of 0.1 N sodium hydroxide per 100 ml.
DETERMI~ATION OF 10 TO TABLE VII. COLORIMETRIC MICROGRAMS OF K +
Ktaken. y Light absorption, % Kfound, y Error, 4
100
100 90
80
70
60
50
40
30
20
TABLEVIII. Foreign Ion
COLORIMETRIC DETERMINATION O F POTASSIUM IN THE PRESENCE OF FOREIGN IONS
Amount of Foreign Ion
Potassium Taken
Potassium Found
Mg
Y
Y
Y
50 50 50 50 50 50 50 50 50 50 26 25 25 25 25
47 48 48 45 48 48 50 54 50 54 27 36 29 29 29
-3 -2 -2 -5 -2 -2
... ... Na
..*
...
0.5 1.0 3.0 0.5 1.0 3.0 1.0 1.0
Mg
Li Ca Na
3.0 1.0
Eg
1.0 1.0
Ca
Error
0
+o“+ 4 +2
+I1
+4 +4 +4
tube and its color matched by adding a standard solution of the potassium salt to 0.001 N sodium hydroide in a second tube, the amount of potassium can be cdcuhted in the unknown. As Beer’s law does not hold, the volumes when the final comparison is made should be the same. Several determinations made by this method yielded results of the same order of accuracy as 100 obtained with the photoelectric colorimeter. 20
10
6 7 . 5 69 6 7 . 0 6 4 . 8 6 2 . 5 5 9 . 8 5 5 . 8 5 0 . 0 4 4 . 1 3 3 . 0 3 4 . 0 1 8 . 0 94 100 90 78 68 59 50 39 31 20 21 9 -6 0 0 -2 -2 -1 0 -1 $1 0 +1 -1
I n the following experiments a Lange photoelectric colorimeter was used for the measurements. A calibration curve had been made with Of the potassium Over a wide range of concentrations. From the light absorption of the unknown and the calibration curve the amount of POtassium was found. The results given in Tables VI1 and VI11 indicate that the method yields satisfactory results with amounts of potassium Varying between 10 and 100 micrograms, even in the presence of relatively large amounts of foreign ions which do not precipitate with the reagent. A photoelectric Colorimeter is not essential for making the determination. If the unknoTvn solution is placed in a Yessler
VOL. 11, NO. 2
Literature Cited 2.ges. Schiess U. Sprengstofe, 5, 16 (1910). ( 2 ) Austens, P. T.,Ber.. 7,1249 (1874). ( 1 ) Anon.,
(3) Feigl, F., “Qualitative Analyse mit Hilfe von Tupfelreaktionen,” 3e Aufl., p. 275, Leipsig, Akademisohe Verlagsgesellschaft, 1938. (4)Hoffman, E. J., and Dame, P. A., J. Am. Chem. Soc., 41, 1013 (1919). (5) Kielland, J., Ber., 71B,220 (1938). (6) Nieuwenburg, C. J. van and Hoek, T. van der, Mikrochemze, 18, 175 (1935). (7) Poluektoff, N. S., I b X , 14,265 (1933-34). (8) portnov, M. A,, and Afanas’ev, S. K., Zavodskaya Lab., 6, 1442 (1937). (9) Sohwars-Bergkampf, E,, z. Chem, 69, 321 (1g26), (IO) SheIntsis, 0. G., Zavodskaya Lab., 4, 1047 (1935). (11) Winkel, A., and Maas, H., Angew. Chem., 49,827 (1936). REOFNVED September 7, 1938. From a master’s thesis submitted by G. H. Bendix to the Graduate School of the Universitv of Minnesota.
Colorimetric Determination of Ascorbic Acid JOHN V. SCUDI’ AND HERMAN D. RATISH Harlem Hospital, New York, N. Y.
R
ECENTLY Basu and Nath ( I ) reported the reduction of 2,6-dichlorophenolindophenol by ferrous salts in the presence of dibasic and hydroxy organic acids. Similarly Lorenz and Arnold (2) show the interference of the ferrous ion on the dye. Some time ago, A. J. Lorenz advised the authors that work from the California Fruit Growers Exchange laboratories indicated that “most canned citrus juices showed about 40 p. p. m. tin, and some iron, tin affecting the iodine titration and iron the 2,6 dye.” A representative of the National Canners Association has advised that “no chemical preservatives are required and none are ever used” in canned grape1 Present address, Squibb Institute for Medioal Research, New Brunswiak, N. J.
fruit juice, and that “heat constitutes the sole means of preservation in canned foods.” The statement made by the authors (3) “the higher values obtained for the canned sample of grapefruit juice may be due to preservatives” is therefore misleading. The above information is submitted as a possible source for the explanation of the discrepancy observed by them.
Literature Cited (1) Basu and Nath, J. Indian Chem. Soc., 15, 133 (1938). (2) Lorenz and Arnold, IND. Exa. CREM.,Anal Ed., 10,687 (1938) (3) Soudi and Ratish, Ibid., 10,422 (1938). RECEIVED December 16, 1938.
