ANALYTICAL EDITION
November 15, 1942
855
Conclusions
and vanadium) using the zinc amalgam (7), Takeno (IO), and Kano (4),using the cadmium amalgam, we have the differential determination of titanium and iron, titanium and uranium, and uranium and iron by Kikuchi (6) and the differential determination of iron and chromium by Kano (6). The indirect determination of phosphorus was described by Hakamori (1). The determination of tin by use of the bismuth amalgam should be better than the usual nickel reduction procedure (3).
The procedure described above for accomplishing the separation of solution and amalgam, using carbon tetrachloride, makes possible the completion of a reduction in 120 seconds and subsequent immediate titration of the reduced solution. I n the oxidimetric determination of iron i t is the most rapid procedure a t present known. The present paper is written for those skilled in the art of analysis. For procedures in the extended use of the Yakazona liquid amalgam reductor, the published results of Someya (9) should be consulted and the new technique applied. By this procedure the published data may be employed in routine procedures in a really practical manner. Besides the applications of Nakazona (iron, molybdenum, uranium, titanium,
(1) Hakamori, J . Chem. SOC.Japan,43, 734 (1922). (2) Hillebrand and Lundell, “Applied Inorganic Analysis”, p. 100, New York, John Wiley & Sons, 1929. (3) Ibid.. DD. 103-6. (4j Kano:j. Chem. SOC.Japan, 43,330,550 (1922). (5) Ibid., 44, 37 (1923). (6) Kikuchi, Ibid., 43, 173,554 (1922). (7) Nakazona, Ibid., 42, 526,761 (1921). (8) Smith and Wilcox, IND. ENQ.CHEM.,ANAL.ED., 9, 419 (1937). (9) ~, Someva. Z.anoru. Chem.. 138. 219 (1924): 145. 168 (1925): 148. 58 (1926) ; 152, 368, 382, 386 (1926) / 160, .355, 404 (1927) f 163, 206 (1927). (10) Takeno, J. Chem. SOC.Japan, 55, 96 (1934). (11) Walden, Hammett, and Edmonds, J . Am. Chem. SOC.,56, 350 (1934).
same set of four results gave, as the average of the closely agreeing results, 16.58ml. of sulfatoceric acid compared to 16.45 ml. using the Jones reductor for the ferric alum reduction. The accumulation of reaction products from a large number of determinations is collected in a suitable container and later separated, using a large separatory funnel, and the liquid amalgam as well as the carbon tetrachloride is used repeatedly.
Molvbdenum Blue Reaction L. T. KURTZ, University of Illinois, Urbana, Ill.
F
LUORIDE ions, even if present in very low concentrations, produce a negative interference in the molybdenum blue reaction ( 6 ) , which is used extensively for determining small amounts of phosphate. The most convenient of the usual methods for removal of fluoride ions is evaporation with perchloric acid. The excess perchloric acid is then neutralized before the determination is made ( 3 , 5 ) . If this neutralization is not carried out with precision, the indicator will cause interference in the photometric procedure. The perchlorate ion, if present in high concentration, may interfere with the development of the molybdenum blue color ( 3 ) . If organic matter is present, varying degrees of hydrolysis and oxidation may occur during the evaporation and thus make the differentiation between organic and inorganic phosphate impossible. Another disadvantage of the usual method is that the evaporation should be carried out in platinum to avoid the larger blank which arises when Pyrex beakers are used (Table I). TABLEI. BLANKDETERMINATIONS ON ALIQUOTSCONTAININQ 0.01 MOLE OF AMMONIUM FLUORIDE
Treatments of Aliauots Evaporated with HClO‘ in Pyrex beakers Evaporated with HClOi in platinum dishes Boric acid added, aliquot not evaporated
Photometer Readinn
Apparent Phosphorus, Equivalent P . P. >I. in Aliauot
93.4
0.340
-
97.6
0.022
98.0
0.017
The fluoride renioval is unnecessary when boric acid is added to the fluoride-containing aliquot before the phosphate determination is made. The boric acid forms with fluoride the fluoborate ion and thus prevents interference by the fluoride ion. Under these conditions, evaporation with perchloric acid may be omitted and accompanying errors avoided, iyith a considerable time saving. Boric acid has also been used to remove fluoride interference in iron determination ( I , 4). This scheme for preventing fluoride interference should be generally applicable where phosphate is to be determined by
Literature Cited
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the molybdenum blue reaction. The procedure of Dickman and Bray ( 2 ) is readily modified to eliminate fluoride interference. Under the conditions of their procedure, neither the excess boric acid nor the fluoborate ions have any appreciable effect on the photometer reading (Table 11). The procedure given below is recommended for determining the amount of phosphate extracted from soils by fluoride solutions (3). TABLE 11. EFFECT OF BORICACIDON FLUORIDE INTERFERENCE Concentration of Phosphorus Added P. p . m. 0.250 0.250 0.250 0.250 0.250 0.250 0.250 0.050 0.500
HaBO; Added Mole/l. None None 0.08 0.08 0.24 0.24 0.24 0.24 0.24
“IF Added Mole/l. None 0.02 0.02 0.20 0.20 0.30 0.60 0.20 0.20
Concentration of Phosphorus Found P. p . m. 0.254 0.00 0.245 0.200 0.246 0.248 0.256 0.050 0,492
ANALYTICALPROCEDCRE. An aliquot of the unknown solution containing less than 0.015 mole of fluoride ion (equivalent to 0.3 mole per liter in the final volume) is pipetted into a test tube, and 15 ml. of 0.8 molar boric acid are added. The volume is then made up to 35 ml. with distilled water and the reagents are added t o give a final volume of 50 ml. The reagents and procedures for development and measurement of the color are those described by Dickman and Bray (a). Maximum color, however, develops less rapidly in the fluoborate procedure and photometer readings should be made between 5 and 10 minutes after the addition of the reagents. In the range of concentrations t o which this reaction has been applied, the photometer calibration curve for known phosphate solutions may be used without modification. With fluoride concentrations greater than those investigated, it may be necessary to construct a calibration curve, using phosphate solutions which contain the same amount of boric acid as will be added to the unknown solutions.
Literature Cited (1) Barnebey, 0. L., J. Am. Chem. Soc., 37, 1481 (1915). (2) Dickman, S. R., and Bray, R. H., IND.ESG. C H n h i . , A s ~ L .ED., 12, 665 (1940). (3) Dickman, S. R., and Bray, R. H., Soil Sci., 52, 263 (1941). (4) Hillebrand and Lundell, “Applied Inorganic Analysis”, p. 776, Kew York, John TTiley & Sons, 1929. (5) Robinson, R. J . , ISD.ESG. CHEM.,AXAL.ED., 13, 465 (1941). (6) Woods, J. T., and Mellon, M.G., Ibid., 13, 760 (1941). CONTRIBUTION from Agronomy Department, University of Illinois.
