Simplified Technique in the Use of Liquid Amalgam Reductors G. FREDERICK SMITH AND L. T. KURTZ, University of Illinois, Urbana, 111.
T
The only reagents required are saturated bismuth, lead, cadmium, or zinc amalgams, carbon tetrachloride, the standard oxidants such as permanganate, dichromate, or cerate solutions, suitable indicators such as ferroin, methylene blue, or ammonium thiocyanate, and ferric alum and titanic sulfate solutions in dilute sulfuric acid. The liquid amalgams are prepared according to the procedures described by Someya (9). The other reagents are prepared by routine analytical procedures.
HE use of liquid amalgams as reducing agents in volu-
metric analysis has many possible advantages. The saturated bismuth, lead, cadmium, and zinc amalgams range in reduction potential from +0.32 to -0.762 volt, in accordance with the following single electrode reactions: BiO+ Pb++ Cd++ Zn++
++ 22e" + 2e= Pb + 2e- = Cd + 2e- = Zn
= Bi
+ H1O
;1 IjESl$
Experimental Procedures
(-0.762 volt)
The determination of iron and titanium only is dealt with here. The conditions for the determination of the other elements follow the procedures of Someya precisely (9) since the only new technique described in this paper deals with the mechanics of the separation of the solutions reduced, from the liquid amalgam reductor. Iron was determined in an approximately 0.1 N ferric alum solution in 0.1 N sulfuric acid as follows: Twenty milliliters of saturated zinc amalgam were placed in a 250-ml. glass-stoppered Erlenmeyer flask, and 100 ml. of sulfuric acid (1 volume of concentrated sulfuric acid to 6 volumes of water) were added, followed by 25.00 ml. of the ferric alum. The flask was stoppered, shaken vigorously during 120 seconds, and then opened, the sides were rinsed, and 30 to 50 ml. of carbon tetrachloride were added. The amalgam forms the lower layer of the three immiscible liquids in the flask. The carbon tetrachloride (specific gravity approximately 1.6) separates the amalgam layer from the solution being analyzed, which forms the top layer. The top layer is then swirled, using a mechanical stirrer that will stir this layer without causing the carbon tetrachloride to stir materially. The reduced iron is then titrated, using an approximately 0.1 N solution of sulfatoceric acid with ferroin as indicator. Using zinc amalgam, 25.00 ml. of ferric alum = 16.35, 16.30, 16.33, and 16.29 ml. of sulfatoceric acid; avera e, 16 32 ml. By titration of 25.00 ml. of the same ferric alum fo%owing reduction using the Jones reductor, 25.00 ml. of ferric alum = 16.44, 16.39, 16.38, and 16.46 ml. of sulfatoceric acid; average, 16.42 ml. I n the presence of air the liquid amalgam gives slightly low results, disclosed by Someya. The low results can be eliminated by filling the flask containing the reactants with carbon dioxide, either from a cylinder of compressed gas or by adding 0.5 gram of sodium bicarbonate to the acid solution before stoppering the flask and shakin to reduce. I n this case 25.00 ml. of ferric alum reduced in a carton dioxide atmosphere using zinc amalgam required 16.47, 16.45, 16.50, 16.48, 16.42, 16.45, 16.34, 16.51,.16.52, 16.51, and 16.35 ml. of H&e(SO&; average, 16.45 ml. It IS thus seen that low results are eliminated through the use of carbon dioxide. The cause of these low results in the absence of carbon dioxide is as yet unknown.
Liquid amalgams, among other applications, have been used in the reduction of vanadium, iron, molybdenum, uranium, tungsten, titanium, and chromium in preparation for their subsequent evaluation using standard oxidizing agents. The indirect determination of phosphorus following the formation of the familiar ammonium phosphomolybdate has been carried out. B y the choice of suitable liquid amalgams many differential reductions have been accomplished. The original use of a liquid amalgam reductor was devised by Nakazona ( 7 ) . Many other applications were subsequently described by Someya (9). The only other liquid metal reductor used (fused Wood's metal) was described by Smith and Wilcox (8). The use of liquid amalgams as reductors following the Xakazona procedure has not proved popular as a practical procedure because of the difficulty of removing the excess of reducing agent from the solution containing the element being determined. While the Nakazona procedure is rapid in the reaction of reFIGURE 1 duction, the apparatus ($) employed in the separation of the liquid amalgam and the reduced solution is extremely awkward to manipulate. The use of liquid amalgam reductors has therefore never been generally adopted and the Jones reductor ($), the Walden silver reductor ( I I ) , or other procedures have always been preferred. The present paper describes a simple and very practical method by which liquid amalgam reducing agents may be rapidly separated from the solutions of metal ions which have been reduced preparatory to their subsequent oxidimetric determination. B y the new procedure a reaction time of 2 minutes is generally ample and the removal of the reducing amalgam from the field of reaction is instantaneous. By comparison with the use of any other metallic reductor the new technique is just as accurate, comparably versatile, and much simpler in mechanical manipulations.
Differential Titration of Iron and Titanium
Apparatus and Reagents
The same procedure was applied to mixtures of 25.00 ml. of the ferric alum solution plus 25.00 ml. of an approximately 0.1 N solution of titanic sulfate. The flask shown in Figure 1 was employed after being filled with carbon djoxide from a cylinder through the side arm. The stopper was inserted, the side arm stoppered, and the flask shaken for 2 minutes. The stoppers were then disengaged, the flask walls rinsed, and 30 to 50 ml. of carbon tetrachloride added. A few drops of 0.1 per cent solution of methylene blue were added as indicator and while a vigorous stream of carbon dioxide gas was passed into the side arm, the titanous ion was titrated, using sulfatoceric acid to the formation of blue by the indicator. A dropnr two of ferroin was then added and the titration of ferrous iron completed.
For reduction of ions which do not require a neutral atmosphere for their final determination, an Erlenmeyer flask of 250or 500-ml. capacity with a tight-fitting glass stopper is all the apparatus required. Such determinations are those of iron and vanadium after reduction to their ferrous and vanadyl states. For reductions such as titanic salts to titanous ions, which must be subsequently oxidized in an oxygen-free atmosphere, the apparatus shown in Figure 1 is employed.
B y this procedure as the average of four consecutive determinations the titration of titanium gave 18.78 ml. of sulfatoceric acid. The comparison of the value of the titanic sulfate solution with the Jones reductor was found to be 18.65 ml. of sulfatoceric acid. These results are not believed to be the best obtainable using the method. Determination of iron in the 854
ANALYTICAL E D I T I O N
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 a r t 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, 16.58 ml. 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 AMMONIUMFLUORIDE 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
’
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.
