Dichromate Determination J. L. HENRY AND
OF
Iron, Using the Silver Reductor
R. W. GELBACH, State College of Washington,
1.
on~~;p,”,”~~f~~;s
were made by introducing in the first series 25-m1. portions of 0.01 molar potassium dichromate, in the second 0.1 molar manganese chloride, and in the third 0.1 molar titanium sulfate. Controls were run on each ion by passing the solution through the reductor in the absence of iron. I n each case one drop of 0.1 normal potassium dichromate gave a distinct end point.
I t has been shown that reliable results can be obtained in the presence of chromium, manganese, or titanium with acidities ranging between 0.5 and 1.5 molar. Checks were obtained with titration acidities as low as 0.1 molar and as high as 2 molar, but a t the extremes of concentration the end points were not sharp. The results shown in Table I indicate that these ions do not interfere with the analysis. In the presence of high concentrations of vanadium an indistinct end point was obtained, the color change being from light green to gray. Titrations were made in the presence of a wide range of concentrations of vanadyl ion; up to concentratiolls of 100 mg. of vanadium per 200 ml. of titrating volume a very sharp end point was obtained. The deep violet color waa not shown as in the absence of vanadium but there was a distinct color change from light green to deep blue. The results of these runs checked well with the accepted value for iron in the standard ferric chloride solution.
Effect of A d d e d Impurities
(Volume of FeCl,, 25 ml.) No. of Solution Added Detns. Fe Found
M1.
Gram
Wash.
a t a rate of about 30 ml. per minute. The reductor was washed with 150 ml. of 1 molar hydrochloric acid added in small portions. To the reduced solution 5 ml. of 85% phosphoric acid and 5 to 6 drops of 0.25% diphenylamine sulfonic acid were added, and the solution was titrated with potassium dichromate. The results, shown in Table I, deviated from those of the Walden method by
H E method of Walden, Hammett, and Edmonds (4) for the Tdetermination of iron using the silver reductor and titrating with ceric sulfate, although highly satisfactory, is expensive because of the relatively high cost of the ceric sulfate and its very high equivalent weight. I n the titration large quantities of furic acid are required. The dichromate method as presented in this paper Overcomesthese objections and seems to retain themore desirable features. I n the analysis of ores and alloys of iron the Common impurities which may interfere with the reduction and subsequent titration are chromium, manganese, molybdenum, titanium, and v a n e dium. Molybdenum may be separated from iron by precipitating iron as the hydrous oxide. I n the silver reductor, chromium is not reduced below the trivalent state, while manganese and tit* nium are not reduced a t all. Vanadium is reduced to the vanadyl ion which is not oxidized by either cerate or dichromate ions. Table
Pullman,
Average Deviatiol P.p. I onn
From previous standardization’ 0.1755 ... None 6 0.1756 0.6 25 0.01M KnCrzOl 6 0.1757 1.1 25 0.1M MnClr 5 0.1760 2.8 25 0.1M Ti(SO4)r 10 0.1756 0.6 6 0 . 1 M NaVO: 2 0.1755 0 10 0.1M NaVO: 2 0.1754 -0.6 1s 0 . 1 M NaVO: 3 0.1754 -0.6 20 0.1M NaVOa 2 0.1753 -1.1 40 10 ml. of each 2 0.1754 -0.6 From the standard method of Walden, Hammett, and Edmonds.
Two samrles obtained from the Bureau of Standards were analyzed: ‘ron ore, B. of S. No. 27, and ferrovanadium alloy, B. of S. No. 61 The samples were dissolved according to recommended procedures accompanying the samples. The removal of molybdenum from the alloy was accomplished by twice precipitating the hydr ted ferric oxide from ammoniacal solution. In each instance tht. rtr,ciitywas adjusted to 1 molar with hydrochloric acid and reduced as before. Results of the analyses are shown in Table 11.
t
The oxidation potential in 1N hydronium-ion concentration of the system,VO,----VO++, is given as 1.2 volts ( I ) , and that of diphenylamine sulfonic acid is 0.83 volt (21, which according to Walden, Hammett, and Edmonds (8) is sufficiently below the vanadate-vanadyl potential to give precise results. Adjusting the acidity to approximately 1N and adding 5 ml. of 85% phosphoric mid, one obtains a condition favorable to the titration of ferrous ion with potassium dichromate even in the presence of vanadium.
Table
B. of 9. Sample
MATERIALS
Standard solutions of ceric ammonium sulfate and potassium dichromate were prepared in the usual manner and standardized against pure iron wire. They were also checked against a gravimetrically standardized solution of ferric chloride, using stannous chloride reduction. A solution of ferric chloride standardized gravimetrically was used in studying the behavior of the dichromate-diphenylamine sulfonic acid titration in the presence Of the ions Of chromium, manganese, vanadium, and titanium. The effect of impurities was studied by adding definite quantities of 0.01 molar potassium dichromate, 0.1 molar manganese chloride, 0.1 molar sodium vanadate, and 0.1 molar titanium sulfate. The latter was prepared by fusing 4 grams of titanium
Iron ore 27 Ferrovanadium alloy c t
II.
Determination of Iron N o of Fe Fe, B. of 8.
Detns. 4
2
Found %
69.30 52.83
Certificate
Average Deviation
%
P.P. 1000
69.26 52.8
0.6
...
SUMMARY
METHOD
Potassium dichromate, using diphenylamine sulfonic acid as indicator, is an oxidizing agent for the determination of iron by the Walden silver reductor method. Manganese, chromium, and titanium do not interfere. Vanadium does not interfere in concentrations of 100 mg. or less in 200 ml. of titrating solution. A more economical method results from the use of only 1N hydrochloric acid instead of the higher concentrations of sulfuric ~ t ~ ‘ acid required in the ceric sulfate method. The stability, purity, low equivalent weight, and comparatively low cost of potassium dichromate render it a very desirable oxidizing agent in this connection.
Analyses were made with 25-m1. portions of the standard ferric chloride solution by the method of Walden, Hammett, and Edmonds (Q), titrating with ceric ammonium sulfate. Similar analyses were then made by titrating with potassium dichromate. A 25-ml. sample of standard ferric chloride solution was pipetted, the acidity was adjusted to 1 molar with hydrochloric acid, and the final volume of 50 ml. was passed through the reductor
(1) Kolthoff and Sandell, “Textbook of Quantitative Inorganic Analysis”, p. 483, New York, Maomillan Co., 1943. (2) Ibid., p. 493. (3) Walden, Hammett, and Edmonds, J. Am. Chem. SOC.,56, 59 (1934). (4) Ibid., p. 350.
, p , ~ 8 ; ~ ~ i ~ ~ ~ ~ g ~ ~ ~ ~ ~ t ~ ~
~ ~ ~ d ~ ~ ~ c ~ ~ c ~ solution to 500 ml. The silver reductor was prepared in the manner described by Walden, Hammett, and Edmonds (4).
LITERATURE CITED
49
