LABORATORY INSTRUCTIONS for STUDENTS of QUANTITATIVE ANALYSIS I.
The Gravimetric Determination of Iron
A. H. KUNZ AND G. STERLING BAILEY University of Oregon at Eugene and Oregon State College at Corvallis
Many textbooks of quantitative analysis contain laboratory di7ectiOn~ that include steps without theoretical foundation, and operations that are time-consuming without yielding improved results. This article points out, with supporting experimental data, a number of sucA cases i n the gravimetric determination of iron.
T
HE TYPE of laboratory instruction, or procedures, given in most textbooks of quantitative analysis is largely responsible for the stigma of superficiality which courses in analytical chemistry so often bear. Essential steps are listed without any indication of their significance; unnecessary operations are included; precautions are advised which may introduce additional sources of error outweighing their advantages; and, when precautions are recommended, the magnitude of the errors they are intended to reduce is not indicated. These defects cannot be tolerated if a course in analytical chemistry is to serve its rightful purpose as a means of investigation of chemical principles. Consequently, a study is being made of those determinations
commonly included in most courses in quantitative analysis, in order to overcome the objections. Despite the fact that the gravimetric determination of iron by precipitation with ammonia has given way to the faster, and usually more reliable, volumetric methods, almost all textbooks of quantitative analysis include this determination. Since the usual directions for the analysis of iron are especially illustrative of the defects cited above, this determination was chosen for the first study. REVIEW AND CRITICISM OF PRESENT METHODS
Ferrous ammonium sulfate, or mixtures of this salt and potassium sulfate, are commonly used for the iron analysis. An appropriate quantity is weighed out, dissolved in dilute hydrochloric acid, and the iron is oxidized to the ferric state with nitric acid or bromine water. Fenic hydroxide is then precipitated with ammonia, either by addition of the ammonia to the iron solution or vice versa. Those procedures recommending the latter, more cumbersome method of precipitation are silent about the magnitude of its advantages.
Some textbooks are especially insistent that freshly distilled ammonia solution be used because of the danger of precipitating, during the course of the determination, some silica dissolved from glass bottles. Others are content with filtered solutions and some do not even mention that precaution. Avoidance of redistillation is desirable if no advantage results from it. After the precipitated femc hydroxide settles, it is washed one or more times by decantation, dissolved with acid, and reprecipitated. The theory of the double precipitation is made clear but again the magnitude of the advantages which will be apparent in the final results is not noted. Before reprecipitation, reoxidation of "any iron reduced by the filter paper" is advised by some authors. Experimental proof of its necessity is required. The reprecipitated hydroxide is washed, first by decantation and then on the filter, until free of chlorides. This operation, although it insures the removal of sulfates, causes the student to assume that the presence of chlorides is detrimental. One author1 goes so far as to explain that on subsequent ignition the ferric oxide reacts with ammonium chloride to produce volatile ferric chloride. Instructions for the remaining operations of drying, igniting, and weighing the preapitate are quite miform and free from obiections. The non-uniform portions of the above procedure which have been investimted are: 1. Comparison of use of ammonia solutions which have stood in glass bottles with the use of solutions prepared by the absorption of ammonia gas in water contained in paraffin- or rubber-lined vessels. 2. The advantages of double precipitation. 3. Comparison of precipitation of femc hydroxide by addition of the ammonia to the ferric salt with the reverse method. 4. The necessity of washing free of chlorides. 5. The possibility of reduction of femc iron by filter paper. EXPERIMENTAL DATA AND DISCUSSION OF
RESULTS
Pure ferrous ammonium sulfate was used for the first four points of this study. Either an approximately 1 or 2.5-gram sample was weighed out for each determination and dissolved in dilute hydrochloric acid. After oxidation with bromine water the solution was diluted to 200 ml., heated to boiling, and the ferric hydroxide was precipitated by the addition of a slight excess of ammonia solution which had been prepared and kept in a paraffin- or rubber-lined bottle. The femc hydroxide was washed twice by decantation with 100-ml. portions of hot water containing a drop of ammonia solution. After solution in acid, the femc hydroxide was again precipitated and washed in the same manner, transferred to the filter, and washed free of chlorides. The dried precipitate was ignited to constant -weight. ~POPOPF, "Quantitative analysis," P. Blakiston's Son & Co.. Philadelphia, 1927, p. 124.
The combined filtrates were evaporated to small bulk, filtered, and barium sulfate was precipitated by adding the sulfate to 250 ml. of boiling solution containing a slight excess of barium chloride and 1 ml. 6 N hydrochloric acid.% This procedure was employed throughout with the exceptions noted. No precautions were omitted even though successive portions of the investigation indicated they were unnecessary. The ferrous ammonium sulfate was analyzed volumetrically, after reduction with stannous chloride and with zinc, with a potassium permanganate solution which in turn had been standardized with both U. S. Bureau of Standards sodium oxalate and iron wire. The percentage of iron conformed with that required by the formula FeS04.(NHa)~S0,.6H~0. TABLE 1 ~ s r e w n r * r z o09 ~
IN F B W ~ J A-omS
IPON AND S-ATB
Vcsrcls A m WI. fa Samslc. No. Pr Melhal Fc(0B)i 8. Dd. Thmraicnl 1 . 0 8 A Pyrex 2.5 8 B Pyrn 2.6 10 C Pvrex c pyrex 1.0 4 2.5 8 C Pt. P P .t ~ I 0 2 . D PY2.5 4 D Pyrex 1.0 4 r, me 9 . F o *
...
-
.
D
pt.
E
PYXX
Pyrex
E
PC
E F
A. B.
C. D. E. F.
1.0 2.5 1.0 2.5 2.5
Pyrex
..
2 4
2 3 2
Mcon Pc
%
14.24 14.45 14.38 14.32 14.37 14.29 1-4 4-. 0 14.31 14.35 1~ 9%
14.37 14.30 14.33 14.29 14.28
No. SO,
Dd.
SDL.ATB Meon %
.. ..
SO4 49.00
2 3 2 2 2
48.06 49.21 49.22 48.83 49.05 'a 17
.. 4 .
$9.07
-R
2
...... . ..
--.-. ... ... 49.20 ...
Double precipitstion, adding to ferric solution ammonivm hydroxide stored thirty days in glass. Double precipitation,adding to ferric solution ammonium hydroxide stored seven days in glass. Double precipitation,adding to ferric solution nmmonium hydroxide stored in paraffin- or rubbedined bottle. same as C, using one precipitation. ~ o u b l eprecipitation, adding ferric solution to ammonium hydroxide from paraffin- or rubber-tined bottle. Double precipitation. washing free of sulfates only.
From the results presented in Table 1 it is apparent that: 1. The use of ammonia solution which has been in contact with glass is very detrimental to the gravimetric iron determination. Even when silica-free ammonia solution is used, the calculated percentage of iron is high, presumably from silica derived from glass vessels used in the analysis. 2. The substitution of platinum vessels for Pyrex provides but little improvement in the iron analysis and decidedly increases the calculated percentage of sulfate. When platinum ware is used, the barium sulfate has a violet tinge, probably due to the formation of a platinum compound from the reduction of the femc iron, which is then adsorbed. This may also account for the high iron value. 3. In all cases, the smaller samples show a greater percentage of iron than the larger samples. 4. No significant advantage can be found for double precipitation of femc hydroxide.
Powpp 1
45 (1930).
AND
N
E
~Ind. , Eng. Chem., Amlyt. Ed.. 2,
5. No advantage is realized by employing the more cumbersome method of adding the femc solution to the ammonia. 6. The presence of chlorides is not detrimental in the ignition of femc hydroxide.
To prove even more definitely that there is no possibility of loss of iron by volatilization when femc hydroxide is ignited in the presence of ammonium chloride, equal volumes of ferric chloride solution were precipitated with ammonia in the usual manner. Three of the samples were filtered, dried, and ignited without washing. Two samples were washed free of chlorides before ignition. Table 2 shows no significant variation in the final results. This is in agreement with the conclusions of Dandt.' TABLE 2
Unwashed
NO.af DII. 2 3
Maon p. Fc 0.2903
0.2901
To show that there is no possibility of reduction of femc ion by the filter paper, a quantity of shredded filter paper was shaken up with a solution 0.1 M with respect to ferric chloride and 0.5 M with respect to hydrochloric acid and allowed to stand for ten days. Portions of the solution, after filtering out shredded paper and adding preventive solution, were titrated with potassium permanganate. Other portions were boiled fifteen minutes before filtering and titrating. In no case was more permanganate used than the volume required in a blank run. Since these conditions are more severe than those existing ., durine the
-
Darror, Ind. Eng. Chem.. 7,847 (1915).
TABLE 3 C o s s s n r o ~m a S n r c A
% FS
Io~molpOP Fe(0H)z wra Awn wraour W*S*SNG Ekes or Cmoaross mhod W-hed
analysis, the "reoxidation of any iron reduced by the filter paper" is obviously a misleading expression, tending to give a false impression of the oxidationreduction potentials in the system involving ferric iron and cellulose. Silica determinations made on ignited femc oxide residues show that silica is responsible for the apparent high iron percentage when Pyrex vessels are used for the precipitation of ferric hydroxide. The residue was dissolved in hydrochloric acid and the silica filtered off and volatilized with hydrofluoric acid in the usual manner. Correcting for the silica, the calculated percentage of iron is in satisfactory agreement with the theoretical.
-
BIfar r a r r c l i n ~ for Si01 14.37 14.31 14.44 14.42
Dz-weo
aaoa
P v ~ s xVs8ssrs
% pa
After conscling for S ~ O Z
14.22 14.22 14.26 14.22
CONCLUSIONS
The gravimetric determination by precipitation with ammonia is unsatisfactory for accurate work unless silica determinations are made on the ferric oxide. The determination of iron and sulfate in ferrous ammonium sulfate is an unsatisfactory example for illustrating the value of double precipitation. Any sulfate retained by the femc hydroxide is not significant with.respect to the high sulfate percentage; and is not significant to the iron value because the sulfate is volatilized on ignition of the femc hydroxide. Many of the precautions advised in the gravimetric determination of iron are not well founded.
