Polarographic Determination of Ferrous and Ferric Iron in Refractory Minerals GEORGE S. BlEN and EDWARD D. GOLDBERG Scrippr Institution of Oceanography, University o f California, La Jolla, Calif.
reduce the number of reagents used in the analytical procedure, preference n-as given to the use of potassium nitrate. The flux used for the breakdown of the minerals was sodium metafluoroborate (6). This flux has advantage over sodium carbonate in that it gives no effervescence even at high temperatures in producing rapid and complete dissolution of minerals ( 1 \.
A polarographic method for the simultaneous determination of ferrous and ferric iron in refractory minerals is described. The minerals are initially decomposed in a flux of sodiitni metafluoroborate. The flux is dissolved in a supporting electrolyte solution containing citrate ion, which complexes both the ferrous and ferric irons. The composite wave of oxidation and reduction of the complex ions gives an anodic current proportional to the concentration of ferrous iron and a cathodic current proportional to the concentration of ferric iron. The procedure can be applied to samples containing both organic materials and manganous compounds.
RECO.MMEYDED PROCEDURE
d weighed sample of 0.1 to 0.2 gram of powdered mineral and about, six t,imes its weight of sodium metafluoroborate are placed in a platinum crucible, the weight of which was previously ascertained. The crucible is covered with an inverted clay crucible through which nitrogen gas enters a t the top. After a %minute nitrogen flush of the apparatus, the flux mixture is gradually brought to a fusion temperature of 1000° to 1050" C. nith a Meker burner under an atmosphere of nit,rogen. The fused condition is maintained for approximately 5 minutes, or until a clear homogeneous melt is obtained. The crucible is alloxed to cool under a stream of nitrogen, weighed again, and the melt removed. The melt is ground under acetone and dried under a stream of nitrogen. An aliquot of 0.3 to 0.4 gram of the powder is weighed and dissolved in 50 ml. of oxygen-free complexing solution under a sheam of nitrogen. An aliquot of this amount gives adequate diffusion currents for minerals containing up t,o 20y0 total iron. After the solution attains a temperatmureof 26.00" C., a polarogram is obtained from +O.l5 to -0.35 volt (Figure 1). The zero current value is ascertained by the pen position n i t h t'h? c1,ropping mercury electrode polarity snit,ch in t,he "of?"' pos1tion.
T
HERE are several serious limitations ( 2 ) in the gencrd extensions t o rocali analysis of the permanganate titration of fttrrous iron. First the oyidizability of manganous ion by permanganate in the presence of even limited amounts of hydrofluoric arid tends to give high results in minerals containing manganese; and second any organic matter present tends to cause high results. Further, the sinililtaneous determination of both ferrous and ferric iron require8 a double titration with the attenciant complicat'ions in the reduction of the ferric iron. A promising rapid polarographic method in which both ferrous and ferric iron are ascertained simultaneously seemed possible from the works of Staclielberg and Freyhold ( 7 ) , Lingane (4,5 ) ) and Kolthoff and Lingane ( 3 ) . The composite wave of oxidation ant1 reduction of the complex ions formed bet,ween ferric and ferrous ions and oxalate ion give an anodic current proportional to the concentration of the ferrous iron and a cathodic current proportional t,o the concentration of the ferric iron. Lingane ( 5 ) indicated that citrate ion might be preferable as a complexing agent for the purpose of determining ferrous and ferric iron. The purpose of this investigation was to develop a general polarographic method for the assay of the absolute ferric and ferrous iron contents of refractory minerals, as ivell as the optimal coritlitions for t,he dissolution of the mineral sample n.ith minimal oxithtion of thc ferrous i:,on. A I ' P A R ~r c s AYD REAGEYTS
All polarographic measurements were made on the Sargent Model XXI polarograph. Standard electrolysis cells were used with the dipping-type saturated calomel or silver-silver chloride cells as the reference anode. All measurements were made a t 25" + 0.05O C The capillary characteristics with a mercury height of 64 em.in the standard complexing solution on the horizontal portion of the current-voltage curve were: m = 8 4 i mg. see.-', t = 3.5 seconds, and m2 3t116= 5.12 mg.2'a All reagents used viere of the highest reagent grade. The distilled nater TTRS the equivalent of that produced by a triple distilling process. The sdpporting electrolyte solution included the conipleving agent and had the folloiving composition: sodium citrate, 0 2 S M ; citric acid, 0 25M; and potassium nitrate, 0.5OX. The pH of this solution is 4 00 and should be maintained at the same value for all solutions inasmuch as the halfwave potentials are determined by the pH (6). The potassium nitrate is added as the carrying electrolyte. Potassium nitrate waa chosen in preference to potassium chloride, because there is a maximum in the ferric wave which was difficult to suppress when potassium chloride was used. Gelatin can be used as a maximum suppressor n.ith potassium chloride. However, to
-2
01
1 0 15
1 t O 05
I
005
E iioIf%l vs
!
I
- 0 I5
- 0 25
- 0 35
S C E.
Figure 1. Typical polarogram Solution contained both divalent and triralent iron in the supporting talectrolyte solution, which contained 0.25.U citric acid, 0.25.1f sodiorli citrate, and 0.50.11 potassium nitrate.
To determine the respective polarographic constants, ~ F and ~ F ~ for ~ o the ~ , equation of quantitative polarographic analysis, i d = k c , where i d is the diffusion current and c the concentration in question, aliquots containing the equivalent of 0 to 8.0 mg. of ferrous and ferric oxides are added to 50-ml. portions of the complexing solution. Reagent grade ferric ammonium sulfate, Fe2(S04)3.( NH4)2SO4.24H20, and reagent 97
~
O
A N A L Y T I C A L CHEMISTRY grade ferrous ammonium sulfate, FeS04.(NH4)~SOa.6H20, were used as standards. Weighed quantities of these salts were introduced directly into the complexing solutions. This procedure bypasses the preparation of standard ferrous and ferric solutions; the concentration of the ferrous standard may change with time. It further eliminates the pipetting of etandard solutions in which oxidation of ferrous iron may occur.
Table I.
Run
Calibration Data for Ferrous and Ferric Oxides“ FeO Concn., Mg./ 50 hll. 0.0 1.25 1.67 2.05 2.71 3.54 3.94 4.82 6.10
F .e..A . Concn., Mg./ +id, 50 MI. #a. ~
--id,
pa. 0.20 0.96 1.24 1.53 1.84 2.46 2.72 3.26 3 92
-idb,
pa. 0 00 0.76 1.04 1.33 1.64 2.26 2.52
3.OB 3 72
h e 0
o:bio
0.624 0.648 0.605 0.639 0.640 0.633 0.610
0 1 1 1
0
13 77 97 3 51 3 71 3.74 4 21 6 71
0.06 0.64 0.98 1.06 1.90 1.93 2.02 2.30 3.60
+idar
pa.
kFeiOa
0.00
0.58 0.92 1.00 1.84 1.87 1.96 2.24 3.54
o:iij
0.519 0.507 0 524 0 520 0 524 0 533 0 528
In 0.5M potassium nitrate, 0.25M sodium citrate, and 0 . 2 5 M citric acid. Dropping mercury electrode u s . S.C.E. where W i a t 1 l 6 = 5.12 mg.213 sec.-1/2. Ferrous and ferric values both obtained in single runs listed above. Corrected for blank values.
The slight increase of volume due to the introduction of the solid salts into the solution is insignificant. The results of one set of these calibrations are given in Table I. The values of the k’s are expressed in microamperes per milligram instead of microamperes per millimole for convenience in the rapid conversion of results to the weight units used in the literature of geology. The standard deviations of kFeo and kFe,or are 2.6 and 1.6%, respectively. There apparently is no effect of the ferric iron produced by the oxidation of ferrous iron upon the measured ferric iron value of the sample. This is shown in Table I where each sample contained both ferrous and ferric iron. There is no trend in the ~ o increasing , ferrous concentration. values of ~ F ~with
Table 111. Determination of Ferrous and Ferric Oxides with Presence of llanganous Ion
Olivine Saponite Hornblende Narcasite bearing rocks
Polarographic,
Volumetric,
%
%
12.4 12.5 12.2 11.2 11.2 5.01 6 55 4.46 19.88 19.15 18.93
11.5 11.1 12 5 11.6 11.6 6.29 6.52 4.51 19 89
Found,
Preqent,
mg.
mg.
mg.
4 4 4 4 4
4 85 4 76
14 4 22 4 44 3
82 78 87 93
4
si
5 03
ae
4 85
2 47 0 0
Found inn. 7 51 0 0
4 09
4 10
2 45 2 44
2 49 1 49
manganate. Replicate analyses were made on the glauconite, olivine, and marcasite samples. I t was found that manganous ion does not interfere with the polarographic determination of either the ferrous or ferric iron inasmuch as the oxidation potential of the manganous ion is far removed from that of the ferrous-ferric waves. Recovery determinations of knom-n quantities of ferrous and ferric oxides with varying quantities of manganous chloride are given in Table 111. The fusion process can be carried out with minimum oxidation of ferrous iron if done rapidly and under a nitrogen atmosphere. IIon-ever, lengthy grinding periods, even under acetone, result in a reduction of the ferrous concentration (Table IF’). To reduce any photochemical reduction of the citrate solutions, it is recommended to carry out the polarographic determinations in low actinic (colored) glass vessels or in the absence of strong light.
Table IV.
Oxidation of Ferrous Iron During Grinding Process Per Cent FeO Fez03 12.60 6.6
Description of Sample Glauconite chips (directly f r o m crucible) Glauconite chips (broken u p by light pounding in a mortar, without acetone) Glauconite powder (ground in acetone, 20 mesh) Glauconite powder (ground in acetone, 100 mesh) ~
Description of Sample Glauconite
FezO,
Present,
0 11 0
___ Table 11. Comparison of Polarographic and Volunietric Determinations of Ferrous Oxide Content in Weight Per Cent
FeO
3InC12 .idded, Mg.
~..
~
~
___.
12 40
11.93 1 1 22 1 1 19
7.1 7.6 7.7 7.6
__
CO\rCLUSION
The use of the polarographic composite oxidation and reduction waves of ferric and ferrous ions provides a convenient method of assay in refractory materials even in the presence of manganous ion and organic matter. The use of the sodium metafluoroborate flus, with the precautions of limited light exposure, fusion time, and grinding, results in niininiuni o\idation of ferrous iron. 4CKNOWLEDG.\lENT
This investigation was supported by a grant from the American Petroleum Institute, Project 51. RESULTS
The polarographic determinations of ferrous iron were compared with the results of a permanganate titration of splits from identical samples (Table 11). I n the volumetric procedure the melt was dissolved in a solution containing 20 ml. of 12N sulfuric acid and 20 ml. of saturated boric acid solution under an atmosphere of nitrogen. The mixture was warmed slightly to effect solution. Upon cooling, 5 ml. of 86% phosphoric acid were added, and the titration was carried out. The results of these titrations are given in Table I1 and show reasonable agreement with the results of the polarographic method. The titration was not carried to completion for the marcasite sample inasmuch as the hydrogen sulfide formed during acidification reduced per-
LITERATURE CITED ( I ) Groves, A . W., “Silicate Analysis,” 2nd ed., p. 184, George h l l e n a n d Unwin, L t d . , London. 1951. ( 2 ) Hillebrand, W. F., a n d Lundell, G. E. F., “Applied Inorganic I n a l y s i s , ” 2nd ed., p. 912, Wiley, N e w York, 1953. (3) Kolthoff, I. AI., and Lingane, J. J., “Polarography,” 2 n d ed., p. 219, Interscience, New York. 1952. (4) Lingane, J. J., Chem. Revs. 29, 17 (1941). (5) Lingane, J. J., J . Am. Chem. SOC.68, 2448 (1946). (6) Rowledge, H. P., J . R o y . SOC.W . Australia 20, 165 (1934). ( 7 ) Stackelberg, 11.von. a n d Freyhold, H. v o n , 2. Elektrochem. 46,
120 (1940). RECEIVED for review August 29, 1955, Accepted October 20, 1955. Contribution f r o m Scripps Institution of Oceanography, New Series S o . 834.
