Colorimetric Determination of Iron with Disodium
1,2-dihydroxybenzene-3,5-disulfonate JOHN H. YOE
and A. LETCHER JONES' of University Virginia, Charlottesville, Va.
A
new sensitive, stable, and widely applicable reagent For the colorimetric determination of ferric iron is presented. The nature of the reaction and chemical behavior of the ferric complex has been studied both visually and spectrophotometrically. The reagent may be used in either acid or alkaline medium. In alkaline solution (pH 9 to 10), it is sensitive to one part of iron in 200,000,000 parts of solution when observations are made in Nessler cylinders (50-ml., tail-form); in acid solution (pH 3.5 to 4.5), the sensitivity
30,000,000. The colored complexes (red in alkaline medium, blue in acid)Obey Beer’s law over the useful range of iron concentrations. A variety of materials has been analyzed with a high degree of accuracy. The number of interfering ions is small. Analyses may be carried out in the presence of fluorides, phosphates, tartrates, citrates, oxalates, etc. Procedures are given for the use of the reagent.
o-dihydroxybenzene derivatives react with ferric resulting in the formation of intensely colored compounds which may be used in the colorimetric determination of iron (2-6, 7, 9, 10). This paper introduces disodium-1,2-dihydroxybenzene-3,5-disulfonate, from this class of compounds, as a new colorimetric reagent for ferric iron. The new reagent is exceedingly sensitive and subject to remarkably little interference by other ions. Ferric iron may be determined colorimetrically with this reagent in the presence of fluorides, phosphates, tartrates, citrates, oxalates, and other ions that normally interfere seriously with colorimetric iron determinations. The procedure for its application in analysis is simple and the results obtained by its use are highly satisfactory. It was observed during a systematic investigation of a series of medicináis, for their color or precipitate formation with about eighty inorganic ions, that “fuadin” (fouadin), an antimony complex of sodium catechol disulfonate, gave an intense blue color in acid solutions of ferric iron. The blue color was found to change to an intense red when the solution was made alkaline. Further investigation revealed that the active constituent of fuadin is disodium-1,2-dihydroxybenzene-3,5-disulfonate, one of the compounds from which fuadin is formed. An extensive study of the reaction of this compound with ferric iron showed that it is well suited for the colorimetric determination of iron. The iron complex is highly soluble in water and is stable to light; the color intensity is deep, permitting detection of a very small amount of iron; and the color reaction is practically specific. Moreover, the reagent is colorless in aqueous solution, a very desirable property of colorimetric reagents. A search of the literature did not reveal that either fuadin or disodium-1,2-dihydroxybenzene-3,5-disulfonate had been reported as a colorimetric reagent for iron.
has been observed. All solutions of the reagent were prepared by dissolving it in distilled water. Some of the solutions were allowed to stand in volumetric flasks for more than 3 months and no difference could be detected between these solutions and freshly prepared ones in either appearance or reactivity with ferric iron. Standard Iron Solution. A standard solution of ferric iron was prepared from ferrous ammonium sulfate, FeS04(NH4)iS04.6HjO, of reagent quality. A sample of 7.022 grams of the salt was dissolved in about 100 ml. of distilled water; 5 ml. of sulfuric acid and 1 ml. of bromine were added, and the solution was boiled until all iron was oxidized and excess bromine expelled. This solution containing 1 gram of ferric iron was made up to 1 liter for use as a stock solution. Solutions of Diverse Ions. The solutions of salts used in studying the effect of various ions on the color reaction were prepared so that each milliliter contained 0.5 mg. of the desired ion. All salts were tested for the presence of iron and if found, either a pure salt was substituted or the contaminated one was recrystallized until iron-free. Buffer Solutions. The most satisfactory buffer solutions for color matchings are: sodium acetate-hydrochloric acid mixtures for the blue complex and disodium phosphate for the red complex. The acetate buffer is prepared by dissolving 136 grams of sodium acetate, NaCsHiOi.SHsO, and 66.6 ml. of 12 N hydrochloric acid in water and diluting to 2 liters. This solution has a pH of 4.0. The phosphate buffer is prepared by dissolving 71.6 grams of disodium phosphate, Na2HP04.12Hi0, adding 4 ml. of N sodium hydroxide, and diluting to a liter. The pH is about 9.5. A carbonate buffer was also investigated for use with the red form of the iron complex. It is prepared by dissolving 10 grams of sodium hydrogen carbonate and 5 grams of sodium carbonate in water and diluting to 1 liter. The pH is 9.8. The red color produced by the iron complex in the carbonate buffer matches that formed in the phosphate buffer but no particular advantage of one buffer over the other is evident. On long standing, however, the phosphate buffer maintains a more stable color. All buffers must be titanium-free as indicated by a colorless blank with the iron reagent.
is one part in
MANY iron,
SPECTROPHOTOMETRIC STUDY OF COLOR REACTION
APPARATUS AND SOLUTIONS
In acid solutions with a pH value below 5, the complex formed by the reaction of disodium-1,2-dihydroxybenzene-3,5-disulfonat,e and ferric iron is deep blue in color. If the blue solution is made alkaline, the color changes sharply to a violet at pH 5.7 to 6.5 and becomes red at a pH of 7. The exact mechanism of this color change is not understood with certainty. It is believed, however, in the light of evidence obtained in this investigation, that it involves a change in the ratio of the pyrocatechol groups to iron as the hydrogen-ion concentration is changed. Considerable investigation has been carried out concerning the composition of the iron complex of pyrocatechol, an analog of this reagent. The only difference structurally between pyrocatechol and the new reagent is the presence of two sulfonic acid groups in the 3,5 positions of the latter. It is unlikely that these groups would influence the molecular ratio of combination of the reagent with iron. It has been reported by Reihlen (6), confirmed by
Instruments. Spectrophotometric measurements were made with a Beckman quartz spectrophotometer, Model D, using 1-cm. Corex glass transmission cells, at a spectral band width of 5
µ.
All pH measurements
were made with a Beckman glass electrode pH meter, Model G. Visual observations were made in matched 50-ml. Nessler cylinders (tail-form) and the Yoe roulette comparator using 100-ml. tubes (160 mm., 12). Acids. Unless otherwise specified, all acids were concentrated o.p. reagents. Reagent Solution. Sodium catechol disulfonate, disodium1,2-dihydroxy benzene-3,5-disulfonate, was obtained through the of the courtesy Winthrop Chemical Company, Inc., New York, N. Y. This salt is highly soluble in water. Its aqueous solutions are colorless and no evidence of their instability on standing 1 Present address, Department of Chemistry, Cornell University, Ithaca, N. Y.
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Vol. 16, No. 2
Weinland and Walter (11), and substantiated by Karrer (3) that the red iron complex of pyrocatechol is a tripyrocatechate ferric anion of the following configuration:
-°xFe/0-f o
o
Reihlen also isolated and analyzed catechol and found it to be as follows:
a
blue salt of ferric pyro-
This configuration corresponds to that found for the antimony analog, fuadin, as determined by Schmidt (8). From these considerations it would seem that the red and blue forms of the complexes of the new iron reagent are tri- and disodiumsulfonate catechates, respectively. Experiments were conducted to determine spectrophotometrically whether or not these configurations exist within the colored solutions.
mp
Figure 2.
Effect of pH
on
Iron Complex
pyrocatechol to iron in the complex is 3 to represented as follows:
1
and that it may be
NaOsS/^—Ox SOsNa
Figure 1.
Effect of Ratio of Reagent to Iron
I. Red complex, 480 mu, II. Blue complex, 620 µ,
pH 9,5 pH 4.0
Two series of solutions were prepared in which the molecular ratio of reagent to iron varied from 1:1 to 7:1. One series was buffered with disodium phosphate at pH 9.5; the other with sodium acetate-hydrochloric acid at 4.0. The absorbency of these solutions was measured spectrophotometrically at the wave lengths of maximum absorbency (—log transmittancy) for the two forms, 480 and 620 µ, respectively, for the red and blue. The results are presented graphically in Figure 1. Curve I shows that the color of the red complex does not inafter a 3 to 1 ratio of reagent to iron has been reached. The break in the curve is sudden at this point, indicating that in solution as well as in the solid form the ratio of the sulfonated crease
Curve II shows no break but increases in a smooth manner with increasing amounts of reagent. This, however, should not be taken to imply that the blue complex is of indefinite composition in solution but rather that its composition is not indicated by this indirect method of determining it. Since there is no sharp point beyond which increasing amounts of reagent produce no further change in color intensity, the blue complex is less desirable from a colorimetric point of view. Weinland and Walter (11) isolated the salt of the blue complex of the pyrocatechol analog and found it to have the composition [Fe(0C6H40)2]Na. The blue complex of the new reagent is probably of similar composition, with two moles of sulfonated pyrocatechol combining with one mole of iron to form the complex
which behaves
as a
singly charged anion.
SPECTROPHOTOMETRIC STUDY OF EFFECT OF pH ON COLORED COMPLEXES
Variations in hydrogen-ion concentration affect both the intensity and hue of the color produced by the new reagent at pH values below 8.5. Three distinct colors exist within this pH region. Below pH 5.7 the solution is blue, between 5.7 and 7 it is deep violet, and above 7 an intense red. This polychromatic property may be applied in analysis to match aliquot parts of a given sample to a red (pH 9.0 to 10.0), violet (pH 5.7 to 6.5), or blue (pH 3.5 to 4.5) standard by the use of suitable buffers to maintain the pH within the range desired. By utilizing this
ANALYTICAL
February, 1944
errors due to color blindness or color fatigue may be reduced. For instance, if the eyes of the observer are not so sensitive to color differences in the red as they are in the blue or violet, or vice versa, standards may be buffered to the color in which differences are most easily detected. Spectrophotometric measurements were made on a series of solutions containing 10 p.p.m. of iron and varying in pH from 4.00 to 9.36. Absorbency measurements were made at 10 µ intervals from 350 to 675 µ. The absorbency (—log of transmittancy) vs. wave-length curves are shown in Figure 2. The curves for the solutions of pH 4.00 to 4.93 represent a blue color; from 5.65 to 6.88, violet; and from 7.20 to 9.36, red. The sudden shift in the wave length of the absorbency maxima occurring just above a pH of 5.65 represents the sudden change in color from blue to violet. At an approximate pH of 7 the solution becomes red. The change in the wave length of the maxima of absorbency is represented graphically in Figure 3. Four of the values shown (pH 5.80, 6.08, 6.34, 6.65) were omitted from Figure 2 to avoid
property,
congestion. The abruptness with which these shifts in the position of maximum absorbency occur strongly indicates a definite change in the molecular structure of the colored complexes at the points where If there were only two complexes and the the shifts occur. violet color resulted from a mixture of the red and the blue, one would expect a gradual color transition. But with increasing pH, the blue changes abruptly to violet; the color remains violet over about 2 pH units, and then changes to red. Since the blue compound is believed to be an inner complex singly charged anion and the red compound a triply charged anion, it is possible that between the two, a doubly charged inner complex anion exists with a violet color. As a result of these spectrophotometric studies it is believed that the three colors, observed with varying pH values, are the result of the formation of three iron-pyrocatechol disulfonate complexes with the following configurations:
NaOsSi^V-Ox
yO
IJ_o/Fe\o SOsNa
Blue Complex
jSOsNa Na
SOsNa
EDITION
113
These studies further show that the new reagent has its greatest sensitivity—i.e., highest value of maximum absorbency—in the red complex. Increases in pH value above 9.5 do not further increase the absorbency or change the position of its maximum with respect to wave length. The possibility of using the iron complex as a pH or redox indicator has been investigated. It has no value as a redox indicator; however, the change in color with change in hydrogen-ion concentration suggests its use as a pH indicator. With increasing pH, there is a sharp change from blue to violet at pH 5.7; but the change from violet to red is not sufficiently sharp for endpoint determination in acid-base titrations. Mixtures of carbonates and bicarbonates may be analyzed by using the complex
in place of
a
double indicator, but the results
are
only roughly
quantitative. BEER'S
LAW APPLIED TO NEW IRON COMPLEXES
Beer’s law is valid for both the red and blue complexes at the wave lengths of maximum absorbency. The violet complex was not tried, owing to failure to find a satisfactory buffer for it. [Spectrophotometric measurements were made using 1-cm. transmission cells over a concentration range of 0.2 to 10 p.p.m. of iron. Neither the red nor the blue complex shows any deviation from Beer’s law within this range.) SENSITIVITY OF REACTION
The limit of sensitivity of the red complex is the detection of part of iron in 200,000,000 parts of solution, when observation is made in either 50-ml. tail-form Nessler cylinders or the Yoe roulette comparator. Most colorimetric reagents for ferric iron have sensitivity limits in the range of 1 part in 10,000,000 to 30.000. 000 under the most favorable conditions. The limit of sensitivity was established in the following way: Solutions containing 1 part of iron in 50,000,000, 100,000,000, and 200,000,000 parts of solution, respectively, were prepared by adding 0.5 ml. of reagent solution (0.113 gram per 100 ml.) to 2.00, 1.00, and 0.50 ml. of a solution containing 0.001 mg. of iron per ml. and diluting to a total volume of 100 ml. with a sodium carbonate-bicarbonate buffer of pH 9.5. In Nessler cylinders it was possible to determine correctly the respective order of concentration and definitely distinguish the solution containing 1 part of iron in 200,000,000 from a blank. 1
The limiting sensitivity of the blue complex was determined in the same manner and found to be 1 part of iron in 30,000,000
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parts of solution. When potassium hydrogen phthalate is used as a buffer (pH 4.0), the sensitivity is decreased to 1 part in 20,000,000.
Optimum concentrations for visual study of the red complex within the range of 0.05 to 2 p.p.m. of iron. The region of greatest sensitivity between fixed small increments is in the range of 0.08 to 0.12 p.p.m. of iron. Within this range, solutions differing from each other by 1 part of iron in 100,000,000 may be identified with certainty. At concentrations in the neighborhood of 1 p.p.m., the color intensity of the red complex prevents detection of differences greater than 1 part in 50,000,000. Increments differing by 1 p.p.m. of iron may be detected by the blue complex. Since the intensity of the blue is less than that of the red, the optimum range for the detection of small increments lies between 0.6 and 1 p.p.m. The sensitivity is greater with sodium acetate-hydrochloric acid buffers than with potassium hydrogen phthalate. are
Spot-plate sensitivity tests were made by transferring 0.05 ml. of standard iron solution to a depression in a white porcelain spot plate, adding 0.05 ml. of reagent solution (0.0036 M) and finally 0.05 ml. of buffer solution. Tests were made with both the red and blue complexes by using disodium phosphate and sodium acetate-hydrochloric acid buffers, respectively. Blanks were prepared by adding buffer solutions to the reagent alone. One drop of solutions containing 1 p.p.m. of iron gave tests which could be distinguished from blanks with certainty, for both the red and blue complexes. This represents the detection of 0.05 microgram of iron. The limit of sensitivity on the spot plate is the same for either complex, red or blue. PERMANENCY OF STANDARDS
The color intensity of both the red and blue complexes increases slightly for the first 18 hours (about 5% transmittancy at the wave lengths of maximum absorbency), after which time it remains constant for many months. Solutions of the colored complexes have been kept in stoppered tubes in the diffuse light of the laboratory for 2 years. These solutions showed no change in intensity when compared visually with freshly prepared solutions that stood for 18 hours. Since there is a slight change in the intensity during the first 18 hours, it is best to prepare fresh standards daily for precise work. The preparation of standards is so simple that artificial color standards offer little or no ad-
Table ion Al
+ +
Co
+ +
Co(NOe)s
Cr
+ *
Cr(NOj)j
Cu
~
~
+ +
*
Mn+
v
-
Sn
+ +
Sn
+
*
+
+
Ti+++* ZrO + +
Phthalate Citrate Maleic Oxalate Tartrate
1
R (if pptd. Ca filtered), B R, B
RX, B
R, B
RX, BX R, B (if done quickly) RX, BX RX, B RX, BX R, B R, BX R, BX
1
R, B
R
(if pptd. Mg filtered), B
>100
R
1000
100 >100
SnCh
>
1
1
25 < 25 < 10
Xa3P04 NasSiOs SnCl2
-
1 1
>
50 > 50
Xi(X03)2
POi" Si03---
>
100
0.01
Zr(N03)< Potassium hydrogen
>
5
>
5
R, B R, B
>
5 5 5 5
R, R, R, R,
phthalate Citric acid Maleic acid Oxalic acid Tartaric acid
R red matches, B blue does not match.
to symbols:
not match, BX EFFECT OF DIVERSE
< 10
KzCrsO?
MnCls
+ +
RX, BX
5 5
< 10 > 25 < 25 > 10
MgSOi
NH4 +
>
> 10
FeSO^NHihSO,
+ +
Mg +
Remarks
>100 > 0.5
KF
Fe
Amount Present
