Thiocyanate Method for Iron J
A Spectrophotometric Study J . T. WOODS
I
WITH
41. G. XIELLON, Purdue University, Lafayette, Ind.
liters. The directions of Snell ( 1 7 ) and l-oe (25) \Tere followed for the solution of thiocyanic acid. Material from the General Chemical Company was used for a solution of ammonium thiocyanate, :is the very small amount of iron in this product could be extracted easily from a slightly acidic solution with four or five portions of a 5 to 2 mixture of amyl alcohol and ethyl ether. The resulting solution, if slightly ammoniacal, is stable in the dark for several weeks. Solutions of recrystallized hexaquocobaltous chloride and potassium chloroplatinate were prepared, according to the directions of the American Public Health Association (1) for use as color standards. Other specially prepared materials used for this purpose, together with the concentration of the solutions, are listed in Table I. During the work on the variable factors studied several variations were used in preparing the solutions to be measured. After deciding upon the best conditions, an operating routine was established. Five milliliters of a solution containing 0.020 mg. of iron per ml. were measured into a 50-ml. volumetric flask. To this were added, in order, 5 ml. of 6 N nitric acid, 2 ml. of 20 per cent ammonium thiocyanate solution, and 30 ml. of acetone. The flask was filled to the mark, the solution mixed, and the spectrophotometric curve determined within 15 minutes. Diverse ions, when used, were added prior to the thiocyanate. Measurement of pH values of the solutions vas made with a glass electrode assembly (12).
S d d l I U C H as the thiocyanate method for iron is generally
accepted as official by the technical organizations having standards of analytical practice (1-3, 22) and the procedure seems to be inferior to certain other colorimetric methods for this element, the operating conditions now recommended were critically studied. I n addition to investigating various proposed modifications of the original procedure, an attempt has been made to evaluate the method in comparison with several other procedures recently suggested for the same determination. Since the thiocyanate method is very old, having been proposed in 1837 by Ossian ( l e ) ,its literature is voluminous. Stokes and Cain (18) pointed out many of t,he errors and suggested as the reagent thiocyanic acid stabilized with mercuric thiocyanate, adding potassium persulfate to oxidize the iron. Hydrogen peroxide and potassium permanganate have been used for the same purpose ( 1 5 ) . Bernhard and Drekter (5) suggested extracting the colored compound by means of a mixture of the monobntyl ether of ethylene glycol and ethyl ether. The extract has a more intense color and does not fade for 24 hours. Addition of acetone increases the sensitivity of the reagent and decreases the error due to phosphates (IO). Winsor ($4)used 2-methoxyethanol for the same purpose. Two mixtures of salts have been proposed as solutions for permanent color standards. Jackson (9) used cobaltous chloride and potassium chloroplatinate, while Velichkovskaya (23) used cobaltous nitrate and ferric chloride. At present the nature of the reaction between ferric and thiocyanate ions remains controversial. The work of Stokes and Cain (18), and later that of Schlesinger and Van Valkenburgh (167, seem to indicate the presence of an anion, Fe(SCNh---. Op osed to this concept is the work of M$ller (13) and of Bent anifFrench (4) who believe the color is due to a cation, such as Fe(SCT\;)++ or Fe(SCN)+.
Color Reaction SELECTIOS OF REAGEKT.The depth of color developed depends upon the amount of reagent used, whether it is thiocyanic acid or ammonium thiocyanate. The ammonium salt was used, because it gives a much deeper color than the acid
Apparatus and Methods All color measurements were made with a General Electric photoelectric spectrophotometer, the spectral band width being 10 mp and the cell thickness 1.000 em. Typical spectral transmission curves were selected for calculating tristimulus values by means of Hardy’s ten selected ordinates ( 7 ) and Swank’s calculator (21). Calculation of the extent of the interference by diverse ions was facilitated by a color slide rule (Keuffel and Esser) . A stock solution cont,aining 100 p. p. m. of iron was made by dissolving pure wire in 10 ml. of 6 N nitric acid and diluting t o 2 -
TABLE I. COMPOSITION Fe Concn. P . p . 7n. 0.2 0.5
1.0 2.0
OF
PERhl.4XENT STANDiRDS
1Iatrrials in Solutian (1Iixture Diluted t o 100 111.) 0 . 2 0 1 If 0.00823 .If COCI? KgPtCla JfZ. Jf 1.
?.GO
1 20 3.03 7 30 20 00
11.50 22.60
0.250 31 COCI?
0.0167 .If FeCls
0.JJ
1.0 2.0
I G ,00 22.00 8 00
0.2 0.5 1.0 2.0
0 010 -1.I Cu (XH3ls( Hz0) Clr 0.40 1.25 4.50 12.00
0,;
4.00 8 50 16.00 31 .OO
I 400
I
I
440
48Q
I S20
I 560
Wffve Length Cmp,
551
I 600
bw
680
552
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 13, No. 8
acetone the range is 0.05 t o 5.0 p. p. m., and the law holds if the p H is maintained in the range 1.2 to 1.5,but not for higher acidities. EFFECTOF OXIDIZIXGAGENTS. The effect of persulfate, periodate, and hydrogen peroxide was studied in the presence of hydrochloric acid and acetone. The first two cannot be used, as they immediately oxidize the reagent and produce a yellow hue. Excess hydrogen peroxide produces the same effect on standing; if carefully controlled, this oxidant may be us:d. Nitric acid is the most practical, since it serves both as the oxidizing agent and for controlling the acidity. EXTRACTION OF COLOREDCOMPOUND.The color of the solution obtained by extracting the ferric thiocyanate complex with a mixture of equal volumes of amyl alcohol and ethyl ether has about the same intensity as that for a nitric acid-acetone solution of the same concentration of iron. The hue of the extract is more purplish, the peak of the absorption band being at 500 mp. Since extraction did not seem to improve the method appreciably, this complication was not continued. EFFECT OF DIVERSEIONS.h study was made of the effect of many of the more common ions upon the development of the iron-thiocyanate color. Unless otherwise stated, the measurements are for a solution containing 60 per cent acetone.
stabilized with mercuric thiocyanate, and is easily freed of any iron. The higher sensitivity is presumably due to the fact that thiocyanic acid and mercuric thiocyanate form a complex, H2Hg(SCn')+ Increasing amounts of reagent shifted the peak of the absorption band toward the red. For 0.5 ml. the minimum is at 460 m p and for 5.0 ml. it is at 480 mp. EFFECTOF SOLVENT. Addition of acetone as solvent increases the sensitivity of the method, the magnitude of the effect being shown in Figure 1. This change has been attributed to the lower dielectric constant of the acetone solution. Also, the error due to fading decreases from about 50 per cent in 20 hours for a water solution t o about 10 per cent in the same time for one containing 60 per cent of acetone by volume. Here, as elsewhere in this article, per cent error refers to the calculated concentration of iron. EFFECT OF p H AND KINDOF ACID. As trials with sulfuric acid seemed to reveal no advantages, most of the work was confined to nitric and hydrochloric acids. The latter is specified in the A. P. H. 8. method, but nitric acid seems preferable. With hydrochloric acid and the thiocyanic acid reagent, the optimum p H range is 1.3 t o 1.8, the color being less intense in either higher or lower acidities. With ammonium thiocyanate and hydrochloric acid the best range is 1.4 to 2.4 in water alone, or 1.2 t o 1.7 in 60 per cent acetone solution. With nitric acid and ammonium thiocyanate the acidity should be maintained below p H 1both in water and in acetone solutions. The lower limit was not accurately found; but since strong nitric acid itself gives a color with thiocyanate, the acidity should not be much greater than 1 N . RANGEOB CONCENTRAT~ON AND CONFORMITY TO BEER'S LAW. The curves in Figure 2 show that the workable range of concentration in water, with hydrochloric acid, is 0.1 t o 10.0 p. p. m. in a 1-cm. cell. A plot of the logarithm of the transmittancies at 478 m p against concentration shows that Beer's law holds, at least u p t o 10.0 p. p. m. With nitric acid and
Cations. Of the 27 cations studied only the aluminum, ammonium, beryllium, cerous, lithium, magnesium, manganous, potassium, and sodium ions do not interfere more than 2 per cent when present in concentrations 250 times that of the iron. Barium, calcium, lead, strontium, and zirconyl ions interfere because of the small solubility of their nitrates in the acetone solution. Silver and mercurous ions form insoluble thiocyanates and must, therefore, be entirely absent. In contrast, several cations interfere by forming soluble colorless complexes with thiocyanate. The presence of these ions, which include antimonous, cadmium, mercuric, and zinc, leads to a distinct bleaching of the color. Several ions, including chromic, nickelous, and uranyl interfere because of the ion's color. In addition to their own color, the interference of copper and cobalt arises from the formation of colored complexes with thiocyanate. Curves for solutions containing these two ions are shown in Figure 3. These complexes are not stable in water. In this colored group the ceric ion inter-
TABLE 11. EFFECTOF DIVERSE IONS Added
Ion Concn.
P. p . m. 0 500 20 20 250 B10z 50 200 500 500 50 50 50 Cr 10 500 50 0 50 50 50 50 10 500 PtOr 50 50 500 50 50 Sn 50 Sn
0 50 10 50
a b
50 10 Zr Calculated from 2 p. p. m. used. T o keep error within 2 per cent.
Iron Concn. P. p . m.a Ppts. 1.96 2.00 1.95 1.94 1.94 2.00 1.95 2.08 New hue New hue New hue New hue 1.95 1.96 Ppts. 1.64 Xew hue New hue 1.82 2.01 1.97 1.72 1.93 1.94 1.56 1.96 1.96 Ppts. Xew hue 1.97 1.9i 1.94 2.01
Permissible Concn. P. p . m.b 0 500 20 15 150 30 200 400 250 0 50
50 0 400 50 0
5 0 50 10
10 700 J
25 300 3 50 50 0
30 10 50
30 10
August 15, 1941
ANALYTICAL EDITION
553
includes the use of 10 ml. of 2 per cent ammonium thiocyanate, 2 ml. of 3 N hydrochloric acid, and the required amount of iron diluted t o 100 ml. Solutions of three mixtures of salts were used: cobaltous chloride and potassium chloroplatinate, each in 2.4 N hydrochloric acid; cobaltous chloride and ferric chloride, in 1 per cent hydrochloric acid; and hexamminocobaltic nitrate and pentamminoaquocobaltic chloride, each in 2.8 per cent ammonia. Table I shows the compositioii of the solutions used t o color match the standards which contained 2.0, 1.0, 0.5, and 0.2 p. p. m. of iron. Visual comparison of the solutions was made in 24-cm. Nessler tubes and the spectrophotometric curves were determined with cells 4.98 cm. thick. The curves for 24-cm. thickness were calculated with the color slide rule from the data for 4.98 cm. Figure 4 shows curves for solutions containing 0.2 and 0.5 p. p. m. of iron, together with the corresponding standards, at a thickness of 4.98 and 24 cm. After the cobaltammine standards stood 110 days, the slight precipitate that developed was filtered off and the curves were redetermined. The standards had faded somewhat in this period. They cannot be recommended, therefore, for use longer than a month after preparation. The calculated trichromatic coefficients and other colorimetric data for the iron thiocyanate and the different permanent standards are shown in Table I11 for a concentration of 0.5 p. p. m. of iron. Using the principle of least squares, it may be seen that the cobaltammine mixture is the best match for this concentration.
feres positively, the presence of 500 p. p. m. causing an error of 4 per cent. Summarized information on the effect of interfering cations is given in Table 11. Anions. Of the 30 anions studied acetate, arsenate, benzoate, bromide, carbonate, chloride, citrate, cyanide, formate, nitrate, phosphate, salicylate, silicate, sulfate, and tartrate do not interfere more than 2 per cent in concentrations 250 times that of the iron. Several ions, such as fluoride, oxalate, and pyrophosphate, form colorless complexes with the iron. Interestingly, the effect of fluoride is very marked in water solution but small in acetone. In order to keep the error within 2 per cent, the concentration must be less than 30 p. p. m. in water and 400 p. p. m. in acetone. Pyrophosphate should not exceed 5 p. p. m., and oxalate must be completely absent in water and not exceed 30 p. p. m. in acetone. Chlorostannite, iodide, nitrite, sulfite, and thiosulfate interfere by reducing the iron. Arsenite, chlorostannate, and tetraborate have a small bleaching effect. A change of hue is roduced by vanadate and dichromate, and the ]Patter oxidizes the reagent. Molybdate, on reduction to quinquivalent molybdenum, forms colored complexes with thiocyanate and must be absent. Tungstate interferes because of the low solubility of tungstic acid. Summarized information on the effect of interfering anions is given in Table 11.
Permanent Standards The permanent standards studied were designed t o match the color obtained by following the A. P. H. A. procedure. This
Vol. 13, No. 8
INDUSTRIAL AND ENGINEERING CHEMISTRY
554
TABLE 111. COLORIMETRIC V.4LCES FOR SOLUTIONS O F I R O N AND THIOCYANATE (0.5 P. P. M.) AND FOR CORRESPONDING COLOR STAND.4RDS
Solutions
-
Fet-SCNCOClZ K2PtC16j COCl*i FeClaJ
la)
Red -in
Least Square SunisC -4 B
Trichromatic Values Green Violet Bb A B 4 B
%
%
%
%
%
%
35.2 34.8
49 6 48.1
34.4 34.0
39.0 38.4
30.4 31.2
11.4 13 3
% ....
%
0.96
7.02
36.4
51.0
32.8
38.2
30.8
10.8 4.16
2.96
33.3
48 4
34.4
39.3
30.3
1 2 . 3 0.02
2 34
....
.\lonochromatic Values Colorimetric Doniiiiant Brightness Purity W a v length A B .4 B .\ B
+
yc
FeitT SCK81.9 COClZ \ 85.1 KzPtCh COCl*j 75.0 FeCla C Co(Ki€s)e(NOa)a o ~ S H a ~ s ~ H 2 0 ) C l a }83.2
%
%
Tc
M p
M p
44.0 51.1
19.0 16.5
69.5 64.0
583 5S4
590 590
43.4 48.6
17.5 19.0
71.0
598 584
59‘2 3 9
67.0
Values for 4.98-cni. cell. b Values for 24.0-em. cell. c Square of difference between values for iron and standard
Ammonium thiocyanate is preferable to thiocyanic acid as a color-forming reagent. Xitric acid is preferable to sulfuric or hydrochloric acid. With this acid Beer’s law is followed through the pH range 1.2 t o 1.5, but not in higher acidities. Many of the 57 diverse ions studied interfere. The sensitivity is increased approximately 100 per cent by using a 60 per cent acetone solution. This compound also improves the stability of the color. -4 solution of two cobaltammine salts is an improved color standard except for a slight fading after several months. In general, the thiocyanate method is inferior to several others, especially those using o-phenanthroline, a,a’-bipyridyl, or mercaptoacetic acid. TABLE IV. COMPARISON OF REAGESTS Itein Compared Fe concii. a t 50% transmission= W‘orking rangc, 1-rin. cell Stability pH range Effect of pH change Color of reagent Effect of excess reagent Conformity to Beer’s law
Discussion I n addition t o seeking the best conditions for carrying out the thiocyanate method, this investigation was undertaken t o evaluate the method in comparison with several other procedures which have been studied recently. Table IV presents comparative data for the following reagents : a,a’-bipyridyl. ferron (19), mercaptoacetic acid (do), o-phenanthroline (6), salicylaldoxime (8), salicylic acid (11), and thiocyanate. The thiocyanate method is inferior in several respects to several of the other methods. For general work o-phenanthroline is probably the best reagent in the list. Mercaptoacetic acid is recommended for total iron. Although either of these reagents is superior, in general, t o thiocyanate, such conclusions may be subject to reservation. A given reagent might be best under some conditions but not under others. I n the determination of iron with thiocyanate the following variables must be kept reasonably constant: amount of reagent, amount and kind of acid, use of excess oxidizing agent, time of standing, presence and amount of certain interfering ions, and dielectric constant of the solvent. Since the intensity of the color formed depends upon the amount of reagent used, this factor must be carefully controlled. The amount and kind of acid, especially with hydrochloric or sulfuric, greatly affect the color intensity. With nitric acid the color is the same over the range 0.3 to 1.2 N , but there is some divergence from Beer’s law. A p H range of 1.2 to 1.5 with this acid yields a more intense color which conforms t o Beer’s law, but the sensitivity to change in p H makes this impractical in analysis. Since the color fades about 10 per cent in 24 hours, matching must be made within 4 to 5 hours to keep the error less than 2 per cent. The use of acetone not only increases the sensitivity but also may decrease interference by some ions, especially fluoride; however, it introduces difficulties in case of salts which are somewhat insoluble in the acetone-water mixture.
Summary By means of a spectrophotometer a critical study has been made of the colored system resulting from the interaction of ferric and thiocyanate ions. From several hundred spectral transmission curves obtained, of which those shown are representative, the following conclusions, relating t o the use of this procedure for the colorimetric determination of iron, are evident :
Fe cpnFii. a t 50% transmission“ Working range, 1-em. cell Btabilitv pH ranie Efiect of p H change Color of reagent Effect of excess reagent
Ferron
Mereaptoacetic Acid
Salicylic Acid
Salicylaldoxime
4.5h
4.3b
1O.lb
3 4b
0.1-54b 9 days 2 7-3.1 Changes hue Tellow Changes hue
0.1-145 14 hours 7-12
0.2-45b 65 hours 2.5-2.7
O.l-4’2h 24 hour5 6.2-6 (i
None
Changes intensity Colorless Changes intensity
None o-Phenanthroline
Close a,a’-Bipyridyl
Close Closr Thiocyaiinte 607, acetone Watrr
1.71,
2.0b
1.2b
0.05-6b 6 months 2-9 None Colorless None
0.2-17b 6 months
0 05-5.0b 0.1-10h 4 hours 2 honrs . ~ ~ 0.3-1.2 S C 0.3-1.2.YC Fades Fades Colorless Colorless Changes Changes intensity intensity
Noiie Colorless
3.5-8.5
None Colorless h-one
Conformity to Beer’s law Close Close Limited a For 1-em. cell and maximum absorption of curve. b P. p , m. of iron. c Sitric acid.
Change hue Colorlrs, Snnc
2.4b
Limited
Literature Cited Am. Public Health Assoc., “Standard Methods of Water Analysis”, p. 74 (1936). Am. Soc. Testing Materials, “Standard AVethods”, Vol. 11, p. 685 (1939). Assoc. Official Agr. Chem., Official and Tentative Methods of Analysis, p. 536 (1940). Bent and French, J . A m . Chem. SOC.,63,568 (1941). Bernhard and Drekter, Science, 75, 517 (1932). Fortune and Mellon, IND. ENG.CHEM.,Anal. Ed., 10, 60 (1938). Hardy, “Handbook of Colorimetry”, Cambridge, Technology Press, 1936. Howe and Mellon, IND. EHG.CHEM..Bnal. Ed., 12,448 (1940). Jackson, Tech. Quart. (M. I. T.), 13,320 (1900). Marriot and Wolf, J . Biol. Chem., 1, 451 (1906). Mehlig, IND.ENG.CHEM.,Anal. Ed., 10,136 (1938). Mellon, “Methods of Quantitative Chemical Analysis”, p. 413, New York, Macmillan Co., 1937. M@ller,Kem. Maanedsblad, 18,138 (1937). Ossian, Pharm. Centr., 13,205 (1837). Peters, MacMasters, and French, IND.ENG.CHEM.,Anal. Ed., 11, 502 (1939). Schlesinger and Van Valkenburgh, J . Am. Chem. SOC., 53, 1212 (1931); Schlesinger, Ibid., 63, 1765 (1941). Snell and Snell, “Colorimetric Methods of Analysis”, Vol. I, p. 295, New York, D. Van Nostrand Co., 1936. Stokes and C a i n , J . Am. Chem. SOC.,29,409 (1907). ENG.CHEY.,Anal. Ed., 9,406 (1937). Swank and Mellon, IND. Ibid., 10,7 (1938). Swank and Mellon, J . Optical Soc. Am., 27, 414 (1937). U. S. Pharmacopoeia, p. 141 (1939), supplement. Velichkovskaya, J. Applied ‘Chem. (U. S . S . R.),12, 1425 (1939). Winsor, IND.ENG.CHEM.,Anal. Ed., 9,453 (1937) Yoe, “Photometric Chemical Analysis”, Vol. I, p. 218, Xew York, John Wiley & Sons, 1928. A B ~ T R A C T from E D a thesis s u b m t t e d t o t h e Graduate School of Purdue University b y J. T. Woods in partial fulfillment of t h e requirements for t h e degree of doctor of philosophy.
.
