The Determination of Iron with Mercaptoacetic Acid H. F.SR.4NK1 WITH 31. G . IIELLON Purdue University. Lafayette, Ind.
Td ) ,
H E reaction betveen mercaptoacetic acid and iron to give a reddish purple color has been known many years (1, but it was not until 1927 that Lyons ( 7 ) applied it to the quantitative determination of this metal. (Mercaptoacetic acid is given a number of names in the literature, such as thioglycolic acid, thioethanoic acid, and thiolacetic acid. Since the reaction occurs in ammoniacal solution, the reagent might more properly be called ammonium mercaptoacetate.) He attributed the color to a complex with ferrous iron and believed that ferric iron is reduced to the ferrous condition by the mercaptoacetic acid, thus accounting for the fact that the color is a function of the total iron present. Cannon and Richardson (3) found that ferric iron in the absence of oxygen gives a red color with this reagent which fades at a rate proportional to the hydrogen-ion concentration over a p H range of 6 to 11, and that ferrous iron gives no color under the same conditions. They concluded that both ferrous and ferric iron form complexes which exist in equilibrium. Various individuals (2, 6, 8, 10) have adapted the method to the determination of iron in biological materials and foods. Lyons ( 7 ) investigated the effect of a number of other ions on the color. Hanael ( 5 ) reported the noninterference of considerable amounts of ortho- or pyrophosphoric acid. The purpose of the present paper is to describe a quantitatire study of the procedure, including the effect of the common ions, made with the more accurate means of color measurement now available.
the interfering ion. The spectrophotometric8 method was supplemented with visual obqervation for ions which produced color.
The Color Reaction A preliminary study of the color reaction showed the intensity of the color to be independent of the form in which the reagent is added and of its concentration. providing a n excess is present. Also the concentration of ammonium hydroxide can be varied over a wide range without any noticeable effect. The particular conditions used for this study gave a p H of 10.1 for the solution after dilution to 100 ml.
Apparatus and Methods A11 color measurements were made with a recording General Electric spectrophotometer in the same manner as described previously (9). A 10 per cent solution, by volume, of mercaptoacetic acid, neutralized with ammonium hydroxide, was used for most of the work. Two milliliters of this reagent and 10 ml. of 3 JI ammonium hydroxide were used for each determination to make a final volume of 100 ml. Standard solutions of iron were prepared by dissolying weighed amounts of iron wire or ferrous ammonium sulfate in sufficient hydrochloric or sulfuric acid to prevent hydrolysis. A concentration of 0.10 mg. of iron per 100 ml. was used for most of the studies on interference. The solutions of cations and anions were prepared from reagent quality salts. Blank determinations were made to ensure absence of iron. All amounts of interfering substances mentioned refer to a volume of 100 ml. The limiting amounts for interfering ions were calculated for a permissible error of approximately 3 per cent of iron. This error, for ions which produced only a change in intensity, was calculated by means of Beer’s law expressed in the form
T1is the transniittancy for an iron concentration of C1, at a selected wave length. T2is the transmittancy for the same wave length and iron concentration with the interfering ion present. C2 is the apparent concentration in the presence of Present address, E. I. du Pont de Nemours & Company, Inc., Buffalo,
N. Y
Various individuali have reported the color to be stable for 30 minutes. According to some obserrers, the original color intensity, after slight fading has occurred, can be restored by agitation with air. The authors found the color to be relatively stable. Solutions protected from light showed no evidence of fading after 12 hours and were stable for a t least 6 hours when exposed to diffuse daylight. These solutions were stored in Erlenmeyer flasks which exposed considerable surface of the liquid to the air. This may account for the greater permanence over that previously reported. Calculations based on the transmittancy data for a wide range of concentrations showed that the color follows Beer’s law very closely, thus allowing the use of a variable depth method of color comparison.
The Effect of Anions The mercaptoacetic acid method is remarkably free of interference by the common anions, many of which must be entirely absent for other colorimetric methods. The following
8
INDUSTRIAL AND ENGINEERING CHEMISTRY
ions, in concentrations of 500 mg. in 100 ml. of solution, had no effect on the color: fluoride, iodide, nitrate, orthophosphate, sulfate, chlorate, tartrate, oxalate, citrate, acetate, bromide, thiocyanate, sulfite, and chloride. Two hundred fifty milligrams of boron trioxide, present 8s tetraborate ion, also had no effect. Pyrophosphate ion, when present in an amount equivalent to 500 mg. of phosphorus pentoxide, decreased the color intensity about 8 per cent. A concentration of 200 mg. gave a decrease of 3 per cent. It is evident that 200 t o 300 mg. can be present without serious error. This quantity is smaller than that reported by Hanzel (5) but still well above that usually encountered in an analysis. Cyanide ion interfered seriously and must be absent. Nitrite ion, when added to an acidic solution of iron, followed by the reagent and ammonium hydroxide, produced a deep orange co!or. When this ion was added after the solution was made ammoniacal, no color appeared, indicating that the color was caused by the nitrous acid. Molybdenum, in the form of molybdate, formed a yellow or orange color a t high concentrations. Up to 2 mg. of molybdenum for 0.10 mg. of iron had no appreciable effect. Tungsten, in the form of tungstate, produced a blue color, but amounts of 2 mg. did not interfere. Arsenic, in the form of arsenate, had no appreciable effect in amounts up to 50 mg. of the metal.
VOL. 10, NO. 1
Figure 2, which limited the amount of lead to 0.10 mg. for
0.10 mg. of iron. Manganous ion produced an amber colorwhich faded rapidly
on standing a few minutes. Stirring caused the color to reappear, followed by fading again. Up t o 10 mg. of manganese caused no interference when the solution was allowed to stand undisturbed for a few minutes. Barium, calcium, and strontium in amounts up t,o 200 mg. caused no trouble when no ion was present which would produce a precipitate in ammoniacal solution.
The Effect of Cations Cobalt produced a yellow or red color, depending upon the concentration. Mercaptoacetic acid is nearly as sensitive t o it as to iron, as shown by transmittancy curves 1 and 2, Figure 1, for 0.10 mg. of each of these metals. Not more than 0.002 mg. of cobalt can be present for 0.10 mg. of iron without changing the hue and introducing an error of more than 3 per cent. Kickel reacted with the reagent to give a color similar in hue t o that produced by iron but not nearly as intense, as shown in curve 3, Figure 1. With 0.10 mg. of iron 0.01 mg. of nickel can be present. Copper, on addition to mercaptoacetic acid in hydrochloric acid solution, formed a white precipitate which dissolved to give a nearly colorless solution after an excess of ammonium hydroxide was added. The presence of copper with iron caused a bleaching effect, shown in curve 4, Figure 1, but this was not appreciable for amounts below 1 mg. Tervalent antimony had no effect in amounts up to 100 mg., but more than this caused a precipitate to form. Tervalent arsenic bleached the color a t concentrations greater than 100 mg. in 100 ml. Bivalent tin, which ordinarily precipitates with ammonium hydroxide, remained in solution in the presence of the reagent, presumably with the formation of a soluble complex ion. Up to 20 mg. of tin did not affect the accuracy, but larger amounts bleached the color. The use of more reagent partly overcame the bleaching action. Zinc did not form a colored complex but did react as shown by curve 2, Figure 2. By adding 6 ml. of reagent t o provide an excess over that used by the zinc and iron, the full intensity of the color was maintained, as shown in curve 3, Figure 2 . If the color intensity were dependent on the exact concentration of the reagent, as in the thiocyanate or ferron methods, the addition of a large excess of reagent could not be used to eliminate interference from metals which react but do not form a color. At least 200 mg. of zinc can be present for 0.10 mg. of iron. Cadmium behaved in the same manner as zinc. Two hundred milligrams caused no interference when 8 ml. of reagent were used. Lead formed a yellow or amber color, shown in curve 4,
Magnesium did not interfere in quantities as high as 250 mg. although it was necessary to add ammonium chloride to prevent the hydroxide from precipitating. Bismuth, which precipitates with ammonium hydroxide, remained in solution in the presence of the reagent. A yellow color formed which limited the amount to about 0.2 mg. for 0.10 mg. of iron. Mercurous ion formed a black precipitate but mercuric salts had no effect up to 30 mg. Uranyl ion produced an intense orange color. More than 0.02 mg. made a color match impossible. Gold gave an amber color which was not appreciable for amounts below 0.2 mg. An amber color which was not reproducible or proportional to the concentration was produced by silver. Only about 0.2 mg. can be present with 0.10 mg. of iron. Aluminum and chromium were precipitated by ammonium hydroxide. Sodium, potassium, and ammonium ions had no effect a t low concentrations. Extremely high concentrations of salts caused a slight decrease in the color intensity.
Discussion It is evident from the data presented that the mercaptoacetic acid method possesses a number of advantages not found in most other colorimetric methods for total iron. Conformity to Beer’s law and independence of the color with respect to the exact reagent concentration and the pH make the method easy and rapid to use. The color, while not extremely stable, does not fade so rapidly that color comparisons are difficult. One set of standards can be used for a number of determinations. At the present time several large manufacturers are using this method for routine analyses. With respect to interference from anions, the method is much superior to those which are based on reaction with ferric iron. It is almost completely free from interference by
JANUARY 15, 1938
ANALYTICAL EDITION
phosphate, pyrophosphate, fluoride, tartrate, citrate, and oxalate ions, all of which exhibit a strong tendency to form stable complexes with ferric iron. For the analysis of materials which contain phosphates, the method is especially to be recommended. A number of metals interfere, but some of these are seldom found in appreciable amounts with iron. A more serious fault is the use of an alkaline solution, which precipitates many metals. The limiting amounts of interfering ions are specified for a volume of 100 ml. and an iron content of 0.10 mg. With smaller amounts of iron, the apparent interference will be greater for some metals, thus lowering the amount that can be present without serious interference.
Conclusions The effect Of the common cations and anions On the mercapt,oacetic acid method for iron has been studied, as well as
9
the general conditions affecting such methods. The lack of interference by nearly all anions and the reproducibility and sensitivity of the color reaction make the method superior to various other colorimetric procedures for iron.
Literature Cited Andreasch, Ber., 12, 1391 (1879). Burrnester, J . Biol. Chem., 105, 180 (1934). Cannon and Richardson, Biochem. J., 23, 1242 (1929). Claesson, Ber., 14, 412 (1881). Hanzel, Proc. SOC.Ezptl. Bid. ‘Wed., 30, 846 (1933). Leave11 and Ellis, IXD.ENO.CHEW,Anal. Ed., 6 , 46 (1934). Lyons, J. Am. Chem. Soc., 49, 1916 (1927). Snell and Snell, “Colorimetric Methods of Analysis,” Vol. I, p. 298, New York, D. Van Nostrand Co., 1936. (9) Swank and hfellon, IXD.Exo. CHEhI., Anal. Ed., 9, 406 (1937). (10) Tompett, Biochem. S.,28, 1536 (1934). (1) (2) (3) (4) (5) (6) (7) (8)
RECEIVED October 7, 1937. Abstracted from a portion of a thesis submitted by H. W. Swank t o the Graduate School of Purdue University in partial fulfillment of the requirements for the degree of doctor of philosophy.
Turbidity in Sugar Products VI.
Generalized Method and Formulas for the Determination of Color and Turbidity in Colored Media F. W. ZERBAN AND LOUIS SATTLER, New York Sugar Trade Laboratory, New York, N. Y.
THE
recent work of the writers (3) on the turbidity and color of white sugars has shown that the absolute turbidity, calculated according to Sauer’s system (d), is directly proportional, within the limits of error of the method, to the turbidity found by the method of the writers with this type of sugars. This fact suggested a reexamination of the data obtained with raw sugars, where no such proportionality had been observed ( 5 ) . The writers have therefore calculated the absolute turbidity of the 21 sirup mixtures containing known by the formula of proportions of color and turbidity (4, Sauer : Absolute turbidity (8) = A f k D t (1)
coefficients and for varying thickness has been published by Landt and Witte (1). When the logarithms of fk are plotted against the extinction coefficient k a t constant thickness, a nearly straight line is obtained, starting a t f k = 0, and k = 0, and satisfying approximately the equation fk
=
(3)
mk
where m is a constant showing slight fluctuations. The values of Sauer’s f k , a t a thickness of 2.455 mm. and corresponding to the extinction coefficients of the 21 mixtures, are shown in Table I, column 5, and the absolute turbidities, S, calculated by Equation 1, in column 6. Three of the mixtures-viz., those highest in turbidity-show absolute turbidities well above unity, which is an impossibility because the intensity of the Tyndall beam cannot be greater
where A is the relative Tyndall beam intensity, measured with the Pulfrich Dhotometer. and equals 0.01 R in the system used by the writers. D is a factor varying with the thickness of the absorption cell, and equals 6.6395 for the 2.455-mm. cells used; t is the abTABLEI. COMPARISON BETWEEN TURBIDITY DATA solute turbidity of the standard glass block of According t o Sauer’s system and the system of Zerban and Sattler, for mixturea known proportions of turbidity and coloring matter. the instrument, in this case 0.00282 for the 1 2 3 4 5 green flter. The factor fk is a function of the 6, 7 “ B&d B&id extinction coefficient (-log T for 1-em. thickComposition of on fk;; on ness) of the solution measured. KO. Mixtures N C fk fk fk’ The relation between f k and k has been de5 U O F O W 0.5155 0.4276 13.288 2.2724 3 . 3 6 7 0.5758 4 U: 1 F : 0 W 0 . 4 3 7 8 0 . 4 1 7 0 10.450 1.5636 3 . 2 6 8 0 . 4 8 9 0 rived by Sauer from theoretical considerations, 8 . 2 0 5 0.7573 3 . 6 9 2 0.3408 3 U, 2 F, 0 W 0.3051 0.4601 6 . 0 6 3 0.4176 3 . 4 8 2 0.2398 2 U 3 F O W 0.2147 0.4395 and is expressed (1) by the following formula: 1 U: 4 F: 0 W 0 . 1 1 7 9 0 . 4 5 2 8 4 . 8 2 6 0 . 1 7 5 8 3 . 6 1 6 0.1317 kd fk
10
-kd
(d- 1) X {1 -
10-kd
2.30269
(42
- l)1
(2)
where d is the depth of layer, in centimeters. The form of this equation would seem to indicate that fk equals 0 when k equals 0. It must be remembered, however, that k is a logarithm; hence log fk equals 0 for k equal to 0, and f k itself equals 1 under that condition. A table of fk values for varying extinction
11 12 13 14 15 16 17 18 19 20 21
containing 9
,_I
Jli
OU,5F,O W 4U,OF,lW 3 U 1F 1 W 2 U: 2 F: 1 W 1 U, 3 F , 1 W
0.0287 0.4175 0,2932 0.2105 0.1259
0.4245 0,3495 0.3764 0.3684 0.3545
3.497 8.245 6.323 4.935 3.768
0.0336 1.3872 0.7113 0.4077 0.1936
3.341 2.764 2.911 2.846 2.737
0.0321 0.4664 0.3275 0.2351 0.1406
from Sauer’a Formula 3.259 3.166 3.563 3.370 3.493 3.219 2.629 2.831 2.770 2.666
OU 4 F 1 W 3 U’OF’2W 2 U’ 1 F ’ 2 W 1U12F’2W O U ’ 3 F ’ ZW 2 U ’ O F ’ 3W 1 U: 1 F : 3 W 0 U, 2 F , 3 W 1 U, 0 F , 4 W 0 U, 1 F , 4 W 0 U, 0 F, o TV
0.0264 0.3200 0.2086 0.1161 0.0303 0.2224 0.1154 0.0151 0.1035 0.0115 0.0036
0.3371 0.2581 0.2609 0.2699 0.2440 0.1510 0.1941 0.1729
2.733 4.924 3.657 2.828 2.138 2.808 2.355 1.684 1.700 1.299 1.000
0.0310 0.8456 0.4060 0,1752 0.0362 0.4542 0.1750 0.0174 0.1530 0.0131 0.0040
2.603 2.081 2.098 2.092 2.001 1.535 1.734 1.635 1.284 1.267 1.000
0.0295 0.3574 0.2330 0.1297 0.0338 0.2484 0.1289 0.0169 0.1156 0.0128 0.0040
2.541 2.044 2.060 2.054 1.966 1.521 1.713 1.615 1.277 1.258 1.000
0.0880
0.0827
0.0000
