INDUSTRIAL -4ND ENGINEERING CHEMISTRY
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TABLEI.
SAPONIFICATION NUMBERSDETERMINED BY PRESSURE-AGITATION METHOD
Sample Crude petroleum acids from still bottoms (black)
7
1 2 3 4
Acidified residues from lubricating oil distillations (black)
1 2 3
4
5
Oleic acid (saponification No. 193.2 by standard method) Stearic acid (saponification Xo. 196.8 by standard method) China wood oil (saponification N o . 193.0 by standard method)
THE
Saponification Number Found 38.6 38.8 38.8 39.0 Av. 38.8 16.0 16.8 16 8 16.6 14.8 Av. 16.2 193.0
1 2 3
Cottonseed oil (saponification No. 210 by standard method)
1 2 3 4
Neats’-foot oil (saponification No. 193.0 by standard method)
1 2 3
197.0 202.0 204.0 202,o Av. 202.6 221.0 219.0 218.0 222.0 Av. 220 0 202.0 200.0 205.0 .4v. 2 0 2 . 3
intimate contact is obtained with the alcoholic layer. This contact between the two layers is obviously increased more than a hundred fold by the continuous agitation. Upon cooling to room temperature (without agitation), the saponified black residue becomes very viscous and sticks to the bottom of the pressure flask, while some of the white oil remains dissolved in the alcohol layer. Thus, in spite of the excellent contact afforded between the alcoholic and the asphaltic layers during the saponification, the solubility of the black material in the alcohol layer remains sufficiently small t o prevent serious discoloration of the top layer. Hence, the supernatant alcoholic layer may be poured from the pressure vessel into an Erlenmeyer flask or a white casserole and titrated with 0.1 N hydrochloric
VOL. 8, NO. 4
acid, using phenolphthalein or alkaline blue as an indicator. When the end point has been reached, some of the liquid from the titration container is carefully poured back into the pressure flask and the black residue on the inside of the flask is washed with this neutral liquid. The rinse is then returned to the caaserole and a new end point is determined. The black insoluble residue, therefore, remains in the pressure flask and is thus kept entirely separate from the alcoholic layer during the titration. In order to obtain a yellowish or a light brownish color of the solution prior to the titration, it is important to add only moderate amounts (1 t o 2 parts) of white oil to the asphalt. Among the advantages of the suggested method the following may be mentioned : 1. The determination of the exact end point is greatly facilitated, since the titration is carried out in the absence of the saponified black residue. 2 . Even for residues of the highest viscosities and molecular weights, a complete saponification is obtained in 30 t o 60 minutes because of the continuous mechanical agitation and higher temperatures employed. 3. Since the saponification is being conducted in a closed system, interference of atmospheric carbon dioxide is eliminated, losses of low-boiling esters are prevented, and errors known to be contributed (2) by cork stoppers are avoided.
Results Obtained Some of the results obtained by the new method are given in Table I.
Literature Cited (1) Albert, K., Albertschrift, No. 15, p. 55. (2) Baader, Chem.-Ztg., 50, 891 (1926); Erddl u. Teer, 4, 234-5, 252-3 (1928). (3) Hicks-Bruun, M. M., and Claffey, L. W., IND. ENG.CHEW, Anal. Ed., 8, 229 (1936).
RECEIVED February 12, 1936.
Determination of Iron in Humates Use of Iodohydroxyquinolinesulfonic Acid NORMAN ASHWELL CLARK
T
AND
DALE H. SIELING, Iowa State College, Ames, Iowa
H E determination of iron in small quantities in organic material involves two procedures : (1) the oxidation of the organic matter, and ( 2 ) the determination of the iron, usually colorimetrically. The removal of the carbon may be carried out by heating (the dry method) or by a wet oxidation. The colorimetric determination most frequently involves potassium thiocyanate; comparison is made between the unknown and a standard iron solution similarly treated. The color produced by the iron and the thiocyanate fades, and fresh standard solutions must be made up a t frequent intervals. In carrying out the dry oxidation of the organic matter in humate material from soils, followed by the potassium thiocyanate determination, some difficulty was experienced in obtaining checks. Burk (1) has pointed out the value of these humic acid extracts from soils, and also of synthetic humates formed from carbohydrates, as carriers of iron, and has shown the availability of this iron to green plants even under alkaline conditions. In inorganic combination iron hydrolyzes very slowly a t about p H 3, but a t pH 5 and above ferric hydroxide is precipitated more rapidly. Unlike iron in the inorganic form, the iron in the humic acid extracts and in the synthetic humates does not precipitate a t pH 5 ; there
is no hydrate of ferric oxide formed, even to pH 8 or 9; t h e iron, therefore, remains in solution over the ordinary plant growth range. Iron can be added to the humate extracted from soils and is either adsorbed or combined; this added iron does not precipitate a t pH values greater than 5 . I n a number of experiments along these lines (2) the authors found it necessary to determine very accurately the quantity of iron in the humate before and after enrichment with that element. Results by the dry oxidation of the carbon followed by the potassium thiocyanate were irregular, and iron added as a check was not always completely recovered. I n 1932, Yoe suggested 7-iodo-8-hydroxyquinoline-5-sulfonic acid, C~HIN(OH)I(SO~H), as a reagent for the colorimetric determination of iron (4). The name “Ferron” has been proposed for the indicator ( 3 ) . With this dye, which is obtainable commercially, ferric iron gave a green color and ferrous did not react. A concentration of 1 in 10 million gave a greenish yellow, with higher amounts more green. The color was very stable, and therefore it was not necessary to prepare standard and sample a t the same time as with the thiocyanate. Yoe found that the color developed best when the solution was acid to methyl orange paper. He tested some
JULY 15, 1936
ANALYTICAL EDITIOY
257
TABLE I. IRON IN POTASSIUM HUMATE p -H --@ ---Total Iron Present Amounts Oxidized 0.1 .y KOH 0.1 N K O H Standard F e Fe in Standard Fe, to bring 10-cc. for 25-cc. (2 mg. per liter) 70-cc. F e in Potassium 2 mg. per aliquot t o p H aliquot to match 25-cc. oxidized F e in Humate solution Blank Sample Solution humate liter 2.75-3.2 (calcd.) aliquot b cc. c c. c c. c c. Cc. Mo . Mg. Mg. M g ./cc. 11.7 29.2 19.0 0 50.0 0.106 0.006 0.100 12.7 31.7 19.0 0 50.0 0.106 0.006 0.100 19.0 13.5 33.7 0 50.0 0.106 0.006 0,100 60.0 0 10.6 26.5 7.0 0.039 0.006 0 033 0.0007 50.0 0 9.3 23.2 7.0 0.039 0.006 0,033 0.0007 50.0 0 9.8 24.5 7.0 0.039 0.006 0.033 0.0007 25.0 11.1 27.7 13.0 25.0 0.073 0.006 0.067 0.0007 11.4 28.5 13.0 25.0 25.0 0.073 0.006 0.067 0.0007 12.3 25.0 30.7 13.0 25.0 0.073 0.006 0.067 0.0007 Potassium hydroxide added until faint blue t o bromophenol blue when diluted t o 40 cc. 0 . 8 , ~ 0.1 ~ . N sulfuric acid brought this t o p H 2.75 t o 3.2. T h e difference, multiplied b y 2.5, gave the net cc. of potassium hydroxide t o be added t o the 2 5 - 0 0 . aliquot before making up t o 100 cc. for the Nessler tube. b Diluted t o 100 cc. a t p H 2.75 t o 3.2; 1 CC. of color reagent, 0.2 per cent. 7
70 other ions and obtained no color. Cupric ion produced a precipitate, and a few salts which hydrolyze, such as tin and titanium, needed removal if present in more than very small amounts. Strong acids and bases destroyed the color. Combined Method It was decided to combine this method with the wet oxidation, by means of hydrogen peroxide, which Koch and McMeekin used for micro-Kjeldahl determinations. (A mixture of perchloric and nitric acids should prove equally effective as an oxidizing agent for removing the organic matter, as it is improbable that the nitrate or perchlorate ions would affect the green color.) The side arms of small Pyrex distillation flasks were sealed off and the flasks calibrated to hold 70 cc. The organic material, containing approximately 0.1 mg. of iron, was placed in the flasks with 5 cc. of 10 per cent sulfuric acid, with a bead to prevent bumping, and heated until white fumes came off; after cooling for a minute, a few drops of 30 per cent hydrogen peroxide were allowed to fall directly into the acid, and the heating was continued until the sulfur trioxide condensation was within 5 cm. (2 inches) of the top of the neck. The hydrogen peroxide treatment was repeated if the li uid was not clear. The solution was made up to 70 CC. a t 24” and aliquots were used for the determination of the iron. Yoe stated that best results were obtained if the solution was made acid to methyl orange paper. Aliquots were therefore treated with 0.4 N potassium hydroxide until just acid to the paper, and methyl orange was also used as an internal indicator, potassium hydroxide being added until a faint pink color was obtained; the same amount of potassium hydroxide was then added to a second aliquot and the iron determined in this. In both cases the results were somewhat unsatisfactory. There was a large color change in the green produced by the reagent when a slight excess of either potassium hydroxide or acid was present.
8.
I n order to find the limits for change in color with acidity, a standard iron solution was made from recrystallized ferrous ammonium sulfate, oxidizing with bromine to the ferric ion, and removing the excess bromine. Varying amounts of sulfuric acid and potassium hydroxide were added and the colors compared. Between p H 2.7 and 3.2 there was a stable green that was constant for each different,amount of iron; more alkaline solutions produced a yellow and more acid a blue-green. The blue-green is stable a t least to p H 2, but for comparisons, the authors preferred the color produced a t the h.igher value. The standard iron solutions were therefore ma’de up within the p H limits 2.7 to 3.2, and if dilution was necessary it was made with distilled water acidified to the same value. Yoe has suggested a hydrochloric acid-potassium acid phthalate buffer ( 3 ) . The treatment of the unknown solutions was also modified to conform with this finding. After digesting the organic matter with sulfuric acid and hydrogen peroxide and making up to 70 cc. a 10-cc. aliquot was removed and 0.1 N potassium hydroxide added until a faint blue color was produced with 2 drops of bromophenol blue as internal indicator, when diluted to 40 cc. with distilled water; 0.8 cc. of 0.1 N sulfuric acid brought this to the range 2.7 to 3.2. The amount of sulfuric acid subtracted from
the potassium hydroxide left the net amount of potassium hydroxide required. A fresh 25-cc. aliquot of the oxidized solution was then used, with t v o and one-half times the amount of potassium hydroxide found for the 10-cc. sample, and brought exactly to 100 cc. for nesslerization. With the use of a glass electrode to adjust the pH, the indicator is not necessary, but the bromophenol blue method is more rapid. Test runs were made on a number of materials, including iron citrate and a mixture of iron citrate and sucrose. Table I gives the results for a n extract of soil with potassium hydroxide-the potassium humate. The alkali extract of soil contained a small amount of silicate, but this did not interfere, as shown in Table I. To check this further a run was made using the standard iron solution, but adding 0.1 to 1mg. of silica as sodium metasilicate. The iron was recovered completely at each silicate level. The standard iron solution, from the ferrous ammonium sulfate oxidized by bromine, deteriorates slowly if the reaction is much greater than p H 2. Two liters were made up a t p H 2.9; these were tested, at first daily and later every 2 days, against a freshly oxidized solution. The standard was constant for 3 days, but the ferric ion slowly decreased thereafter. The permanency of the green color, when this standard is treated with the iodohydroxyquinolinesulfonic acid a t p H 2.7 to 3.2, is almost indefinite. Solutions were made up containing 0.02, 0.04, and 0.06 mg. of iron in 100 cc., and the green color was produced with the reagent. No change in color was found at any level of iron during the 25 days these were tested against freshly made solutions. The method seems to be a valuable addition to the colorimetric determination of iron because of this color stability, and because of the fact that there is so little interference by other ions-a very desirable characteristic in dealing with soil extracts.
Literature Cited (1) Burk, D., Lineweaver, H., and Horner, C. K., Soil Science, 33, 413-53 (1932). (2) Clark, N. A., and Roller, E. M., Ibid.,31, 299-309 (1931). (3) Hahn, P. F., and Whipple, G. H., Am. J . M e d . Sci., 191, 24-42 (1936). (4) Yoe, J. H., J . Am. Chem. Soc., 54, 4139-43 (1932).
RECEIVED April 27, 1936. Presented befoie the Division of Physical and Inorganic Chemistry a t the 9lst Meeting of the American Chemical Society, Kansas City, M a , April 13 to 17, 1936.
CORRECTION.In the article on “Spectrophotometric Determination of Copper in Ores and Mattes” [IND.ENG.CHEM., Anal. Ed., 7, 388 (1935)] the third line should read: “The ratio
2
may be evaluated.” J. P. MEHLIG
