did not require initial irradiation hardening since the two pieces were held together by aluminum wire. The chromatographic strips are placed in the polyethylene boron brick and inserted into a vertical irradiation tube containing a 20-mil cadmium liner. After irradiation, the strips are scanned for the location of the **Ppeaks from the 32S(n,p)32Preaction. The strips are then irradiated with no shielding to locate the 32Ppeaks from the 31P(n,y)32P reaction. Irradiation Effects on Boron-Polyethylene Material. Irradiation containers constructed from the boron-polyethylene should be limited to relatively short irradiation periods because of two reasons. First, the boron-10 nuclear reaction, 1oB(n,~)7Li, results in tremendous deposition of energy into the polyethylene matrix by the recoil energy of the alpha particle and the lithium-7 nucleus. An experiment was conducted in which the above mentioned irradiation brick was exposed to a total thermal neutron dose of about 5 X loLenvt over a three-hour period. Some melting and fusing of the material occurred caused primarily by the heating effect of these recoiling activation products. In the other measurements involved in this work, irradiation doses did not exceed 3 X 1015nvt each time over a 9-minute period. In these tests, no such heating effects were observed. The second factor that limits the use of these irradiation
containers is the deterioration of the polyethylene matrix with accumulative neutron doses in excess of about 10’’ nvt. This deterioration results in a serious swelling and loss of tensile strength. Therefore, the authors have limited the use of this material to the construction of containers for short irradiation periods. Suggested Methods for Improving This Technique. The first step in improving the manufacturing of these rabbits and other types of polyethylene-boron irradiation containers would be to obtain material made from boron instead of the boron carbide. In addition, if the boron is enriched with boron-10, the effectiveness of the containers as absorbing material would be improved without increasing the amount of this foreign material in the polyethylene. Another approach which might help solve the problems of neutron moderation in irradiation containers and irradiation deterioration of the containers and also obtain larger epithermal neutron absorption capabilities might be to produce such containers from boron carbide or boron nitride. Modern sintering technology could be employed to produce such containers from these ceramic compounds.
RECEIVED for review March 16, 1970. Accepted November 20,1970.
Determination of Iron by EDTA Titrimetry of the Thiocyanate Complex Antonio de A. Figueiredo Instituto de Tecnologia Alimentar, Rua Jardim Botanico, 1024, ZC-20, Rio de Janeiro, B r a d
THETHIOCYANATE IRON COMPLEX is not satisfactory for analytical purposes since the color is neither stable nor proportional to the concentration and may be influenced by various factors (1-3). These inconveniences and interferences can be eliminated if the [Fe(SCN)n](3-n)f complex is extracted with an organic solvent, such as isoamyl alcohol. The complexometric titration with EDTA in acid medium (pH 2-3) is selective for iron since the majority of metal ions will not undergo complex formation at such low pH values ( 4 ) . The association of the two mentioned conditions-extraction of the iron(II1)-thiocyanate complex with isoamyl alcohol, and EDTA titration in acid medium-enables a simple and accurate volumetric determination of iron. In the proposed method, water is added to the isoamyl alcohol solution of the iron(II1)-thiocyanate complex and the conditions are adjusted to optimize the complexation of iron. The titration is performed directly, with both the aqueous and organic layers present in the titration flask. The end pointdecoloration of the organic phase-is easily observed. EXPERIMENTAL
Reagents. Reagent grade chemicals were used throughout : 25 (v/v) aqueous hydrochloric acid ; concentrated nitric
z
z
acid; isoamyl alcohol; 20 aqueous potassium thiocyanate; sodium acetate; and 0.01M EDTA solution (obtained by dilution of 0.1M stock solution standardized against zinc chloride). Procedure. Three or four drops of concentrated nitric acid are added to about 20 ml of iron solution which is heated on the water bath for 30 minutes in order to ensure total oxidation of the iron. After cooling, the solution is transferred to a 125-ml separatory funnel; 10 ml of isoamyl alcohol and 5 ml of 20% potassium thiocyanate are added, upon which the characteristic red color of iron(II1)-thiocyanate appears immediately (Equation 1). Fe3f
+ nSCN-
+
[Fe(SCN)n](3-n)+
The contents of the separatory funnel are well shaken for approximately 1 minute and afterward decanted. The almost colorless aqueous layer is transferred back to the same beaker in which the solution was heated; the colored organic layer is transferred to an Erlenmeyer flask for subsequent titration. The aqueous phase is re-extracted twice with isoamyl alcohol (1 X 10 ml, 1 X 5 ml). To the combined organic solutions, 25 ml of water and a small amount of sodium acetate (300400 mg) are added. This results in a decoloration of both phases caused by the equilibrium (Equation 2).
+ Acetate e
[F~(SCN)TZ](~ ‘)+ (1) C. A. Peters, M. M. MacMaster, and C. L. French, IND. ENG.CHEM., ANAL.ED.,11,502 (1939). (2) M. E. Kahane, Bull. Chim. France, 41, 1403 (1927). (3) H. W. Winsor, IND.ENG.CHEM., ANAL.ED.,9,453 (1937). (4) G. Schwarzenbach, “Complexometric Titrations,” 2nd ed., Methuen & Co. Ltd., London, 1960, p 17-78. 484
ANALYTICAL CHEMISTRY, VOL. 43, NO. 3, MARCH 1971
(1)
Aceto complexes
+ nSCN-
(2)
Hydrochloric acid, 2 5 % (v/v), is added carefully until the pH reaches a value between 2 and 3. The red color will reappear. The solution is heated on a water bath to 40-50 “C and titrated with 0.01M EDTA solution until the color dis-
appears in both phases (Equation 3).
+
[Fe(SCN)n](3-n)f Y4- -+ Fey-
+ nSCN-
Table I. Precision of Method in Terms of Relative Standard Deviation No. of Re1 std Iron taken, pg detn Iron found, pg dev 84.89 3 83.78 1.3 169.78 3 167.55 1.3 424.45 4 418.88 1.5
(3)
For amounts of iron below 1 mg, a microburet should be used. When the end point approaches, the amyl alcohol layer will be slightly pink. (A few more drops of the titrant will promote complete decoloration.) From this point on, after each drop of titrant, the flask must be agitated vigorously and then allowed to stand for 20 to 30 seconds and the color of the amyl alcohol layer observed. The end point is reached when this is colorless. The result of the determination is calculated from the volume of EDTA consumed.
Table 11. Levels of Interference from Various Cations Iron taken, Diverse ion Iron found, Difference, Pg added fig Pg 169.78 co 16 173.13 1.9 169.78 Ni 25 167.55 1.3 169.78 Mn 44 167.55 1.3 169.78 cu 36 167.55 1.3 Ag 80 167.55 1.3 169.78 169.78 Zn 41 167.55 1.3
z
RESULTS AND DISCUSSION
Good results were obtained with amounts of iron in the range 50 to 1000 pg with almost the same precision. The values of several determinations are presented in Table I. The interference due to the presence of phosphates, fluorides, and oxalates is eliminated by extraction with isoamyl alcohol (5). The small amounts of foreign ions such as Ag+, CuZf, Cozf, ZnZ+,etc., do not interfere as can be seen in Table 11. The method has been successfully used in the determination of iron in samples of coarse salt and calcinated bone meal used as supplements in cattle feeding. Thirty-nine ppm of iron could be determined in a sample of coarse salt. It was also used for the determination of iron in plants. In this case, the previous mineralization was made after Ward and Johnston (@.
The proposed method is probably suitable for determination of iron in foods, feeds, pharmaceuticals, and minerals. ACKNOWLEDGMENT
(5) G. W. Monier-Williams, “Trace Elements in Food,” Chapman & Hall, Ltd., London, 1949, pp 257-261. (6) G. M. Ward and F. B. Johnston, “Chemical Methods of Plant Analysis,” Canada Department of Agriculture; Publication 1064, Feb. 1960.
The author wishes to express his indebtedness to Dr. David Goldstein, Laboratbrio da Produ@o Mineral, Rio de Janeiro, for his experienced help in preparing the final manuscript, RECEIVED for review September 21, 1970. Accepted November 19, 1970.
Confirmation of the High Aromaticity of Anthracite by Broadline Carbon-13 Magnetic Resonance Spectrometry H. L. Retcofsky and R. A. Friedel U. S. Department of the Interior, Bureau of Mines, Pittsburgh Energy Research Center, 4800 Forbes Aue., Pittsburgh, Pa. 15213
ANTHRACITIC COALS are generally considered to be highly aromatic substances. The aromaticity of coal as well as of other carbonaceous materials is defined as the number of aromatic carbon atoms divided by the total number of carbon atoms and is designated fa. Literature values of J;1 for anthracites lie in the range 0.90 to 1.00 ( I ) . A large variety of physicochemical techniques including measurements of sound velocity, heat of combustion, proton magnetic resonance, and graphical densimetric methods have been used to arrive at these values. Detailed discussions of these and other methods of estimating fa for coals have appeared in several books ( 1 4 ) . The main objections to each of these
methods are that none of them yields a truly direct measure of and that all fa’s calculated from the data obtained from the particular measurement involved require a priori assumptions about the structure of coal. The present investigation was undertaken to explore the use of broadline carbon-13 nuclear magnetic resonance (13C NMR) spectrometry of solids in studies of coal structure. For this purpose, spectra of an anthracitic coal and the completely saturated hydrocarbon adamantane were obtained. NMR of solids, a relatively new technique, is potentially a method for the direct determination of aromaticities of coals and other organic materials of limited solubility.
fa
(1) D. W. van Krevelen, “Coal,” Elsevier, Amsterdam, 1961, p 447. (2) H. Tschamler and E. DeRuiter, “Chemistry of Coal Utilization,” SUPPI. v O l . 3 H.H.Lowry, Ed., Wih’, New York, 1963, P
EXPERIMENTAL
1 C JJ.
(3) W. Francis, “Coal,” 2nd ed., Edward Arnold, London, 1961,
720.
(4) H. Tscharnler and E. DeRuiter, “Coal Science,” R. F. G o ~ l d , Ed., Amer. Chem. SOC.,Washington, D.C., 1966, p 332.
NMR spectra were obtained using a Varian Associates Model ~ p - 6 0spectrometer operating at 15,085 M H ~and equipped with a time-averaging computer and Fieldial scanning unit. Broadline detection of the dispersion mode was employed throughout, thus all spectra produced are first derivative curves. The modulation amplitude was 0.156 G
ANALYTICAL CHEMISTRY, VOL. 43, NO. 3, MARCH 1971
485
