RESULTS
The 38” C. curve in Figure 1 is used. Mixtures of sodium phosphate and various amounts of tetrasodium pyrophosphate were made and analyzed by this method (Table I). Results obtained were within ~ t 0 . 0 4 7 , of actual content, which is of sufficient precision for present needs. The method is fast and gives results in a lower range than any other method that has come to the authors’ attention. This method works equally well with ammonium, potassium, and other sodium salts of orthophosphoric acid. The sample ‘weight must be adjusted to give the same orthophosphate ion concentration as in the above procedure, and the quantity of acetic acid must be changed to buffer the resulting solution to a p H of 3.6. Although not normally found in
Table
1.
Taken 0.02 0.05 0.08
0.11 0.13 0.15 0.21 0.27 0.28 0.31 0.33 0.43
0.53 0.62
Determination o f Pyrophosphate
Sodium
NarPZO?, % Found Difference 0.03 +0.01 0.07 $0.02 0.08 0 0.15 +O. 04 0.10 -0.03 0.15 0 -0.04 0.17 0.24 -0 03 -0.04 0.24 0.30 -0 01 0.34 10.01 0.40 -0.03 0.52 -0.01 0.58 -0.04
orthophosphate, the higher condensed phosphates, if present, would appear as a positive interference, as they tend to
complex iron also. Calcium and magnesium, if present, render the method useless, as their complexes with pyrophosphate are more stable than the iron-pyrophosphate complex. LITERATURE CllED
(1) Bell, R. s., IND.ENG CHEII.,~ ~ N A ED.,19, 97 (1947). 121 A. B.. Miles. F. T 13& . , Gerber. 10, 519 (1938). (3) Karl-Kroupa, Editha, Zbid., 28, 1091 (1956). (4) Snell, F. D., and Snell, C. T., “Colorimetric Methods of Analysis,” 3rd ed., Vol. 11, p. 314-15, Van Nostrand, New York, 1949. (ti) Wirth, H. E., IND.ENG.CHEW,ANAL., ED.14, 722 (1942.) I
RECEIVEDfor review March 18, 1957. Accepted August 30, 1957. Division of Analytical Chemistry, 131st Meeting, ACS, Miami, Fla., April 1057
Analysis of High-Purity Iron R. E. HEFFELFINGER, D. L. CHASE, G. W. P. RENGSTORFF, and W. M. HENRY Baftelle Memorial Insfitufe, Columbus, Ohio
b
The methods used a t Battelle Memorial Institute for determining impurities in high-purity iron are described. Separation o f metallic impurities b y ether extraction o f iron and spectrographic analysis o f the remaining aqueous solution are used for manganese, copper, nickel, chromium, vanadium, cobalt, aluminum, lead, titanium, tungsten, and zirconium. Sulfide precipitation and subsequent spectrographic analysis are used for elements in the sulfide group. Direct arcing of ferric chloride is used for silicon, magnesium, and calcium. Detection limits b y these procedures range from 0.1 to 10 p.p.m. Boron i s determined b y standard spectrographic methods. Carbon i s determined by a combustion method, and sulfur and phosphorus are determined spectrophotometrically. Oxygen, hydrogen, and nitrogen are determined b y vacuum-fusion analysis.
I
on the complete analysis of high-purity iron is not readily available. The impurity content of iron prepared by purifying the best available grades of electrolytic iron with hydrogen is so low that standard analytical techniques are unsatisfactory. Therefore, when the American Iron and Steel Institute asked Battelle Memorial Institute to set up a “bank” of highpurity iron to be available to researchers NFORMATION
112
ANALYTICAL CHEMISTRY
for studies on basic properties of iron, steel, and other iron-containing alloys (11, la), it was necessary to develop adequate analytical methods. Nom that iron, which is being distributed by the American Iron and Steel Institute, is being further purified by zone mclting, the need for special techniques suitable for low levels of impurities is becoming even more important. This paper describes some of the procedures used a t Battelle for obtaining a complete analysis of high-purity iron. It was necessary to use special concentrating techniques, such as ether separation and sulfide precipitation, in conjunction with spectrographic analysis. Analysis for phosphorus required extensive modifications of known methods. On the other hand, analysis of carbon required only the substitution of solid samples for the usual turnings to adapt standard high-precision methods. Techniques developed for oxygen and hydrogen in other high-purity metals were also applied to iron with only slight changes. SPECTROGRAPHIC METHODS
Chemical means of determining impurities of the order of 10 p.p.m. or less, while sometimes sensitive enough, are too time-consuming for most elements. Fast, accurate methods, such as those involving spectrographic analysis, are therefore needed.
Wolfe and Fowler ( I S ) used a solution of iron dried in a graphite-cup electrode in the spectrographic analysis of pure iron. The authors have found that the detection limits by such a method are of the order of 10 p.p.m, Because the concentration of most impurities in highpurity iron is below this limit, a more sensitive method is required. Kolfe and Fowler ( I S ) and Scribner [see Moore ( 9 ) ] used an ether extraction of iron as a method for separating impurities before a spectrographic determination of the impurities. Because some of the impurities are extracted with the iron, two additional methods are used a t Battelle Institute for a complete analysis-sulfide precipitation and direct arcing of ferric chloride. Because of the very lorn impurity concentration involved, the separation techniques are the most critical parts of the analysis of high-purity iron. Ether Extraction Method. The ether extraction method is useful for determining the following impurities in high-purity iron: manganese, copper, nickel, chromium, vanadium. cobalt, aluminum, lend, titanium, tungsten, zirconium, and zinc. I n principle, this method is similar to that described by Wolfe and Fowler (15). It differs because of the use of ethyl ether, rather than isopropyl ether, in a continuous extraction and in treatment of the aqueous portion of the extraction process. The process is
L
presented here to give a more complete technique for high-purity iron.
PROCEDURE. A 1-gram sample of the high-purity iron is dissolved in about 20 ml. of redistilled 6N hydrochloric acid and oxidized with a slight excess of concentrated nitric acid (about 1 ml.). The solution is evaporated to near dryness to expel excess nitric acid, then diluted 11-ith about 40 ml. of redistilled 6 9 hydrochloric acid. The ferric chloride is then extracted with diethyl ether in a continuous extractor ( I ) or by standard batch techniques (6, 7 ) . If a batch extraction is used, the ether should be distilled before use. The aqueous portion from the extraction process is added to a beaker containing bismuth (0.1 mg. added in solution) as a n internal standard and 100 mg. of pure graphite powder, and is evaporated to dryness. A 30-mg. portion of this graphite is placed in a graphite-cup electrode and excited in a direct current arc. The arc spectra from the standards and samples are recorded on the same film and compared visually. (In each of the spectrographic methods described in this paper, the spectra are photographed in the region of 2200 to 4400 A.) Standards are made in concentrations of 10 to 500 p.p.m.; these correspond to actual impurity concentrations of 1 to 50 p.p.m., as the extraction process causes a tenfold concentration of impurities. The standard itself is made by adding the element (as its oxide) to be determined to pure graphite powder a t several levels in the desired range. Table I shows the elements that are detected by this method. Typical recovery data for elements determined after removal of iron by ether extraction are given in Table 11. The accuracy is of the order of i 5 0 % . Poor results
Table 1.
hln FeC1, Ether extraction HIS separation
10
0.1
...
for the determination of silicon, magnesium, and calcium are due to pickup of trace amounts during handling of the sample. S a d e Precipitation Method. As elements not detectable by the ether extraction method are usually present, it is necessary t o employ other highsensitivity methods. The sulfide group includes many of t h e impurity elements in pure iron, and the precipitation of these elements as sulfides is the basis for their separation and subsequent determination.
PROCEDURE. One gram of iron is dissolved in 24 ml. of 10% sulfuric acid. Copper sulfate solution equivalent to 10 mg. of copper is added to serve as a carrier for the sulfide group. The solution is adjusted to 100 ml. of 0.5N sulfuric acid, heated to just boiling, and treated with hydrogen sulfide for 30 minutes. The solution is heated to just under boiling, and the precipitate is collected on a filter paper, washed several times to remove iron, and ignited a t 450' C. for 2 hours. The entire precipitate is placed in a graphite-cup electrode and excited in a direct current arc. The sample and standard spectra are recorded on the same photographic film. Spectrographic standards are made by adding elements in the sulfide group to pure copper oxide, bearing in mind that the conversion factor from iron to copper is 100. Visual comparison of the standard with sample spectra gives a probable accuracy of about =!=50%. The recovery is checked by adding impurities in solution to the sulfuric acid solution of iron. Recoveries of arsenic, tin, and molybdenum may also be checked by use of Kational Bureau of Standards samples 21d and 55d. The recovery data are given in Table 111.
PROCEDURE. Thirty grams of iron are dissolred in 300 ml. of redistilled 6*\- hydrochloric acid and oxidized with about 15 ml. of concentrated nitric acid. The solution is evaporated t o moist dryness and diluted with concentrated hydrochloric acid to make a 30y0 iron solution (each milliliter will contain 0.3 gram of iron). A 0.05-ml. portion of the solution (15 mg. of iron) is added to a graphite-cup electrode which has been made nonporous with a mineralfree organic coating. It is then necessary t o dry slowly to prevent mechanical loss of ferric chloride. Samples and standards are excited in the direct current arc and the spectra recorded on photographic film. The determination is made by means of a
Detection Limits of Impurities in Iron b y Various Methods (P.P.M.) cu Si Cr v Mo co A1 10 10 30 30 40 10 30 0.1 1.0 1.0 1.0 ... 1.0 1.0 ... ... ... ... ... 5 ... ...
Si 10 High blank
1Ig
Pb
Sb
5 0. l a ...
10 1.0
...
Fecia Ether extraction HZS separation Blank usually higher.
As the elements determined by sulfide precipitation are not common laboratory contaminants, ordinary precautions of cleanliness are sufficient. Ferric Chloride-Dried Salt Method. -4 direct approach is useful for the determination of silicon, magnesium, and calcium because these elements give a high blank in the ether extraction method and are not precipitated in the sulfide method. Although iron metal could be used directly in spectrographic analysis, solution is used because a larger amount of iron can be sampled and because standards are readily made by dissolving a sample of pure iron and then adding impurity elements a t known levels from stock solutions. The chloride of iron is used rather than some other anion because of its high sohbility in concentrated hydrochloric acid. Also, metal chlorides are more volatile than most other salts, and, hence, vaporize into the arc more readily.
...
...
5
Ti 10 0.5 ...
11-
100 10 ...
Zr 10
0 5 ...
Be
Ca
.~
5
10
...
10'J
J
...
, . .
Sn 50
Zn
-4s
Ga
I3
100
...
...
.. ..
10
...
...
10
r\.d data
Ge
Ctl
...
~d
... ...
brtta
Table II. Recovery Data from Ether Extraction Method (Per Cent) Si Cu Cr Ni Mo V Co Sn A1 Ca Ti Zr Zn M g Pb 0.008 0.0005 0.02 0.002 0.008 0.004 0.001 . . 0 . 0 0 2 0.001 0.001 0.001 . . 0.001 0.001 0.005 0.005 0.02 0.002 0.005 . . . 0.0005 0:002 . . 0.002 0.005 0.002 0.001 . . 0.002 0.005 0.01 . . . 0.01 0.01 0.01 0.01 0.01 . . . . . . . . . . . . . . . . . . . . . . 0.005 . . . 0.01 0 . 0 1 0.01 . . . 0.005 o.'oi . . . . . . . . . . . . . . . . . . . . . . 0.001 . . . 0.001 0.001 0.001 0 001 0.001 0.001 . . . . . . . . . . . . . . . . . . . . . . 0.001 . . . 0.001 0,001 0.001 . . . 0.001 0.001 . . . . . . . . , . . . . . . . . . . . . . 0.016 0.001 0.04 0.003 0.016 0.004
