Table 1.
Limiting Cclncentrations of Interfering Ions
Interference Cr(V1) V(V) Ce(IV)
Mn(VI1)
Limiting concentration, pg./50 ml. SO 51 _-
408 0.55
one month after extraction showed no change in absorbance. Nitric acid was used :‘or the oxidation of iron to its trivalent ~ t a t ein the NBS samples. This was utsed rather than peroxide because of the lower reduction potential of nitric acid. It will oxidize iron to its higher oxidation state, but will not oxidize many other metals-i.e., Cr and Mn-to their higher oxidation states. In their higher oxidation states, these metals would also oxidize the reagent. Solutions of reagent heated to boiling in 0.1M HX03 showed no oxidation. The rate of the oxidtttion reaction is temperature dependent. The temperature effect was studied and results are given in Figure 5. Solutions were maintained a t the temperature indicated for a period of 5 minutes and then were cooled to room temperamre. The reaction is complete a t a temperature of 90” c. Beer’s law is obeyed over the concentration range 1.5 to 150 pg. Fe(II1) in a 50-ml. sample. Sensitivity of the method, expressed as the weight of iron corresponding to an absorbance of 0.010 a t 550 mp, is 1.48 pg. This corresponds to a sensitivity of 0.030 p.p.m. if a 50-ml. sample of unknown iron solution is used, as #suggestedin the procedure. The molar absorptivity is 2 X lo3 mole-’ cm.-l liter. This is calculated from the absorbance of the aqueous solution of the oxidation product and the concentration of iror. in the aqueous solution. The sensitivity is increased tenfold in the suggested procedure by extracting the oxidation product from a 50-ml. sample into 5 m: . of nonaqueous solvent. Sensitivity of the 8-.aminoquinoline method compares favorably to the standard colorimetric methods used for iron, such as 1,lO-phenanthroline (1) and thiocyanate (4). I3owever, as is
generally true with organic reducing agents, 8-aminoquinoline is less specific. Any oxidizing agent with a reduction potential more positive than that of iron is capable of reacting with the reagent. However, many other oxidizing agents have pH curves which are different from that of iron (Figure 2A), and if these oxidations are carried out in the pH interval 1.25 to 1.50 they are incomplete. Rates of the oxidation reactions are also dependent upon the concentration of the oxidizing agent. As the concentrations are decreased, the rates become so slow that the reactions tend to become very incomplete. The combination of the pH effect and the reaction rates tends to increase the limiting concentration of other Oxidizing agents as interferences. Table I contains several Oxidizing agents that were studied as
sample, it must be removed before the analysis for iron is performed. The method was checked using two different NBS aluminum alloy samples. The results are given in Table 11. LITERATURE CITED
B., hIellon, ?v[. U., IND.Exa. CHEM.,ANAL. ED. 10, 60
(1) Fortune, 1%’.
(1938). (2) Popa, G., Paralescu, I., Mircea, D., 2. Anal. Chem. 184, 353 (1961). (3) Sandell, E. B., “Colorimetric Determination of Traces of Metals,” 3rd ed., p. 135, Interscience, Xew York, 1950. (4) Snell, F. D., Snell, C. T., “Colorimetric Methods of Analysis,” 1’01. 11, p. 307, Van Xostrand, New York, 1949.
RECEIVED for review December 17, 1962. Accepted June 14, 1963.
Corrections Analysis of NBS Aluminum Alloy Samples Sample %Fe, NBS % Fe
Table II.
NO.
value
found
55a
0.208
0.207 0.211 0.209 0.211 Av. 0.210 0.244 0.239 0.241 0.242 Av. 0.241
55b
0.24
interferences in the iron deterniiiiatioii Limiting concentration is defined here as the concentration of interference needed to produce a change in the absorbance of 0.010. These reactions were carried out under the conditions given in the procedure for iron. In addition to the amount of interference indicated, all solutions also contained 62.5 pg. of Fe(II1). When added to a sample containing 1.00 p.p.m. of Fe(III), 50 p.p.m. of Cu(II), Co(II), Ki(II), Mn(II), Cr(III), Zn(II), nIg(II), Ca(II), Cd(II), and Sn(I1) ions caused no interference. The prcaence of traces of phosphate ion results in an incomplete reaction. Therefore, if phosphate is present in a
X-Ray Emission Analysis of Plutonium and Uranium Co mpound Mixtures In this article by Oscar Menis, E. K. Halteman, and E. E. Garcia [ANAL. CHEM.34, 1049 (1963)] reference to the potassium pyrosulfate flux (p. 1051) should include citation of the early work by T. J. Cullen [AXAL.CHEM.32, 516 (1960)l and [ANAL. CHEM. 34, 862 (1962) 1.
The Chromatographic Determination of Trace Am o unts of Polynuclea r Hydrocarbons in Petrolatum Mineral Oil, and Coal Tar I n this article by William Lijinsky etal. [ANAL.CHEM.35, 952 (1963)l on page 953, the first sentence under Experimental should read: “Procedure. The mineral oils and white petrolatums (U.S.P. grade) and amber petrolaturns (S.F. grade) were obtained commercially, as wa5 the creo,?ote.”
VOL. 35, NO. 10, SEPTEMBER 1963
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